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 THE GERM-PLASM.
 
 THE GERM-PLASM 
 
 A THEORY OF HEREDITY. 
 
 /AUGUST WEISMANN, 
 
 Professor~in the University of Frciburg-in- Baden. 
 
 TRANSLATED BY 
 W. NEWTON PARKER, Ph.D., 
 
 Professor in the University College of South Wales anti Monmouthshire, 
 AND 
 
 HARRIET RONNFELDT, B.Sc. 
 
 WITH 24 ILLUSTRATIONS. 
 
 LONDON: 
 WALTER SCOTT, LTD., 24 WARWICK LANE, 
 
 PAT EU MOST KK ROW, 
 I8 93 .
 
 TO THE MEMORY 
 
 CHARLES DARWIN.
 
 NOTE TO THE ENGLISH EDITION. 
 
 T N preparing the English edition of the present work, I have 
 ^ had the great advantage of being able to consult Professor 
 Weismann personally with regard to many of the more difficult 
 passages. Only those who have attempted to make a translation 
 of an abstruse work from German manuscript, can appreciate 
 the difficulties of rendering such a work into good English, and 
 at the same time of keeping closely to the text. As the time in 
 which the translation had to be prepared was a comparatively 
 short one, I have been unable to revise the style as thoroughly 
 as I could have wished, but trust that the author's meaning has 
 been expressed with tolerable accuracy. 
 
 In the case of special technical terms which have no recog- 
 nised English equivalents, I have in all cases added the German 
 word in brackets the first time they are used. For the extremely 
 useful and untranslateable word 'Anlage,' the somewhat awkward 
 term 'primary constituent' has been used when it refers to the 
 concrete vital units : in other cases, it has been rendered by 
 ' rudiment ; ' or, when it has a more abstract meaning, by ' pre- 
 disposition.' The words ' Eigenschaft,' ' Charakter,' 'Merkmal,' 
 and ' Qualitat,' are often used synonymously by the author, and 
 have therefore been indiscriminately translated by ' character- 
 istic,' ' character,' ' peculiarity,' and ' quality.' 
 
 I must express my thanks to Dr G. H. Parker, of Harvard 
 University, Cambridge, Mass., who kindly undertook a first 
 revision of Chapters XIII. and XIV., and thereby rendered an 
 earlier publication of the book possible ; as well as to my friend 
 and colleague Mr Franck Arnold, for help in elucidating some 
 of the more complicated sentences, and for many suggestions. 
 
 W. N. PARKER. 
 
 CARDIFF, Nov. a&th 1892.
 
 quate m this respect : owing to the comparatively limited 
 number of facts then at his disposal, it could not but be 
 what we may call an ideal theory; that is to say, it is 
 founded upon certain principles without inquiring how far 
 they are based upon actual facts. In themselves, such 
 theories can hardly be looked upon as suggestive, for if 
 once the assumed principle is accepted, all the phenomena are 
 thereby explained, and the matter is open to no further 
 doubt. 
 
 Let us assume that the germ contains millions of the 
 primary constituents ('Anlagen') of all the most minute 
 portions of the body ; moreover, that these constituents are 
 always present at the right place and in the right combina- 
 tion during the process of development ; and, further, that 
 they are capable of giving rise in their turn to the parts or 
 organs to which they severally correspond. Such a theory 
 explains everything, or nothing the premises alone can be 
 attacked. No new problems can arise from it till it has 
 been placed upon a sound basis; the premises must be 
 shown to be correct, and it must be proved that the germ is 
 actually composed of primary constituents, which by some 
 means or other become combined into groups and are 
 capable of giving rise to the various parts and organs in 
 question. Then, and then only, would the theory serve as 
 an incentive to further investigations into the phenomena of 
 heredity of all kinds, and experiments might be made which 
 would support or contradict it. 
 
 There is no doubt a natural tendency to base experiments 
 upon certain preconceived ideas ; but it is one thing to be 
 guided solely by such phenomena as may at the moment 
 appear of especial importance, and another to base opera- 
 tions upon the completed outline of a theory founded upon
 
 PREFACE. XI 
 
 the principal data bearing upon the question. I have myself 
 more than once abandoned a line of research undertaken in 
 connection with the problem of heredity, because I felt that 
 to proceed without the guidance of a theory more or less 
 complete in itself, and developed on a basis of ascertained 
 facts, would be little better than groping in the dark. The 
 importance of such a theory lies primarily in its suggestive- 
 ness, by which alone it becomes a step towards the ideal at 
 which we aim, viz., the formulation of the true and complete 
 theory. 
 
 The growth of this book has been very gradual. What 
 first struck me when I began seriously to consider the 
 problem of heredity, some ten years ago, was the necessity 
 for assuming the existence of a special organised and living 
 hereditary substance, which in all multicellular organisms, 
 unlike the substance composing the perishable body of 
 the individual, is transmitted from generation to generation. 
 This is the theory of the continuity of the germ-plasm. My 
 conclusions led me to doubt the usually accepted view 
 of the transmission of variations acquired by the body 
 (soma) ; and further research, combined with experiments, 
 tended more and more to strengthen my conviction that in 
 point of fact no such transmission occurs. Meanwhile, the 
 investigations of several distinguished biologists in which I 
 myself have had some share on the process of fertilisation 
 and conjugation, brought about a complete revolution in 
 our previous ideas as to the meaning of this process, and 
 further led me to see that the germ-plasm is composed of 
 vital units, each of equal value, but differing in character, 
 containing all the primary constituents of an individual. 
 These ' ancestral germ-plasms ' (Ahnenplasmen'), or ' ids,' as 
 I now prefer to call them, afforded additional matter where-
 
 with to construct a theory of heredity, though much was 
 wanting to render it complete. 
 
 In my last essay I certainly suggested the possibility of 
 solving one of the most difficult problems in heredity viz., 
 the co-operation of the hereditary substance of the parents 
 in sexual reproduction by assuming the existence of these 
 ' ids ' ; but I did not for a moment suppose that in doing 
 so I had propounded a complete and elaborated theory of 
 heredity, as some of my readers have thought to be the 
 case ; much still remained to be done first. I had as yet 
 not touched upon such phenomena of heredity as have no 
 direct bearing on the question of sexual reproduction, and 
 had also abstained from any mention of the fundamental 
 point of my theory of heredity namely, the constitution of 
 the ids. Although I pointed out that they must possess 
 a complex structure which undergoes gradual and regular 
 changes during the development of the individual from the 
 egg-cell, I did not enter into any further details. This 
 question remained in abeyance, for I was by no means sure 
 whether the conception that I had formed on a priori 
 grounds of the minute structure of the ids would prove 
 tenable when viewed in the light of all the many phenomena 
 of heredity. No conclusion could be arrived at respecting 
 the structure of the ids till these phenomena had been 
 individually considered. 
 
 All my investigations on the problem of heredity were so 
 far only links, to be some day united into a chain which had 
 as yet no existence. The question of the ultimate elements 
 on which to base the theory was the very point on which I 
 remained longest in doubt. The ' pangenesis ' of Darwin, 
 as already mentioned, seemed to me to be far too inde- 
 pendent of facts, and even now I am of the opinion that the
 
 PREFACE. Xlii 
 
 very hypothesis from which it derives its name is untenable. 
 There is now scarcely any doubt that the entire conception 
 of the production of the ' gemmules ' by the body-cells, their 
 separation from the latter, and their ' circulation,' is in reality 
 wholly imaginary. In this regard I am still quite as much 
 opposed to Darwin's views as formerly, for I believe that 
 all parts of the body do not contribute to produce a germ 
 from which the new individual arises, but that, on the 
 contrary, the offspring owes its origin to a peculiar substance 
 of extremely complicated structure, viz., the 'germ-plasm.' 
 This substance can never be formed anew; it can only grow, 
 multiply, and be transmitted from one generation to another. 
 My theory might therefore well be denominated 'blasto- 
 genesis' or origin from a germ-plasm, in contradistinction 
 to Darwin's theory of ' pangenesis ' or origin from all parts 
 of the body. 
 
 My doubts as to the validity of Darwin's theory were for 
 a long time not confined to this point alone : the assumption 
 of the existence of preformed constituents of all parts of the 
 body seemed to me far too easy a solution of the difficulty, 
 besides entailing an impossibility in the shape of an abso- 
 lutely inconceivable aggregation of primary constituents. I 
 therefore endeavoured to see if it were not possible to 
 imagine that the germ-plasm, though of complex structure, 
 was not composed of such an immense number of particles, 
 and that its further complication arose subsequently in the 
 course of development. In other words, what I sought was 
 a substance from which the whole organism might arise by 
 epigf nests, and not by evolution* After repeated attempts, in 
 
 * The theory of ' evolution ' or ' preformation ' of the early physiolo- 
 gists supposed that all parts of the fully-formed animal or plant were 
 present, in a minute form, in the germ. The rival theory of 'epigenesis '
 
 which I more than once imagined myself successful, but 
 all of which broke down when further tested by facts, I 
 finally became convinced that an epigenetic development is 
 an impossibility. Moreover, I found an actual proof of the 
 reality of evolution, which will be explained in the chapter 
 on the structure of the germ-plasm. It is so simple and 
 obvious that I can scarcely understand how it was possible 
 that it should have escaped my notice so long. 
 
 It is gratifying to me to find myself at one with the great 
 English naturalist Darwin, as well as with de Vries and 
 Wiesner, at all events in the main point at issue ; and this 
 agreement seems to me to point to the possibility of solving 
 in the end the problem of heredity, which might seem to be 
 open only to the wildest speculations : we may now perhaps 
 hope to succeed in recognising the probable explanations 
 among the many possible ones, and in finally selecting from 
 among these the real solution of the problem. This will 
 assuredly be the work of time, and our approach to the truth 
 will be a very gradual one. But our path is marked out ; 
 reasoning supported by observation will lead us to the goal. 
 We are led by the observation of facts to form an opinion as 
 to their bearing on each other. This gives rise to further 
 problems and fresh investigations, which in their turn lead 
 to a new interpretation. In this way light has before now 
 been thrown on many a problem that seemed to baffle 
 explanation. I need only mention the insight that we have 
 now gained into the phenomenon of sexual reproduction. 
 
 taught that there is no preformation of parts in the germ, but that the 
 fully-formed organism is produced by a gradual process of differentiation. 
 It will be seen that the word ' evolution," as here used, has no connec- 
 tion with the doctrine of descent with which it is usually connected. 
 W. N. P.
 
 PREFACE. XV 
 
 In the same way we shall succeed in obtaining a firmer and 
 firmer grasp of the problem of heredity, which but a short 
 while ago appeared so utterly unapproachable. 
 
 What in this particular question appears to afford additional 
 promise of success is the fact that we can in a sense approach 
 it from two sides ; namely, by observations, firstly, on the 
 phenomena of heredity, and secondly, on the hereditary sub- 
 stance itself, with which we are now of course acquainted. 
 We can now form an estimate as to whether an explanation 
 of any particular phenomenon of heredity is of a merely hypo- 
 thetical nature, or whether it may attain to the value of an 
 established fact, inasmuch as we are in a position to judge, 
 within certain limits at all events, whether it is consistent 
 with the actual behaviour of the hereditary substance. 
 Hitherto this has not been possible, and hence all previous 
 theories, including both that of Darwin's gemmules and of 
 Herbert Spencer's units, were up to a certain point purely 
 speculative. We are now better off in this respect; and 
 I have no doubt that further research will enable us to 
 penetrate far more deeply still into the complicated processes 
 connected with the idioplasm, if we are prepared to reason 
 on the results of our observations, and to utilise every 
 theoretical advance as an incentive to fresh questions 
 regarding the processes in connection with the distribution 
 of the mysterious nuclear substance. 
 
 We are still far from having attained a complete insight 
 into the matter, but I trust nevertheless that the present 
 attempt at a theory of heredity is no mere work of the 
 imagination ; and though it still be no more than an attempt, 
 which will be followed by better ones, I venture to believe 
 that time will prove it to contain more definite points, 
 forming the centre of numerous possibilities, than many will
 
 XVI PREFACE. 
 
 for the present be prepared to admit. Nevertheless I am 
 well aware that it is but the beginning of a theory, and for 
 this reason I have presented it in the form of an inquiry 
 rather than of an established system. My plan has been 
 not so much to advance doctrines as to propound questions, 
 and to answer them with a greater or lesser degree of certainty, 
 or in some cases even to leave them to be decided by future 
 researches. I do not regard my theory as a complete and 
 perfect one, but trus, 4 - that it is of such a nature as to be 
 capable of improvement and further development. 
 
 It has been my endeavour to write as simply and intel- 
 ligibly as possible ; not as a specialist writing for specialists, 
 but as one who desires to make his case clear to all inter- 
 ested in biological problems. For this reason a number of 
 figures have been inserted, which, though perhaps super- 
 fluous for specialists, will, I trust, assist all who are less 
 conversant with the subject, such as physiologists, medical 
 men, and indeed all interested in natural science, to a clearer 
 conception of the matters under discussion. 
 
 As a zoologist, I have naturally, in the first instance, 
 considered the phenomena in their relation to animals, for 
 every one must base his ideas on the facts most familiar to 
 him. I have, however, done my best to lay due weight on 
 the data afforded by the study of plants, and to take into 
 account the views of botanists. It will be seen that the 
 very facts which are furnished by certain hereditary pheno- 
 mena in plants afford a strong support to certain fundamental 
 points in my theory, and that even those which are at first 
 sight in apparent contradiction, are in reality in perfect 
 accordance with it. 
 
 It may perhaps be considered by medical men that I 
 ought to have brought forward more evidence with regard
 
 to diseases. We certainly possess a rich material on which 
 observations concerning the transmission of diseases might 
 be based, and this I have made use of so far as seemed 
 expedient. It must, however, not be forgotten that the 
 transmission of so-called hereditary diseases is not always 
 due to a true process of heredity, but in some cases, at any 
 rate, results from an infection of the germ. Unfortunately, 
 we are not always able to distinguish between these two 
 causes ; and as long as this is the case, the data furnished 
 by diseases can only be used with great caution, as will 
 be shown in Chapter XII. 
 
 The manuscript of this book was practically completed by 
 the end of April last, but as the translation had then to be 
 made, its publication was delayed for some months. This 
 will account for the fact that no mention, or only a brief 
 one, has been made of researches which have appeared in 
 the interval. My sincerest thanks are due to the translator 
 Professor W. N. Parker, whose task has -been by no 
 means an easy one : apart from the mere knowledge of 
 the two languages, an intimate acquaintance with the facts 
 treated of and with the whole science of biology is essential 
 in order to render the meaning of this complicated subject 
 clear, and at the same time to reproduce the original text 
 with anything like accuracy. I am of course unable to 
 judge how far Mr Parker has succeeded in clothing my ideas 
 in good English, but am glad to state that they have been 
 given very correctly, so far as I can judge from those parts 
 which we have discussed together. 
 
 In conclusion, I must express my warmest thanks to the 
 Government under which I have the good fortune to live, 
 for the efficient way in which they have seconded my 
 endeavours, by releasing me from my academical duties
 
 xviii PREFACE. 
 
 during two winter sessions. My hearty thanks are also due 
 to my friends and colleagues Professors Baumann, Liiroth, 
 Wiedersheim, and Ziegler, as well as to Professor Goebel, of 
 Munich, for information of various kinds; and I am no 
 less indebted to Miss Else Diestel, who, in addition to 
 much help of a technical nature, has also been at the great 
 pains of preparing an alphabetical index. 
 
 I thus venture to bring into the light of day a work which 
 is the fruit of many years labour and of many doubts ; and 
 even though but few of my results should remain unmodified, 
 I hope nevertheless that my work has not been in vain ; for 
 even error, if it originate in correct deductions, must become 
 a step towards truth. 
 
 AUGUST WEISMANN. 
 
 FREIBURG, I/BR., 
 May \gth 1892.
 
 CONTENTS. 
 
 INTRODUCTION. 
 
 PAGE 
 
 A. HISTORICAL PART - i 
 
 B. -DESCRIPTIVE PART 20 
 
 PART I THE MATERIAL BASIS OF HEREDITY. 
 
 CHAPTER I. 
 THE GERM-PLASM 
 
 1. The fundamental units 37 
 
 2. The control of the cell 45 
 
 3. The determinants - 53 
 
 4. The id in ontogeny 60 
 
 5. Summary of sections 1-4, relating to the structure of 
 
 the germ-plasm 75 
 
 6. The mechanism for the phyletic changes in the germ- 
 
 plasm 77 
 
 7. The magnitude of the constituents of the germ-plasm - 85 
 
 PART II. HEREDITY IN ITS RELATION 
 TO MONOGONIC REPRODUCTION. 
 
 CHAPTER II. 
 REGENERATION 
 
 1. Its cause and origin in the idioplasm - 93 
 
 2. The phylogeny of regeneration - 1 14 
 
 3. Facultative or polygenetic regeneration 126 
 
 4. Regeneration in plants 132 
 
 5. Regeneration in animal embryos, and the principles 
 
 of ontogeny - - 134
 
 XX CONTENTS. 
 
 PAGE 
 
 CHAPTER III. 
 MULTIPLICATION BY FISSION 
 
 1. Preliminary remarks - ... 145 
 
 2. The process of fission in the NaiJce - 14 
 
 3. The process of fission in the Microstomida - - 149 
 
 4. The phylogeny of the process of multiplication by fission 151 
 
 CHAPTER IV. 
 MULTIPLICATION BY GEMMATION 
 
 1. The proces 5 of gemmation in animals - - 154 
 
 a. Ccelenterata - - 154 
 
 b. Polyzoa - - 158 
 
 c. Tunicata - 1 60 
 
 2. The process of gemmation in plants - - 163 
 
 3. Comparison of the process of gemmation in animals 
 
 and plants - - 166 
 
 4. The phylogeny of the process of multiplication by 
 
 gemmation - - 168 
 
 CHAPTER V. 
 ALTERNATION OF GENERATIONS IN ITS RELATION TO THE 
 
 IDIOPLASM - 173 
 
 CHAPTER VI. 
 THE FORMATION OF GERM-CELLS 
 
 1. The continuity of the germ-plasm - - - 183 
 
 2. The germ-tracks - - 192 
 
 3. Historical account of the theory of the continuity of 
 
 the germ-plasm - - 198 
 
 4. Objections to the theory of the germ-plasm - - 202 
 
 5. Galls - - 218 
 
 CHAPTER VII. 
 SUMMARY OF PART II. (CHAPTERS II. -VI.) - - 225 
 
 PART III. THE PHENOMENA OF HERE- 
 DITY RESULTING FROM SEXUAL RE- 
 PRODUCTION. 
 
 INTRODUCTORY REMARKS ON THE NATURE OF SEXUAL 
 
 REPRODUCTION - - - 230
 
 CONTENTS. Xxi 
 
 PACK 
 
 CHAPTER VIII. 
 
 MODIFICATIONS OF THE GERM-PLASM CAUSED BY AM- 
 PHIMIXIS 
 
 1. The necessity of a halving of the germ-plasm - - 235 
 
 2. Proof that the essential part in the process of ' reducing 
 
 division ' consists in the extrusion of ids - - 240 
 
 CHAPTER IX. 
 
 ONTOGENY RESULTING FROM THE UNION OF THE GERM- 
 PLASM OF TWO PARENTS 
 
 1. The nature of the offspring as determined by the 
 
 process of fertilisation - - 253 
 
 2. The share taken by the ancestors in composition of 
 
 the germ-plasm - - 256- 
 
 3. The struggle of the ids in ontogeny - - 260 
 
 a. Plant-hybrids - - 260 
 
 b. Intercalary remarks on variation - 271 
 
 c. The struggle of individual characters - - 274 
 
 4. The force of heredity - - 290 
 
 5. Summary of Chapter IX. - . 293 
 
 CHAPTER X. 
 
 THE PHENOMENA OF REVERSION IN THEIR RELATION 
 TO AMPHIMIXIS 
 
 1. Reversion to racial characters in plant-hybrids - 299 
 
 2. Reversion to individual characters in man - - 308 
 
 3. Reversion to the characters of ancestors far removed 
 
 in animals and plants - 316 
 
 4. Reversion to rudimentary characters - - 333 
 
 5. Preliminary summary of sections 1-5 - - 335 
 
 6. Reversion in asexual reproduction - - 338 
 
 a. Reversion in the process of gemmation - 338 
 
 b. Reversion in parthenogenesis - - 344 
 
 7. Proof that the determinants become disintegrated 
 
 into biophors - 348 
 
 CHAPTER XI. 
 DIMORPHISM AND POLYMORPHISM 
 
 1. Normal dimorphism - - 352 
 
 2. Pathological dimorphism : haemophilia 370 
 
 3. Polymorphism - - 374 
 
 4. Dichogeny in plants 380
 
 Xxli CONTENTS. 
 
 PAGE 
 CHAPTER XII. 
 
 DOUBTFUL PHENOMENA OF HEREDITY 
 
 1. ' Xenia ' and telegony - - - 383 
 
 2. The influence of temporary abnormal conditions of 
 
 the parents on the offspring - - 386 
 
 3. The transmission of diseases - - 387 
 
 PART IV. THE TRANSFORMATION OF 
 SPECIES: ITS ORIGIN IN THE IDIO- 
 PLASM. 
 
 CHAPTER XIII. 
 
 THE SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS 
 
 1. Difficulties in the way of a theoretical basis for this 
 
 assumption - - 392 
 
 2. The hypothesis tested by facts - 396 
 
 3. Climatic variation in butterflies - - - 399 
 
 CHAPTER XIV. 
 VARIATION 
 
 1. Normal individual variation - - - -410 
 
 2. Pathological variation - - 428 
 
 3. Summary of the above two sections, and conclusions - 431 
 
 4. Variations on a larger scale - - 435 
 
 a. The origin of these variations - - 435 
 
 b. The transmission of these variations - - 444 
 
 SUMMARY AND CONCLUSION 450
 
 LIST OF FIGURES. 
 
 Fig. I. Diagram of nuclear division. 
 
 2. Two idants of Ascaris megalocephala. 
 
 3. Diagram of the cell-generations in the fore-limb of a newt. 
 
 4. Multiplication by fission in a marine worm (Myrianida). 
 
 5. Transverse section through the 'zone of gemmation' in a 
 
 fresh-water worm (Nais). 
 
 6. An early stage in the formation of a bud in a Hydroid polype 
 
 ( Eudendriu m ) . 
 
 7. Apex of a shoot of a fresh-water Alga (Ckara). 
 
 8. Female water-flea (Daphnia), with summer eggs. 
 
 9. Nauplius larva of Leptodora hyalina. 
 
 10. Alternation of generations in Bougainvillea. 
 
 11. Strobilation of a Medusa. 
 
 12. Degeneration of a Medusa into a mere gonophore. 
 
 13. Early stages of development of the primitive germ-cells in 
 
 Sagitta. 
 
 14. Early stages in the development of the primitive germ-cells in 
 
 Moina. 
 
 15. Early stage of the embryo of Rhabditis nigrovenosa. 
 
 16. Diagram of the germ-track in Rhabditis nigrovenosa. 
 
 17. Volvox and Pandornia. 
 
 18. Process of fertilisation in Ascaris megalocephala. 
 
 19. Diagram to illustrate the composition of the idants out of 
 
 individually different ids. 
 
 20. Formation of spermatozoa in Ascaris megalocephala. 
 
 21. Formation of ova in Ascaris megalocephala. 
 
 22. Diagram to illustrate the combination of idants in hybrids. 
 
 23. The two varieties of Cypris reptans. 
 
 24. Bonellia viridis, male and female.
 
 INTRODUCTION. 
 
 A. HISTORICAL PART. 
 
 HERBERT SPENCER was practically the first in the 
 present generation to attempt a theoretical explanation 
 of heredity when he propounded his theory of ' physiolo- 
 gical units.' The regeneration of lost parts, e.g., of a leg or the 
 tail of a salamander, led him to the conception of these units, 
 ' in all of which there dwells the intrinsic aptitude to aggregate 
 into the form of that species ; just as in the atoms of a salt 
 there dwells the intrinsic aptitude to crystallise in a particular 
 way.' He calls this aptitude the ' polarity of the organic units,' 
 and defines the latter as being intermediate between the 
 ' chemical units ' or molecules and the ' morphological units 
 or cells. They must be ' immensely more complex than the 
 chemical units,' and must therefore correspond to groups of 
 molecules. It is very interesting at the present day, now that 
 we have advanced somewhat further towards a theory of 
 heredity, to summarise the various aptitudes and forces which 
 Herbert Spencer thought it necessary to ascribe to his ' physio- 
 logical units,' in order to arrive at an explanation of the 
 phenomena. Although the sections on Heredity and Regene- 
 ration constitute only a small portion of his great work on the 
 ' Principles of Biology,' and cannot therefore contain a detailed 
 treatment of the phenomena of heredity, his opinions on this 
 subject are evident. 
 
 Spencer considers, on the one hand, that the whole organism 
 is composed of these units, which are all alike in kind, and on 
 the other, that the germ-cells also contain small groups of them. 
 The former supposition makes regeneration possible to each 
 sufficiently large portion of the body, while the latter gives the 
 germ-cell the power of reproducing the whole : inasmuch as the
 
 2 THE GERM-PLASM. 
 
 'polarity' of the 'units' leads to their arrangement in such a 
 way that the whole ' crystal ' the organism is restored, or 
 even formed anew. The mere difference in the arrangement of 
 units alike in kind determines the diversity of the parts of the 
 body, while the distinction between different species and that 
 between different individuals is due to a diversity in the con- 
 stitution of the units. 
 
 The units of an individual are therefore to a certain extent 
 protean. They are capable of arranging themselves in an 
 immense variety of ways, and so form the most diverse cells, 
 tissues, organs, and parts of the body. But they only do this under 
 the directing influence of the whole, in such a way that the whole 
 forces the units of one part to arrange themselves in just such 
 a way as is necessary for the perfection of that part, a perfec- 
 tion required for the harmony of the whole. Spencer himself 
 says very rightly, ' It seems difficult to conceive that this can 
 be so, but we see that it is so.' As a matter of fact, groups of 
 units removed from an organism possess the power of construct- 
 ing the whole anew ; and we are thus obliged to admit that the 
 tendency to take a specific form is present in all parts of the 
 organism. The ' units ' are physiologically variable quantities, 
 which in every case act in such a manner as the whole demands. 
 
 The assumption of these 'physiological units' does not 
 suffice as an explanation of heredity : it proves insufficient even 
 as interpreting the differentiation of organs in simple ontogeny, 
 quite apart from the question of amphigonic heredity. But it 
 has the merit of having utilised the smallest vital particles as 
 constituent elements of the organism, and of having made them 
 the basis of a theory of heredity. 
 
 Ernst Briicke was the first to admit the existence of small vital 
 particles of this kind, and to give cogent reasons for so doing. 
 Although he did not denote them by any special name in his 
 extremely important paper entitled ' Elementar Organismen,'* he 
 was the first to oppose the old theory of the cell, especially with 
 regard to its fluid contents, and to show that its body must pos- 
 sess an organisation, quite distinct from the molecular structure 
 of the organic compounds. 
 
 Darwin's theory of ' pangenesis ' was stated in the final chapter 
 oi his great work on 'The Variation of Animals and Plants 
 
 ' Wiener Sitzungsberichte,' Oct. 10, 1861, Bd. 44, ii., p. 381.
 
 INTRODUCTION. 3 
 
 under Domestication,' which appeared only a few years after 
 Spencer's ' Principles of Biology.' The enormous wealth of facts 
 bearing on heredity which is accumulated in this book is in itself 
 sufficient to show how the gifted author felt himself urged on all 
 sides to consider this extremely difficult and complicated pro- 
 blem. For although Darwin modestly described his theory as 
 a provisional hypothesis, his was, nevertheless, the first com- 
 prehensive attempt to explain all the known phenomena of 
 heredity by a common principle. The theory has so often been 
 discussed and is so well known that a brief account of its 
 substance will suffice here. 
 
 A multicellular organism, whether animal or vegetable, is 
 gradually built up by cell-division : but it is assumed that 
 this method of multiplication is not the only one. Each cell 
 possesses in addition, at each stage of its development, the 
 power of giving off invisible granules or atoms, which, at a later 
 period and under certain conditions, can develop again into cells 
 similar to those from which they originated. Numbers of these 
 'gemmules' are being given off continually from all cells of 
 the body and conveyed into the blood, and thus circulate 
 through the body, finally settling down in some part, principally 
 in those regions in which the development of offspring will take 
 place later on, i.e., in buds or germ-cells. As gemmules from 
 all the cells of the body are aggregated in these cells, they 
 invest the latter with the power of developing into a new and 
 complete organism. This occurs as follows : each gemmule 
 reproduces the cell from which it is derived, and the gemmules 
 of the different cells become active in the same order as that 
 in which the corresponding cells followed each other in the 
 ontogeny of the parent. 
 
 The germ is not by any means composed exclusively of 
 gemmules which have been derived from the organism in 
 which they were formed, but consists, at the same time, of a very 
 large number of gemmules which are derived from parents and 
 ancestors even of very remote generations ; and hence a great 
 many more gemmules take part in each case of ontogeny than 
 there are cells formed. Each cell and each part is represented 
 by a great variety of gemmules. A selection must therefore take 
 place, as only one gemmule can form the required cell, and the 
 rest must remain dormant. In this way a number of gemmules, 
 which have been hitherto dormant, are transferred from one
 
 4 THE GERM-PLASM. 
 
 generation to the next : they may, under certain conditions, be- 
 come active, and thus again bring into existence ancestral traits 
 which had disappeared in the parents. 
 
 This is, in brief, the theory of pangenesis. It does not take 
 into account the physical nature of the gemmules. They are 
 capable of multiplying, and do so continually : but the question 
 as to whether they have any definite arrangement, and if so, 
 what the nature of that arrangement is, is not touched upon ; 
 nor is any mention made of the causes and mechanism by 
 which it comes about that they are always in the right place 
 and develop into cells at the right time. 
 
 I do not say this by any means as a reproach, but only to 
 bring out clearly the speculative character of the whole 
 hypothesis. Darwin did not go on to inquire whether all these 
 assumptions were possible : he only asked what it was necessary 
 to assume in order to explain this or that fact of heredity, with- 
 out troubling himself to consider whether the assumption were 
 borne out by facts or not. And he was right in doing so, for at 
 the time when he propounded his hypothesis it was not possible 
 to found any theory of heredity on the only sound basis, that 
 of a knowledge of the most minute cell-structure. I have 
 already pointed out how extremely important and fruitful his 
 theory of pangenesis has been : it drew attention for the first 
 time to all the phenomena that needed explanation, and showed 
 what assumptions must be made in order to explain them. 
 
 It will be shown later on that, in spite of the fact that a con- 
 siderable number of these assumptions are untenable, a part of 
 the theory still remains which must be accepted as fundamental 
 and correct, in principle at any rate, not only now, but also 
 for all time to come. I refer to the most general portion of these 
 assumptions only, namely, that presupposing the existence of 
 material particles in the germ which possess the properties of 
 the living being, and each of which is to be regarded as the 
 primary constituent (' Anlage ') of one portion of the organism. I 
 must honestly confess to having mentally resisted this funda- 
 mental point of the Darwinian doctrine for along time. It appeared 
 almost impossible to me that such an enormously large number 
 of individual primary constituents as we must suppose to exist, 
 according to Darwin's view, could be contained in the minimum 
 of substance which, as will be shown hereafter, we have to 
 regard as the actual bearer of heredity. I tried in several ways
 
 INTRODUCTION. 5 
 
 to arrive at a satisfactory epigenetic theory,* which, starting 
 from a germ-substance of comparatively simple structure, should 
 exhibit the various differentiations of the organism as due to 
 regular changes brought about by the division of this primary 
 structure. But the more I considered the problem as time 
 went on, the more I was convinced that such a solution was 
 impossible. And in this book I trust that I shall be able to 
 give a satisfactory proof that only one theory of evolution in 
 Darwin's sense, i.e., the assumption of minute primary constitu- 
 ents in the germ, is in accordance with the facts ; and the objection 
 which for a long time prevented me from accepting this very 
 simple assumption, disappears with the discovery that what is 
 apparently impossible does really occur. 
 
 I certainly consider even now that Darwin's theory must be 
 looked upon, and that he probably considered it, rather as an 
 inquiry into the problem of heredity than as a solution of the 
 problem. His assumptions do not, properly speaking, explain 
 the phenomena. They are to a certain extent a mere para- 
 phrase of the facts, an explanation of a purely formal nature, 
 based on speculative assumptions, which were made not because 
 they seemed possible, or even likely, but because they provided 
 a formal explanation of all the phenomena on one principle. 
 If we suppose that each cell arises from a special gemmule, and 
 that these gemmules are present wherever they are wanted, it is 
 easy to see how that structure, the origin of which we wish to 
 explain, may appear in any given position. Further, when a 
 large number of cells is to arise in regular succession from one 
 egg-cell, the desired sequence of cells must of course result 
 if we assume that the gemmules present become active in the 
 required order. But this supposition does not really explain 
 the phenomena. Even at the present day our explanations are 
 imperfect enough, and are far from going to the bottom of the 
 matter, but they differ from Darwin's provisional hypothesis in 
 that they attempt to find out the actual facts concerned in the 
 processes, and to arrive at a real, and not merely a formal, solu- 
 tion of the problem. The great naturalist's merit in having at 
 once found the right foundation on which to base a real solution 
 is not diminished by the fact of his having been less startled by 
 
 * The indication of such a theory is given, e.g., in the essay entitled 
 'Die Continuitat des Kcii.iplasma's,' Jena, 1885, p. 38 et seq. (pp. 207 ct 
 scq. of the English translation.)
 
 6 THE GERM-PLASM. 
 
 the consequences of his 'gemmule'-hypothesis when seeking for 
 a purely formal explanation, than he would have been had he tried 
 to adapt his hypothesis to the facts. The hypothesis, as stated 
 by him, could not be regarded as a real solution of the problem 
 of heredity, if only because it leaves unexplained the giving off 
 of the gemmules into the blood, their circulation through the 
 body, and intrusion into the germ- and other cells. All these 
 are assumptions without a basis in fact. This is evidently the 
 reason why modifications of the theory of pangenesis were 
 repeatedly made very soon afterwards. * 
 
 Before considering these modifications, I should like once 
 more to state clearly the relation of Spencer's ' physiological 
 units' to Darwin's 'gemmules.' Darwin himself considered the 
 former to be closely related to his gemmules ; and, in fact, he 
 would have regarded Spencer's ideas as essentially coinciding 
 with his own, had he not noticed certain passages in Spencer's 
 book which seemed to point to something quite different.* 
 
 It will be apparent, I think, from what has already been said, 
 that these two views are entirely different. What is common 
 to both is that they assume the existence of minute living units, 
 multiplying by fission : but the part taken by them in the con- 
 stitution of the body is quite differently conceived. Spencer's 
 units are the elements which exclusively compose the living 
 body ; while Darwin's gemmules only give rise to cells, i.e., 
 they are elements which are present for the special purpose 
 of bringing about heredity, without anything being specified as 
 to their share in the composition of the living body. As will 
 be shown more clearly later on, Spencer's hypothesis is superior 
 in this respect to Darwin's. On the other hand, Spencer's simi- 
 lar units are the bearers of all the characters of the species, 
 owing to their complex molecular structure ; while the Dar- 
 winian gemmules are primary constituents of individual cells, 
 which are to be considered as differing in a manner correspond- 
 ing to the difference of the individual cells. Spencer's theory 
 is epigenetic, Darwin's evolutionary ; in this respect the latter is, 
 in my opinion, superior to the former. 
 
 Galton t was the first to make an attempt to improve on the 
 
 * Charles Darwin, ' The Variation of Animals and Plants under Domes- 
 tication,' and ed., vol. ii., London, 1888 ; note on p. 371. 
 
 + Francis Galton, ' A Theory of Heredity,' Journal of the Anthropologi- 
 cal Institute, 1875.
 
 INTRODUCTION. 7 
 
 theory of pangenesis. In a short but suggestive essay he 
 accepted the hypothesis of the gemmules, but rejected the 
 doctrines of their circulation through the blood and of the 
 aggregation in tho germ-cells of gemmules given off by the 
 body-cells. Now as the gemmules which have been converted 
 into body-cells are used up, it follows that the germ-cells can 
 only contain those gemmules which are left, those, out of the 
 enormous number contained in a germ-cell, which have not deve- 
 loped further. For each germ-cell, as both Gallon and Darwin 
 assume, contains each kind of gemmule in many modifications, 
 originating from the different ancestors of the organism. The 
 theory of the origin of the germ-cells from the remains of the 
 germ mass not used up in ontogeny ('the residue of the stirp') 
 has been compared to, and regarded as the precursor of, the 
 conception of the continuity of the germ-plasm which I origin- 
 ated long afterwards. A certain resemblance does, it is true, 
 exist between the two conceptions, but it will be shown in the 
 section on the continuity of the germ-plasm that the similarity 
 is only a superficial one. 
 
 Herbert Spencer defines heredity as the capacity of every 
 plant and animal to produce other individuals of a like kind, 
 and states expressly that in this fact, which is perfectly 
 familiar to us, and for this reason seems to be a matter 
 of course, lies the real essence and principle of heredity, 'the 
 phenomena commonly referred to it being quite subordinate 
 manifestations.' Thus the blending of the individual ' characters' 
 of the parents in the children has, as a rule, been placed 
 in the foreground in considering questions of heredity, and it 
 has been overlooked that this is quite a secondary pheno- 
 menon, important no doubt in many respects, and interesting 
 in a high degree, but still only the result of a certain mode of 
 multiplication, i.e., sexual reproduction, and by no means an 
 essential phenomenon of heredity. Darwin recognised this 
 distinctly, and concerned himself primarily with the theoretical 
 explanation of individual development (ontogeny). But the majo- 
 rity of writers on heredity, including Gallon, have turned Iheir 
 whole allenlion lo ihe blending of ihe qualilies of ihe parenls 
 in Ihe children, a problem which is doublless well worthy of 
 invesligalion, bul which, al ihe same time, deals only wilh a side 
 issue of the processes of reproduction. How litlle I under- 
 rate the significance of amphigonic heredity, even in its theo-
 
 THE GERM-PLASM. 
 
 retical relations, will be evident in a later part of this book, in 
 which I attempt to derive the existence of the germ-plasm 
 from the phenomena of this form of heredity ; but to me it 
 seems dangerous to investigate heredity theoretically from the 
 point of view of amphigonic descent exclusively, because one 
 has here to deal with the most complex of all the phenomena, and 
 the main point may easily be overlooked in a mass of confusing 
 secondary considerations. Even Galton, in my opinion, allowed 
 himself to be too much influenced by this aspect of the question. 
 Excellent as are his later researches on the laws relating to 
 the blending of characters of the parents in the children, I 
 consider his theoretical deductions on the fundamental pheno- 
 mena of heredity unsatisfactory. The few hints that he gives as 
 to the cause of ontogeny seem to me by no means equal to Dar- 
 win's simple but truly penetrating and accurate deductions. It 
 is quite conceivable that the phenomena of the blending of the 
 characters of the parents in the children would be the most inter- 
 esting to a statistician and anthropologist like Galton, but they 
 have kept him within the limited range of these phenomena, and 
 have prevented him from arriving at really general principles 
 and at a comprehensive theory of heredity. 
 
 Galton has, however, the merit of having been the first to 
 deny the circulation of the gemmules, and, in connection with 
 this, to cast doubt upon the general validity of the doctrine of 
 the transmission of acquired modifications. He certainly be- 
 lieves the latter to be ' faintly heritable,' and assumes, in order 
 to explain this transmission, that no general 'circulation of 
 the gemmules' takes place, but that each cell sets free some 
 gemmules which get into the circulation and eventually penetrate 
 into the sexual elements. 
 
 Gallon's essay was published only a few years after the 
 appearance of Darwin's theory of pangenesis ; but it cannot 
 be said that it exercised any influence on the subsequent 
 development of the theory of heredity. Apparently it was 
 not much noticed even in England, and on the Continent it 
 remained unknown for a long time. This must be my excuse for 
 being ignorant of the existence of this paper, and consequently 
 for not referring to it in my essays which appeared nearly ten 
 years later.* In one of these essays 'On Heredity' (1883), 
 
 * These essays first appeared separately in the years 1881-91. The
 
 INTRODUCTION. 9 
 
 I contested at first in general terms not only the existence but 
 also the theoretical possibility of the transmission of acquired 
 characters, and tried to release the theory from the necessity of 
 an explanation which deprived it of any further development. 
 In this essay I further assumed the existence in the germ-cell 
 of a reproductive substance, the germ-plasm, which cannot be 
 formed spontaneously, but is always passed on from the germ-cell 
 in which an organism originates in direct continuity to the germ- 
 cells of the succeeding generations. The difference between the 
 'body ' in the narrower sense (soma) and the reproductive cells 
 was also emphasised, and it was maintained that the germ-cells 
 alone transmit the reproductive substance or germ-plasm in 
 uninterrupted succession from one generation to the next, while 
 the body (soma) which bears and nourishes the germ-cells, is, 
 in a certain sense, only an outgrowth from one of them. 
 
 A second attempt to improve upon the theory of pangenesis 
 must be considered here. I have already referred elsewhere* to 
 the interesting and ingenious book on ' The Laws of Heredity,' t 
 by W. K. Brooks. The author retains the fundamental points 
 of this theory, viz., the formation of gemmules in all the cells 
 of the body, their circulation through the latter, and their aggre- 
 gation in the germ-rells or buds : he differs, however, from Darwin 
 principally in ascribing to the male germ-cell a particularly 
 strong power of attraction for the gemmules, so that it collects a 
 special mass of them and stores them up. As this assumption 
 is made chiefly for the purpose of explaining variation, I shall 
 postpone any further consideration of it to the section which 
 treats of this subject. 
 
 In the following year, Nageli's ' Mechanico-physiological 
 
 only complete edition of the collected essays which has hitherto appeared 
 is the English translation, ' Essays upon Heredity and Kindred Biological 
 Problems' (edited by Poulton, Schonland, and Shipley, Oxford, 1889), 
 containing Essays I. VIII. A second edition appeared in 1891 as Vol. 
 I., and Essays IX. XII. follow this year as Vol. II. A French trans- 
 lation of all these essays, with the exception of the last on 'Amphimixis,' 
 &c., has also appeared with the title, ' Essais sur I'Hereditd et la Selection 
 Naturelle,' traduits par Henry de Varigny, Paris, 1892. 
 
 * ' The Significance of Sexual Reproduction in the Theory of Natural 
 Selection.' 'Essays upon Heredity,' p. 326. 
 
 t W. K. Brooks, ' The Laws of Heredity, a Study of the Cause of 
 Variation and the Origin of Living Organisms,' Baltimore, 1883.
 
 10 THE GERM-PLASM. 
 
 Theory of Descent ' * appeared. This book, which abounds in 
 ingenious deductions and important suggestions, doubtless 
 exercised a great influence on the views of that time. Its 
 importance cannot be denied, even if, as I believe to be the case, 
 only a small portion of its theoretical propositions can be 
 retained. Many as are the fruitful ideas and anticipation of 
 facts afterwards proved which we owe to Nageli, his own theory 
 of heredity has already became untenable. For this reason, and 
 also because the theory is so well-known, I will not describe it 
 fully here, but will only refer to the remarks which I made on 
 the subject some years ago,t and to the recent detailed criticism 
 by Wiesner.J Although I do not consider that Nageli's 
 hypothesis leads us towards a true theory of heredity, it never- 
 theless contains an important suggestion, that of the idioplasm, 
 which gives us a further insight into the problem. I had already 
 assumed the existence of a special reproductive substance the 
 germ-plasm on the changes of which development depends, 
 while heredity rests on its continuity : and now Nageli 
 independently postulated a special reproductive substance, an 
 ' Anlagenplasma ' or ' idioplasm,' which although much smaller 
 in bulk than the rest of the living substance of the body 
 the trophoplasm (' Ernahrungsplasma ') determines the detailed 
 construction of the latter. The correctness of this conjecture 
 has not as yet, so far as I know, been disputed, although it was 
 very soon shown that Nageli was wrong as regards the form in 
 which he imagined the idioplasm to exist. He represented it as 
 consisting of very fine parallel fibres which, by uniting into 
 bundles and crossing each other so as to form a network, 
 traverse the substance of the cell, and being continuous from 
 cell to cell, pervade the whole body as a connected network. 
 
 At the time when Nageli's book appeared, it was already 
 suspected that the reproductive substance is not contained in 
 the body of the cell but in its nucleus, and several discoveries 
 were made shortly afterwards which rendered it certain that the 
 idioplasm is to be looked for in the ' chromosomes ' of the nucleus, 
 
 * C. v. Nageli, c Mechanisch-physiologische Theorie der Abstam- 
 mungslehre,' Munchen and Leipzig, 1884. 
 
 t Vide 'The Continuity of the Germ-plasm,' 1885 (pp. 180 et seg., 
 192, &c.). 
 
 : Julius Wiesner, ' Die Elementarstructur und das Wachsthum der 
 lebenden Substanz,' Wien, 1892.
 
 INTRODUCTION. II 
 
 those rod-like, coiled, or grain-like structures which are dis- 
 tinguished by their remarkable affinity for certain colouring 
 matters. I shall return to the proof of this fact in the following 
 section. 
 
 From this time onwards each subsequent theory of heredity 
 was based on a firm foundation of fact. It was now not only known 
 that the phenomena of heredity among the higher organisms are 
 connected with a definite substance, but the seat of the latter 
 had also been ascertained. I now therefore adopted this firm 
 basis for my theory of the germ-plasm, if I may call the imper- 
 fect form in which it then existed by such a name : I localised 
 the germ-plasm in the nuclear substance of the germ-cell, and 
 supposed that ontogeny was due to a qualitative change in 
 it, which hands the idioplasm on from one generation to the next 
 by means of nuclear- and cell-division. But I soon went further. 
 From the fact of sexual reproduction, which brings together 
 equal amounts of paternal and maternal germ-plasm at each fer- 
 tilisation, I inferred not only the composition of the germ-plasm 
 out of a number of units, the ' ancestral germ-plasms' (' Ahnen- 
 plasmen '), but also the necessity of a reduction of the germ-plasm 
 each time to one-half of its bulk, as well as a reduction of the 
 number of the ancestral germ-plasms contained in it.* The 
 hypothesis of the ' reducing divisions of the germ-cells ' has been 
 thoroughly substantiated by subsequent observations : in fact it 
 has even been proved that in many cases this reduction occurs 
 exactly as I had foretold and had represented in a diagrammatic 
 figure ;t that is to say, by the non-occurrence of the longitudinal 
 division of the chromosomes which occurs in the ordinary nuclear 
 division, and by the distribution of these in the daughter-nuclei. 
 This holds good for the ovum as well as for the sperm-cell in 
 animals, and, as far as is known, in plants also. The germ-cell 
 must in all cases by division get rid of half of its nuclear rods, 
 that is to say, of its germ-plasm, in order to become capable 
 of fertilisation. This fact supports the other assumption of the 
 construction of the germ-plasm from ancestral germ-plasms, 
 which are not minute vital particles analogous to Spencer's 
 physiological units but rather bodies of a highly complex 
 constitution, each containing all the primary constituents which 
 are necessary to the formation of an organism. Each ancestral 
 
 * ' On the Number of Polar Bodies,' &c., 1887. f Ibid.
 
 12 THE GERM-PLASM. 
 
 germ-plasm seemed to me to be of a 'special kind,' and 
 just as many ' different kinds of idioplasm ' are removed by the 
 reducing division ' from the ovum as are afterwards introduced 
 by the sperm-nucleus' on fertilisation. It will be seen that I 
 retain this essential basis of the theory of the germ-plasm in its 
 further development as presented here, and I trust that I may 
 now succeed in refuting the objections which have been urged 
 against the ' ancestral germ-plasms,' or, as I now call them, the 
 ' ids.' In any case it cannot be denied that they help to throw 
 an important light on the subject. 
 
 De Vries has so far been my most powerful opponent as 
 regards the ancestral germ-plasms, but his opposition is founded 
 on the misunderstanding I have already referred to, for he looks 
 upon them as the ultimate vital particles an idea which was 
 foreign to me from the beginning. Of this, however, I do not com- 
 plain, as at that time I had left the question as to the construction 
 of the ancestral germ-plasms unanswered.* This omission is sup- 
 plied in the present book, and it will be shown that, although 
 each ancestral germ-plasm is in my opinion a bearer of all the 
 primary constituents required for the construction of an organism, 
 my assumption does not exclude the possibility of its being 
 composed of these constituents in the form of minute vital 
 particles. The 'ancestral germ-plasm' is indeed a unit, but 
 one of a higher order. For this reason alone it cannot be 
 compared with Spencer's 'physiological units,' because the 
 latter, as ultimate vital particles, compose the whole body ; while 
 the ancestral germ-plasms only form the nuclear matter, and 
 merely serve the mechanical purpose of the processes of 
 heredity. 
 
 De Vries has in a significant manner developed a theory of 
 heredity in his essay on ' Intracellular Pangenesis.'t The 
 opinions there expressed really contradict the title of the 
 
 * De Vries is also mistaken in ascribing to me the opinion that ' there 
 is only one hereditary substance only one material bearer of the hereditary 
 tendencies in each individual.' The sentence quoted by him (' On the 
 Number of Polar Bodies,' p. 355) does not deal with this question ; it 
 runs as follows : from several reasons already stated ' at least one certain 
 result follows, viz., that there is an hereditary substance, a material bearer 
 of hereditary tendencies, and that this substance is contained in the nucleus 
 of the germ-cell, ' &c. 
 
 t ' Die Intracellularc Pangenesis,' Jena, 1889.
 
 INTRODUCTION. 1 3 
 
 paper ; for pangenesis, in Darwin's sense, means the develop- 
 ment of gemmules throughout the body, the composition of 
 the hereditary substance from gemmules which are derived from 
 all the cells of the body. This very point in Darwin's hypothesis 
 is set aside completely by de Vries : the most characteristic 
 part of it is removed, and what remains is of a more general 
 nature, consisting of principles which, in one form or another, 
 must form the basis of every theory of heredity, at the present 
 day at any rate. Some ideas of his own, however, are then added, 
 and it is these which give a characteristic stamp to his whole 
 series of conceptions. If we regard his hypothesis, as de 
 Vries himself does, as an alteration of the Darwinian theory 
 of pangenesis, it is certainly a radical one, and is of such a kind 
 as at one stroke to infuse new life into the latter, which had 
 become untenable in its original form. 
 
 De Vries distinguishes two parts in Darwin's theory of pan- 
 genesis, one of which he rejects, while he retains the other. He 
 calls the former portion the 'transport hypothesis,' meaning 
 thereby the assumption of the origin of the gemmules in all the 
 cells of the body, their separation from the cells, circulation in 
 the blood, and ultimate aggregation in the germ-cells. And 
 relying on my rejection of the heredity of ' somatogenic' charac- 
 ters, he shows that the assumption of the transportation of 
 the gemmules from all the cells of the body to the germ-cells 
 is superfluous. He thus does away with that portion of the 
 hypothesis of pangenesis which makes it unacceptable to most 
 people, and places the theory on a new and firmer foundation 
 on which it is capable of further development. 
 
 De Vries nevertheless goes too far if he looks upon the 
 'transport hypothesis' as necessary only for explaining the 
 transmission of somatogenic qualities. It must not be for- 
 gotten that the idea of the continuity of the germ-plasm did not 
 exist in Darwin's time. How could the gemmules of all the 
 cells of an organism enter its germ-cells unless they are formed 
 in the body-cells, migrate therefrom, circulate through the body, 
 and come together in the germ-cells ? A direct connection 
 between the fertilised egg-cell and the germ-cells of the organism 
 arising from it was not supposed to exist by any one at that 
 time, nor does it do so except in isolated cases. The ' transport 
 hypothesis ' was therefore also necessary in order to explain the 
 production of germ-cells of each kind, which must again
 
 14 THE GERM-PLASM. 
 
 contain the gemmules of the parents. Galton, who also rejected 
 the 'transport hypothesis,' thus found himself in the peculiar 
 position of being obliged to suppose that the germ-cells which 
 the organism produces can only contain the unused remainder 
 of the gemmules and their successors, i.e. y those gemmules which 
 had been unable to take part in the construction of the organism, 
 and which had remained dormant and were individually of a 
 different nature from the other gemmules. He made use of this 
 supposition to explain the difference between children of the 
 same parents, but found himself obliged to resort to a very 
 artificial assumption to account for the main problem of the 
 resemblance between such children and their parents. 
 
 That part of Darwin's theory which de Vries retains is the 
 existence of an hereditary substance composed of ' gemmules,' or 
 minute vital particles which are capable of growth and multi- 
 plication by fission, and which become active consecutively in 
 ontogeny, and so build up the organism. The theory is thus 
 deprived of its merely speculative elements, and by transferring 
 the gemmules, in accordance with the most recently ascertained 
 facts, to the germ substance, which, as we know, is passed on 
 by division from cell to cell, the theory of pangenesis is placed 
 on a firm footing. 
 
 De Vries, however, was not content with simply modify- 
 ing Darwin's theory of pangenesis in a negative manner, by 
 doing away with one almost the greater portion of it ; he 
 also reformed it positively by giving a new meaning to the 
 'gemmules.' There is an essential difference between Dar- 
 win's gemmules of cells, and de Vries's pangenes, which are 
 gemmules of elements much smaller than cells that is to say, 
 of the smallest parts of which a single cell is composed. 
 These pangenes are the bearers of the individual qualities or 
 ' characters ' of the cell. 
 
 The train of thought which led de Vries to imagine the con- 
 struction of the hereditary substance from such ' bearers of the 
 qualities ' ('Eigenschaftstrager')of the cells is too interesting to be 
 passed over. He bases this idea on the assumption of ' a mutual 
 independence of the hereditary qualities? According to his view, 
 all species consist of a sum of ' hereditary qualities' ; very few, 
 or none of these, are peculiar to any one species, the character of 
 which is determined by the way in which they are combined. 
 The same quality recurs in many species, but in different com-
 
 INTRODUCTION. 15 
 
 binations. ' We constantly see how one and the same hereditary 
 quality, or how a definite small group of such, may be combined 
 with all kinds of other hereditary qualities ; and how the differ- 
 ent characters of individual species are due to the extreme 
 variety of these combinations.' The different organs of a species 
 stand in the same relation to one another in this respect, as 
 do the different species themselves. They exhibit the same 
 qualities, but in different combinations. The individual qualities 
 which constitute a species ' can almost all vary independently 
 of each other,' and can therefore be increased even by artificial 
 selection according to the fancy of the breeder, without requir- 
 ing a corresponding change in the remaining qualities of the 
 species. But the qualities too are ' miscible in almost any pro- 
 portion,' as experiments in hybridising are intended to show : 
 ' in no other way can we so clearly demonstrate the secondary 
 importance of a specific type (' Bild'), regarded as a whole as 
 opposed to the independent factors which constitute it.' The 
 qualities, or rather their material substratum, are therefore 
 independent of one another, and miscible to almost any extent. 
 
 Those ultimate vital particles or pangenes, which de Vries 
 substitutes for Darwin's gemmules, are therefore the bearers of 
 constituent qualities of the species. 
 
 The fundamental idea of de Vries's whole deduction is 
 doubtless perfectly correct. Some ten years ago, when I first 
 began to devote my attention to the problem of heredity, I fully 
 believed in the possibility of an epigenetic theory, but, as will 
 be seen in the course of this book, have long since given up 
 this idea as untenable. I too now believe that the hereditary 
 substance is composed of primary constituents, and even trust that 
 I can prove this assumption to be not only sound, but inevit- 
 able. But, at the same time, I do not imagine that it suffices 
 as an explanation of the phenomena of heredity. According 
 to de Vries, the germ-substance is formed of a number of 
 different kinds of pangenes, of which as many are present as 
 there are qualities in the species. He does not consider these 
 pangenes as arranged in any definite grouping, but as freely 
 miscible, in accordance with the assumed 'free miscjbility of 
 the qualities.' He contests as superfluous the assumption of 
 higher units, such as might be formed by a certain number of 
 pangenes in a definite order ; and this view seems to me to be 
 the weak point in his argument.
 
 1 6 THE GERM-PLASM. 
 
 In the section on the control of the cell by the nuclear sub- 
 stance, I shall adopt what seems to me to be a remark- 
 ably happy idea on the part of de Vries, who supposes that 
 material particles leave the nucleus, and take part in the 
 construction of the body of the cell. These particles correspond 
 to the 'pangenes;' they are the 'bearers of the qualities' of 
 the cell, the specific character of which I believe to be stamped 
 upon it by the nature, the different varieties, and the propor- 
 tional numbers of these particles. 
 
 But does the character of a species depend only on 
 these primary qualities of the cell ? Are there not qualities 
 of various degrees primary, secondary, and so on ? The 
 pangenes are primary ' bearers of qualities ' ; their mere 
 presence in the hereditary substance gives no indication, or at 
 most, only a very slight one. as to the character of a species. 
 If, for instance, ' chlorophyll-pangenes ' are present in the egg- 
 cell of a plant, the only conclusions we can draw as to the 
 specific character of the latter are that it will have green cells 
 of some sort : but we cannot thereby determine where they 
 will be situated, or which portions of the plant will be green, 
 and which variegated ; or again, whether its flowers will be 
 green, white, or of some other colour. Not until we were 
 able to find groups of pangenes in the germ-substance, some 
 of which were destined to give rise to leaves, and others to 
 flowers, should we be able to say whether the latter will be green 
 or otherwise. 
 
 In the course of his remarks, de Vries mentions the stripes 
 of a zebra. How can these be hereditary if the different kinds 
 of pangenes merely lie close together in the germ without being 
 united into fixed groups, hereditary as such f There can be no 
 'zebra pangenes,' because the striping of a zebra is not a cell- 
 character. There may perhaps be black and white pangenes 
 whose presence causes the black or white colour of a cell : 
 but the striping of a zebra does not depend on the development 
 of these colours within a cell, but is due to the regular 
 alternation of thousands of black and white cells arranged in 
 stripes. 
 
 De Vries, in another place, refers to the long-stalked variety 
 of the alpine Primula acautis, which is due to reversion to a 
 remote ancestral form of the species. In this case, again, the 
 special peculiarity cannot depend on ' long-stalk pangenes,'
 
 INTRODUCTION. 17 
 
 because the possession of a long stalk is not an intracellular 
 character. The specific form of the leaves and other parts of a 
 plant is likewise not due to the character of the individual cells 
 composing them : the serrated margin of a leaf, for instance, 
 cannot depend on the presence of ' serration-pangenes,' but is 
 due to the peculiar arrangement of the cells. The same argu- 
 ment would apply to almost all the obvious ' characters ' of the 
 species, genus, family, and so on. For instance, the size, 
 structure, veining, and shape of leaves, the characteristic and 
 often absolutely constant patches of colour on the petals of 
 flowers, such as orchids, may be referred to similar causes : 
 these qualities can only arise by the regular co-operation 
 of many cells. The characteristics of the human race may 
 be taken as another illustration. The peculiarities as regards 
 the shape of the skull, nose, &c., cannot depend on the mere 
 presence in the germ of pangenes, which are destined to form 
 the hundreds and thousands of different cells constituting 
 the respective qualities ; but they must be due to a fixed 
 grouping of pangenes, or some other primary elements of the 
 germ, which is transferable from generation to generation. 
 
 The character of a species cannot depend only upon 
 the number and relation of the pangenes in the germ. It is 
 quite possible to conceive of two different species of totally 
 different structure in which the pangenes of the germ were alike 
 in nature and amount, the difference being solely due to the 
 grouping of the pangenes in the germ. De Vries, it is true, 
 traces ' systematic difference to the possession of different kinds 
 of pangenes,' and considers that ' the number of similar pan- 
 genes in two species is the real measure of their affinity;'* 
 but this statement seems to me to be somewhat at variance with 
 his fundamental view, according to which ' a number of here- 
 ditary qualities constitute the character of each individual 
 species, though by far the greater majority of them recur in 
 innumerable other species! Does he not, in so many words, 
 emphasise the fact that the almost formidable number of 
 different pangenes which are required for 'the construction of a 
 single species' does not necessitate the existence of an incon- 
 ceivably large multitude of different pangenes in the entire 
 organic world, because 'the number of individual hereditary 
 
 Loc. cit., p. 73.
 
 1 8 THE GERM-PLASM. 
 
 qualities required for the construction of the latter is rela- 
 tively small when compared with the number of species ' ! 
 Each species appears to us as an extremely complicated struc- 
 ture : the whole organic world, however, seems to be the result 
 of innumerable different combinations and permutations of 
 relatively few factors. 
 
 The idea which is here so clearly and decidedly expressed of 
 the construction of innumerable species by various combinations 
 of relatively few pangenes, shows that, even from de Vries's 
 point of view, it is not the 'pangene material' 1 as such, which is 
 the main factor in determining the character of the species, but 
 rather its arrangement, or, as I shall afterwards express it, the 
 architecture of the germ-plasm. 
 
 De Vries certainly speaks frequently of ' groups of pangenes,' 
 but he only just touches upon this idea, and postpones entering 
 into details until further discoveries are made with regard to the 
 mechanism of nuclear division. Important as his fundamental 
 view as regards the composition of the germ substance out 
 of primary constituents undoubtedly is, it may easily seem 
 to explain more than it really does ; without assuming the 
 formation of groups of such primary constituents for a number 
 of orders each included in the other, even the simplest case 
 of ontogeny cannot be explained, quite apart from reversion 
 or any other complicated phenomenon of amphigonic heredity. 
 Darwin's theory of pangenesis accomplishes more in this 
 respect than does de Vries's modification of it, inasmuch as the 
 former at least deals with the primary constituents of cell- 
 structures. The mere presence of a certain collection of 
 pangenes in the germ does not necessitate the formation in the 
 offspring of similar cells to those which existed in the parent ; 
 for the character of the individual cell is determined by a definite 
 selection of pangenes. If, indeed, it be assumed that the 
 required pangenes always lie close together, and are always 
 ready at hand whenever they are wanted, an explanation of any 
 particular phenomenon of heredity is no longer difficult, but it 
 seems to me that it would then be necessary to show how the 
 nature of the germ can determine that the right primary 
 constituents are always at the right spot. 
 
 As already stated, de Vries occasionally speaks of groups of 
 pangenes, but, at the same time, he looks upon the view of there 
 being any 'higher units' in the germ as a superfluous one.
 
 INTRODUCTION. 19 
 
 I can only explain this inconsistency by supposing that he 
 regards the ' qualities ' as independent and perfectly freely 
 miscible, and, in fact, postulates a germ-mechanism which 
 admits of their separation in any manner required. If this 
 were really the case, and the primary constituents were not 
 combined into fixed groups in the germ, how could composite 
 characters composed of many different kinds of cells with a 
 definite arrangement e.g., the eye-like spot on a certain feather 
 of a bird become a fixed specific character ? I am of opinion 
 that the view which entails an independence and uncontrolled 
 miscibility of the qualities is a fallacy, originating in the con- 
 ception of amphigonic reproduction as a necessary element in 
 heredity. The chapters on amphigonic heredity, reversion, &c., 
 will show how I imagine the idea of the uncontrolled miscibility 
 of the separate qualities to have arisen. 
 
 It will frequently be apparent in the course of this book that 
 my point of view is identical with that of the Dutch botanist in 
 many of the most important particulars. I believe, however, 
 that his ' pangenes ' or similar minute elements do not suffice 
 in themselves for the construction of a theory of heredity, but 
 that something more must be added to make the phenomena 
 comprehensible at any rate in principle. 
 
 The manuscript of the present book had already been written 
 for some time when Wiesner's work on the elementary structure 
 and growth of living substance* appeared. Although this mono- 
 graph does not contain, and is not intended to offer, a theory of 
 heredity, it is nevertheless of great importance in this respect, 
 for it treats of the fundamental points of such a theory, viz., 
 the composition of living matter out of very small units. 
 Wiesner remarks that theories of heredity have hitherto always 
 adopted units invented for the purpose, whereas the same units 
 which make life possible at all, and which control assimilation 
 and growth, must also be the agents in bringing about the 
 phenomena of heredity. Spencer's ' physiological units,' Dar- 
 win's 'gemmules,' Haeckel's ' plastidules,' and my 'ancestral 
 germ-plasms,' are all, in fact, elements of this kind, assumed 
 for the explanation of the problem of heredity. De Vries stands 
 alone in considering all living matter to be actually composed 
 
 * J. Wiesner, ' Die Elementarstruktur und das Wachsthum der lebenden 
 Substanz," Wicn, 1892.
 
 20 THE GERM-PLASM. 
 
 of his 'pangenes,' though I have already indicated that my 
 ' ancestral germ-plasms ' are also composed of similar primary 
 units, which do not exist in them alone. The minute vital 
 particles or ' plasomes,' adopted by Wiesner from Briicke, cor- 
 respond in all essential points to the ' biophors ' or bearers of 
 life assumed by myself. 
 
 B. DESCRIPTIVE PART. 
 
 By the term heredity is simply meant the well-known fact 
 that living organisms are able to produce their like, and that the 
 resemblance between a child and its parent, although never 
 perfect, may nevertheless extend to the most minute details of 
 construction and functions. 
 
 The fundamental phenomena of heredity are familiar in all 
 existing organisms : the transmission of the character of the 
 species from parent to offspring results whether the multiplica- 
 tion takes place by the halving of a unicellular organism or by 
 the process which occurs in multicellular organisms, which 
 consists in a complex succession of continually increasing 
 groups of cells, z>., in development. These fundamental pheno- 
 mena of heredity are, however, complicated in all the higher 
 organisms by the connection of reproduction with that process 
 which may be described as amphimixis.* This consists in the 
 mingling of two individuals or of their germs, and owing to its 
 constant connection with reproduction in multicellular organisms 
 it is usually spoken of as c sexual reproduction.' As will be 
 shown in greater detail further on, the various phenomena, such 
 as the blending of parental characters in the offspring, and 
 reversion, depend exclusively on the hold which amphimixis has 
 taken on the life of the species. Similar phenomena must occur 
 amongst unicellular organisms, in which amphimixis is widely 
 spread if not universal, in the form of conjugation, and is, 
 therefore, not directly connected with reproduction. At present, 
 however, we are ignorant of the details of heredity in these 
 forms, and are therefore compelled to base our conclusions 
 entirely upon what we know to occur in multicellular organisms. 
 These phenomena have only been observed in detail in the 
 higher plants and animals, more particularly in man. In the 
 
 * August Weismann, 'Amphimixis, oder die Vermischung der In- 
 dividuen,' Jena, 1891. See ' Essays upon Heredity.' vol. ii., 1892.
 
 INTRODUCTION. 21 
 
 case of the higher forms of life a large number of facts have 
 now been accumulated which can be used for the purpose of 
 theoretical analysis. 
 
 Although the study of heredity is greatly complicated by 
 amphimixis, this mingling of the hereditary tendencies of two 
 parents, and even the process of sexual reproduction which 
 accompanies it, afford us a much deeper insight into the actual 
 processes of heredity than we could ever have obtained in any 
 other way. We may thus hope in time to penetrate further into 
 its nature by carrying out more detailed investigations of the 
 phenomena. 
 
 In order to do so, however, we must not forget that this form 
 of reproduction is neither the only nor the original one, and that 
 even in multicellular organisms reproduction is not necessarily 
 connected with amphimixis ; it must also be borne in mind that 
 so-called asexual (monogontc) reproduction forms the basis 
 of the amphigonic method. The fundamental phenomena of 
 heredity had already shaped their course in the living world 
 before the introduction of amphimixis, and have, therefore, no 
 connection with amphigonic descent and the complications 
 arising from it. This fact has often been overlooked or left out of 
 consideration, and thus the solution of the problem of heredity 
 has been rendered much more difficult. A whole series of the 
 phenomena of heredity can be investigated theoretically without 
 considering the complications arising from amphimixis, though, 
 in point of fact, it is always a factor, and thus the problem to 
 be solved is very considerably simplified. 
 
 The natural course of such an investigation would be to pass 
 from the simple to the complex, but it is not advisable at present 
 to begin the study of heredity by a consideration of the simplest 
 beings, and to ascend from the unicellular to the multicellular 
 organisms. For besides the fact that we know nothing of the 
 individual phenomena such as the transmission of the in- 
 dividual characters in the lower forms, the principal reason 
 for not following the ordinary course in this case is the 
 fact that amphigonic reproduction, or the processes of fer- 
 tilisation and the complicated development of multicellular 
 organisms, affords us, as already stated, a deep insight into 
 the processes of heredity. The same is true in this case as in 
 almost all physiological processes, investigation cannot pro- 
 ceed from the simple to the more complex without taking into
 
 22 THE GERM-PLASM. 
 
 consideration the objects and processes for which it was first 
 undertaken. It must, on the contrary, avoid the densely over- 
 grown path and skirt the hedge which surrounds the enchanted 
 castle of the secret of Nature, in order to see if there be not 
 somewhere a gap through which it is possible to enter and 
 obtain a firm foothold. 
 
 Such a gap in the hedge which encloses the secret of heredity 
 may be found in the processes of fertilisation, if we connect 
 them with the facts of heredity as observed in the organisms 
 which have adopted sexual reproduction. 
 
 As long as we were under the erroneous impression that 
 the fertilisation 01 the ovum by the spermatozoon depended 
 on an aura seminalis which incited the egg to undergo de- 
 velopment, we could only partially explain the fact that the 
 father as well as the mother is able to transmit characters to 
 the children by assuming the existence of a spiritus rector, con- 
 tained in the aura seminalis which was transferred to the ovum 
 and united with that of the latter, and thus with it directed the 
 development. The discovery that development is effected by 
 material particles of the substance of the sperm, the sperm-cells, 
 entering the ovum, opened the way to a more correct inter- 
 pretation of this process. We now know that fertilisation is 
 nothing more than the partial or complete fusion of two cells, 
 the sperm-cell and the egg-cell, and that normally only one of 
 the former unites with one of the latter. Fertilisation thus 
 depends on the union of two protoplasmic substances. More- 
 over, although the male germ-cell is always very much smaller 
 relatively than the female germ-cell, we know that the father's 
 capacity for transmission is as great as the mother's. The 
 important conclusion is therefore arrived at that only a small 
 portion of the substance of the ovum can be the actual hereditary 
 substance. Pfliiger and Nageli were the first to follow out this 
 idea to its logical conclusion, and the latter observer stated 
 definitely that it is impossible to avoid the assumption that no 
 more hereditary substance is contained in the egg-cell than in 
 the male germ-cell, and that consequently the amount of the 
 substance must be infinitesimal, for the sperm-cell is, in most 
 cases, many hundred times smaller than the ovum. 
 
 The numerous and important results of the investigations of 
 many excellent obseivers on the process of fertilisation have 
 now rendered it almost certainin my opinion, absolutely so
 
 INTRODUCTION. 23 
 
 that by far the larger part of the egg-cell does not consist of 
 hereditary substance, and that the latter only constitutes a 
 small portion even of the sperm-cell. From his observations on 
 the egg of the star-fish, Oscar Hertwig had suspected that the 
 essential part of the process of fertilisation consists in the union 
 of the nuclei of the egg- and sperm-cells, and as it is now 
 known that the hereditary substance is undoubtedly contained 
 in the nucleus, this view has, in this respect at least, proved to 
 be the right one. It is true that the nucleus of the male cell 
 is always surrounded by a cell-body, and that Strasburger's 
 opinion to the contrary is incorrect. We now know, through 
 the researches of Guignard, that even in Phanerogams a small 
 cell-body surrounds the nucleus, and that a special structure, the 
 ' centrosome,' which is absolutely essential for the commence- 
 ment of development, is contained within it. This structure 
 will be treated of in further detail presently, but I must here lay 
 stress upon my view, that the ' centrosome ' with its ' sphere of 
 attraction ' cannot in any case be the hereditary substance, and 
 that it is merely an apparatus for the division of the cell and 
 nucleus. 
 
 Both in animals and plants, however, essentially the same 
 substance is contained in the nucleus both of the sperm-cell and 
 egg-cell : this is the hereditary substance of the species. There 
 can now be no longer any doubt that the view which has been held 
 for years by Strasburger and myself is the correct one, according 
 to which the nuclei of the male and those of the female germ-cells 
 are essentially similar, i.e., in any given species they contain the 
 same specific hereditary substance. 
 
 The splendid and important investigations carried out by 
 Auerbach, Biitschli, Flemming, and many others, on the detailed 
 processes of nuclear division in general, and those dealing more 
 particularly with the fertilisation of the egg in Ascaris by 
 van Beneden, Boveri, and others, have given us the means 
 of ascertaining more definitely what portion of the nucleus is the 
 substance on which heredity depends. As already remarked, 
 this substance corresponds to the ' chromosomes,' those rod-like, 
 looped, or granular bodies which are contained in the nucleus, 
 and which become deeply stained by colouring matters. 
 
 As soon as it had been undoubtedly proved that the nucleus, 
 and not the body of the cell, must contain the hereditary sub- 
 stance, the conclusion was drawn that neither the membrane of
 
 24 THE GERM-PLASM. 
 
 the nucleus, nor its fluid contents, nor the nucleoli which 
 latter had been the first to attract attention could be regarded 
 as such, and that the 'chromatic granules' alone were 
 important in this respect. As a matter of fact several investi- 
 gators, Strasburger, Oscar Hertwig, Kollicker, and myself, 
 reasoning from the same data, arrived at this conclusion 
 independently, within a short time of one another. 
 
 It will not be considered uninteresting or superfluous to 
 recapitulate the weighty reasons which force us to this con 7 
 elusion, for it is clear that it must be of fundamental importance 
 in a theory of heredity to know for certain what the substance is 
 from which the phenomena which are to be explained proceed. 
 
 The certainty with which we can claim the 'chromatin 
 granules ' of the nucleus as the hereditary substance depends 
 firstly, on the process of amphimixis ; and secondly, on that of 
 nuclear division. We know that the process of fertilisation 
 consists essentially in the association of an equal number of 
 chromatin rods from the paternal and maternal germ-cells, and 
 that these give rise to a new nucleus from which the formation of 
 the offspring proceeds. We also know that in order to become 
 capable of fertilisation each germ-cell must first get rid of half of 
 its nuclear rods, a process which is accomplished by very peculiar 
 divisions. Without entering into further particulars here, 
 amphimixis may be described as a process by means of which 
 one-half of the number of nuclear rods is removed from a cell 
 and replaced by an equal number from another germ-cell. 
 
 The manner, however, in which the chromatin substance is 
 divided in nuclear division strengthens the above view of its 
 fundamental nature. This method of division leaves no doubt 
 that it is a substance of the utmost importance. I need only 
 briefly recapitulate the main points of the wonderfully com- 
 plicated process of the so-called mitotic or karyokinetic cell- 
 division, which follows a definite law even as regards the 
 most minute details. 
 
 When the nucleus is going to divide, the chromatin granules, 
 which till then were scattered, become arranged in a row, 
 and form a long thread, which extends through the nucleus in 
 an irregular spiral, and then divides into portions (chromosomes) of 
 fairly equal length. The chromosomes have at first the form of 
 long bands or loops, but afterwards become shortened, thus 
 giving rise to short loops, or else to straight rods or rounded
 
 INTRODUCTION. 25 
 
 granules. With certain exceptions, to be mentioned later, the 
 number of chromosomes which arise in this way is constant for 
 each species of plant or animal, and also for successive series of 
 cells. By the time the process has reached this stage a special 
 mechanism appears, which has till now remained concealed in 
 the cell substance. This serves to divide the chromatin elements 
 into two equal parts, to separate the resulting halves from one 
 another, and to arrange them in a regular manner. At the 
 opposite poles of the longitudinal axis of the nucleus two clear 
 bodies, the ' centrqsomes,' each surrounded by a clear zone, 
 the so-called ' sphere of attraction ' now become visible. The 
 importance of these was first recognised by Fol, van Beneden, 
 and Boveri. They possess a great power of attraction over the 
 vital particles of the cell, so that these become arranged around 
 them like a series of rays. At a certain stage in the preparation 
 for division, the soft protoplasmic substance of the cell-body as 
 well as of the nucleus gives rise to delicate fibres or threads : these 
 fibres are motile, and, after the disappearance of the nuclear 
 membrane, seize the chromosomes whether these have the 
 form of loops, rods, or globular bodies with wonderful certainty 
 and regularity, and in such a way that each element is held 
 on either side by several threads from either pole. The 
 chromatin elements thus immediately become arranged in a 
 fixed and regular manner, so that they all come to lie in the 
 equatorial plane of the nucleus, which we may consider as a 
 spherical body. The chromatin elements then split longitudin- 
 ally, and thus become doubled, as Flemming first pointed out. 
 It must be mentioned that this splitting is not caused by a pull 
 from the pole threads (spindle threads), which attach themselves 
 to the chromatin rods on both sides ; the division arises rather 
 from forces acting in the rods themselves, as is proved by the 
 fact that they are often ready to divide, or indeed have already 
 done so, some time before their equatorial arrangement has 
 taken place by means of these threads. 
 
 The splitting is completed by the two halves being gradually 
 drawn further apart towards the opposite poles of the nuclear 
 spindle, until they finally approach the centre of attraction or 
 ccntrosome, which has now fulfilled its object for the present, 
 and retires into the obscurity of the cell-substance, only to 
 become active again at the next cell-division. Each separated 
 half of the nucleus now constitutes a daughter-nucleus, in which
 
 26 THE GERM-PLASM. 
 
 it immediately breaks up, and becomes scattered in the form of 
 minute granules in the delicate nuclear network, so that finally 
 a nucleus is formed of exactly the same structure as that with 
 which we started. Similar stages to those which occur in the 
 aggregation of the chromatin substance in the mother-nucleus 
 preparatory to division are passed through during the separa- 
 tion of the daughter-nuclei, but in the reverse order. 
 
 It is evident, as Wilhelm Roux was the first to point out, 
 that the whole complex but wonderfully exact apparatus for the 
 division of the nucleus exists for the purpose of dividing the 
 chromatin substance in a fixed and regular manner, not merely 
 quantitatively, but also in respect of the different qualities which 
 must be contained in it. So complicated an apparatus would 
 have been unnecessary for the quantitative division only : 
 if, however, the chromatin substance is not uniform, but is 
 made up of several or many different qualities, each of which 
 has to be divided as nearly as possible into halves, or accord- 
 ing to some definite rule, a better apparatus could not be 
 devised for the purpose. On the strength of this argument, we 
 may therefore represent the hereditary substance as consisting of 
 different ' qualities! The same conclusion is arrived at on purely 
 theoretical grounds, as will be shown later on when we follow 
 out the consequences of the process of amphimixis. 
 
 For the present it is sufficient to show that the complex 
 mechanism for cell-division exists practically for the sole pur- 
 pose of dividing the chromatin, and that thus the latter is 
 without doubt the most important portion of the nucleus. Since, 
 therefore, the hereditary substance is contained within the 
 nucleus, the chromatin must be the hereditary substance. 
 
 De Vries's objection to this view is, in my opinion, only an 
 apparent one; for it has not been asserted that 'the nucleus 
 alone is the bearer of the hereditary characters,' as de Vries 
 thinks, but that the nucleus alone contains the hereditary sub- 
 stance, or that substance which is capable of determining not 
 only the character of a particular cell, but also that of its 
 descendants. This is never contained in the cell-body, but 
 always in the nucleus in multicellular organisms, and doubtless 
 the same holds good for unicellular beings. It is quite possible 
 that in certain lower Algae a few of the structures in the cell 
 such as vacuoles and chlorophyll bodies pass directly from the 
 egg-cell into the daughter-cells, although this cannot by any
 
 INTRODUCTION. 
 
 FIG. i. DIAGRAM OF NUCLEAR DIVISION. 
 
 A. Cell with nucleus n, and centrosomes 
 cs, preparatory to division. The chromatin 
 has become thickened so as to form a spiral 
 thread-cAr. 
 
 B. The nuclear membrane has disappeared. 
 Delicate threads radiate from the centrosomes, 
 and form the ' nuclear spindle,' in the equator 
 of which eight chromosomes or nuclear loops 
 (fhr) are arranged : these have been formed by 
 the spiral thread of cbromatin in A becoming 
 broken up. 
 
 C. The chromosomes have each become 
 split longitudinally into two, and are about to 
 be drawn apart by means of the spindle-tlirtaiis. 
 ( For the sake of clearness only four of the eight 
 chromosomes are indicated. 
 
 D. The daughter-loops pass towards the 
 poles of the spindle. 
 
 E. The body of the cell has undergone divi- 
 sion ; each of the resultant cells contains a 
 centrosome and eight nuclear loops.
 
 28 THE GERM-PLASM. 
 
 means be considered as proved. In any case such a direct 
 transmission plays only a very insignificant part in plants, and 
 practically none at all in animals, for specific structures are 
 not present in the egg-cells of animals : there may at most be 
 deposits of nutrient material. These, however, are not living 
 structures of the cell, but only passive chemical substances. So 
 far from denying that the nucleus contains the hereditary sub- 
 stance, de Vries bases his whole theory on this incontestable 
 fact. The last doubts on this point were dispelled by the expe- 
 riments of Boveri,* who, after artificially removing the nucleus 
 from the eggs of a species A of sea-urchin, and then pouring 
 over them the sperm of another species B, found that these 
 eggs developed into larvae of the latter species. In this case 
 therefore the substance of the maternal germ-cell acted as 
 nutrient material only, whilst the paternal germ-cell impressed 
 the character of the species on the larva. None of the maternal 
 specific characteristics were transmitted, and in this case, at all 
 events, the question of any ' heredity apart from the nucleus ' is 
 therefore excluded. 
 
 Several objections have been recently raised to my view that 
 the nucleus is the seat of heredity. Verworn,t for instance, 
 repeats the opinion, previously expressed by Whitman, that the 
 cell-body, quite as much as the nucleus, must be looked upon as 
 the hereditary substance, because the nucleus cannot exist with- 
 out the cell-body ; and also because, in his opinion, which is 
 undoubtedly a correct one, the life of a cell consists in a continual 
 interchange of substance between the cell and the nucleus. But 
 is the question as to whether the closest physiological relations 
 exist between the nucleus and the cell, so that neither can exist 
 apart from the other, synonymous with that as to whether the 
 hereditary substance is contained in the nucleus or in the cell- 
 body? We must at least be allowed to make the hypothesis 
 that the 'store of the primary constituents' (' Anlagenmagazin ') 
 of the hereditary substance is contained and preserved in the 
 nucleus ; for, as has already been indicated, and will subsequently 
 be shown more clearly, this substance can hardly be stored up 
 in two different places, seeing that a very complicated apparatus 
 
 * Boveri, ' Ein geschlechtlich erzeugter Organismus ohne miitterliche 
 Eigenschaften.' Gesellsch. f. Morph. u. Physiol. Mtinchen, 16 Juli 1883. 
 
 t MaxVerworn, 'Die physiologische Bedeutung des Zellkerns,' Bonn, 
 1891 (Archiv. f. ges. Physiol., Bd. 51).
 
 INTRODUCTION. 29 
 
 is required for its distribution : a double apparatus would 
 certainly not have been formed by nature if a single one suffices 
 for the purpose. Only as long as the phenomena of heredity 
 and the meaning of these phenomena are still far from being 
 known, is it possible to hold such opinions as that which pre- 
 supposes the distribution of the hereditary substance amongst 
 both cell and nucleus. As soon as a further insight into these pro- 
 cesses is obtained, it will no longer be possible to doubt that the 
 structure of the hereditary substance must be so complex that 
 we can only wonder how it could ever have been developed at 
 all. We know that the nucleus contains a substance which, even 
 with the imperfect means of observation at our disposal, is seen 
 to be extremely complex, and that it becomes modified in a very 
 remarkable manner after every cell division, only to be again 
 transformed at the approach of the following division. We 
 can, moreover, observe that the cell is provided with a special 
 apparatus which evidently enables it to halve this substance very 
 accurately. The statement that this substance is the hereditary 
 substance can, therefore, hardly be considered as an hypothesis 
 any longer. 
 
 It has also been supposed that fresh evidence against the 
 view that the chromosomes are the hereditary substance, has 
 been furnished by the recent observations of Fol* and Guignard.t 
 which prove that the centrosome and its ' sphere of attraction,' 
 which belong to the cell-body, and constitute the apparatus for 
 division, pass into the ovum along with the sperm-nucleus in 
 the process of fertilisation. Suppose I take two heaps of grain 
 from different places, and load them on two carts, harness a 
 horse to each cart, and drive them to the same place ; does this 
 prove that the horses consist of grain ? They are merely the 
 means by which the grain is transferred from one place to 
 another, just as the centrosomes are the means whereby the 
 sperm-nucleus is transferred to the ovum : whether they are any- 
 thing more than this z'.e., whether they contain hereditary sub- 
 stance still remains to be proved, and such a conjecture is 
 hardly more probable than that the horses, besides being the 
 means of transport of the corn, should actually consist of corn ! 
 
 * 'Le Quadrille des Centres,' Archiv. Sc. Phys. et Nat., Gen&ve, 
 15 Avril 1891. 
 
 t ' Sur 1' Existence des Spheres Attractives dans les Cellules Vegetales,' 
 Compt. Rend. Sc., 9 Mars 1891.
 
 30 THE GERM-PLASM. 
 
 It might, however, be urged that the transference not only of 
 the hereditary substance or store of primary constituents, but 
 also of the centrosome or means of transference of this substance, 
 implies the transference of the rate of cell-division, which is 
 regulated by the centrosome and essentially decides the cell- 
 sequence in the offspring, and which consequently also takes 
 part in heredity. But I consider this also to be an incorrect 
 deduction, because the periods of activity of the apparatus for 
 division must obviously be dependent on the conditions of the 
 cell itself; these conditions, however, apart from nutrition, 
 depend on the ultimate specific structure of the cell. As, 
 according to my view, this structure is impressed on the cell by 
 the nuclear substance, the periodicity of cell-division must also 
 be dependent on the nucleus. The law that only a certain part 
 of the nuclear matter is to be regarded as the hereditary substance 
 appears to me to receive fresh support from all the more recent 
 observations.* 
 
 Chromatin substance is not only contained in the nucleus of 
 the germ-cells and of the fertilised ovum, but also in all the cells 
 of the entire organism in each phase of its development, at any 
 rate as long as they are capable of multiplying, and are pos- 
 sessed of vitality. The chromatin in all the cells of the body is 
 derived from that in the fertilised ovum, while the development 
 of the body from the egg-cell is effected by aseries of cell- 
 
 * Many seem inclined to regard the process observed by Fol, which he 
 described as ' le quadrille des centres,' as a proof that the centrosomes 
 nevertheless must or at any rate might contain a kind of hereditary sub- 
 stance. I believe, however, that this process is quite similar to that which 
 occurs in every nuclear division, except that in fertilisation owing to the 
 fact that the first segmentation nucleus receives a centrosome from each of 
 the two conjugating cells it is a double, and not a single process. Each of 
 these centrosomes divides and passes to the region of the two poles of the 
 future spindle, just as would occur if only a single centrosome were present 
 in the cells. I should be surprised if this were not the case, and if the 
 centrosome of the egg-cell passed to one pole, and that of the spermatozoon 
 to the other ! Guignard is of the opinion that even if the nucleus is of great 
 importance as regards the transference of transmissible qualities, we must 
 nevertheless attribute to the 'spheres directives,' Me role primordial dans 
 1'accomplissement de la fecondation." This is true if it only indicates that 
 the beginning of embryonic development depends as does every nuclear 
 division on the presence of the apparatus for division. But the view is not 
 thereby refuted that fertilisation consists in the union of two nuclear 
 substances.
 
 INTRODUCTION. 31 
 
 divisions, each of which includes a division of the nucleus in the 
 manner just described. In the process of ontogeny the chro- 
 matin of the first nucleus undergoes repeated subdivisions into 
 two parts of equal volume, and it would very soon become so small 
 as to be invisible even under the highest powers of the micro- 
 scope, if it did not continue to grow, as does the cell-body. 
 This occurs just as much in the case of numerous animal eggs 
 to which no nutrient material is supplied during the develop- 
 ment of the embryo, as in that of those which are nourished 
 from the beginning, or of plants which as a rule begin to obtain 
 their own nutriment at a very early stage. The chromatin, or 
 hereditary substance of the fertilised ovum, enters upon a long 
 and complex process of growth, which only ceases when no 
 new cells are produced either for the formation of new parts, or 
 to replace old ones, that is to say, at the end of the life of the 
 individual. This growing hereditary substance may be com- 
 pared to a tree whose branches arise in strict dichotomy, except 
 for the fact that the chromatin does not consist of one con- 
 tinuous mass, but of a number of separate particles not actually 
 contiguous with one another ; for at each cell-division the two 
 halves of the chromatin rods separate never to unite again in 
 one nucleus. Each is finally contained in a special nucleus, 
 which is separated from the rest by being enclosed in a special 
 cell-body. The question now arises as to whether all these 
 fragments of the hereditary substance which compose the 
 chromatin 'tree' of an organism are similar to, or different 
 from, one another, and it can easily be shown that the latter 
 must be the case. 
 
 In order to prove this, we take as our basis the well-grounded 
 assumption that the chromatin in the nucleus of the fertilised 
 egg is the substance on which heredity depends. Thus we 
 know that the possibility of the offspring resembling its father, for 
 example, in a thousand different physical and mental characters 
 depends on the minute mass of a few chromatin granules in 
 the nucleus of the sperm-cell, and that the characters of a fully 
 formed organism depend as a whole, as well as in detail, on the 
 arrangement, number, and nature of the cells which compose 
 it. The influence therefore which the minute mass of paternal 
 chromatin in the nucleus of the fertilised egg-cell exerts on the 
 course of development, can only be such as to regulate the 
 nature and the rate of multiplication of the cells in the body of
 
 32 THE GERM-PLASM. 
 
 the offspring in such a manner as to cause them to resemble 
 the cells of the paternal body. The chromatin is therefore in a 
 condition to impress the specific character on the cell in the 
 nucleus of which it is contained. 
 
 As the thousands of cells which constitute an organism pos- 
 sess very different properties, the chromatin which controls them 
 cannot be uniform ; it must be different in each kind of cell. 
 
 The chromatin, moreover, cannot become different in the cells 
 of the fully formed organism ; the differences in the chromatin 
 controlling the cells must begin with the development of the egg- 
 cell, and must increase as development proceeds ; for otherwise 
 the different products of the division of the ovum could not give 
 rise to entirely different hereditary tendencies. This is, how- 
 ever, the case. Even the two first daughter-cells which result 
 from the division of the egg-cell give rise in many animals to 
 totally different parts. One of them, by continued cell division, 
 forms the outer germinal layer, and eventually all the organs 
 which arise from it, e.g., the epidermis, central nervous system, 
 and sensory cells ; the other gives rise to the inner germinal 
 layer and the organs derived from it, the alimentary system, 
 certain glands, &c. The conclusion is inevitable that the 
 chromatin determining these hereditary tendencies is different 
 in the daughter-cells. 
 
 This holds good in all subsequent stages of ontogeny; the 
 difference between the developmental tendencies of the cells 
 resulting from the division of the ovum is in exact proportion to 
 that between the chromatin substance of their nuclei. Ontogeny, 
 or the development of the individual, depends therefore on a 
 series of gradual qualitative changes in the nuclear substance of 
 the egg-cell. 
 
 The fundamental principle of the view which has just been 
 briefly sketched was put forward by me some years ago, and I 
 then made use of the term idioplasm to represent the substance 
 which is contained in the chromatin bodies of the nucleus, and 
 which determines the nature of the whole cell. Oscar Hertwig 
 also independently adopted this term, which had first been intro- 
 duced by Nageli with a somewhat different meaning. As stated 
 in the first section of this book, Nageli defined idioplasm as the 
 guiding and controlling substance of the body, in contrast to the 
 more passive and controllable trophoplasm. It is open to doubt 
 whether the latter term should be retained, but the former is
 
 INTRODUCTION. 33 
 
 certainly happily chosen. It is true Nageli did not mean to 
 indicate any definite substance visible under the microscope when 
 he used the word idioplasm, for the facts of nuclear division and 
 fertilisation were then unknown. But these facts are so convincing 
 that no doubt as to what is to be regarded as the idioplasm is any 
 longer possible, and Nageli's conception of an idioplasm forming 
 a network, traversing and connecting the contents of all the cells 
 in the organism, may be regarded as abandoned. We are 
 therefore justified in transferring the term introduced by him to 
 the nuclear substance which determines the nature of the cell. 
 
 We now therefore understand by the term idioplasm the 
 nuclear substance controlling any particular cell. This is at the 
 same time the hereditary substance, for it is never formed afresh, 
 but is always derived from the idioplasm of another cell ; more- 
 over, it not only determines the actual characters of the parti- 
 cular cell, but also those of all of its descendants. 
 
 Hence we must assume a difference in the idioplasm not only 
 in dealing with two cells differing in structure and functions, 
 but also in all cases in which we know that different primary 
 constituents are contained in two cells. This has often been 
 overlooked in using the term ' embryonic cells ' merely in 
 the sense of equivalent elements ' which may give rise to any 
 parts,' simply because they frequently resemble one another, 
 assuming that they must therefore always be actually equivalent. 
 It is quite true that the idioplasm of such cells appears similar, 
 at least we can recognise no definable differences in the 
 chromatin rods of two cells in the same animal. But this is no 
 argument against the assumption of an internal difference. The 
 perfect external resemblance between two eggs is not a sufficient 
 reason why two identical chickens should be hatched from them. 
 The eggs may have been produced by different mothers, or they 
 may have been fertilised by two different males. We cannot 
 perceive these slight differences in either case, and we could not 
 even do so by attempting to analyse the idioplasm concealed in 
 the nuclei of the two eggs by the aid of our most powerful 
 objectives. Theoretical considerations will show later on that 
 it must be so, and that the units of the idioplasm on which the 
 nature of the latter depends are far too numerous, and therefore 
 far too small, to be visible. 
 
 If therefore the two halves into which the chromatin rods are 
 split in karyokinesis look exactly alike, and even if the divided
 
 34 THE GERM-PLASM. 
 
 portions of the granules (microsomes), of which the rods often 
 visibly consist, resemble each other exactly, there is still no 
 reason why they should not be different in their nature ; in some 
 cases one, and in others another occur. 
 
 We shall consequently in this connection have to assume two 
 kinds of nuclear division which are externally indistinguishable 
 from one another, in one of which the two daughter-nuclei con- 
 tain similar idioplasm, while in the other they contain different 
 kinds of idioplasm. These kinds of division we may speak of as 
 homceokinesis and heterokinesis, that is, as a division into parts 
 similar or dissimilar to each other with regard to the hereditary 
 tendencies they contain (' erbgleich' and ' erbungleich'). The 
 former must depend on a perfectly uniform distribution of the 
 primary constituents in the two halves of the rods, and will 
 consequently have been preceded by a duplication in the process 
 of growth ; in the latter this growth will be connected with a 
 heterogeneous grouping of these constituents. 
 
 Although we cannot ascertain anything directly about the 
 forces which cause this splitting of the chromatin rods, it may at 
 any rate be asserted that they must be contained within the sub- 
 stance of the latter, and be connected with the actual develop- 
 ment of the qualities of the idioplasm : for otherwise it could not 
 be understood how the qualities, which are changed during the 
 division of the nucleus, become separated sharply from one 
 another and arranged in the two daughter-nuclei. And yet this 
 must be the case if different cells with different kinds of idioplasm 
 can all arise from one mother-cell, which is an undoubted fact. 
 
 It appears to me, therefore, that the regular ontogenetic 
 changes of the idioplasm, as they begin with the division of the 
 egg-cell and cease with the natural death of the organism, 
 depend on purely internal causes, which lie in the physical 
 nature of the idioplasm. In obedience to these, a division of the 
 nucleus accompanies each qualitative change in the idioplasm, 
 in which process the different qualities are distributed between 
 the two resulting halves of the chromatin rods. I shall speak of 
 the different kinds of idioplasm arising in this way as the onto- 
 genetic stages of the idioplasm, or shortly, the onto-idic stages. 
 
 Hereditary substance, in the full meaning of the term i.e., 
 that substance which contains all the primary constituents 
 of the whole organism, is merely the idioplasm of the germ- 
 cell, and it is advisable for practical purposes to denote
 
 INTRODUCTION. 35 
 
 this first onto-idic stage by the short term germ-plasm, which I 
 suggested for it at a time when the idea of idioplasm had not 
 been introduced. At that time I meant by the term ' germ- 
 plasm' the hereditary substance of a germ-cell capable of 
 development, without expressing any opinion as to its position 
 or nature. We can now state that the germ-plasm is the first 
 ontogenetic stage oj the idioplasm of an animal or a plant, 
 whether the cell, in the nucleus of which it is contained, is 
 sexually differentiated or not. 
 
 We must next attempt to form an idea of the constitution and 
 nature of the germ-plasm, and of the ontogenetic stages of the 
 idioplasm, or onto-idic stages.
 
 PART I. 
 
 THE MATERIAL BASIS OF HEREDITY. 
 
 CHAPTER I. 
 
 THE GERM-PLASM. 
 
 i. THE FUNDAMENTAL UNITS. 
 
 Now that the conception of the germ-plasm as the hereditary 
 substance contained in the germ-cells has been fully established, 
 and since it has been shown in general terms that this form of 
 the idioplasm must become changed during ontogeny and con- 
 verted into the idioplasm of the cells which constitute the 
 mature organism, we must attempt to form some idea of its 
 nature ; for it would otherwise be impossible to construct a 
 theory of heredity. In attempting this, we shall for the present 
 entirely neglect the complication due to sexual reproduction, 
 and take as our starting-point a germ-plasm which does not 
 contain the primary constituents of two parents, but those of 
 one only, that is to say, one which is constituted just as it 
 would be in a species which had at all times multiplied 
 asexually. 
 
 Before venturing to express an opinion concerning the consti- 
 tution of the germ-plasm, and to derive therefrom the pheno- 
 mena of heredity, I should like to premise that it is not my 
 intention to attempt an explanation of life. It is necessary to 
 distinguish between a theory of life and one of heredity. De 
 Vries has pointed out very clearly that the former is impossible 
 at present, but that it seems by no means impossible to arrive 
 at a satisfactory explanation of the phenomena of heredity if one
 
 30 THE GERM-PLASM. 
 
 takes for granted the essential phenomena of life, nutrition, 
 assimilation, and growth. 
 
 These functions, together with the associated ones of sensa- 
 tion and movement, are connected in all organisms with which 
 we are familiar, from the simplest unicellular forms to the 
 highest plants and animals, with at least two different sub- 
 stances, viz., the idioplasm of the nucleus, z>., the hereditary 
 plasm in the more general sense, and the protoplasm of the 
 cell-body. These two differ as regards their functions, though 
 they resemble each other in being composed of living substance : 
 that is to say, the primary vital forces, nutrition and growth, are 
 developed within them. As the term ' protoplasm ' is used in a 
 far too indefinite sense, I shall follow Nageli's example, and call 
 the vital substance of the cell the 'formative plasm' or 
 morphoplasm (Nageli's ' trophoplasm '), in contrast to the idio- 
 plasm. The latter is the active element in the process of for- 
 mation, and the former the passive one. As we now know that 
 the idioplasm is situated in the nuclei only, we cannot regard 
 the cell-bodies which determine the form of all parts of the 
 organism as mere ' nutrient plasm.' 
 
 Both forms of the living substance are included in the term 
 'protoplasm,' and we have now to decide how we are to 
 imagine its constitution in detail. 'Protoplasm' has often 
 been conceived as a ' modification of albumen ' ; till quite recently, 
 in fact, this was the general idea. Briicke, however, pointed 
 out a considerable time ago that albumen does not possess 
 the power of assimilation, and has therefore no vitality ; 
 it has moreover been proved by the study of physiological 
 chemistry that other substances besides albumen are also 
 obtained from protoplasm, and that these cannot be assumed to 
 be insignificant without further proof. Although compounds of 
 sulphur and phosphorus, for instance, only exist in protoplasm 
 in comparatively small quantities, we must not infer from this 
 fact that they are of slight importance. In any case, we cannot 
 say that protoplasm is a modification of albumen, because we 
 can only examine it chemically when dead, and in this condi- 
 tion it has lost its most important properties, and has become 
 changed in a manner which we need not here consider further. 
 As de Vries expresses it, protoplasm is not a chemical, but a 
 morphological conception. That is, it does not consist of a 
 confused mass of certain chemical molecules, but of morpho-
 
 TFIE GERM PI. ASM. 39 
 
 logical units, which are themselves composed of molecules, 
 or, as Briicke first expressed it, protoplasm is 'organised.' 
 As I have shown in the historical introduction to this book, 
 Herbert Spencer, and more recently de Vries and Wiesner, 
 have assumed the existence of such organic units. 
 
 De Vries, moreover, points out that protoplasm possesses 
 certain 'historical' properties besides its physical and chemical 
 ones. It may certainly be doubted, as de Vries states, whether 
 it will ever be possible to produce 'living protoplasm other- 
 wise than in a phylogenetic manner,' that is to say, to make it 
 artificially in the laboratory ; but it cannot be admitted that this 
 is so improbable, merely because the conception of protoplasm 
 demands that it should be derived from pre-existing proto- 
 plasm. This would exclude for ever not only the possibility of 
 its production in our laboratories, but also its logically inevit- 
 able and indispensable primary formation in the great 
 laboratory of Nature. Most, in fact probably all, kinds of 
 protoplasm with which we are acquainted possess historical 
 qualities, not in addition to, but "within their physico-chemical 
 ones ; that is, they contain special modifications of construction 
 peculiar to themselves which arose in adaptation to the condi- 
 tions of life, and have been transmitted for a long period of 
 time. But protoplasm which does not yet possess 'historical' 
 i.e., inherited qualities, does not seem to me to be inconceiv- 
 able. It would be the simplest form of living matter which, in 
 virtue of its constitution, possessed the primary vital forces, 
 assimilation, metabolism, and so on. The historical qualities 
 of the protoplasm, its special hereditary tendencies, are not con- 
 nected with these primary vital forces. The latter must exist 
 independently in all protoplasm. 
 
 All those writers * who have assumed the existence of units on 
 which the vital forces of protoplasm depend, have pointed out 
 that they are not chemical molecules, for the latter do not pos- 
 sess the power of assimilation and reproduction. Hence it 
 follows that protoplasm is a complex substance which is not 
 homogeneous, but which consists of different kinds of molecules. 
 There is therefore no molecule of protoplasm, but we have to 
 imagine that even in its simplest modifications, protoplasm 
 invariably consists of groups of molecules, each of which is 
 
 * Brucke, Herbert Spencer, de Vries, and Wiesner.
 
 4O THE GERM-PLASM. 
 
 composed of different kinds of chemical molecules. I shall 
 call these units the ''bearers of vitality' (' Lebenstrager') or 
 ' biophors] because they are the smallest units which exhibit 
 the primary vital forces, viz., assimilation and metabolism^ 
 growth, and multiplication by Jission. 
 
 As living protoplasm cannot be subjected to chemical analysis, 
 we cannot describe its chemical constitution more precisely ; 
 but what has so far been determined by the analysis of dead 
 protoplasm certainly indicates that the albuminoids are not the 
 only bearers of vitality, as has generally been assumed, but that 
 other substances play a no less important part in living proto- 
 plasm, a fact which has been insisted on by Hoppe-Seyler and 
 Baumann. Besides albuminoids, compounds containing phos- 
 phorus, such as lecithin and nuclein, which are not related 
 chemically to albumen, but enter into combination with it, are 
 known to occur in dead protoplasm ; and besides these, proto- 
 plasm also contains cholesterin, which is probably a product of 
 destructive metabolism, and carbohydrates, such as glycogen, 
 starch, inulin, and dextrin, as well as compounds of potassium.* 
 Although we cannot at present guess from what chemical com- 
 pounds in living protoplasm these bodies have been derived, 
 there can be no doubt that ' a relation exists between them and 
 the vital processes' (Hoppe-Seyler), and that albumen, or dif- 
 ferent kinds of albumen, do not alone bring about the vital 
 processes, but that several other substances, such as salts, and 
 compounds containing phosphorus, and more particularly water, 
 are just as essential : in short, life depends simply on the 
 interaction of molecules, differing chemically from one another, 
 but defined within certain limits. 
 
 After long consideration, I have decided to designate such a 
 group of molecules on which the phenomena of life depend by 
 the special term ' biophor? This seemed to be advisable, because 
 the various terms introduced previously by others were either left 
 too vague for these minute vital particles to be identified with them, 
 or if defined more exactly, were used with a different meaning. 
 It would certainly be a mistake to make use of a name already 
 introduced, in another sense from its original one. Herbert 
 Spencer's t ' physiological units ' are similar to the biophors, 
 
 * Cf. Hoppe-Seyler, 'Allgemeine Biologic,' Berlin, 1877, p. 75 (Parti, 
 of the ' Lehrbuch der physiologischen Chemie'). 
 + Herbert Spencer, 'Principles of Biology,' vol. i., p. 183.
 
 THE GERM-PLASM. 41 
 
 and he looks upon them as being intermediate between the 
 chemical units (molecules) and the morphological -units (cells). 
 But he supposes their function in heredity to be different from 
 that which I ascribe to my biophors. Haeckel* understands by 
 the term 'plastidule,' introduced by Elsberg,t the hypothetical 
 ultimate particles of which 'protoplasm' is composed ; he re- 
 gards them as equivalent to the ' molecules ' of inorganic matter, 
 but supposes them to possess 'vital qualities' as well. Of 
 course this definition is in itself ^sufficient proof, as de Vries 
 very correctly remarks, that Haeckel's plastidules are not 
 molecules in the physical sense; these very 'vital qualities' 
 are the point in which they differ from them. I could not 
 adopt Nageli's term either, because a ' micella ' differs essentially 
 in its construction and properties from a biophor. It is defined 
 as 'a minute crystal, microscopically invisible, consisting of a 
 larger or smaller number of molecules, and is, when turgid, 
 surrounded by a layer of water.' I As regards the absolute 
 size of the micella, Nageli calculates that it may consist of 
 one hundred molecules, or on the other hand, of only a single 
 molecule of albumen. As in the case of Haeckel's plasti- 
 dules, we have here therefore to deal with a unit the vital 
 character of which does not depend on a peculiar grouping 
 of several or even many different kinds of molecules. Indeed 
 Nageli draws attention in another part of his book (p. 63) to 
 the unstable chemical composition of the proteids so far as 
 can be made out by analyses, and very correctly considers it 
 extremely probable ' that there are various molecules of albumen 
 which differ from one another in containing unequal quantities 
 of hydrogen, oxygen, &c.' This leads him to the further 
 assumption 'that the micellae of the proteids consist of a mix- 
 ture of two or more different kinds of molecules of albumen. In 
 each proteid the different molecules of albumen would be mixed 
 in special proportions, and, further, each would contain special 
 quantities of phosphates, salts of magnesia, lime, and so on.' This 
 conception, however, hardly agrees with that of the 'crystalline' 
 
 * Ernst Haeckel, ' Die Pcrigenesis der Plastidule,' Berlin, 1876. 
 
 t Louis Elsberg, ' Regeneration ; or, The Preservation of Organic 
 Molecules; a Contribution to the Doctrine of Evolution.' Proceed. Am. 
 ASSM. for the Advancement of Science, Hartford Meeting, Aug. 1874. 
 
 Carl Nageli, ' Mechanisch-physiologische Theorie der Abstammungs- 
 ehre,' Munchen u. Leipzig, 1884, p. 35.
 
 42 THE GERM-PLASM. 
 
 nature of the micella, for crystals are not ' mixtures,' but chemi- 
 cally pure substances. And apart from this, we should be wrong 
 in inferring from this passage that Nageli considers the vital 
 properties of a micella dependent on the co-operation of different 
 molecules united into a single group ; for in the passage quoted 
 above he also states that one molecule of albumen is sufficient 
 for the constitution of a micella. 
 
 For this reason alone it will be seen that the conceptions of the 
 biophor and of the micella do not coincide. They differ also as 
 regards the mode of multiplication : the fundamental importance 
 of this will become apparent later on. The biophors, as bearers 
 of vitality, possess the power of growth and of multiplication by 
 fission, just as is the case in all orders of vital units on which 
 direct observations have been made, beginning with the micro- 
 somata, which constitute the chromatin of the nucleus, and 
 passing through the chlorophyll granules, nuclei, and cells, up 
 to the simpler plants and animals. Nageli's micellae also mul- 
 tiply, but the multiplication occurs 'by the free interposition of 
 new micellae, similar to, or identical with, those already present,' 
 in the same manner as he supposed the addition of new particles 
 to take place in a starch grain, or as crystals separate from the 
 mother liquor. These new micella? would certainly have to be 
 formed by an influence, exerted by those already present, which 
 cannot be further defined. 
 
 The ' pangenes ' of de Vries correspond almost exactly to 
 my biophors, for they are also accredited with the functions of 
 growth and multiplication by division, and play a similar part 
 in heredity. The biophors, as will be explained in the following 
 pages, only differ from the pangenes in being constituents of 
 higher units of the hereditary substance. 
 
 The minute vital particles or ' plasomes,' recently assumed by 
 Wiesner, resemble both pangenes and biophors as regards their 
 properties. The part they take in heredity is, however, only 
 hinted at, and it is therefore better for me to use the special 
 term biophor than to press the plasomes into the service of my 
 theory of heredity. 
 
 The biophors play the same part with respect to heredity as 
 that which de Vries ascribes to his pangenes, t'.e., they are the 
 ' bearers of the qualities or ' characters' of the cells;' or more 
 accurately, the bearers of the cell-qtialities. As all living matter 
 consists of biophors, the differences in it can only depend on
 
 THE GERM-PLASM. 43 
 
 the differences in the biophors composing it ; an animal cell 
 containing, for example, transversely striped muscular sub- 
 stance, or delicate nervous or glandular structures, or again, a 
 vegetable cell enclosing chlorophyll bodies, must contain 
 several different kinds of biophors of which these various cell- 
 structures are composed, and which constitute the germ-plasm 
 of a species. 
 
 There must be a great number of different kinds of biophors, 
 for otherwise they could not give rise to so great a variety of 
 cells as exists in the organic world. Nor is it difficult to infer 
 the possibility of an almost unlimited number of different kinds 
 of biophors from their assumed composition. 
 
 As the biophors are not individual molecules, but groups of 
 molecules, nothing prevents us from tracing a large number 
 of variations in them to the widely varying number of their 
 molecules. But even the cJiemical constitution of the molecules is 
 not by any means necessarily the same in all cases, although the 
 possible fluctuations are certainly confined within certain limits. 
 
 Numerous facts show that at any rate in the two main divi- 
 sions of the organic world, the animal and vegetable kingdoms, 
 several of the molecules composing the biophors differ chemi- 
 cally from one another, so that substitutions occur. Whereas 
 glycogen is a constituent which is never absent from animal 
 protoplasm, provided that the latter possesses amoeboid move- 
 ment, this carbohydrate has not yet been discovered in plants, in 
 which, as Hoppe-Seyler suspects, it is probably replaced by 
 amylum, dextrine, or gum. Similarly, the crystalline proteids 
 in plants, which are known as aleurone grains, are chemically 
 different from the yolk-granules in animals. 
 
 A difference in the biophors can, moreover, be conceived 
 without a change in their atomic composition, by regarding as 
 possible a re-arrangement of the atoms in the individual mole- 
 cules. The molecule of albumen in particular has, according to 
 the conclusions of modern chemistry, a molecular weight of at 
 least 1,000, so that innumerable isomeric molecules of albumen 
 seem to be conceivable. It is, however, impossible to state 
 how many of them actually exist. 
 
 In order to give as complete an explanation as possible of the 
 phenomena of heredity with the aid of the biophors, the latter 
 must be invested with the capacity for a further change, 
 namely, a rearrangement of the molecules, analogous to the
 
 44 THE GERM-PLASM. 
 
 isomeric rearrangement of the atoms in a single molecule. 
 This assumption is not unfounded, inasmuch as several instances 
 of molecular compounds are known in chemistry, e.g., the double 
 salts and the water of crystallisation of salts, in which definite 
 numbers of molecules are always present : this number is even 
 retained in spite of substitution. Thus alum always contains 
 twenty-four molecules of water of crystallisation, and this evidently 
 indicates a degree of affinity between the molecules. We shall 
 have to assume this property for the biophor also, for without it 
 the latter would not be a real unit at all. We shall, moreover, 
 be able to conclude that these degrees of affinity are of various 
 kinds, and that the molecules can combine in many different 
 ways and form groups, so that isomeric molecular compounds 
 are formed. Such isomeric compounds, however, will possess 
 other properties, just as in the isomeric arrangement of atoms 
 in the individual molecule ; and thus we conclude that the 
 special properties of a biophor are to be considered dependent 
 not only on the physico-chemical constitution of the molecule, 
 but also, very essentially on their position and relation to one 
 another ; so that one biophor can be changed into another by 
 an alteration in the arrangement of its molecules. 
 
 According to this statement there are several kinds of 
 biophors, the difference between which depends on either the 
 absolute relative number of molecules, their chemical constitu- 
 tion (isomerism included), or their grouping ; in fact we may 
 say that the number of possible kinds of biophors is unlimited, 
 just as is the number of conceivable organic molecules. We 
 shall, at any rate, meet with no theoretical difficulties on this 
 score, however large the number of different kinds of biophors 
 may be which we require to explain the theory of heredity. 
 
 The biophors are not, I believe, by any means mere hypo- 
 thetical units ; they must exist, for the phenomena of life must 
 be connected with a material unit of some sort. But since the 
 primary vital forces assimilation and growth do not proceed 
 spontaneously from either atoms or molecules, there must be a 
 unit of a higher order from which these forces are developed, 
 and this can only consist of a group consisting of a combina- 
 tion of dissimilar molecules. I emphasise this particularly, 
 because a theory of heredity requires so many assumptions which 
 cannot be substantiated that the few fixed points on which we 
 can rely are doubly valuable.
 
 THE GERM-PLASM. 45 
 
 These biophors constitute all protoplasm the morphoplasm 
 which is differentiated into the cell-substance, as well as the 
 idioplasm contained in the nucleus. It will be shown sub- 
 sequently in what manner these two kinds of protoplasm differ 
 as regards their constitution, and I will only remark here that the 
 idioplasm must have a far more complex structure than the 
 morphoplasm. The latter, as the cell-substance of a muscle or 
 gland-cell shows, can assimilate, grow, and also divide, but it is 
 not able to change into anything different from itself. The 
 idioplasm, on the other hand, is capable of regular change 
 during growth ; and ontogeny, or the development of the indi- 
 vidual in multicellular organisms, depends upon this fact. The 
 two first embryonic cells of an animal arise from the division 
 of the ovum, and continually give rise to differently constituted 
 cells during the course of embryogeny. The diversity of these 
 cells must, as I have shown, depend on changes in the nuclear 
 substance. 
 
 It now remains to be considered how we are to imagine this 
 capacity on the part of the idioplasm for regular and spontane- 
 ous change. The fact in itself is beyond doubt, when once it is 
 established that the morphoplasm of each cell is controlled, 
 and its character decided, by the idioplasm of the nucleus. 
 The regular changes occurring in the egg-cell and the pro- 
 ducts of its division in each embryogeny must then be referred 
 to the correspond.ng changes of the idioplasm. Bui what 
 is the nature of these changes, and how are they brought about f 
 
 2. THE CONTROL OF THE CELL. 
 
 In order to answer the question which has just been asked, it 
 will be necessary to consider the manner in which the idioplasm 
 of the nucleus determines the characters of the cell. At 
 present we only know that the idioplasm consists of a large 
 number of different biophors of various kinds. To exert a 
 determining influence on the minute structure of the cell-body 
 and on the chemical composition of its different components, 
 it must either be capable of exerting an emitted influence 
 (' Fernwirkung ') or else material particles must pass out of the 
 nucleus into the cell-body. 
 
 Strasburger* has endeavoured to prove a dynamical effect of 
 
 * E. Strasburger, ' Neue Untersuchungen iiber den Befruchtungs- 
 vorgang bei den Phanerogamen,' 1884, p. in
 
 46 THE GERM-PLASM. 
 
 the nuclear matter. In his opinion 'molecular stimuli are 
 transmitted from the nucleus to the surrounding cytoplasm, and, 
 on the one hand, control the processes of metabolism in the 
 cell, and on the other, give a definite specific character to 
 the growth of the cytoplasm, this growth being caused 
 by nutrition.' Although transmission of the molecular stimuli, 
 proceeding from the nucleus to the rest of the cell, is cer- 
 tainly conceivable, de Vries has rightly shown that this is 
 not a sufficient explanation of the phenomena, because it takes 
 for granted the fundamental point of the matter requiring 
 explanation. If the cell of any plant is to acquire the hereditary 
 property of forming malic acid, those pangenes in the cell- 
 body which can produce this acid could, it is true, come into 
 play by molecular stimuli being transmitted to them from the 
 nucleus ; but this hypothesis takes their presence for granted, 
 and the main question as to how these producers of malic acid 
 get into the cell remains unanswered. 
 
 Haberlandt* has attempted to trace the control of the cell by 
 the nucleus to the enzymatic action of the latter, i.e., to the giving 
 off from the nucleus of certain chemical compounds which cause 
 the cell-substance to become changed in a given manner ; but 
 this explanation is regarded by de Vries as insufficient, because 
 here again it is necessary to presuppose a definite differentiation 
 of the cell body. 
 
 De Vries himself gives a solution of the problem, and his 
 hypothesis has, at any rate, the advantage of great simplicity 
 and lucidity. He supposes that some of the pangenes which 
 constitute the nuclear matter pass into the body of the cell 
 through the nuclear membrane, and there form its parts and 
 structures, of the qualities of which they are the special 
 bearers. 
 
 Although I formerly inclined towards Strasburger's view, it 
 always appeared to me rather as a formal than as a real ex- 
 planation of the problem, and I regarded it more as a 
 provisional formulation than as a solution of the difficulty. In 
 my opinion de Vries's idea of the migration of minute, specific, 
 vital particles from the nucleus into the cell-body affords an 
 extremely happy solution of the apparently inexplicable manner 
 
 * G. Haberlandt, ' Uber die Beziehungen zwischen Funktionen und 
 J,age des Zellkerns,' 1877.
 
 THE GERM-PLASM. 47 
 
 in which the cell is controlled by the nucleus. It, moreover, fits 
 in very well with my other views. 
 
 As long as I was engaged in seeking for an epigenetic theory 
 of heredity, an explanation of this sort was naturally impossible, 
 but as soon as I assumed that the germ-plasm consisted of 
 biophors, the various kinds of which are required for the various 
 characters of the respective cells, it was not only possible to 
 suppose that the particles exerted an influence of this nature on 
 the cell, but such an explanation of the phenomena became the 
 most natural and satisfactory one. Much may of course be 
 urged against this fundamental assumption, and it is not in 
 itself a sufficient explanation ; but it is not only fruitless to 
 attempt a satisfactory explanation from the other point of view, 
 but as will appear later on, de Vries's conception alone agrees 
 with certain fundamental biological principles. 
 
 If the nuclear substance exerted an emitted influence on the 
 cell body so as to give rise to the structures characteristic of this 
 particular kind of cell, they would be formed by a kind of 
 ' generatio equivoca' 1 ; they would have arisen by the operation 
 of an external influence on the given substance in the cell, just 
 as would be the case in primordial generation. Particularly 
 favourable influences would have operated on certain combina- 
 tions of inorganic substances in such a way as to give rise to a 
 vital particle. 
 
 We know nothing of such a primordial generation as far as 
 our experience extends, and even if it must be considered to be 
 logically necessary, we have every reason to suppose that it has 
 no share in the origin of those forms of life with which we are 
 acquainted, but that these always arise by division from others 
 similar to themselves. Moreover, what is true of the independent 
 organisms familiar to us must also hold good for all the different 
 orders of vital units which have united to form higher organisms, 
 for each "of the earliest and lowest organisms must have been 
 neither more nor less than the equivalent of one biophor. 
 If, then, in order to explain the presence of life on the 
 earth, we must assume that such individual biophors arose 
 at one time by primordial generation, they must have been 
 capable of reproduction by division immediately after their 
 origin, for such multiplication is caused directly by the 
 primary forces of life, assimilation and growth. We can only 
 imagine the very simplest biophors as having been produced
 
 48 THE GERM-PI.ASAf. 
 
 by primordial generation : all subsequent and more complex 
 kinds of biophors can only have arisen on the principle of 
 adaptation to new conditions of lifej they must have been 
 developed gradually by the long - continued co-operation of 
 heredity and selection. All these biophors of a higher order, 
 which are adapted to the special conditions of existence and 
 which in endless varieties form organisms as we see them around 
 us, possess ''historical* qualities ; they can, therefore, only arise 
 from others like themselves, and cannot be formed spontaneously. 
 This fact is confirmed by experience. Not only does a cell 
 always arise from a cell, and a nucleus from a nucleus, as de 
 Vries, and more recently Wiesner, have shown, but all the other 
 constituents which occur in the cell-body and determine its 
 structure never arise, so far as we know, by l generatio equi- 
 voca,' or, as de Vries expresses it, ' neogenetically.' They are 
 always produced by the division of similar structures already 
 present. This is apparently true of the green chromatophores 
 and the ' vacuoles ' of plant-cells, as well as of the ' sphere of 
 attraction,' or centrosome, which controls the division of the 
 nucleus : the same must also hold good for those invisible vital 
 units, the various kinds of biophors, which have arisen during the 
 course of the earth's history by gradual adaptation to continually 
 new conditions of life. 
 
 If then, each vital unit in all organisms, from the lowest to the 
 highest grade, can only arise by division from another like itself, 
 an answer is given to the question with which we started ; and 
 we see that the structures of a cell-body, which constitute the 
 specific character of the cell, cannot be produced by the emitted 
 influence of the nuclear substance, nor by its enzymatic action, 
 but can only arise owing to the migration of material particles 
 of the nucleus into the cell-body. Hence the nuclear matter 
 must be in a sense a storehouse for the various kinds of biophors 
 which enter into the cell-body and are destined to transform it. 
 Thus the development of the ' undifferentiated ' embryonic cell 
 into a nerve-, gland-, or muscle-cell, as the case may be, .s de- 
 termined in each case by the presence of the corresponding 
 biophors in the respective nuclei, and in due time these 
 biophors will pass out of the nuclei into the cell-bodies, and 
 transform them. 
 
 To me this reasoning is so convincing that any difficulties we 
 meet with in the process of determining the nature of the cell
 
 THE GERM-PLASM. 49 
 
 hardly come into account. We are still far from being able to 
 describe in detail the entire histological process of the differen- 
 tiation of a cell. The passage of invisible 'biophors' through 
 the pores of the nuclear membrane is probably just as admis- 
 sible an assumption as that of the independent power of motion 
 thereby necessitated in these bearers of vitality ; but the histo- 
 logical structure of a cell is nat completed by the mere emission 
 into the cell-body of a few kinds of biophors with great powers of 
 multiplication. Numerous questions suggest themselves in this 
 connection, all pointing to the fact that forces are at work of 
 which we are at present ignorant. The immigrating biophors 
 are the mere material which forms the histological structure of a 
 cell, only when subjected to the guiding forces presumably 
 those of attraction and repulsion which must be located in the 
 biophor. 
 
 We can as yet form no more exact conception of this process 
 than we can of the manner in which the biophors already con- 
 tained in the cell-body behave in respect to those which have 
 migrated into it from the nucleus. Presumably a struggle of 
 the parts occurs, in which the weaker are suppressed and serve 
 as nutritive material for the stronger ones. But although much 
 remains to be decided by future investigation, the main point at 
 issue, at any rate, viz., that the nature of the cell is really decided 
 by the elements of the nucleus, is definitely established. By the 
 nature of the cell must be understood not only the histological 
 structure of the cell as a whole and its mode of reacting to 
 external influences, but more particularly its mode of division in 
 respect of time and place. It is true that the c&ll-body itself and 
 its apparatus for division (the centrosome) primarily determine 
 whether a cell is to divide sooner or later, and into equal or 
 unequal parts ; but these processes always depend finally on the 
 nucleus, which controls the cell-body and impresses on the 
 latter its definite nature. 
 
 The most plausible objection which can be urged against the 
 migration of the particles of the idioplasm into the cell-body is 
 that the substance of the latter is chemically quite different from 
 that of the nucleus. Their behaviour as regards taking up 
 colouring matters is certainly different, as the terms chromosome 
 and chromatin indicate ; but even if a difference in their chemical 
 composition could be inferred from this fact, it would still fail to 
 constitute a decisive proof against the hypothesis of migration :
 
 50 THE GERM-PLASM. 
 
 for it is well known that the affinity of the chromosomes for 
 colouring matter varies markedly at different periods, and this 
 indicates that slight changes, which are beyond our control, 
 take place in the constitution of this substance, and are suffi- 
 cient to cause its most striking reaction with regard to colouring 
 matters to disappear for a time. Chemical analysis of the 
 substance contained in the nucleus has certainly established 
 the presence of ' nuclein ' ; but although it is probable from 
 Miescher"s * excellent observations on the sperm of the salmon 
 that nuclein is derived from the nuclei of the sperm-cells, it is 
 not by any means certain from what part of the nucleus it 
 originates : if one supposes that over 48 per cent, of the dried 
 sperm consists of nuclein, it is doubtful whether this is con- 
 tained in the small mass of chromatin which we see in the form 
 of chromosomes. 
 
 Another recent observation may be mentioned here, which 
 proves at any rate that matter is actually transferred from the 
 chromosomes of the nucleus into the cell-body just at the time 
 when the characteristic structure of the cell-body is being 
 formed. I refer to Riickert's observations on the remarkable 
 alteration in the size oj the chromosomes of the nucleus during 
 the growth of the ovum of the dog-fish.t One of the youngest 
 ova observed in the ovary which measured 2 mm. in dia- 
 meter contained from 30 to 36 chromosomes, each of which 
 was 12 microns \ long, and 2 cubic microns in bulk : later on, in 
 nearly ripe eggs, the length of a chromosome reaches 100 /*, 
 and its cubic contents 7,850 cubic /*, or more accurately, since 
 it has meanwhile become doubled Sy division, 15,700 cubic /*. 
 Still later, just before the formation of the first polar body, 
 when the ovum is ripe and has attained its full size, the length of 
 the individual chromosome diminishes to 2 /*, and the cubic 
 contents of a double rod to 3 cubic p. Riickert infers from 
 these facts that the chromosomes give off a great amount of 
 substance to the ovum during the gradual ripening of the latter 
 and we can only agree with him on this point. But the questio 
 arises as to how this transference of substance takes place, 
 
 * Miescher-Riisch, ' Statist, u. biolog. Beitrage zur Kentniss vom Leben 
 des Rheinsalm,' 1880; Schweiz. Literatursamml. z. internal on ilen 
 Fischereianstcll. in Berlin. 
 
 f J. Riickert, ' Anat. Anzeiger,' loth March, 1892. 
 
 t A micron (/A) is the yo^th of a millimetre,
 
 THE GERM-PLASM. 51 
 
 whether it occurs in the ordinary way, fluid nutrient material 
 being given off, and then assimilated by the cell-body, or in 
 some other manner. There seems to me to be no reason why 
 we should not assume that minute, specific, vital particles, and 
 not merely nutritive substances, are produced by the chromo- 
 somes during the growth of the egg, and are then emitted 
 through the nuclear membrane into the cell-body. Further 
 facts must be ascertained before we can attempt to explain the 
 details of the curious morphological transformations which the 
 chromosomes undergo during this period. We are already, 
 however, in a position to state that the extremely interesting 
 processes described by Riickert must have a wide significance, 
 and must occur in all cells which become histologically differen- 
 tiated as well as in all animal ova. But they cannot appear so 
 distinctly in these other cells, for no animal cell grows to such 
 an enormous size as does the egg-cell. I shall again refer to 
 the process in a later section, in order to emphasise one of the 
 consequences which results from it still more strongly. 
 
 Let us now suppose with de Vries that the nature of a cell 
 depends on the extrusion of minute vital particles of different 
 kinds from the nucleus into the cell-body, and that these subse- 
 quently multiply and become regularly distributed and arranged 
 in groups according to the forces of attraction and repulsion 
 situated within them. On this supposition, heredity could be 
 simply and easily accounted for in unicellular organisms, for in 
 them multiplication depends on a division of the whole body 
 and of the nucleus into two parts, and thus each product of the 
 division receives a similar supply of latent biophors which form 
 its nucleus, and from which it can then provide the necessary 
 material to the cell-body. 
 
 As the influence of amphimixis is not taken into account in 
 the present connection, I may here leave out of consideration 
 the fact that the nucleus may be differentiated into two different 
 kinds of nuclei. This arrangement is practically universal 
 amongst the highest unicellular forms the Infusoria and is 
 merely an adaptation for conjugation. In the unicellular forms 
 heredity will therefore depend, firstly, on the fact that all the 
 different kinds of biophors which are required for the construc- 
 tion of the body are present in the nucleus in a latent condition, 
 and in definite proportions very probably they have also a 
 definite style of architecture ; and secondly, on the period!-
 
 52 THE GERM-PLASM. 
 
 cal or occasional migration of these biophors into the cell-body, 
 where they multiply, and become arranged in obedience to the 
 forces acting within them. The difficulty of ascertaining the 
 actual mode of arrangement is nowhere greater than in the 
 case of the higher unicellular forms. How is it possible that the 
 nucleus should always allow only those kinds of biophors to 
 migrate which are required to replace those structures lost by 
 division ? And why do these biophors always move either in the 
 direction of the missing oral region, or towards the posterior end 
 of the body, according to which parts are wanting in the two 
 daughter-animals ? For the present these questions are un- 
 answerable ; and in the meantime we must be content with 
 having shown how the materials for the construction of the 
 cell-substance are transmitted from mother to daughter, and in 
 what way they are placed at the disposal of the forces acting in 
 the cell-body. 
 
 The experiments made by Nussbaum * and Gruber t on the 
 artificial division of Infusoria prove that the nucleus really 
 controls the cell-body. These observers found that only those 
 portions which contained a part of the nucleus were capable of 
 giving rise to a complete animal ; the other pieces lived for a 
 time, and then perished. One of ember's observations also 
 tends to show that when regeneration of missing parts occurs, 
 the nucleus sends out invisible material particles into the 
 cell-body. He cut a large Stentor which was preparing for 
 division transversely into two parts, so that the posterior por- 
 tion contained no trace of the nucleus, and then observed that 
 regeneration of the missing parts nevertheless took place, espe- 
 cially in the oral region. If the control of the cell depended on 
 the emitted influence of the nucleus, this regeneration would be 
 totally inexplicable ; if, however, biophors proceed from the 
 nucleus into the cell-body when regeneration is to take place, 
 this might have already occurred in an animal preparing for 
 division, as this one was before it was artificially divided. 
 
 The descendants of unicellular animals are similar to their 
 ancestors : two daughter-cells are produced by the division of 
 
 * Nussbaum, ' Ueber die Theilbarkeit der lebenden Materie," Archiv. 
 f. mikr. Anat., 1886. 
 
 t Gruber, ' Ueber kitnstliche Teilung bei Infusorien, 1 ' Biol. Central- 
 blatt," Bd. iv. ; and ' Beitrage zur Kentniss der Physiologic und Biologic 
 der Protozoen,' Ber. d. naturf. Gesellsch. zu Freiburg i/Br., 1886.
 
 THE GERM-PLASM. 53 
 
 the mother-cell, and thus the nuclear substance is always com- 
 posed of different kinds of biophors. But how does this apply 
 to multicellular forms in which so large a number of different 
 kinds of cells, each presupposing a different structure of the 
 nuclear matter, arises from the germ-plasm of the ovum ? Thus 
 we find ourselves brought back to the question asked at the 
 end of the last section : on what do the regular series of 
 changes in the germ-plasm during ontogeny depend ? 
 
 3. THE DETERMINANTS. 
 
 As has just been shown, the nuclear matter of an Infusorian 
 must be composed of a great number of different kinds of 
 biophors, each of which corresponds to the primary constituent 
 of a definite portion of the unicellular organism. If the cells of 
 a multicellular animal were represented in the germ-plasm by all 
 the kinds of biophors occurring in them, such an enormous 
 aggregation of biophors would result that, even if they were ex- 
 tremely small, the minute quantity of matter in the germ-plasm 
 would not be able to contain them. It was this consideration 
 more than any other which for many years made me persevere 
 in my attempt to discover an epigenetic theory of heredity. I 
 thought that it must be possible to imagine a germ-plasm which, 
 although highly complex, nevertheless did not consist of such an 
 inconceivably large number of separate particles, but which was 
 of such a structure as to become changed in a regular manner 
 during its growth in the course of ontogeny, and, finally, to yield 
 a large number of different kinds of idioplasm for the control 
 of the cells of the body in a specific manner. 
 
 Hatschek,* too, has recently put forward the view that ' the 
 egg-cell may be supposed to contain a relatively small number 
 of qualities,' and that this number is not larger than that which 
 is to be assumed in the case of any other histologically differen- 
 tiated cell of the body. The diversity in structure seen in 
 multicellular organisms is due, in his opinion, to the fact that 
 in spite of the limited diversity as regards the qualities con- 
 tained within a single cell (including the ovum), a far greater 
 complication of the body as a whole is attained by the variation 
 of these few qualities (' des einen Grundthemas')- 
 
 * B. Hatschek, ' Lehrbuch der Zoologie,' 2te Lieferung, Jena, 
 p. 232.
 
 54 THE GERM-PLASM. 
 
 If in considering a theory of heredity we had only to deal with 
 an explanation of the transmission of an unalterable structure 
 from the parent to the offspring from generation to generation, 
 there would be theoretically no objection to the assumption of 
 such a structure of the germ-plasm. We have, however, to deal 
 with the transmission of parts which are variable, and this 
 necessitates the assumption that just as many independent and 
 variable parts exist in the germ-plasm as are present in the fully 
 formed organism. It is impossible that a portion of tLe body 
 should exhibit an independent variation capable of transmission 
 unless it were represented in the germ-plasm by a special par- 
 ticle, a variation in which is followed by one in the part under 
 consideration. If this were represented, together with other 
 parts of the body, by one particle of the germ-plasm, a change 
 in the latter would be followed by a variation in all the parts of 
 the body determined by it. The independently and hereditarily 
 variable parts of the body therefore serve as an exact measure for 
 determining the number of ultimate particles of which the germ- 
 plasm is composed: the latter must contain at least as great a 
 number as would be arrived at by such a computation. 
 
 An example may make it clear that the independently variable 
 parts are not identical with those which are merely hereditary. 
 
 It is well known that butterflies pass through a metamor- 
 phosis in the course of development, the stages of which are 
 independently variable from the germ onwards : that is to say, 
 a variation in the caterpillar is not necessarily followed 
 by one in the butterfly, and vice versa. The caterpillars 
 of a species may be dimorphic, some being green, and 
 others brown, but both of these forms nevertheless give 
 rise to butterflies with a similar coloration. If, therefore, the 
 phyletic modifications depend on changes in the minute 
 structure of the germ-plasm, there must be at least two inde- 
 pendently variable units in the germ-plasm of such a butterfly ; 
 for if there were only one, the butterfly as well as the caterpillar 
 would be affected by a variation in it. But a comparison of 
 nearly related species shows us that the individual parts of the 
 caterpillar or butterfly must also be variable from the germ 
 onwards : the limbs, for instance, of two species may be very 
 similar, while their wings are different, and even the separate 
 parts of the wings may vary independently of one another. We 
 must therefore assume that the germ-plasm contains a large
 
 THE GERM-PLASM. 55 
 
 number of units, on the variation of which the independent 
 changes of certain parts of the body depend. 
 
 In all the higher animals the number of these units must be 
 very large, because the parts which are independently variable 
 from the germ onwards is large also. 
 
 A consideration of the individual and hereditary characters in 
 the human species will show most clearly how great this number 
 may be. I know of a family in which a depression of the size 
 of a pin's head in the skin in front of the left ear has been trans- 
 mitted through three generations. This slight abnormality 
 must therefore have been contained potentially in the germ- 
 plasm of the respective individuals, and their germ-plasm must 
 differ from that of other people in the slightly abnormal form of 
 the element which determines this peculiarity. We are logi- 
 cally compelled to assume a particular element of the germ- 
 plasm for each peculiarity of this sort, not because heredity may 
 be manifested in details so minute, but because the transmission 
 of such details may be independent. If all people possessed such 
 a depression in front of one ear, we could not thereby conclude 
 that it must be represented by a special element in the germ- 
 plasm merely because it is hereditary. It might conceivably be 
 represented, together with the skin of half the face, by one 
 element or biophor, which in the course of ontogeny became 
 divided into a number of secondary ones of divers sorts, one of 
 which proved to be abnormal and came to be situated at that 
 particular spot in the skin. What compels us to accept the 
 above assumption is the fact that all people do not possess this 
 depression, and that two persons might conceivably resemble 
 one another in all other respects except in the possession of this 
 abnormality. The germ-plasm of both these persons would be 
 almost identical, but not perfectly so, for it would contain a 
 certain element which differed in the two cases. This simply 
 means that this particular character which is independently 
 "variable from the germ onwards is also represented by a special 
 element in the germ-plasm. It -.vould not have been pos- 
 sible to infer this from its transmissibility alone. A hundred 
 different characters might conceivably be determined by a single 
 element in the germ-plasm ; the whole hundred would then be 
 transmitted as soon as the determining element was present in 
 the latter, but not one of them would be independently variable 
 from the germ onwards ; but if the determining element varied,
 
 56 THE GERM-PLASM. 
 
 all the hundred characters would vary at the same time. The 
 capacity for transmission and that of independent variation 
 from the germ onwards are distinct from one another. 
 
 The germ-plasm must consequently be composed of as many 
 units as there are transmissible parts in the body which are in- 
 dependently variable from the germ onwards. Each of these 
 units cannot be smaller than a biophor, and they can therefore 
 not be simple molecules within a biophor; for variation is a 
 biological conception, and a biological element does not pre- 
 suppose a one that is merely physical. 
 
 What parts of the body of a multicellular organism are 
 represented in the germ by special particles of the minimum 
 value of one biophor ? Is each cell, or even each part of a cell ? 
 Darwin adopted the former, and de Vries the latter of these 
 two alternatives. Darwin's gemmules are germs of cells, 
 so that every cell of the body would be represented in the 
 ovum by these units ; while de Vries's pangenes are in a sense 
 germs of the characters or structures ('Zellorganen') of 
 the cell. There is no doubt that the hereditary variations in 
 plants and animals manifest themselves in alterations of the 
 individual parts or structures of the cell, and not only in the 
 number, relative arrangement, and the changes in the form, 
 size, and nature of the cells as a whole. The variegated varieties 
 of our ornamental plants possess similar cells to those of their 
 ancestral forms, but the green colour of the leaf is absent in 
 certain of the cells : the red tint of the leaves of the coppei 
 beech, and other varieties of plants, depends on the red coloui 
 of the sap in a certain layer of cells, and this colour is trans- 
 missible. The coloured pattern on a butterfly's wing or a 
 bird's plumage depends on cellular elements which were pro- 
 bably all alike in remote ancestors, but which afterwards became 
 gradually changed by hereditary variations in the individual 
 components or in the structure of the cell. Although the entire 
 phyletic transformation of a species does not by any means alone 
 depend on its z'/rcz-cellular variation, the latter has, nevertheless, 
 constantly accompanied the other variations, and has shared to 
 a greater or less extent in the transformation of the species. 
 Hence it cannot be doubted that even in multicellular forms 
 not only the cells as a whole, but also their parts, are determined 
 from the germ onwards. 
 
 It seems therefore impossible to avoid the stupendous
 
 THE GERM-PLASM. 57 
 
 assumption that each of the millions of cells in a multicellular 
 organism is represented in the germ-plasm by several or many 
 different kinds of biophors. There is, however, a simple and 
 natural way out of this dilemma, as soon as we inquire whether 
 every cell of a plant or an animal is independently variable at 
 all, and whether consequently it must be represented by special 
 elements in the germ-plasm. 
 
 I shall designate the cells or groups of cells which are 
 independently variable from the germ onwards as the ' hereditary 
 parts' or ' determinates? and the particles of the germ-plasm corre- 
 sponding to and determining them, as the 'determining parts' 
 or ' determinants.' It is evident that many of the cells in the 
 higher animals are not represented individually in the germ- 
 plasm by a determinant. The millions of blood-corpuscles 
 which are formed during the life of a Vertebrate might possibly 
 be controlled in the germ-plasm by a single determinant. At any 
 rate no disadvantage to the species would result from this, because 
 the capacity for being independently determined on the part of 
 the individual blood-corpuscles, or even individual thousands of 
 them, would be of no value to the animal. They are not 
 localised : one of them has the same value as another, and their 
 variability therefore might well be controlled from a single 
 point. In conformity .with the law of economy, Nature would 
 not have incorporated more determinants than was necessary 
 into the germ-plasm. 
 
 Thus there are probably many groups of cells in the higher 
 animals, the constituents of which are not represented in- 
 dividually in the germ-plasm. All the nerve-cells of the brain 
 do, it is true, possess their special determinants, as otherwise 
 the transmission of such fine shades of mental qualities in man 
 would be inexplicable ; but it can matter little whether each 
 fibre of a muscle, or each cell of the epidermis or of the 
 epithelial lining of the alimentary canal, has its special 
 determinant : in the last-mentioned cases larger or smaller 
 groups of cells are presumably controlled by a single determinant. 
 The manner in which the epithelium of the alimentary canal is 
 renewed amongst insects may perhaps be taken as pointing to 
 this assumption. In flies and butterflies, for instance, as I 
 have proved long ago, the alimentary canal of the larva under- 
 goes disintegration, and that of the imago, which has a very 
 different structure, is developed out of its remains. Kowalewski
 
 58 THE GERM-PLASM. 
 
 and van Rees have since shown that the process takes place 
 as follows : the formation of portions of the new alimentary 
 canal begins in certain cells which are separated by fairly 
 regular intervals; these then spread until they come into 
 contact with one another. The idioplasm of the new in- 
 testinal cells is consequently only contained in these formative 
 cells, and it is natural to suppose that each of them contains 
 only one kind of determinant. 
 
 The same appears to be the case with the hair of mammals. 
 Every hair does not possess a special determinant in the germ, 
 but more or less extensive regions of the hairy covering are 
 represented each by one determinant. These regions are not 
 large, as is shown by the stripes and spots on the coat of such 
 animals as the tiger and leopard. The recurrence in the son, 
 on exactly the same part of the head as in the parent, of an 
 abnormal tuft of white hair, has been observed in the human 
 subject. 
 
 Similar hereditary parts or determinates may be observed in 
 butterflies, in which the colours on the wings often form very 
 complicated lines and spots of slight extent but of great 
 constancy. Such regions are often limited to quite a few 
 scales (cells) : Lycana argus, for instance, possesses a black 
 spot on a particular part of the anterior wing consisting of 
 only ten scales, while the surrounding parts are blue. In this 
 case we may therefore conclude that the black cells are 
 represented in the germ-plasm by at least one determinant. 
 The determination may possibly be carried out in still further 
 detail in this instance, and each cell in the black spot may be 
 determined from the germ onwards ; and possibly it is only 
 the constant intermingling of two hereditary tendencies in sexual 
 reproduction, and the consequent variability in the number of 
 scales, which prevents us from recognising the fact. We can 
 at any rate, however, find instances of the determination of 
 single cells in other species of animals. For example, in many 
 Crustaceans a number of sensory organs are situated on the 
 anterior antennas : each of these corresponds to one cell. The 
 number, position, and form of these ' olfactory ' setas is deter- 
 mined exactly for each species. The Ostracod Cypris possesses 
 only one olfactory seta on each antennule, while in the common 
 fresh-water species of Gammarus, there are about twenty 
 of these structures, each of which is separately attached to one
 
 THE GERM-PLASM. 59 
 
 of the consecutive joints of the feeler. In many blind Crusta- 
 ceans which live in the dark, the number of these setae is 
 greater than in the case of related forms which possess the 
 sense of sight. And though in all these instances individual 
 deviations occur, we may nevertheless suppose them to be here- 
 ditary, for otherwise the increase in the number of olfactory 
 setae incident on a life in darkness, could not have been 
 established as a specific character. 
 
 In smaller and simpler organisms each individual cell may 
 well have been determined from the germ onwards, and not 
 merely with the result that the number of cells is a definite one, 
 and the position of each definitely localised : the determination 
 may also have caused individual peculiarities of each cell, in so 
 far as they depend on changes in the germ-plasm at all i.e., are 
 ' blastogenic,' to reappear in the corresponding cell in the next 
 generation, just as in the case of a birthmark in the human sub- 
 ject which recurs in precisely the same place on the same side 
 of the body. This may also be true of animals as simple as the 
 Dicyemidce or the Tardigrada, although it is not possible to 
 prove it positively. 
 
 In all the more highly differentiated animals there can be 
 little doubt that the number of determinants is always very much 
 less than that of the cells which are the factors in the process 
 of ontogeny. If we compare this statement with Darwin s 
 assumption of the presence of a gemmule or rather of several 
 gemmules for each cell, it is evident that the genii-plasm is 
 thus to some extent relieved of a burden. 
 
 We must not forget, however, that a cell may vary as regards 
 transmission not only as a whole but also in its parts, so that 
 not one but several biophors must be assumed for each deter- 
 minant of a cell or group of cells ; we must, in fact, suppose just 
 as many to be present as there are structures in the cell which 
 are variable from the germ onwards. We ought, properly 
 speaking, to speak of these bearers of qualities, which corre- 
 spond to de Vries's pangenes, as determinants also, for they 
 letermine the parts of a cell. As the name of biophor has 
 been given to them, however, it is better to retain this term, 
 and to define a determinant as a primary constituent of a 
 cell or group of cells. Thus a determinant is always a group of 
 biophors, and never a single one. 
 
 It may now, I believe, be proved without difficulty that the
 
 60 THE GERM-PLASM. 
 
 biophors determining a cell not only lie close together in the 
 germ-plasm so as to form a group, but that they also combine to 
 form a higher unit. The determinant is not a disconnected mass 
 of different biophors, but a -vital unit of a higher order than the 
 biophor, possessed of special qualities. 
 
 The fact that the determinants must possess the power of 
 multiplication is in itself a sufficient proof of this. We know 
 how greatly the nuclear matter contained in the fertilised egg- 
 cell increases in volume during development, and this can only 
 be due to the multiplication of its vital particles, the biophors. 
 Such a multiplication could never occur with as much precision 
 and regularity as is necessary for the preservation of the 
 character of a certain cell, if the biophors which determine it 
 were scattered at random instead of being definitely separated 
 from those of other cells. Hence the multiplication of the 
 biophors must occur within the fixed limits of the determinant, 
 and must be preliminary to the division of the determinant 
 itself. And consequently the latter is also a vital unit. 
 
 In accordance with our assumption, which can scarcely be 
 refuted, a single determinant of the germ-plasm frequently con- 
 trols entire groups of cells : this is a further proof that the 
 determinants as such must multiply. This is only possible if 
 they do so in the process of ontogeny. It is very probable, 
 moreover, that the nucleoplasm of any cell in the body never 
 contains one specimen only of the determinant controlling it, 
 but several ; otherwise, how could such a cell be visible at all 
 under our microscopes ? Biophors, at any rate, are far beyond 
 the limit of vision, and even determinants can hardly come 
 within it. 
 
 Thus the assumption made by the gifted propounder of 
 the theory of pangenesis is so far justified. ' Gemmules ' of 
 cells really exist, and multiply by fission ; but they are not the 
 ultimate vital units, nor are special gemmules of all the cells of 
 the body already present in the germ-plasm. 
 
 We have next to deal with the question as to how these two 
 elements of the germ-plasm, which have now been formulated, 
 are instrumental in the process of ontogeny. 
 
 4. THE ID IN ONTOGENY. 
 
 We can now make an attempt to solve the problem stated at 
 the close of the last section concerning the way in which the
 
 THE GERM-PLASM. 6 1 
 
 germ-plasm is capable of giving rise to the various kinds of 
 idioplasm required in the construction of the organism. 
 
 As we have seen, the germ-plasm contains the primary con- 
 stituents of all the cells in the body in its determinants, and it 
 only remains to inquire how each kind of determinant reaches 
 the right part in the right number. Although we do not know 
 what forces are called into play for this purpose, the elements of 
 the germ-plasm now formulated, and the processes and course 
 of ontogeny, nevertheless enable us to draw certain conclusions 
 as to the structure of the germ-plasm and the nature of the 
 changes it undergoes ; and I trust that these conclusions will 
 not lead us too far from the truth. 
 
 We can, in the first place, state with certainty that the 
 germ-plasm possesses a fixed architecture, which has been trans- 
 mitted historically. In working out the idea of determinates, it 
 was stated that probably not nearly all the cells of the higher 
 organisms are represented in the germ-plasm by special deter- 
 minants : possibly all the blood-corpuscles, or the thousands of 
 fibres in a particular muscle, for instance, are represented each 
 by one determinant. But it does not therefore follow that all 
 the cells of a similar kind which exist in the body can be repre- 
 sented by one common determinant : this would be equivalent 
 to abandoning the conception of determinants altogether. If, 
 for instance, all the transversely striped muscles of a Vertebrate 
 were represented in the germ-plasm by a single determinant, 
 each variation in the latter would also produce a corresponding 
 change in all the muscles, and the independent variation of 
 which each individual muscle is actually capable would then be 
 impossible. 
 
 Several, or even many, similar determinants must therefore 
 exist in the germ-plasm of tin animal. Muscle-cells and nerve- 
 cells are repeatedly formed even in the fully developed organism, 
 and, in so far as they can vary individually at all from the germ 
 onwards, will be represented by identical or by very similar 
 determinants in the germ-plasm. 
 
 If such identical determinants represent a single fixed cell or 
 group of cells, they cannot be situated anywhere in the germ- 
 plasm, nor can they change their position according to varying 
 influences : the determinants must be definitely localised, for 
 otherwise they would not be certain to reach the right cell and 
 the right position in the course of ontogeny. I have already
 
 62 THE GERM-PLASM. 
 
 mentioned the olfactory setae of Gammarus, which are situated 
 individually on particular segments of the feeler. Each of these 
 can vary hereditarily, and thus it is necessary to assume special 
 determinants for them in the germ-plasm ; these, however, will 
 all be similar to one another. This is also true of the black 
 spots on the wings of certain butterflies, already referred to. In 
 Lyccena argus, for instance, there is a spot on that part of the 
 wing which is known to entomologists as ' cell I b,' and this 
 spot is independently variable : it may be larger or smaller, and 
 the variations in it can be transmitted quite independently of 
 the numerous other black marks on the wing. The particular 
 spot referred to may have disappeared entirely in another 
 species of Lyccena, while a precisely similar spot in ' cell 4 ' has 
 become much larger. We have also decided indications that 
 homologous parts in the two halves of the body in bilaterally 
 symmetrical animals can vary independently of one another. 
 The human birthmark mentioned above was always inherited on 
 the left side, and never on the right. 
 
 If each determinant occupies a fixed position in the germ- 
 plasm, it cannot have an indefinite or -variable size and form, 
 but must form a complete unit by itself, from which nothing can 
 be removed, and to which nothing can be added. In other 
 words, we are led to the assumption of groups of determinants, 
 each of which represents a separate vital unit of the third degree, 
 since it is composed of determinants, which in their turn are 
 made up of biophors. These are the units which I formulated 
 on different lines long ago, and to which the name of ancestral 
 germ-plasms was then given. I shall now speak of them as 
 ' ids,' * a term which recalls the ' idioplasm ' of Nageli. 
 
 I assume that just as the individual biophor has other quali- 
 ties than those of the determinant, which is composed of 
 biophors, so also does the id possess qualities differing from 
 those of its component determinants. The fundamental vital 
 properties growth and multiplication by division must how- 
 ever be attributed to the id as to all vital units. Several reasons, 
 more especially those furnished by the phenomena of heredity 
 
 * I have already used this term in my essay on ' Amphimixis ' (' Amphi- 
 mixis, oder die Vermischung der Individuen,' Jena, 1891, p. 39). In my 
 earlier essays the ids were spoken of as 'ancestral germ-plasms,' the mean- 
 ing and derivation of which term will be explained in the chapter on 
 amphigonic heredity.
 
 THE GERM- PL ASM. 63 
 
 in sexual reproduction, lead us to assume that the germ-plasm 
 does not consist of a single id, but of several, or even many of 
 them, and this assumption must be made even in the case of 
 asexual reproduction. 
 
 I shall therefore assume that each idioplasm is composed of 
 several or many ids, which are capable of growth and multipli- 
 cation by division. If animals existed, in the whole series of 
 ancestors of which sexual reproduction had never occurred, these 
 ids would be exactly similar to one another. But in all cases 
 every id of the germ-plasm contains the whole of the elements 
 which are necessary for the development of all subsequent 
 idic stages. Theoretically, therefore, one id would suffice for 
 ontogeny. 
 
 We assume that the changes in the id of germ-plasm during 
 ontogeny consist merely in a regular disintegration of the 
 determinants into smaller and smaller groups, until finally only 
 one kind of determinant is contained in the cell, viz., that which 
 has to determine it. It is highly improbable that all the deter- 
 minants in the id of germ-plasm are carried along through all 
 the idic stages of the ontogeny. In discussing regeneration and 
 gemmation later on, I shall have to show that, under certain cir- 
 cumstances, groups of determinants are supplied to certain series 
 of cells, and that these are not actually required for determining 
 the cells ; this arrangement, however, depends, I believe, on 
 special adaptations, and is not primitive, at any rate not in the 
 higher animals and plants. Why should Nature, who always 
 manages with economy, indulge in the luxury of providing all 
 the cells of the body with the whole of the determinants of the 
 germ-plasm if a single kind of them is sufficient ? Such an 
 arrangement will presumably only have occurred in cases in 
 which it serves definite purposes. The enormous number of 
 determinants contained in the germ-plasm also stands in the 
 way of such an assumption, for in the higher animals they can 
 be reckoned by hundreds of thousands at the very least ; and 
 although we may assume that they all remain in a latent condi- 
 tion in every cell, and so need not interfere with the activity of 
 the determinants which control the cell, they nevertheless 
 deprive the active determinants which we must also suppose 
 to exist in large numbers of a considerable space. 
 
 If we wished to assume that the whole of the determinants of 
 the germ-plasm are supplied to all the cells of the ontogeny, we
 
 64 THE GERM-PLASM. 
 
 should have to suppose that differentiation of the body is due to 
 all the determinants except one particular one remaining 
 dormant in a regular order, and that, apart from special adapta- 
 tions, only one determinant reaches the cell, viz., that which has 
 to control it. This latter supposition is undoubtedly less likely 
 than the former. 
 
 If however we do make this assumption, the question then 
 arises as to what factors can cause the gradual disintegration of 
 the id of germ-plasm into smaller and smaller groups of deter- 
 minants, that is to say, into ids which contain fewer and fewer 
 kinds of determinants. 
 
 This disintegration I believe to be due to the co-operation of 
 three factors : these are the inherited architecture of the germ- 
 plasm, in which each determinant has its definite position ; 
 the unequally -vigorous multiplication of the various deter- 
 minants ; and possibly also, the forces of attraction which are 
 situated within each determinant, and result from its specific 
 nature as a special and independent vital unit. The architecture 
 of the germ-plasm has already been discussed in general terms : 
 for the present, at any rate, we can hardly conjecture the actual 
 details of its structure. In order to do so, it would be necessary 
 to suppose that hundreds of thousands, or millions, of deter- 
 minants, which are all definitely localised, take part in the forma- 
 tion of the higher organisms. The fact that the right and left 
 halves of the body can vary independently in bilaterally sym- 
 metrical animals, points to the conclusion that all the deter- 
 minants are present in pairs in the germ-plasm. As, moreover, 
 in many of these animals, e.g. the frog, the division of the ovum 
 into the two first embryonic cells indicates a separation of the 
 body into right and left halves, it follows that the id of germ-plasm 
 itself possesses a bilateral structure, and that it also divides so 
 as to give rise to the determinants of the right and left halves of 
 the body. This illustration may be taken as a further proof of 
 our view of the constant architecture of the germ-plasm. An id 
 is evidently not constituted like the sediment of a complicated 
 and well-shaken mixture, in which the heavier particles come to 
 lie at the bottom and the lighter ones at the top ; nor is it con- 
 stituted in such a manner that the respective positions of the 
 particles are only determined independently by the forces acting 
 on them and between them momentarily. Its structure may be 
 compared to that of a complicated ancient building, the stones
 
 THE GERM-PLASM. 65 
 
 of which we may suppose to be alive, so that they can grow 
 and increase, and thus cause displacements and fissures in the 
 walls, in which process the forces of attraction present within 
 these living stones take part. The historical transmission of the 
 architecture of the germ-plasm forms the basis of the entire 
 ontogenetic development of the idioplasm. 
 
 If however the id has a right and left half in bilateral animals, 
 we must not thereby infer that it is merely a miniature of the 
 fully -formed animal, and that therefore we are once more deal- 
 ing with the old theory of preformation. Quite apart from all 
 conjectures as to the detailed architecture of the id of germ- 
 plasm, it is at any rate certain that the arrangement of the 
 determinants in it is quite different from that of the correspond- 
 ing parts in the fully-formed organism. This is proved by a study 
 of development, and need scarcely be treated of in detail here. 
 Any one with a knowledge of animal embryology knows how 
 great a difference there is between the mode of development of the 
 parts from one another in the embryo and their respective relation 
 in the mature organism. The early stages of segmentation of 
 the ovum show that groups of determinants have been formed in 
 the id of germ-plasm, and that these, moreover, correspond to 
 the parts of the body which arise from one another consecutively, 
 though they can have no resemblance to them either in form or 
 in their degree of perfection. 
 
 In some worms the two first blastomeres do not give rise 
 respectively to the right and left sides of the body, but to the 
 entire ectoderm and endoderm. In these cases the id of germ- 
 plasm must break up into two groups, one of which contains all 
 the determinants of the ectodermal organs, and the other all 
 those of the endoderm : it is evident that this arrangement has 
 no analogy to that which obtains as regards the organs of the 
 fully-formed animal. If in any species we knew the ' value 
 in primary constituents ' (' Anlagenwerth ') if I may use such a 
 term of each cell in the ontogeny, we could give an approximate 
 representation of the architecture of the germ-plasm ; for, begin- 
 ning with the last formed cells, we could infer the nature of the 
 determinants which must have been contained in each previous 
 mother-cell, passing gradually backwards to the ovum ; thus we 
 should reach the two first blastomeres, and finally the egg-cell 
 itself. The groups of determinants which are present at each 
 stage would thus be known, and we might in imagination then 
 E
 
 66 THE GERM-PLASM. 
 
 arrange them in such a way that it would be possible to picture 
 their disintegration into the respective series of smaller and 
 smaller groups. 
 
 Such a representation of the architecture of the id of germ- 
 plasm would, however, never be an accurate one, because its 
 parts must be subjected to incessant slow displacement during 
 the growth of the idioplasm and in the course of development. 
 
 This brings us to the second factor which takes part in the 
 ontogeny of the idioplasm, viz., the uneven rate of multiplication 
 of the determinants. An id of germ-plasm composed entirely 
 of similar determinants, would have to retain its original archi- 
 tecture even during vigorous growth and continued division ; 
 just as would be the case in one of the lowest forms of life a 
 Moner consisting of a number of identical biophors, which must 
 remain the same throughout all the divisions which it undergoes. 
 In a germ-plasm consisting of a number of different determin- 
 ants, a perfectly even rate of multiplication cannot be assumed in 
 the case of all of them. For the difference between two deter- 
 minants depends presumably on the differences in the nature, 
 number, or arrangement of their constituent biophors, and the 
 latter differ again in their molecular structure, i.e. in their essen- 
 tial physico-chemical properties. Hence the determinants will 
 behave differently as regards their reaction to external influences, 
 more especially in respect of their rate of growth and increase, 
 according to their constitution. The same conditions of nu- 
 trition will therefore stimulate one to a faster, and another to a 
 slower, growth and corresponding multiplication, and thus an 
 alteration in the proportional numbers in which the individual 
 kinds of determinants are present in the germ-plasm must occur 
 continually in the course of embryogeny ; for the latter is con- 
 nected with a constant growth of the idioplasm, and therefore 
 also with a continual increase of the determinants. This 
 must cause a disarrangement in the architecture of the germ- 
 plasm, in which process the third factor concerned in these 
 changes, viz., the forces of attraction in the determinants, may 
 take part. 
 
 The assumption of such forces can scarcely be avoided. For 
 it is very probable, a priori, that vital units do act upon one 
 another in different degrees, and this view is supported by a con- 
 sideration of the processes of nuclear division, together with the 
 distribution of the primary constituents in ontogeny.
 
 THE GERM-PLASM. 67 
 
 So far I have not touched upon the question as to what 
 observable parts of the idioplasm are to be regarded as ids. 
 This point cannot be decided with certainty at present, but I 
 have elsewhere expressed the opinion that those rod-like, loop- 
 like, or granular masses of chromatin in the nucleus, the 
 chromosomes, are to be considered equivalent, not to single ids, 
 but to series or aggregations of ids. I have therefore proposed 
 to call the chromosomes idants* in order to keep up a certain 
 uniformity in the nomenclature. It is probable that the ids 
 correspond to the small granules hitherto called ' microsomata,' 
 which are known to form the individual idants in many animals : 
 \ve may mention as an example, Ascaris megalocephala^ as in it 
 the nuclear structure is best known. These 
 microsomata, although lying very close 
 together in one row, are nevertheless separ- 
 ated by a thin layer of intermediate sub- 
 stance ; the whole idant cannot therefore be 
 equivalent to one id, for the latter is a clearly 
 defined vital unit possessing a fixed archi- 
 tecture, and cannot consist of completely 
 separated parts. 
 
 The great variety as regards size, number, Two idants with their 
 
 ... * _ . , ,-rr contained Ids of As- 
 
 and form of the chromosomes in different carh megahcephala. 
 species of animals, indicates that they (After Boveri -' 
 possibly have not always a similar morphological value. As 
 however there is no reason for assuming that the number of ids 
 must always be the same in all species, and as, on the contrary, 
 it is much more probable that their number varies greatly, it is 
 impossible to make use of the above fact as a decisive argument. 
 We can only state that the individual chromosome or idant in 
 all probability represents a different number of ids in different 
 species. 
 
 Division of the nucleus depends on the longitudinal splitting 
 of the idants, in which process each of the spherical ids assum- 
 ing these to correspond to the microsomata becomes halved. 
 Each half then becomes rounded off, and passes, together with 
 the idant to which it belongs, into one of the two daughter- 
 nuclei. 
 
 In the ordinary process of cell-division in tissues, which 
 
 * ' Amphimixis,' pp. 39, 40.
 
 68 THE GERM-PLASM. 
 
 results in the formation of daughter-cells similar to those from 
 which they arose, the ids produced by the division naturally 
 contain precisely similar determinants ; in embryogeny, on the 
 other hand, divisions occur which ensure that the two daughter- 
 nuclei contain combinations of determinants which are usually 
 entirely different from one another. We have an example of 
 such a nuclear division in the segmentation of the ovum in the 
 case, for instance, of certain worms already referred to, in which 
 two cells are formed by the first division of the egg-cell, one of 
 which contains all the determinants of the internal, and the 
 other all those of the external germinal layer. A division of this 
 latter kind we may speak of as differential or dissimilar as re- 
 gards heredity ('erbungleich'), in contrast to the former, which is 
 integral or similar as regards heredity (' erbgleich'). As in the 
 case of the entire idants, the ids are split by an internal force, 
 and are not pulled apart mechanically by the threads of the 
 ' nuclear spindle ' which are attached to them. Flemming has 
 shown that this splitting often takes place long before the spindle- 
 threads become active. The forces of attraction in the deter- 
 minants must therefore take part in this process, just as they 
 must be assumed to act between the biophors which constitute 
 the body of a dividing cell. 
 
 It appears to me, therefore, that the inherited architecture of 
 the id of germ-plasm undergoes a gradual change, owing to the 
 uneven rate of multiplication of the determinants, and that it is 
 further regulated by the forces of attraction which we must 
 suppose to act between them. We might represent the archi- 
 tecture of the id by a very complicated geometrical figure, which 
 gradually becomes changed during the growth of the id ; this 
 change does not occur in the first division, the preparation for 
 which has been accurately made in the original figure, but in 
 the subsequent stages of ontogeny. As the greater number of 
 these divisions is connected with a diminution in the number of 
 kinds of determinants, the geometrical figure representing the id 
 gradually becomes simpler and simpler, until finally it assumes 
 the simplest conceivable form, and then each cell will contain 
 the single kind of determinant which controls it. The dis- 
 integration of the germ-plasm is a wonderfully complicated 
 process ; it is a true ' development,' in which the idic stages 
 necessarily follow one another in a regular order, and thus the 
 thousands and hundreds of thousands of hereditary parts are
 
 THE GERM-PLASM. 69 
 
 gradually formed, each in its right place, and each provided with 
 the proper determinants. 
 
 The construction of the whole body, as well as its differentia- 
 tion into parts, its segmentation, and the formation of its organs, 
 and even the size of these organs, determined by the number 
 of cells composing them, depends on this complicated disin* 
 tegration of the determinants in the id of germ-plasm. The 
 transmission of characters of the most general kind that is to 
 say, those which determine the structure of an animal as well as 
 those characterising the class, order, family, and genus to which 
 it belongs are due exclusively to this process. The slight 
 differences only, namely those which distinguish species from 
 species, and individual from individual, depend partly on the 
 characters of the individual cells. De Vries has overlooked 
 this in his attempt to explain all the facts of heredity by the 
 theory of ' infra-cellular ' pangenesis. As was mentioned in 
 the ' Historical Introduction ' to this book, it must be borne in 
 mind that most of the ' characters ' of any of the higher forms 
 of life result not from the characters of the individual cells, 
 but from the way in which they are combined. On the other 
 hand, the construction of a living organism is not conceivable 
 unless we presuppose the determination of the characters of 
 each cell. 
 
 We have therefore to give an explanation of this concluding 
 part of the process of ontogeny : this has already been done to 
 a great extent above, in the section treating of the control of the 
 cell by the idioplasm. I there assumed, as de Vries has also done, 
 that this determination depends on the migration of minute vital 
 particles from the nucleus into the cell-body. We have now seen 
 by what means the biophors characteristic of any particular cell 
 reach that cell in the requisite proportion. This results from 
 the fact that the biophors are held together in a determinant 
 which previously existed as such in the germ-plasm, and which 
 was passed on mechanically, owing to its ontogenetic disintegra- 
 tion, to the right part of the body. In order that the deter- 
 minant may really control the cell, it is necessary that it should 
 break up into its constituent biophors. This is an inevitable 
 consequence of the assumed mode of determination of the cell. 
 We must suppose that the determinants gradually break up into 
 biophors when they have reached their destination. This 
 assumption allows, at the same time, an explanation of the
 
 70 THE GERM-PLASM. 
 
 otherwise enigmatical circumstance, that the rest of the de- 
 terminants, which are contained in every id except in the last 
 stages of development, exert no influence on the cell. As each 
 determinant consists of many biophors, it must be considerably 
 larger than a biophor, and is probably therefore unable to pass 
 out through the pores of the nuclear membrane, which we must 
 suppose to be very small and only adapted for the passage of 
 the biophors. Although it is impossible to make any definite 
 statement with regard to the internal structure of the deter- 
 minants, it must be owing to this structure that each determinant 
 only breaks up into biophors when it reaches the cell to be 
 determined by it. We may suppose that, just as one fruit on a 
 tree ripens more quickly than another, even when the same 
 external influences act on both, so also one sort of determinant 
 may mature sooner than another, although similar nourishment 
 is supplied to both. 
 
 It must not, however, be overlooked, that a difference in the 
 time of maturation of the determinants in the embryogeny of 
 animals is chiefly to be assumed only in the case of the actual 
 embryonic cells ; for the histological differentiation of the cells 
 of the body, and the differentiation of the parts of the latter, 
 occur at about the same time ; that is, not until the organs 
 already exist as definite groups of cells. This is equivalent to 
 saying that the disintegration into biophors occurs when the id 
 only contains the single determinant which controls that par- 
 ticular kind of cell. It is well known how suddenly the histolo- 
 gical differentiation of the cells occurs in the embryogeny of an 
 animal. For a long time the various parts and tissues are very 
 similar to one another, though not perfectly so, and then histo- 
 logical differentiation suddenly sets in. This is very markedly 
 the case as regards the transversely striped muscles of Arthropods 
 and Vertebrates, in which the contractile substance is first seen 
 as a mere narrow ring around the cell, and then gradually be- 
 comes thicker, so as to replace the greater portion of the cell- 
 body, just as one would expect if it were caused by muscle- 
 biophors which had migrated into the cell-substance and there 
 multiplied. 
 
 The assumption of a 'ripening' of the determinants, which 
 though not simultaneous, is yet exactly regulated, nevertheless 
 remains indispensable ; or, to express it differently, we must 
 assume that the determinants pass through a strictly regulated
 
 THE GERM-PLASM. 71 
 
 period of inactivity, at the close of which the disintegration into 
 biophors sets in. The determinants certainly continue to grow 
 and multiply without interruption during this period, as may be 
 deduced from the fact that the amount of the nuclear substance 
 in the individual cell does not decrease during embryogeny, 
 although such an enormous increase in the number of cells 
 takes place. No accurate and methodical observations have at 
 present been made with regard to the comparative size of the 
 chromosomes in the various stages of development and in the 
 different organs of the body, but it may nevertheless be taken as 
 certain that the entire mass of the nuclear substance grows 
 considerably during embryonic development. It appears to me, 
 however, to follow from the observations of Riickert I have 
 already referred to concerning the chromosomes of the ovum 
 of the dog-fish,* that the most marked growth of the deter- 
 minants takes place immediately before, and during, their activity. 
 During the period of growth and histological differentiation of 
 the egg in this fish the idants grow enormously, and towards 
 the completion of these processes they gradually decrease in 
 size, until finally, when the ovum is ripe, they have become 
 almost as small as they were originally. 
 
 This may be expressed, in the terms we have adopted, as 
 follows : the determinants which control the histological struc- 
 ture of the egg \ 'multiply enormously during the growth of the 
 ovum, into the body of which they transmit their numerous 
 biophors. After this has occurred, only those determinants of 
 the germ-plasm are left which have in the meantime been in- 
 active, and which have only increased to a slight extent ; these 
 are thus contained in those idants which are not much larger 
 than they were in the young egg-cell. From the beginning of 
 ontogeny and onwards, one determinant after another becomes 
 active, and during their activity they also multiply. It has for 
 a long time appeared to me probable that the determination of 
 
 * Anat. Anzeiger. loth March 1892. 
 
 t These determinants of the ovum correspond to the 'oogenetic nucleo- 
 plasm ' of my earlier essays, and constitute the substance which deter- 
 mines the growth and histological differentiation of the egg. For a long 
 time I believed that this substance was extruded from the ovum at the 
 close of the period of maturation by means of the polar bodies. We now 
 see that such an extrusion is not required, as this substance is used up in 
 the differentiation oi' the egg.
 
 72 THE GERM-PLASM. 
 
 a cell does not take place, as one might suppose, by the agency 
 of a single determinant, but by that of many determinants of 
 a similar kind ; and I imagine that that kind of determinant 
 which has to control a particular cell, multiplies considerably 
 by division before and perhaps even during the process of 
 determination. This view is completely borne out by Riickert's 
 interesting observations. 
 
 Every cell during the whole period of ontogeny is, however, 
 controlled not only as regards its structure, but also in respect 
 of its mode of division by a single determinant only. The in- 
 active determinants remain without exerting any influence on 
 the cell-body ; they however determine the architecture of the 
 id, and therefore the further formation of the embryo also. For, 
 indeed, the mode of disintegration of the id into smaller groups 
 of determinants is necessitated by its architecture. 
 
 I have above attributed to the determinants forces of attrac- 
 tion which take part in the configuration of the structure of the 
 ids. Such forces must be present, for otherwise the id could 
 not possess a definite architecture ; but I do not wish it to be 
 understood that these forces are the principal factors in the 
 arrangement of the determinants. They are concerned in con- 
 necting together the parts of which the determinants are com- 
 posed, and not in their continual rearrangement during the 
 course of ontogeny. It is primarily always the inherited definite 
 architecture of the id of germ-plasm which results mechanically 
 in the idic figure of the subsequent stages ; disarrangements in 
 this architecture are due to the unequally vigorous increase of 
 the various kinds of determinants, all of which naturally are 
 definitely determined beforehand. The arbitrary or accidental 
 action of the forces of attraction takes no part at all in this 
 process. 
 
 I must emphasise this view particularly, in contrast to that of 
 Galton, who speaks of ' repulsions and affinities ' of the gemmules 
 which compose the ' stirp.' He compares the masses of these 
 gemmules, which undergo active and incessant changes of their 
 mutual positions owing to attraction and repulsion, to a swarm 
 of flying insects, in which ' the personal likings and dislik- 
 ings of an individual may be supposed to determine the posi- 
 tion that he occupies in it.' With this view I can by no means 
 agree, for it rests on the assumption that the germ-substance is 
 composed of many homologous gemmules (' competing germs ')
 
 THE GERM-PLASM. 73 
 
 which struggle for the supremacy, only those which are success- 
 ful determining the character of the future organism. From 
 the very first Gallon takes into consideration the complications 
 of the germ-substance caused by sexual reproduction, which, as 
 will be shown subsequently, are due essentially to the fact that 
 the germ-plasm contains many, and not a single specimen of 
 each primary constituent, and that these are present in various 
 modifications. It is this struggle between the homologous 
 primary constituents which Gallon refers to in the passage just 
 quoted, which indicates that first one, and then another, reaches 
 the desired spot, without any definite order being observed. 
 This conception appears still more plainly in another passage, 
 in which he compares the germ-plasm (the ' stirp ') to a nation, 
 and those gemmules ' that achieve development,' i.e. become 
 transformed into the corresponding parts of the body ' to the 
 foremost men of that nation, who succeed in becoming its 
 representatives.' 
 
 Excellent as these similes are in themselves, I cannot help 
 thinking that they lead to error if intended as an explanation 
 of ontogeny. If we take up the position which Gallon occupies 
 with regard to the essenlial parl of ihe iheory of pangenesis, we 
 musl suppose that a large number of gemmules many more 
 than are necessary for the construclion of ihe body are con- 
 lained in ihe slirp ; lhal is, in ihe germ-subslance of ihe fer- 
 lilised egg. For only one gemmule is required for each cell of 
 Ihe body, bul neverlheless a large number are present ; and 
 these, so to speak, struggle for the precedence, the successful 
 gemmule alone becoming convened inlo ihe cell which is lo be 
 formed. In this conceplion the fact is entirely overlooked 
 thai onlogeny ilself cannot possibly depend on this slruggle, 
 bul would lake place jusl ihe same if only one gemmule were 
 presenl in ihe ' stirp ' for each cell, and lhal ihe cause for the 
 progress of development must therefore be sought elsewhere 
 lhan in ihe rivalry belween homologous gemmules : il musl be 
 due lo Ihe righl succession of the gemmules. Gallon considers 
 lhal ihe ' purely step-by-slep-developmenl ' assumed by Darwin 
 in his Iheory of pangenesis is insufficienl, but I ihink, neverlhe- 
 less, lhal Darwin's opinion is ihe more correcl one. 
 
 Neilher does Gallon's simile of ihe swarm of insecls seem to 
 me to be appropriate as an explanalion of ihe slruggle belween 
 homologous gemmules derived from differenl anceslors. Even
 
 74 THE GERM-PLASM. 
 
 if the gemmules in the 'stirp' were in perpetual motion, and if 
 on this depended the decision as to which of them obtained the 
 privilege of taking part in the formation of the organism, how 
 could one explain the existence of identical twins, about which 
 we have received such valuable information from Galjton him- 
 self? How would it be possible for the exactly corresponding 
 gemmules in two individuals in the flying and ever-changing 
 swarm always to reach the most favourable position, even if the 
 ' stirp' contained precisely similar gemmules ? 
 
 In a subsequent section I shall attempt to show that this 
 struggle between homologous but individually different primary 
 constituents can be proved in quite another manner in connec- 
 tion with the idioplasm. It was necessary to mention Gallon's 
 view here, in order to show that the forces of attraction and 
 repulsion, assumed by him, are introduced for an entirely 
 different purpose from that which I have stated with regard 
 to the similar forces in connection with the biophors of the 
 idioplasm. 
 
 Two physiological conditions of the elements of the idio- 
 plasm exist, an active and an inactive. In the former, these 
 elements become disintegrated into their constituent parts ; 
 while in the latter, they remain entire, although they are capable 
 of multiplication. When determinants are active, they become 
 disintegrated into biophors, and are then capable of controlling 
 the cell in the nucleus of which they are situated. The activity 
 of entire ids depends on a disintegration into determinants, 
 which, though certainly successive, is often very slow ; it must 
 be contrasted with the inactive state, which in both elements of 
 the idioplasm depends on the fact that their constituent parts 
 do not become separated from one another, but remain in their 
 primarily entire condition. In the immature ovum, for instance, 
 only one kind of determinant the 'oogenetic' determinant is 
 active, and this controls the growth and histological differentia- 
 tion of the egg ; all the other kinds remain inactive, as do also 
 the ids which are formed from them. Only when fertilisation 
 has occurred do they become active, that is to say, one kind of 
 determinant after another begins to separate itself from the 
 entire id. We shall see later on, however, that ids of the germ- 
 plasm also exist which remain inactive even after fertilisation 
 has occurred, and are passed on from cell to cell in what we 
 may call an unalterable (' gebundenem'} condition, so as to form
 
 THE GERM-PLASM. 75 
 
 subsequently the germ-cells of the embryo. We know as little 
 about the cause of this condition as we do about that of the 
 state of the brain during sleep, or of the latent period of certain 
 fertilised animal eggs, which, after beginning to undergo develop- 
 ment, remain inactive at a certain stage for months. The facts 
 with which we are acquainted, however, render the assumption 
 of an active and an inactive state of the ids and determinants 
 unavoidable, as will become more evident in the course of this 
 book. A similar assumption has been made by all those who 
 have formulated vital units : thus Darwin has assumed these 
 conditions in connection with his ' geinmules,' and de Vries with 
 regard to his ' pangenes.' 
 
 Two forms of heredity, which we call homotopic and homo- 
 chronic, may be deduced from the theory given above. As the 
 individual determinants from the germ-plasm onwards, through- 
 out all the stages of ontogeny take up a definite position in the 
 id, they must reach the right place in the body, and there cause 
 the development of a structure corresponding to that of the 
 parent. As, moreover, the period of maturation of each deter- 
 minant is decided by the nature of the latter, the determinant 
 will become active in the individual and will cause the forma- 
 tion of any particular part of the body at the same stage of 
 development as in the parent. Exceptions to this rule occur 
 in the case of abnormalities, and also in that of phylogenetic 
 displacements. 
 
 5. SUMMARY OF SECTIONS 1-4, RELATING TO THE STRUCTURE 
 OF THE GERM-PLASM. 
 
 According to my view, the germ-plasm of multicellular or- 
 ganisms is composed of ancestral germ-plasms or ids, the vital 
 units of the third order, each nuclear rod or idant being formed 
 of a number of these. Each id in the germ-plasm is built up of 
 thousands or hundreds of thousands of determinants, the vital 
 units of the second order, which, in their turn, are composed 
 of the actual bearers of vitality (' Lebenstrager '), or biophors, 
 the ultimate vital units. The biophors are of various kinds, and 
 each kind corresponds to a different part of a cell : they are, 
 therefore, the 'bearers of the characters or qualities' (' Eigen- 
 schaftstrager') of cells. Various but perfectly definite numbers
 
 76 THE GERM-PLASM. 
 
 and combinations of these form the determinants, each of which 
 is the primary constituent (' Anlage') of a particular cell, or of a 
 small or even large group of cells (e.g., blood-corpuscles). 
 
 These determinants control the cell by breaking up into 
 biophors, which migrate into the cell-body through the pores of 
 the nuclear membrane, multiply there, arrange themselves 
 according to the forces within them, and determine the histolo- 
 gical structure of the cell. But they only do so after a certain 
 definitely prescribed period of development, during which they 
 reach the cell which they have to control. 
 
 The cause of each determinant reaching its proper place in 
 the body depends on the fact that it takes up a definite 
 position in the id of germ-plasm, and that the latter, there- 
 fore, exhibits an inherited and perfectly definite architecture. 
 Ontogeny depends on a gradual process of disintegration of 
 the id of germ-plasm, which splits into smaller and smaller 
 groups of determinants in the development of each individual, 
 so that in place of a million different determinants, of which we 
 may suppose the id of germ-plasm to be composed, each 
 daughter-cell in the next ontogenetic stage would only possess 
 half a million, and each cell in the next following stage only a 
 quarter of a million, and so on. Finally, if we neglect possible 
 complications, only one kind of determinant remains in each 
 cell, viz., that which has to control that particular cell or 
 group of cells. This gradual disintegration of the id of germ- 
 plasm into smaller and smaller groups of determinants in the 
 subsequent idic stages does not consist in a mere division of the 
 id into portions, but as occurs in all disintegrations of vital 
 units is accompanied by displacements in the groups of these 
 units, brought about by the unequally vigorous multiplication of 
 the various individual determinants, and regulated by the forces 
 of attraction acting within them. In spite of all the alterations 
 in the arrangement of the determinants which must occur, owing 
 to the differential nuclear divisions together with unequal 
 growth of the various kinds of these units of the second order, 
 the original position of each determinant in the extremely com- 
 plex structure of the id of germ-plasm renders it necessary that 
 it should take up a definite and fixed position in each idic stage ; 
 and also that it should traverse the preciselyregulated course from 
 the id of germ-plasm, through perfectly definite series of cells, to 
 the cell in which it reaches maturity in the final stage of develop-
 
 THE GERM-PLASM. 77 
 
 ment. In this cell it breaks up into its constituent biophors, 
 and gives the cell its inherited specific character. Each id, in 
 every stage, has its definitely inherited architecture ; its structure 
 is a complex but perfectly definite one, which, originating in the 
 id of germ-plasm, is transferred by regular changes to the subse- 
 quent idic stages. The structure exhibited in all these stages 
 exists potentially in the architecture of the id of germ-plasm : to 
 this architecture is due, not only the regular distribution of the 
 determinants, that is to say, the entire construction of the body 
 from its primary form to that in which its parts attaia their final 
 arrangement and relation, but also the fact that the determin- 
 ant, of a small spot on a butterfly's wing, for example, reaches 
 exactly the right place ; and that, to take another instance, the 
 determinant of the fifth segment in the feeler of a Gammarus 
 reaches this particular segment. The determination of the 
 character of the individual cell depends on the biophors which 
 the corresponding determinant contains, and which it transmits 
 to the cell. 
 
 6. THE MECHANISM FOR THE PHYLETIC VARIATIONS IN 
 THE GERM-PLASM. 
 
 The causes of phyletic development will be treated of in the 
 chapter on Variation : the present section merely gives an 
 account of the mechanism existing in the idioplasm in connec- 
 tion with this process. I shall here attempt to show how the 
 phyletic changes in the idioplasm follow mechanically from its 
 assumed ultimate structure. 
 
 Since all parts of the organism are determined from the germ 
 onwards, permanent variations in these parts can only originate 
 from variations in the germ. Each phyletic variation must 
 therefore be due to a variation in the structure of the id of 
 germ-plasm. If we suppose, with Darwin, that the transforma- 
 tion of species is a gradual one, originating in individual varia- 
 tions which become increased and directed by selection, it 
 follows that the corresponding process in the idioplasm cannot 
 be due to a sudden and complete variation in the entire id, 
 but must begin with changes in the individual biophors or 
 in individual determinants and groups of determinants also, 
 and must then extend gradually to more numerous groups, until
 
 78 THE GERM-PLASM. 
 
 finally the nature of the id becomes entirely, or to a great 
 extent, changed. 
 
 The basis of the process must be sought in the variability of 
 the biophors, which is followed in turn by that of the units of a 
 higher order, the determinants and ids. These variations are 
 not by any means confined to the structure of the individual 
 cell, but concern primarily the number of cells of which an 
 organism consists. A leaf of a plant, or a bird's feather, may 
 increase considerably in size during the course of phylogeny, 
 without a change necessarily occurring in the cells which form 
 these parts. The variation will depend primarily on a multipli- 
 cation of the respective determinants. If the primitive eye of a 
 lower animal consisted of a single cell, constituting a visual rod, 
 and the power of multiplication of its determinants gradually 
 increased in the course of phylogeny, the number of identical 
 determinants which would arise during development by the 
 multiplication of the single determinant in the germ-plasm 
 would gradually increase so as to suffice for two cells instead of 
 one. The eye would then possess two visual rods, and if the 
 power of multiplication increased still more, a whole group of 
 visual rods would be controlled by one determinant. We are 
 unable to conjecture on what internal variations in the deter- 
 minant such an increase in the power of multiplication de- 
 pends ; but the fact that every individual cell in the body does 
 not possess a special determinant, while large groups of cells are 
 controlled by a single one, proves that such variations must be 
 possible. 
 
 Such a very simple phyletic variation, resulting in the local 
 increase of the number of cells, will be followed by a further 
 variation as soon as the multiplication of the determinant of, 
 e.g., an undifferentiated sensory cell, is not confined to the later 
 stages of ontogeny, but occurs also in the germ-plasm itself; 
 that is, when the doubling of the determinant has already taken 
 place in the id of germ-plasm. For in this case the group of 
 sensory cells, which have become developed phyletically from 
 the originally single cell, will now be controlled by two deter- 
 minants, each of which can vary independently of the other, and 
 can transform the group of cells under its control. Thus one of 
 these groups might give rise to auditory cells, and the other to 
 gustatory or olfactory cells. 
 
 Thus the increase in the differentiation of the body depends
 
 THE GERM-PLASM. 79 
 
 primarily upon the multiplication of the determinants in the id 
 of germ-plasm, but this differentiation is only rendered complete 
 by variations in the determinants of similar origin taking place 
 in different directions. The mere addition of a new ontogenetic 
 stage can very easily be conceived without an increase occurring 
 in the determinants of the id ; but as soon as the double number 
 of cells which are present in the new idic stage have to become 
 differentiated in various ways, the differentiation must be pre- 
 ceded by a doubling of the determinants in the id of germ- 
 plasm. A higher degree of differentiation will therefore be 
 primarily connected with an increase in the number of cells of 
 which the organism is constituted. It is known that the extreme 
 prolongation of development, due to the constant addition of 
 new generations of cells at the end of ontogeny, can be 
 neutralised by the abbreviation and reduction of the ontogenetic 
 stages : this process may be also to some extent understood if 
 we trace it to its origin in the structure of the idioplasm. The 
 reduction in the number of generations of cells from two or 
 more to one, depends on the fact that the process of multiplica- 
 tion and rearrangement of the determinants takes place more 
 rapidly during these particular stages, than does that of cell- 
 division ; so that several idic stages, each of which formerly 
 characterised a particular stage of the cell, pass into one 
 another during the same stage of the cell. The respective idic 
 stages have not here disappeared completely : they only follow 
 one another more rapidly, and therefore disappear as recog- 
 nisable stages in development. 
 
 In lowly organised beings the differentiation of the body may 
 become increased by a simple reduction of the hereditary parts 
 or determinates, without an increase taking place in the cell- 
 generations. If a determinant which controls a region con- 
 sisting of a hundred cells divides into two, each of which only 
 controls fifty cells, the two resulting groups of cells can vary 
 independently of each other from this point onwards, and may 
 give rise to very different structures. In this way a continued 
 division of the determinants, and consequently also a constantly 
 increasing differentiation of the species, may occur, without 
 necessitating an increase in the total number of cells present in 
 ontogeny. 
 
 Each additional differentiation denotes an increase in the 
 degree of organisation. But the phyletic development of the
 
 8o THE GERM-PLASM. 
 
 organism is by no means invariably connected with an increase, 
 or, in fact, with any other change in the degree of organisation. 
 The species of a genus, and often the genera of a family, cannot 
 be distinguished from one another by the number of cells com- 
 posing them, nor by an increase in the variety of these cells, 
 but only by qualitative differences in the structure of the various 
 parts. Hence the phyletic development of living beings cannot 
 simply be due to the augmentation of the number of determinants 
 in the id of germ-plasm, but must also be attributed to a change 
 in the nature of the determinants and in that of their com- 
 ponent biophors. 
 
 The structure of the idioplasm which we have here assumed, 
 also offers an explanation of the phenomena of parallelism 
 between ontogeny and phylogeny, which depend on the law of 
 biogenesis as well as on the relegation of the final characters to 
 earlier and earlier ontogenetic stages in the course of phylogeny. 
 Let us first consider the former of these phenomena. We have 
 assumed that each ontogenetic stage is characterised by a 
 definite ' determinant figure,' i.e., a sort of geometrical structure 
 composed of the determinants. The nature of each individual 
 cell is certainly controlled by those determinants in the nucleus 
 which have reached maturity, that is to say, have arrived at the 
 stage in which they break up into biophors and migrate into 
 the cell-body. But the manner in which the embryonic develop- 
 ment of an animal occurs does not by any means depend only 
 on the histological structure of the individual cells in each stage, 
 it rests to a much greater degree on the manner in which 
 these cells divide and on the rate of their division, and also 
 primarily on the way in which the 'unripe' determinants of the 
 nuclear substance, which are still latent, are grouped together 
 and distributed by means of the cell-divisions. This distri- 
 bution of the primary constituents among the different cells is 
 of the first importance in determining the character of the 
 ontogeny ; and one could easily imagine a case of animal 
 embryogeny in which ten or twenty generations of similarly 
 constituted ' embryonic cells ' followed each other, and in which 
 a perfectly definite distribution of the primary constituents 
 (determinants) had nevertheless occurred, although only now 
 apparent for the first time. It is well known how close a 
 resemblance exists between the cells of the embryo in various 
 stages in the case of the higher animals.
 
 THE GERM-PLASM. 8 1 
 
 The regular distribution of the determinants which are still 
 latent or ' unripe' 1 must therefore decide the course of ontogeny; 
 and the manner of this distribution finds expression in the 
 architecture of each idic stage, or, as I have expressed it, in each 
 ' determinant-figure? 
 
 It is obvious that the same geometrical figure may be con- 
 structed out of different elements, just as the same form of 
 crystal may be produced from molecules of a different nature. 
 Thus the resemblance between the ontogenetic stages of nearly 
 allied species is to be explained by the degree of similarity 
 between their respective ' determinant figures,' which persists 
 although the individual determinants constituting the figure 
 differ more or less from one another. As the study of develop- 
 ment shows, an explanation is thus offered of the fact that the 
 earlier ontogenetic stages are so very much alike in allied 
 species, and that the differences only appear later on ; for in 
 the early idic stages, the differences as regards the nature or 
 power of multiplication of single determinants, or groups of 
 determinants, can exert no marked influence, because the entire 
 number of determinants is still very large, and thus the archi- 
 tecture of the id will be practically the same in corresponding 
 stages. But the further ontogeny advances, and the smaller 
 the groups become into which the determinants separate, the 
 greater also will be the diversity in the architecture of the id, 
 and in the further distribution of ' unripe ' determinants resulting 
 from this architecture. Thus a certain part will be longer or 
 shorter, a spot of colour larger or smaller, and the final stages 
 of ontogeny in which the cells possess only one determinant 
 will differ according to the degree of difference which obtains 
 in the respective determinants. This explains the fact that the 
 segmentation cells in allied species are frequently exactly alike, 
 and also that the resemblance between many mammalian 
 embryos in their earlier stages, though not complete, is never- 
 theless a very close one. 
 
 The law of biogenesis, as far as it applies at all, depends on 
 the fact that phyletic development is partly due to new onto- 
 genetic stages being added at the end of ontogeny. In order 
 that these new stages may be reached, the stages which were 
 previously the final ones must be passed through in each 
 ontogeny. This may be expressed in terms of the idioplasm as 
 follows : the determinants of the id of germ-plasm become 
 F
 
 82 THE GERM-PLASM. 
 
 endowed with a greater power of multiplication, so that each 
 one of them causes the addition of one or more cell-generations 
 to the end of the ontogeny. At the same time, the determinants 
 in the germ-plasm increase in number, and each of them be- 
 come* differentiated in a fresh manner. As, however, every two 
 new determinants always follow the same course from the id 
 of germ-plasm to the final stage in ontogeny as was taken by 
 the single original determinant, they will pass through the same 
 determinant figures as before, and only lead to the formation of 
 new structures in the final stages, when they become separated 
 from one another. 
 
 The ontogenetic stages of the ancestors will be repeated less 
 accurately the nearer development approaches its termination. 
 
 The disappearance of a character or of a part which has 
 become use/ess, may also be traced to the mechanism of the 
 idioplasm. The group of determinants which gives rise to a 
 particular character, will have to be removed entirely from the 
 germ-plasm if the corresponding part is to disappear com- 
 pletely. But this is a very complicated process, and one of long 
 duration as regards more complex organs, such as, for instance, 
 the limbs of Vertebrates. For the determinants which take part 
 in the formation of an extremity are very numerous, and of many 
 different kinds ; and moreover, they cause the rudiment of the 
 limb to appear very early in ontogeny. Hence the determinants 
 will have to suffer successively many retrogressive and simplify- 
 ing changes before a noticeable reduction of the organ occurs. 
 The functionless and rudimentary wings of the New Zealand 
 Kiwi (Apteryx), which are concealed by the plumage, possess all 
 the bones of the perfect wing, though these are very much re- 
 duced in size. This is to be explained by supposing that the 
 entire group of determinants for the wing still remain in the id 
 of germ-plasm, but that it has decreased in strength, that is to 
 say, its elements no longer increase so rapidly, and they there- 
 fore can only control smaller groups of cells. If the process of 
 degeneration continued, the organ would not only grow smaller 
 and smaller, but its component parts would also disappear at 
 different rates, and, losing their characteristic form, would 
 appear as indistinguishable rudiments. Such a degeneration 
 has occurred in certain species of whales, in which the rudi- 
 ments of the posterior extremity lie concealed beneath the skin ; 
 while in other species, the form of the separate bones has been
 
 THE GERM-PLASM. 83 
 
 to some extent preserved, and those of the thigh and shank can 
 still be plainly distinguished. In these cases, many of the 
 determinants which were formerly present must have dis- 
 appeared entirely from the id of germ-plasm, and the remainder 
 must have lost the power of multiplication to a greater extent 
 than has occurred in the case of the wing of Apteryx. 
 
 We know, however, that even in such animals as snakes, in 
 which the extremities have in most cases disappeared com- 
 pletely in previous geological periods, the rudiments of the 
 limbs arise in the form of ' muscle-buds ' in the earlier stages of 
 development, and then disappear very shortly afterwards.* 
 This fact may be expressed in terms of the idioplasm as 
 follows : the power of multiplication in the small remnant of 
 the group of determinants of the extremity which still exists in 
 the id of germ-plasm, has decreased so considerably that it only 
 suffices for these early embryonic stages. The youngest deter- 
 minants, and consequently the most recent hereditary structures, 
 are the first to disappear, the loss of the older ones taking place 
 gradually, until even the oldest of all are no longer present. 
 This must be due to the manner in which the deter- 
 minants increase, although the actual connection between the 
 two phenomena is not apparent. It may perhaps be traced to 
 the fact that those determinants which are the youngest phy- 
 letically are destined for the latest ontogenetic stages, in which 
 only therefore they become ' ripe,' and undergo disintegration 
 into biophors. If then, their power of multiplication decreases 
 considerably during the process of degeneration, the number of 
 determinants required for the control of any particular group of 
 cells will not be reached, nor will the determinants even become 
 ripe. Although still present, they are unable to exert any in- 
 fluence ; whereas the determinants of the older phyletic stages 
 which are still passed through, ripen in the earlier stages of 
 ontogeny. 
 
 The process of degeneration of an organ may be represented 
 as depending on the fact that the determinants first become 
 changed in such a manner as to cause a decrease in their power 
 of multiplication, and this then leads to a very gradual reduction 
 
 * Cf. J. van Bemmelen, ' Over den oorsprong von de vorste lede- 
 mateti en de tongspieren bij Reptilen.' Ron. Akademie de Wetenschappen 
 tc Amsterdam, 3oth June 1888.
 
 84 THE GERM-PLASM. 
 
 affecting an increasing number of determinants belonging to 
 the group in question. At the same time, the power of mul- 
 tiplication in the remaining determinants also diminishes, so 
 that the groups which they constitute gradually extend a less 
 distance into the ontogeny, until finally they drop out of it 
 altogether. 
 
 It must not be understood that I have given a mechanico- 
 physiological explanation of the process of degeneration because 
 I have connected it with the theory of determinants. As long 
 as we know practically nothing about the forces which act 
 within and among the biophors, it will be impossible to offer an 
 explanation of this kind. I have only attempted to show that 
 this doctrine does not contradict the facts, but that, on the con- 
 trary, it agrees with them up to a certain point. The phenomena 
 of degeneration have not hitherto been considered from this 
 point of view. When a deeper insight into the actual phenomena 
 has been obtained, we may perhaps be able to make further 
 theoretical deductions, and it would then be possible to develop 
 the theory of determinants more fully. 
 
 A few words may now be said as regards correlated varia- 
 tions. Darwin has shown what an important part these varia- 
 tions play in the transformation of species, and how changes 
 which we must consider to be primary are followed by a number 
 of others in various parts of the organism. Thus an increase in 
 size in a stag's antler necessitates a thickening of the skull, and 
 a strengthening of other parts, viz., the muscles of the neck, the 
 spines of the cervical vertebrae, the ligamentum nuchae, and 
 even the thoracic skeleton and fore-limbs. Referring all these 
 variations to the processes which take place in the idioplasm, 
 they will be seen to depend on changes in the corresponding 
 groups of determinants in the id of germ-plasm, which cannot 
 be due directly to the change and increase in the group of 
 determinants of the antler : they must have arisen secondarily, 
 owing to the occurrence of variations in the determinants upon 
 which selection could act. There is also an entirely different 
 kind of correlation, in which the variation in one part is accom- 
 panied by that in another, the latter having no anatomical or 
 functional connection with the former. Thus Darwin states, for 
 instance, that cats with blue eyes are generally deaf, and that 
 pigeons with feathered legs have a web between the outer tcos. 
 
 I do not think such correlations can be traced to a connection
 
 THE GERM-PLASM. 85 
 
 of the parts by means of the nervous system : it is perhaps 
 more likely that they are due to the contiguity of the determinants 
 in the id of germ-plasm of those parts which vary correlatively. 
 It will be shown later on that local differences in nutrition occur 
 in the id, and that these may cause changes in the determinants 
 affected by them. If, now, the determinants controlling regions 
 of the body which are far apart, are situated close together in 
 the id, they might easily be affected simultaneously by influences 
 producing variation. But the perfectly definite architecture of 
 the id of germ-plasm, on which we base our argument, does not 
 only permit of a vicinity of the determinants of parts of the 
 body far removed from one another, but actually requires it. 
 For, according to our assumption, the id of germ-plasm is not a 
 representation of the body in miniature, but a structure of a 
 special kind, in which the individual component parts are 
 arranged in the order in which they are passed on subsequently 
 in the process of ontogeny to their final destination, viz., to the 
 determinates or hereditary parts. This however requires that the 
 determinants of the ectoderm should be closely adjacent to 
 those of the endoderm in the id, if they are to be distributed to 
 a primary ectoderm and a primary endoderm cell in the first 
 division of the ovum. A cell-division which leads to the 
 separation of widely differing groups of determinants, admits 
 of a close aggregation of these different groups in the id of 
 the mother-cell. This may give some slight insight into the 
 above-mentioned phenomena of correlation. 
 
 7. THE MAGNITUDE OF THE CONSTITUENTS OF THE 
 GERM-PLASM. 
 
 The assumption that the germ-plasm is composed of biophors, 
 determinants, and ids, implies the existence within a narrow 
 space of a large number of ultimate vital units (biophors) 
 in all the higher organisms. The question arises whether a 
 sufficient number of these units can be contained within an id. 
 Although I believe it is at present quite impossible to obtain 
 anything like a reliable answer to this question by calculating 
 the relative sizes of the elements of the germ-plasm, it may 
 perhaps not be uninteresting to attempt to make such a cal- 
 culation. 
 
 In order to solve the problem with any approach to accuracy,
 
 86 THE GERM-PLASM. 
 
 it would at least be necessary to know the sizes of a biophor and 
 of an id, and also the number of determinants in a given species. 
 Unfortunately, however, we are completely ignorant on these 
 points, nor do we even know how many molecules take part in 
 the construction of a biophor : even the computed size of the 
 molecule is somewhat uncertain. 
 
 The diameter of a molecule has been estimated at between 
 the 5335^1 and the io ^ ooo th of a millimetre by four different 
 lines of reasoning, 'founded respectively on the undulatory 
 theory of light, on the phenomena of contact electricity on 
 capillary attraction, and on the kinetic theory of gases.'* 
 O. E. Meyer has calculated the size of a molecule 'from the pro- 
 perties and behaviour of vapours. From the constant of friction 
 and the comparison between the space occupied in the liquid 
 and gaseous conditions, together with the deviations from Boyle 
 and Mariott's law, we can approximately calculate, firstly, the 
 volume of all the particles contained within a given space ; 
 secondly, that of a single particle ; thirdly, the number of 
 particles ; and finally, the weight of a single particle.' The 
 result of such a calculation agrees with that given above. 
 
 If we take the average diameter of a molecule to be a(X ^ 000 th 
 mm., and reckon that each biophor, which we will suppose to be a 
 cubical structure, is composed of 1,000 molecules, the biophor 
 would measure 10 molecules in length. A row of 200 biophors 
 would therefore measure I p., and 8,000,000 biophors would occupy 
 the space of I cubic fi. A human blood-corpuscle measures 
 7.7 p. in diameter ; if we imagine it to be enlarged so as to form 
 a cube of 7.7 /z. in diagonal length, this space would contain 
 703,000,000 biophors. Let us further assume that those por- 
 tions of the cell which, according to the facts at our disposal, 
 must contain the idioplasm, viz., the chromosomes, are mostly 
 a great deal smaller than the nucleus in which they are situ- 
 ated, and that the germ-plasm is composed not of one but of 
 several ids, each of which contains all the biophors required 
 for the construction of the entire body, it will then be evident 
 that only a limited number of biophors can be contained in 
 one id. 
 
 The chromosomes in the germ-plasm of Ascaris megaloce- 
 
 * Sir William Thomson, ' Popular Lectures and Addresses, 1 Vol. I., 
 1889, p. 148.
 
 THE GERM-PLASM. 87 
 
 phala are the largest which are at present known to us. Each 
 nucleus in this animal contains two or four rod-like chromosomes 
 (see fig. 2), each of which is composed of ' six thickened granu- 
 lar or disc-shaped portions, which become deeply stained with 
 colouring matter, and which are separated by portions staining 
 less deeply' (Boveri). If we connect this fact with the hypo- 
 thetical composition of the germ-plasm out of ids, it follows 
 that an id cannot in any case be larger, and is probably smaller, 
 than one of these granules or microsomata. It cannot be 
 larger, because the id is a unit which is capable of division into 
 two daughter-ids, but which cannot remain permanently sepa- 
 rated into several parts by a different kind of intermediate sub- 
 stance. If we suppose the id to be as large as it can possibly 
 be, that is to say, to correspond in size to a microsome, it 
 will measure, according to Boveri's drawing and scale of enlarge- 
 ment, .0,008 mm., or not quite i //, in diameter. Only the ter- 
 minal granules of the rods, however, are as large as this ; the 
 greatest diameter of those in the middle measures .0,006 mm., 
 while their shorter diameter is about .0,003 .0,004 mrn. The 
 terminal granules, looked upon as spherical bodies, would 
 be capable of containing about two million biophors of the size 
 given above. 
 
 This number is certainly a very considerable one, and it would 
 apparently be sufficient to make up the number of determinants 
 in such a lowly organised animal as Ascaris. But even in 
 Arthropods the number of determinates, and therefore that of 
 the determinants also, is considerably greater. Each of the 
 olfactory setae on the feelers of Crustaceans, which were 
 mentioned above, must be capable of being determined from the 
 germ onwards ; and this is also true of the spots and lines on 
 a butterfly's wing, each of which represents at least one deter- 
 minant, and in case of all the large markings several, or even 
 many, of these units. If we consider that the pattern on the 
 wing is often very complicated, and frequently differs on the 
 under and upper surfaces, it is evident that hundreds of deter- 
 minants must exist for this pattern alone. But there are, again, 
 several peculiarities in the structure of the wing-scales, and thus 
 it is probable that almost every scale can vary independently 
 from the germ onwards. In some males of the family Lycaenidce, 
 e.g., Lycaena adonis, small guitar-shaped odoriferous scales (the 
 'androconia' of Scudder) are distributed regularly amongst
 
 88 THE GERM-PLASM. 
 
 the colour-scales, while these are entirely absent in other nearly 
 allied species, such as Lycaena agestis : hence we must conclude 
 that these androconia have arisen by the transformation of 
 ordinary scales. This, however, presupposes the independent 
 variability of the scales which are to become changed phyleti- 
 cally, and consequently also their capability of being deter- 
 mined from the germ onwards. Were this not the case, a 
 single scale could never have varied from the others hereditarily. 
 In Lycaena adonis there are 30,830 scales on the upper surface 
 of the wing.* If each of these is to be looked upon as cor- 
 responding to a determinate, the enormous number of about 
 240,000 determinants of the germ-plasm would result merely 
 from the scales covering the wings, provided that the upper and 
 under surface of the four wings possess each about the same 
 number of scales. 
 
 I have endeavoured by direct experiment to ascertain the 
 lowest limit to the size of a determinate, that is to say, the 
 size of the smallest determinates for a particular character of a 
 certain species. For this purpose I selected one of the Ostra- 
 coda, Cypris reptans, which multiplies parthenogenetically, and 
 in which it is easy to compare the different green spots on the 
 shell in the mother and daughter. It appears that the larger 
 spots are strictly transmitted, though this is not the case as 
 regards the very small ones, which consist of only one or two 
 pigment-cells. The form of these larger spots, which consist of 
 fifty or a hundred pigment-cells, also varies to some extent, so 
 that the number is here also somewhat inconstant. If partheno- 
 genetic reproduction could be looked upon as being purely 
 uniserial, it might be inferred that the determinates are not in 
 this case single cells, but groups of cells. Unfortunately, how- 
 ever, this experiment cannot be considered conclusive, for as 
 will appear later on the germ-plasm is here composed, just as in 
 the case of sexual reproduction, of dissimilar, and not of similar 
 ids, and consequently variations in heredity may thus arise. 
 
 We must conclude, even from the external coloration, that a 
 very considerable number of determinates exists in the case of 
 the higher Vertebrates. Thus most, if not all, of the contour- 
 feathers of a bird must be controlled by special determinants in 
 
 * My assistant, Dr V. Hacker, was good enough to make this calcula- 
 tion for me.
 
 THE GERM-PLASM. 89 
 
 the germ-plasm, for they are independently variable hereditarily. 
 The number of wing- and tail-quills is nevertheless definitely 
 fixed for every species of bird, and each of these feathers pos- 
 sesses a definite form, size, and coloration. We must assume 
 that more than one determinant is necessary for an entire 
 feather, for a feather is formed from thousands of epidermic 
 cells, which are not by any means all similar to one another, 
 either as regards form, mode of combination, or colour. 
 Many feathers are striped, while others have a brilliant orna- 
 mental spot at the tip ; as in the case, for instance, of the peacock, 
 many humming-birds, and certain birds of paradise. The cells 
 to which these stripes and spots owe their origin, must contain 
 determinants which differ from those of the rest of the cells 
 which take part in the construction of the feather. We must 
 therefore conclude that at least one, and often several, deter- 
 minants of the germ exist for each of these two kinds of 
 cells ; for, as is well known, ornamental spots of this kind are 
 often formed of several colours, and are very complex. 
 
 It would be erroneous to suppose that the contour feathers are 
 not determined individually in such birds as the raven, in which 
 the plumage is all of one colour ; but in such cases the qualita- 
 tive differences refer less to colour than to form and size. The 
 fact that each part of the feather is determined hereditarily, 
 even as regards its colour, is proved by the variation which 
 occurs, and which in individual species has resulted in certain 
 feathers being partially or entirely white, or being brilliantly 
 coloured, as in the case of the bird of paradise, which is allied 
 to the raven. One need only look through a collection of 
 humming-birds, and compare the females, which are so often 
 plainly coloured, with the wonderfully variegated males, in order 
 to become convinced that almost every contour-feather can vary 
 in almost any direction as regards coloration, form, size, and 
 minute structure. 
 
 As has already been remarked, the internal organs are ap- 
 parently by no means so specially determined from the germ 
 onwards as are the external parts : their determinants must 
 therefore control larger regions of cells, as in the case, e.g., of 
 blood-corpuscles and epithelial cells. The number of deter- 
 minants in the germ-plasm of the higher animals is nevertheless 
 an enormous one, and it might certainly be doubted whether 
 such a large number of biophors as must be required for the
 
 9 THE GERM-PLASM. 
 
 construction of an id of the germ-plasm could be contained 
 within a single id. 
 
 It is impossible, as we have already seen, to obtain a satis- 
 factory answer by means of a calculation. But let us assume 
 for the moment that we possess reliable data as to the number 
 of determinants and the size of an id in a particular species. 
 We will further assume that each determinant is composed of, 
 let us say, fifty biophors, and each biophor of a thousand mole- 
 cules, and that the average diameter of a molecule is j^-^th mm. 
 Supposing we found that all these units could not be contained 
 within an id of the size we have assumed, we should be forced 
 to conclude that one or more of these quantities had been over- 
 estimated. This result would not weaken the theory of deter- 
 minants, for minute particles must exist in the germ-plasm for 
 each hereditary and independently variable part of the body. 
 I therefore consider it fruitless to attempt a more accurate 
 estimation of the number of determinants in individual species, 
 and to endeavour to find a support for this fundamental theory 
 by means of such calculations. The theory is correct in any 
 case, although our conception of the structure of the germ- 
 plasm may be very incomplete. 
 
 The object of making the above calculation was simply to 
 arrive at this result. The germ-plasm is an extremely delicately- 
 formed organic structure, a microcosm in the true sense of the 
 word, in which each independently variable part present 
 throughout ontogeny is represented by a vital particle, each of 
 which again has its definite inherited position, structure, and 
 rate of increase. A theory of evolution appears to me to be only 
 possible in this sense. The constituents of the germ-plasm are 
 not miniatures of the fully-formed parts, or even particles exist- 
 ing solely for the formation of the corresponding parts in the 
 body. But each of these particles (the biophors and deter- 
 minants) has a definite and important share in the preceding 
 stages of development, for it takes part in determining the 
 architecture of each idic stage, and consequently also assists in 
 the further ontogenetic disintegration and distribution of the 
 determinants amongst the subsequent cell-stages. All the more 
 essential differences in the structure of organisms depend on 
 this fact. The determinants are particles on whose nature that 
 of the corresponding parts in the fully-formed body depends, 
 whether the latter consists of a single cell or of several or many
 
 THE GERM-PLASM. 91 
 
 cells. The assumption of such particles is inevitable in a theory 
 of heredity, and it alone necessitates an almost inconceivable 
 complexity in the architecture of the germ-plasm. But if we 
 suppose that the number of ultimate particles of which the 
 germ-plasm is composed is less than the number of parts in the 
 body which are independently variable hereditarily, it would 
 then follow that several minute parts of the body must become 
 changed simultaneously with the variation of one of these 
 particles, that is to say, the number of determinants then 
 become too small theoretically.
 
 PART II. 
 
 HEREDITY IN ITS RELATION TO MONOGONIC 
 REPRODUCTION. 
 
 INTRODUCTORY REMARKS. 
 
 In the following part those phenomena of heredity will be 
 considered which do not result directly from the composition of 
 the germ-plasm as already described, but which would also 
 occur if there were no such thing as sexual reproduction. In 
 considering the phenomena of the regeneration of lost parts, of 
 multiplication by fission and gemmation, of the production of 
 unicellular germs ; and of the continuity of the germ-plasm, it will 
 materially facilitate the attainment of clear results if the inves- 
 tigation is conducted throughout as though these phenomena 
 occurred in organisms in which the process of multiplication is, 
 and always has been, entirely an asexual one. The complica- 
 tions resulting from sexual reproduction can be considered 
 afterwards, and it will then be easy to connect them with all 
 these phenomena of heredity.
 
 REGENERATION. 93 
 
 CHAPTER II. 
 REGENT RATION. 
 
 i. ITS CAUSE AND ORIGIN IN THE IDIOPLASM. 
 It does not follow directly from what has already been said 
 with regard to the structure of the germ-plasm, that lost parts 
 can be more or less completely replaced. The only deduction 
 that can be made so far is, that all the parts of which the entire 
 organism is composed are formed once during the development 
 of the organism from the egg : no explanation is given of the 
 fact that individual parts can be produced a second time, when 
 they have been lost by the action of external influences. 
 During ontogeny, the determinants of the part in question pass 
 from the ovum into the segmentation-cells, from these into 
 embryonic cells of a later stage, and finally into those cells 
 which constitute the fully formed part. If this part is forcibly 
 removed from the organism to which it belongs, its determinants 
 are removed along with it : this follows from what has already 
 been assumed with regard to the ontogenetic stages of the 
 idioplasm. We must now therefore attempt to explain the fact 
 that a part of the body can nevertheless be reconstructed. 
 
 If the capacity for regeneration were possible at all, it is 
 obvious that it would have to be introduced by Nature, for its 
 physiological importance is apparent. The power of replacing 
 larger or smaller parts of the body must in all cases be useful to 
 the organism, and is often indeed indispensable to its further 
 existence. Arnold Lang* is certainly right in considering 
 the faculty of regeneration in animals to be one of the arrange- 
 ments for protection which prevent the species from perishing. 
 The capability of completely restoring those parts of the body 
 which have become injured by the bite of an enemy, forms a 
 
 * ' Ueberden Einfluss der festsitzenden Lebensweise smf die Thiere.'&c. 
 Jena, 1888, p. 108.
 
 94 THE GERM-PLASM. 
 
 more efficient protection in many of the lower animals more 
 especially in polypes and worms than would the possession of 
 shells, stings, poison-organs, and all other kinds of weapons, or 
 even protective coloration. For although all these arrange- 
 ments certainly serve as a protection from many enemies, and 
 from various dangers, they are not always effective, and there- 
 fore the capability of restoring losses of substance would cer- 
 tainly be extremely valuable in any case. This fact must not be 
 forgotten in any inquiry with regard to the question of regenera- 
 tion. If we consider how highly important regeneration is from 
 a physiological point of view, its wide and even general distribu- 
 tion in the animal kingdom need not surprise us, and we shall 
 be able to understand why it has been introduced even into the 
 course of normal life : for the functions of certain organs depend 
 on the fact that their parts continually undergo destruction, and 
 are then correspondingly renewed. In this case it is the process 
 of life itself, and not an external enemy, that destroys the life 
 of a cell. I refer, of course, to the process of physiological 
 regeneration. 
 
 Our knowledge of histology is not yet sufficient for us to be 
 able to determine what tissue-cells in the higher animals become 
 worn out by use during life, and have therefore to be continually 
 replaced ; but it has been proved in many cases that the wear- 
 ing away of the cells goes on incessantly, and that life could not 
 last if these cells were not constantly replaced. Such a con- 
 stant loss and renewal of the cells occurs in the cases of the 
 epidermis of the higher Vertebrates, the human finger-nails, 
 blood-corpuscles, hairs and feathers, claws and hoofs, the epi- 
 thelial lining of the respiratory and other passages, and even in 
 the antlers of stags. In all these cases a continual or periodic 
 wasting away or shedding of groups of cells occurs normally, 
 and a corresponding replacement of these cells is one of the 
 normal functions of the body, and is therefore provided for. 
 
 It is not difficult to explain the simplest of these cases of 
 physiological regeneration theoretically. If a tissue such as the 
 human epidermis, for instance, consists of one kind of cell only, 
 it is only necessary, in order that regeneration may take place, 
 that all the cells should not be thrown off simultaneously, and 
 that the tissue should be composed of cells of various ages, the 
 youngest of which, under certain influences of nutrition and 
 pressure, always retain the power of reproduction, and so form
 
 REGENERATION. 95 
 
 a stock in which the necessary substitutes for the older cells can 
 constantly be produced. The whole supply of the corresponding 
 determinants is not therefore removed from the body simul- 
 taneously by the loss of the worn-out cells, for the young cells 
 which remain contain determinants of the same kind. In the 
 human epidermis, this stock of young cells constitutes the so- 
 called ' rete Malpighii ' or ' mucous layer,' in which new cells are 
 constantly being formed by division ; these, in proportion as 
 they become older, are gradually pushed upwards mechanically 
 from the deeper into the superficial layers, while the deepest 
 layer of all consists entirely of young cells which are capable of 
 division. 
 
 No special theoretical assumption need be made to explain 
 this process. We must only suppose that the first formed 
 epidermic cells are endowed in advance with a capacity for 
 reproduction during many generations. It must be assumed 
 that the reproductive power of a cell is regulated by the idio- 
 plasm, because the power and rate of multiplication are essen- 
 tial qualities of a cell, and, as we have seen, are controlled by the 
 nuclear substance. But we cannot at present even form a con- 
 jecture as to which qualities of the idioplasm the degree and 
 rate of the capacity for reproduction are due. We must be 
 satisfied with attributing to the cells which form the epidermis 
 of the embryo an idioplasm possessing a definite reproduc- 
 tive power, which gradually decreases. We can further 
 only suppose that the idioplasm retains its constitution during 
 life, or, in other words, that the determinant of a particular part 
 of the epidermis is always retained in the permanent stock of 
 young cells. Regeneration depends simply on a regular increase 
 of those cells which contain epidermic idioplasm. 
 
 The nature of the epidermis is not the same in different parts 
 of the human skin : thus it differs on the volar and on the dorsal 
 surfaces of the fingers ; and, again, on the two basal and on the 
 ungual segments. But this fact does not stand in the way of 
 the theoretical explanation of regeneration, for the determinants 
 of different parts must differ somewhat from one another. Even 
 in places where two or more dissimilar parts are situated close 
 together, the retention of the limits between them, during their 
 continual regeneration, may be explained simply by the fact that 
 the different regions of the tissue are regenerated by formative 
 cells possessing different determinants.
 
 g6 THE GERM-PLASM. 
 
 Many tissues, even in the highest animals, when they have 
 suffered an abnormal loss of substance, are renewed in precisely 
 the same way as in the cases of physiological regeneration 
 already mentioned. Thus in mammals, for instance, portions 
 of muscular tissue, of epithelium covering an organ or lining 
 the duct of a gland, and of bone, can be replaced by cellular 
 elements of a similar kind ; and recent researches in pathological 
 anatomy render it almost certain that all these regenerative 
 processes originate in the cells of the tissue which is to be 
 replaced. Hence these tissue-cells retain the power of multi- 
 plying by division, but they only begin to exercise this power in 
 response to certain external stimuli, more particularly to that 
 which is produced by a loss of substance in their immediate 
 vicinity. Thus epithelial cells multiply around a defect in the 
 epithelium ; and in an injured muscle, the nuclei multiply and 
 cause the surrounding protoplasm to be transformed into cells, 
 which become spindle-shaped, and give rise to muscle-fibres. 
 In both these cases we must merely attribute to the idioplasm 
 the capacity for multiplication : the cells in question only begin 
 to divide when influenced by a stimulus due to the loss of sub- 
 stance, or, as it would be expressed in the language of modern 
 pathology,* 'by the removal of the resistances to growth.' 
 Thus in these very simple cases of the abnormal loss of parts, 
 the rest of the tissue gives rise to a stock of determinants 
 from which replacement of the part can occur. 
 
 In more complicated tissues, the process of regeneration is 
 less simple. Thus Fraisse has shown that in the Amphibia 
 the entire epidermis, together with the slime-glands and the 
 integumentary sense-organs, is regenerated by the epidermic 
 cells in the vicinity of the defect. In this case also, the new 
 material is furnished by the deeper uncornified layers of the 
 epidermis. But the newly-formed cells do not all develop into 
 the same kind of tissue. The main mass of them gives rise to 
 the stratified epidermis, while others ' unite to form pearl-shaped 
 masses in the deeper part of the epidermis, the cells becoming 
 grouped around an imaginary centre.' ' Connective tissue-cells 
 then migrate from the rutis, and these masses, each consisting of 
 from ten to twenty cells, thus become marked off from the 
 epidermis.' 'At the same time pigment-cells wander into the 
 
 * Cf. E. Ziegler, ' Lehrbuch der pathologischen Anatomic,' Jena, 
 1890.
 
 REGENERATION. 07 
 
 epidermis, and finally the development of smooth muscle fibres 
 takes place.' "* New integumentary sense-organs arise in a 
 similar way. A number of young cells become arranged so as 
 to form a rounded mass in the deeper portion of the newly- 
 formed epidermis : these then become elongated in a direction 
 vertical to the surface of the epidermis, the central element 
 undergoing differentiation into sensory cells, while the peripheral 
 ones form an investment around these. 
 
 It is evident that the process is rendered more complicated in 
 this case by the fact that the young epidermic cells, formed by the 
 proliferation of those already present, give rise to cells of various 
 kinds, viz., to ordinary epidermic cells, to gland cells, and to 
 sensory and 'investing' cells ; and the complication is further 
 increased by all these cells being arranged and localised in a per- 
 fectly definite and more or less prescribed manner. We certainly 
 must not assume that the formative cells which undergo these 
 various differentiations are really identical, although they may 
 appear so. It cannot possibly depend on external influences 
 alone whether one of these subsequently becomes transformed 
 into a horny, glandular, or sensory cell ; for we cannot assume 
 the existence of such a regular and localised difference in the 
 external influences. The various differentiations of the formative 
 cells must therefore depend on their own nature that is to say, 
 on the determinants contained within them, which have hitherto 
 been latent but which have now become ripe, and have im- 
 pressed a specific character upon each cell. These formative 
 cells must have contained different sorts oj determinants from the 
 first. 
 
 Fraisse compares the processes which can be observed in the 
 regeneration of the skin in Amphibia with those which occur in 
 the embryogeny of this class, and shows that they are essentially 
 similar. We shall therefore be justified in imagining these pro- 
 cesses which are invisible to us even under the highest powers 
 of the microscope to be homologous with those which take 
 place during the development of the embryo. 
 
 We can thus further assume that the stratified cells in the 
 ' mucous layer' of the epidermis, although apparently all alike,-- 
 as are those cells which form the first rudiment of the embryonic 
 
 * Cf. Fraisse, ' Die Regeneration von Geweben u. Organen bei den 
 Wirbelthieren, besonders bci Amphibien u. Rcptilien.' Cassel and 
 Berlin, 1885. 
 
 G
 
 98 THE GERM-PLASM. 
 
 integument, must nevertheless possess several kinds of deter- 
 minants. We can hardly venture to say svhether the three kinds 
 of determinants with which we are here concerned arc all 
 present together in the formative cells, and only become dis- 
 tributed amongst special cells when regeneration sets in, or 
 whether they are distributed amongst special cells from the 
 first. Either arrangement is possible. Hence we may assume 
 that some of the young formative cells contain determinants for 
 the glands, and others those for horny or sensory cells, and 
 that the proportional numbers and topographical arrangement 
 of these are definitely fixed from the first. A precisely similar 
 assumption is also necessary in the case of embryogeny. 
 
 If, for instance, the sensory organs of the lateral line in a 
 fish or amphibian occur only along the lateral lines and their 
 branches, we must suppose that the subdivision of the idioplasm 
 of the ectodermic cells occurs during the development of the 
 epidermis in such a way that the cells containing the deter- 
 minants of these sensory organs come to be situated only along 
 the lateral lines, and only in definite places on these lines. If 
 now, all the formative cells of the sensory organs do not undergo 
 further development at once, but some of them, on the contrary, 
 remain undeveloped in the immediate neighbourhood i.e. in the 
 deep layer of young cells until a necessity for regeneration 
 arises, we can understand in principle why a similar topogra- 
 phical arrangement and numerical relation of the sensory organs 
 to the remaining epidermic elements occurs in the case of 
 regeneration, as well as in that of the primary formation of the 
 epidermis in the embryo. 
 
 The idioplasm of the cells does not alone decide what will 
 happen in regenerative processes of this kind. This is shown 
 by the fact that the occurrence of regenerative cell-multiplication 
 depends on a loss of substance, and that the cells cease to pro- 
 liferate as soon as the defect is made -good. The stimulus to 
 the further proliferation of the cells ceases at the same time. 
 These facts, however, only give us a very vague insight into 
 the causes of the limitation of the regenerative process ; and we 
 shall presently see that the above explanation is insufficient in 
 more complicated cases of regeneration, and that we must, 
 indeed, assume in addition the existence of other regulating 
 factors, which are situated within the active cells, and not 
 outside them.
 
 REGENERATION. 99 
 
 It is well known ihat the limbs of a salamander grow again 
 after they have been cut off, and we owe our accurate knowledge 
 of the regenerative processes concerned mainly to the researches 
 ofGotte*and Fraisse.t The investigations of these observers 
 show that the regeneration of the limbs and their forma' ion in 
 embryogeny take place in a similar manner: the individual parts 
 and segments of the extremity become developed in the same 
 order, and are formed of similar cell-material in each instance. 
 Both here and in the case of the epidermis described above, the 
 regeneration is paltngenettc. 
 
 If we take as our basis the law, which holds good at any rate 
 as regards Vertebrates, that in regeneration each specific tissue 
 can only reproduce its own specific cells, we can test the theory 
 of regeneration by taking as an example a single tissue of an 
 extremity. It is certain as regards the bones, for instance, that 
 regeneration always proceeds from the injured bone, or rather 
 from its periosteum. If the extremity is disarticulated from the 
 shoulder-girdle, for example, and the bones are uninjured, these 
 latter do not become re-formed. Although it cannot be denied 
 that the various tissues which are required for the regeneration of 
 the entire limb have an influence upon one another, especially 
 when pressure is exerted by one part on another situated near 
 to it, it is clear that the formation of new bones depends entirely 
 on the bones present in the stump of the amputated limb, which 
 not only determine the quality of the tissue, but also regulate 
 the size and shape of the bone which is to be formed anew. 
 These last-mentioned facts are the most important of all in 
 explaining the phenomenon of regeneration of a limb. From 
 what has already been said, it is evident that the bony tissue, 
 including the periosteum, can be formed from the cells of the 
 corresponding pre-existing parts. All that is necessary in order 
 that the process may take place is a supply of cell?, capable of 
 proliferation, which contain 'bone-idioplasm,' and which are 
 incited to multiply by the stimulus due to the injury in the 
 tissue surrounding them. The regeneration of the epidermis 
 may be explained in a similar manner. But as regards these 
 bones, it is not merely the production of bony tissue of a definite 
 structure .which has to be considered, but the formation oj 
 
 * Gotte, ' Uber Entwickelung u. Regeneration des Glicdmassen- 
 Skelelts der Molche,' Leipzig, 1879. 
 t Fraisse, loc. cit., on p. 97.
 
 100 THE GERM-PLASM. 
 
 (i definite number of bones of a definite shape and size, arranged 
 in a definite series, must also be taken into account. What 
 assumptions must we make in order to explain such an ac- 
 curately prescribed and complex mode of construction of 
 these parts? If the fore-limb of a newt (Triton} is cut off 
 between the shoulder and elbow, not only does the lost portion 
 of the humerus become formed afresh, but the radius and 
 ulna, and all the bones of the wrist and hand, are regenerated 
 accurately, even as regards the number of segments. It 
 seems hardly possible that so complex a structure could be 
 produced merely by the co-operation of proliferating cells, and 
 it might be supposed that an invisible power a spiritus rector 
 or a vis forjnativa must be present to direct their mode of 
 increase and arrangement. We are nevertheless probably right 
 in assuming that no such external direction takes place, and 
 that the complex structures in living beings are produced merely 
 by the agency of the forces which are present in the individual 
 cells. 
 
 We can understand these processes to some extent in the 
 case of embryogeny if we base our reasoning on the principle of 
 the gradual transformation of the idioplasm, which has already 
 been treated of in connection with ontogeny. This principle 
 may be roughly illustrated with respect to the skeleton of the 
 anterior extremity in the following manner. 
 
 When the fore-limb of a Triton begins to arise as a small 
 blunt elevation of the skin, it consists of cells of the external 
 and middle embryonic layers. The whole of the former, and 
 that portion of the latter which forms the cutis, may be left out 
 of consideration ; they together give rise to the integument. 
 The rest of the mesoderm now forms a mass of cells which have 
 not yet begun to undergo differentiation, and which individually 
 do not apparently differ essentially from one another. They 
 must, nevertheless, be very different as regards the primary 
 constituents which they contain, for some of them will sub- 
 sequently give rise, for instance, to muscles, others to connective 
 tissue or to blood-vessels, and others, again, to bones. These 
 cells, which are so differently predisposed, must therefore con- 
 tain various determinants, which, when they obtain control over 
 them in the course of further cell-divisions, impress on the sub- 
 sequent generations the character of muscle- or bone-cells. 
 Each of these kinds ot cells must be present from the first in a
 
 REGENERATION. IOI 
 
 perfectly definite number, and must occupy a perfectly definite 
 position. 
 
 Let us follow out this line of reasoning with regard to one 
 system of organs, namely, the bones, and assume for the sake of 
 simplicity that only a single bone-forming cell is present in the 
 first rudiment of the limb. This cell would virtually contain the 
 entire skeleton of the limb ; and we should have to attribute to 
 its idioplasm the power not only of giving the succeeding cells 
 of a certain number of generations the character of bone-forming 
 cells, but also of determining the entire sequence of these cells 
 as regards quantity, quality, and mutual arrangement, as well as 
 the rhythm in which the divisions will follow one another. For 
 the particular point at which an interruption occurs in the con- 
 tinuity of the bone, and consequently also the boundary line 
 between two segments of the bony chain, might essentially 
 depend, indeed, on this rhythm. 
 
 We must therefore suppose that the composition of the idio- 
 plasm of the first primordial bone-cell of the limb causes all 
 these sequences to take place : in other words, the idioplasm 
 must contain the determinants of all the succeeding bone-cells. 
 This may be illustrated by the following diagram (fig. 3), in 
 which the actual processes, which concern hundreds of thousands 
 of cells, are represented as greatly abbreviated, and the different 
 generations of cells are indicated arbitrarily by a genealogical 
 tree, which, however, does not by any means always represent 
 their actual connection. 
 
 Each primary cell of the individual bones is represented in 
 the figure by a circle, and is supposed to be so simple that it 
 can be controlled by one determinant. Thus the primary cell 
 of the entire series of bones is controlled by determinant I, but 
 also contains the determinants 2-35 in its ids. In the first cell- 
 division this cell divides into two, the primary cells of the upper 
 arm (humerus), and of the fore-arm and hand. The former 
 contains determinant 2, and its further division is indicated by 
 the cells containing determinants 2a-2x~. The latter contains 
 the remaining determinants 3-35, which become separated into 
 smaller and smaller groups in e.-.'.i cell-division, until finally 
 each cdl only contains a single determinant. The diagram only 
 represents the main bones of the extremity, the individual 
 carpals are omitted. 
 
 Let us now return to the question of regeneration. If each
 
 THE GERM-PLASM. 
 
 cell in the fully-formed bone only Contains that kind of idioplasm 
 which controls it, and which is therefore the molecular ex- 
 pression of its own particular nature, it would be impossible to 
 understand how the regeneration of the bone could be effected 
 when, for instance, it had been cut through longitudinally. 
 Supposing that a stimulus, produced by the injury, caused the 
 
 Manet, 
 
 FIG. 3. D 
 
 PludM 
 
 THE FORE-LlMI! OF 
 
 cells of the injured part to undergo multiplication : bony tissue 
 would then, indeed, be developed, but a bone of a definite shape 
 and size would not necessarily be formed. The formation of a de- 
 finite bone can only take place if the proliferating cells possess, in 
 addition to their active determinants, a supply of determinants 
 which control the missing parts which have to be renewed. If
 
 REGENERATION. 103 
 
 therefore we wish to suppose that Blumenbach's ' nisus joniui- 
 tivus' is situated in the idioplasm of the cell, it appears neces- 
 sary to assume that each cell capable of regeneration contains an 
 accessory idioplasm, consisting of the determinants of the parts 
 which can be regenerated by it, in addition to its primary idio- 
 plasm. Thus, for instance, the cells in the bone of the upper- 
 arm must contain, in addition to their controlling determinant 2, 
 the determinants 3-35 as accessory idioplasm, which can cause 
 all the bony parts of the fore-arm to be formed anew ; the 
 cells of the radius, again, must contain the determinants 4-20 
 as accessory idioplasm for the reconstruction of the radial 
 portion of the wrist and hand. 
 
 This theoretical illustration may be looked upon, indeed, as 
 representing the phenomena as they occur in reality. It is very 
 possible that the required accessory idioplasm becomes separated 
 from the disintegrating embryonic idioplasm in the earliest stage 
 of development of the entire organ. According to our assumption, 
 the individual determinants are present singly in the germ-plasm, 
 and their multiplication increases the further ontogeny advances. 
 As only those determinants which correspond to parts to be 
 formed subsequently are required in the accessory idioplasm, the 
 material for the latter is always present ; and we need only 
 assume that in each division of the primary cell of any bone, a 
 portion of the determinants required for the formation of the 
 subsequent parts becomes split off as secondary idioplasm, and 
 remains inactive within the cell until a cause for regeneration 
 arises. 
 
 I shall speak of this group of determinants as accessory idio- 
 plasm (' Neben-Idioplasma'), and its component determinants 
 as supplementary determinants (' Ersatz- Ueterminanten '). We 
 may imagine that these form a special and minute group en- 
 closed within the id in the neighbourhood of the determinants 
 which control the cell in question. A similar assumption may 
 be made as regards the individual bones of the entire limb. 
 The regeneration of the bisected humerus can be explained by 
 supposing that each cell capable of regeneration possesses an 
 accessory idioplasm, containing the determinants of the cells which 
 will subsequently be formed in a distal direction ; this formation 
 will be possible because the necessary 'determinant-material' is 
 present. The process only depends on the fact that in each 
 differential cell-division a certain number of determinants, which
 
 104 THE GERM-PLASM. 
 
 ripen later on, become split off from the rest, and are retained 
 in the cell as accessory idioplasm. The mechanism for regene- 
 ration is certainly a very complicated one, for each separate 
 bone is controlled by a number of different determinants, and 
 not by a single one ; and all these special determinants are 
 contained in the accessory idioplasm. As far as. we can judge 
 from the investigations made hitherto, the bones are at any rate 
 regenerated in detail fairly exactly. The complexity of the 
 mechanism accounts, in my opinion, for the fact that the fore- 
 limb, which has such a marked power of regeneration in the 
 salamander, has lost this power completely in the higher 
 Vertebrates, for in them the mechanism would have become too 
 complex. 
 
 A simpler mechanism than that which we have supposed to 
 exist can only be conceived, if, with Herbert Spencer,* we attri- 
 bute to each of the units composing the body the power of com- 
 bining to form any necessary organ just when it is wanted. We 
 might then compare the entire animal to a large crystal, in the 
 individual parts of which ' there dwells the intrinsic aptitude 
 to aggregate into the form of that species ; just as in the atoms 
 of a salt there dwells the intrinsic aptitude to crystallise in a 
 particular way.' The only difference between the particles of 
 the crystal and those of the organism would be that the 
 former are all permanently alike ; and that the latter, in order 
 that regeneration may be possible, are arranged in many differ- 
 ent ways, according to whether an entire limb, a tail, a gill, or 
 a single toe, fore-arm, or finger is to be replaced. How are the 
 ' units ' shown in each individual case what part is missing, and 
 what form their arrangement is to take in order to produce the 
 part anew ? We are thus once more brought back to Blumen- 
 bach's ' nisus formativus? Spencer himself says : ' If in the 
 case of the crystal we say that the whole aggregate exerts over 
 its parts a force which constrains the newly-integrated atoms to 
 take a certain definite form, we must, in the case of the organism, 
 assume an analogous force.' This force would correspond to what 
 was formerly spoken of as the ' spiritus rector' or ' nisns forma- 
 thmsf and even supposing it to exist, it does not in the least 
 help us in the attempt to explain the mechanism of the pheno- 
 mena. Spencer adds that his view ' in truth is not a hypothesis.' 
 
 * Herbert Spencer, ' The Principles of Biology,' Vol. i, \>. 181.
 
 REGENERATION. 105 
 
 but only ' a generalised expression of facts ; ' and remarks in 
 another passage, that although it is ' difficult ' to imagine regene- 
 ration as a sort of process of crystallisation, ' we see that it is so} 
 It is just this point that I must object to. We see that it is so, 
 or rather appears to be so, sometimes, but we also see that it is 
 often not so. If the units of the body were capable of becom- 
 ing modified at will under the influence of the whole, and of 
 crystallising into the missing part, they must be able to do so 
 in all species and in all organisms. This, however, is not the 
 case. The limb of a salamander can be regenerated, and that 
 of a lizard cannot. In a special section of this chapter I shall 
 be able to show in greater detail that regeneration depends 
 on special adaptation, and not on a general capacity of the 
 animal-body. 
 
 It will be unnecessary to give a special diagram illustrating 
 the regeneration of a single bone, such as that of the upper 
 arm, and showing the supplementary determinants of each of 
 the cells composing the bone which are necessary in order 
 that regeneration may set in at any point. The diagram given 
 for the entire limb is sufficient to make the general principle 
 clear an approach to an explanation of the actual details is 
 out of the question, as is evident if we compare the number of 
 cells given in the diagram with that of which the bones actually 
 consist. For this reason I have not attempted to enter into 
 minute histological details, or to define the quality of the cells 
 which are capable of regeneration, that is, to state whether 
 they belong to the periosteum or to the bone itself, and whether 
 all or only certain cells take part in the process. We only re- 
 quire a diagram which can be adapted to the actual details of 
 the processes when these are known. It is sufficient at present 
 to show that regeneration may be understood by considering 
 the activity of the cells themselves, without having recourse 
 to the assumption of an unknown directive agency. The 
 ' nisus formations ' descends from its previous position as a 
 single force directing the whole, and breaks up into an un- 
 limited number of material particles which are situated in the 
 individual cells, and each of which prescribes the course of life 
 of the cell. These particles arc determined as regards iheir 
 kind, and are distributed to their proper places so accurately, that 
 by their united effect they give rise to a composite whole, such 
 as, for instance, a series of bones, t-gether with their articular
 
 I06 THE GERM-PLASM. 
 
 capsules and ligaments, and the muscles, nerves, blood-vessels, 
 connective tissue, and integument which come into relation with 
 them. The diagram I have given to illustrate the regeneration 
 of a bone can obviously be adapted to represent any other part 
 or tissue. We must not look upon the bone as something quite 
 disconnected from the rest of the limb, as we may very likely be 
 inclined to do if we are specialists. The bone is in reality con- 
 nected most intimately along its entire surface with the surround- 
 ing tissues, the periosteum and loose connective tissue external 
 to the latter, the numerous blood-vessels which penetrate into 
 the substance of the bone, the nerves, and so on. The first 
 rudiment of the limb consists, in fact, of a mass of mesodermic 
 cells, which give no indication of the various structures which 
 will later be developed from them. Nevertheless, their differen- 
 tiation does not, in my opinion, depend on their accidental posi- 
 tion within the limb, or in fact on any other external influences, 
 but is primarily due to their individual nature, that is, to the 
 constittdion of their idioplasm. The determinants composing 
 the id control the subsequent development of the cell and of its 
 successors. The further changes which the id undergoes in the 
 course of cell-division, and the manner in which the deter- 
 minants undergo disintegration in the ids of the daughter-cells 
 of all the subsequent generations, is decided by the composition 
 of the id. 
 
 We can thus understand, at least to some extent, how it is 
 possible that such a complicated part as a limb, the structure of 
 which is so accurately prescribed, can arise by degrees from a 
 mass of cells which are apparently all similar to one another. 
 The harmony of the whole is primarily brought about by the 
 variation and increase of the cells, the kind and rhythm of 
 which respectively, is prescribed by the idioplasm of each in- 
 dividual cell, rather than by the mutual influence of the cells 
 during their gradual differentiation. A muscle becomes de- 
 veloped at any definite spot, because one particular cell amongst 
 all the apparently similar cells in the first rudiment of the limb 
 contained the determinants which are capable of giving to a 
 large number of the successors of this cell the special character 
 of muscle-cells ; and because, again, the id of this particular 
 cell caused a rhythm of multiplication to set in, which, on 
 mechanical grounds, rendered it necessary that certain succes- 
 sors of this cell which contained muscle-determinants should
 
 REGENERATION. 1 07 
 
 take up their position in the precise region of the limb in which 
 this particular muscle is situated. 
 
 We must not, however, imply from what has been said above, 
 that external influences are of no importance whatever in onto- 
 geny, but merely that they certainly only play a secondary part 
 in the process. A limb will certainly grow crooked if a corres- 
 ponding external pressure is brought to bear upon it. Growing 
 cells do not cease to multiply directly they are subjected to 
 abnormal external influences, for they can accommodate them- 
 selves to circumstances. It is such cases as the regeneration of 
 broken bones and the formation of new joints under abnormal 
 external conditions, which prove that the cells continue to 
 perform their functions of growing and of giving rise to organs 
 under circumstances which deviate very markedly from the 
 normal. These false joints also show what a considerable 
 power of adaptation is possessed by the cells, and how efficient 
 may be the parts which these cells are able to produce under 
 abnormal conditions. But although the principle formulated 
 by Roux * of the struggle of the parts, or as it might well be 
 called ' intrabiontic selection] is certainly a very important one, 
 I think it would be a great mistake to refer the normal process 
 of ontogeny mainly to this principle. The groups and masses 
 of cells must certainly press upon one another during the process 
 of differentiation : in the process of the formation of a joint, for 
 instance, proliferating connective-tissue cells do actually force 
 themselves amongst the cartilage cells in one part of the rudi- 
 mentary bone, in order to separate them from one another. But 
 this proliferation and pressure are taken account of, just as 
 much as are the processes of dissolution or absorbtion that 
 occur in those cells in the primordial cartilage which are situ- 
 ated in the region of the joint. It might be supposed that 
 the existence of so-called ' identical ' human twins contradict 
 my conception of ontogeny ; for although they are undoubtedly 
 derived from a single ovum and sperm-cell, and hence possess 
 the same kind of germ-plasm, they are never really identical, 
 but only very similar to one another. But apart from the fact 
 that the absolute identity of the germ-plasm has not been proved 
 in these cases, the very close resemblance between these twins 
 shows how slightly the diversity of external influences affects 
 
 * W. Roux, ' iVr Kr\m;if d.-r Theilc im Organismus,' Leipzig, 1881.
 
 I08 THE GERM-PLASM. 
 
 the development of an organism. How wonderfully accurately 
 the course of ontogeny must be prescribed, if it can be kept to 
 so closely, through thousands of generations of cells, that 
 ' identical ' twins result ! We may compare the process of 
 development of such twins with the course taken by two ships, 
 which, starting from the same place, proceed along the same 
 devious route which has been carefully mapped out beforehand 
 in all its thousands of definite changes in direction, until each 
 finally reaches the same distant shore independently, within a 
 mile of the other. 
 
 A careful consideration of such a case as this leaves no doubt 
 that a very exact and definite course is mapped out for the egg- 
 cell by its idioplasm, which, again, directs the special course 
 to be taken by each of the innumerable generations of cells, 
 in the direction of which course external influences can only 
 play a very subordinate part. If this consideration be borne in 
 mind, it will be less likely that the objection may be made that 
 a much too complicated structure has been attributed to the 
 idioplasm. Its structure must be far more complex than we can 
 Possibly imagine; and in this respect, its construction, as we have 
 represented it theoretically, must certainly be far simpler than is 
 the case in reality. For the same reason, it is less probable that 
 similar objections may be made to the theory of regeneration as 
 here stated. Complicated phenomena cannot possibly depend on 
 a simple mechanism. The machines in a cotton factory cannot 
 be constructed of a few simple levers, nor can a phonograph 
 be manufactured from two lucifer matches. 
 
 That form of regeneration which has been considered above 
 may be described as palingenetic, for it pursues the course 
 taken by the primary or embryonic development ; but as soon 
 as it leaves this course and takes a shorter one, it may be 
 distinguished as ccenogenetic. 
 
 Ccenogenetic variations of the primary process of development 
 probably always occur in cases of regeneration of complex 
 structures ; and even the reconstruction of the extremities, 
 which we have chosen above as an example, will hardly take 
 place in exactly the same way as occurs in the primary develop- 
 ment of these parts, although it may resemble the latter in its 
 principal phases. 
 
 Even if mere abbreviation of the development of a part can 
 be easily conceived by supposing an aggregation and redistribu-
 
 REGENERATION. IOQ 
 
 tion of the determinants to occur in the idioplasm, the process 
 of idic division becomes very complicated when the primary 
 and secondary development take place along different lines ; 
 for in the latter process the combinations of supplementary 
 determinants in the id of the cell-generations must be different 
 from those which occur in the former. But this difference is 
 evidently due merely to a greater complication of the process, 
 and it does not stand in the way of the theory. In all cases of 
 regeneration, the mode in which the supplementary determin- 
 ants become split off must be previously arranged for in the id. 
 The assumption of a mere increase in the power of multiplica- 
 tion of certain determinants might seem sufficient in the case 
 of palingenetic regeneration, for this would lead to one portion 
 of a certain group of determinants becoming separated off as 
 accessory idioplasm at a particular ontogenetic stage. In 
 cctnogenetic regeneration, however, we can only assume that a 
 double or still greater number of determinants are present in the 
 germ-plasm, one set of which are destined for embryonic deve- 
 lopment and the others for regeneration ; and these are previ- 
 ously arranged with reference to their internal forces, particularly 
 that of multiplication, so that at a certain stage of development 
 they become split off as ' accessory idioplasm,' either alone or 
 together with the adjacent ' regenerative determinants.' 
 
 It seems to me, however, that palingenetic regeneration 
 cannot be satisfactorily accounted for unless we assume the 
 existence of special regenerative determinants, for it would 
 otherwise be impossible to explain the phyletic origin of the 
 crcnogenetic variations in the process of regeneration. These 
 latter must, indeed, depend on variations in a determinant of the 
 germ-plasm. If however the latter contained only the one 
 determinant destined for embryogeny, variations must occur in 
 the latter process at the same time. But this is not the case, 
 and consequently a kind of double determinant must be con- 
 tained in the germ for those hereditary parts (determinants) 
 which are capable of becoming regenerated : that is to say, two 
 originally identical determinants must be present, one of which 
 becomes functional in embryogeny and the other in regenera- 
 tion. This will be made apparent if we take some examples. 
 
 In most existing amphibians the caudal region of the vertebral 
 column may undergo regeneration, although its embryonic 
 foundation, the notochord, is never formed anew. The cartila-
 
 110 THE GERM-PLASM. 
 
 ginous sheath of the notochord has an important share in the 
 primary formation of the vertebral column, but it disappears 
 to a greater or less extent at a later stage. If it became pos- 
 sible for the vertebras to undergo regeneration after a portion 
 of the tail had been lost without a renewal of the notochord 
 taking place, the result would be a useful abbreviation of the 
 process of regeneration. Such an abbreviation has occurred, 
 and everything supports the assumption that at an earlier stage 
 of phyletic development the notochord was capable of undergoing 
 regeneration, and that it has only lost this capacity secondarily. 
 In the case of frog-tadpoles, the power has been retained of 
 regenerating the tail when it is cut oft together with the notochord. 
 We must not assume that the notochord does not become 
 restored in other amphibians because it no longer persists in the 
 full-grown animal ; for it is entirely absent only in a few of 
 them (e.g., Salmandrina), and the notochord of the larval sala- 
 mander cannot be regenerated any more than that of the adult. 
 Thus the capacity for regenerating the notochord has been lost 
 by most amphibians in the course of phylogeny. Such a process 
 of degeneration is certainly to be explained most easily by 
 assuming the existence of special regenerative determinants, 
 which may gradually disappear without in the least affecting 
 their embryogenetic partners. 
 
 The necessity of this assumption is shown still more conclu- 
 sively in the case, for instance, of the restoration of the solid 
 axis of the tail in reptiles. The tail of a lizard quickly becomes 
 restored after it has been cut off, but its structure is then different 
 from that of the original tail ; for, according to the statements 
 of Leydig and Fraisse, the spinal cord and vertebral column 
 are not renewed. The former is, however, represented by an 
 epithelial tube, but gives off no nerves ; and the latter is replaced 
 by an unsegmented cartilaginous tube. As Fraisse points out, 
 this tube does not correspond to the regenerated notochord, but 
 is a new structure which is substituted for it. 
 
 A phyletic development, tending essentially towards a sim- 
 plification of the parts, has taken place in this case as 
 regards the processes oj regeneration. A gradual degeneration 
 has occurred, just as may take place in the tail or any other 
 organ of an animal in the course of phylogeny. The caudal 
 region of the vertebral column has undergone a reduction, 
 which does not influence its primary (embryonic) ontogeny,
 
 REGENERATION. Ill 
 
 but only its secondary formation by regeneration. A vertebral 
 column is formed primarily ; but if the re-formation of a part of 
 it becomes necessary, in consequence of the loss of the tail, the 
 secondary reduced process for the development of the axis 
 comes into play, and a simple cartilaginous tube is formed. 
 This process recalls the phenomena of 'dichogeny' which take 
 place so frequently in plants, and in which the same group of 
 cells may develop in either of two different ways, according to 
 the nature of the external stimulus which is applied to them. 
 Thus a shoot of ivy will produce roots on a certain side if it is 
 shaded, and leaves if it is exposed to light. The determination 
 of the sex of an animal may perhaps be referred to similar 
 causes, if, at least, we may assume that the sex is not always 
 universally decided by the act of fertilisation, and that influences 
 exerted upon the organism subsequently may have an effect 
 in this determination. In the case of certain parasitic Crus- 
 taceans, the CymathoidcE, the male sexual organs are developed 
 first ; and when the animal has fulfilled its function as a male, 
 the female organs become developed, and give the animal the 
 character of a female. The two developmental tendencies here 
 come into operation temporarily, one after the other ; just as in 
 the case of the lizard's tail, in which the tendency to form the 
 vertebral column first comes into play, and then that to form 
 the secondary cartilaginous tube. The necessity for the forma- 
 tion of this tube certainly need not arise at all ; just as that 
 side of the shoot of ivy from which the roots arise need not 
 necessarily be subsequently exposed to the light, and give rise 
 to leaves ; the possibility of such an occurrence is, however, 
 foreseen by Nature. It might be urged that there is an im- 
 portant difference between the regeneration of a lizard's tail 
 and the successive development of the two kinds of sexual 
 organs in the Cymathoids, since in the latter case the rudiments 
 of these organs are present in the embryo, and it is only 
 their final development which takes place successively. This 
 is certainly a difference, but it is just such a one as to in- 
 dicate to us how these cases of supplementary substitution 
 may be explained theoretically. The cells in the tail of a lizard 
 which give rise to the secondary cartilaginous tube must con- 
 tain determinants which differ from those of the embryonic for- 
 mative cells of the caudal vertebras, just as the idioplasm of 
 the formative cells must contain different determinants for the
 
 112 THE GERM-PLASM. 
 
 testes and ovaries. The supplementary determinants -with which 
 the idioplasm of certain cells of the vertebral column was pro- 
 vided for the purposes of regeneration, must have become changed 
 in the course of phylogeny. 
 
 A transmissible variation of this kind must, however, also have 
 had some effect on embryogeny, if only one and the same deter- 
 minant were present in the germ-plasm for the two modes of 
 development. Hence each determinant of these caudal vertebra 
 must be doubled in the germ-plasm. 
 
 It would be premature to go beyond this assumption, and to 
 attempt to decide anything about the manner in which the 
 various supplementary determinants which are required for the 
 restoration of one of the larger parts such as, for instance, the 
 caudal vertebra? come together, and how and when they be- 
 come separated from the primary determinants. The processes 
 of regeneration have not as yet been examined from the point 
 of view which I have here suggested ; and in many cases it is not 
 even known for certain from what cells regeneration proceeds. 
 
 Hitherto we have not discussed in detail the question as to the 
 kind of cells which contain the supplementary determinants, and 
 from which regeneration thus takes place. May these deter- 
 minants be present in any kind of cell belonging to any tissue, 
 or is their distribution always limited to young and apparently 
 undifferentiated cells of the so-called 'embryonic type'? 
 
 If we only consider Man and the higher Vertebrates, we shall 
 be disposed to look upon the latter of these two alternatives as 
 the one which is in general correct. Even recently, in fact, 
 many authors seemed to be in favour of this view : ' embryonic 
 cells ' were supposed to be contained in all those tissues which 
 are capable of regeneration, and it was, indeed, believed by 
 many that the leucocytes are cells of this nature. The latest 
 investigations, however, lead us to the conclusion that this is 
 not the case, and that although the white blood corpuscles are 
 extremely important as conveyers of nutriment in the process of 
 regeneration, they do not serve as formative elements in the 
 construction of a tissue. In his text-book on Pathological 
 Anatomy, Ziegler speaks of a formal ' law of the specific 
 character of the tissues,' which he explains as follows: 'the 
 successors of the various germinal layers which separate from 
 one another at an early embryonic stage, can only give rise to 
 those tissues which belong to the germinal layer from which
 
 REGENERATION. 113 
 
 they were developed." But this statement can only be true in the 
 case of the highest Vertebrates, for, as the brothers Hertwig 
 have shown, the germinal layers of the Metazoa are not primi- 
 tive organs in the histological sense ; and moreover, in the lower 
 animals, several, if not all of the tissues, can be formed from 
 each of the germinal layers. In lower animals, not only all the 
 varieties of tissue, but under certain circumstances even rows of 
 cells of one primary germinal layer, and even indeed the entire 
 animal, may arise from young cells belonging to the other 
 germinal layer. In the chapters on multiplication by fission 
 and gemmation, this process will be traced to its origin in the 
 idioplasm. At present we have only to deal with the question 
 as to whether the determinants of the various kinds of cells 
 which are required for regeneration are contained within young 
 cells only, or whether they are also present in those which have 
 become differentiated histologically. 
 
 Although the supplementary determinants are certainly in 
 many cases contained in young cells without any specially 
 marked histological character, their distribution can neverthe- 
 less hardly be limited to these cells exclusively. It may happen 
 as will be shown in greater detail subsequently that cells, which 
 are fully developed histologically, both in plants and in the 
 lower animals, contain all the determinants of the species ; 
 that is to say, they may contain germ-plasm as supplementary 
 idioplasm. Hence there is no reason to assume that smaller 
 groups of determinants may not have been supplied to specific 
 tissue cells wherever they were required, although I am unable 
 to give a definite example of such a case. 
 
 Although regeneration may originate in most cases in young, 
 or so-called ' embryonic ' cells, it is nevertheless quite a mistake 
 to connect the idea of the undifferentiated state of these cells 
 with this fact, as is so often done. These ' embryonic cells ' are 
 not ' capable of giving rise to anything and everything,' for each 
 of them can only develop into that kind of cell the determinant 
 of which it contains. Under certain circumstances such a cell 
 may contain several different determinants at the same time, 
 which are only distributed amongst the individual cells in sub- 
 sequent cell-generations ; but the structure which can and will 
 become developed from it always depends on the cell itself, and 
 its fate is determined by the idioplasm it contains, and can only 
 be affected secondarily by external influences. Cells moreover 
 H
 
 114 THE GERM PLASM. 
 
 exist, the idioplasm of which permanently retains the possi- 
 bility of development along one of two lines. 'Dichogeny' in 
 plants, which has already been mentioned, is likewise deter- 
 mined by the idioplasm, inasmuch as the latter must contain 
 two kinds of determinants, one or the other of which either 
 remains inactive owing to the nature of the external influences 
 acting upon the cell, or else becomes active and determines the 
 cell. 
 
 There are, however, no such things as 'embryonic cells ' in, 
 the sense in which this term is used by authors. In the fresh- 
 water polype (Hydra\ for instance, cells which are young 
 and histologically undifferentiated the so-called 'interstitial 
 cells ' are present in the deeper part of the ectoderm : these 
 can certainly give rise to various structures, viz., to ordinary 
 ectoderm-cells, nettle-cells, muscle-cells, sexual-cells, and in all 
 probability to nerve-cells also. It would nevertheless be absurd 
 to suppose that any particular interstitial cell is capable of 
 developing into any one of these structures. It obviously con- 
 tains either germ-plasm, i.e., the whole of the determinants, in 
 which case it can develop into a sexual cell, or only the deter- 
 minants of a thread cell or of one of the other kinds of cells, and 
 then it can only give rise to one of the corresponding structures, 
 and can never develop into a sexual cell. 
 
 2. THE PHYLOGENY OF REGENERATION. 
 
 It may, I believe, be deduced with certainty from those phe- 
 nomena of regeneration with which we are acquainted, that the 
 capacity for regeneration is not a primary quality of the organism, 
 but that it is a phenomenon of adaptation. 
 
 The power of regeneration has hitherto been practically always 
 regarded as a primary quality of the organism, that is to say, 
 as a direct result of its organisation : it has been looked upon 
 as a faculty for which no special arrangements are required, 
 but which naturally results as an unintentional secondary effect 
 of the organisation which exists independently of it. 
 
 This view is based on the idea, which is in general a correct 
 one, that the regenerative power of an animal is inversely 
 proportional to its degree of organisation.* If this were univer- 
 
 * Cf. Herbert Spencer (loc. cit., p. 175), who, however, expresses him- 
 self very cautiously with regard to this difficult subject as follows:
 
 REGENERATION. 115 
 
 sally true, it would nevertheless not be a convincing Argument 
 for the above view, although it would certainly support it. But 
 a closer examination into the facts shows that this statement is 
 not absolutely correct. Although the capacity for regeneration 
 is never so far-reaching in the highest animals as it is in the 
 case of the lower ones, and this must be due to some cause, 
 the regenerative power may nevertheless even vary widely in 
 animals of the same degree of organisation, and may in fact 
 be far greater in one of the higher than in one of the lower 
 forms. Thus fishes are unable to regenerate a lost pectoral or 
 pelvic fin, while the much more highly organised salamander 
 has been known to regenerate a limb six times in succession 
 (Spallanzani). 
 
 The regenerative power often varies in degree even within 
 the same group of animals. In Triton and Salamandra the 
 entire limb grows again after amputation, but apparently, so far 
 as I have been able to observe, this does not occur in Proteus. 
 The tail, too, is only replaced slowly and imperfectly in the latter 
 animal, whereas it easily becomes restored in the salamander. 
 In the year 1878 I received a living Siren lacertina^ the fore-limb 
 of which had been torn off, so that only the stump of the upper 
 arm remained, and the entire limb did not grow again in the 
 course of the ten years during which I kept the voracious 
 animal, and gave it abundant food. In this case again the 
 power of regenerating the extremities seems to be less than in 
 that of salamanders, which are much younger phyletically, and 
 much more highly organised. 
 
 It is well known also that the limbs of a frog do not grow 
 again when they have been cut off, even when the animal is in 
 the larval condition. The fact that the regenerative power can 
 vary so considerably within the same genus is still more re- 
 markable. Schreiber observed that the power of regeneration 
 in Triton marmoratus is relatively very slight as compared with 
 that which is possessed by all other species of Triton which 
 have been examined for this purpose. ' In captivity, at any rate, 
 even slight injuries in such parts as the crest are never re- 
 
 ' so that the power of reproducing lost parts is greatest where the 
 organisation is lowest, and almost disappears where the organisation is 
 highest. And though we cannot say that between these extremes there is 
 a constant inverse relation between reparative power and degree of orga- 
 nisation, yet we may co ' that there is some approach to such a relation,"
 
 Il6 THE GERM-PLASM. 
 
 placed, while the animal invariably succumbs to greater injuries.' 
 Fraisse gives similar instances. Thus ' an amputated extremity 
 never grew again to its normal size : merely a somewhat 
 deformed protuberance was formed on the stump. The tail also 
 was only reproduced to a very slight extent.' * 
 
 With regard to reptiles, Fraisse points out that the regenera- 
 tive capacity obtains to a much slighter extent in some groups 
 than in others. Chelonians, crocodiles, and snakes are unable 
 to regenerate lost parts to any extent, while lizards and geckos 
 possess this capacity in a high degree. 
 
 The dissimilarity, moreover, as regards the power of regene- 
 ration which exists in -various members of the same species, also 
 indicates that adaptation is an important factor in this process. 
 In Proteus, which in other respects possesses so slight a capa- 
 city for regeneration, the gills grow again rapidly when they 
 have been cut off. In lizards, again, this power is confined to 
 the tail, and the limbs cannot become restored : in these ani- 
 mals, however, the tail is obviously far more likely to become 
 mutilated than are the limbs, which as a matter of fact are 
 seldom lost, although individuals with stumps of limbs are occa- 
 sionally met with. The physiological importance of the tail of a 
 lizard consists in the fact that it preserves the animal from total 
 destruction ; for pursuers will generally aim at the long trail- 
 ing tail, and thus the animal often escapes, as the tail breaks 
 off when it is firmly seized. It is, in fact, as Leydig was the first 
 to point out, specially adapted for breaking off, the bodies of the 
 caudal vertebrae from the seventh onward being provided with 
 a special plane of fracture, so that they easily break into two 
 transversely. Now if this capability of fracture is provided for 
 by a special arrangement and modification of the parts of the tail, 
 we shall not be making too daring an inference if we regard the 
 regenerative power of the tail as a special adaptation, produced by 
 selection, of this particular part of the body, the frequent loss of 
 which is in a certain measure provided for, and not as the out- 
 come of an unknown 'regenerative power' possessed by the 
 entire animal. This arrangement would not have been pro- 
 vided if the part had been of no, or only of slight, physiological 
 importance, as is the case in snakes and chelonians, although 
 these animals are as highly organised as lizards. The reason 
 
 * Loc. cit., p. 1152.
 
 REGENERATION. 117 
 
 that the limbs of lizards are not replaced is, I believe, due to 
 the fact that these animals are seldom seized by the leg, owing 
 to their extremely rapid movements. But if a lizard does 
 happen to be caught by one of its limbs, it must fall a prey to 
 its pursuer, and the capacity for regenerating the limb would 
 be useless. The case is very different with regard to such ani- 
 mals as Tritons. Their movements are much less rapid, and 
 their assailants, being too small to swallow the whole animal, 
 frequently bite off a limb. They are often attacked by members 
 of the same species, which gnaw off a gill, limb, or the tail of a 
 weaker comrade, bit by bit. If a considerable power of regene- 
 ration were possible at all, it would certainly be provided in this 
 case. This power is possessed in a much smaller degree by 
 Proteus ; but these animals are only found in the caves of 
 Carniola, where enemies larger than themselves do not exist, 
 and in which there is no great competition for food, and there- 
 fore, at least as far as my observations extend, they do not bite 
 one another. 
 
 Spallanzani has stated that nature doesnotreproduceeverypart 
 that is cut off; expressed in theoretical terms, this simply means 
 that the various organs of an animal possess the power of regen- 
 eration in different degrees. If we inquire further into the question, 
 we shall find that those parts which are most frequentlyexposed to 
 injury or loss must possess the power of regeneration in the highest 
 degree. So far as I can judge from the facts with which we are 
 at present acquainted, this remark appears to me to be a per- 
 fectly correct one. Unfortunately Spallanzani gives no instance 
 in support of the above statement, so that we do not know what 
 parts he referred to. I have myself, however, made some investi- 
 gations in order to ascertain whether the degree of regeneration 
 of a part bears any relation to its liability to injury. 
 
 If regeneration is a phenomenon of adaptation, the internal 
 organs which are not exposed to injury in the natural life of 
 the animal cannot possess any regenerative power, even in 
 those animals in which the external parts which are exposed 
 to the attacks of enemies possess it in a high degree. 
 
 The following experiment bears upon this point : I cut open 
 a large newt (Triton cristattis\ removed about half of the right 
 lung, and sewed the skin together again. The animal soon 
 recovered from the effects of the chloroform, and its wounds 
 healed : it was then well cared for for fourteen months, and
 
 Il8 THE GERM-PLASM. 
 
 afterwards killed. An examination showed that the right lung 
 had not become restored : it was only half as long as the left one, 
 and its end was blunt, and not pointed as in the normal lung. 
 Four other similar experiments yielded like results : in one of 
 these it was doubtful whether a growth of the lung had not taken 
 place, but even in this case it had not recovered its long, pointed 
 form. 
 
 These experiments are still being continued, but we may 
 already deduce from them that a striking disproportion exists 
 between the regenerative power of the external parts of a newt 
 and that of its lungs. This difference seems even more marked 
 if we bear in mind that in the case of a limb the process of 
 regeneration is a very complex one, for complicated parts, con- 
 sisting of many entirely different portions, have to be reproduced ; 
 whereas a lung is a simple hollow sac, which has no joints, and 
 the histological structure of which is relatively simple. 
 
 We therefore infer that the internal parts, which are not ex- 
 posed to injuries of an ordinary kind, do not possess a greater 
 capacity for regeneration in these species than they do in the 
 highest Vertebrates, which are so exceptionally inferior to them 
 as regards the regenerative power of the external parts. Hence 
 there is no such thing as a general power of regeneration : in each 
 kind of animal this power is graduated according to the need of 
 regeneration in the part under consideration : that is to say, die 
 degree in which it is present is mainly in proportion to the 
 liability of the part to injury. 
 
 This conclusion is closely connected with the fact that the re- 
 storation of a part which possesses the power of regeneration in a 
 high degree, can only take place as the result of definite injuries 
 which are in a manner provided for, and not from any kind of 
 injury. Philippeaux was the first to discover that the limb of a 
 Triton does not grow again when it has been removed at the 
 joint, and that, in fact, it only does so when it is cut or torn off, 
 so that the bone is injured. This fact has been explained by 
 referring it to the law of the specific nature of the tissues, accord- 
 ing to which bone can only be formed from bone, and the 
 bone of the limb must be injured before it can become capable 
 of being formed anew. It seems to me that this explanation is 
 insufficient, although it is founded on a correct principle, accord- 
 ing to which the injury to the bone causes the stimulus by which 
 the cells of the stump are incited to proliferate. This is certainly
 
 REGENERATION. 1 19 
 
 corrc t, and may be expressed according to our theory by say- 
 ing that the supplementary determinants which are present in a 
 passive condition in the cells, are prompted to become active by 
 the stimulus. But if an articular cavity is exposed, a stimulus 
 is likewise produced, which must affect the cells of the articular 
 cartilage, and doubtless also those of the underlying bone or 
 periosteum. If, therefore, all the cells in this region were 
 capable of reproducing the missing bones, and if the exposure 
 of the articulation were the ordinary form of injury, these cells 
 would certainly be just as much adapted for and capable of 
 responding to this stimulus, by a formative growth, as would 
 those situated at the broken ends of a bone. But the disarticu- 
 lation of a limb, or of apart of a limb, hardly ever takes place 
 in the natural conditions of life, and therefore could not have been 
 provided for by the organism ; the respective cells of the exposed 
 articular cavity could not consequently have been supplied with 
 the supplementary determinants necessary for regeneration. 
 Hence these cells are incapable of reacting in an adequate 
 manner to the stimulus due to the disarticulation. 
 
 In spite of all the facts already mentioned, it might still 
 appear doubtful whether regeneration really depends on a special 
 adaptation of the part in question, and whether it does not result 
 from the degree of organisation of the animal, or at any rate from 
 ^general regenerative force possessed by the entire organism. 
 The following considerations must, however, I think, set aside 
 all doubts on the question. Physiological and pathological 
 regeneration obviously depend on the same causes, and often 
 pass one into the other, so that no real line of demarcation 
 can be drawn between them. We nevertheless find that in 
 those animals in which the power of regeneration is extremely 
 great physiologically, it is very slight pathologically. This 
 proves that a slight power of pathological regeneration cannot 
 possibly depend on a general regenerative force present within 
 the organism, but rather that this power can be provided in 
 those parts of the body which require a continual or periodic 
 regeneration : in other words, the regenerative power of a fart 
 depends on adaptation. Let us take a few examples. It was 
 mentioned above that fishes are said to possess a very slight 
 ' general regenerative power,' because they are unable to replace 
 lost external parts, especially such structures as fins. Neverthe- 
 less many fishes arc provided with teeth which are very liable to
 
 120 THE GERM-PLASM. 
 
 become worn out, and consequently they possess the power of 
 constantly producing new teeth to replace the old ones. In the 
 mouth of a ray or dog-fish the teeth are arranged in several 
 rows along the edges of the jaws, the outer rows containing 
 those which are worn out, and the inner the younger teeth 
 which take their place. Birds, again, possess a very slight power 
 of repairing defects which have arisen accidentally, and hence 
 they are considered to have a very slight capacity for regenera- 
 tion. But their power of physiological regeneration with re- 
 spect to certain parts is nevertheless extraordinarily great : all 
 the feathers are cast off and renewed once a year. Pathological 
 regeneration occurs to a very slight extent in mammals ; defects 
 in the superficial epithelium, the epithelium of the ducts of 
 glands, the various supporting tissues, including bone, and in 
 nerve-fibres, can be repaired from the elements of the respective 
 tissues ; but in no mammal does a segment of a finger or an 
 eyelid grow again when once it has been cut off. In certain 
 mammals, however, the power of physiological regeneration 
 with respect to certain parts is unusually marked. Male stags 
 shed their antlers annually, and new ones are formed in four or 
 five months. If we take into consideration the mass of organic 
 tissue which is thus formed in such a short time, this feat out- 
 strips even the regenerative performances of the full-grown 
 salamander. For according to Spallanzani, it takes a salaman- 
 der more than a year to restore an amputated limb to its normal 
 size and strength. Young individuals can, however, certainly 
 reproduce a limb in a few days ; and this gifted experimenter 
 observed in the case of a young Triton that the four limbs and 
 tail when they were cut off grew again six times in the space of 
 three summer months ! 
 
 In one respect, however, viz., as regards the complexity of the 
 part replaced, this remarkable regenerative power in stags and 
 birds is far inferior to that which obtains in the Triton. 
 Although a bird s feather is a very wonderful structure, it is formed 
 merely from epidermic cells, and a stag's antler is only a der- 
 mal bone covered over by the epidermis. But the limb of a 
 Triton, on the other hand, consists of every kind of tissue with 
 the exception of endodermal epithelium, viz., of skin, muscles, 
 a large number of skeletal parts, connective tissue, blood- 
 vessels, nerves, and so forth ; and all these have a very definite 
 arrangement, number, and form. There is no doubt therefore
 
 REGENERATION. 121 
 
 that the regeneration of a limb is a greater feat than the renewal 
 of feathers or antlers ; and the fact has been long recognised, 
 that the more complex organs are regenerated less easily than 
 those which have a simpler structure. A series of carefully 
 performed experiments, made with the view of testing this 
 somewhat vague statement, would be of great value theoreti- 
 cally. We may predict that in one sense it would be confirmed, 
 and that we should find that under similar conditions the 
 simpler organs are on the whole regenerated much more easily 
 than the more complex ones in any particular species. Even 
 in the human race, many simple tissues such as the connective 
 substances, epithelia and nerves can be repaired, and it is 
 only the cells of the glands and ganglia, which are the most 
 highly differentiated histologically, which are not replaced at 
 all, or at most only to a very slight extent. We can see from a 
 theoretical point of view that a far less complex apparatus is 
 required in these cases than in those which concern a regenera- 
 tion of entire parts of the body, such as the tail or limbs ; for it 
 is only necessary that the respective tissues should contain cells 
 which are capable of multiplying, in response to the stimulus 
 produced by the loss of substance in their immediate neighbour- 
 hood, and which continue to do so until the loss is made good. 
 When, however, several kinds of cells take part in the restora- 
 tion, and a strict regulation as to their arrangement in groups, 
 their direction of growth, and rate of reproduction is required, 
 it becomes necessary for the individual cells from which the 
 restoration takes place to be accurately provided with supple- 
 mentary determinants of various kinds ; and it is clear that this 
 will gradually become more difficult and complex, the greater 
 the complexity of the part to be regenerated, and the more 
 accurately all the details of its structure have to be preserved. 
 
 If, however, we review the facts known to us concerning 
 regeneration in animals of various degrees of organisation, we 
 meet with such marked differences even as regards the regenera- 
 tive power of homologous parts, that we cannot help receiving 
 the impression, which has affected all writers on this subject, 
 that in general the regenerative power is greater in less highly 
 organised animals than in those of a more complex structure. 
 The question thus arises as to how this view is to be interpreted 
 and presented in a scientific form. 
 
 Even in Vertebrates, certain facts seem to indicate that the
 
 122 THE GERM-PLASM. 
 
 'lower' forms, as such, always possess the power of replacing 
 lost parts in a greater degree than do the higher ones. 1 1 is true 
 that the capacity for regeneration is certainly much slighter in 
 fishes than in the more highly organised amphibians ; but 
 although the limb of a Triton becomes restored, and the fin of 
 a fish does not, it must not be forgotten that the physiological 
 importance of the two organs is somewhat unequal. On the 
 other hand, the fore-limb of a Triton and the arm of a man are 
 not only homologous structures, but are also of almost equal 
 physiological importance, and yet their power of regeneration is 
 very unequal. We must therefore inquire into the causes of this 
 dissimilarity. The power of regeneration in any particular part 
 cannot depend only on the conditions which exist as regards the 
 species under consideration : it must also be due to arrangements 
 fojcj-egeneration which have been transmitted by the series of 
 ancestors of this species. Leaving this question aside, and re- 
 garding the power of regeneration as merely depending in each 
 individual case on adaptation, we should arrive at some such 
 conclusion as the following : the provision of the cells of a 
 certain part with supplementary determinants for the purposes 
 of regeneration, depends primarily on the liability of this part to 
 frequent injury that is to say, on the degree of probability of 
 injury. Precautions are not taken for injuries which seldom 
 occur, even though these may be very disadvantageous to the 
 individual ; for the loss thereby resulting to the species as regards 
 the number of individuals would be extremely small and unim- 
 portant, and therefore processes of selection would not take 
 place in order to counterbalance this loss. 
 
 In the second place, the physiological or biological importance 
 of the organ itself must be taken into consideration. A useless 
 or almost useless rudimentary part may often be injured or torn 
 off without causing processes of selection to occur which would 
 produce in it a capacity for regeneration. Thus, so far as is 
 known, the minute limbs of Siren and Proteus, which are often 
 bitten off, are not icplaced ; while the gills of these animals and 
 of the Axolotl, which are equally liable to similar injuries, become 
 regenerated : in the latter case the organs are physiologically 
 valuable, while in the former they are not. The tail of a lizard, 
 again, which is very liable to injury, becomes regenerated, be- 
 cause, as we have seen, it is of great importance to the individual, 
 and if lost its owner is placed at a disadvantage.
 
 REGENERATION. 123 
 
 Finally, the complexity of the individual parts constitute the 
 third factor which is concerned in regulating the regenerative 
 power of the part in question ; for the more complex the struc- 
 ture is, the longer and more energetically the process of selection 
 must act in order to provide the mechanism for regeneration, 
 which consists in the equipment of a large number of different 
 kinds of cells with supplementary determinants, which are accur- 
 ately graduated, and regulated as regards their power of multi- 
 plication. Thus we can understand, for instance, why the fore- 
 limb of a Triton becomes regenerated, while that of a bird does 
 not, although the wing is of far greater importance and is much 
 more indispensable to its owner than is the fore-limb in the case 
 of the Triton. Although there are fewer bones in a bird's 
 wing than in a Triton's limb, the former is by far the more 
 complicated structure ; for it is covered with feathers, and 
 as each quill has a special size, form, and coloration, the wing 
 must contain a large number of special determinants in its for- 
 mative cells. These determinants must all be contained and 
 arranged in the germ-plasm, so that they can be passed on 
 during embryonic development through a certain series of cells, 
 first into the outer germinal layers, then into the epidermis 
 of the fore-limb, and finally, by the agency of further series of 
 cells arising in the course of growth, to the region to which they 
 specially belong. It is difficult enough to imagine how the dis- 
 tribution of the determinants can possibly take place in so 
 accurate and certain a manner as must be the case in reality, so 
 that not only the shape of the feather but even every speck of 
 colour on it is accurately repeated in every individual of the 
 species ; and it might well, indeed, be considered impossible that 
 the whole of this complex mechanism should also be capable of 
 becoming modified in such a manner, that the entire wing, with 
 all its feathers and patches of colour, could be regenerated from 
 a cut surface in any part. Did this occur, the cells of any 
 section of the wing would, according to our theory, have to con- 
 tain the whole of the determinants of all the cells required for the 
 construction of the portion of the wing distal to the cut surface 
 as supplementary determinants, in addition to their own special 
 idioplasm ; and moreover, these determinants must then be dis- 
 tributed proportionately among the cells of the radial and ulnar, 
 and of the upper and under surfaces of the wing, and the power 
 of multiplication of each cell and its successors would have to be
 
 124 THE GERM-PLASM. 
 
 accurately adjusted. Although we cannot easily judge as to 
 what is possible in nature, and are so often impressed by the 
 discouraging conviction that many vital processes are still 
 incomprehensible to us, we may perhaps in this case feel justified 
 in inferring the impossibility of such an occurrence from the fact 
 that it does not take place; that is, to infer that the regeneration 
 of a bird's wing is impossible on account of the complexity of 
 the mechanism required for it, because it does not actually 
 occur. 
 
 We cannot, however, regard this as a formal proof of the fact 
 that regeneration does not take place in this case. This would 
 be inadmissible, if only because the first of the three factors 
 which, as we have assumed, produces the mechanism of re- 
 generation that is, the probability of loss is not present. In 
 the state of nature, at any rate, a bird's wing is seldom injured 
 without loss of life ensuing at the same time. For this reason 
 alone, selective processes in connection with a regenerative 
 mechanism could not be introduced. I have not brought for- 
 ward the above example for the purpose of proving the case for 
 this instance in particular, but because it seemed to me to be 
 specially fitted to show how extraordinarily the complexity of 
 the regenerative mechanism must increase along with the 
 greater complexity of the part. But this brings us back to the 
 consideration of the general power of regeneration possessed by 
 the lower, in contrast to the higher, animals. 
 
 The supposition that this power exists, may, I believe, be con- 
 ceded in a certain sense : that is to say, in consequence of the 
 slighter complexity in structure of all the parts in one of the 
 lower groups of animals, any particular part may also become 
 capable of regeneration more easily than in the case of the higher 
 groups. We must, however, always presuppose that the two 
 other factors the probability of injury, and the physiological 
 importance of the organ are present in the required degree ; 
 so that in speaking of the greater power of regeneration pos- 
 sessed by animals of a lower type, we are only using another 
 expression for the third factor which takes part in the process, 
 viz., the complexity of the organ to be regenerated. 
 
 The question, however, arises as to whether the capacity of 
 each part for regeneration results from special processes of adap- 
 tation, or whether regeneration occurs as the mere outcome 
 which is to some extent unforeseen of the physical nature of
 
 REGENERATION. 125 
 
 an animal. Some statements which have been made on this 
 subject seem hardly to admit of any but the latter explanation. 
 Thus, according to Spallanzani, the jaw of a Triton may become 
 regenerated along with its bones and teeth. Bonnet states that 
 even the eye of this animal is replaced after it has been extir- 
 pated. It has never come before my notice that in the natural 
 state Tritons frequently lose the lower jaw in combat ; but some 
 of these animals which I had put for a short time in a small 
 vessel attacked each other vigorously, and several times one of 
 them seized another by the lower jaw, and tugged and bit at 
 it so violently that it would have been torn off if I had not 
 separated the animals. The loss of part of the jaw or eye 
 may therefore occur not infrequently in the natural state, and 
 we may thus perhaps assume that these parts are adapted 
 for regeneration. Kennel, moreover, gives an account of a 
 stork, the upper beak of which had accidentally been broken 
 off in the middle, the lower one then being sawn off to the 
 same length, and both were subsequently regenerated. Such 
 cases, the accuracy of which can scarcely be doubted, indicate 
 that the capacity for regeneration does not depend only on the 
 special adaptation of a particular organ, but that a general 
 power also exists which belongs to the whole organism, and to 
 a certain extent affects many, and perhaps even all, parts. By 
 virtue of this power, moreover, simpler organs can be replaced 
 even when they are not specially adapted for regeneration. 
 
 From our point of view, such cases are not incomprehensible 
 in principle. We need only assume that in all, or at any rate 
 in many, of the nuclear divisions in the embryo, some of the 
 earlier determinants remain associated with later generations 
 of cells as accessory idioplasm. It only remains to trace this 
 arrangement which is a more or less universal one, and affects 
 the whole body to its origin ; for no arrangement can be pro- 
 duced which is not useful, especially when it concerns such a 
 complicated mechanism as that for supplying the idioplasm 
 with accessory determinants. We are therefore led to infer 
 that the general capacity of all parts for regeneration may have 
 been acquired by selection in the lower and simpler forms, and 
 that it gradually decreased in the course of phylogeny in corre- 
 spondence "with the increase in complexity of organisation; but 
 that it may, on the other hand, be increased by special selective 
 processes in each stage of its degeneration, in the case of certain
 
 126 THE GERM-PLASM. 
 
 parts which are physiologically important and are at the same 
 time frequently exposed to loss. In all probability this view is 
 the correct one. 
 
 3. FACULTATIVE OR POLYGENETIC REGENERATION. 
 
 The tail of a lizard or the limb of a Triton grows again when it 
 has been cut off, but the part amputated does not reproduce the 
 entire animal. In some segmented worms, on the other hand, 
 such as Nats and Lumbricuhis^ not only does the amputated tail 
 end become restored, but this end itself reproduces the anterior 
 part of the body, so that two animals are formed from one. 
 
 This fact evidently cannot be deduced merely from the 
 assumption we have made with regard to supplementary deter- 
 minants ; for were this the case, determinants of one kind only 
 viz., those which are necessary for the construction of the lost 
 part would be present in the cells. But in the above instances 
 the same cells give rise to entirely different parts, according to 
 whether they are situated on the surface which is anterior or 
 posterior to the plane of amputation : in the former case they 
 reproduce the tail-end, and in the latter the head-end. The 
 fact that both parts grow again when the worm is cut into two 
 through any region of the body, proves that regeneration 
 in either direction may proceed from the same cells ; it there- 
 fore follows that the cells situated in any particular transverse 
 plane of the body are not merely provided with the supplemen- 
 tary determinants for the formation of the head- or tail-end 
 only, but every cell can react in either way, according to 
 whether it is situated anteriorly or posteriorly to this plane. In 
 order therefore to explain the twofold action of these cells in 
 accordance with our fundamental view, which presupposes 
 that the cells taking part in regeneration are arranged and con- 
 trolled by the forces situated within them, and not by an external 
 agency, it seems necessary to assume that each cell possesses 
 two different supplementary determinants, one for the construc- 
 tion of the head-end, and one for that of the tail-end ; and that 
 the one or the other becomes active according to whether the 
 stimulus, due to the exposure of the cell, is applied to its ante- 
 rior or to its posterior surface. 
 
 Before attempting to verify this assumption, I must mention 
 certain cases in which the regenerative activity of the cells may 
 even be threefold.
 
 REGENERATION. 127 
 
 It appears to me that the regenerative processes which have 
 been observed in the fresh- water polype Hydra and in the sea- 
 anemones (Actinia;) are instances of this kind of regeneration. 
 If a worm is cut through in the median or any other longitu- 
 dinal plane, neither part grows again, and each soon dies. The 
 case is different in Hydra. If this animal is cut through 
 longitudinally, the two parts grow again into entire individuals, 
 irrespective of the plane of section. As the transverse section 
 of the animal at any point is likewise followed by the restoration 
 of each part, it follows that Hydra, in every part of the body, 
 must be capable of a threefold regeneration, i.e. of regeneration 
 in the three directions of space. And as the body is differently 
 constructed in these three directions, we are compelled to assume 
 that each cell contains groups of determinants of three different 
 kinds, viz., those which are concerned in the formation of the 
 proximal and distal ends, and in the completion of the body- wall. 
 An individual cell * must therefore be capable of dividing in 
 three different planes, and of giving rise to a part of one of three 
 different regions of the body ; and, moreover, the plane in which 
 division actually occurs, and consequently the kind of deter- 
 minants which become active and control the cell, is decided 
 not by the quality, but by the kind of division resulting from the 
 stimulus produced by the injury. 
 
 The processes of regeneration in Hydra can, I think, to a 
 certain extent be understood on this assumption. If, for in- 
 stance, the group of supplementary determinants of the proxi- 
 mal end of the body becomes active, it will cause the develop- 
 ment of linear rows of cells extending in the direction of the axis 
 of the body and united laterally so as to give rise to a tube ; these 
 cells, moreover, will have the tendency to close in towards the 
 centre as soon as possible, so as to form the disc or foot, and will 
 also cause the differentiation of the ectoderm cells of the foot 
 into glandular cells which secrete slime : the determinants for 
 the formation of tentacles are wanting in this group. If, again, 
 the group of supplementary determinants of the distal end 
 becomes active, rows of cells arise which will tend to close in to 
 form the oral disc, leaving a large space in the centre for the 
 mouth. Tentacles will then grow out from certain points around 
 
 * I shall not refer to the histological details with regard to the pro- 
 cess of regeneration in Hydra, as the necessary data appear to be too 
 uncertain and incomplete.
 
 128 THE GERM-PLASM. 
 
 the mouth, and it is certainly not easy to explain why the de- 
 terminants which cause their formation become active at these 
 points only. It will, however, be shown later on that the 
 cells of Hydra and probably those ef all animal tissues 
 are in a certain sense polarised ; that is to say, they are 
 differently constituted in the three directions of space. The 
 fact that the determinants of the tentacles which we must sup- 
 pose to exist in all regions of the body only become active in 
 certain cells around the margin of the mouth, may be due to the 
 polarisation of the cells as well as to the peculiar conditions oi 
 pressure within the cellular dome of the oral disc. 
 
 What has just been said can certainly not be looked upon as 
 anything more than the merest provisional explanation of the 
 facts, but it appears to me to be impossible to give a better one 
 at present. It nevertheless, I think, penetrates somewhat further 
 into the problem than does Herbert Spencer's hypothesis, in 
 which regeneration is compared in general to crystallisation, 
 and the capacity of arranging itself on every occasion under 
 the influence of the whole aggregate in the manner required 
 for the renewal of the missing part, is attributed to every ulti- 
 mate particle. If we take the fresh-water polypes alone into 
 consideration, one of these explanations seems just as good as 
 the other ; but if other groups of animals are included, it is at 
 once apparent that this capacity is not by any means always 
 possessed by the particles, but that even the cell may give rise 
 by regeneration sometimes to various parts of the whole aggre- 
 gate, at other times only to one certain part, and at others again 
 only to those similar to itself, and that it must therefore contain 
 something which makes it specially capable of one or of the 
 other kind of regeneration. This something is the group of 
 supplementary determinants. 
 
 If a polype or worm is cut through transversely, or if a loss of 
 substance is caused artificially in any organism, the conditions 
 of pressure previously existing in the cell in the region of the 
 injury become changed, the pressure previously exerted by 
 the lost part suddenly ceasing. This induces a change in the 
 vital conditions of the cells thus affected, which must have a 
 definite morphological and physiological result. We are unable 
 at present to state more precisely what this change is ; but as we 
 know that such losses of substance are followed by the multipli- 
 cation of the cells, we may safely assume that it exerts a stimu-
 
 REGENERATION. I 29 
 
 lus on the cell, and more especially on its idioplasm, which 
 forces the latter to undergo multiplication. This view is main 
 tained by those who have the greatest opportunity of investi- 
 gating the details of such processes, I refer to the pathological 
 anatomists. The proliferation which ensues in the surrounding 
 tissue after a loss of substance, is not explained by them as 
 being due indeed to a stimulus in the ordinary sense of the 
 word exerted on the surrounding cells, but rather to a cessa- 
 tion of the ' resistance to growth] and this may in one sense also 
 be described as a 'stimulus,' inasmuch as it is an 'incitement' 
 to growth. 
 
 If the cells were constituted alike in the three directions of 
 space, the effect on the idioplasm would be the same whether 
 the stimulus due to the loss of substance acts from before, from 
 behind, or from the side. One of the three groups of deter- 
 minants could not possibly be alone affected by the stimulus 
 and thus rendered active in one case, the second only in another, 
 and the third only in a third instance. We have, however, every 
 reason to suppose that the structure of one of these tissue-cells 
 is not the same in the three directions of space, and that they 
 are, in fact, variously differentiated according to each of these, 
 and consequently respond to, stimuli in different ways according 
 to the direction in which the latter act upon them. Vochting* 
 has proved that at any rate in higher plants, ' a different upper 
 and lower, anterior and posterior, and right and left half, can be 
 distinguished in each living cell in the root and stem.' Portions 
 of the root of the poplar transplanted on to the stem, or por- 
 tions of the stem transplanted on to the root, only grew and 
 flourished when they were fixed in a certain position ; in the 
 reverse position they sometimes indeed grew, but soon showed 
 phenomena of degeneration. Vochting infers from this obser- 
 vation that the cells are 'polarised,' this term being taken 
 merely in an analogous sense to that in which it is generally 
 used. The root and stem behave in a certain sense like a 
 cylindrical magnet, which is composed of sections equally 
 magnetised in the radial and longitudinal directions. Such 
 a magnet, like the stem and root, may be separated into 
 pieces. If the smooth adjoining surfaces of the portions of 
 
 * H. Vochting, ' Uber Transplantation am Planzenkorper," Tubingen, 
 1889, p. 400. 
 
 I
 
 130 THE GERM-PLASM. 
 
 the magnet are placed with their opposite poles as close to- 
 gether as possible, the entire magnet is once more formed. 
 Similarly, if the root of a poplar is cut in half transversely, each 
 half produces buds and roots at the corresponding poles; but 
 if, on the other hand, the two portions are joined together in 
 the same relative position as that which they occupied origi- 
 nally, they become united together, so that a single piece of 
 root, with its two poles, results, quite similar to the original 
 piece. 
 
 These important results which Vochting has obtained by his 
 experiments on transplantation, are mentioned in this place 
 because they can be utilised in considering the phenomena of 
 regeneration in animals, which have just been discussed. We 
 may in this respect compare a fresh-water polype with a poplar 
 root. After a Hydra has been cut in half transversely, the dis- 
 tal portion gives rise to a new foot at its proximal end, and the 
 proximal portion produces an oral region at its distal end. We 
 might therefore in this case speak of pedal- and oral-poles, instead 
 of root- and stem-poles, as in the case of the poplar. And, in fact, 
 if a Hydra is cut transversely into three portions, the distal part 
 or oral pole of the middle piece develops a new oral region, and 
 its proximal part or pedal pole gives rise to a new foot. It 
 might not be impossible for a clever experimenter to cause this 
 middle piece to unite with the two terminal portions of the 
 body before the former had had time to develop into a complete 
 animal, by joining the three portions together with bristles. 
 This would result in a union just as in the case of the poplar. 
 
 It would be a mistake to try to deduce that one of the poles of 
 the poplar root must grow shoots and the other roots merely 
 from the fact of its polarisation : one might as well try to deduce 
 it from the fact of the polarisation of a real magnet. Something 
 more is required before this can take place : the cells of the 
 poplar root must contain the primary constituents for the forma- 
 tion of shoots and roots ; that state of the cells which Vochting 
 describes as polarisation only produces the conditions under 
 which one or other of the primary constituents becomes active, 
 and thus undergoes development. The hypothesis of the polar- 
 isation of the cells does not, therefore, relieve us in the least 
 from the necessity of making a theoretical assumption to explain 
 how it comes about that the primary constituents of different 
 kinds of structures are present in one and the same cell.
 
 REGENERATION. 131 
 
 According to my view, we must assume in the case of the poplar 
 root that the cells are provided with two different kinds of idio- 
 plasm, which remain inactive until the adequate stimulus arises 
 and causes the idioplasm of either the root or of the stem to 
 become active. In both cases the loss of substance must be re- 
 garded as the stimulus, and the direction in which it acts must 
 decide the quality of the reaction. 
 
 If the idioplasm of the tissue-cells were capable in itself of 
 responding to the effect of this stimulus by causing a regenera- 
 tion of the missing parts of the body, worms possessing the 
 regenerative power in a high degree, such as Nats and Ltimbri- 
 cu/us, would be capable of regeneration in a lateral as well as in 
 the anterior and posterior directions. This, however, as Bonnet 
 has previously proved, is not the case : when cut in half longi- 
 tudinally, the missing right or left half is not reproduced, and the 
 cells of these animals must therefore be wanting in that sub- 
 stance viz., in the antimeral supplementary determinants 
 which renders this kind of reproduction possible. 
 
 From our point of view, it is not surprising that these deter- 
 minants are absent in worms ; for in the natural state these 
 animals are never torn in half longitudinally, and there was there- 
 fore no need for Nature to provide for such a contingency. 
 
 If we consider that the groups of supplementary determinants 
 must become more complicated in proportion as the organism 
 and the part to which they give rise increase in complexity, we 
 can understand why facultative regeneration only occurs in rela- 
 tively simple organisms, and that it apparently takes place in 
 three dimensions in Polypes and Flat- worms only, in two dimen- 
 sions in Annelids and Starfishes, and merely in one dimension 
 in Arthropods, Molluscs, and Vertebrates. 
 
 It must not be supposed that other factors do not also take 
 part in limiting the capacity for regeneration, such as, in 
 particular, the vulnerability of the higher organisms, and the 
 fact that they are dependent on the circulation and tem- 
 perature of the blood, even apart from the influence of the 
 nervous system, of which we are practically still very ignorant. 
 The relatively small quantity of substance in the part removed, 
 as compared with that of the rest of the body, would also pre- 
 vent the amputated limb of a salamander, for instance, from 
 becoming regenerated into an entire animal. All these con- 
 siderations help to explain why bi-dimensional regeneration
 
 132 THE GERM-PLASM. 
 
 that is, regeneration in two directions cannot take place in the 
 higher animals. 
 
 If, then, regeneration depends on the distribution of supple- 
 mentary determinants to certain cells, which occurs whenever it 
 is necessary or possible, the process must be primarily traceable 
 in the case of the Metazoa to the doubling of the ids in a certain 
 ontogenetic stage. And since a division and doubling of the 
 idants takes place in every mitotic nuclear division, this hypothesis 
 is supported by actual fact, even although we are still far from 
 being acquainted even with the general details of the processes 
 of growth and doubling of the ids and determinants, not to 
 mention the systematic transference of such inactive determinants 
 to definite cells and series of cells. Here again, however, 
 Nature will have caused an advance from the simple to the more 
 complex; and it therefore follows that, just as complicated 
 organisms could only arise in the course of innumerable series 
 of generations and species, so also the complex apparatus for 
 regeneration in the tail or limb of a newt could not have been 
 developed suddenly, but must have arisen in consequence of 
 similar modifications in innumerable ancestors. 
 
 It might be possible to picture to one's self approximately the 
 series of modifications which the apparatus for regeneration has 
 gradually undergone, beginning at the lowest multicellular 
 forms, and passing upwards to those animals in which the power 
 of regeneration is the most highly developed and complex. I 
 shall not, however, attempt to do so. At some future date it 
 may perhaps be found that differences occur as regards the 
 number of ids contained in the cells of those which have, and in 
 those which have not, a marked capacity for regeneration : it 
 will not be worth while to trace in detail the courses which the 
 development of the power of regeneration has taken, until our 
 knowledge of the idioplasm is sufficiently complete to furnish a 
 basis for the theory in fact. 
 
 4. REGENERATION IN PLANTS. 
 
 The process which may be described as regeneration in the 
 case of the lower plants the algae, fungi, and mosses will be 
 treated of in greater detail subsequently. In this place, I merely 
 wish to point out that true regeneration only occurs in a very 
 slight degree in all the higher plants which are regarded as 
 cormophytes or plant-stocks. If a piece is cut out of a
 
 REGENERATION. 133 
 
 leaf of a tree or of any other Phanerogam, the leaf does not 
 become regenerated. If, again, an anther or a stigma is cut off 
 from a flower, the corresponding filament or style will not give 
 rise to a new anther or stigma. The cells of these organs are 
 therefore not adapted for regeneration, and do not contain 
 ' supplementary determinants.' 
 
 Botanists might be inclined to explain this fact by supposing 
 it to be due to the cells having already reached their full size, 
 and having therefore lost their power of multiplication. This is 
 certainly the case, but it does not explain matters in the sense 
 I mean : the question still remains as to why these cells have 
 not been provided with supplementary determinants. The large 
 number of cases in which adult cells of leaves or other parts, 
 which have reached their full size, may under certain circum- 
 stances begin to multiply, and form buds from which entire plants 
 arise (e.g., Begonia), proves that such a provision is possible. 
 
 The solution of the above problem is to be sought for in the 
 fact that it would have been of far too slight importance to the 
 plant to be able to restore such defects in its leaves, as it pos- 
 sesses the power of producing new leaves. Buds can be 
 formed and undergo further development in many parts, and 
 thus the plant gains much more than it could possiby do by 
 mere regeneration. Regeneration can be dispensed with, as the 
 far more important power of budding is possessed by the plant. 
 
 The fact that the higher plants are unable to restore such 
 parts as portions of leaves, furnishes an additional important 
 proof that regeneration is dependent on external circumstances, 
 and that it is a phenomenon of adaptation. True regeneration, 
 however, occurs in those cases in which the losses or injuries 
 would be harmful to the plant, and cannot be made good by the 
 development of buds. Thus a loss of substance in the bark of a 
 tree becomes replaced by the formation of callus, which arises 
 from the edges of the wound, and grows over it, and thus the 
 underlying wood is protected from injury. The cut or broken 
 surface of a branch, even in the case of many herbaceous 
 stems, becomes covered over in a similar manner by a mass of 
 proliferating callus, which may even give rise to new growing 
 points of shoots and roots, and thus become the place of origin 
 of new individuals.* The stimulus to proliferation, as in the 
 
 * J. Sachs, ' Lectures on the Physiology of Plants,' Leipzig, 1882, 
 p. 709. (English edition, translated by H. Marshall Ward, Oxford, 1887.)
 
 134 THE GERM-PLASM. 
 
 case of regeneration in animals, is due to the removal of the 
 opposition to growth ; the cells must, however, be adapted for 
 this reaction, otherwise the proliferation cannot take place ; the 
 stems as well as the roots and veins of herbaceous plants do not 
 by any means always respond to an injury by the formation of 
 callus. This process is therefore not a primary quality of the 
 plant, but an adaptation, due, in my opinion, to the association 
 of certain supplementary determinants with the active idioplasm 
 of certain kinds of cells. 
 
 The formation of callus is probably the only process in plants 
 which can be regarded as an actual regeneration. 
 
 5. REGENERATION IN ANIMAL EMBRYOS, AND THE 
 PRINCIPLES OF ONTOGENY. 
 
 The theory of heredity which has now been formulated, and 
 more especially that portion of it which concerns the composition 
 of the germ-plasm out of determinants, and the gradual disin- 
 tegration of the mass of determinants in the germ-plasm during 
 the course of ontogeny, is based on the assumption that the 
 cells control themselves: that is to say, the fate of the cells is 
 determined by forces situated within them, and not by external 
 influences. The primary cells of the ectoderm and of the endo- 
 derm arise by the division of the fertilised egg-cell and its con- 
 tained germ-plasm, because the determinants of the ectoderm 
 are passed into one cell and those of the endoderm into the 
 other, and not because some external influence, such as the 
 force of gravity, affects the cells in a different manner. Simi- 
 larly a certain cell in a subsequent embryonic stage does not 
 give rise to a nerve-,. a muscle-, or an epithelial-cell because it 
 happens to be so situated as to be influenced by certain other 
 cells in one way or another, but because it contains special 
 determinants for nerve-, muscle-, or epithelial-cells. 
 
 This conception of the predestination of the individual cells, 
 the fate of which, together with that of their successors, is deter- 
 mined by the idioplasm they contain, was first imperfectly ex- 
 pressed in the theory formerly propounded by His,* in which he 
 formulated the existence of ' special regions in the germ, which 
 give rise to special organs.' His imagined that 'the primary 
 
 * Wilhelm His, 'Unsre Korperform u. das physiologische Problem ihrer 
 Entstehung,' Leipzig, 1874.
 
 REGENF.RATION. 135 
 
 constituents of the organs of a chick \ve>e present in superficial 
 extension in the germinal disc,' i.e. in the cell-body of the ovum, 
 and that each organ is therefore represented by a definite part 
 of the body of the egg. As has already been mentioned in the 
 historical introduction, subsequent investigations, made in the 
 course of the following ten years, proved that the ' primary con- 
 stituents ' of the various structures are to be found in the nuclear 
 substance. The special form in which His expressed his views 
 was thus certainly contradicted, although the fundamental prin- 
 ciple of his theory was not thereby affected in its general sense, 
 which indicates that the differentiating principle of ontogeny is to 
 be looked for in the cells themselves, and not in external influ- 
 ences. Wilhelm Roux * was the first to prove definitely that the 
 differentiation of the egg into the embryo is certainly not caused 
 by influences existing apart from the egg, but that it is due to 
 causes originating in the egg itself. PfKigert showed with regard 
 to the ovum of the frog, that whatever position the egg is forced to 
 take up the upper side always gives rise to the animal pole of the 
 embryo, and it was thought that this must be due to the force of 
 gravity. Roux, however, proved that frogs' eggs which are 
 rotated slowly in a vertical direction, develop just as well as 
 those on which the force of gravity is not interfered with. It has 
 further been proved by Born J that, although when an egg 
 undergoes development in a fixed position the substance of the 
 cell-body does not become disp'aced at first, the nucleus never- 
 theless changes its position, for it very soon passes to the upper 
 pole of the egg, at which point development then begins. 
 These observations undoubtedly proved that the formative 
 forces are situated in the egg itself; but they still left it 
 undecided whether the differentiation of the ovum is due essen- 
 tially to the action of the individual cells alone, that is to say, 
 whether differentiation occurs independently in each individual 
 cell, so that it would, if necessary, be capable of passing through 
 its prescribed course of development apart from the rest of the 
 embryonic cells, or whether the various cells of the embryo 
 
 * Wilhelm Roux, ' Beitrage zur Entwicklungsmechanik des Embryo, ' 
 Munchen, 1885. 
 
 t Pfliiger, ' Ueber den Einfluss der Schwerkraft auf die Thielung der 
 Zellen u. auf die Entwicklung des Embryo,' Arch. f. Physio!., Bd. xxxii., 
 1883, p. 68. 
 
 Born, ' Biologische Untersuchungen,' (I.) Arch. f. mikr. Anal. Bd. 24.
 
 136 THE GERM-PLASM. 
 
 become differentiated by their mutual interaction : or, in other 
 words, whether a determinating influence is to a certain extent 
 exerted by the whole on its parts and thus prescribes the fate of 
 the various cells. 
 
 The experimental proof of the self-differentiation or predis- 
 position of the individual cells was, I believe, furnished by 
 Roux,* whose ingenious experiments are always accompanied 
 by keen deductions. Roux destroyed a single segmentation- 
 cell in each of a series of frogs' eggs by means of a hot 
 needle, and then observed that eggs treated in this manner 
 developed into ' half or three-quarter embryos,' that part being 
 absent which corresponded to the cell thus destroyed. When 
 one of the first two segmentation-cells was demolished, half of 
 the embryo was formed, and this corresponded either to a lateral 
 or to the anterior or posterior half, according to whether the first 
 segmentation had resulted in a division of the hereditary sub- 
 stance into portions belonging to the right and left, or to the 
 anterior and posterior halves. The process of segmentation in 
 the frog is known to vary in this respect. When one of the first 
 four segmentation-cells was destroyed, three-quarters of the 
 embryo was formed. 
 
 These experiments must be regarded as affording a proof of 
 the self-differentiation of the cells. Observations have since been 
 made which seem to contradict this deduction ; and although 
 these are still incomplete, and can only be regarded as the 
 preliminaries to more detailed investigations, they must not be 
 passed over in silence, especially as I am convinced that they do 
 not really contradict the hypothesis of the self-determination of 
 the cells. 
 
 Chabry's t experiments on the eggs of Ascidians must be 
 mentioned first. By means of a special apparatus, he destroyed 
 one of the first two segmentation-cells, and then observed that 
 the remaining cell continued to develop, and eventually gave 
 rise, not indeed to half an embryo, but to an entire one of 
 half the normal size. Such embryos were certainly not quite 
 perfect, but only organs of slight importance were wanting in 
 them. Chabry himself has drawn no theoretical conclusions 
 
 * Wilhelm Roux, ' Beitrage zur Entwicklungsmechanik des Embryo,' 
 (V.) Virch. Arch. Bd. 94. 
 
 t L. Chabry, ' Embryologie normale et teratologique des Ascidies,' 
 Paris, 1887.
 
 REGENERATION. 137 
 
 from his observations ; Driesch,* however, has made certain 
 deductions from a series of similar experiments on the eggs of 
 Sea-urchins. By continued shaking, Driesch effected a mechani- 
 cal separation of the two first segmentation-cells, and observed 
 that at first each of them continued to undergo further segmenta- 
 tion just as would occur in the entire egg, but that later on the 
 resulting //*//-blastula became completed to form an entire one. 
 In some of these hemi-b! astute development proceeded still 
 further, the invagination taking place to form the primary 
 digestive cavity of the gastrula, so that eventually a rudimentary 
 pluteus-larva which, though small, was in other respects nor- 
 mal could be recognised. 
 
 Driesch sums up his results in the following words : 'These 
 experiments therefore show that under certain circumstances 
 each of the two first segmentation-cells of Echinus micro-tuber- 
 culatus can give rise to a larva of the normal form, which is 
 entire as regards its shape ; and that a partial formation, and 
 not a semi-formation, occurs in this case.' The author con- 
 cludes that his results ' fundamentally disprove the existence of 
 special regions in the germ which give rise to special organs,' 
 and adopts the following view stated by Hallez t : ' II n'est pas 
 des lors permis de croire que chaque sphere de segmentation 
 doit occuper une place et jouer un role, qui lui sont assignes a 
 Pavance.' 
 
 Although I am far from wishing to assert that we are at 
 present in a position to give a perfectly reliable and detailed 
 explanation of the extremely interesting and important results 
 of the experiments just described, I nevertheless cannot help 
 thinking that they do not in the least necessitate the giving up 
 of the view which entails a predestination of the individual 
 segmentation-cells, and, in fact, of cells in general. Other than 
 experimental methods may lead us to fundamental views, and an 
 experiment may not always be the safest guide, although it may 
 at first appear perfectly conclusive. Even Driesch himself 
 doubts whether the above-mentioned experiments made by Roux 
 are really conclusive, though, in my opinion, he is wrong in 
 doing so : he asks, in fact, whether the uninjured segmenta- 
 tion-sphere of the frog would not behave exactly in the same 
 
 * H. Driesch, ' Entwicklungsmechanische Studien, Zeitschrift f. wiss. 
 Zoologie, Bd. 53, 1891. 
 t Hallez, ' Rechcrches sur Pembryologie des N&iiatodes,' Paris, 1885.
 
 138 THE GERM-PLASM. 
 
 manner as that of the sea-urchin if it could be actually isolated, 
 instead of remaining in close connection with the other injured 
 sphere. Thus even the apparently incontrovertible result of this 
 experiment may be doubted. 
 
 It" seems to me that careful conclusions, drawn from the 
 general facts of heredity, are far more reliable in this case than 
 are the results of experiments, which, though extremely valuable 
 and worthy of careful consideration, are never perfectly 
 definite and unquestionable. If what was said in support 
 of the theory of determinants in the first chapter of this book be 
 borne in mind, the conviction that ontogeny can only be ex- 
 plained by evolution, and not by epigenesis, seems to force itself 
 upon us. It would be impossible for any small portion of the 
 skin of a human being to undergo a hereditary and independent 
 change from the germ onwards, unless a small vital element 
 corresponding to this particular part of the skin existed in the 
 germ-substance, a variation in this element causing a corre- 
 sponding variation in the part concerned. Were this not the 
 case, ' birth-marks' would not exist. If, however, determinants 
 are contained in the germ-plasm, these can only take part in 
 controlling the formation of the body if, in the course of embry- 
 ogeny, they reach those particular cells which they have to 
 control, that is to say, if the differentiation of a cell depends 
 primarily on itself, and not on any external factor. 
 
 If therefore ontogeny is not, as Roux aptly expresses it, a 
 ' new formation' 1 of multiplicity, or an epigenesis, but is merely 
 the unfolding of multiplicity, i.e. an evolution, or, as it might 
 also be called, the appearance of a previously in-visible multi- 
 plicity, the principle of self-determination is certainly only 
 established with regard to the egg as a whole : the self-deter- 
 mination of each cell, and its control of ontogeny, do not neces- 
 sarily follow from this conclusion. We can only thereby arrive 
 at the very simple assumptions, that the primary constituents of 
 the germ-plasm are distributed by means of the processes 
 which can actually be observed in the nuclear divisions, so that 
 they come to be situated in those regions which correspond to 
 the various parts of the body, and that those primary consti- 
 tuents are present in each cell which correspond to the parts 
 arising from it. 
 
 As has just been shown, it is also possible to make the reverse 
 hypothesis, and to suppose that although the whole of the idio-
 
 REGENERATION. 139 
 
 plasm is contained in each cell, only that particular primary 
 constituent which properly concerns the individual cell has any 
 effect upon it. The activity of a primary constituent would thus 
 dependnot on theidioplasm of the cell, but on the influences arising 
 from all the cells of the organism as a whole. We should thus 
 have to suppose that each region of the body is controlled 
 by all the other regions, and should therefore practically be 
 brought back to Spencer's conception of the organism as a 
 complex crystal. This simply means giving up the attempt to 
 explain the problem at all, for we cannot form any conception 
 of such a controlling influence exerted by the whole on the 
 millions of different parts of which it consists, nor can we bring 
 fonvard any analogy to support such a view, the acceptance of 
 which would render a great number of observations on the 
 phenomena of heredity totally incomprehensible. What ex- 
 planation, for instance, could be given of the fact that a certain 
 human birthmark is always inherited on the left side only? 
 According to this hypothesis, the germ-plasm contained in the 
 cells of this region would be present on the right side just as 
 much as on the left : as the two halves of the body are alike 
 in other respects, we cannot suppose that the whole aggregate 
 exerts different influences as regards this region on the left and 
 on the right sides. 
 
 It seems to me, therefore, that we must not give up the hypo- 
 thesis of the self-determination of the cells, in spite of its 
 apparent refutation by the facts described by Chabry and 
 Driesch. Moreover, I think these facts can be explained in 
 principle at any rate in another manner, viz., by attributing the 
 processes observed to regeneration, the arrangement for which, 
 however, has not been provided for the first stages of segmenta- 
 tion, but for a later period of ontogeny. 
 
 It is hardly to be expected that the first stages of segmenta- 
 tion should be in a sense purposely arranged for regenera- 
 tion. Both in Ascidians and sea-urchins the number of eggs 
 produced is so large, that it probably matters very little whether 
 a segmenting ovum perishes or becomes regenerated when one 
 half of it has been eaten by a small enemy. I do not, however, 
 wish to do away entirely with the idea that the eggs of certain 
 animals may conceivably be protected in this manner from 
 numerous enemies, but in this place I must refrain from includ- 
 ing such a possible occurrence in the argument.
 
 14 THE GERM-PLASM. 
 
 The following explanation of the phenomena, however, still 
 remains. The first division of the ovum separates the group of 
 determinants into two, viz., that for the right and that for the 
 left half of the body ; each of these groups does not constitute 
 a perfect germ-plasm, as each determinant it contains is not 
 doubled ; but it is very probable that the ids are capable under 
 certain circumstances of dividing in such a way that each be- 
 comes doubled. Such a germ-plasm could not contain in 
 potentia a birthmark, or any other asymmetrical peculiarity of 
 the other side of the body, but it would be able to give rise to a 
 complete animal. The destruction or mechanical removal of 
 one segmentation-cell in the first stage of segmentation may be 
 the primary cause of the doubling of the ids in the other cell. 
 
 The capability of becoming doubled, which the undivided 
 germ-plasm possesses in certain cases, may be mentioned in 
 support of this view of the regeneration of an isolated cell in the 
 first stage of segmentation. The fact that in each integral 
 division of the cell and nucleus, a longitudinal splitting of the 
 nuclear rods and their contained macrosomes occurs, shows 
 that the ids are as a rule capable of growth and of doubling 
 their number by division. The assumption of a doubling of the 
 ids of germ-plasm must be made in dealing with the origin 
 of identical twins, i.e. those twins in which we must suppose 
 that the division of the nucleus of the ovum from which they 
 arise occurs after and not before fertilisation ; for otherwise the 
 embryos could not be identical, as two spermatozoa would then 
 take part in the process. In the case of facultative partheno- 
 genesis, a doubling probably also occurs in the ids and idants 
 of the ovum, half of them having previously been removed 
 by the ' reducing divisions.' 
 
 The formation of an entire embryo by the regeneration of one 
 of the two first blastomeres admits, however, of another inter- 
 pretation. Ascidians multiply very freely by budding, and not 
 only by sexual reproduction. It is true that this is not the case 
 with sea-urchins, but the power of regeneration which these 
 animals possess is unusually great. This fact was explained in 
 the present chapter by assuming that certain idic stages of 
 ontogeny are provided with an ' accessory idioplasm,' consisting 
 of the determinants required for regeneration. In a subsequent 
 chapter I shall have occasion to show that we must make a 
 similar assumption in the case of budding. Such assumptions
 
 REGENERATION. 141 
 
 are indispensable if we accept the hypothesis of the germ-plasm 
 and determinants. The accessory idioplasm required for 
 budding causes the reproduction of the entire animal, and must 
 therefore contain all the determinants of the germ-plasm, and 
 must exist in the ovum before segmentation, remaining in a 
 latent condition in a definite series of cells during all the stages 
 of development. If now this accessory idioplasm were capable 
 of becoming active under certain abnormal influences, such as 
 that produced by the destruction of the other blastomere, a 
 regeneration of the whole embryo might thus result. 
 
 These explanations are, however, only possible ones, and I 
 should not have been sorry to leave them out of consideration 
 altogether, for I am fully aware of their incompleteness and 
 unreliability : I merely wish to show that the observations men- 
 tioned above do not render an explanation impossible, even 
 although we are not able at present to state that any particular 
 interpretation of the phenomena is the correct one, because the 
 observations themselves are far too incomplete and deficient. 
 For this reason I shall not attempt to give a more precise ex- 
 planation of the peculiar development of these embryos. 
 
 I must, however, draw attention to the different behaviour of 
 the eggs in the case of the frog and in that of Ascidians and 
 sea-urchins. Leaving aside the question of ' post-generation,' 
 we have seen that only half an embryo arises from one blasto- 
 mere of a frog's ovum, while an entire animal becomes developed 
 from one blastomere in the case of either of the other two ani- 
 mals mentioned. However imperfect the explanation I have 
 offered may be, the fundamental assumption on which it is 
 based must in general be a correct one, viz., that the first 
 blastomeres of the egg of an Ascidian or sea-urchin must possess 
 a capacity which is absent in the case of the frog's egg. As, 
 however, forces are dependent on substances, it is probable 
 that the blastomeres of an Ascidian and of a sea-urchin contain 
 an excess of substance the accessory idioplasm which gives 
 them the power of regeneration, and that this substance is wanting 
 in the blastomeres of the frog. Driesch, as already mentioned, 
 expresses a doubt as to whether the blastomere of a frog would 
 not behave in a similar manner to that of a sea-urchin, if, like 
 the latter, it could be completely separated and isolated from its 
 injured fellow blastomere. This doubt seems, however, to be 
 hardly justified, as such an isolation was not effected in Chabry's
 
 142 THE GERM-PLASM. 
 
 experiments on the ascidian ovum, but nevertheless the de- 
 velopment into a complete animal ensued just as in the case of 
 the egg of the sea-urchin. 
 
 Although the half of a frog's egg develops into half an 
 embryo only in the first place, the latter may subsequently 
 become completed by a very peculiar regenerative process, which 
 was first observed by Roux in ' half and ' three-quarter embryos,' 
 and which he designated as ' post-generation.' 
 
 Roux observed that a segmentation -cell of a frog's egg may 
 be ' re-animated ' after it has been deprived of its capacity for 
 development. A considerable number of nuclei pass into the 
 vitellus of the injured part from the normally developed half of 
 the egg, and there increase and give rise to cells. ' The post- 
 generative formation of the germinal layers takes place from 
 the cell-material subsequently formed, while the process of 
 differentiation continues to advance in the quiescent cell-mate- 
 rial.' Roux thought he observed that a complete restoration of 
 the embryo may take place in this manner, so that it can con- 
 tinue to live ; and, in fact, he actually succeeded in keeping such 
 an embryo alive for some time. 
 
 Considerable attention has naturally been drawn to these 
 observations, which are certainly of the greatest interest ; but 
 I doubt whether in their present state they are sufficiently com- 
 plete to form the basis of fundamental theoretical conclusions. 
 With all respect for Roux's accuracy of observation and skill 
 in research, I cannot help thinking that the half embryos which 
 were subsequently ' post-generated ' to entire animals, were pos- 
 sibly those in which the thrust with the hot needle had not 
 affected the nucleus of the segmentation-cell. 
 
 In any case, it was only possible to observe the actual effect 
 of the operation and its result on the whole series of processes 
 which followed, and which led to the restoration of individuals 
 other than those which ultimately became complete. To pierce 
 a segmentation-cell with a hot needle must be a tolerably rough 
 operation, and something different may be destroyed each time 
 it is performed : not only the nuclear matter as a whole, but 
 also the individual idants, might possibly remain uninjured. 
 The idants, again, might subsequently increase to the normal 
 number by doubling, and so bring about the development of the 
 half of the egg. Roux certainly states that ' post-generation ' 
 does not occur in the same manner as does the normal develop-
 
 REGENERATION. 143 
 
 ment of the two primary halves, that is to say, the germinal 
 layers are not formed independently in each ; but the processes 
 which take place in the interior of the ovum can only be fol- 
 lowed out by means of sections, the preparation of which neces- 
 sitates the killing of the embryo. 
 
 In such experiments, moreover, no two cases are alike, and it 
 would be necessary to examine a very large amount of material 
 before stating with any degree of certainty that the egg which 
 has been cut into sections, and that in which the development 
 and post-generation have been followed out, have a precisely 
 similar internal structure. 
 
 Roux observed a ' re-animation ' of three different kinds in the 
 halves of the eggs'operated upon, one of which consisted in a growth 
 of the cells in the external layer of the living half around the dead 
 half. In this instance, however, post-generation did not result : 
 it only occurred in certain, but not all, of those cases mentioned 
 above in which nuclei passed from the living half into the part 
 which had been operated upon, and in which only slight patho- 
 logical changes had occurred in the yolk. It is therefore natural 
 to suppose that post-generation only occurred when the injury 
 was a slight one, and when some nuclear matter remained 
 and subsequently caused a formation of cells. This, however, 
 does not imply that living 'nuclei' did not penetrate into the 
 injured half of the egg ; the segmentation-cells, even in normal 
 development, have to undergo an enormous increase, and it is 
 therefore not surprising that after the opposition to growth has 
 been removed by the operation on the other half of the egg, they 
 should increase at the expense of the latter. In those cases in 
 which the other half of the embryo was subsequently completed, 
 this completion must have resulted from a kind of infection 
 of the cell, of such a nature that mere contaet with ectoderm 
 or mesoderm cells, for example, caused the undifferentiated 
 cells of the injured half of the egg to become correspondingly 
 differentiated into ectoderm and mesoderm cells. But I could 
 only accept such a revolutionary hypothesis as this if it could be 
 proved by incontestable facts. 
 
 Roux himself has, however, only looked upon his contribu- 
 tions to this subject as ' a first instalment of a large work,' and 
 has led us to expect a continuation of his experiments. But as 
 long as the processes which he describes admit of more than 
 one interpretation, we cannot reject the hypothesis of the pre-
 
 144 THE GERM-PLASM. 
 
 destination of the cells by means of the distribution of certain 
 determinants and groups of determinants to them, for this 
 view is supported by so large a number of facts, and even by the 
 earlier experiments of Roux himself. It would certainly, how- 
 ever, have to be rejected if we could prove that the cells of 
 the germinal layers were really capable of being determined in 
 their nature by the region which they accidentally reach, or by 
 their accidental surroundings. 
 
 Further research along the line opened up by Roux will, I am 
 convinced, show us the facts in another light, and will enable us 
 to reconcile them to the rest of our conceptions as to the causes 
 of ontogeny. But I do not consider it worth while at present to 
 enumerate all the possible causes which must be taken into 
 account in an attempt to explain ' post-generation.'
 
 MULTIPLICATION BY FISSION. 145 
 
 CHAPTER III. 
 
 MULTIPLICATION BY FISSION. 
 
 i. PRELIMINARY REMARKS. 
 
 Until a short time ago the process of multiplication by gem- 
 mation was looked upon as having been derived phyletically from 
 the corresponding process by fission, and the two were thought 
 to be closely related, and connected by gradual transitions. Von 
 Wagner* has, however, recently attempted to contest this 
 opinion, and to show that the two processes should be more dis- 
 tinctly separated from one another than they have hitherto been, 
 and that they are, in fact, genetically distinct. By the term 
 1 fission,' Wagner means to indicate a process of multiplication 
 which is preceded by a symmetrical growth of the parent 
 organism, by means of which the individuality of the latter be- 
 comes changed and to a certain extent abolished : the term 
 'gemmation,' on the other hand, he takes to mean a process of mul- 
 tiplication which is preceded by an unsymmetrical (differential) 
 growth of the parent organism, in which the individuality of the 
 latter is not abolished and its place taken by a new individual. 
 
 This view I agree with in go far as I am convinced that in 
 multicellular organisms the processes of multiplication by fission 
 and budding have not arisen genetically from one another : 
 these processes differ so essentially that it will be advisable to 
 discuss them separately. 
 
 Following von Wagner's example, I shall include under this 
 head of fission all the processes of asexual multiplication which 
 occur in the flat-worms (Turbellaria, Cestoda), the annelids 
 proper (Syllidce, Natda, Tubificidce, &c.), and also in the higher 
 Medusae (strobilation). In all these cases multiplication is 
 effected by the division of the parent animal into two or more 
 
 * Franz von Wagner, 'Zur Kentniss der ungeschlechtlichen Fort- 
 pflanzung von Microstoma nebst allegemeinen Bemerkungen iiber Thielung 
 u. Knospung im Thierreich,' Zool. Jahrbucher, Abth. f. Anat. u. Onto- 
 genie, Bd. iv., Jena, 1890. 
 
 K
 
 146 THE GERM-PLASM. 
 
 parts : this therefore necessitates a regeneration of one or of the 
 other end of the body, or even of both ends. This process may 
 begin to take place either after (Ltunbricuhis) or before the 
 
 division has taken place, 
 and in the latter case is 
 more or less complete 
 before the fission begins. 
 The actual process of the 
 formation of the new 
 organism is essentially 
 the same in both cases, 
 and important, differences 
 only occur as regards the 
 various groups of animals. 
 We are particularly well 
 acquainted with these pro- 
 cesses of regeneration 
 which may either precede 
 or succeed fission in the 
 case of various kinds of 
 worms, and we will there- 
 fore first illustrate them in 
 their main features by 
 reference to these animals. 
 
 FIG. ^. Myrianida, a marine worm which mul- 
 tiplies by fission (after Milne-Edwards, from 
 Hatschek's ' Lehrbuch der Zoologie '). The 
 letters a g indicate the relative ages of the 
 daughter-individuals resulting from the di- 
 vision of the parent. 
 
 2. THE PROCESS OF FISSION IN THE 
 
 The process of .fission in these small fresh-water segmented 
 worms has been very accurately followed out by Semper. An 
 individual undergoes division into two, or usually into several, 
 daughter-individuals at the same time, the fission being regu- 
 larly preceded by a circular growth of cells taking place 
 around the circumference of the body at one or more definite 
 regions, each of such ring-like thickenings eventually giving 
 rise to a new head- and tail-end respectively. These rings of 
 cells have hitherto been spoken of as 'zones of gemmation ;' 
 but it would be better to call them ' zones of regeneration/ 
 as they are not concerned with budding in the true sense of the 
 word. Two of these rings are as a general rule formed in each 
 animal, and when the anterior and posterior ends of each of the 
 resulting three sections are fully developed, the separation into
 
 MULTIPLICATION BY FISSION. 147 
 
 the corresponding three daughter-individuals takes place by a 
 constriction in the middle of each zone of regeneration. 
 
 In Nat's the zones of regeneration are always formed at the 
 boundary line of two segments : that is to say, they arise from 
 the contiguous margins of two segments, in the following way. 
 Cells of the epidermis first begin to multiply, and give rise to a 
 circular layer of small stratified cells, which is thickest on the 
 ventral side. The cells have at first no definite histological 
 character. At the same time an increase in the length of the 
 internal organs takes place : this is rendered necessary by the 
 growing zone penetrating between the segments from which it 
 arises and thus forcing them apart. The alimentary canal, how- 
 ever, is the only internal organ which becomes regenerated from 
 its own cells : all the other new formations, including the ventral 
 nerve cord, muscles, blood-vessels, 'liver '-cells, and excretory- 
 organs, are developed from the ring of proliferating epidermic 
 cells. 
 
 As Semper has pointed out, the process of the reconstruction 
 of the anterior and posterior ends which prepares the way for 
 fission, may in a sense be compared with the embryonic develop- 
 ment of the animal subsequent to the gastrula stage, in which 
 the two primary germinal layers are already distinct. 
 
 In these regenerative processes two layers of formative cells 
 are likewise produced, owing to the proliferation of the ectoderm 
 cells on the one hand, and those of the alimentary canal on the 
 other ; the epithelial lining of the latter only is formed from the 
 internal layer, the outer layer giving rise to all the other organs, 
 including the mesodermic structures as well as those which 
 belong to the ectodermic part of the integument. In fact the 
 resemblance between the processes which take place in embryo- 
 geny and in regeneration is so close, that in both cases the 
 mesoderm becomes split off from the mass of formative 
 ectoderm cells in the form of two longitudinal bands, from which 
 the blood-vessels, muscles, &c., are then differentiated. 
 
 In order to explain these processes theoretically from our 
 point of view, we must suppose that those cells of the epidermis 
 from which the formative cells arise possess an ' accessory idio- 
 plasm] containing the determinants of those organs which are 
 formed from them in regeneration in addition to their own 
 specific idioplasm. The rate of division of each of these cells, 
 as well as the manner in which the groups of determinants con-
 
 148 THE GERM-PLASM. 
 
 tained m them becomes disintegrated in the course of the sub- 
 sequent divisions, is strictly definite, and determines the number 
 of successors which each cell produces, as well as the relative 
 position and combination into organs, and histological differen- 
 tiation of the cells. When the process of proliferation begins, 
 the newly-formed cells no longer retain the specific epidermic 
 character, and their successors may, indeed, be said to possess 
 an 'embryonic character,' in the sense in which that term 
 has usually been used, if it is not thereby understood that they 
 must all contain similar primary constituents. Their further 
 development shows that this cannot be so : the cells of one 
 particular region give rise, for instance, to the dorsal vessel ; 
 those of another to the nerve cord ; those of a third region to 
 certain muscles, and so on. 
 
 We must therefore suppose that the various epidermic cells 
 of the parent animal are pro- 
 vided with active accessory 
 idioplasm, somewhat in the 
 manner I have indicated in 
 the accompanying diagram. 
 The cells marked ;/, for in- 
 stance, would contain the 
 groups of inactive determin- 
 ants for the formation of the 
 ventral nerve-cord ; those of 
 the epidermis marked ' m, 
 
 Fu;.5.-Transver S e section through a Nais r U P S <>f determinants for 
 in the region of the zone of regeneration the mesoderm Structures, in 
 
 (modified from Semper). Ekt, integu- addition to their own proper 
 
 ment ; Ent, Ep^elium of the all- idioplasm, W l Containing 
 
 mentary canal; N, Nerve-cord; J\fs, , , , , , , 
 
 Visceral mesoderm; Vd, Dorsal blood- those for the lateral muscles, 
 
 vessel ; m, Cells with accessory deter- 1H~ those for the Ventral 
 
 minants for the mesoderm : n Cells blood-VCSSel, /// 3 tllOSC for the 
 
 with accessory determinants for the ( .. ., . . , 
 
 nerve-cord. liver-cells and mesodermic 
 
 part of the intestine, w 4 those 
 
 for the segmental organs and the adjacent system of muscles, 
 m 5 those for the dorsal vessel, ;// 6 those for the dorsal muscles, 
 and so on. For the sake of simplicity I have supposed that the 
 epidermis consists of only one layer Of cells, although in reality 
 there are two layers in many parts: the diagram, in fact, is not 
 by any means intended to represent the actual structure of the
 
 MULTIPLICATION BY FISSION. 149 
 
 animal in detail, or even to indicate accurately the part which 
 the individual cells take in the process of budding. 
 
 The question of the origin of the supplementary determinants 
 which we have assumed to exist in the cells of the epidermis, 
 does not stand in the way of this explanation of the regenera- 
 tive process ; for, as already stated, a similar course is in the 
 main followed both in embryogeny and regeneration. In 
 both processes the primary mesoderm arises from the primary 
 ectoderm. The definitive ectoderm cells have therefore an 
 opportunity during their embryonic development of taking over 
 certain primary mesoderm determinants as accessory idioplasm 
 from the primary ectoderm cells : these can then become 
 separated into several groups during the multiplication of 
 the ectoderm cells, so that the epidermic cells around the 
 circumference of the body are provided with an accessory 
 idioplasm consisting of various mesoderm determinants. 
 
 It must, however, also be borne in mind that the growth in 
 length of the worm only takes place at the posterior end of the 
 body, just as occurs in the regeneration which prepares the way 
 for division. In both cases a new body-segment is formed 
 between the last segment and the last but one, in which process 
 the epithelium of the intestine alone arises from the endoderm, 
 the integument and all the mesodermic structures being formed 
 from the ectoderm. Thus the accessory determinants which we 
 have assumed to exist in the epidermic cells, and which render 
 the subsequent regeneration possible, are not derived from the 
 embryo directly, but from the zone of growth in the tail-end, 
 into which again they have passed during embryogeny. 
 
 3. THE PROCESS OF FISSION IN THE MICROSTOMIDJE. 
 
 It is not, then, in the nature of every ectoderm cell to give rise 
 to all the possible kinds of cells and organs with the exception 
 of the epithelium of the alimentary canal : each one must be 
 specially equipped for the purpose. This is proved by the fact 
 that the ectoderm by no means always performs this function 
 in animals which multiply by fission : even in some worms this 
 is not the case. 
 
 According to von Wagner's* excellent researches, the cells 
 of the epidermis in a certain flat-worm, Microstoma h'neare, take 
 
 * Loc. ctt.
 
 150 THE GERM-PLASM. 
 
 a very small share in the reconstruction of the anterior and 
 posterior ends of the animal during the process of fission, the 
 restoration being effected in this case chiefly by the mesoderm, 
 or so-called ' connective tissue ' cells, which ' are suspended in 
 large numbers in the perivisceral fluid between the supporting 
 trabeculae.' These cells begin to increase in number when the 
 animal is preparing for division, and by their multiplication 
 they form a ventral mass of so-called ' embryonic ' cells, which 
 gives rise to the pharynx, the pharyngeal and prostoinial glands, 
 all the parts generally known as ' parenchymatous ' or ' meso- 
 dermic structures,' and also apparently to certain parts of the 
 nervous system. Kennel * found that a similar mode of develop- 
 ment of these parts occurs in a Planarian. In such cases we 
 must therefore suppose that the accessory determinants re- 
 quired for regeneration are supplied to the mesoderm cells, 
 instead of to those of the ectoderm. We cannot at present 
 determine whether this is effected by each of these cells being 
 provided with all the supplementary determinants for the 
 mesoderm, which only become disintegrated and distributed 
 amongst the other cells when these begin to multiply, or as 
 we assumed in the case of the ectoderm cells of Naisby the 
 distribution of the different determinants to a number of these 
 cells before proliferation occurs. 
 
 The regularity with which all organs are formed in the proper 
 position and mutual relation, may perhaps be taken as a proof 
 of the assumption that they contain latent determinants which 
 are from the first separate, and which differ according to the 
 topographical position of the organ. It is hardly possible that 
 the contrary assumption can be the correct one, for this would 
 render it necessary to suppose that although all the deter- 
 minants are certainly present in every formative cell, only 
 that one can undergo development which corresponds to the 
 region in which the cell happened to be situated. 
 
 Here, again, we meet with no serious difficulty as regards 
 the derivation of the required supplementary determinants in 
 ontogeny : in fact there is less difficulty in this case than in that 
 of A'ai's, for the cells of the different layers of the body contain 
 the determinants for the corresponding organs. 
 
 * J. Kennel, ' Untersuchungen an neuen Turbellarien,' Zool. Jahr- 
 bucher, Bd. 3. Abth. f. Anat. u. Ontog. d. Thiere., p. 447.
 
 MULTIPLICATION BY FISSION. 151 
 
 4. THE PHYLOGENY OF THE PROCESS OF MULTIPLICATION 
 BY FISSION. 
 
 There can be little doubt that the process of spontaneous 
 division which occurs in flat-worms and in annelids is to be 
 derived phylogenetically from regeneration, as Kennel* has 
 recently attempted to prove. He has rightly, I believe, shown 
 that multiplication by a spontaneous separation into parts, such 
 as occurs regularly in the freshwater worm Lumbriculus, must be 
 looked upon as a preliminary stage of that kind of fission, 
 accompanied by regeneration, which occurs in the Naida, for 
 instance. The difference between the two processes consists 
 essentially in the fact that in Nats the separation into parts is 
 preceded and prepared for by the formation of new head- and 
 tail-ends, which appear between the old segments at the point 
 at which the separation is to take place. Such a preparatory 
 process does not occur in Lumbricitlus : the region in which 
 division will take place in this worm cannot previously be dis- 
 tinguished, and the new head- and tail- ends are formed subse- 
 quently, after the division has occurred. 
 
 The capacity for division of an individual into parts must 
 naturally be looked upon as an adaptation, and it presupposes 
 some kind of histological and physiological arrangement of 
 which we are at present ignorant. It is, however, quite conceiv- 
 able that when fission had once occurred in a species, it may 
 have been advantageous for a more thorough preparation for the 
 process to take place, and for the structures necessary for the 
 completion of the individuals thus formed to become developed 
 beforehand. Such a capacity for multiplication by spontaneous 
 division necessitates, moreover, the previous possession of the 
 power of regeneration. Hence the latter must have existed in the 
 animal before spontaneous division could take place regularly 
 in the species, and we must thus conclude that the capacity for 
 regenerating portions of the body which had been accidentally 
 torn asunder was first acquired very early in the phylogeny of 
 multicellular animals ; and that the special arrangements for 
 multiplication by fission subsequently originated from this 
 capacity for regeneration, and was followed by the formation of 
 new head- and tail-ends. The formation of the new parts pre- 
 
 * J. Kennel, ' Uber Theilung u. Knospung der Thiere,' Dorpat, 1888.
 
 152 THE GERM-PLASM. 
 
 viously to the division must be looked upon as a still later 
 modification of the process. 
 
 This conclusion receives further support from the fact that, as 
 already shown, the capacity of regeneration is not by any means 
 an inherent quality of the organism : that is to say, it is not a 
 direct and inevitable result of a particular degree of organisa- 
 tion, but is due to an adaptation produced by natural selection, 
 and constitutes a special arrangement which may exist in 
 different degrees of perfection, or which may, again, be entirely 
 absent. If an earthworm is cut into two, the anterior portion 
 develops a new tail-end, but the posterior portion does not 
 give rise to a new head-end : the arrangement existing in 
 Lumbriculus and Nais is therefore absent in this case. This 
 fact I should explain by assuming that in the last-named animals 
 the determinants required for the formation of the head-end 
 are supplied to the cells of the integument and alimentary canal 
 as accessory idioplasm, while in the earthworm these cells 
 only possess the determinants required for the formation of the 
 tail-end. 
 
 It is very possible that the arrangement for the regeneration 
 of the tail-end may have taken place more easily than that 
 for the restoration of the head in the case of segmented 
 worms, owing to the fact that the last segment possessed the 
 power of giving rise to entire new segments. The growth of 
 the animal is effected by the formation of new segments at the 
 posterior end of the body, which would therefore be already 
 provided with the requisite accessory determinants, and it would 
 then only be necessary that these should be transferred to the 
 corresponding cells of the other body-segments. This might 
 have taken place in a relatively simple manner in the course of 
 phylogeny, by a portion of the accessory determinants being 
 left in the cells of each new body-segment as it became formed. 
 The determinants of the head-end, on the other hand, can only 
 have been supplied to the respective cells as accessory idio- 
 plasm before or during embryonic development ; we can there- 
 fore understand why the capacity for forming a new head-end 
 was only acquired later, and that some worms are able to re- 
 generate the posterior, but not the anterior end by the body 
 when it is cut in half. 
 
 We can therefore trace a series of stages of gradually increas- 
 ing complexity in the development of the process of regenera-
 
 MULTIPLICATION BY FISSION. 153 
 
 tion in worms, beginning with the formation of segments at the 
 growing tail-end, and then passing from the regeneration first 
 of the tail, and then of the head-end, to the actual fission of 
 Lumbriculus, and finally to that of Nais. And according to 
 our view, this course of development depends on the regular 
 distribution of certain accessory determinants to particular 
 tissue-cells, and the gradual increase in the complexity of this 
 distribution. 
 
 The regenerative process which renders fission possible must 
 be traced to the doubling of certain groups of determinants in 
 Ae idioplasm, so that the half of them remain latent. I imagine 
 that this doubling need not necessarily take place and be fol- 
 lowed by the subsequent multiplication of the inactive groups 
 of determinants in the germ-plasm itself : such a multiplication 
 would, in fact, be a useless encumbrance to the germ-plasm. 
 The latter need only contain the determinants for fission when 
 this process leads to an alternation of generations ; that is to 
 say, when the animals formed by division have a different struc- 
 ture from those which arise directly from the ovum : for in the 
 latter case the forms which arise by fission are independently 
 variable hereditarily. This must be the case in the alternation 
 of generations in certain marine annelids, such as the Syllidce, 
 and also in the strobilation of polypes, which will be discussed 
 later on. In all such cases, two kinds of ids must be assumed 
 to exist in the germ-plasm. In the ordinary kind of fission 
 which occurs in the fresh-water annelids, on the other hand, the 
 separation of the groups of determinants necessary for the re- 
 generation of a part may take place during embryogeny : 
 nothing definite, however, can be said on this point at present, 
 but in any case the process of splitting off of the accessory deter- 
 minants may conceivably be thrown back from the later to the 
 earlier stages of ontogeny, until it finally takes place in the 
 fertilised ovum, so that double ids are present in the germ- 
 plasm. It will be assumed in the next section that certain forms 
 of budding owe their origin to the presence of these double ids.
 
 154 THE GERM-PLASM. 
 
 CHAPTER IV. 
 
 MULTIPLICATION BY GEMMATION, 
 i. THE PROCESS OF GEMMATION IN ANIMALS. 
 
 If, with von Wagner, we look upon gemmation as 'a process 
 in which entire individuals are formed anew,' and which depends 
 ' exclusively on a special (differential) growth differing from the 
 normal one,' we must include under this term the processes of 
 asexual multiplication which occur in most of the Ccelenterata, 
 the Polyzoa, and the Tunicata. 
 
 A. C&lenterata. 
 
 Hitherto it has been considered that we were fully acquainted 
 with the process of gemmation in the Ccelenterata, especially in 
 the case of the Hydrozoa. It had been observed that the two 
 layers of cells which form the body-wall of these animals are 
 present even in very young buds of Medusas and Hydroid- 
 polypes. These layers surround the digestive-cavity just as they 
 do in the parent animal, and since the body-wall as well as the 
 cavity it encloses are in direct connection with those of the 
 parent, nothing was more natural than to suppose that the bud 
 arises as an evagination of the body-wall of the parent, both layers 
 of the latter taking part in its formation from the first. A doubt 
 as to the correctness of this statement was less likely to arise 
 owing to the fact that even in the youngest buds of a Hydroid- 
 polype, before they become hollow, the ectoderm and endoderm 
 were seen to consist of a number of cells engaged in active 
 multiplication. I myself made such a statement in connection 
 with my investigations on the formation of the sexual cells in 
 Hydroids,* and no doubt has yet been raised as to its correctness, 
 or rather as to its interpretation. 
 
 The assumption that both germinal layers of the parent take 
 part in the formation of the bud is nevertheless an incorrect one ; 
 for the bud arises from the ectoderm only, and the young cells 
 
 * ' Die Enstehung der Sexualzellen bei den Hydromedusen ' (with 25 
 plates), Jena, 1883.
 
 MULTIPLICATION BY GEMMATION. 155 
 
 which form the endoderm in these buds are not derived from the 
 endoderm of the parent, but have migrated from the ectoderm. 
 
 Purely theoretical considerations first led me to suppose that 
 this must be so. The origin of the process of gemmation in the 
 idioplasm can only be brought into agreement with the theory 
 of the continuity of the germ-plasm, if the cells of the parent- 
 organism from which buds arise collectively contain all the 
 determinants of the species as accessory idioplasm. If this were 
 not the case, an entire animal, capable of reproduction, could 
 never arise from the bud. If, now, a certain cell of the ectoderm 
 contained all the determinants for the outer, and one of the endo- 
 dermal cells all those for the inner layer, a bud could only be 
 formed when these two cells happened to lie exactly opposite to 
 one another in the body-wall. As, however, the endoderm cells 
 form a definite and continuous epithelial layer, and have a fixed 
 relative position, and, moreover, the position of the ectoderm cells, 
 although not quite so definite, is still on the whole a fixed one, I 
 found it difficult to imagine how budding could take place at 
 perfectly definite parts of the polype and of the stock in such a 
 regular manner as actually occurs in many cases. The assump- 
 tion that all the cells of the ectoderm and endoderm are equally 
 provided with the necessary accessory idioplasm is excluded by 
 the fact that budding occurs in such a regular manner. I am 
 therefore led to suppose that the distribution of the 'blastogenic* 
 germ-plasm ' might possibly be confined to one germinal layer 
 only; and since it is known that in Hydroids the germ-cells are 
 always developed from the ectoderm, it is natural to conclude that 
 the blastogenic idioplasm is contained in the cells of this layer. 
 
 This conclusion has now been confirmed by investigations 
 carried out by Mr Albert Lang, in trfe Zoological Institute at 
 Freiburg. In various Hydroid-polypes (Eudendrittm, Pluinu- 
 laria, and Hydra) the bud arises in the following way. The 
 cells in a certain small circumscribed region of the ectoderm 
 first begin to multiply, the ' supporting lamella,' which separates 
 the two layers of the body- wall, gradually becoming thinner and 
 softer at the same time, and then a few of the newly-formed cells 
 penetrate into the endoderm through this membrane. Here they 
 
 * The term ' blastogenic idioplasm ' is here used in the special sense of 
 ' Knospungs-Idioplasma,' and not in the more general sense in which 
 it is usually used by the author (cf. , e.g. , the chapter on ' The Supposed 
 Transmission of Acquired Characters ') W. N. P.
 
 156 THE GERM-PLASM. 
 
 form a layer of young actively dividing cells, such as I had for- 
 merly observed in very young buds : this layer forces the older 
 endoderm cells away from the supporting membrane, in conse- 
 quence of which they loosen their connection with the rest of 
 the endoderm, undergo disintegration, and gradually become 
 absorbed. The cells, however, which have migrated from the 
 ectoderm then give rise to the endoderm of the bud. 
 
 Now that these facts have been proved by Lang's investiga- 
 tions,* it is easier to give a theoretical explanation of the process 
 
 Mk-' 
 
 FIG. 6. Diagrammatic section through the rudiment 
 of a bud of Eudendrium. (Modified from a figure 
 by Albert Lang). Ps , the horny perisarc ; Ps , 
 portion of the perisarc which has become very 
 thin owing to the pro'Kferntion of the underlying 
 ectoderm (Ekti); Enti, the region of the endo- 
 derm, in which a number of proliferating ectoderm 
 cells have broken through the supporting lamella 
 (si), migrated into the endoderm, and caused the 
 latter to project into the gastric cavity. 
 
 of gemmation in Hydroids. We must, however, still assume 
 that certain cells and series of cells in the ectoderm are provided 
 with an accessory idioplasm, which contains all the determinants 
 of the species, and which is therefore a kind of germ-plasm, 
 though perhaps not quite identical with the germ-plasm proper : 
 
 * Albert Lang, ' Ueb. die Knospung bei Hydra u. einigen Hydro- 
 polypen,' Zeitschr. f. wiss. Zool., Bd. 54, 1892, p. 365.
 
 MULTIPLICATION BY GEMMATION. 157 
 
 I therefore speak of it as 'blastogenic idioplasm.' It cannot be 
 stated with certainty which cells of the ectoderm contain this 
 idioplasm ; it seems, however, that the growth of the bud 
 originates in the deeper layer, i.e., in the 'interstitial' cells. 
 We may therefore suppose that some of these interstitial cells 
 contain inactive blastogenic idioplasm, which, after a certain 
 series of cell-divisions necessary for the growth of the polype, 
 obtains the control of one of the offspring of these cells, and so 
 causes budding to take place. Each bud must originally arise 
 from one cell only, although this fact has not as yet been actually 
 proved ; and in the first division, or at any rate in the early divi- 
 sions of this cell, the group of determinants of the ectoderm must 
 become separated from that of the endoderm, the ' bearers ' of 
 the latter group migrating into the old endoderm through the 
 disintegrating supporting lamella. The remaining details of the 
 process require no further explanation. 
 
 In the Hydromeduscs, then, each bud originates in a single 
 cell, and the process of multiplication by gemmation therefore 
 differs essentially from that of reproduction by fission. For 
 gemmation owes its origin to the entire mass of the determinants 
 of the species, which only undergo disintegration at a later stage ; 
 while the new structures which arise by fission originate simul- 
 taneously from numerous smaller groups of determinants, corre- 
 sponding with those of the later stages of ontogeny. 
 
 We should nevertheless be mistaken in supposing that the 
 essential difference between fission and gemmation is due 
 altogether to this difference as regards the group of determinants 
 concerned in the two processes. This is rendered evident by a 
 comparison with the processes of budding in other groups of 
 animals. It still remains to be shewn whether the process of 
 gemmation in other Ccelenterates, viz., in the Actinozoa, the 
 higher Medusa, and the Ctenophora, also only apparently origi- 
 nates from both layers of the body-wall, or whether it actually 
 arises from one layer only. As the possibility of the latter mode 
 of origin has not till now been considered, it is very possible that 
 the migration of cells may have been overlooked in thiscase also. 
 
 If we now turn our attention to the other groups of the animal 
 kingdom in which gemmation occurs, viz., to the Polyzoa and 
 Tunicata, we shall find that we possc-s the results of very excel- 
 lent investigations on which our arguments can be based ; and 
 the histological structure of these animals is such as to render
 
 158 THE GERM-PLASM. 
 
 it unlikely that any oversight as regards the migration of cells 
 can have occurred. 
 
 B.Polyzoa. 
 
 The small stocks or colonies formed in the Polyzoa arise by 
 a process of gemmation ; and even the small number of species 
 which do not form stocks multiply vigorously by budding, but 
 in these cases the buds become detached from the parent sooner 
 or later. 
 
 The process of gemmation seems to be essentially similar in 
 all Polyzoa. A proliferation, which primarily originates in one 
 cell, takes place in a certain region of the epidermis ; the masses 
 of cells which are thus produced form a hollow invagination, 
 which extends into the body-cavity of the animal and gives rise to 
 the entire alimentary canal including the fore-, mid-, and hind- 
 guts, as well as to the preoral ' atrium ' with the tentacles 
 (' lophophore ') Certain ' free mesoderm cells ' are then said to 
 migrate from the body-cavity of the parent into the bud, in 
 which they give rise to the muscles and sexual organs, and also 
 in certain groups of Polyzoa to the outer (serous) layer of the in- 
 testine ; while in others again they form a subcutaneous layer of 
 cells. This is at any rate the case according to the recent obser- 
 vations made by Seeliger,* which are undoubtedly very accurate 
 and trustworthy. But one point, however, still seems to be 
 doubtful, viz., whether the sexual organs may not perhaps, after 
 all, arise from the primary proliferation of the epidermic cells. 
 
 These processes of gemmation interfere very considerably 
 with the ordinarily accepted and extremely conventional ideas 
 of the germinal membranes ; for the epithelium of the alimentary 
 canal, which characteristically belongs to the inner germinal 
 layer, here arises from the ectoderm. This, however, causes no 
 difficulty from the point of view of the theory of the germ- 
 plasm : we need only assume that the group of determinants for 
 the endoderm is passed on to certain cells of the epidermis as 
 accessory idioplasm. This transference must take place at an 
 early stage in embryogeny, before the separation of the primary 
 endoderm and ectoderm occurs. 
 
 Nitsche, whose means of observation were comparatively im- 
 
 * O. Seeliger, 'Die ungeschlechtliche Vermehrung der endoprokten 
 Bryozoen,' and ' Bemerkungen zur Knospenentwicklung der Bryozoen,' 
 Zeitschr. f. wiss. Zool., Bd. 49 and 50, 1889 and 1890.
 
 MULTIPLICATION BY GEMMATION. 159 
 
 perfect, but whose researches are nevertheless of great value, 
 concluded that the whole bud was derived from the prolifera- 
 tion of the ectoderm. Had his statement proved correct, the 
 explanation of the process of budding in the Polyzoa, based on 
 the idioplasm, would be just as simple as in the case of Hydroids : 
 it would then only have been necessary to suppose that the cell 
 in which proliferation first began contained accessory idioplasm 
 in the form of ' blastogenic idioplasm.' Seeliger was, however, 
 unable to support Nitsche's statements, and the most recent 
 observations of Oka,* Davenport,t and Braem,| prove beyond 
 doubt that the ' mesoderm cells ' of the parent take part in the 
 formation of the buds. We must therefore suppose that certain 
 mesoderm cells, provided with definite groups of determinants 
 for muscles, endothelia, and sexual organs, migrate into the bud. 
 It is quite conceivable that muscles, and more especially endo- 
 thelia, should be developed in this manner, but it would be 
 difficult to understand how free cells from the body-cavity of the 
 parent could migrate into the bud, and there give rise to sexual 
 organs at perfectly definite regions : were this so, we must sup- 
 pose that in reality certain of the cells only, and not any of 
 them, are concerned in the migration. Such an assumption is, 
 however, contradicted by the abnormal processes of budding 
 which occur, for instance, in Pedicillina. I therefore do not 
 consider that the question of the origin of the sexual organs is 
 yet decided, but I suspect, nevertheless, that one or two of the 
 mesoderm cells of the bud are derived from the primary proli- 
 feration of the ectoderm. This view is supported by Seeliger's 
 statement that he considers such a derivation of individual 
 mesodern cells of the bud possible, at any rate, in the case of 
 Loxosoma.% 
 
 As, however, we are not specially concerned with the process 
 of budding in the Polyzoa in particular, but are only making 
 use of it as an example of gemmation in which two germinal 
 
 * A. Oka, ' Observations on Freshwater Polyzoa,' Journ. of College of 
 Science, Imperial University, Japan, Vol. iv., Pt. i, 1890. 
 
 t C. B. Davenport, ' Observations on Budding in Paludicilla and some 
 other Bryozoa,' Bull, of the Museum of Comp. Zool. at Harvard College, 
 Vol. xxii., No. i, 1891. 
 
 t I". Braem, ' Untersuch. iiber d. Bryozoen des sussen Wassers,' Bibl. 
 Zool., Cassel, 1890. 
 
 Seeliger, ' Bemcrkungen zur Knospenentwicklung der Bryozoen,' 
 Zeitschr. f. wiss. Zool., Bd. 50, p. 564.
 
 l6o THE GERM-PLASM. 
 
 layers are primarily concerned, we can leave this question aside. 
 It is at any rate true that in the Polyzoa parenchymatous cells, 
 as well as a certain ectoderm cell, take part in each process of 
 budding. We must therefore assume that the determinants of 
 the species cannot be contained altogether in one cell as blasto- 
 genic idioplasm, as in the case of the Hydrozoa, but that a 
 number of them including those for the muscles, endothelia, 
 blood-corpuscles, and perhaps those for the sexual organs also 
 are supplied to certain mesoderm cells of the parent. The 
 development of sexual cells renders it necessary that those 
 cells from which they arise shall also contain germ-plasm ; and 
 the formation of the epidermis of the bud, which results to 
 some extent on purely mechanical grounds, presupposes the 
 existence of determinants for the ectoderm in the epidermic cells 
 of the parents. 
 
 The disintegration of the determinants, which is necessary 
 before budding can take place, is obviously, however, of a very 
 different kind from that which occurs in embryonic develop- 
 ment. Seeliger, indeed, has called attention to the fact that the 
 ontogeny which results from gemmation is a much shorter pro- 
 cess than that which occurs when an embryo and larva are 
 formed. In the former case, not only are the whole series of 
 stages of segmentation and development of a free-swimming 
 larva absent, but even in the later periods of development none 
 of the stages in embryogeny and in gemmation exactly corre- 
 spond to one another. Without following out these two pro- 
 cesses in detail, I should be inclined to explain them in general 
 by assuming that the groups of latent supplementary deter- 
 minants, with which certain cells are provided in the course of 
 embryogeny, contain combinations of determinants different 
 from those which lead to the development of the embryo. 
 
 C. Tunicata, 
 
 The fixed Ascidians usually multiply very extensively by 
 gemmation, and thus give rise to stocks, the individual persons 
 of which are more or less closely connected with one another. 
 
 We owe our detailed knowledge of the process of budding 
 in the genus Clavelina to the researches of Seeliger.* The 
 
 * O. Seeliger, ' Zur Entwicklungsgeschichte der Ascidien ; Eibildung 
 u. Knospung von Clavelina lepadiformisj Sitzungsber. d. Wien. Aka- 
 demie, Bd. 85, 1882.
 
 MULTIPLICATION BY GEMMATION. l6l 
 
 parent, which was developed from an ovum, produces long stalk- 
 like processes or stolons, on which new animals are produced 
 by budding. Each of these stolons is made up of three layers 
 of cells an outer ectoderm, an inner endoderm, and an inter- 
 mediate layer of motile ' mesoderm cells.' The ectoderm layer 
 gives rise merely to the epidermis of the bud ; the epithelium of 
 the alimentary canal and its accessory organs, the branchial sac 
 (' peribranchial tube '), and the pericardial tube, being developed 
 from the endoderm ; and the muscles, ganglion (?), and sexual 
 glands from the ' free mesoderm cells.' 
 
 The endodermal tube mainly determines the form of the 
 animal in these processes : it becomes definitely segmented, 
 and on it the growing ectodermal tube is moulded, so to speak. 
 We may thus conclude that a series of homologous formative 
 zones of structure are to be found in the endodermal tube of 
 the stolon, each of which may consist originally of a single 
 circular layer of cells. At the point where a bud will arise, the 
 corresponding zone of cells grows out to form a bladder-shaped 
 enlargement, which becomes detached from its point of origin 
 on the endodermal tube of the stolon and regularly differen- 
 tiated, so as to give rise to the peribranchial tube, the intestine, 
 and so on. The cells of this endodermic vesicle cannot all be 
 equivalent, nor can they contain exactly similar determinants : 
 were that the case, such a differentiation could not occur, and 
 the walls of the peribranchial chamber could not arise from one 
 part, and the intestine from another. But even as regards the 
 primitive intestinal vesicle itself, one cell must contain the deter- 
 minants of the stomach, another those of the hind-gut, and so 
 on. In short, we must assume that just as occurs in principle, if 
 not as regards actual details in the case of embryogeny a dis- 
 integration of the idioplasm and a distribution of the groups of 
 determinants among the different cells takes place during develop- 
 ment. The determinants of all parts arising in connection with 
 each endodermal vesicle, must be collectively contained in each 
 zone of cells of the endodermal tube from which such a vesicle 
 is developed. 
 
 The formation of those organs which arise from the 'free 
 mesoderm cells' of the stolon is the most difficult to understand. 
 There is certainly no reason why we should not suppose that 
 these cells contain very different kinds of idioplasm : one, for 
 instance, might contain ' muscle-determinants,' another ' nerve- 
 L
 
 162 THE GERM-PLASM. 
 
 determinants,' and a third ' blood - corpuscle determinants.' 
 Various kinds of these cells may easily be distinguished while 
 they still float freely in the blood of the stolon. The difficulty only 
 consists in ascertaining the exact part they play in the formation of 
 the developing bud. Those which are to give rise to the longitudi- 
 nal muscles become arranged in rows, which, diverging obliquely 
 from one or two definite points, extend over the animal from 
 behind forwards, and are attached at more or less definitely 
 fixed points anteriorly. The ganglion and the sexual glands 
 have also perfectly definite positions in the animal. In em- 
 bryogeny, as well as in the development of the endodermal 
 vesicle of the bud, the position of every cell is assigned to it 
 mechanically, in consequence of its origin from previous cell- 
 generations, that is, by the rhythm of the cell-divisions. In the 
 case, however, of the ganglion for instance, the cells of which it 
 is composed must come together at the right place by means of 
 their power of locomotion. A similar process is known to occur 
 in embryogeny in the case of several groups of animals, such as 
 the Echtnodermata, for instance ; and until we know more of 
 the actual facts concerned, we can only however unsatisfac- 
 tory such an assumption may be attribute to the cells a ten- 
 dency to become attached at definite points according to the 
 manner in which they have previously been determined. The 
 reverse assumption that these cells develop into muscle-, 
 nerve-, or sexual-cells according to their point of attachment 
 seems to me at any rate a less likely one. 
 
 If we compare the processes of gemmation and embryo- 
 geny in Ascidians, important differences are seen to exist 
 between them. In the former, all the stages of segmentation 
 of the egg and gastrulation, together with the formation of 
 the mesoderm, are omitted ; and many parts, again, arise 
 from the ectoderm in the embryo and from the mesoderm 
 in the bud. These differences are perhaps still more marked 
 in the free-swimming Salpce. These animals also multiply by 
 buds produced on a kind of stolon ; and, as in the other Ascidians 
 referred to, the ectoderm forms practically nothing except the 
 epidermis, and the endoderm gives rise to only a few structures, 
 by far the greater number of parts arising from the ' mesoderm- 
 cells.' Seeliger * explains this by supposing that ' the mesoderm 
 
 * Seeliger, 'Die Knospung der Salpen,' Jena, 1885.
 
 MULTIPLICATION BY GEMMATION. 163 
 
 of the mother-animal which passes into the buds, practically 
 corresponds only to the future sexual apparatus.' This, however, 
 can merely be taken as an explanation of the fact in so far as it 
 indicates the possibility of the ' mesoderm cells ' of the stolon 
 and bud containing the groups of determinants required for these 
 different structures. For all the determinants must be present 
 in the sexual cells, and, owing to their disintegration during cell- 
 division, they may become arranged in very varied groups, so 
 that certain mesoderm cells may become furnished with one 
 group and others with another. This certainly presupposes 
 that the process of the distribution of the determinants in this 
 case is entirely different from that which takes place during 
 embryogeny, and this difference, again, can only depend on 
 a difference in the original architecture of the idioplasm. In 
 discussing the process of alternation of generations I shall 
 once more return to this point, which, from a theoretical point 
 of view, is a very fundamental one. 
 
 2. THE PROCESS OF GEMMATION IN PLANTS. 
 
 Our conception of the process of gemmation has been in the 
 first instance derived from the vegetable kingdom : all the higher 
 plants correspond to stocks or corms which arise by copious and 
 regular budding, much as occurs in the case of such animal- 
 stocks as those of the Hydrozoa, for instance. Although the 
 physiological individuality of separate 'persons' in a plant is 
 often less defined than in the case of many animal colonies, 
 there can nevertheless be no doubt as to the morphological 
 value of a shoot as a ' person,' in the sense in which Haeckel 
 uses the term. 
 
 Although as regards animal colonies, it has not yet in all 
 instances been possible to ascertain with absolute certainty the 
 actual origin of the processes of budding in connection with the 
 cell-generations of the first person of the colony, this has been 
 done very accurately in the case of plants ; a theory of heredity 
 can therefore be much more safely applied to the process of 
 gemmation in plants than to that in animals. 
 
 In many plants, at any rate, budding originates from a single 
 cell, situated at the apex of the growing shoot, and known as the 
 1 apical cell? This cell grows and undergoes a series of divisions, 
 much as occurs in the development of the ovum, and thus gives 
 rise to a group of cells, the number, form, and arrangement of
 
 164 THE GERM-PLASM. 
 
 which is perfectly definite. The primary constituents of the 
 entire new shoot are contained in this group, and it is possible to 
 predict what parts of the shoot will be formed from each of its 
 cells. The successors of this group of cells continue to multiply 
 up to a certain limit, and have then only to become elongated in 
 one or more directions, and more highly differentiated, in order 
 to give rise to a fully developed 'person' of the stock. This 
 person does not undergo any further essential changes, but it 
 is capable of giving rise to a new person from its apical cell ; 
 for the latter is always being renewed, or, in other words, 
 it always remains the same. 
 
 I'u;. 7. The apex of a shoot of Chara, in longitudinal section. (From Sachs' 
 ' Lectures on the Physiology of Plants.') 
 
 Let us take as an example the alga Chara. A glance at Fig. 
 7 will at once make it apparent that the idioplasm of the apical 
 cell (v) cannot undergo separation into different groups of deter- 
 minants in the first division, because one of the resulting two 
 cells remains as the apical cell, while the other, or 'segmental 
 cell,' gives rise to an entire shoot, that is to say, to that very 
 structure which the apical cell is capable of producing. The
 
 MULTIPLICATION BY GEMMATION. 165 
 
 next division of the lower of the two daughter-cells, however, 
 separates the determinants into two dissimilar groups, for it re- 
 sults in the production of an upper biconcave ' nodal cell,' from 
 which the leaves (b\ b n , b"\ 6"), the lateral shoot (), and the 
 sexual organs (a and o), will subsequently arise ; and of a lower 
 biconvex cell, which does not undergo further-division, but only 
 grows considerably in length, so as to form a segment of the 
 main axis (/', z", i 1 ", * nv ). The idioplasm of this 'internodal 
 cell' does not therefore undergo further disintegration ; the nodal 
 cell, however, divides vertically, so as to form cells which, since 
 they give rise to other parts of the shoot, must contain various 
 groups of determinants. Thus a comparison of the younger 
 with the older segments of the shoot, shows that the outer of the 
 five nodal cells in the figure gives rise to a whole leaf, together 
 with the sexual organs, the inner ones forming the actual node. 
 The division of the outer cell is accompanied by constant though 
 usually unimportant changes as regards its idioplasm : a glance 
 at the structure of the leaf, in which similar segments are 
 repeated many times over, will make this evident. If we now 
 leave out of consideration the accessory idioplasm which is 
 present in the cells along with the primary idioplasm, it will be 
 seen that the distribution of the group of determinants derived 
 from the apical cell must simply take place so as to result in each 
 cell, as it is formed, receiving that group of determinants only, 
 the individual constituents of which are required by its successors 
 for the control of the individual cells. We must therefore sup- 
 pose that the internodal cells of the stem only contain their own 
 specific idioplasm, composed of ' internodal determinants,' for 
 they do not give rise to any other structures. The primary 
 nodal cell, on the other hand, must contain an entire group of 
 determinants, as it gives rise to a number of cells which have 
 various forms and perform various functions. 
 
 Although the cells of plants are often apparently very much 
 alike, and no essential difference can be observed between them, 
 such a difference must exist if the origin of the specific leaf, 
 stem, and reproductive organs can be proved theoretically at 
 all. For the origin of these structures can only be explained, at 
 any rate in principle, by supposing that each of these centres of 
 vitality is controlled by a specific idioplasm ; that is, by a deter- 
 minant which differs in some way or other from those in the 
 other cells.
 
 1 66 THE GERM-PLASM. 
 
 3. COMPARISON OF THE PROCESS OF GEMMATION IN 
 ANIMALS AND PLANTS. 
 
 Various stages may be recognised in the different kinds of 
 gemmation with regard to the kind of idioplasm concerned in 
 the process. The simplest form of budding is seen in those 
 plants in which the production of a new 'person' by budding 
 always originates from a single cell. We must therefore 
 assume that the idioplasm of this cell contains all the deter- 
 minants of the shoot, and very probably those of the root 
 also. For most of the shoots of a plant, when they have been 
 cut off from the stem, are capable of giving rise to roots under 
 favourable circumstances. This does not as a rule occur under 
 normal conditions, that is to say, while the shoot is still con- 
 nected with the parent-plant. The ' blastogenic idioplasm ' 
 cannot be quite identical with germ-plasm proper ; for although 
 precisely the same parts may arise from it as from the ferti- 
 lised egg-cell, the different succession of cells which results in 
 embryogeny and in gemmation indicates that the determinants 
 must at any rate be differently arranged in the idioplasm, and 
 that possibly their proportional number is also different. ' Blas- 
 togenic idioplasm ' and germ-plasm may in a sense be regarded 
 as 'isomeric' idioplasms, using the term in an analogous sense 
 to that of isomeric chemical compounds. 
 
 The same would be true as regards such animals as Hydroids, 
 in which the formation of a bud originates from a single cell. 
 In this case, again, the resemblance between embryonic develop- 
 ment and the process of gemmation, although to a certain extent 
 approximate, is not a complete one ; and it must again be 
 assumed that the whole of the determinants of the species are 
 contained in the blastogenic idioplasm, not only those which 
 as a rule undergo development, but also those required for the 
 formation of the attached ends in the case of Hydroids, or of 
 roots in the case of plants. This conclusion is supported by the 
 phenomena of budding in polypes like Hydra, in which the buds 
 regularly become detached, and carry on an independent exist- 
 ence. In such cases the daughter-polypes do not develop a 
 ' foot ' until they become detached from the parent. 
 
 The next stage in the process of budding is seen in the 
 Polyzoa. All the determinants of the species from which the 
 bud is formed are no longer contained in a single cell, but are
 
 MULTIPLICATION BY GEMMATION. 167 
 
 arranged into main groups, one of which is supplied as accessory 
 idioplasm to one cell of the ectoderm, and the other to one or 
 more cells of the mesoderm. The single ectoderm cell gives 
 rise to the entire endoderm, but it must nevertheless not be 
 considered equivalent to those cells of the embryo which give 
 rise to the endoderm by invagination, for it forms parts which 
 are either not developed at all in the embryo, or else arise from 
 other ectoderm cells. Without entering into details here, the 
 facts may be expressed in terms of the idioplasm by supposing 
 that the ectoderm-cell from which the bud arises is provided 
 with an idioplasm which contains the whole of the determinants 
 for the endoderm, as well as a number of others, and that this 
 combination of determinants does not occur in embryogeny. 
 The mesoderm-cells of the parent which gives rise to the endo- 
 thelia, muscles, &c., of the bud, must also contain a peculiar 
 combination of determinants which is not exactly similar to that 
 which occurs in embryogeny. The gemmation must therefore 
 be prepared for in embryogeny by certain series of cells in the 
 ectoderm and mesoderm being provided with these groups of 
 determinants in the form of accessory idioplasm. 
 
 A third stage is represented by the gemmation of fixed Asci- 
 dians and Salps. In these the bud originates in the fully- 
 formed animals, or in those which are still undergoing develop- 
 ment from three kinds of cells, viz., those of the ectoderm, 
 mesoderm, and endoderm. And here again those groups of 
 determinants which must be supposed to exist in the three 
 kinds of cells do not correspond exactly to those which must be 
 contained in the primary ectoderm, endoderm, and mesoderm 
 cells. In fact, no group of cells which occurs in embryogeny 
 can contain precisely the same group of determinants as does 
 the endoderm cell of the bud. A collection of determinants 
 especially adapted for budding must therefore be provided on a 
 large scale during embryogeny in this case, so that eventually 
 certain cells may receive their supply from it in the form of 
 accessory idioplasm. 
 
 This last kind of budding resembles regeneration very closely 
 as regards the idioplasm concerned in the process. It must 
 not, however, be therefore implied that the former process 
 has been derived phylogenetically from the latter. The re- 
 semblance only consists in the formation of a new ' person,' 
 which in both cases originates in several cells provided with
 
 1 68 THE GERM-PLASM. 
 
 different groups of determinants, these eventually completing one 
 another, and interacting in such a manner that a fully-formed 
 person must result. 
 
 4. THE PHYLOGENY OF THE PROCESS OF MULTIPLICATION 
 BY GEMMATION. 
 
 In all probability the phylogeny of gemmation has taken place 
 along different lines. The process most likely arose independ- 
 ently in animals and in plants, and perhaps even in different 
 groups of animals it has had a different origin. 
 
 In many of the lower plants, the cells and organs of which 
 are only slightly differentiated, all, or at any rate many, of the 
 cells can individually give rise to a new plant under certain cir- 
 cumstances. In such cases we might be inclined to suppose that 
 each cell contained originally, i.e. at the time of its phyletic origin, 
 the entire mass of determinants of the species, or, in other 
 words, contained germ-plasm. The various differentiations of 
 the cells on the upper and lower surfaces, for instance, would 
 consequently depend on the different determinants becoming 
 active in response to different external stimuli : some, for in- 
 stance, might be stimulated by a bright light, and others by 
 a dim light. 
 
 This explanation would hardly suffice in the case of the higher 
 plants, the differentiation of which is far too complicated to be 
 due to the effect of external causes. A large number of the 
 cells must nevertheless contain germ-plasm, which, however, is in 
 the unalterable ('gebundenen') state, that is to say, it is not 
 merely inactive, but is incapable at the time of undergoing disin- 
 tegration. This stage in the phylogeny of gemmation may be 
 derived from the earliest stage. As the plant underwent an 
 increasing differentiation, cells appeared which only contained 
 special determinants, in addition to those with germ-plasm 
 proper; and this may have led to the condition which we now 
 find in the highest plants, and which is distinguished by the 
 fact that many cells only contain specific determinants, while a 
 large number of others possess in addition germ-plasm in the 
 unalterable condition, which only becomes active under certain 
 influences. I shall have occasion to return to this subject 
 later on. 
 
 In the case, again, of the various groups of the lower animals
 
 MULTIPLICATION BY GEMMATION. 169 
 
 which multiply by gemmation, we cannot assume that this pro- 
 cess has a common origin. But although it may have arisen 
 independently in the various subdivisions of the animal king- 
 dom, the history of its origin will have been essentially the 
 same in all cases, for ' blastogenic' idioplasm must have become 
 differentiated from the germ-plasm even in the egg-cell, as all 
 the determinants of the species are contained only in the latter. 
 Even at the present day the blastogenic idioplasm must be 
 present as such in the germ-plasm, for otherwise it could not 
 have undergone independent and hereditary variation : the for- 
 mation of medusae from polypes by gemmation, and many other 
 cases of alternation of generations, prove that this has actually 
 occurred. 
 
 Balfour attempted to derive the process of budding from a 
 division of the fertilised ovum into two separate parts, such as 
 has been observed in certain animal forms, and which leads to 
 the formation of two individuals. He imagined that if this 
 process of doubling were transferred to a later ontogenetic stage, 
 budding would result, and expressed his views as follows : 
 'While it is next to impossible to understand how production 
 of a bud could commence for the first time in the adult of a 
 highly organised form, it is not difficult to form a picture of the 
 steps by which the fission of the germ might eventually lead to 
 the formation of buds in the adult state.' * Unfortunately this 
 gifted observer did not work out this idea in detail : it seems to 
 me, however, that the derivation of budding from the doubling 
 of the fertilised ovum by division is not so simple or self-evident 
 as we might expect at first sight. 
 
 Let us suppose that a fertilised ovum became capable of 
 dividing into two parts : these two first segmentation - cells 
 would not then be blastomeres, but would correspond to egg- 
 cells, each of which could give rise to an entire animal. But this 
 could not be called gemmation, nor would the latter process 
 occur if the doubling were transferred to a later stage : this 
 would only cause a multiplication of the egg-cell, which would 
 result in the formation of four, eight, sixteen, &c., ova, instead 
 of two. 
 
 If, however, we suppose that the division of the egg is of such 
 
 * F. M. Balfour, 'Comparative Embryology,' Vol. i., Introduction, 
 P- 13-
 
 170 THE GERM-PLASM. 
 
 a kind that the two halves at first remain together so as to form 
 only one embryo, the condition observed in a certain earth- 
 worm (Lumbricus trapezoides} by Kleinenberg would result. In 
 this animal the development is apparently single up to the gas- 
 trula stage, at which the separation of the two embryos first 
 occurs. Did this separation take place at a much later stage, 
 perhaps not until the two individuals are fully developed, the 
 process would not be one of budding, but only of a doubling of 
 the embryo. 
 
 An essential modification of this process is indispensable if 
 gemmation is to result from it, and this consists in the postpone- 
 ment of the development of one half of the egg. Let us suppose 
 that one of the two equivalent blastomeres of an ovum did not at 
 once undergo development at the same time as the other, but 
 remained in a unicellular condition enclosed within the embryo 
 formed from the active blastomere, and subsequently began to 
 develop when the latter had already given rise to a full-grown 
 animal : this would be a true process of gemmation. I do not 
 wish to assert definitely that the phylogeny of budding might 
 not have taken place pn similar lines. A postponement and 
 subsequent transference to a later stage of ontogeny of the de- 
 velopment of one of the blastomeres is not actually inconceiv- 
 able. But such a transference must have undergone a still 
 further modification, before even the simplest form of budding 
 with which we are acquainted could arise. The shifting must 
 have occurred in a backward as well as in a forward direction ; 
 that is to say, the division of the egg into two separate ones must 
 have been suppressed, and represented by the mere division of the 
 germ-plasm. 
 
 Thus in Hydroids and other animals which multiply by bud- 
 ding, we see, in fact, that one of the two blastomeres into which 
 the egg-cell divides does not serve, so to speak, as a reserve cell 
 for subsequent gemmation ; both blastomeres, on the contrary, 
 continue to divide, and together give rise to the embryo : and even 
 in the latter none of the cells can be distinguished as 'blastogenic- 
 cells ' : the cells which take part in the formation of the buds 
 only appear at a much later stage, when the polype is fully 
 formed. If therefore gemmation has in this case originated 
 from the doubling of the egg, the latter process must itself have 
 become degenerated, only the essential part of it remaining : the 
 germ-plasm concerned in it must have remained associated with
 
 MULTIPLICATION BY GEMMATION. 17 I 
 
 that of the egg-cell in the form of ' unalterable ' germ-plasm, and 
 must then have been passed into certain series of cells in the 
 course of ontogeny. 
 
 Whether the process of budding has actually been derived 
 from that of the doubling of the egg or not, it seems to me to be 
 certain at any rate that the first process undergone by the idio- 
 plasm must have been that of the doubling of the ids of the 
 germ-plasm in the fertilised egg-cell, and that this was not con- 
 nected with the division of the egg-cell ; one half of the germ- 
 plasm consequently remained in an unalterable and inactive 
 condition, in which, however, it was capable of development. 
 This blastogenic germ-plasm was then supplied to one of the 
 first segmentation-cells in the form of accessory idioplasm ; 
 and from these it was passed on through certain series of cells 
 in an unalterable condition, only becoming active when it had 
 reached certain parts in the fully formed animal, in which it then 
 caused gemmation to occur. 
 
 It does not seem to be inconceivable that the process of bud- 
 ding owes its origin phyletically to such a spontaneous division 
 and doubling of the germ-plasm, and that this was originally 
 connected with the inactivity of half the germ-plasm : its 
 connection with the doubling of the ovum was consequently not 
 such as was indicated above, that is to say, gemmation did not 
 owe its origin to the doubling of the egg, but both processes origin- 
 ated primarily in the division and doubling of the germ-plasm of 
 the egg-cell, to which in any case the doubling of the egg must 
 be due. The difference between the two processes would then 
 consist in the fact that in budding one-half of the germ-plasm 
 would pass into the inactive condition, while in the doubling of 
 the egg both halves would at once become active. 
 
 The modifications of the idioplasm which result in gemmation 
 must become more complex as soon as two, or all three, of the 
 germinal layers take part in the process, instead of one only. 
 In such cases the blastogenic germ-plasm must undergo disin- 
 tegration at certain ontogenetic stages, e.g., at the separation of 
 the ectoderm from the endoderm, and again at the separation of 
 the mesoderm from one of the two primary germinal layers. 
 Precisely the same combination of determinants need not neces- 
 sarily be produced by the disintegration of the accessory germ- 
 plasm into two or three groups of unalterable accessory 
 idioplasm, such as are formed in embryogeny. We can thus
 
 172 THE GERM-PLASM. 
 
 explain the origin of endodermal organs from the ectoderm cells 
 of the bud, as occurs in the Polyzoa for instance, and also even 
 the co-operation of three germinal layers in the formation of 
 the bud. 
 
 It seems to me to be improbable that the phylogeny of gem- 
 mation in animals has taken place in the reverse manner. We 
 might assume that in the lowest Metazoa, which no longer exist 
 at the present day, all or many of the cells also contained germ- 
 plasm proper, just as in the case of the lower multicellular 
 plants. Under certain circumstances a perfect animal might 
 have been produced from each of these cells. But this assump- 
 tion would only suffice as long as the individual formed by 
 budding was exactly similar to that arising from the egg. Even 
 the slightest difference between these would necessitate the 
 presence of special ids in the germ-plasm. For such a differ- 
 ence can only depend on the fact that the two kinds of indi- 
 viduals are capable of independent variation from the germ 
 onwards. We should therefore have to assume further, that in 
 the course of phylogeny the germ-plasm of these somatic cells 
 from which the buds originated became doubled in the earlier 
 stages of ontogeny, and that it was consequently present in the 
 germ-plasm of the egg-cell in the form of a special group of ids. 
 But this, to say the least, is a very involved assumption, and can 
 hardly be considered very probable : that which presupposes a 
 primary doubling of the ids of germ-plasm is certainly far pre- 
 ferable to it. The following chapter will make this point still 
 more evident.
 
 ALTERNATION OF GENERATIONS. 173 
 
 CHAPTER V. 
 
 ALTERNATION OF GENERATIONS IN ITS RELATION TO 
 THE IDIOPLASM. 
 
 Starting from the germs specially adapted for amphimixis 
 (sexual intermingling), we have designated as germ-plasm the 
 definitely arranged group of determinants which must be con- 
 tained in the sexual cells. By this term is meant an idioplasm 
 which contains all the determinants of the species. At the 
 same time a large number of species exist in which the sexual 
 cells are not the only ones which contain all the determinants, 
 and in which the development takes place, for the second time 
 during the course of the life-cycle, from a single cell; the idio- 
 plasm of this cell must therefore also be composed of all the 
 determinants of the species. This is the case in most of the 
 lower plants, such as mosses, horse-tails, and ferns, in all of 
 which sexual reproduction alternates with the formation of 
 asexual 'spores,' as well as in those groups of animals in which 
 that form of alternation of generations which is known as 
 heterogeny occurs. But even in the case of alternation of 
 generations in the more restricted sense, t.e., the alternation of 
 sexual reproduction and gemmation, the development of an 
 individual may take place twice successively from a single cell, 
 as was mentioned above with regard to the stocks or colonies of 
 plants and of Hydroid-polypes. In all these a cell, the idio- 
 plasm of which contains all the specific determinants, occurs 
 twice in the course of the life-cycle from one fertilised ovum to 
 the next one. The question therefore arises as to whether the 
 idioplasm in each case is to be considered identical, and may 
 merely be described as germ-plasm. 
 
 This question has already been discussed in the section on 
 the process of gemmation in plants ; and it was there concluded 
 that the idioplasm of the apical cell and that of the fertilised 
 ovum cannot be assumed to be perfectly identical, owing to the 
 fact that the course taken by embryogeny in which process the 
 first shoot and roots are formed is different from that followed by
 
 174 THE GERM-PLASM. 
 
 the cell-divisions which result in the apical cell producing a new 
 shoot. The same is true as regards the formation of a new 
 polype from a blastogenic cell and from an ovum. In both 
 cases the final result is the same, or at any rate very similar, 
 though the method by which it is attained is different. Although 
 a precisely similar organism might be produced by either of 
 the two methods of development, and the primary cells would 
 therefore contain the same determinants in both cases, the 
 grouping of the latter in the two idioplasms at any rate must 
 be different, for they must pass through different groups during 
 ontogeny before their ultimate disintegration into single deter- 
 minants. Even in this very simple case it is necessary to dis- 
 tinguish the 'germ-plasm proper' of the egg- and sperm-cells 
 from the ' apical plasm ' or ' blastogenic germ-plasm.' It is 
 convenient, however, to speak of every kind of idioplasm which 
 contains all the determinants of the species as germ-plasm in 
 the wider sense, and to distinguish its various subdivisions as 
 ' blastogenic ' and ' sporogenic ' germ-plasm, and so on ; these 
 latter may all be included under the term accessory germ-plasms 
 or para-germ-plasms, in contrast to the primary or ancestral 
 germ-plasm. 
 
 Wherever two kinds of germ-plasm occur in the life-cycle of 
 a species, we might be inclined to assume that they change into 
 one another in the course of life. But this view is untenable, as 
 has been shown above, and we are on the contrary forced to 
 assume that both kinds of germ-plasm continually pass simul- 
 taneously along the germ-tracks, and that each of them becomes 
 active in turn. 
 
 This assumption is unavoidable, for the phyletic development 
 of the species shows that the individual generations in cases of 
 alternation of generations can vary independently and heredi- 
 tarily. It, however, presupposes that special determinants are 
 present in the germ-plasm in each generation, for otherwise 
 both generations would be affected at the same time by a varia- 
 tion in the germ. A similar assumption must be made in the 
 case of metamorphosis. The wings of a butterfly must be re- 
 presented in the germ-plasm by a group of determinants. If 
 the wings were formed by the transformation of some of the 
 determinants of the caterpillar, they could never vary without 
 at the same time producing a variation in some parts of the 
 caterpillar, and vice versa.
 
 ALTERNATION OF GENERATIONS. 
 
 175 
 
 It will not be unin- 
 teresting to give some 
 examples by way of 
 illustration. 
 
 We will first take a 
 case of hcterogeny, or 
 alternation of gener- 
 ations in which the 
 two generations do not 
 differ at all from one 
 another in the full- 
 grown condition. As 
 a rule, the difference 
 between the two suc- 
 cessive generations in 
 \.\\eDaphnida:or water- 
 fleas, for example, con- 
 sists in the fact that 
 one generation is de- 
 veloped from summer- 
 eggs, which contain a 
 small amount of yolk, 
 while the other arises 
 from winter-eggs, in 
 which the yolk is very 
 abundant. From both 
 of these two kinds of 
 eggs similar females 
 are developed : the 
 complication arising 
 from the periodic ap- 
 pearance of the male 
 may be neglected for 
 the present. The sum- 
 mer-eggs are nourish- 
 ed by the blood of the 
 mother, while the win- 
 ter-eggs are not ; for 
 the amount of yolk in 
 the latter necessitates 
 a different kind of 
 
 FIG. 8. -A female of Daphni 
 
 with two parthenogcnetic eggs in 
 he brood-cham^r (/). (After R.
 
 176 THE GERM-PLASM. 
 
 ontogeny, and this presupposes not only a difference in 
 the arrangement of the determinants in the germ-plasm as 
 compared with the meta-germ-plasm (' Nach-Keimplasma '), but 
 also the presence of different determinants for some of the 
 embryonic stages. The case becomes still clearer if we take one 
 particular species of Daphnid (Lcptodora hyalina) into con- 
 sideration. In this animal the embryogeny of the winter-eggs 
 
 FIG. 9. Nauplius larva of Lcptodora. hyalina. (After Sars, from Korschelt and 
 Heider's ' Lehrbuch der vergleichenden Entwicklungsgeschichte. ') 
 
 only extends as far as to the formation of the primitive crustacean 
 larva, or nauplius, which possesses three pairs of limbs : the 
 summer-eggs, on the other hand, develop at once into the adult 
 form of the animal, in which all the limbs are present. The 
 summer-eggs certainly also pass through the stages from the 
 ripe ovum to the nauplius, but these are abbreviated, and 
 though this nauplius also possesses three pairs of limbs, these 
 are only rudimentary, and are useless as swimming organs. 
 There must therefore be two kinds of germ-plasm in Leptodora, 
 one of which still contains all the determinants of the nauplius, 
 while the other only contains a portion of them, and even these 
 have probably undergone some change. The two kinds of 
 germ-plasm must be passed separately along the germ-tracks 
 from one generation to another, so that each must always con- 
 tain the other, which is, so to spca'c, stored away in it in an 
 inactive condition. It seems to me impossible to explain the 
 facts in any other way, for it is inconceivable that the germ- 
 plasm of the summer-eggs, which has undergone reduction, 
 and possesses comparatively few determinants, should be able to 
 develop the lost determinants out of Us own substance.
 
 ALTERNATION OF GENKRATIONS. 177 
 
 The phyleti* development of these two kinds of germ-plasm 
 would be very enigmatical if we were compelled to assume that 
 only a single unit of the germ-plasm is present in the nucleus of 
 the germ-cell. We have, however, made the reverse assump- 
 tion from the first, and it will be shown later on that a con- 
 sideration of sexual reproduction, or amphimixis, leads us to 
 assume that several, and in fact probably a large number of 
 units or ids must be contained in the germ-plasm of every 
 species which multiplies sexually. If now a reduction of the 
 determinants for the nauplius in the summer generation of 
 Leptodora were advantageous, it would have appeared, increased, 
 and become fixed in the course of generations by selection, and 
 an abbreviation of embryogeny would thus have resulted. This 
 would only have occurred gradually, so that at first the summer- 
 eggs would contain more reduced than unreduced ids only in the 
 case of a few individuals ; and if the original unabbreviated form 
 of embryogeny were of greater advantage to the winter genera- 
 tion, the determinants for the nauplius would not become lost or 
 modified in all the ids, but only in certain of them. A balance 
 of the two kinds of ids would finally take place from the struggle 
 between the modified ids, which were more advantageous in 
 summer, and the unmodified ones, which were of greater advan- 
 tage in winter, and this would result in the germ-plasm of the 
 species being composed of an equal number of modified and 
 unmodified ids ; these would alternately control the cell, so {hat 
 each would remain inactive and unalterable during a certain 
 number of generations, and become active during certain others. 
 
 This regular alternation between definite periods of activity 
 and inactivity in the two kinds of germ-plasm can be directly 
 observed, for we can determine how many generations occur 
 which give rise to summer-eggs before one again appears in 
 which winter-eggs are produced. As I was able to prove a con- 
 siderable time ago, this regularity varies very much in different 
 species of Daphnidae, and stands in close relation to the mode 
 of lite of the species. In those species which live in very small 
 bodies of water which are liable to become rapidly dried up, the 
 formation of the two kinds of eggs alternates very frequently ; 
 this is due to the fact that the extermination of the animals by the 
 sudden drying up of the pond is only prevented by the thick 
 shells by which the winter-eggs are surrounded. On the other 
 hand, all the species which live in large bodies of water, such as 
 If
 
 178 THE GERM-PLASM. 
 
 pools and lakes which never become dried up, produce summer- 
 eggs alone for a large number of generations, and only give 
 rise to eggs of the other kind on the approach of winter ; and 
 these, on the death of the animals which produced them, ensure 
 the continuance of the species in the following spring. 
 
 The occurrence of changes in the final stages of ontogeny must 
 be accounted for in a similar way. 
 
 In plant-lice belonging to the genus Aphis, the fertilised egg 
 gives rise to females, which are, however, incapable of being fer- 
 tilised, for the receptaculum seminis is wanting, and this is 
 essential in the process. Their eggs are, however, capable of 
 undergoing development in the ovary parthenogenetically. The 
 resulting offspring give rise to similar females possessing no 
 arrangements for fertilisation, and these again produce others of 
 the same kind. Ultimately, however, one of these gives rise 
 parthenogenetically to females which are capable of fertilisation, 
 as well as to males. The two sexes as a rule differ as regards 
 the shape and colour of the body, apart from the structure of the 
 reproductive organs and sexual products, but the embryogeny of 
 these sexual animals is similar to that of the others. 
 
 In this case, therefore, the determinants of the mature animal 
 become modified in the parthenogenetic generations, for sexual 
 reproduction is the more primitive of the two forms of the pro- 
 cess. If therefore we make the assumption, which, however, is 
 not a strictly correct one, that the sexual generations have 
 remained quite unaltered since the introduction of alternation of 
 generations in these animals, we should have to represent the 
 phyletic change in the germ-plasm as being of such a nature as 
 to cause the degeneration of the determinants of the seminal 
 vesicle in one half of the ids, and to produce a change in other 
 determinants, such as those which control the colour of the in- 
 tegument, for instance. The modified, as well as the unmodified 
 ids, must be contained u ithin the same germ-plasm, but they 
 control the egg alternately, and never become active at the same 
 time. 
 
 In this instance the generations which have been interpolated 
 have only suffered a slight change as regards the structure of the 
 whole body. But in many cases of alternation of generations 
 very important differences of structure occur, so that not infre- 
 quently one might easily believe that the different generations 
 belong to two entirely different groups of animals.
 
 ALTERNATION OF GENERATIONS. 
 
 179 
 
 This is the case in the alternation of generations in medusa. 
 The polype is the original form, and even at the present day the 
 fertilised ovum of the medusa gives rise to a polype in most 
 species. By the budding of this polype, or at any rate of the off- 
 spring which have been produced by gemmation, medusa^ are 
 again developed. If, for the sake of simplicity, we neglect the 
 slight differences which may exist between the germ-plasm of 
 
 FIG. 10. Bougainvillca rantosa. (After Allman.) Polype stock with gastro- 
 zooids (A) and medusoid buds (mk) ; tti, young Medusa (Marf fit's ramosa), 
 which has become free. (From A. Lang's ' Lehrbuch der vergleichenden 
 Anatomic.') 
 
 the egg and that of the bud, it is evident that two germ-plasms 
 take part in the cycle of development of the species, and 
 these must differ as regards very many, if not almost all, of 
 their determinants, for the medusa is provided with a number 
 of parts and organs which the simple polype does not possess. 
 Thus we must assume that there are two different kinds of ids
 
 l8o THE GERM-PLASM. 
 
 of which the germ-plasm is composed in equal numbers, the 
 periods of activity of which alternate with one another. The ids 
 of the accessory germ-plasm, which arose subsequently, must be 
 larger than those of the ancestral germ-plasm, because they 
 contain more numerous determinants. If at some future time it 
 should be definitely ascertained that those granules or micro- 
 somes, which are arranged like beads in a necklace in the 
 nuclear rods, really correspond to ids, we may possibly, perhaps, 
 be able to prove by the aid of the microscope that such differ- 
 ences in size actually exist. A knowledge of the entire number 
 of nuclear rods or idants may also possibly help to confirm the 
 theory, for it is probable that in species in which alternation of 
 generations occurs, the ids, and therefore the idants also, have 
 been doubled during the development of the species. For if my 
 view is correct that a definite amount of germ-plasm is necessary 
 for the normal development of a certain kind of egg, the 
 periodical inactivity of half the ids must have been accompanied 
 by a corresponding doubling of these structures. 
 
 The mechanism of the idioplasm in alternation of generations 
 becomes somewhat different, and rather more complicated, as 
 soon as the second generation arises, not from a single 
 cell, but from several cells at the same time, derived from differ- 
 ent layers of the body. This is the case as regards the stro- 
 bilation of the higher medusa and that oj tape-worms, and an 
 intermediate stage is seen in the gemmation of the Salpce. 
 
 In the last-named animals, two generations differing as regards 
 the form of the body and mode of reproduction follow one 
 another. A number of individuals are united into a ' chain ' in 
 the first generation, in which sexual multiplication occurs ; and 
 in the second generation the individuals are separate, and mul- 
 tiply by budding. It has already been pointed out in the chapter 
 on gemmation that this budding is produced by a co-operation 
 of the ectoderm and mesoderm cells. We must imagine that in 
 this case, again, the germ-plasm of the egg- and sperm-cells is 
 composed of two kinds of ids, which alternately become active, 
 one of which contains the determinants for gemmation, and the 
 other those for embryogeny. In the case of the Hydroid-polypes 
 and medusae, the determinants of the ' blastogenic ' ids remain 
 together in one cell, but in the single form of Salpa they must 
 be separated into groups during embryogeny, and these groups 
 would be supplied in part to the ectoderm, and in part to the
 
 ALTERNATION OF GENERATIONS. 
 
 181 
 
 mesoderm and endoderm .is inactive and 'unalterable' accessory 
 idioplasm. These only become active, and cause budding, when 
 they have reached some definite part, such as the ovary or pro- 
 liferating stolon. 
 
 The development of the higher medusa? or Acalepha: by 
 strobilation can easily be traced from the above processes. In 
 these animals the sexual forms arise asexually : the polype 
 becomes divided into disc-shaped portions, and so comes to 
 resemble a pile of saucers, each disc eventually being trans- 
 formed into a medusa. If the medusa underwent division, the 
 
 IMC. ii. Development of JMedusa by strol!lation\, the young larva ; 2 5, its 
 development into a polype ; 7, a polype viewed from the oral pole ; 6, 8, 
 and 9, transverse division of a polype into disc-shaped portions ; 10, the con- 
 striction of these portions into young Moil usa: ; n and 12, a young Medusa. 
 (From Hatschek's ' Lehrbuch der Zoologie.') 
 
 process would be one of simple regeneration : the differentiation 
 of one of these discs into a medusa depends on a mechanism in 
 the idioplasm exactly similar to that which gives rise to the process 
 of regeneration in a worm the anterior end of which has been 
 cut off, or which has undergone spontaneous division. The
 
 182 THE GERM-PLASM. 
 
 various cells in the body of the polype must be provided with 
 different groups of determinants of the medusa? in the form of 
 inactive accessory idioplasm, and these must become active in 
 the process of strobilation, and cause the development of highly 
 complex medusae with eight or more radii, and provided with eyes, 
 auditory organs, and olfactory pits. The difference between 
 this process and that of simple division followed by regeneration, 
 consists in the fact that in the latter the supplementary deter- 
 minants of the cells of the body are of the same kind as those 
 from which the body was constructed : in strobilation, on the 
 other hand, the germ-plasm of the egg- and sperm-cells, which 
 gives rise to the sexual generation or medusa, must contain not 
 one, but two kinds of ids, viz., those of the polype and those of 
 medusa ; the latter, although they remain inactive during the 
 ontogeny of the polype, and take no part in the control of the 
 cell, are nevertheless not absolutely unalterable, for they break 
 up during ontogeny into many different groups of determin- 
 ants, and at the same time become distributed among different 
 cells in a regular and perfectly definite manner. It is very 
 probable, however, that all the cells of the polype those of the 
 ectoderm as well as of the endoderm are provided with ac- 
 cessory determinants, so that each cell of the polype contains in 
 addition the primary constituents of some cell of the medusa. 
 We have, however, no positive knowledge on this point, for no 
 investigations have as yet been made with regard to the succes- 
 sion of the cells which lead to the formation of the medusa from 
 the polype. 
 
 The basis of the alternation of generations as regards the 
 idioplasm must therefore in all cases consist of a germ-plasm 
 composed of ids of at least two different kinds ; which ultimately 
 take over the control of the organism to which they give rise.
 
 THE FORMATION OF GERM-CELLS. 183 
 
 CHAPTER VI. 
 THE FORMATION OF GERM-CELLS. 
 
 i. THE CONTINUITY OF THE GERM-PLASM. 
 
 If heredity depends on the presence of a substance, the 
 germ-plasm, which causes the production of the new individual 
 by directing the process of division in ontogeny, in the course of 
 which it becomes changed in a regular manner, the question 
 arises as to how unaltered germ-plasm can nevertheless reap- 
 pear in the germ-cells of the new individual. The transmission 
 of characters from parent to child can only depend on the germ- 
 cell from which the offspring arises containing ids of germr 
 plasm precisely similar to those of the germ-cell from which the 
 parent was developed. The germ-plasm, however, undergoes 
 an enormous number of changes during the development of the 
 ovum into the parent : how is it possible therefore that this 
 substance can reappear in the germ-cells of this parent ? 
 
 There are obviously two possible solutions of this problem. 
 The changes which the germ-plasm undergoes during the con- 
 struction of the body must either be of such a kind that they 
 can take place in the reverse order when the idioplasm of all, or 
 at least of a portion of, the somatic cells is re-transformed into the 
 germ-plasm from which it was, in fact, indirectly derived ; or, if 
 such a reversal is impossible, the germ-plasm of the germ-cells 
 must be handed on directly from parent to offspring. This 
 latter hypothesis was suggested by me some years ago under the 
 name of the continuity of the germ-plasm* A third solution of 
 the problem is impossible, for it is quite out of the question that 
 the germ-plasm can be entirely formed anew. 
 
 The hypothesis of the continuity of the germ-plasm depends 
 on the assumption of a contrast between the somatic and the 
 reproductive cells, such as can be observed, in fact, in all multi- 
 cellular plants and animals, from the most highly differentiated 
 forms to the lowest heteroplastids amongst the colonial Algae. 
 
 * ' Die Continuitat des Keimplasma's als Grundlage einer Theorie det 
 Vererbung," Jena, 1885 (English translation, and ed., p. 163).
 
 184 THE GERM-PLASM. 
 
 I assume that germ-cells can only be formed in those parts of 
 the body in which germ-plasm is present, and that the latter is 
 derived directly, without undergoing any change, from that 
 which existed in the parental germ-cell. Hence, according to 
 my view, a portion of the germ-plasm contained in the nucleus 
 of the egg-cell must remain unchanged during each ontogeny, 
 and be supplied, as such, to certain series of cells in the develop- 
 ing body. This germ-plasm is in an inactive condition, so that 
 it does not prevent the active idioplasm of each cell from im- 
 pressing a specific character on the latter in a greater or less 
 degree. It must, moreover, differ from ordinary idioplasm, 
 inasmuch as the determinants it contains are kept closely to- 
 gether, and are not distributed in groups among the daughter- 
 cells. This accessory germ-plasm is thus passed on in an 
 unalterable condition through longer or shorter series of cells, 
 until it ceases to be inactive in a certain group of cells, more or 
 less remote from the egg-cell, and then impresses upon the par- 
 ticular cell the character of a germ-cell. The transmission of 
 the germ-plasm from the ovum to the place of origin of the re- 
 productive cells (' Keimstatte ') takes place in a regular manner, 
 through perfectly definite series of cells which I call germ-tracks. 
 These are not actually recognisable, but if the pedigree of the 
 cells in the embryogeny is known, they may be traced from their 
 termination in the germ-cells backwards to the ovum. 
 
 This assumption is supported by the fact that a direct, or at 
 any rate a very close, connection can be proved to exist, although 
 only in rare instances, between the germ-cells of two consecu- 
 tive generations. In the Diptera the first division of the egg- 
 cell separates the nuclear material of the subsequent germ-cells 
 of the embryo from that of the somatic-cells, so that in this 
 case a direct continuity can be traced between the germ-plasm 
 in the germ-cells of the parent and offspring. 
 
 The process in this case must certainly, however, be looked 
 upon, not as a primary one which has been passed on unchanged 
 from very ancient times, but as a special arrangement peculiar 
 to this order of insects. It nevertheless proves the possibility 
 of each generation of germ-cells being derived directly from the 
 preceding one, and also that the germ-plasm which has been 
 prevented from taking part in the construction of the somatic 
 portion of the embryo is not required in this process. 
 
 We may next take the case of the embryogeny of the
 
 THE FORMATION OF GERM-CELLS. 185 
 
 Daphnida. In these animals the primary germ-cells become 
 separated from the somatic cells in the first stages of the seg- 
 mentation of the egg ; and in Sagitta again, this separation 
 takes place at the gastrula-stage. In Vertebrates this process 
 occurs much later, although it always takes place within the 
 first half of embryogeny ; while in Hydroids both in colonial 
 and solitary forms the germ-cells do not appear in the ' per- 
 son' which is developed from the ovum at all, and only arise in a 
 much later generation, which is produced from the first by con- 
 tinued budding. The same is true as regards the higher plants, in 
 which the first shoot arising from the seed never contains germ- 
 cells, or even cells which subsequently become differentiated 
 into germ-cells. In all these last-mentioned cases the germ- 
 cells are not present in the first person arising by embryogeny 
 as special cells, but are only formed in much later cell-genera- 
 tions from the offspring of certain cells of which this first person 
 was composed. These ancestors of the germ-cells cannot be 
 recognised as such : they are somatic cells, that is to say, they, 
 like the numerous other somatic cells, take part in the construc- 
 tion of the body, and may be histologically differentiated in 
 various degrees. 
 
 A series of organic species might therefore be formed in 
 which the formation of the germ-cells begins at very different 
 degrees of remoteness from the egg-cell. This would admit of 
 the interpretation that the fertilised egg-cell of the earliest 
 Metazoa first divided into two cells, one serving for the forma- 
 tion of the body (soma), and the other for that of the germ- 
 cells ; and that a shifting had occurred subsequently, owing 
 to a separation of the material for both parts in the germ-plasm, 
 so that the portion of the germ-plasm which remained un- 
 changed was supplied in an inactive condition, in the form of 
 accessory idioplasm, to one of the somatic halves of the egg- 
 cell, and was transmitted by the latter to a somatic cell of the 
 second, third, or fourth generation. The shifting of the 
 process of separation into germ-cells and somatic-cells finally 
 reached its extreme limit, as in the case of the Hydroids, and 
 the unchanged germ-plasm of the fertilised ovum then only led 
 to the formation of germ-cells after passing through a long series 
 of somatic cells. 
 
 These facts do not, however, as yet constitute an actual proof 
 of the correctness of this interpretation : they might be taken
 
 1 86 THE GERM-PLASM. 
 
 as indicating that the series has been developed in the reverse 
 direction, the late differentiation of the germ-cells being the 
 primary condition and the earlier separation of the two parts 
 then having arisen gradually in individual cases. There can 
 hardly be any doubt, indeed, that the early differentiation of the 
 germ-cells of the Diptera and Daphnidae is of a secondary 
 nature ; and it will presently be shown that in the case of 
 Hydroids such a shifting of the formative areas (' Bildungs- 
 statte ') of the germ-cells i.e., the fact of their earlier differen- 
 tiationcan be actually proved. But the facts which have been 
 stated still support the interpretation of them given above, in so 
 far as they show that the germ-cells are by no means formed at 
 the time and in the place where they are actually wanted, and 
 that the time of their formation, in fact, varies very much, and 
 must have been changed in the course of phylogeny. The 
 direction in which this shifting originally took place that 
 is to say, whether it proceeded from the egg to the close of onto- 
 geny or in the reverse direction must be decided when our 
 knowledge of the facts is more complete. 
 
 We might here lay stress on the fact that the destruction of 
 the sexual glands in an animal, however low in the scale, is not 
 followed by the formation of sexual cells in any other part of the 
 body. Castration might be expected to have this effect if germ- 
 cells could be formed from any young cells of the body. But 
 just as in the case of any other highly specialised organs, such 
 as the liver, kidneys, and central portions of the nervous system 
 in Vertebrates, such a replacement never occurs. This fact is 
 to be explained according to our present view by supposing that 
 the formation of these latter organs anew is impossible, because 
 the determinants necessary for such a development are not 
 present in any other cells of the body. The same conclusion 
 will, it seems to me, be inevitable in the case of the germ-cells ; 
 the idioplasm necessary for the formation of germ-cells i.e., 
 germ-plasm must be absent in these cases, and germ-plasm at any 
 rate cannot be formed from somatic idioplasm. 
 
 The case of the Hydroids * is probably still more convincing, 
 for here a natural shifting of the place of origin of the germ-cells 
 has actually taken place. As has already been mentioned, the 
 germ-cells of Hydroids first arise very late in the life-cycle, and 
 
 * Weismann, ' Die Enstehung der Sexualzellen bei den Hydromedusen,' 
 Jena, 1883.
 
 THE FORMATION OF GERM-CELLS. 187 
 
 hundreds or even thousands of cell-generations are passed 
 through from the fertilised egg-cell onwards before they appear. 
 In species which exhibit a complete alternation of generations, 
 they are first formed in particular parts of the medusae which 
 have arisen from the polype-stock by budding usually in the 
 ectoderm of the manubrium. No trace of them is to be seen in 
 
 FIG. 12. Diagram of the degeneration of the Medusa into a mere gonophore. 
 A, Medusoid bud ; B, a Medusa shortly before it is set free ; C, degenerated 
 Medusa, in which the manubrium is present, but the mouth and tentacles are 
 wanting ; D and E, further stages in degeneration. (From Hatschek's 
 ' Lehrbuch der Zoologie.') 
 
 the young bud, and in many cases they only become differentiated 
 from the other ectoderm-cells after the medusa has become de- 
 tached from the stock, and has developed into an independent,
 
 1 88 THE GERM-PLASM. 
 
 free-swimming animal. Some of the ectoderm cells of the part 
 in question then become transformed into egg- or sperm-cells. 
 
 In the case of certain species of polypes, free sexual medusae 
 were produced in the earlier period of the development of the 
 species, but at the present day these do not become detached, 
 but always remain attached to the stock : they thus no longer 
 serve for the dispersal and ripening of the sexual products but 
 only for their production and ripening. These species illustrate 
 the different stages in the process of degeneration of the medusae 
 to mere gonophores, or sexual sacs. In some species the form 
 of the medusa is completely retained in the sexual persons of the 
 stock, only the eyes and marginal tentacles being absent ; in 
 others, the bell has become degenerated into a closed sac, the 
 walls of which still retain the circular and radial canals ; and in 
 other species again, these canals have also disappeared, only the 
 three characteristic layers of the medusa remaining, and even 
 these have become so thin that their presence can only be 
 detected in microscopic sections. Finally, these three layers 
 also undergo degeneration, the wall then consisting of a single 
 layer, so that the derivation of the sac from the bell of the 
 medusa can only be proved indirectly. Throughout all these 
 stages of degeneration, however, the ova or spermatozoa always 
 ripen in the gonophores. 
 
 The behaviour of the germ-cells is the chief point of interest 
 to us in the course of this process of degeneration. For the 
 entire degeneration of the medusa proceeds from its germ-cells, 
 and is due to the fact that the development of the latter has 
 gradually to be thrown back to earlier stages, so that the sexual 
 elements are ripened more quickly. 
 
 It will not be necessary to enter into the reasons for this 
 hastening of the sexual maturity ; it is sufficient to know that in 
 some species in which the medusae become detached, e.g. Podo- 
 coryne cornea, the egg-cells are developed earlier than the 
 medusas in which they subsequently ripen, and in proportion as 
 the degeneration of the medusa advances the place of origin 
 of the germ-cells recedes more and more into the older 
 and earlier formed parts of the stock. The advantage of this is, 
 that the germ-cells develop earlier, and afterwards enter the 
 germ-sacs in a riper stage : they thus reach maturity much more 
 quickly. 
 The remarkable thing about this process is the fact that
 
 THE FORMATION OF GERM-CELLS. 189 
 
 active migrations of the germ-cells take part in it. Originating 
 in the ectoderm, these cells wander into the endoderm, and 
 subsequently back again into the ectoderm ; and this remark- 
 able process occurs in a definitely prescribed and regular 
 manner. In spite of the relegation of their place of origin to 
 earlier persons of the stock, the germ-cells always originate from 
 the same layer of cells as that from which they arose in the 
 ancestors of the species. It may thus be said that they are 
 developed ontogenetically from the ancestors of those cells from 
 which they would have arisen if the polype stock still produced 
 free medusae ; or, in other words, they arise lower down on 
 the germ-track at present than they did formerly. Thus in 
 Hydractinia echinata, for instance, the youngest egg-cells first 
 become visible in the endoderm of certain polypes in the same 
 regions from which gonophores (degenerate medusae) subse- 
 quently bud out. The egg-cells then migrate into the latter, 
 and enter the ectoderm of the manubrium as soon as it is formed ; 
 and in this way they return to the old place of ripening* 
 which in earlier times was also the place in which they were 
 formed. At the present day, however, the egg-cells only appa- 
 rently originate in the endoderm of the polype : it can indeed 
 be proved that they are derived from the ectoderm, but migrate 
 into the endoderm while still in a very young condition, before 
 they exhibit the definite character of egg-cells. They therefore 
 originate in the same region in which at an earlier phyletic 
 period the ectodermal layer of the manubrium of the medusa 
 was developed ; or, in other words, the same ontogenetic series of 
 cells which produces the egg-cell at the present day did so informer 
 times. This fact probably only admits of one interpretation, 
 and this is, that only certain series of cells contain the primary 
 constituentsof the germ-cells, and wherever it became useful in the 
 course of phylogeny for the germ-cells to be situated in another 
 position and in another layer of the body-wall, this change of 
 position could only be effected by the cells of the germ-track 
 becoming transformed into germ-cells at an earlier stage, and at 
 the same time migrating into the other layer of the body-wall. 
 If any I will not say all of the cells could give rise to germ- 
 cells, this complicated mode of procedure would be quite inex- 
 plicable, for Nature always takes the shortest possible course. 
 
 If this reasoning is correct, the hypothesis of the germ-tracks, 
 as I have formerly stated it, is inevitable ; and the fact that the
 
 iqo THE GERM-PLASM. 
 
 cells lying in these tracks are alone capable of giving rise to 
 germ-cells, can hardly be explained otherwise than by assuming 
 that these cells alone contain germ-plasm along with their 
 special idioplasm. If germ-plasm could be produced from the 
 idioplasm of ordinary somatic cells, it would be impossible to 
 see why germ-cells should not be formed in Hydroids in case of 
 need by the transformation of young ectoderm cells : but this 
 never happens. And even if we wished to assume that the 
 endoderm cells, as such, possessed an idioplasm which could not 
 be transformed into germ-plasm, while the nature of the ecto- 
 derm cells rendered such a transformation possible, this assump- 
 tion would be contradicted by other facts ; for, as far as we 
 know, the germ-cells arise exclusively from the endoderm cells 
 in the higher medusas, and in the polypes nearly related to 
 them. In this case therefore the germ-tracks are situated in 
 the endoderm, that is to say, the germ-plasm is only passed 
 into certain series of cells in the endoderm, and the reserve 
 material of unalterable germ-plasm, which will serve for the 
 formation of the germ-cells, is passed into the primary endo- 
 derm cell only in the process of segmentation of the ovum, and 
 is handed on by it. In the Vertebrata the germ-cells become 
 differentiated from certain groups of mesoderm cells, and they 
 are never found in any other part of the body. In this case 
 the germ-track passes from the fertilised egg-cell into those 
 segmentation cells from which the primary cells of the whole 
 mesoderm are formed, in which latter it follows a closely con- 
 fined course. 
 
 All these facts support the assumption that somatic idioplasm 
 is never transformed into genii-plasm, and this conclusion forms 
 the basis of the theory of the composition of the germ-plasm as 
 propounded here. It is obvious that its composition out of de- 
 terminants which gradually split up into smaller and smaller 
 groups in the course of ontogeny, cannot be brought into 
 agreement with the conception of the re-transformation of 
 somatic idioplasm into germ-plasm. If, as we have assumed, 
 each cell in the body only contains one determinant, the 
 germ-plasm which is composed of hundreds of thousands 
 of determinants could only be produced from somatic idio- 
 plasm if cells containing all the different kinds of determinants 
 which are present in the body were to become fused together 
 into one cell, their contained idioplosm likewise combining to
 
 THE FORMATION OF GERNf -CELLS, igi 
 
 form one nucleus. And, strictly speaking, even this assumption 
 would be by no means sufficient, for it does not account for the 
 architecture of the germ-plasm : the material only would be 
 provided. Such a complex structure can obviously only arise 
 historically. 
 
 The fact that somatic idioplasm cannot again give rise to 
 germ-plasm serves as an additional support for the theory of 
 the germ-plasm as here developed. Invisible, or at any rate 
 unrecognisable, masses of unalterable germ-plasm must have 
 been contained in the body-cells in all cases in which such a 
 transformation has apparently occurred. 
 
 These masses need not necessarily be invisible, for they 
 cannot be smaller than ids ; and if it should subsequently be 
 proved that the microsomes of the nuclear rods do actually cor- 
 respond to ids, we may hope to ascertain the exact number of 
 these ids in the individual species. An extensive field would 
 then be opened out for further investigation, for it would be 
 possible to decide by direct investigation whether the cell-series 
 of the germ-tracks carry along with them a larger or a smaller 
 number of ids than is contained in the fertilised egg-cell, and 
 also the relative proportion of the number of ids in the somatic 
 cells in the germ-track. We may thus hope that facts will 
 come to light which can be utilised in connection with this 
 theory. 
 
 Observations of this kind have already been made which indi- 
 cate an actual continuity of the germ-plasm. Boveri * observed 
 that the differentiation of the somatic cells from the primary 
 sexual cell in the segmenting egg of Ascaris megalocephala is 
 accompanied by a peculiar diversity in the nuclear structure. 
 The nuclei of the somatic cells throw off a large part of their 
 chromatin, in which process each idant loses a similar amount 
 of substance. Further facts and illustrations of the process are 
 still wanting, but even did we possess them it would be neces- 
 sary to postpone the detailed theoretical explanation of such 
 observations until we were able to judge as to the universal 
 occurrence of the process. Observations which my assistant, 
 Dr V. Hacker,+ has made on the segmenting ovum of a crus- 
 
 * Theodor Boveri, ' Anatom. Anzeiger II. Jahrgang,' No. 22, 1887 ; and 
 'Zellen-Studien,' Heft 3, Jena, 1890, p. 70. 
 
 t Valentin Hacker, Zool. Jahrbucher, Abth. f. Anat. und Ontog., Bd. 
 v., 1892; and Archiv. f. mik. Anat., Bd. 40, 1892.
 
 I 92 THE GERM-PLASM. 
 
 tacean (Cyclops), have indeed also proved that the behaviour of the 
 somatic segmentation cells is different from that of the primary 
 sexual cell, but the process differs essentially from that which 
 occurs in Ascaris. When we are in possession of similar 
 observations on various types of animals, we shall be able to 
 recognise the essential parts of the process, and shall then be 
 in a position to offer an explanation of them. 
 
 From a theoretical point of view, we must expect that the ids 
 of germ-plasm become doubled in the nucleus of the fertilised 
 egg-cell or even previously, one half of such a double id being 
 in the active condition in which it can undergo disintegration, 
 and the other being in the inactive and unalterable condition. 
 The former direct ontogeny, and the latter are passed on in a 
 passive condition to the primary sexual cells. As these, 
 however, behave at first like somatic cells, that is to say, they 
 multiply in a regular manner, and are distributed amongst de- 
 finite series of cells to definite parts of the body, they must 
 possess active idioplasm in addition to unalterable germ-plasm. 
 They must therefore contain more ids in their nuclear matter 
 than do the somatic cells. The above-mentioned observations 
 on Ascaris can thus be explained in accordance with our 
 theory up to this point, but more than this cannot be stated at 
 present. 
 
 2. THE GERM-TRACKS. 
 
 Taking sexual reproduction only into consideration for the 
 present, the course of the germ-tracks in existing Metazoa 
 apparently varies both as regards its length and the direction it 
 takes. The germ-track is shortest in the Diptera, in which the 
 primary germ-cell becomes separated off in the first division of 
 the ovum, so that in this case we might speak of a division of the 
 ovum into a primary germ-cell and a primary somatic-cell. In 
 the Daphnidce the germ-track is longer ; for, counting from the 
 ovum, five successive divisions occur before the primary germ- 
 cell is formed. In the free-swimming marine worm Sagitta it is 
 longer still, two primary germ-cells only appearing after ten or 
 more successive divisions have occurred, and the mass of 
 embryonic cells has already given rise to a gastrula-larva. In 
 other worms, such as the Nematodes, the primary germ-cells 
 become separated from the somatic cells in a still later genera- 
 tion of cells, which has so far not been actually determined ; and
 
 TUE FORMATION OF ( IKR.M-CI 
 
 '93 
 
 in most of the higher Metazoa this only occurs after the forma- 
 tion of hundreds or thousands of cell-generations. 
 
 The position of the germ-track may also vary. In tho Diptera 
 it is quite distinct from the somatic cell-tracks, and the genea- 
 logical trees of these two kinds of cells separate at the root. In 
 the Duphnida: the germ-track passes through each of the first four 
 segmentation-cells, and then branches off from the somatic 
 A a C 
 
 Fir,. 13. Three early stages in the development of Sagitta. (After O. Hertwig). 
 In /} the differentiation of two primitive germ-cells (g) from the endoderm, and 
 in /> and C the multiplication and separation of these cells is shown. (From 
 Lang's 'Lehrbuch der vergleichenden Anatomic.') 
 
 tracks. In Sagitta it passes through the primary endoderm 
 cell, and the primary germ-cells separate from the primary 
 endoderm before the definitive endoderm of the alimentary canal 
 has been formed from the latter. In Rhabditis nigro-vcnosa the 
 germ-track extends through three generations of endoderm cells, 
 passes into the primary mesoderm, and after several generations 
 branches off from two of the mesoderm cells. In most of the 
 Metazoa, however, the formation of the primary germ-cells is 
 postponed to a still later period, so that the separation of the 
 germ-branch from the somatic branch takes place at a much 
 higher level on the genealogical tree of the cells, and often first 
 occurs in the younger and smaller lateral branches. The primary 
 germ-cells do not always branch off from the track of the endo- 
 derm, but may just as often diverge from that of the ectoderm. 
 In the lower Medusas, for instance, in which the development is 
 a direct one, the germ-cells become differentiated at a very late 
 stage from the ectoderm cells of the body, which is already 
 fully formed and often independent and self-supporting ; while 
 in the higher Medusae and Ctenophora the primary genii-cells 
 are derived from the endoderm. We thus see that the germ- 
 tracks or series of cells which lead from the egg-cell to the 
 
 N
 
 i 9 4 
 
 THE GERM-PLASM. 
 
 primary germ-cells frequently take very different courses : they 
 are in some cases very short, and in others longer sometimes 
 so long that they pass through very different embryonic cells ; 
 in some instances they branch off from the primary endoderm- 
 cells, and in others from those of the mesoderm, and they may 
 even arise from later generations of the mesoderm, ectoderm, or 
 endoderm-cells. 
 
 FIG. 14. Three Stages in the development of the summer eggs of Moina. 
 (After Grobben.) A, Stage of segmentation viewed from the vegetative pole, 
 in which thirty-two cells are present : B, Blastula stage, from the vegetative 
 pole ; C, Gastrula stage, in longitudinal section, g, the primitive germ-cells. 
 (From Korshelt and Heider's ' Lehrbuch der vergleichender Entwickelungs- 
 geschichte.') 
 
 The same genn-track is always strictly followed in each of 
 these cases respectively, no deviation ever taking place : thus 
 the primary germ-cells never arise from endoderm-cells in a 
 group in which the normal germ-track lies in the ectoderm, 
 and vice versa. We consequently cannot help arriving at the 
 conclusion that the cells in the germ-track imist have some 
 advantage over the rest of the cell -tracks in ontogeny, for they
 
 THE FORMATION OF GERM-CELLS. 
 
 alone are capable of giving rise to the primary germ-cells. More- 
 over, if we remember that in the case of the Hydroid polypes 
 the period of the separation of the primary germ-cells can be 
 relegated to earlier or later stages, it will be clear that not only 
 the cells at the terminations of the germ-track in which this 
 separation actually occurs in indi- >j 
 viclual cases, but also the entire ^" 
 preceding series of cells, possess 
 qualities which are absent in the 
 other cells of the organism, and 
 which, sooner or later, render the 
 cells of the germ-track capable of 
 giving rise to primary germ-cells. 
 
 The cells of the germ-track do not 
 themselves correspond to primary 
 germ-cells, the character of which 
 latter is not as yet apparent ; they 
 are cells of a mixed character, that 
 is to say, they contain different 
 primary constituents, which are 
 gradually separated off until even- 
 tually only two of them remain, 
 and these then separate from one 
 another by means of a final cell- 
 division. 
 
 The embryogeny of a parasitic 
 worm (Rhabditis nigrovenosa) from 
 the frog's lung may serve to illus- 
 trate this point. In fig. 15, A to D 
 represent the first four stages in 
 segmentation up to the differentia- 
 tion of the primary mesoderm-cell 
 (me*). This and the following stages 
 are represented diagrammatically 
 in fig. 1 6, which shows the genea- 
 logical tree of the cells and the 
 germ-track. The ovum (Eiz) must of course be considered as 
 containing the whole of the primary constituents of the organism 
 before its first division into a primary ectoderm (iirEkf) and a 
 primary endoderm cell (urEnf). The latter retains all the 
 primary constituents of the mesoderm and primary germ-cells, 
 
 FIG. 15. Stages in the segmen- 
 tation of the ovum and for- 
 mation of the germinal layers 
 in Rhabditis nigrovenosa. 
 (After Gotte). ect, Ectoderm ; 
 ent, Endoderm ; mes, Meso- 
 derm.
 
 196 THE GERM-PLASM. 
 
 in addition to those of the endoderm, and is therefore not 
 merely a primary endoderm cell. This then divides again and 
 forms two cells, of which the one marked 3 on the left side of 
 the figure only contains primary constituents of the endoderm, 
 and is therefore an endoderm cell proper ; while that marked 3' 
 represents the first rudiment of the mesoderm and of certain 
 portions of the endoderm, and contains in addition the primary 
 constituents of the primary germ-cells. This cell (3') divides into 
 two (4' and 4"), thus separating the above-named rudiments into 
 those for the right and the left sides of the body; and finally, the 
 
 FIG. 16. Diagram of Me germ-track of Rhabditis nigrovenosa.Thz various 
 generations of cells are indicated by Arabic numbers, the cells of the germ- 
 track are connected by thick lines, and the chief kinds of cells are distinguished 
 by various markings : the cells of the germ-track by black nuclei, those of the 
 mesoblast (Mes) by a dot in each, those of the ectoderm (Ekt) are white, those 
 of the endoderm (Enf) black ; in the primitive germ-cells (nr Kz) the nuclei 
 are white. The cells are only indicated up to the twelfth generation. 
 
 complete separation of the rudiments of the mesoderm and endo- 
 derm occurs, and results in one daughter-cell (5")> containing the 
 primary constituents of the mesoderm and primary germ-cells, 
 while the other gives rise to a cell of the endoderm proper. 
 The primary constituents of the primary germ-cells remain con-
 
 THE FORMATION OF GERM-CELLS. 197 
 
 nected with certain of those of the mesoderm during several 
 generations of cells, and in each subsequent division certain of 
 the latter pass out alone into one of the daughter-cells, the 
 other retaining the primary constituents of the primary germ- 
 cells in addition to those of the mesoderm. Finally, in the 
 ninth series of cells in the diagram in which the processes are 
 represented as greatly abbreviated the separation of these two 
 sets of primary constituents occurs, and the first primary germ- 
 cell (ur Kz) is formed. 
 
 So much is certain, and does not depend on any hypothesis. 
 Opinions may differ as to whether the cells situated in the germ- 
 track are to be described as real somatic cells. I have called 
 them so, because in many cases the germ-tracks extend far 
 beyond the period of embryogeny into the fully-developed func- 
 tional tissues, and because it can be proved that even cells which 
 are histologically differentiated may produce germ-cells under 
 certain circumstances. This occurs amongst plants in the 
 prothallus of ferns, for instance and also in the cells of certain 
 Polyzoa from which gemmation may take place, and which must 
 therefore contain inactive germ-plasm. In these cases it is 
 certain that real somatic cells are situated along the germ- 
 tracks ;' in all cases the cells of the germ-track are not 
 germ-cells from the first, and they always take part in the con- 
 struction of the body. And if we further consider that a large 
 number of somatic cells must contain accessory idioplasm of 
 some kind, either that which will serve for simple regeneration, 
 or for the regeneration of more complex parts, or again, for the 
 formation of buds, we can hardly assume that the character of a 
 somatic cell is thereby abolished : I can see no advantage in 
 objecting to describe a cell of the germ-track as a somatic cell. 
 
 The change which the idioplasm of the cells constituting the 
 germ-track undergoes, can obviously only consist in its active 
 portion gradually becoming separated off in the course of the 
 ontogenetic cell-divisions, {so that ultimately the cell contains 
 germ-plasm only, which then stamps it as a germ-cell. Even 
 then the germ-plasm remains unalterable as long as this first 
 or primary germ-cell continues to produce others similar to 
 itself. The cells only become differentiated into spermatozoa 
 and ova when this multiplication ceases, and this presupposes 
 the splitting off of special spermatogenetic or ontogenetic de- 
 terminants. The disintegration of the germ-glasm which results
 
 198 THE GERM-PLASM. 
 
 in a new embryogeny provided that the necessary conditions 
 have been fulfilled can only begin when this has occurred. 
 
 3. HISTORICAL ACCOUNT OF THE THEORY OF THE CON- 
 TINUITY OF THE GERM-PLASM. 
 
 When my essay on the 'Continuity of the Germ-plasm' 
 appeared seven years ago,* I was under the impression that I 
 was the first to give utterance to this conception. Since then, 
 however, I have found that similar ideas had arisen, in a more 
 or less distinct form, in other brains ; and I gradually discovered 
 that a number of authors had independently recognised more or 
 less clearly the distinction between the body-cells and germ- 
 cells and the direct connection between the germ-cells of dif- 
 ferent generations : some had merely made the assertion, and 
 others had attempted to support it by facts. I shall here give 
 an account of those theories which preceded mine, taking them 
 in chronological order. 
 
 As early as 1849, Sir Richard Owen had indicated that 
 differences may arise in the developing germ-cells between 
 those which become greatly changed and form the body, and 
 those which only undergo a slight change and form the repro- 
 ductive organs.t 
 
 Francis Galton was the first to express certain ideas which 
 bore some resemblance to the conception of the continuity of the 
 germ-plasm. In a paper which appeared as early as 1872, the 
 individual is conceived 'as consisting of two parts, one of which 
 is latent, and only known to us by its effects on his posterity, while 
 the other is patent, and constitutes the person manifest to our 
 senses. The adjacent and, in a broad sense, separate lines of 
 growth in which the patent and latent elements are situated, 
 diverge from a common group, and converge to a common 
 contribution, because they were both evolved out of elements 
 contained in a structureless ovum, and they jointly contribute 
 the elements which form the structureless ova of their off- 
 spring.'! 
 
 * 'Der Continuitat des Keimplasma's,' Jena, 1855 (Essay iv., p. 163, in 
 the second English edition). 
 
 t I quote this statement from Geddes and Thomson's ' Evolution of 
 Sex' (London, 1889), p. 93, in which the original authority is not given. 
 
 t Proc. Roy. Soc., No. 136, p. 394.
 
 THE FORMATION OF GERM-CELLS. 199 
 
 A few years later, Gallon changed his opinion and adopted 
 Darwin's theory of pangenesis, which he modified considerably, 
 and only used 'as a supplementary and subordinate part of a 
 complete theory of heredity.' This theory has already been dis- 
 cussed in the Historical Introduction to this book. The 
 'gemmules' which are contained in the fertilised ovum together 
 constitute the 'stirp' or stock, which by means of the egg-cell 
 gives rise to a new individual. Each ' sort of gemmule ' is repre- 
 sented by a number of gemmules which differ somewhat from and 
 compete with one another ; and since the successful ones in the 
 competition for taking part in the construction of the body form 
 the various parts of the body and are therefore contained in 
 them, the rest remain unused, thus constituting the ' residual 
 germs.' These, then, are ' the parents of the sexual elements and 
 buds.' The ' dominant ' gemmules may also take a part, though 
 only a slight one, in the formation of the germ-cells, ' as they 
 are the least fertile in the production of gemmules.' The germ- 
 cells are therefore mostly formed from gemmules which have 
 remained latent, and this accounts for the fact that the offspring 
 usually do not exhibit the most marked peculiarities of the 
 parent. As this hypothesis only accounts for the dissimilarity 
 between parent and child, so far as it exists, and not for the far 
 commoner resemblance between them, Gallon assumes that the 
 parls of ihe body can also give off gemmules which become dis- 
 Iribuled and extend beyond the boundaries of ihe cells in which 
 they arose, and so may even penelrate inlo the sexual elemenls. 
 He ihus subslilules ihe idea of a locally restricled dislribulion of 
 the gemmules for Darwin's view of their ' free circulation.' If we 
 attempt to make this somewhat vague and unrealistic idea rather 
 more comprehensible, by considering the ' residue of the stirp ' 
 as equivalent to the ' unalterable ' reserve germ-plasm, Gallon's 
 hypolhesis will be found lo bear some resemblance lo Ihe iheory 
 of the conlinuily of Ihe germ-plasm. But there is still a 
 fundamental difference between them, for Gallon's idea is only 
 conceivable on ihe presupposilion of ihe occurrence of sexual 
 reproduclion, while ihe Iheory of Ihe conlinuily of ihe germ- 
 plasm is enlirely independenl of any assumplion as lo whelher 
 each primary consliluent is presenl in Ihe germ singly or in 
 numbers. According to my idea, the active and the reserve 
 germ-plasm conlain precisely similar primary consliluenls, gem- 
 mules, or determinanls ; and on ihis Ihe resemblance of a child
 
 200 THE GERM-PLASM. 
 
 to its parent depends. The theory of the continuity of the germ- 
 plasm, as I understand it, is not based on the fact that each 
 ' gemmule ' necessary for the construction of the soma is present 
 many times over, so that a residue remains from which the germ- 
 cells of the next generation may be formed : it is founded on the 
 view of the existence of a special adaptation, which is inevitable 
 in the case of multicellular organisms, and which consists in the 
 germ-plasm of the fertilised egg-cell becoming doubled primarily, 
 one of the resulting portions being reserved for the formation of 
 germ-cells. 
 
 Gustav Jager* was the first to express the idea that in the 
 higher organisms the body consists of two kinds of cells, which 
 he calls respectively ' ontogentic ' and ' phylogentic ; ' and that 
 the latter, or reproductive cells, are not a product of the former, 
 or body-cells, but are derived directly from the germ-cell of the 
 parent.* He took it for granted that the 'formation of repro- 
 ductive substances occurs in an animal during the early em- 
 bryonic stages,' and imagined that he had thus proved the 
 existence of a connection between the germ-cells of the parent 
 and those of the child. Although these opinions were not 
 founded on fact or followed out in detail, they ought to have led 
 to further ideas on the subject. They, however, together with 
 the book in which they were contained, remained unnoticed. 
 
 A few casual remarks made by Rauber,* in a paper on ' Form- 
 bildung und Form stoning in der Entwicklung von Wirbelthieren,' 
 
 * Gustav Jager, ' Lehrbush der allgemeinen Zoologie,' Leipzig, 1878, 
 II. Abtheilung. 
 
 + The praiseworthy attempt to do justice to my predecessors in this par- 
 ticular subject has perhaps been carried too far. In Geddes and Thomson's 
 ' Evolution of Sex ' (p. 93), for instance, a. quotation is given from Jager 
 which seems to prove that he anticipated me with regard to the theory 
 under consideration. The quotation in which this idea is expressed is, 
 however, not taken from the book published in 1878, but from an essay 
 written ten years later, and it concludes with the following words : ' This 
 reservation of the phylogenetic material I described as the continuity of the 
 germ-plasm.' But no mention is made by Jager of the continuity of the 
 germ-plasm in his book which appeared in 1878, in which a connection 
 between the germ-cells of different generations is supposed to exist : and 
 this is not the case. The entirely new statement of his ideas has been 
 influenced by those contained in my essays which had appeared in the 
 meanwhile. 
 
 J ' Morphol. Jahrbuch,' Bd. 6, 1880.
 
 THE FORMATION OF GERM-CELLS. 2OI 
 
 suffered the same fate. This author states that ' as regards the 
 effect of fertilisation, it can only convert a portion of the egg, 
 viz., the personal part, into the form of a person ; the other 
 portion does not experience this effect, for it has a stronger 
 power of persistence.' 
 
 Finally, Nussbaum * was likewise led to the idea of the con- 
 tinuity of the germ-^//^. He, too, assumed that ' the segmented 
 ovum divides into the cell-material of the individual and the 
 cells for the preservation of the species,' and he supports this 
 statement by quoting the cases already mentioned of the very 
 early differentiation of the sexual cells. 
 
 I will conclude this section with the words which appeared in 
 the preface to a short paper intended as a defence against the 
 accusation of plagiarism which had been made against me. 
 'A fertile scientific idea has rarely appeared without having 
 been contested on the one hand, and set down as already known 
 on the other. The former k certainly a perfectly justifiable and 
 even necessary course of proceeding, for a clear and definite know- 
 ledge of the truth can only result from the contest of opinions ; 
 and even the latter is to some extent justifiable, for an idea of 
 this kind probably very rarely arises without having been pre- 
 ceded by similar attempts directed towards the same object ; 
 and it is only natural that those who first made such attempts 
 should overlook the difference between these struggles towards 
 the desired object and its attainment.' 
 
 Others may decide the reason why no attention had been 
 drawn to the suggestions mentioned above as having been made 
 previously to my theory of the continuity of the germ-plasm, 
 and why these did not exert any influence on scientific thought. 
 This is certainly the case : and it practically follows from the 
 fact, that all the objections which have been -made have been 
 directed against me. Some of these objections will be discussed 
 in the following chapter. That I am far from desirous of de- 
 tracting from the merit of others, has, I trust, been shown by 
 the fact that as soon as I became aware of previous views on 
 the subject I brought them forward. Jager's ideas, for instance, 
 might have long remained unnoticed, had not I brought them 
 to light. But an historical account of the various previous views 
 
 * M. Nussbaum, 'Die Differenzirung des Geschlechts ira Thierreich," 
 Archiv. f. mikr. Anatomic, Bd. xviii., 1880.
 
 202 THE GERM-PLASM. 
 
 on this subject* cannot be considered to be an impartial one, 
 if no mention is at the same time made of the fact that all these 
 suggestions remained unnoticed, and had no effect on the pro- 
 gress of scientific thought. That this is the case there can be 
 no doubt. And although it may be a satisfaction to every one 
 to have expressed a correct idea, no such idea can be con- 
 sidered as fertile, and as having an important influence on 
 the progress of scientific thought, unless its meaning is si 
 obvious that it results in further progress. Such a result, how 
 ever, only followed after my essays had appeared. 
 
 4. OBJECTIONS TO THE THEORY OF THE GERM-PLASM. 
 
 Important objections to this theory have been raised by 
 several botanists ; and at first sight the facts on which these 
 are based may easily give rise to the impression that the 
 theory cannot be carried out in the case of plants. If this were 
 so, however, its correctness would be altogether doubtful, for the 
 hereditary mechanism cannot be totally different in plants and 
 animals. We must therefore make a closer examination into the 
 facts as they concern plants, and I hope to be able to show that 
 the fundamental ideas \\ihich I have assumed are applicable to 
 plants as well as to animals, although they did not originate 
 from the botanical point of view. 
 
 Certain misconceptions and inaccurate representations must 
 first be put on one side. Many botanists deny the existence of 
 the germ-plasm entirely. 
 
 Vines t considers the assumption of a special ' reproductive 
 substance' unnecessary, as the capacity for reproduction is a 
 fundamental property of protoplasm. A cutting gives rise to a 
 complete plant, just as a broken crystal becomes complete when 
 immersed in the mother-liquor, for it produces the missing 
 parts, viz., roots. It is not necessary to assume the existence of 
 a special ' reproductive substance ' in either case. 
 
 I need not especially emphasize the fact that this stimulus 
 which results in the completion of a part is not by any means a 
 universal phenomenon, and that, for instance, some parts of 
 plants cannot be reproduced from cuttings. I shall simply 
 
 * Cf. the account given in Geddes and Thomson's ' Evolution of Sex,' 
 pp. 93 and 94. 
 
 t Cf. ' Nature, ' Oct. 24, 1889 ; and ' Lectures on the Physiology of 
 Plants,' Cambridge, 1886.
 
 THE FORMATION OF GERM-CELLS. 203 
 
 confine myself to calling attention to the fact that even if a 
 universal reproductive power existed in protoplasm, it certainly 
 would not explain matters. For this power is just what has to 
 be explained. 
 
 If we know, for instance, that Infusoria are able to replace 
 great losses of substance, so that when the oral region is cut 
 off, it, together with all the cilia and other minute structures, can 
 be formed anew, a proof is thereby obtained that these uni- 
 cellular organisms actually possess the universal reproductive 
 power which Vines wishes to ascribe to vegetable protoplasm. 
 But does this help us in the slightest degree to understand the 
 fact, or to explain why the ultimate particles of the cell-body 
 become rearranged and transformed after a loss of substance 
 has occurred, just as is necessary for the reappearance of the 
 species ? Do we thereby gain the faintest idea as to how and 
 why the residual particles of the cell-body are compelled to give 
 tip their previous form and connection, and to reconstruct 
 exactly that part which is required in order to render the whole 
 complete? The assumption of a 'reproductive power' simply 
 amounts to the statement of the fact that regeneration occurs ; 
 and this, it seems to me, is equivalent to saying that the re- 
 productive power is a fundamental property of vegetable 
 protoplasm. 
 
 In the case of the unicellular Infusorian we can, however, 
 hardly venture at present to attempt an explanation of this 
 problem, as we know very little of the vital units of which the 
 cell-body is composed, and of the forces situated within them. 
 But the case is different with regard to those organisms which 
 consist of many physiologically differentiated cells : in these we 
 are at any rate acquainted with the form and function of one 
 arrangement of the vital units of which the whole aggregate is 
 composed, and so we can attempt to deduce the functions of 
 the whole body from those of the units, and conversely to refer 
 the latter processes to a distribution of the forces amongst the 
 units composing the whole. We need not therefore confine 
 ourselves to the mere statement of the fact that a process 
 occurs by means of which the whole is completed, but we may 
 further inquire as to when this occurs, from what elements it 
 proceeds, how the whole body arises at all, and how so complex 
 a structure can be formed from the apparently simple substance 
 of the germ.
 
 204 THE GERM-PLASM. 
 
 In order to give a satisfactory answer to these questions, I 
 have assumed the existence of a germ-plasm, but have not 
 primarily regarded this as a ' special reproductive substance ' 
 which is very different from all other substances in the body ; 
 I have looked upon it, on the contrary, as the substance which 
 gives rise to all the other formative substances of the entire 
 individual. Every part of the body contains a portion of this 
 substance, and the whole organism can only be formed anew 
 when all the portions of this controlling substance (the idio- 
 plasm) are combined ; that is to say, when germ-plasm is 
 present. The assumption of germ-idioplasm or germ-plasm is, 
 I consider, quite unavoidable, for we must at the present 
 day take it as proved that the hereditary tendencies are con- 
 nected with a substance. In my opinion, it is also an irrefutable 
 fact that this germ-plasm undergoes regular changes from the 
 ovum onwards : it must, indeed, undergo change from cell to 
 cell, for we know that the individual cell is the seat of the forces 
 which give rise collectively to the functions of the whole. The 
 forces which are virtually contained in the germ-plasm can 
 therefore only become apparent when its substance undergoes 
 disintegration, and its component parts, the determinants, be- 
 come rearranged. The difference in function seen in the 
 various groups of cells in the body compels us to suppose that 
 these contain a substance which acts in various ways. The 
 cells are therefore centres of force of different worth, and the 
 substance (idioplasm) vvhich controls them must be just as 
 dissimilar as are the forces developed by them. 
 
 The apparent similarity of many young plant-cells may account 
 for the vagueness with which Vines, following Sachs's example, 
 speaks of an * embryonic substance* from which reproduction is sup- 
 posed to proceed in all cases, and which is assumed to be present 
 in all ' young ' cells. In my opinion the hereditary value of a cell 
 can be estimated as little by its age as by its appearance. The 
 mass of cells resulting from the segmentation of an animal egg 
 certainly possess the character of youth, and in a certain stage of 
 development these cells are all of the same age and all look 
 alike. They have, however, entirely different hereditary values ; 
 and if we are accurately acquainted with the ontogeny of the 
 animal in question, we can tell what hereditary tendencies lie 
 hidden in each cell. The primary constituents of the entire endo- 
 derm, for instance, may be contained in one cell, and that of the
 
 THE FORMATION OF GERM CELLS. 205 
 
 ectoderm or mesoderm in another ; or, again, in a later stage, 
 only a rudiment of a particular part, organ, or portion of an 
 organ belonging to the germinal layer in question, may be present 
 in an individual cell. But if we inquire whether the entire body 
 could arise from each of these cells, known facts give a very 
 decided answer in the negative. Only one, or a few perfectly 
 definite cells amongst them, which we speak of as germ-cells, 
 can reproduce the whole animal under favourable circumstances. 
 This is true of all the higher Metazoa : the cells of the segment- 
 ing ovum are completely dissimilar as regards their hereditary 
 value, although they are all ' young* and ' embryonic] and are 
 not infrequently quite similar in appearance. It therefore 
 seems to me to follow from this, as a logical necessity, that the 
 hereditary substance of the egg-cell, which contains all the 
 hereditary tendencies of the species, does not transmit them in 
 toto to the segmentation-cells, but separates them into various 
 combinations, and transmits these in groups to the cells. I have 
 taken account of these facts in considering the regular distribu- 
 tion of the determinants of the germ-plasm and the conversion 
 of the latter into the idioplasm of the cells in the different stages 
 of ontogeny. All these cells contain ' embryonic substance,' but 
 the detenninants contained in one set differ from those in 
 another, and therefore contain different hereditary tendencies. 
 Hence it is comparatively meaningless to speak merely of an 
 ' embryonic substance.' 
 
 De Vries regards some of my views in a very different way, 
 and from an entirely different aspect. In an extremely able 
 manner he brings forward a number of facts concerning 
 heredity in plants, and finds that they usually do not fit in with 
 my views. I have followed his deductions with great interest, 
 and have gratefully made use of the facts which he has brought 
 forward ; but I nevertheless believe that the chasm which 
 separates his views from mine can be bridged over. 
 
 In the first place, de Vries accuses me of having taken a one- 
 sided view of the question by considering the processes as they 
 occur in animals only : in these it may be possible, as I have 
 assumed, to draw a sharp line of distinction between somatic- 
 and germ-cells, but this cannot be done in the case of plants. 
 In the latter, those series of cells which I have called germ- 
 tracks may give rise to many other cells besides germ-cells, 
 although this as a general rule is only exceptionally the case :
 
 206 THE GERM-PLASM. 
 
 that is to say, it occurs in response to definite external influences. 
 It would not, however, only take place in those parts of the 
 plant which might be assumed to be specially adapted for this 
 capacity, but might also occur in those in which adaptation is 
 out of the question. We are therefore compelled to assume that 
 most, if not all, of the cells contain all the primary constituents 
 of the species in a latent condition. 
 
 I will first discuss the manner in which de Vries applies my 
 hypothesis of the germ-tracks to the case of plants, and the con- 
 clusions at which he has arrived and has illustrated by describing 
 a number of genealogical trees representing the various series of 
 cells in plants. 
 
 De Vries draws a distinction between 'primary' and 'accessory' 
 germ-tracks. The former correspond to the germ-tracks I have 
 already assumed : that is to say, to those cell-series which nor- 
 mally lead from the fertilized egg-cell to the new germ-cells (ova, 
 spermatozoa, pollen-grains). By 'accessory germ-tracks' are 
 meant those cell-series which lead to germ-cells ' through ad- 
 ventitious buds.' These accessory germ-tracks are, according 
 to de Vries, absent in the higher animals, but are of common 
 occurrence amongst plants, and I am accused of not having 
 taken them sufficiently into account. The 'accessory germ- 
 tracks,' if I understand the term aright, are regular germ-tracks, 
 which do not, however, always come into use. In many of the 
 lower plants, such as mosses and fungi, ' almost all of the cells 
 may develop into new individuals ; ' and in the higher plants, 
 buds, from which entire plants possessing germ-cells may arise, 
 can, under certain circumstances, be formed at any rate from 
 certain kinds of tissue, which may consist of young (meristematic) 
 cells or indeed even of full-grown cells. 
 
 Let us first consider the 'primary germ-tracks.' De Vries 
 thinks that their behaviour is essentially different in the higher 
 animals and in plants : in the former, the genealogical tree of the 
 cells of the germ-track 'is straight, and only slightly branched at the 
 apex,' while in the higher plants ' the branches are so numerous 
 and subdivided from the base upwards that they frequently over- 
 top the main stem ; or, more accurately, the main stem is hardly 
 recognisable.' No objection can certainly be raised to this state- 
 men^ which we may illustrate by a blossoming apple tree, in 
 which the blossoms which crown the top may be taken as cor- 
 responding to the germ-cells. But how is this difference to be
 
 THE FORMATION OF GERM-CELLS. 207 
 
 pcoved, and on what does it depend? It is not based on the 
 animal or vegetable nature of the organisms, for as de Vries 
 himself incidentally acknowledges, we find a similar kind of 
 branching of the primary germ-tracks in the Hydroid-polypes. 
 It simply depends on the fact that a higher individuality of the 
 stock exists in these animals, just as in the case of the higher 
 plants. In both cases we have to deal not with a single person 
 and the formation of its germ-cells, but with a number of per- 
 sons which arose from the primary one by budding, each of 
 which has a body of its own, and gives rise to its own germ- 
 cells. The germ-track is concealed within the first person of 
 the stock produced from an egg, and gives off a lateral branch 
 as soon as this first polype develops a bud. Shortly afterwards, 
 the polype gives rise to a second bud, into which a lateral germ- 
 track likewise extends ; and when these two buds have developed 
 into complete polypes, they again give rise to buds, into which 
 germ-tracks are once more given off, and so on. The copious 
 branching of the germ-track is thus accounted for, and it is quite 
 immaterial whether the separate persons of the stock are more 
 or less independent and perfect, and to what extent they may be 
 regarded as ' individuals.' In those cases in which a periodic 
 segmentation of the body into serially homologous segments or 
 metameres takes place, each of which has almost a similar 
 origin and is able to produce germ-cells, the type of the genea- 
 logical tree of the germ-track, as described above, results. 
 
 If, however, we inquire as to the conclusions which can be 
 drawn from the course taken by the germ-tracks in animals and 
 plants, we receive a reply from de Vries which is very significant 
 of the way in which this problem is at present regarded by 
 many botanists : the whole question which I have raised with 
 regard to the continuity of the germ-plasm is an idle one. In 
 his opinion, 'the whole question as to whether somatoplasm can 
 become transformed into germ-plasm has no basis in fact.' ' A 
 germ-track,' says de Vries, ' never arises from a somatic-track,' 
 and 'a continuity of the germ-cells exists, not merely in the 
 very rarest cases, but universally and without exception, although 
 it often takes place through a long series by means of the germ- 
 tracks.' 
 
 With the exception of the last one, these sentences merely 
 repeat my own views, and the apparent contradiction of the latter 
 is simply due to the fact that de Vries adopts expressions which
 
 208 THE GERM-PLASM. 
 
 1 have used in another sense. In stating that germ-cells arise 
 from somatic cells in innumerable cases, I referred to the soma- 
 tic cells which are situated along the germ-track, the existence 
 of the latter being assumed for this special purpose. De Vries, 
 however, disputes the somatic character of these cells, because 
 he considers that they also contain ' germ-substance.' I should 
 attach slight importance to a mere name, if a very definite idea 
 did not depend on this name, the abandonment of which would 
 lead to confusion. It appears to me to be dangerous to intro- 
 duce a third category of cells viz., those of the germ-track 
 between the somatic and the germ-cells. In the first place, it is 
 unpractical to do so, for the appearance of a cell does not reveal 
 to us whether it is situated in the germ-track or not ; and 
 secondly, it would lead to a total confusion of the ideas of 
 somatic and germ-cells ; for, as has been shown in the previous 
 chapters, there are a number of cells in plant- and animal- 
 stocks which are undeniably somatic, and which must therefore 
 contain germ-plasm. Since we regard the ' blastogenic ' idio- 
 plasm of plants and Hydroids as a modification of germ-plasm, 
 we must also look upon a very considerable number of the cells 
 which constitute these organisms as cells of the germ-track, 
 and we should therefore arrive at the absurd conclusion that a 
 soma (body) is not present at all in these cases. The soma 
 nevertheless is present, and a contrast also exists between it and 
 the germ-cells in plants as much as in animals. 
 
 De Vries contradicts himself when he states that a universal 
 ' continuity of the germ-cells' exists through the germ-track ; for 
 in other passages he emphasises the fact that germ-cells do not 
 as a rule arise directly from one another (p. 84), and that a dis- 
 tinction must be made between germ-cells and cells of the germ- 
 track. The somatic character of the cells of a fern-prothallus, 
 for instance, cannot be denied, for they function as somatic cells, 
 and at first are all similar in appearance, so far as we are able 
 to observe. But nevertheless some of them are situated on the 
 germ-track, and give rise to male and female germ-cells. 
 
 If de Vries puts aside the whole question of the continuity of 
 the germ-plasm because he is able to prove that germ-cells 
 always arise from cells of the germ-track, it is evident that he 
 must be labouring under a similar delusion to that which induced 
 Sachs to claim precedence as regards the theory of the con- 
 tinuity of the germ-plasm. Both these observers consider it
 
 THE FORMATION OF GERM-CELLS. 209 
 
 self-evident that each apical cell contains the germ-substance of 
 the ovum, because in plants all growth takes place from the 
 growing point and originates in the apical cells, which are 
 derived directly from the egg-cell. 
 
 This, however, is at any rate only self-evident in the case of 
 the first apical cell of the main shoot, and is certainly not so in 
 that of the lateral shoots, which are, indeed, only derived in- 
 directly from the former. All the cells of a plant are undoubtedly 
 descended in a direct line from the ovum ; but this fact does not 
 imply that they must all give rise to apical cells or must all con- 
 tain germ-plasm, nor does it in any way explain the fact that 
 only relatively few of them can become germ-cells and the rest 
 cannot. These were the very facts which the hypothesis of the 
 continuity of the germ-plasm was intended to make compre- 
 hensible, to some slight extent at any rate. The origin of a cell 
 from the ovum gives no clue to its nature ; and, as de Vries him- 
 self says, the entire description in detail of the cell-series leading 
 from the ovum to the first apical cell, although very interesting 
 in itself, gives us no information as to the origin of the germ- 
 substance present in certain parts of the plant-body. I do not 
 understand therefore how de Vries can look upon the fact of the 
 existence of these cell-series as constituting in itself an important 
 explanation of the problem, without attempting to explain it 
 further. It seems to me that the cell-series can only be of any 
 explanatory value when they are regarded as germ-tracks in the 
 sense in which I use the term, that is to say, as those series 
 through which the germ-substance is transmitted from the egg- 
 cell to the remotest parts of the plant. 
 
 I must say that it seemed to me to be a somewhat crude idea 
 to suppose that the same kind of idioplasm is contained in all 
 the cells of the germ-track, including the apical cell, and that it 
 is equivalent to ' germ-substance.' Why do not the apical cells 
 in the sterile shoots of the horse-tail give rise to germ-cells, 
 while those of the fertile shoots do so ? This must be due to a 
 difference in the idioplasm of the apical cells of these shoots. And 
 although structures bearing germ-cells may become developed 
 from the apical cell of a fertile shoot of the plant, all the cells of 
 the latter do not nevertheless give rise to germ-cells : only certain 
 cell-series lead from the apical cell to the new germ-cell, and 
 these are the cells of the germ-track which contain germ-plasm ! 
 The process of the formation of a shoot from an apical cell is 
 O
 
 210 THE GERM-PLASM. 
 
 analogous to that of the production of a single polype by budding 
 from a polype-stock. But both these processes are essentially 
 the same as that of the development from the ovum in a higher 
 animal. In all three cases the formation of the new animal 
 originates in one cell. The latter must therefore possess an 
 idioplasm which contains all the primary constituents of the 
 organism ; and, moreover, if the organism is to be ' fertile,' in the 
 sense in which this term is used by botanists, the original cell 
 from which it is derived must contain the primary constituents 
 of all the structures characteristic of the species in its idioplasm: 
 that is to say, it must contain germ-plasm. If we trace the 
 development of such a shoot or organism, we shall find that it 
 follows a precisely similar course to that which we have already 
 described in the case of embryogeny ; and that at each cell- 
 division the primary constituents break up into smaller and 
 smaller groups, until at last each cell only contains one such 
 element. And yet all these very different kinds of cells are 
 descended in a direct line from the original cells. How, then, 
 can we account for the fact that one or several of them contain 
 all the primary constituents of the species in a latent condition, 
 in addition to one specific primary constituent of a particular 
 kind of somatic cell, as must be the case in those which give rise 
 to germ-cells ? It would, indeed, be a very simple matter if a 
 continuous series of cells which contain ' genii-substance ' only, 
 led from the original cell to the new germ-cells. But as simple 
 a case as this only occurs in the Diptera : in all other instances 
 the intermediate cells which constitute the germ-track can be 
 proved to contain perfectly definite somatic elements in addition 
 to the germ-plasm j and this is the case in plants as well as in 
 animals. 
 
 To make this clear, it is only necessary to glance at the genea- 
 logical tree representing the ontogeny of Rhabditis mgrovenosa 
 (fig. 1 6). How does it comes to pass, for example, after the 
 division of the primary endoderm cell into the first endoderm 
 and first mesoderm cell, that the latter is nevertheless capable 
 of producing cells subsequently which contain ' germ-substance,' 
 i.e., germ-cells ? At its origin this cell gave up the primary con- 
 stituents of the endoderm to the sister-cell ; by what means do 
 these primary constituents even those of the ectoderm which 
 were previously given up reach the germ-cells which eventually 
 arise from this cell ? My answer to these questions has already
 
 THE FORMATION OF GERM-CEI.LS. 211 
 
 been given, and is as follows : in addition to their active meso- 
 derm-idioplasm, the cells which will give rise to germ-cells carry 
 along with them a certain amount of germ-plasm in an unalterable 
 condition. De Vries, and those botanists who agree with him, 
 consider my answer superfluous. Every one, of course, is at 
 liberty to reject the solution of a problem, but in that case he 
 must not claim to have explained it. 
 
 I now come to the consideration of de Vries's 'accessory 
 germ-tracks.' As has already been stated, this term is used to 
 describe those series of cells which give rise to germ-cells 
 through the agency of ' adventitious buds.' According to 
 Sachs,* adventitious buds correspond to those growing points 
 which are not derived from those already present, or 'from 
 embryonic tissue already present,' but which ' originate at places 
 where the tissue has already passed over into the permanent 
 condition, in fully-developed roots, in the interfoliar parts of 
 shoot-axes, and more particularly in foliage leaves, the tissues of 
 which are already completely differentiated and developed.' 
 
 In my former essays I have endeavoured to account for these 
 ' adventitious ' buds, such as are formed, for instance, in a 
 Begonia leaf when it is placed on damp soil, by supposing them 
 to be adaptations of particular species of plants to this peculiar 
 method of reproduction : I assumed that certain series of cells 
 which in these species take part in forming the leaves contain 
 unalterable and inactive germ-plasm in addition to their own 
 active idioplasm. 
 
 In opposition to this interpretation much may be, and in fact 
 has already been, said, and the principal objections must now 
 be considered. 
 
 It has, in the first instance, been urged that the capacity pos- 
 sessed by leaves, roots, and so on, for producing adventitious buds, 
 cannot be regarded as an adaptation, because so many cases 
 are known in which this process only occurs exceptionally, and is 
 of no advantage to the plant. There can be no doubt, however, 
 that the power possessed by Begonia, Bryophyllum, Cardamine 
 pratensis, and Nasturtium officinale, of giving rise to buds in 
 those parts in which they are not formed in most plants, is due 
 to an arrangement peculiar to these plants. Neither in Begonia 
 nor in Bryophyllum can the buds and young plants arise from 
 
 * ' Lectures on the Physiology ot Plants,' p. 477.
 
 212 THE GERM- PLASM. 
 
 all parts of the leaf ; they are only formed in perfectly definite 
 regions, e.g., on the margins of the leaves in Bryophyllum and 
 in the angles between the points of origin of the large veins in 
 Begonia. All the cells of the leaf do not, therefore, as de Vries 
 supposes, possess this capacity, which is limited to perfectly 
 definite though numerous cells. These therefore correspond to 
 somatic cells, quite as much as do those which produce the 
 several cells in the prothallus of a fern, which contain unalter- 
 able germ-plasm in addition to the active somatic idioplasm, 
 the former only becoming active by the influence of particular 
 external influences. 
 
 These conditions may be fulfilled in thousands of other leaves 
 without resulting in the production of young plants. There are 
 indeed a whole series of observations which apparently prove 
 that ' every small fragment of the members of a plant contains 
 the elements from which the whole complex body can be built 
 up, when this fragment is isolated under suitable external condi- 
 tions.' Phenomena of this kind are exhibited by cuttings and 
 adventitious buds which arise on a twig the apex of which has 
 been cut off. In the higher plants, the development of roots on 
 a cutting, or the formation of adventitious buds, does not take 
 place in all parts of the plant, but only in those which contain 
 ' a number of cambium cells.' These cells alone therefore con- 
 tain accessory idioplasm, which, according to the nature of the 
 stimulus acting on them, renders them capable of growing in a 
 manner which is very different from the normal. There can be 
 very little doubt that the whole of the cambium layer of these 
 plants is endowed with the capacity for reproduction. The only 
 question is, whether this is a result of special adaptation, or 
 only the outcome of the normal constitution of each plant-cell. 
 
 I should still, however, be inclined to consider it as a special 
 adaptation, and will endeavour to state my reasons for this 
 view ; not being a specialist in botany, however, I am unable to 
 deal with the various groups of the vegetable kingdom in such 
 detail as I could wish. 
 
 The question to be decided is, whether each cell was pro- 
 vided with all the specific primary constituents in a latent con- 
 dition at the time when the multicellular plant arose from the 
 unicellular form ; or whether, owing to the diversity of the 
 differentiation of the idioplasm, a sharp distinction first arose 
 between the somatic cells and the germ-cells, and the idioplasm
 
 THE FORMATION OF GERM-CELLS. 213 
 
 of the somatic cells was only subsequently provided with germ- 
 plasm in a latent condition in those cases in which this arrange- 
 ment was a useful one. I hold the latter view, and de Vries the 
 former one. It is important for the theory of the germ- plasm 
 to decide between these two opinions ; for it would be incom- 
 patible with this theory for germ-substance to be present as the 
 idioplasm of the somatic cells at the phyletic origin of the 
 soma. According to my conception of the germ-plasm, the 
 phyletic origin of the somatic cells depends on the determinants 
 contained in the germ-plasm being separated into groups. It 
 would entirely contradict this assumption if those somatic cells 
 which were phyletically the first to be formed, had contained all 
 the other characters of the species in a latent condition in addi- 
 tion to their manifest specific characters. De Vries thinks that 
 the marked distinction which actually exists between the somatic 
 and germ-cells of the higher animals has led me to assume 
 the universal existence of this contrast, which is not nearly 
 of such a decided nature in the case of plants in which gradual 
 transitions from somatic-cells to germ-cells can be proved to 
 exist. 
 
 This, however, I believe is not the case : transitions between 
 somatic and germ-cells never occur, and de Vries's opinion 
 simply rests on the fact that he confuses germ-cells with the 
 cells of the germ-track. That the latter must be regarded as 
 somatic cells has already been shown. 
 
 In my opinion, germ-cells were sharply distinguished from 
 somatic cells on their first appearance in phylogeny, and this 
 distinction has since persisted. In no species, whether animal 
 or vegetable, can there be any doubt as to the cells which are to 
 be looked upon as germ-cells ; and as regards the somatic cells, 
 such a doubt can only arise when cells in the germ-track are 
 regarded as germ-cells. 
 
 I know of no more convincing proof of my view than that 
 which is furnished by the Volvocinoe, These organisms consist 
 of communities of cells which may or may not exhibit a division 
 of labour, and in which a contrast between the somatic and 
 germ-cells may or may not exist. In Pandorina all the cells of 
 the colony are similar to one another, and each performs all the 
 vital functions. In Vol-vox the cells are differentiated : some 
 of them have the function of maintaining the individual, and 
 others that of preserving the species : that is to say, they are
 
 214 
 
 THE GERM-PLASM. 
 
 differentiated into somatic cells and germ-cells. The hetero- 
 plastid genus Volvox must have arisen phyletically from a 
 homoplastid form ; but we can hardly imagine that there can be 
 
 FIG. 17. \.Pandorina. morum. A colony of swarming cells. II. A colony 
 which has given rise to daughter-colonies : all the cells are similar to one 
 another. (After Pringsheim.) III. A young individual of Volvox minor, 
 still enclosed within the parent (after Stein) : the cells are differentiated into 
 somatic- and germ-cells.
 
 THE FORMATION OF GERM-CELLS. 215 
 
 many intermediate stages between these two, for at the present 
 day the two kinds of cells in Volvox hardly differ as much as do 
 the somatic- and germ-cells in the case of the higher organisms. 
 The somatic cells have nevertheless entirely lost the capacity of 
 reproducing the entire organism. 
 
 Transitions between these two kinds of cells could naturally 
 only arise by the germ-cells first becoming only slightly differ- 
 entiated from the somatic cells, and could not have been pro- 
 duced, as de Vries thinks, owing to all the cells containing 
 germ-substance in a more or less latent condition from the first. 
 There is no germ-substance in the somatic cells of Volvox 
 which, figuratively speaking, have only just become differentiated 1 
 from the germ-cells. If the latter are artificially removed from 
 a colony, the somatic cells continue to exist for a long time, but 
 they do not give rise either to new germ-cells or to a new colony. 
 
 And why should they do so ? Of what advantage would this 
 be to the species, since millions of individuals, each of which 
 again produces daughter-individuals, exist in the same pond ? 
 The ordinary process of multiplication is so vigorous that special 
 means for ensuring the existence of the species are unnecessary. 
 
 Such means have, however, been adopted in very many, if no I 
 by far the greater number, of the more highly organised plants. 
 
 The power possessed by fungi and mosses of reproducing a 
 new individual from any bit of the plant under favourable condi- 
 tions, has been supposed to contradiet my view. But I do not 
 see what prevents us from regarding this power as an adaptation 
 for ensuring the existence of a species surrounded by dangers 
 of all kinds. When the top of a toadstool is knocked off, a new 
 one is formed (Brefeld) ; and this arrangement is obviously of 
 great use in the preservation of the species. An entire liverwort 
 can be regrown from the smallest fragments of the plant (Voch- 
 ting). Why therefore should the assumption be improbable that 
 this power has been acquired in order to insure the persistence 
 of a species the existence of which is threatened by every sudden 
 drought ? My knowledge of plant life is not sufficient for me to 
 be able to support this statement in detail, but other facts will, I 
 think, to some extent confirm my opinion from the opposite point 
 of view. 
 
 Why is this power of regeneration not possessed by adult 
 ferns and horse-tails ? If a frond of a fern is cut off, it is not 
 reproduced from the stalk, and even the individual pinnae cannot
 
 2l6 THE GERM-PLASM. 
 
 be formed anew. In answer to this it might be urged that the 
 somatic cells of ferns have become too highly differentiated : but 
 this is contradicted by the fact that many, although by no 
 means all, ferns can produce bulbils on their fronds. I must 
 leave botanists to decide why this occurs ; but were I asked 
 whether the power of producing entire plants from somatic cells 
 would not have been of advantage to the other ferns, and would 
 therefore be expected to be possessed by them, I should be in- 
 clined to reply that all ferns are able to replace lost fronds by 
 forming new ones, not by the regeneration of the injured leaf, 
 but by budding from the stem. This suffices to restore the plant 
 when it has been injured. 
 
 We must now consider the Phanerogams in this connection. 
 In these plants, again, ' accessory germ-tracks ' are usually absent 
 in the leaves : that is to say, the cells of the leaves are not 
 capable of producing buds or even of restoring a piece which has 
 been cut out. The axes of the shoots, on the other hand, possess 
 this power in a high degree, and it depends on the presence of 
 cambium cells, all of which are apparently capable of giving rise 
 to new growing points, which produce new shoots with leaves 
 and blossoms, and consequently also germ-cells. The power of 
 regenerating the leaves is, as a rule, useless; for the formation 
 of new persons of the plant stock can take place to an unlimited 
 extent by means of the cambial layer; and this mode of compen- 
 sation for losses sustained is more effectual than the restoration 
 of defects in the leaves would be. The power of growing adven - 
 titious buds is probably unnecessary in the case of most leaves, 
 on account of the enormous number and certain dispersion of 
 the seeds produced by the plant. Amongst animals the same 
 is true as regards polype-stocks. Cells are distributed through- 
 out the stock which have the appearance and functions of 
 ordinary somatic cells, but which can give rise to new persons 
 under certain circumstances, such as, for instance, when the 
 stock has become injured. In a living stock of Ttibidaria 
 mesembryanthemum, which I once brought from Marseilles to 
 Freiburg, the crowns of the polype died one after the other 
 within a week, probably on account of want of nourishment ; 
 but within a few days afterwards all the stalks had given rise to 
 new crowns ; and though these were very small, they would un- 
 doubtedly have grown to the full size had it been possible to sup- 
 ply them with food. The capacity for regeneration is apparently
 
 THE FORMATION OF GERM-CELLS. 217 
 
 provided for in this case ; it is at any rate stated by Loeb* in a 
 recent paper that a shedding and new formation of the crowns 
 occurs periodically. The same writer also showed by experi- 
 ment that, under favourable circumstances, crowns may bud out 
 at any point of the stem, either at the distal end or at the base. 
 On the other hand, he never succeeded in getting the root-like 
 organs of attachment to be produced at the apical end. 
 
 No one will be surprised that such a growth did not occur who 
 agrees with me in looking upon all these processes of regenera- 
 tion and budding as resulting from adaptation. Under natural 
 conditions the apex of a stem could hardly be situated in such a 
 position as to render the formation of roots necessary, for it 
 never comes to lie upside down in the earth, and consequently 
 none of the cells in the apex contain ' rhizogenic idioplasm ' 
 ('Wurzel-Idioplasma'). On the other hand, however, it is easy 
 to understand why the power of budding exists in such a marked 
 degree in polypes, if one considers how liable the soft body is to 
 be attacked by crabs, worms, gasteropods, Pycnogonids, and 
 other small enemies. If these polype-stocks did not possess the 
 power of continually producing new crowns, i.e. new individuals, 
 when the old ones have been bitten off, the whole colony 
 would soon perish owing to the absence of ' nutritive persons.' 
 The fact that regeneration is possible to such an enormous ex- 
 tent results, at any rate in Part, from the aggregation of persons 
 to form the higher stage of individuality of the stock. For such 
 a combination of individuals procures the advantage of perman- 
 ent nutrition as long as all the individuals of the stock have not 
 fallen victims to their enemies, and thus it is favourable to the 
 production of new buds. 
 
 Amongst tfie Polyzoa the case is very similar. In many of 
 these animals the normal form of gemmation takes place with 
 great regularity, and the region at which the next bud will arise 
 can be predicted beforehand : on this fact depends, as in the 
 case of the Hydroid polypes, the characteristic form of the 
 stock in the different species, which is sometimes branched like 
 a foliage tree, and sometimes like a fir-tree or a feather. In these 
 animals, therefore, definite cells must be provided with ' blasto- 
 genic' idioplasm in advance, and merely the stimulus due to ordi- 
 
 * Jacques Loeb, ' Untersuchungen zur physiolog schen Morphologic dcr 
 Thiere,' I. ' Uber Hetermorphose,' Wiirzburg, 1891.
 
 2l8 THE GERM-PLASM. 
 
 nary nutrition is required in order to incite them to form buds. 
 The series of cells which lead directly to the cells of these buds 
 must be looked upon as the main germ-tracks, using the term 
 in de Vries's sense. According to Seeliger's researches, how- 
 ever, budding takes place from other regions as well as the 
 ordinary ones in certain Polyzoa, e.g., Pedicellina. If the crown 
 is lost in this animal, so that only a stump of the stalk remains, 
 new crowns are produced on the end of the stalk ; and in this 
 case, therefore, budding originates in the flat epithelial cells 
 characteristic of the ectoderm, which did not previously appear 
 to be capable of proliferating at all. This is another instance of 
 the presence of accessory germ-tracks. The cells of the epi- 
 dermis are provided with blastogenic germ-plasm, although they 
 do not as a rule take part in the formation of buds, but only 
 give rise to them in response to unusual stimuli. These cells are 
 just as much exposed to destruction as are those of the Hydroid 
 polypes, and we need therefore not be suprised that arrangements 
 for budding should have been made in the stalk, even were we 
 not aware of the fact that in Pedicellina, under normal condi- 
 tions, the crowns drop off periodically from the stalk, and are 
 replaced by others which bud out afresh. This process cer- 
 tainly occurs at the upper end of the stalk, but it is quite com- 
 prehensible that it would be advantageous for the lower end of 
 the stalk also to be provided with blastogenic idioplasm. 
 
 The arrangement which exists universally in the higher plants 
 for the production of adventitious buds is, in my opinion, to be 
 explained in a similar manner. In this group of organisms the 
 cambium layer in particular is provided with the means of re- 
 placing lost leaves and entire shoots. This obviously affords an 
 important protection against numerous enemies such as insects 
 more especially the number of which is often incalculable. It 
 is therefore not surprising that such an arrangement viz., the 
 addition of unalterable germ-plasm to the cambium cells is 
 here met with. 
 
 5. GALLS. 
 
 De Vries has also brought forward the question of the formation 
 of galls as furnishing an additional argument against my views. 
 In his opinion the production of galls proves that a vegetable 
 cell, even when it exhibits a specific histological differentiation, 
 contains the primary constituents of every other kind of cell in
 
 THE FORMATION OF GERM-CELLS. 2 19 
 
 a latent condition, and these are ready to become active as soon 
 as a suitable stimulus is brought to bear upon them. This 
 proof he considers to be incompatible with the assumption of 
 the existence and continuity of a germ-plasm. 
 
 The development of galls is undoubtedly a highly interesting 
 problem, which, in my opinion, has not yet been fully explained, 
 in spite of the numerous and excellent researches on the subject 
 which have been made within the last ten years. Amongst 
 these, the contributions of Adler * and Beyerinck t in particular 
 have materially helped to throw light on the problem. 
 
 The most important point in the consideration of this question 
 is the fact that galls are not by any means exclusively composed 
 of those kinds of cells which occur in the organs of the plants 
 upon which they arise, but may also contain cells of other kinds. 
 ' Cells which are usually only developed in the bark of a plant 
 may also frequently be found in the galls produced by those 
 Cynipidce and Diptera which infest leaves.' It is therefore cer- 
 tain that the power of producing forms of cells which do not 
 usually occur in the leaf, for instance, ' is not confined to those 
 organs in which they are developed normally,' but is present also 
 in certain cells of the leaf, and even indeed, de Vries thinks, ' in 
 all other parts of the plant.' 
 
 This is not surprising if we look upon the formation of the gall 
 as due to an adaptation of the plant to its parasites, such as we 
 may assume to have occurred with regard to the peculiar 
 arrangements exhibited by certain tropical plants for the pro- 
 tection of ants, which in their turn again protect the plant. 
 Reciprocal adaptation has taken place in this case ; the animal 
 has become adapted to the plant, and the plant to the animal, 
 because a joint existence is advantageous to both of them. In 
 the case of the galls of the Cynipidce and Tenthredinidce, the 
 advantage which might result to the plant from the presence of 
 the parasite is not apparent, and we may therefore be inclined 
 
 * Adler, ' Beitrage zur Naturgeschichte der Cynipiden,' Deutsche 
 entomolog. Zeitschr. xxi., 1877. p. 209; and ' Uber den Generations- 
 wechsel der Eichengallenwespen, 1 Zeitschr. f. wiss. Zool., Bd. xxxv., 1880, 
 
 P- 151. 
 
 t M. N. Beyerinck, ' Beobachtungen liber die ersten Entwicklungs- 
 phasen einiger Cyidipidengallen," Akademie d. Wiss. zu Amsterdam, 1882 ; 
 'Die Galle von Cecidomyia poae,' Bot. Zeitung, 1885; ' Uber das Cecidiuro 
 von Nematus capreas,' Bot. Zeitung, 1888, No. i.
 
 220 THE GERM-PLASM. 
 
 to explain their formation as due to a reaction of the plant in 
 response to the stimulus exerted by the animal. If, as was 
 formerly supposed, the gall resulted from the action of a 
 poison which is inserted into the tissues of the plant by the 
 female during oviposition, this explanation would be totally 
 inadequate ; for it is not conceivable that the infusion of a 
 poison, which happens only once, could with such regularity 
 produce a gall which grows slowly, and only gradually attains its 
 definitive and often complex structure. Moreover, several kinds 
 of galls, differing very much from one another, may be produced 
 from the same substratum, such as an oak-leaf, for instance. We 
 know, however, from the researches of Adler and Beyerinck, 
 that the formation of the gall is not due to the sting of the 
 parent animal, but to the activity of the larva after it has been 
 hatched. We must therefore suppose that this peculiar specific 
 proliferation of the tissues of the plant is due, in the first in- 
 stance, to the stimulus produced by the bodies of the larvae 
 when they begin to move about and to feed, the specific secre- 
 tion of their salivary glands then also having an effect. The 
 diversity of the galls arising from the same substratum must 
 therefore be due to differences in these factors ; and the con- 
 spicuous adaptations of the galls, which serve to protect, sup- 
 port, and nourish the parasite, must depend on adaptations of 
 the latter as regards its mode of feeding and movement, and 
 the chemical composition of its salivary secretion. We cannot 
 help accepting this interpretation of the facts since no other is 
 forthcoming ; and we must therefore suppose that natural selec- 
 tion has operated so long on these factors, and has gradually 
 effected such an improvement, that the kind of gall which pro- 
 vided the best protection and nourishment for the species was 
 ultimately produced by the larva. 
 
 Beyerinck has, in fact, proved that cells and tissues often 
 occur in galls which very closely resemble those in different 
 parts of the plant, but which do not exist in the substratum (e.g. 
 a leaf) on which the gall is produced. De Vries infers from 
 this fact that the primary constituents of such tissues must have 
 been contained in the cells of the leaf, although they could not 
 previously be recognised as such. This inference does not seem 
 to me to follow of necessity, for the stimulus produced by the 
 parasite might conceivably have modified the idioplasm of the 
 cells of the leaf so as to result in the formation of cells
 
 THE FORMATION OF GERM-CELLS. 221 
 
 differing from those ordinarily present in the leaf. It will be 
 shown in the chapter on Variation that changes of this kind 
 do occur, and that somatic idioplasm may at times, owing to 
 known or unknown causes, become so modified as to produce a 
 deviation from the inherited form of the cells of the series. The 
 sudden appearance of such peculiarities as those exhibited by 
 the moss-rose may be taken as an instance. It is very possible, 
 therefore, that owing to the specific stimulus produced by the 
 larva, and more especially by its secretion, the idioplasm of 
 certain layers of cells in the gall becomes modified and causes 
 the cells to assume another character, such as that of woody 
 fibres. 
 
 This view receives decided support from the circumstance 
 that the gall is by no means only composed of those kinds of cells 
 which occur in other parts of the plant. A similar statement to 
 this is, indeed, made by de Vries, who, however, makes an 
 exception in the case of ' the peculiar layer of sclerenchyma 
 in some Cynipid galls, which afterwards becomes modified into 
 thin-walled, nutritive tissue.' I cannot look upon this as being 
 ' only an apparent exception ' to the rule, for it seems to me to 
 be a very valuable proof that no such rule exists, and that the 
 above instance is to be explained as an apparent reversion to 
 inherited forms of cells, such as were already contained in a latent 
 condition in the cells of the leaf. I should rather be inclined to 
 regard these ' exceptions ' as a proof that definite new forma- 
 tions occur in galls, and thai these are due to modifications of the 
 cells from which they arise in response to the stimulus produced 
 by the larva. 1 1 can hardly be a matter of surprise that a marked 
 resemblance exists between these cells and those occurring in 
 other parts of the plant, for the changes produced by the larva 
 take place in an idioplasm consisting of determinants of the 
 species in question ; these changes would not therefore at first 
 result in combinations of biophors (determinants) very different 
 from those which ordinarily occur in the plant. The new com- 
 bination of the biophors in different ways results, I believe, from 
 the action of the larva, and thus modifications of the detenninants 
 are produced. 
 
 The galls of Cecidomyia pace, which de Vries mentions when 
 contesting my views, are probably to be accounted for in the same 
 manner. In response to the stimulus produced by the larva, 
 these stalk-galls become covered with a thick felt of rootlike
 
 222 THE GERM-PLASM. 
 
 outgrowths, which doubtless serve as a protection : these, when 
 they gain access to the soil, become branched like ordinary 
 roots. The assumption that under certain circumstances the 
 idioplasm of certain somatic cells becomes modified in response 
 to the stimulus produced by the parasite, so as to give rise to a 
 structure similar to that of another tissue or even organ of the 
 same plant, seems to me by no means to prove that the primary 
 constituent of this tissue must previously have been contained 
 in these cells. In animal tissues transformations of this kind 
 are certainly not known to occur. Pathological anatomists are 
 now of the opinion that only those kinds of cells occur in 
 tumours which actually belong to the sort of tissue from which 
 the tumour arises. This is not surprising, for tissues of animals 
 are far more highly differentiated than those of plants, and 
 corresponding elements in the idioplasm must also differ in a 
 corresponding degree; and consequently, in spite of the displace- 
 ments and re-arrangements which may be produced by stimuli, 
 they can never form precisely the same combinations as those 
 which occur in the various other tissues of the body. 
 
 I shall not discuss the case of the gall of Nematus, as Beyer- 
 inck's observations with regard to it are not yet complete. If it 
 should be shown that a complete willow can be produced from 
 the leaf-gall of the plant, as de Vries considers probable, it will 
 then certainly have been proved that the cells in the leaf contain 
 germ-plasm, just as in the case of the leaves of Begonia. At 
 present, however, it is only known that the gall can give rise to 
 roots, and although normal roots are always capable of forming 
 adventitious buds, it cannot be said at present whether these 
 abnormal roots are able to do so. In the willow, in any case, 
 the primary constituents of roots are distributed throughout the 
 stem in the form of invisible determinants, contained within 
 visible cells, and this accounts for the fact that the production 
 of new individuals by means of cuttings takes place exceptionally 
 easily in this plant. This may perhaps be accounted for by a 
 wider distribution than usual of the merely ' unalterable ' group 
 of determinants for roots taking place in the plant, in connec- 
 tion with the wide distribution of the corresponding primary 
 constituents. But I do not by any means imagine that in all 
 these cases in which the cells of a plant possess inactive germ- 
 plasm, its presence is actually useful at the present day. If the 
 distribution of unalterable germ-plasm once took place in an
 
 THE FORMATION OF GERM-CELLS. 223 
 
 organism to such a considerable extent as has occurred in most 
 plants, it would be a matter of slight importance in the economy 
 of the plant whether the cells of those organs which at the 
 present day are no longer in a condition to make use of this 
 substance were provided with a minimum of germ-plasm or 
 not. Such a provision might have been of advantage to the 
 ancestors of the species ; and if this were not the case, we know 
 so little of the processes by means of which the various qualities 
 of the idioplasm become separated mechanically in nuclear 
 division, that we cannot altogether reject the assumption of an 
 occasional chance admixture of germ-plasm to somatic idio- 
 plasm, especially in the case of the higher cormophytes, which 
 must in any case possess a number of cells containing germ- 
 plasm throughout the entire plant. Time will show whether we 
 require this assumption. 
 
 The difference between my view and that of de Vries does 
 not consist in the fact that I am compelled to deny the admix- 
 ture of germ-plasm in the case of a large number of cells in the 
 body on principle, but in my assumption that each somatic cell 
 contains a definite somatic idioplasm consisting of a limited 
 number of definite determinants, to which any other ' unalter- 
 able ' accessory idioplasm may be added if required. De Vries, 
 on the other hand, considers that the whole of the primary con- 
 stituents of the species are contained in the idioplasm of every, 
 or nearly every, cell of the organism. But he does not explain 
 how it is that each cell nevertheless possesses a specific histolo- 
 gical character. A new assumption, which would not be easy to 
 formulate, would therefore be required to explain why only a 
 certain very small portion of the total amount of idioplasm 
 which is similar in all parts of the plant becomes active in 
 each cell. This theory explains the differentiation of the body 
 as being due to the disintegration of the determinants accumu- 
 lated in the germ-plasm, and requires a special assumption, 
 viz., that of the addition of accessory idioplasm when necessary, 
 in order to account for the formation of germ-cells, and the 
 processes of gemmation and regeneration. The reconstruction of 
 entire plants or of parts from any point can be easily accounted 
 for by de Vries's hypothesis, just as it can by Darwin's theory of 
 pangenesis, for the pangenes or gemmules are present wher- 
 ever they are wanted. But de Vries is unable, on the basis of 
 his hypothesis, to offer even an attempt at an explanation of the
 
 224 THE GERM-PLASM. 
 
 diversity of the cells in kind and of the differentiation of the 
 body. 
 
 These two assumptions appear to me to be of equal value in 
 explaining the fact that in many of the lower plants each cell, 
 under certain circumstances, can apparently reproduce an entire 
 individual. The differences between the somatic cells are here 
 only slight ones, and are so few in number, that we might be 
 inclined to consider them as due to reactions of the same idio- 
 plasm to different kinds of influences exerted by the environ- 
 ment. Such is the case, for instance, in liverworts. But this 
 assumption ceases to be tenable as soon as the soma can be- 
 come variously differentiated, and any explanation must in the 
 first place account for this differentiation : that is to say, the 
 diversity which always exists amongst these cells and groups of 
 cells arising from the ovum must be referred to some definite 
 principle. De Vries's principle is of no use at all in this case, 
 for it only accounts for the fact that entire plants may, under 
 certain circumstances, arise from individual cells, and does not 
 even touch the main point. In fact, no one could even look 
 upon it as giving a partial solution of the problem, if differentia- 
 tion is supposed to be due to that part alone of the germ-plasm 
 always becoming active, which is required for the production of 
 the cell or organ under consideration. But the higher we ascend 
 in the organic world, the more limited does the power of pro- 
 ducing the whole from separate cells become, and the more do 
 the numerous and varied differentiations of the soma claim our 
 attention and require an explanation in the first instance. 
 
 The presence of idioplasm in all parts containing all the 
 primary constituents does not help us in this respect ; and even 
 in attempting to explain the formation of germ-cells, it is of 
 very little use to assume that they arise from cells which, like 
 the rest, contain all the primary constituents of the species. 
 How is it that these cells, and these alone, in the entire soma of 
 the animal, give rise to germ-cells? In the lower plants the 
 fact of the differentiation of the soma is liable to be overlooked 
 or underrated, but this cannot possibly be the case as regards 
 the higher animals.
 
 SUMMARY OF PART II. 225 
 
 CHAPTER VII. 
 
 SUMMARY OF PART II. 
 
 WE have now seen that the idioplasm of the fully-formed indivi- 
 dual animal- or plant-cell may exhibit a considerable amount of 
 difference as regards its decree of complexity ; and before going 
 further, it may be as well to state clearly in what this difference 
 consists. 
 
 We suppose that the process in the idioplasm which brings 
 about the ontogeny of a multicellular organism is due to the 
 thousands of determinants, which constitute the germ-plasm of 
 the fertilised ovum, becoming systematically separated into 
 groups, and distributed among the successors of the egg-cell. 
 This separation into smaller and smaller groups of deter- 
 minants continues to take place, until each cell contains deter- 
 minants of one sort only, and these then either control a single 
 cell, or, in case the hereditary character ('determinate') is 
 constituted by a group of cells with a common origin, the 
 control is exerted over this whole group. 
 
 All the determinants are not active at the same time ; every 
 cell, in fact, which appears in the entire course of ontogeny is 
 controlled by one determinant. This is effected by the disin- 
 tegration of the determinant into its constituent biophors, which 
 migrate into the cell-body. Even in the earlier stages of onto- 
 geny, in which the idioplasm of a cell consists of a still larger 
 number of different kinds of determinants, the cell is also con- 
 trolled by only one of them in this manner. The rest of the 
 determinants have in any case an important function with regard 
 to the course taken by ontogeny, for each of them has its own 
 rate of increase, and thus an alteration is produced in the pro- 
 portion of the various determinants originally present in the 
 germ-plasm, and consequently its definite architecture also 
 undergoes alteration, the subsequent disintegration being con- 
 trolled by the determinants which have thus been rearranged. 
 These determinants arc therefore only inactive with respect to 
 P
 
 226 THE GERM-PLASM. 
 
 the cell in which they are situated, and not as regards develop- 
 ment as a whole. 
 
 Various circumstances may, however, produce complications 
 in this simple course of development of the idioplasm. 
 
 In the first place, it is necessary that the organism should be 
 able to replace losses of substance. In order that this may be 
 possible, the final cells of ontogeny, at any rate, i.e. those of 
 the various tissues, must be rendered capable of producing 
 others similar to themselves. The possession of this power 
 necessitates that each cell shall be capable of unlimited multi- 
 plication, so that the single determinant which controls it can 
 likewise grow and multiply. This would result in the cells 
 being able to produce others of a like character by cell- 
 division. 
 
 We find, however, that regeneration is not limited to this very 
 simple form of restoration : more complex tissues can also be 
 reproduced; and even entire organs, such as limbs, and still 
 larger parts of the body, such as the head and tail, may, in 
 certain groups of animals, undergo restoration when they have 
 been lost. These facts are to be explained in terms of the idio- 
 plasm by supposing that in these cases also the determinants for 
 the groups of cells which are to be capable of regeneration have 
 undergone an increase, and have been supplied to certain cells 
 in the course of ontogeny in the form of inactive accessory idio- 
 plasm. The equipment of such cells with determinants for re- 
 generation is due to adaptation, and is only connected with the 
 degree of organisation of the animal in so far as the difficulty of 
 providing a large number of cells with accurately graduated deter- 
 minants for regeneration increased as the number of cells from 
 which regeneration had to proceed became larger, and as the num- 
 ber and degree of differentiation of the organs to be restored also 
 increased. An increase, both as regards cells and organs, takes 
 place in correspondence with the complexity of structure, and 
 the ' regenerative power,' therefore, as a rule, gradually under- 
 goes a proportionate decrease. 
 
 Cells which possess the regenerative power are therefore 
 those which contain, in addition to their own active determi- 
 nants, a larger or smaller group of inactive determinants : these 
 latter belong to those cells and cell-series which are capable of 
 taking part in the reconstruction of that part of the body which 
 has been lost, and which is situated distally to them when the
 
 SUMMARY OF PART II. 227 
 
 determinants become active. The occurrence of regeneration in 
 a high degree is only rendered possible by the cells in a definite 
 transverse plane of the body being regularly equipped with 
 various groups of suitable supplementary determinants which are 
 capable of acting together as a whole. 
 
 This form of regeneration leads directly to the process of 
 reproduction by fission^ which simply consists in the employment 
 of a marked power of regeneration for the purpose of increasing 
 the number of individuals. 
 
 While the possession of the regenerative power in a low, as 
 well as in a high degree, is due to the equipment of cells with 
 certain larger or smaller groups of determinants in the form of 
 'unalterable' accessory idioplasm, the development of new per- 
 sons by gemmation depends either on the fact that a single cell 
 contains all the determinants of the species in an inactive and 
 unalterable condition, in the form of accessory idioplasm ; or 
 else is due to two or three cells in the different layers of the 
 body containing large groups of determinants as accessory 
 idioplasm, which together constitute all the determinants pos- 
 sessed by the species i.e. germ-plasm. 
 
 In cases where budding originates in a single cell, as in the 
 Hydroid-polypes, the blastogenic idioplasm concerned in the 
 process must be regarded as a modification of the germ-plasm, 
 which consists of all the determinants of the species, though 
 these have a different arrangement to that which obtains in the 
 germ-plasm proper of the fertilised egg-cell. The fact that a 
 bud may originate in two or three cells does not show that these 
 cells contain exactly those groups of determinants which corre- 
 spond to those of the two or three germinal layers of the 
 Metazoon in question. In fact, the combination of the deter- 
 minants differs more or less in all known cases, and is adapted 
 to the circumstances under which budding occurs. This proves 
 that in the embryogeny of the species, divisions of the accessory 
 idioplasm occur quite independently of the ordinary divisions in 
 the mass of determinants, and these result in certain cells being 
 provided with a definitely constituted accessory idioplasm. 
 
 The blastogenic germ-plasm must be contained in the germ- 
 plasm of the sexual cells in the form of special ids, for buds can 
 vary independently of the persons which produce them. On the 
 other hand, primary idioplasm must also be supplied to the bud 
 during its development, and this may be effected by means
 
 228 THE GERM-PLASM. 
 
 either of special cells which contain the idioplasm in an unal- 
 terable condition, or else of certain ids of the primary idioplasm 
 being added in an unalterable condition to the blastogenic 
 germ-plasm when it becomes separated from the primary idio- 
 plasm. 
 
 As, apart from plants, gemmation only occurs in comparatively 
 low forms of animals, viz., in the Ccelenterata, Polyzoa, and 
 Tunicata, we may infer that the addition of this blastogenic 
 idioplasm, consisting of accurately graduated groups of deter- 
 minants, eventually reaches a limit, owing to the increasing com- 
 plexity of the structure of the animal and to the surprising extent 
 to which the number of determinants increases. 
 
 According to our view, the cells of the Metazoa and Metaphyta 
 may not only be provided with the above-mentioned accessory 
 idioplasm, but may in addition contain primary germ-plasm, and 
 these cells are to be found along the * germ-tracks] that is to 
 say, they are situated in the direct line of development which 
 leads from the ovum to the germ-cells which are eventually 
 formed from it. As each somatic cell is only controlled by one of 
 the large number of determinants belonging to the germ-plasm, 
 and as determinants cannot be produced spontaneously, those 
 cells which are to give rise to germ-cells must contain unalterable 
 germ-plasm in addition to the active determinants which control 
 them ; and the former can only be derived from the cell to 
 which the whole organism owes its origin, for this alone contains 
 the whole of the determinants organically united to form germ- 
 plasm. A series of cells containing germ-plasm in the form of 
 unalterable accessory idioplasm, must therefore be traceable 
 from the egg-cell to that region of the body which sooner or 
 later gives rise to germ-cells ; that is, there must be a continuity 
 of the germ-plasm. 
 
 The number of germ-tracks in the lower plants and animals 
 is a very large one ; under normal circumstances, germ-cells are 
 not only found in very many parts, especially in the case of 
 animal and plant-stocks, but new persons may in exceptional 
 cases be formed in many regions by budding, especially when 
 injuries to the stock have occurred ; and these persons can again 
 produce germ-cells. In- the case of the Hydroid-polypes and 
 Polyzoa, a large number of cells of the stock must be provided 
 with germ-plasm, although it is impossible to say whether these 
 are the same as those which contain blastogenic idioplasm, or
 
 SUMMARY OF PART II. 2 29 
 
 whether the latter is situated in other adjacent cells which also 
 take part in the formation of the bud. In any case blastogenic 
 idioplasm and the germ-plasm of the ovum are not identical, 
 even if, as in plants and Hydroid-polypes, the former contains 
 the whole of the determinants of the species. The deter- 
 minants must at any rate have another arrangement : not 
 infrequently, indeed, the blastogenic idioplasm must consist of 
 entirely different kinds of determinants, and in the case of the 
 alternation of generations of the Medusae of far more numerous 
 ones. 
 
 The cells of the germ-tracks are somatic cells ; that is to say, 
 each of them is controlled by a special determinant, and con- 
 tains germ-plasm in an inactive as well as in an unalterable 
 condition. In the latter state, it only again becomes capable of 
 disintegration when the cells in which it is situated give rise to 
 germ-cells, and begin to undergo development into embryos. 
 This germ-plasm, like the unalterable blastogenic idioplasm, 
 may be contained in young cells with only a slight amount of 
 histological differentiation, as well as in cells with a sharply 
 defined histological character. 
 
 We thus see that in many cases the cells of the adult organism 
 contain an accessory idioplasm in addition to the determinants 
 which control their special character, structure, and physio- 
 logical activity for the moment : the former may become active 
 in the ordinary course of development, as occurs in the 
 normal formation of germ-cells and in multiplication by fission 
 and gemmation ; or its activity may be due to abnormal causes 
 only, such as those resulting from injuries and mutilations, 
 from which the processes of regeneration or gemmation in the 
 first place originated.
 
 PART III. 
 
 PHENOMENA OF HEREDITY RESULTING 
 FROM SEXUAL REPRODUCTION. 
 
 INTRODUCTORY REMARKS ON THE NATURE OF SEXUAL 
 REPRODUCTION. 
 
 THE phenomena of heredity have so far been considered in 
 connection with a purely asexual form of reproduction only : the 
 complications of the germ-plasm arising from the intermingling 
 of the hereditary parts of two parents have been left aside, and 
 the composition of the germ-plasm has been assumed to be of 
 such a nature as would result if monogonic reproduction were 
 the only form in which the process existed. The advantage of 
 this method of procedure is seen in the fact that it has only been 
 necessary for us to bear the essential part of the processes in 
 mind when analysing the fundamental phenomena of heredity, 
 and this essential part has therefore not been lost sight of in the 
 confusing and ever changing intermixture of individual varia- 
 tions which result from amphigonic reproduction. The course 
 we have followed is justified by the fact that fundamental pro- 
 cesses such as ontogeny, regeneration, and multiplication by 
 fission and gemmation cannot owe their origin to amphigonic 
 reproduction, but would take place even if this form of multi- 
 plication did not exist at all. 
 
 Bearing this fact in mind when considering the complications 
 arising from sexual reproduction with which we have to deal 
 in analysing the phenomena of heredity and their material sub- 
 stratum, it will now be profitable to consider the facts concerning 
 this form of reproduction, and to see how they can be explained. 
 I will now therefore give a short account of the processes in
 
 AMPHIGONIC HEREDITY. 231 
 
 question as far as is necessary in order to render comprehensible 
 the complications in the phenomena of heredity resulting from 
 them. 
 
 Until far into the present century 'sexual reproduction' was 
 considered to be the essential and primary form of the process ; 
 and although it gradually became more and more evident that 
 several kinds of ' asexual reproduction ' may also occur, these 
 nevertheless only take place in the lower forms of animals and 
 plants. As the details of the phenomena of reproduction were 
 for a long time known almost exclusively with regard to the 
 higher animals, and as in them sexual reproduction alone 
 occurs, the special peculiarities of the latter were naturally 
 considered to be necessary and indispensable in the process. 
 Fertilisation was looked upon as an essential part of this process, 
 and it alone was supposed to render life from one generation 
 . to another possible at all ; in short, fertilisation was regarded 
 as a ' process of rejuvenescence,' and sexual reproduction was 
 considered to form the foundation from which all forms of 
 reproduction have arisen. The existence of different forms of 
 asexual reproduction was explained as an ' after-effect ' of the 
 process of fertilisation or rejuvenescence occurring in sexual 
 reproduction. 
 
 This view appeared to receive support from the fact that 
 sexual reproduction is of universal occurrence, from the lowest 
 to the highest forms of animals and plants ; and also that asexual 
 reproduction never takes place in the higher organic forms, and 
 even in the lower ones it only alternates with the sexual form of 
 the process. 
 
 The present state of our knowledge of the process of fertilisa- 
 tion, however, justifies us in considering these earlier views to be 
 totally erroneous. In no case does fertilisation correspond to a 
 rejuvenescence or renewal of life, nor is its occurrence neces- 
 sary in order that life may endure : it is merely an arrangement 
 which renders possible the intermingling of two different heredi- 
 tary tendencies. We shall deal later on with the question as to 
 why such a mingling has been introduced and so extensively 
 adopted by Nature ; at present it is only necessary to prove that 
 this is the case. Fertilisation consists in the union of two here- 
 ditary substances, i.e., of the germ-plasms of two individuals : 
 all the complicated and varied phenomena of differentiation 
 beginning with that of the two different kinds of reproductive
 
 232 THE GERM-PLASM. 
 
 cells, usually known as male and female, up to that of the in- 
 dividuals themselves into males and females, and including the 
 innumerable other resulting adaptations and phenomena take 
 place solely for the purpose of rendering possible the union of 
 the primary constituents of two individuals. 
 
 This process of the fusion of two germ-plasms, which con- 
 stitutes the essential part of fertilisation and is as a rule 
 connected with the fusion of two cell-bodies, I have desig- 
 nated as amphimixis. It is not always connected with repro- 
 duction, for these two processes take place independently of one 
 another in all unicellular organisms. In the Infusoria, for in- 
 stance, two individuals in the course of their life-history come 
 into contact with one another, and thers either fuse completely 
 into one, or else undergo a partial or temporary fusion ; in both 
 of which cases half the hereditary substance is transferred from 
 one individual into the other, and thus amphimixis is brought 
 about. The latter process is only invariably connected with 
 reproduction in the case of multicellular forms : this is necessi- 
 tated by the fact that the union of two different germ-plasms 
 cannot take place by the fusion of entire individuals, as the 
 germ-plasm is enclosed in separate cells, a male and a female, 
 the fusion of which takes place in a similar manner to that 
 which occurs in the process of conjugation in unicellular organ- 
 isms. This act of amphimixis must then be followed by the 
 multiplication of the fertilised egg-cell, accompanied by the 
 differentiation of its successors, or, in other words, by the onto- 
 geny of a new individital ; for did this not result, the process 
 of amphimixis would be useless. Amphimixis is therefore 
 always connected with reprodttction in all multicellular forms, 
 and these two processes together constitute ' sexual reproduc- 
 tion' ' or ' amphigony* (Haeckel). 
 
 The process of amphimixis, as it occurs in amphigonic repro- 
 duction, is briefly as follows. The two kinds of germ-cells 
 mutually attract one another, and then fuse together, the smaller 
 male element always entering the larger female one. The 
 nuclei of the two cells then approach each other, and so come 
 to be situated close together, each being accompanied by its 
 ' centrosome,' i.e., that remarkable body, enclosed in a clear 
 sphere, which, as already stated, constitutes the apparatus 
 for division. The germ-plasm in both nuclei is at first dis- 
 tributed in the form of fine threads, as is represented in the case
 
 AMPHIGONIC HEREDITY. 
 
 2 33 
 
 of the female nucleus in Fig. 18, I. ; it subsequently, however, 
 becomes contracted, so as to give rise to nuclear rods or idants 
 
 FIG. 18. Diagram of the fertilisation of the egg in Ascaris megaloccphala. 
 (Compounded from the figures and descriptions of Boveri and others.) 
 
 I. The sperm-cell (?p) is about to enter the ovum, which contains a nucleus (nov) 
 and centrosome (cr). Rk I and Rk If the two primary polar-bodies, the first 
 of which has divided into two ; each contains two idants. 
 
 II. The sperm-nucleus (JT/) has passed into the egg, near the nucleus of which 
 it is situated. Each of these nuclei contains two idants, and also a centro- 
 some, which has divided into two. 
 
 III. The two nuclei are now close together : the centrosomes, with their ' spheres 
 of attraction,' are connected together in pairs, and are situated at the poles of 
 the spindle, which is already visible. 
 
 IV. The nuclear membrane has disappeared, and the first embryonic nuclear 
 division is now taking place. 
 
 (Fig. 18, II.). Edouardvan Beneden was the first to prove that 
 the number of these idants is the same in both of the conjugal-
 
 234 THE GERM-PLASM. 
 
 ing cells, and this discovery which has since been confirmed in 
 the case of a large number of species of animals, and has been 
 proved quite recently by Guignard to apply to plants also is of 
 decided importance in connection with the conception that 
 idants constitute the hereditary substance. As the two nuclei 
 are approaching one another, their centrosomes become doubled, 
 and the corresponding pairs unite to form the two poles of a 
 nuclear spindle (Fig. 18, III.), which direct the first cell-division 
 leading to the formation of the embryo ; this usually only 
 occurs after the nuclear membrane has completely disappeared 
 (Fig. 1 8, IV.). 
 
 The process of fertilisation therefore consists in the union of 
 the nuclei of the two sexual cells within the maternal germ- 
 cell, and also of the bodies of the cells, together with their 
 apparatus for division. One half of the genii-plasm of the 
 ' combination-nucleus ' (' Copulationskern ') thus formed by the 
 union of the sexual nuclei consists of idants derived from the 
 mother, and the other half of those derived from the father, and 
 the resulting combination of two hereditary substances directs 
 the ontogeny and controls the building-up of the new individual. 
 The entire number of idants nevertheless always remains the 
 same in all the cells of the body : thus, for instance, if eight 
 paternal and eight maternal idants were brought together in the 
 process of amphimixis, there would be sixteen idants in every* 
 cell in the body of the individual arising from the fertilised ovum ; 
 and if, again, as represented in Fig. 18, there are only two idants 
 in each germ-cell, each somatic cell will contain four idants. 
 
 The nature of sexual reproduction depends therefore on the 
 intermingling of two hereditary tendencies which are individually 
 different from one another ; or, to pass from the abstract to the 
 concrete, it depends on the union of two hereditary substances in 
 the first rudiment of the individual. We must next investigate 
 the manner in which this combination of hereditary substances 
 affects the composition of the germ-plasm. 
 
 * A recent observation renders it doubtful whether ' every' cell contains 
 the same number of idants ; but this need not here be taken into con- 
 sideration, as its importance cannot at present be estimated.
 
 EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM. 235 
 
 CHAPTER VIII. 
 
 MODIFICATIONS OF THE GERM-PLASM CAUSED BY 
 AMPHIMIXIS. 
 
 i. THE NECESSITY OF A HALVING OF THE GERM-PLASM. 
 BY the process of amphimixis the hereditary substances of two 
 individuals become united into one substance in the offspring. 
 If the process is repeated in every generation, a doubling of 
 these individually different hereditary substances must take 
 place each time, and the mass of germ-plasm and the number of 
 idants must likewise be doubled. As a matter of fact this can- 
 not and does not occur, for in every species the number of idants 
 remains the same throughout all generations. The unlimited 
 increase of the germ-plasm must therefore be prevented in some 
 way or other. 
 
 The mass of germ-plasm might possibly remain constant if 
 its growth stopped in the young germ-cells when only half the 
 normal quantity had been formed. It is quite conceivable that 
 a continual increase in mass might in this way be prevented, if, 
 contrary to the theory of the germ-plasm here propounded, we 
 were to imagine that the idioplasm merely consists of ultimate 
 vital particles ' pangenes,' ' primary constituents,' or whatever 
 else we choose to call them which are not combined into units 
 of a higher order. 
 
 If, however, we assume the existence of a germ-plasm in the 
 sense in which I use the word, t'.e., an idioplasm in which the 
 ultimate bearers of vitality (biophors) are combined to form 
 units of a higher order, the determinants and ids, having a 
 definite structure and size, it is evident that the amount of germ- 
 plasm would not remain constant, or at most it would only 
 remain so for a few generations, as long, that is, as each kind of 
 germ-plasm is represented by several ids. As soon as this 
 stage was reached, a decrease in growth could no longer 
 prevent a doubling of the mass ; this could, in fact, only be 
 prevented by the removal of half of the number of ids present 
 in the cell.
 
 236 THE GERM-PLASM. 
 
 This actually occurs before the germ-cells unite in the process 
 of ' reducing division ' of the nuclear matter of the germ-cells. 
 This fact may probably be taken as indicating the correctness 
 at any rate of the fundamental idea on which the theory of the 
 germ-plasm is based, viz., that the hereditary substance is com- 
 posed of ids. These parts of this substance, the existence of which 
 I formerly concluded from purely theoretical considerations, and 
 which I have called ' ancestral germ-plasms,' must exist in reality. 
 I venture to make this assertion with all the more assurance, 
 owing to the fact that at the time when I postulated the 
 ' reducing division ' merely on theoretical grounds, the existence 
 of such a process could not be gleaned from recorded observa- 
 tions even in the case of the female germ-cells of animals, in 
 which it can be observed comparatively easily, quite apart from 
 that of the male cells of animals, or of the germ-cells of both 
 sexes in plants. 
 
 We now know that this reduction of the number of ids by one- 
 half is of general occurrence, and is effected b\ means of the 
 nuclear divisions which accompany cell-division. The divisions 
 which result in the formation of the polar bodies perform the 
 function of the ' reducing divisions ' as regards the ovum, and 
 the final divisions of the sperm mother-cells have this function 
 in the case of the spermatozoa. In both cases the reducing divi- 
 sion does not consist in the idants becoming split longitudinally, 
 and in their resulting halves being distributed equally amongst 
 the two daughter-nuclei as in ordinary nuclear division, but in 
 one-half of the entire number of rods passing into one daughter- 
 nucleus, and the other half into the other. The process is some- 
 what more complicated than would appear from this statement, 
 and it will be discussed more fully later on ; but the final result 
 is the same. 
 
 The following considerations may perhaps help to explain 
 why the constant doubling of the germ-plasm could only be 
 prevented by this method of removing entire nuclear rods, and 
 will at the same time indicate what are the primary causes of 
 the changes in the structure of the germ-plasm caused by 
 amphimixis. 
 
 As already remarked, the nuclear rods must, before the intro- 
 duction of the process of amphimixis into the organic world, 
 have consisted of a number of identical ids, each correspond- 
 ing exactly to the individuality of the organism in question.
 
 EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM. 237 
 
 These ids must have been united into idants, which were all 
 equal in value, their number, as well as that of the ids, 
 remaining the same in subsequent generations. When sexual 
 reproduction first arose, the same number of idants from both 
 parents became enclosed in one nucleus, the total number of 
 idants and mass of germ-plasm of which were thereby doubled. 
 This may have been of no disadvantage if it occurred once only, 
 but as the process was repeated, an arrangement for preventing 
 the germ-plasm from increasing to an unlimited extent became 
 necessary each time amphimixis took place. 
 
 Were the germ-plasm an unorganised, or even a perfectly 
 homogeneous substance with no internal differentiation, z>.,were 
 it not composed of units of different orders, its doubling every 
 time amphimixis occurred might have been prevented simply by 
 a limitation of its growth in each germ-cell, so that the latter 
 would contain only half the mass of germ-plasm formerly pre- 
 sent. But as soon as the germ-plasm came to consist of a 
 definite number of units, a diminution of the latter could not 
 result from a mere limitation as regards growth, for their num- 
 ber would nevertheless remain the same. This result could 
 only be attained by the appearance of a process by means of 
 which the number of units was reduced to half, and we have 
 seen that such a process occurs in the form of the remarkable 
 ' reducing divisions ' already described. 
 
 It is not difficult to ascertain what changes must result in 
 the composition of the germ-plasm by the combination of this 
 process with continued amphimixis. 
 
 Let us suppose that before the introduction of the latter process 
 the germ-plasm of a species consisted of sixteen idants. When 
 amphimixis, accompanied by the 'reducing division,' occurred 
 for the first time, eight paternal idants A would unite with eight 
 maternal idants B in the fertilised egg-cell to form the segmen- 
 tation nucleus. In consequence of the reducing division, each 
 of the germ-cells of the next generation would contain a com- 
 bination of the idants A and B, e.g., 4 A + 4 B. These 
 would again unite in the next amphimixis with eight idants 
 e.g., 4 C + 4 D in the germ-cell of another individual 
 with different hereditary tendencies ; and the ontogeny of 
 the third sexual generation would therefore be controlled by 
 a germ-plasm composed of the idants 4A+4B + 4C + 4D. 
 Let us assume, for the sake of simplicity, that the reduction
 
 238 THE GERM-PLASM. 
 
 always affected every kind of idant to the same extent ; the 
 germ-plasm of the fourth generation would then consist of the 
 idants 2 A + 2 B + 2 C + 2 D + 2 E + 2 F + 2 G + 2 H, and that 
 of the fifth, of a number of individually different idants, provided, 
 of course, that interbreeding had not occurred. The germ-plasm 
 of this fifth generation would therefore consist of the idants A Q. 
 
 This naturally does not imply that the process would really 
 take place in such an even and systematic manner ; it must, on 
 the contrary, be a very irregular one. But although it may not in 
 five generations have resulted in the germ-plasm being composed 
 of a number of different ids, this result must certainly follow in 
 the course of a greater number of generations. 
 
 The modification of the germ-plasm will not, however, then 
 have reached its limit. If my view of the composition of idants 
 out of ids is a correct one, and the id is really a unit which con- 
 tains all the primary constituents of the species, that is to say, 
 if it contains all the determinants required for the construction 
 of a single individual, it follows that the composition of the 
 individual idants must gradually have become changed, so that 
 each idant ', instead of being made up of similar ids, comes to be 
 constituted by dissimilar and individually different ids. 
 
 The idants are not, in my opinion, perfectly invariable quan- 
 tities ; certain phenomena of heredity have led me to conclude 
 that they are in any case only relatively constant, and that their 
 composition becomes modified from time to time, so that the 
 ids which previously belonged to the idant A may later take 
 part in the composition of the idant B or C. Our present know- 
 ledge of the processes of the division of the nuclear substance 
 vloes not enable us to say how frequently and regularly this 
 occurs ; but even if it only takes place at irregular intervals, during 
 long periods of time, it must nevertheless have resulted in a very 
 varied composition of the idants in the course of the enormous 
 number of generations which have ensued since the introduction 
 of the process of amphimixis into the organic world. As new 
 idants are always added to those already present in one of the 
 parents each time amphimixis occurs, a continual interpolation 
 of new ids can take place in the idants ; and as this process is 
 repeated an indefinite number of times, a single idant must 
 ultimately if we neglect the repetition of similar ids which 
 results from interbreeding come to consist of a number of 
 individually different ids.
 
 KKKKCTS OK AMPHIMIXIS ON THK GKRM-PLASM. 239 
 
 The process of mingling the icb would proceed most rapidly 
 if the paternal and maternal ids regularly combined each time 
 amphimixis took place, so as to bring together the half of the 
 different kinds of ids in the idants of both parents. If, for in- 
 
 GENERATION IV. 
 
 FIG. 19. Diagram illustrating the composition of the idants out of individually 
 different ids. (From Weismann's 'Essays,' Vol. I., p. 369.) 
 
 stance, each idant in an individual consisted of sixteen ids which 
 were all similar to one another on the first appearance of sexual 
 reproduction, the first occurrence of amphimixis would result in 
 idants consisting of eight paternal and eight maternal ids, which 
 are represented respectively by the black and white parts in the
 
 240 THE GERM-PLASM. 
 
 accompanying diagram (Fig. 19). (The boundaries between the 
 single ids are only indicated in generation IV. in the figure.) In 
 the second generation four groups, each consisting of four 
 similar ids, would be combined ; in the third, eight groups of 
 two ids ; and in the fourth, sixteen groups, each consisting of 
 only one id. The accompanying diagram illustrates this pro- 
 cess : the two parental idants are shown on the left, and their 
 fusion to one idant in the offspring on the right. The different 
 kinds of shading and dotting indicate the individual differences 
 between the ids. 
 
 The mingling of the ids in the individual idants, just as in the 
 case of the mingling of the idants themselves, will not have 
 occurred so quickly and regularly as is indicated in the diagram ; 
 but the final result is the same, whether the process takes place 
 more quickly or more slowly. 
 
 The introduction of sexual reproduction will thus have gradu- 
 ally resulted in a greater degree of complication of the germ-plasm, 
 so that it is no longer composed ofszmi'/arids, but is mainly made 
 up of ids which are individually different from one another. All 
 those phenomena of heredity which are spoken of as the inter- 
 mingling of the characters of ancestors, such as degeneration or 
 atavism of all kinds and degrees, depend, I believe, on this 
 complicated structure of the germ-plasm. 
 
 In the following chapter an attempt will be made to explain 
 these phenomena theoretically. It will, however, first be neces- 
 sary to glance for a moment at the process of the reduction of 
 the ids, as far as we are acquainted with it. 
 
 2. PROOF THAT THE ESSENTIAL PART IN THE PROCESS OF 
 'REDUCING DIVISION' CONSISTS IN THE EXTRUSION OF IDS. 
 
 In the chapter on the architecture of the germ-plasm, it was 
 pointed out that the ids are probably identical with the 'micro- 
 somes' which are known to exist in many cases in the nuclear 
 rods, and not with the entire rods, or idants. This conjecture 
 was based on the fact that the rod-like chromosomes, the struc- 
 ture of which we are best acquainted with, consist of a series of 
 granules, or microsomes, which are separate and independent 
 structures. The composition of these rods evidently excludes 
 the possibility of considering each of them to be equivalent to a 
 single id. For an id is a vital unit, with a definite structure, and
 
 EFFECTS OF AMPHIMIXIS ON THE GERM PLASM. 241 
 
 cannot be composed of a row of loosely-connected spherical 
 bodies, each containing only a portion of its determinants. 
 Moreover, the fact that the number of idants is on the whole 
 a small one, speaks against their being regarded as ids : the 
 phenomena of reversion alone, it seems to me, require the 
 assumption of a larger number of ids. 
 
 The chromosomes are not, it is true, in all cases rod-like, and 
 may have a more spheroidal form ; the existence of microsomes 
 has, moreover, not been definitely proved in all cases. We might 
 therefore be inclined to look upon the chromosomes as structures 
 which are not always and absolutely equivalent, and to regard 
 some of them as single ids, and others as rows of ids. This 
 conception receives support from the fact that a considerable 
 variation as regards the number of chromosomes is seen in 
 nearly allied species, in which we might expect the processes of 
 heredity to occur in almost the same way. Thus, for instance, 
 the usual number of nuclear rods in Ascaris lumbricoides is 
 twelve, and in Ascaris megalocephala two or four ; in other 
 worms belonging to the same order the normal number of rods 
 may be eight, twelve, or sixteen. I should not, however, con- 
 sider these differences sufficiently great to warrant the assump- 
 tion that these rods have a different value in different cases ; 
 and this view receives support from the observations of Boveri 
 and Oscar Hertwig, which prove that in the same species 
 (Ascaris megalocephala} two varieties occur, in one of which two, 
 and in the other four, nuclear rods are present in the cells. In 
 this case, then, the one variety likewise possesses twice as many 
 microsomes as the other ; and although it is not always easy to 
 determine the number of microsomes in the case of other 
 Nematodes, we may infer their existence from the form of the 
 idants. For these reasons I am inclined to regard the micro- 
 somes as corresponding individually to ids, and the nuclear rods 
 as representing groups of ids; for this reason I have called them 
 idants. 
 
 The number of idants, and even that of the ids contained in 
 each of them, is a definite one for each individual species, but it 
 varies considerably in different species. Each id of any particular 
 germ-plasm could direct the entire ontogeny if it were present 
 in sufficient numbers ; that is to say, every id contains all the 
 determinants required for one individual : but, as has already 
 been remarked, the ids contained in the idants of a species
 
 242 THE GERM-PLASM. 
 
 which multiplies sexually do not contain precisely identical 
 determinants, but these differ more or less from one another, 
 at any rate to such an extent that they correspond to the 
 individual differences existing in the species at the present day. 
 It results from the mechanism for nuclear division that all the 
 different kinds of ids pass into all the cells throughout ontogeny, 
 and therefore the character of every individual cell occtirring in 
 ontogeny must be determined by an aggregate of ids; so that all, 
 or at any rate the greater portion of the ids of which the idants 
 are made up, determine the constitution of the cell in question, 
 this determination resulting from the forces within the cell. 
 These preliminary remarks will serve as a general basis for the 
 following considerations on the effects of sexual reproduction. 
 
 We can now consider the process of the ' reducing divisions ' 
 somewhat more closely. We require to know what influence the 
 reducing division exerts on the composition of the germ-plasm, 
 and of what kind are the ids which are consequently respectively 
 removed from, and retained in, the germ-plasm. 
 
 Direct observation of the process is not alone sufficient to 
 explain it ; for not only do the ids and idants appear alike to our 
 eyes, but we cannot even determine whether the idants of the 
 young germ-cells of a new individual are the same as those of 
 the fertilised egg-cell from which this organism arose ; that 
 is to say, whether an idant is a permanent structure, and whether 
 a particular idant remains the same from one generation to 
 another. 
 
 We know that during the process of amphimixis the paternal 
 and maternal idants are situated close together, and are en- 
 closed within a common nuclear membrane. There is often a 
 small, though distinct space between the two groups of rods ; 
 and did this remain distinct during the whole period of onto- 
 geny until new germ-cells were formed and underwent reducing 
 divisions, we might be able to determine directly whether the 
 paternal and maternal groups became separated, or whether half 
 the number of the paternal rods remained in connection with the 
 maternal ones, or also whether different combinations of rods 
 are removed by the reduction. 
 
 The matter is, however, not so simple as this : the idants of 
 the fertilised ovum only retain this form during the first division 
 of the egg-cell, and then become broken up into a number of 
 minute granules, which are distributed throughout the nuclear
 
 EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM. 243 
 
 substance, and only recombine to form nuclear rods when the 
 second division begins to take place. This process of disin- 
 tegration and subsequent re-combination of the idants is repeated 
 every time a cell is formed by division during ontogeny, and 
 thus it is impossible to decide whether a certain idant of any 
 particular cell is derived from the father or from the mother. And 
 further, we cannot even ascertain with any degree of certainty 
 by mere observation, whether the idants of the subsequent cells 
 are the same as those of the fertilised egg-cell, that is to say, 
 whether they contain the same kinds of ids in the same order. 
 It is very possible that the ids may become entirely separated 
 from each other whenever the idants undergo disintegration, and 
 then become arranged in some other order subsequently. The 
 number and nature of the ids contained in the entire idioplasm 
 would then certainly remain the same as before, but the indi- 
 vidual idants would differ, because the combination of ids would 
 be different. It would then be immaterial whether the idants 
 on the right were separated from those on the left in the reducing 
 division, or whether the halving of the number of idants were 
 effected in some other way ; all the idants would consist of new 
 combinations of ids already present, and their combination 
 would necessarily differ completely from that of the idants of the 
 fertilised egg-cell, which is almost always separated by a number 
 of cell-generations from the new germ-cells, in each of which a 
 rearrangement of the ids must have taken place. The removal 
 of entire idants in the reducing division would obviously there- 
 fore be unnecessary, for the mere qualitative division of the whole 
 of the idioplasm into two halves would be sufficient for the 
 purpose. 
 
 As, however, the reducing division actually consists in the 
 removal of half the number of idants, and as, moreover, this 
 division is, as we shall see, a double one, I conclude that the 
 disintegration of the idants atter every nuclear division is 
 only an apparent one, and that the separate ids of the idant, on 
 the contrary, remain connected together by fine threads of the 
 cementing substance, or ' linin ' ; and at the approach of nuclear 
 division, they become rearranged in the same order as before. 
 
 That this is the case may be concluded from certain pheno- 
 mena of heredity ; a child, for instance, not unfrequently takes 
 after one parent, e.g., the father only, or at any rate to such an 
 extent that the resemblance to the mother is unnoticeable. We
 
 244 THE GERM-PLASM. 
 
 must therefore suppose that the fertilised ovum from which the 
 child arose contained a very similar combination of ids and 
 idants to that which controlled the ontogeny of the father. It 
 must therefore be possible, and cannot be altogether a matter of 
 chance, that the germ-cell of the father contains these paternal 
 or maternal idants, or, in other words, almost precisely the 
 same ids as those which directed the development of the father 
 or mother, arranged in almost the same order. This is only 
 conceivable, it seems to me, if the combination of ids into idants 
 usually, at any rate, persists even during the disintegration of the 
 latter in the nucleus. 
 
 Many recent observations support this conclusion, inasmuch 
 as they show that fine threads of ' linin ' connect the individual 
 microsomata (ids), even when the idant has apparently undergone 
 disintegration. In fact, Dr Otto vom Rath* has just shown that 
 such connecting threads even extend between the idants. It is 
 therefore probably not too bold an hypothesis to assume the 
 existence of such an arrangement for connecting the ids together. 
 
 I am therefore of the opinion that the idants only apparently 
 undergo disintegration into granules during the ' resting- 
 stage' of the nucleus, and I agree with van Beneden and 
 Boveri in considering the idants to be essentially permanent 
 structures. I do not, however, as already mentioned, wish this 
 statement to be taken too literally : it must not be supposed that 
 the structure of an idant must always remain the same through- 
 out all generations, or that the reconstruction of an idant after 
 its disintegration must in all cases result in the ids being re- 
 arranged in the same order, I imagine, on the contrary, that 
 deviations from the original serial arrangement frequently occur 
 in the ids. The fact of the constant change of individuality and 
 non-recurrence of the same individual which can actually be 
 observed in the human race in the course of generations, indi- 
 cates, in my opinion, that an occasional change of the ids 
 within the idants can take place in the course of generations, 
 although this does not occur every time the idants are recon- 
 structed. 
 
 If this is the case, and essentially the same idants persist 
 during ontogeny from the fertilised ovum to the germ-cells of 
 
 * 'Zur Kentniss des Spermatogenese von Grylloptalpa vulgaris.' Arch, 
 f. Mikr. Anat., Bd. 40, p. 120.
 
 EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM. 245 
 
 the new organism,* we may conclude from certain phenomena 
 of heredity that the reduction of the number of ids to one half 
 does not result in the separation of groups of ids which are 
 always the same, and are definitely determined beforehand, but 
 in the removal of different groups on different occasions. The 
 germ-cells of one and the same organism must consequently 
 contain very different combinations of ids, and consequently 
 also of primary constituents, than those which were present 
 in the parents of this organism. The reduction affects the 
 paternal and maternal idants in a precisely similar and equal 
 way ; it takes place in such a manner that any combinations 
 may result from the halving of the number of idants. Let 
 us take, for instance, four idants a+b and c + d ; not only may 
 the paternal group a + b and the maternal group c + d, as well as 
 combinations of a + b and c + d, be present in the fully-formed 
 germ-cell, but also the combinations a + c and b + d, or a + d and 
 b + c, that is to say, combinations each of which consist of one 
 paternal and one maternal element. 
 
 A moderate amount of difference between the germ-cells of 
 an organism as regards their contained primary hereditary con- 
 stituents will thus result. In the case of the four idants taken 
 above as an example, only six combinations would be possible, 
 and consequently there could only be six kinds of germ-cells 
 differing from one another in respect of their primary consti- 
 tuents. The number of possible combinations, however, in- 
 creases very considerably with the increase in the number of 
 idants ; for example, 70 combinations are possible with eight 
 idants, 12,870 combinations with sixteen idants. 
 
 * Appearances certainly seem to contradict this assumption, and I am 
 fully aware of the fact that Oscar Hertwig, and more recently Guignard, 
 have stated their opinion to the contrary. In many conditions of the 
 nucleus it is, in fact, impossible to recognise the idants, and they certainly 
 do not exist as such, that is, in the form of compact rods. But it is quite 
 conceivable that the connection of the ids in an idant may nevertheless 
 persist, and that the individual ids are connected together by fine threads 
 of 'linin.' An observation made by my assistant, Dr Hacker, supports 
 this view. He noticed that the microsomes of the rod-like idants of the 
 growing egg in Copepods become separated from one another, but always 
 remain connected by a delicate thread of linin, which in this instance 
 can be stained : the linear arrangement of the microsomes certainly persists 
 in this case. (Cf. Hacker, 'Die Eibildung bei Cyclops und Cantho- 
 cam plus,' Zool. Jahrbucher, Abth. f. Anat. und Ontog., Bd. v. p. 237.)
 
 246 THE GERM-PLASM. 
 
 In Ascaris megalocephala the number of idants is only two or 
 four ; but as far as we know, a greater number is present in the 
 case of all other animals, and also in that of plants : thus there 
 maybe eight, sixteen, thirty-two, and even a hundred or more.* A 
 simple and single reduction, such as we have hitherto assumed,, 
 will therefore in general secure a very considerable amount of 
 variety as regards the combinations of primary constituents 
 caused by the reducing division. Nature seems, however, to 
 have aimed at a far greater degree of variety, at any rate in the 
 case of animals, in which a double instead of a single reduction 
 of the number of idants to one half always occurs ; and this, 
 as I have recently attempted to show, must have the effect of 
 increasing the number of possible combinations of idants very 
 considerably.t 
 
 The facts as they concern the Metazoa may be briefly sum- 
 marised as follows. In all those species which have been 
 investigated for this purpose, the germ-cells are formed by the 
 mother-cell undergoing two consecutive divisions, each of which 
 results in a halving of the number of idants, one half passing into 
 the one daughter-cell, and the other half into the other. In the 
 second division this would lead to the presence of only a quarter 
 of the original number of idants, if the number in the mother-cell 
 were not doubled by each idant becoming split into two before 
 the first division takes place. Thus there is first a doubling, and 
 then a halving, of the number of idants. It is a matter of 
 secondary consideration in the question of heredity that in the 
 formation of the female germ-cell or ovum three of the cells pro- 
 duced by the division of the mother-cell give rise to the evanes- 
 cent 'polar-bodies,' one cell alone becoming an ovum capable 
 of development, while all four of the male germ-cells become 
 functional. The chief point which now concerns us is the pro- 
 cess of doubling, and the two subsequent halvings of the number 
 of idants : this is known to occur in all classes of the Metazoa 
 from the lowest to the highest forms, and, as far as we know, is 
 only wanting in those eggs which are adapted for partheno- 
 genesis. Even in these cases the doubling also occurs, but it is 
 followed by only a single halving of the number of idants, in 
 
 * Dr vom Rath informs me that in the crayfish (Astacus fluviatilis) the 
 number of idants reaches 108-125. 
 
 t Cf, 'Amphimixis,' Jena, 1891 (Essay xii. in the English Translation, 
 Vol. ii., p. 105).
 
 EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM. 247 
 
 correspondence with the absence of amphimixis. For the full 
 number of idants only appears a second time in an ovum adapted 
 for fertilisation, by the union of the nucleus of the sperm-cell with 
 that of the ovum. 
 
 Rain 
 
 FIG. 20. Diagram of the formation of spermatozoa in Ascaris megalocephala, 
 var. bivalent. (Modified from O. Hertwig.) A, primitive sperm-cells ; B, 
 sperm-mother-cells; C, first 'reducing division'; D, the two daughter- 
 cells ; E, second ' reducing division ' ; F, the four granddaughter-cells (the 
 sperm-cells). 
 
 I consider this remarkable and apparently useless * process 
 of the doubling and two subsequent halvings of the idants as a 
 method of still further increasing the number of possible com- 
 binations of idants in the germ-cell of one and the same indivi- 
 dual, and have given reasons for this opinion in the above-named 
 
 * It might be supposed from the observations of Ruckert on the ovum of 
 the dog-fish, which were described in Chapters I. and II., that this doubling 
 is simply concerned with a doubling as regards mass, and consequently 
 with the activity of the idants : their activity must be very considerable in 
 this case, for the egg of the dog-fish is very large, and requires a consider- 
 able amount of multiplication of the 'oogenetic' determinants. But a 
 doubling of the idants occurs also in all other animal eggs, even in the
 
 248 
 
 THE GERM-PLASM. 
 
 essay. As already stated, a single halving of four idants can 
 only result in six combinations. But if, as actually occurs, each 
 
 RedJ 
 
 FIG. 21. Formation of ova in Ascaris megalocephala, var. bh'alens. A, primi- 
 tive germ-cell ; B, fully-developed egg-cell, the number of the idants in which 
 have increased from four to eight ; C, first ' reducing division ' ; D, the egg 
 with the first polar-body, immediately succeeding the stage represented in C ; 
 
 E, the first polar-body has divided into two daughter-cells (2 and 3), the four 
 idants which remain in the egg giving rise to the second ' reducing spindle ' ; 
 
 F, stage immediately succeeding the second ' reducing division' i, the ripe 
 egg-cell ; 2, 3, and 4, the three polar-cells ; each of the four cells only contain- 
 ing two idants. 
 
 smallest in which a very small amount of yolk is contained as well as in the 
 sperm-mother-cells, which never attain to such a size or structural differen- 
 tiation as do the ova. The process cannot be concerned with an increase 
 of the germ-plasm contained in the idants, for in the formation of the ova 
 three-quarters of the mass of germ-plasm passes into the polar bodies and 
 is again lost. The explanation of the process here given seems therefore 
 to be the only possible one.
 
 EFFECT'S OF AMPHIMIXIS ON THE GERM-PI ASM. 249 
 
 idant \vere doubled before the division, ten combinations would 
 be possible. This means that one individual of any species 
 possessing four idants in each of its cells can produce ten kinds 
 of ova and spermatazoa differing from one another as regards 
 individual hereditary tendencies. Two new idants are added to 
 such an ovum when one of them is fertilised by the spermatozoon 
 of another individual ; and since each parent produces ten 
 different kinds of germ-cells, as many offspring differing in 
 character from one another may arise from these two parents as 
 there are possible combinations of the ten kinds of spermatozoa 
 of the father with the ten kinds of egg-cells of the mother, i.e., 
 10 x 10 = 100. 
 
 With eight idants, 70 combinations are possible without, and 
 266 with, doubling ; and following this up, twelve idants will 
 thus give 924, or 8,074 combinations ; sixteen, 12,870, or 258,570 ; 
 twenty idants, 184,756, or 8,533,660; and with thirty-two idants, 
 about five hundred times as many combinations would be 
 obtained with doubling as without it. 
 
 Since the same number of idants from each of the conju- 
 gating cells come together in the process of fertilisation, 
 and each of the parental germ-cells only contains one 
 of the many possible combinations of idants, the number 
 of variations in the germ-plasm which it is possible for 
 two parents to produce must be an enormous one. It can 
 be calculated by multiplying the number of possible combina- 
 tions in the two conjugating cells together : thus in the case of 
 twelve idants only, it would amount to 8,074 x 8,074. Unfor- 
 tunately we are unacquainted with the number of idants in the 
 human subject, in which we are best able to recognise individual 
 differences in most minute detail. We may, however, suppose 
 that this number is more than four. If, for instance, it were as 
 high as twelve, we need not wonder that two children born con- 
 secutively are never identical, as must be the case if they had 
 originated from the same combination of ids of the germ-plasm. 
 Approximately identical children only occur in the case of twins, 
 and we have every reason to believe that these originate from 
 one sperm-cell and one ovum. 
 
 We cannot as yet judge with certainty as to how far the entire 
 idants pass unchanged as regards their constituent ids from the 
 germ-cells of one generation into those of the next. The pheno- 
 mena of the reduction in the germ-cells which have recently been
 
 250 THE GERM-PLASM. 
 
 made known to us in the case of various Arthropods by the re- 
 searches of Henking, vom Rath, and Hacker, indicate that even 
 the idants may become changed during the process. If we suppose 
 that in the mother germ-cell, when it is preparing for the first 
 reducing division, the ids become arranged in their original 
 order so as to form a long thread which doubles back on itself, 
 and thus gives rise to a ring, the latter would become trans- 
 versely divided in certain places. If the transverse divisions 
 could take place at different points, it would be possible either 
 for the old idants to be accurately restored, or for the new ones 
 to differ from them to a greater or lesser extent. 
 
 This assumption is not, however, essential for a theory of 
 amphigonic heredity, and we may here disregard it, although it 
 will doubtless be found to apply to some extent, as was indicated 
 above with regard to such a slow and slight change of the idants 
 due to the disarrangement in the combinations of ids contained 
 in them. It must be left to future researches to follow out this 
 process in detail, and to show whether the differences in the 
 combination of ids is merely due to the halving and rearrange- 
 ment of the idants, or whether regular, or at any rate frequent, 
 changes in the composition of the idants out of ids also occur. 
 For the present we must be content with knowing that the germ- 
 cells of an individual contain very many different combinations 
 of idants, and that a frequent repetition of amphimixis never 
 indeed results in the germ-cells of the same parents containing the 
 same combinations. It therefore follows that the combination of 
 parental and ancestral characters continually varies, and this 
 variation is characteristic of amphigonic heredity. 
 
 This statement also holds good for plants, in which we know 
 that a reduction to half the number of idants takes place in the 
 germ-cells. According to the researches of Guignard,* the 
 
 * L. Guignard, Compt. rend. May n, 1891, and ' Nouv. etudes sur la 
 fecondation,' Ann. scienc. nat. Bot. Vol. xiv., 1891, p. 163. 
 
 Particulars regarding Guignard's valuable researches cannot be entered 
 into here. They have not only proved that in plants the mature germ- 
 cells likewise contain only half as many ids as do the somatic cells, and 
 that the normal number is again produced by the union of the nuclei of the 
 male and female cells, but have also shown that the centrosome passes on 
 from one generation to the next. In spite of the fact that these observa- 
 tions are obviously perfectly accurate, I cannot help doubting whether the 
 reduction in the number of idants actually occurs without a nuclear divi- 
 sion, as Guignard states is the case. I have arrived at this conclusion net
 
 EFFECTS OF AMPHIMIXIS ON THE GERM-PLASM. 251 
 
 somatic cells, as well as the mother-cells of both kinds of germ- 
 cells in Lilium marlagon, contain twenty-four idants, while the 
 mature germ-cells contain twelve only. We do not as yet know 
 whether this reduction is effected by a single reducing division, 
 or by two such divisions preceded by a doubling, as in the case 
 of animals. For evident theoretical reasons I consider it ex- 
 tremely unlikely that the reduction occurs in the mother-cell 
 while it is preparing for division, as Guignard thinks is the case. 
 It is very possible, however, that only one reducing division 
 takes place in this instance. 
 
 The details of these processes in the lower plants are quite 
 unknown, probably owing to the minute size of the idants, which 
 till now has rendered the difficulties of such investigations insur- 
 mountable. It has, however, at any rate been ascertained that 
 in many marine algas (Fttcoidea) the development of the egg- 
 cells is accompanied by the formation of ' polar bodies,' which 
 certainly correspond to stunted and phyletically degenerated 
 ova : this was first shown to be the case by Biitschli and Giard, 
 and the fact has long been recognised by other zoologists besides 
 myself. In the genus Fucus these polar bodies do not occur, 
 and eight eggs are formed from the primary ovum, if I may 
 venture to apply this term to the original cell of the so-called 
 oogonium or ovary ; in an allied species of wrack, Ascophyllum 
 nodosum, only four eggs are formed from the primary ovum, but 
 four polar bodies are also produced ; in Pelvetia canaliculata 
 the primary ovum gives rise to two eggs and six polar bodies ; 
 
 merely from a comparison of the analogous process in animals, but also 
 because I cannot help thinking that it is possible, and even probable, that 
 in this respect these otherwise admirable observations are not quite com- 
 plete. In the formation of the male germ-cells, a reducing division may 
 perhaps take place between the ' cellules meres primordiales ' and the 
 'cellules mferes definitives;' and as regards the female germ-cell, it will 
 occur in the division which gives rise to the ' cellule mere du sac embry- 
 onnaire.' In both cases even the most acute observer might fail to notice 
 the reducing division if his attention were not specially directed to this 
 point. Why should an arrangement for a ' reducing division ' have been 
 made in the case of animals if the reduction could take place without 
 nuclear division, and could produce the same result ? Of all the other 
 numerous observations which have been made on the process of karyo- 
 kinesis, not a single one supports the view that the single (?) chromatin band 
 of the 'skein ' stage can become disintegrated into half as many idants as 
 were previously present in the nucleus.
 
 252 THE GERM-PLASM. 
 
 and in Himanihalia lorea only a single egg and seven polar 
 bodies are formed.* 
 
 We have here, however, no information as regards the reducing 
 divisions, which, as I pointed out long ago, need not by any 
 means be connected with the degeneration of several germ-cells. 
 We can only state that the three successive divisions of the 
 primary ovum, which occur in all the above-mentioned cases, 
 affords more than sufficient opportunity for one or even two 
 reducing divisions, and that it is extremely probable that one, at 
 least, actually occurs. 
 
 We may therefore assume that in plants very varied com- 
 binations of the germ-plasm derived from each parent usually 
 take place in the germ-cells of the offspring, and that perfectly 
 1 identical* germ-cells can very rarely occur either in plants or 
 in animals. 
 
 * Cf. Oltmans, ' Beitiaje zur Kentniss der Fucaceen,' Cassel. 1889.
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 253 
 
 CHAPTER IX. 
 
 ONTOGENY RESULTING FROM THE UNION OF THE 
 GERM- PLASM OF TWO PARENTS. 
 
 i. THE NATURE OF THE OFFSPRING DETERMINED BY THE 
 PROCESS OF FERTILISATION. 
 
 The first question which presents itself in the consideration of 
 'amphigonic heredity' is concerned with the relative share taken 
 by the germ-plasm of each parent in the control of ontogeny : 
 whether the paternal and maternal ids always co-operate simul- 
 taneously, and the forces contained within them together form a 
 single resultant, or whether one group only is active while the 
 other remains passive. This question cannot at present be de- 
 cided from the results of observations on the nuclear substances 
 themselves ; the phenomena of heredity, together with what we 
 know concerning the composition of the idioplasm resulting 
 from amphimixis, can alone help to elucidate this problem. 
 These phenomena must therefore be analysed as accurately and 
 minutely as possible. 
 
 We must base our analysis on the fact which we have already 
 proved, that the germ-cells of an individual differ from one 
 another as regards the hereditary substance they contain, and 
 that the proportion of paternal and maternal ids in a germ-cell 
 varies between wide limits, the degree of variability being greatly 
 increased by the union of the germ-cells of two individuals in 
 the process of amphimixis. This fact is sufficient to account for 
 the difference existing in the human race between children of 
 the same parents. The fundamental law of amphigonic heredity 
 enunciated by Victor Hensen follows directly from this fact : 
 ' the individual is determined at the time of fertilisation ; ' or, in 
 other words, the individuality of an organism results from the 
 "^actthat the germ-plasm is composed of the paternal and maternal 
 ids which are brought together in the egg-cell. 
 
 This law is not self-evident, for we might have believed, a priori, 
 that the development and mingling of parental characters in 
 the offspring is due entirely, or at any rate to a great extent, to
 
 254 THE GERM-PLASM. 
 
 external influences of nutrition, &c., to which the germ is sub- 
 ject after fertilisation. The existence of 'identical' human 
 twins, however, proves the contrary. Some twins do not re- 
 semble one another more closely than do children of the same 
 parents which are born consecutively ; and, in fact, this is 
 apparently true of the greater number of twins, in which 
 the dissimilarity may even be very considerable. We have 
 every reason to suppose that such 'dissimilar' twins are usually 
 derived from two ova, which must of course have been fertilised 
 by two different spermatozoa. On the other hand, in the case 
 of those twins which I speak of as ' identical,' the resemblance, 
 although not perfect, is much closer than has ever been 
 observed in children born successively. There is every reason 
 to suppose that such identical twins are derived from a single 
 ovum and spermatozoon. If this is actually the case, it fur- 
 nishes a proof of the above statement that heredity is potentially 
 decided at the time of fertilisation, or, expressed in terms of the 
 idioplasm, that the nature of the combination of the parental 
 ids which takes place during fertilisation predetermines the 
 whole subsequent ontogeny. The slight differences which exist 
 between identical twins would therefore probably indicate to 
 what extent the course of development may be affected by 
 external influences. These differences are generally so slight that 
 'A is difficult to observe them at all, unless they are specially 
 sought for ; as a rule such twins can only be individually recog- 
 nised by the parents or brothers and sisters, and cannot be 
 distinguished from one another by strangers. 
 
 These slight differences might, however, be due to an imper- 
 fect predetermination of the influence which is exerted at every 
 ontogenetic stage by the idioplasm of each of the parents. We 
 can hardly decide between these views from the consideration of 
 identical twins only. Mr Otto Ammon, of Karlsruhe, has kindly 
 furnished me with two photographs of identical twins taken in 
 consecutive years at the ages of seventeen and eighteen, as well 
 as with exact measurements of all parts of their bodies. In 
 spite of a striking resemblance, not only in face but in all parts 
 of their bodies, certain differences are nevertheless recognisable 
 between them. For instance, the height of the one marked 
 No. 507 on Mr Ammon's list, measured, when lying down, 172 
 cm., and that of No. 508 only 170 cm. ; and again, although 
 the length of the hand and of the left arm is the same in both,
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 255 
 
 the latter measuring 74 cm., the right arm of No. 507 is only 
 71 cm. long, while that of No. 508 reaches 74 cm. The relative 
 lengths of the upper arm and fore-arm are also different, that 
 of the left upper arm of No. 507 being 27 cm., while in No. 508 
 it is 27*5 cm. ; and consequently the length of the fore-arm in No. 
 507 is also 27 cm., while in No. 508 it is only 26 cm. Even if 
 we possessed the measurements of the parents at the same age, 
 we should probably be unable to draw any definite conclusions 
 as to whether these slight differences in size are due to a corre- 
 sponding difference in the combination of the germ-plasm, such 
 as might arise from a slightly inexact division of the nucleus of 
 the fertilised ovum in the process of doubling or at a later stage, 
 or whether they simply owe their origin to slight general or 
 local differences of nutrition taking effect during ontogeny. 
 
 Other facts are, however, known, which prove that although 
 the nature of the combination of the parental idioplasms during 
 ontogeny is in general, as a matter of fact, determined at the 
 time of fertilisation, it is nevertheless liable to slight individual 
 fluctuations. Instances of this kind are furnished by the hybrids 
 of certain species of plants, many parts of which exhibit a con- 
 siderable degree of variability, and fluctuate between the specific 
 characters of the two parents. The blossoms of the hybrid 
 plants obtained by crossing Digitalis lutea and D. purpurea, for 
 instance, ' vary in colour ; in some instances they are pale, with 
 a slight pink tinge, which latter, again, may be entirely absent ; 
 and in others they have a more or less bright purple colour.'* 
 These observations appear to me to be particularly important, 
 owing to the fact that we may assume with certainty in this case, 
 in which two distinct and sharply defined species were crossed, 
 that both parents possessed the specific characters in the same 
 degree of purity and strength, and that consequently the relative 
 proportion of the parental idioplasms does not remain quite con- 
 stant during ontogeny, owing either to slight irregularities in the 
 nuclear division, or and this is less probable to inequalities 
 in nutrition and in the growth of the idants derived from the 
 two parents. Owing to the kindness of Professor Hildebrandt 
 of Freiburg i. Br., I have had an opportunity, in the case of 
 hybrids of two species of Oxalis, of observing how extremely 
 detailed the process of predetermination is. The flowers of one 
 
 * Focke, ' Die Pflanzen-Mischlinge,' Berlin, iSSi. p. 316.
 
 256 THE GERM-PLASM. 
 
 of the parent-species were large, and of a pale lilac colour, 
 while those of the other were smaller, and their colour was red, 
 with a dark crimson ground. The flowers of the different 
 hybrids were by no means quite similar, but three principal forms 
 could be distinguished according to the combination of colours 
 in the flowers, which I shall not describe in detail : the flowers 
 of the same hybrid, however ; resembled each other in their most 
 minute details. One plant, for instance, had violet petals of a 
 rather pinker tint than those of one of the parent-species, and 
 all the petals were strongly tinged with red on one and the same 
 lateral margin. As far as I could observe, all the flowers were 
 similarly coloured on this stock. On another stock, all the sepals 
 had brown rims, and on a third there was a narrow dark orange- 
 coloured band in the centre of each flower. In these cases, there- 
 fore, the combination of the colours of the parents which appeared 
 in the petals of the hybrids must have been decided at the time of 
 fertilisation. It will be shown later on how this combination 
 may vary somewhat in different plants. 
 
 Even although the slight differences in identical human twins, 
 which can be proved to exist, are certainly due in part to minute 
 differences in the idioplasm itself, some of them must neverthe- 
 less with equal certainty be attributed to the effect of various 
 external influences. My photographs of the above-mentioned 
 identical twins shows that No. 507 has particularly white hands, 
 while those of No. 508 are much browner. No one would 
 attribute this fact to dissimilarity in the respective germ-plasms, 
 or to an alteration in the proportion of paternal and maternal 
 idioplasm which occurred during ontogeny : it must be due to 
 the fact that the hands of No. 508 had been more exposed to 
 the sun than those of No. 507 ; and, as it happens, the former 
 of the two had been more employed in the open air than the 
 latter before the photograph was taken. Several differences 
 in the proportional sizes of parts of the body may possibly have 
 been brought about in a similar way. 
 
 2. THE SHARE TAKEN BY THE ANCESTORS IN THE 
 
 COMPOSITION OF THE GERM-PLASM. 
 
 If, then, it is certain that the characters of the developing 
 offspring are essentially decided by the mingling of parental 
 idioplasms which takes place in the process of fertilisation, we 
 must next try to ascertain whether the entire pat ental idioplasm,
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 257 
 
 with all its constituent determinants, or only a portion of it, is 
 passed into the germ-cell which will give rise to a new individual ; 
 and also what proportion of it is constituted by the germ-plasm 
 of the grandparents, great-grandparents, and more remote ances- 
 tors. The fact that the reducing division, which takes place 
 both in the male and female germ-cells before fertilisation, 
 removes half of the idants from each, leads us to conclude 
 that only half the normal number of ids can be contained in 
 each germ-cell ; and this could only be the case if two of each 
 kind of parental idant were present in the genii-plasm, and 
 if the reduction resulted in each individual germ-cell containing 
 a similar group of idants. But this cannot be so, for the germ- ' 
 plasm must consist of a number of entirely different idants, 
 unless, in consequence of interbreeding, two of the same kind 
 are present in certain of the groups. The whole of the idants 
 of both Barents evidently cannot possibly be contained in any one 
 germ-cell, because the total number would be twice as great as 
 that which actually occurs in a ripe germ-cell. If in Man, for 
 example, there were thirty-two idants in the fertilised ovum, 
 sixteen of them would be derived from each parent. Of this 
 latter number, sixteen at most could be derived from one grand- 
 parent, and this could only occur if no idants at all from the 
 other grandparent had passed into the germ-cell in question. 
 It is evidently more than inaccurate to fix the limit of the here- 
 ditary power as is done by animal-breeders of a parent at 
 \, of a grandparent at , of a great-grandparent at \, and so 
 on.* These numbers do not even represent the maximum or 
 minimum share in heredity which may be taken by the respec- 
 tive ancestor in the constitution of the fertilised egg. The 
 
 * Gallon has also emphasised this fret in the concluding chapter of his 
 book on 'Natural Inheritance" (p. 187 et seq.). According to his view, 
 the 'personal heritage' of each parent =J, and the heritage of 'latent 
 elements' of the parent likewise=J, the two together thus making up $. 
 Naturally I cannot agree with this calculation, for in my opinion the 
 latency of the characters of a parent does not result from the ' primary 
 constituents ' of these qualities, but from the struggle between the primary 
 constituents of both parents ; and I do not in the least suppose that the 
 primary constituents which practically give rise to the individual become 
 separated from those which form the latent germ for the germ-cells of the 
 next generation. But I fully agree with Gallon that all the ' characters ' 
 of the ancestor the grandparent, for instance are never present in every 
 germ-cell from which a grandchild may arise. 
 R
 
 258 THE GERM-PLASM. 
 
 parent is certainly always represented by one-half, but the share 
 varies even in the case of the grandparent ; in the instance just 
 given it would vary between o and 16. For the reducing divi- 
 sion may, for instance, cause the sixteen paternal idants result- 
 ing from the reduction of the thirty-two originally present 
 in the sperm-cell, to contain idants derived from the grandfather 
 only, and none from the grandmother ; or, again, there might 
 be fifteen from the grandfather, and one from the grandmother, 
 or fourteen pp and two mm, or thirteen pp and three mm, and 
 so on.* This would be so, at least, if any kind of combination 
 of the idants may result from the reducing division. It may 
 perhaps not be the case absolutely, but the capriciousness with 
 which reversion to a grandparent may occur nevertheless in- 
 dicates that a considerable latitude exists with regard to this 
 combination. 
 
 In passing back to the third, fourth, and fifth generations, we 
 cannot in the least determine, a priori, to what extent an indi- 
 vidual ancestor of the animal in question is still represented in 
 the germ-plasm of a germ-cell ; we can only state the maximum 
 which might be possible in the most favourable case. In the 
 above-mentioned instance, an ancestor of the third generation 
 might still be represented by sixteen idants, for the sixteen 
 idants which this ancestor furnished for the purposes of amphi- 
 mixis in the second generation might, all in fact, possibly have 
 passed into one germ-cell in the process of reducing division in 
 this generation, and the same, again, might have occurred in the 
 first generation. Such a case can only be of rare occurrence, 
 but it apparently accounts for the instances of reversion in Man 
 to ancestors more remote than grand-parents, which, though 
 rare, certainly occur occasionally. The more remote the gene- 
 ration, the greater are the chances against the entire half of 
 the total number of idants remaining together through several 
 generations in individual germ-cells, and the probability of such 
 an occurrence will very soon be reduced to zero. 
 
 We may suppose that, as a general rule, the number of ances- 
 tral idants contained in a fertilised egg-cell becomes less in pro- 
 
 * I shall now denote the paternal ids or idants by the letter /, the 
 maternal ones by m, and those derived from the grandfather by // or 
 pm, and from the grandmother by mp or mm, &c. The first letter in each 
 case signifies the parent, the second the grandparent, the third the great- 
 gvandparent, and so on.
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 259 
 
 portion as the ascendancy of the ancestor concerned decreases. 
 Any more exact calculation of the share taken by a certain 
 remote ancestor in the composition of the germ-plasm of its 
 descendant would be erroneous. Hitherto the customary 
 method of making such a calculation has been to assume that 
 the following shares are taken by the various ancestors in the 
 predisposition of the offspring : parents, 2 x ; grandparents, 
 4 x J, and so on ; that of the sixth generation of ancestors being 
 32 x ^V- Thus, in the last-mentioned generation, one idant out 
 of the thirty-two assumed to be present in Man would, according 
 to the theory of the germ-plasm, still remain. This does not by 
 any means imply that each of the thirty-two ancestors of the 
 sixth generation is still represented by one idant in the germ- 
 plasm of the descendant ; it is quite as probable that thirty or even 
 twenty of these ancestors take part in its constitution, and the 
 number may possibly, though improbably, be still less than this. 
 In treating of the phenomena of reversion, I shall have occasion 
 to refer to this subject again. 
 
 It is at any rate certain that in no case can more than the half 
 of the idants of one parent be present in the germ-plasm of the 
 fertilised egg-cell. 
 
 This statement is, however, apparently contradicted by certain 
 facts. 
 
 Plant-hybrids frequently keep to the mean between the two 
 ancestral species ; that is to say, they contain all the characters 
 of these two species in equal proportions. Thus all the primary 
 constituents of each parent would be contained in the fertilised 
 egg-cell, although, according to our theory, only half of the 
 parental idants are concerned in its constitution. This contra- 
 diction is easily accounted for, if it be borne in mind that we are 
 here concerned with the mingling of the characters of two specie 
 and not of those of two individuals of the same species. The 
 characters of the species must be contained in the majority oj ids 
 in each idant, if not in every id, and half the idants may in this 
 case produce the same effect as would result if all the idants were 
 present: that is to say, they contain every specific character. 
 In cross-breeding, specific characters are opposed to specific 
 characters, and in comparison with the greater differences 
 between these, the lesser individual ones disappear. 
 
 The reverse is true in the case of reproduction in Man, espe- 
 cially within one and the same race. The specific characters
 
 260 THE GERM-PLASM. 
 
 are probably contained in all the ids of the father as well as ot 
 the mother, and the differences between the parents refer to 
 individual characters only. Our theoretical conception of the 
 idants as a collection of ids seems incompatible with the above- 
 mentioned statement that the child can only closely resemble 
 one parent, for only half of the idants of this parent take part in 
 the construction of the child. We shall, however, be able to 
 explain this apparent contradiction later on. 
 
 The facts of the case may be stated in general terms as 
 follows. Half the number of parental idants always reach the 
 germ-cells of the offspring, but this half may consist of all 
 possible combinations of the parental idants : that is to say, 
 either of idants derived from the grandfather or grandmother 
 only, or of a combination derived from both, in which one or the 
 other may predominate. Nothing will be gained by taking the 
 ancestors of the third or fourth generation into consideration 
 until we come to consider the phenomena of reversion. 
 
 3. THE STRUGGLE OF THE IDS IN ONTOGENY. 
 a. Plant- Hybrids. 
 
 The structure of the offspring results from the struggle of all, 
 the ids contained in the germ-plasm. 
 
 That this statement must be in general correct is to some 
 extent indicated by the fact that all parts of hybrid plants, pro- 
 duced by crossing two species or varieties, usually possess the 
 characters of both parents. The details concerning hybrids are 
 of far greater value for theoretical purposes than are those re- 
 lating to the normal offspring of any particular species, as we 
 know for certain that the characters which compete with one 
 another or combine, so as to result in the production of a hybrid, 
 must be contained in every idant of one or other of the parents ; 
 for these characters are those of the species. 
 
 The difference as regards the idioplasm between individual 
 and specific characters, seems to me to be due to the determin- 
 ants of the latter being present in an overwhelming majority in 
 all the ids of every idant of the germ-plasm, while the deter- 
 minants controlling the structure of individual characteristics 
 are only contained in a portion of the idants of which the germ- 
 plasm consists : at most they can only be present in all the 
 idants of one of the parents, that is, in half the entire number
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 261 
 
 of idants. The extent to which the determinants of any indivi- 
 dual character are represented whether they are contained in 
 all the ids, or only in a small portion of them could only be 
 ascertained from the phenomena of heredity if we knew the 
 cause of the predominance of any particular ' successful ' charac- 
 ter. This, however, can only be inferred from crosses between 
 species, the great constancy of the specific characters in which 
 leads us to presuppose that their determinants predominate in all 
 the idants of the parental germ-cell. 
 
 In plant-hybrids, paternal and maternal idants come together 
 in the process of amphimixis, and each group of them may be 
 assumed to consist of similar idants. The effect this arrange- 
 ment will produce on the phenomena of heredity must now be 
 considered, and conclusions drawn from these considerations. 
 
 From a very large number of observations on hybrid-plants, 
 we find that, in the first place, parental characters may be 
 variously intermingled. From a comparison of all the cases 
 observed up to the year 1881, Focke* concludes that these com- 
 binations of characters may be divided into three principal 
 groups, viz. : (i) a mean between both parents is maintained 
 in all parts of the plant ; (2) the characteristics of the father or 
 mother predominate ; and (3) certain parts of the hybrid exhibit 
 the maternal, and others the paternal, characters, 
 
 The first-mentioned case is by far the most frequent : we 
 may take as an example the hybrid obtained by Kollreutter from 
 two species of the tobacco plant, Nicotiana rustica 9 and N. 
 paniculata <j . Kollreutter t himself stated that this plant was 
 exactly intermediate between its parents, while it was considered 
 by Gartner to bear a slightly greater resemblance to N. panicu- 
 lata, and by Focke to N. rustica. Kollreutter's opinion is probably 
 therefore a fairly accurate one, for in any case the point can 
 only be estimated, and cannot be decided with mathematical 
 precision. According to Focke, the corolla-tube of N. rustica 
 is 14 mm. long, that of N. paniculata 26 mm., and that of the 
 hybrid 19 mm. The exact mathematical mean between these is 
 20 mm. ; and hence in this character the hybrid approaches N. 
 paniculata. As regards the diameter of the widest part of the 
 
 * ' Die Pflanzen-Mischlingen,' Berlin, 1881. 
 
 t Joseph Gottlieb Kollreutter, ' Vorlaufige Nachricht von einigen das 
 Geschlecht der Pflanzen betreffenden Versuchen u. Beobachtungen,' 1716.
 
 262 THE GERM-PLASM. 
 
 corolla-tube, on the other hand, the hybrid inclines more to IV. 
 rustica, while in the narrowest part it exhibits the exact mathe- 
 matical mean. This case is instructive, for we cannot reL'^nise 
 the true physiological mean, because the length, as well as the 
 diameter, of the corolla-tube is determined by the same cells. 
 Any estimation of the mean between the colours must be still 
 less precise, for the different shades depend on entirely dif- 
 ferent morphological constituents. If the yellow and red of two 
 different species were blended in the flower of a hybrid, the 
 intensity of both these colours might conceivably be as great as 
 in the parent plant, and yet one of them might predominate 
 because it happened to cover the other. For the yellow is due 
 to special pigment-granules, while the red occurs only in the 
 cell-sap, which might possibly be nearly hidden by a superficial 
 layer of chromatophores. 
 
 In any inquiry with regard to the factors which control the 
 struggle of the parental characters, it must, above all, be 
 borne in mind that the cells are always the determining agents. 
 The determinants of the father and mother come together in the 
 cell, and in the cell only ; and all characters, whether relating to 
 a large part of the organism, or merely to a single cell, can only 
 be determined by processes taking place within the substance of 
 one or of many cells. 
 
 This does not mean that the visible differentiation of every 
 individual cell always constitutes a 'character' of the organism. 
 The nature of the histological differentiation of the cells that is 
 to say, whether muscle- or nerve-substance, or chlorophyll 
 granules, for instance, are produced in the cell-body or not 
 only comes into consideration in connection with the number of 
 cells in the definitive cell-aggregate constituting the organism. 
 Very many characteristic qualities cannot be due to this fact, 
 but must chiefly depend on the number and arrangement of the 
 cells in an organ, which again must be due to qualities of the 
 embryonic cell which are invisible to us principally those which 
 relate to its method of division, and its vigour and rate of multi- 
 plication, 
 
 We must suppose that these factors are wholly determined by 
 the idioplasm of the cells quite as much as is the visible differen- 
 tiation of the latter. The division of a cell certainly originates 
 in its apparatus for division, and primarily in the 'sphere of 
 attraction ' and its contained centrosome ; but we should have to
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 263 
 
 give up the whole idea of the controlling influence of the nuclear 
 substance were we to suppose that this apparatus actually directs 
 the process of division. The entire process of development of 
 the animal from the ovum depends so essentially on the rate of 
 division of the cells, that the nuclear substance could no longer 
 be considered to correspond to the hereditary substance if it 
 merely caused the visible differentiation of the cell. But in 
 the Introduction I have already stated the reasons which 
 prove beyond a doubt that the nuclear matter actually contains 
 the primary hereditary constituents, and it follows from this fact 
 that there can be no question of a self-determination of the 
 apparatus for division. We must, in fact, suppose that the ulti- 
 mate structure of the cell-body, which is invisible to our eyes, 
 controls its entire growth as well as its method and rate of 
 division, this structure itself being controlled by the nuclear sub- 
 stance or idioplasm. Ultimately, therefore, everything depends 
 on the determinant of the cell ; and in the case of sexual 
 reproduction, the co-operation of the paternal and maternal de- 
 terminants determines the character of the cell, whether this cha- 
 racter is visible or invisible. As every cell in the entire ontogeny 
 is, according to our view, controlled by one kind of determinant 
 only, irrespective of the fact whether it also contains other 
 determinants in a latent condition or not, the co-operation of 
 maternal and paternal determinants always determines the 
 character of the cell, and controls the development of the 
 individual as far as the influence of this cell extends. In spite 
 of its greater histological differentiation, a slighter influence is 
 therefore obviously exerted by one of the final cells in ontogeny, 
 that is, one of the tissue-cells, than by one of the first 
 four segmentation-cells, or by the primary cell of the entire 
 germinal layer, or, again, by any cell from which many and 
 various kinds of cells may subsequently arise. On the other 
 hand, we must not forget that each embryonic cell only 
 determines its own method of division, and not necessarily 
 that of its daughter-cells, and that consequently in these a new 
 counter-balance or co-operation of the paternal and maternal 
 determinants again takes place. 
 
 The determinants which control the daughter-cells are, how- 
 ever, derived from the latent ids of the mother-cell, and it there- 
 fore essentially depends on the methods of division and on the 
 architecture of these ids as to which determinants are to control
 
 264 THE GERM-PLASM. 
 
 the daughter-cells. That portion of the ids of the mother-cell, 
 therefore, which is for the time latent, exerts a definite influence 
 on the determination of the subsequent development; on it alone, 
 indeed, depends the number and order of succession of the 
 determinants which will subsequently become active, and all 
 those characters which result from the number and grouping of 
 the cells, as well as from the histological nature of the individual 
 cell, are also determined in the first instance by their ids. 
 
 We can thus explain why cross-breeding is only successful in 
 the case of nearly-related species, and not in that of members of 
 entirely different families. If it were possible, for instance, for 
 the ovum of a sea-urchin to be fertilised by the spermatozoon of 
 a worm, such as Rhabditis nigrovenosa, the disintegration of 
 the ids of germ-plasm derived from the father and mother 
 respectively would take place in an entirely different manner 
 even in the first stage of segmentation ; the maternal ids might 
 become disintegrated into the groups of determinants for the 
 right and left halves of the body, while the paternal ids became 
 separated into the groups for the external and internal germinal 
 layers. But dissimilar groups of determinants of this kind could 
 not co-operate and give rise to intermediate structures ; and 
 even if the ontogeny advanced a few more stages, embryonic 
 structures could never be produced which would work together 
 harmoniously. 
 
 The term homologous determinants and ids may be applied to 
 those which control homologous cells and groups of cells. It 
 may therefore be stated that structures which are intermediate 
 between those of the two parents may arise whenever homologous 
 determinants and ids come together. If a small spot is present 
 on a certain region of the wing in two allied species of butter- 
 flies, for instance, and this is represented in the germ-plasm by 
 a single determinant, the homologous determinants of the two 
 cells in which this spot was first contained will be brought to- 
 gether when these two species are crossed, and will eventually 
 be able to share in the control of the cell in question. They 
 need not, however, necessarily be exactly alike ; the spot may 
 be brown in species A, and red in species B. In this case the 
 determinants would be homologous but not homodynamous, and 
 might possibly combine to form a reddish-brown spot. The 
 essential point in amphigonic heredity is, that the idioplasm in 
 each cell in the entire ontogeny contains ids which are individually
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 265 
 
 different from one another, and does not consist of a number of 
 identical ids; and that the cell may take on an intermediate 
 character in consequence of their co-operation. In normal repro- 
 duction the active ids of the idioplasm are all homologous, that 
 is to say, their function is to control the same part of the body, 
 but they differ among themselves, i.e., are heterodynamous, 
 or, in other words, they tend to impress a somewhat different 
 character on the same part of the body. In the crosses between 
 different species, the idioplasm of a cell in many stages will be 
 composed of homologous as well as of heterologous ids, and, 
 as has just been shown, it then remains to be decided whether 
 a common control of the cell is possible at all, and if so, over 
 how many generations of cells it can extend. 
 
 The scales of butterflies are cells belonging to the final stage of 
 ontogeny, and their ids are made up of determinants of one kind 
 only. The further back we pass towards the beginning of ontogeny, 
 the more numerous are the determinants composing the ids ; 
 only one of them, however, breaks up into biophors and controls 
 the cell. If the determinant which actually controls the cell is 
 homologous in both parents, an intermediate form of cell may 
 result ; but as soon as the rest of the determinants in the id no 
 longer correspond to one another, the further development 
 gradually becomes checked, and will ultimately be brought to a 
 stand-still. The processes which must be supposed to take 
 place from the beginning of ontogeny onwards in the case of the 
 supposed cross between a sea-urchin and a worm may very 
 possibly be only partial, i.e., they may refer merely to special 
 parts. Let us suppose that a certain species of insect, which is 
 normal in other respects, possesses limbs on the dorsal side 
 instead of wings, and that this is crossed with another insect 
 possessing normal wings, development of the fertilised egg 
 taking place up to the stage in which the wings arise. The 
 idioplasm of the primary cell of the wings and ' dorsal limbs ' 
 respectively would then contain two perfectly heterologous ids, 
 one derived from the father and the other from the mother, and 
 none of the determinants in these two ids would be respectively 
 homologous. In the first stage of the development of the wings 
 or ' dorsal limbs,' as the case may be, antagonistic determinants 
 would be opposed to one another in each cell-generation, and 
 would prevent a common determination of the cell. 
 
 Such extreme cases do not actually occur, as very different
 
 266 THE GERM-PLASM. 
 
 species do not interbreed ; but the principles which may be 
 deduced from these imaginary cases may be applied to the 
 production of hybrids. Homologous determinants and ids 
 co-operate, while heterologous ones do not ; and the larger the 
 number of heterologous determinants present in the homologous 
 ids of the parents, the more do the hereditary tendencies of the 
 latter tend to nullify one another. 
 
 We must now attempt to give a theoretical explanation of the 
 first of the three kinds of combination of parental characters in 
 the offspring mentioned above, viz., that in which ' a mean between 
 both parents is maintained in all parts of the plant? 
 
 If we assume that the two parental species are so closely allied 
 that each determinant in the one corresponds to a homologous 
 determinant in the other, a form exactly intermediate between 
 the two species must result, supposing that the number of ids in 
 either parent is the same, and that a similar controlling force 
 acts on the homologous determinants. 
 
 If the idioplasm of one parent is represented by a larger 
 number of ids, its controlling force must obviously be greater ; 
 and as regards the controlling force of the individual determin- 
 ants, we may state as follows: the control of the cells is in 
 our opinion effected by the disintegration of the determinant 
 into biophors, which, like the pangenes of de Vries, migrate 
 into the cell-body, multiply at its expense, and give rise to 
 definite cell-structures. This multiplication must take place 
 with a certain amount of energy, the degree of which varies in 
 the different kinds of biophors. Thus, whenever such con- 
 trolling biophors, possessing the energy of growth in different 
 degrees, migrate into the same cell-body, a struggle of the 
 parts (Roux) must ensue, in which the stronger part will be 
 successful, and the weaker will be more or less, or even com- 
 pletely, suppressed. 
 
 The formation of structures which are strictly intermediate 
 between those of the two parents, implies that the homologous 
 determinants possess a similar controlling force. All the deter- 
 minants of any two species are, however, never homologous : 
 this follows from the fact that the number of cells in homologous 
 parts is often very different. The characters of the species do 
 not by any means depend only on the histological nature of the 
 individual cells, but, as already stated, are due in almost a 
 greater degree to the number and arrangement of the cells, to
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 267 
 
 the repetition and position of certain organs, and so on. Thus 
 the flower of Nicotiana paniculata is decidedly longer than 
 that of N. rustica, and the former species is more extensively 
 branched and possesses a larger number of glands than the 
 latter. The above assumption that the individual determinants 
 correspond to one another in the two species cannot therefore 
 be quite an accurate one ; the germ-plasm of N. paniculata 
 must, on the contrary, contain a larger number of determinants 
 than that of N. rustica, and the process of disintegration of the 
 two species must differ in many ways. 
 
 If an equal number of ids from each of these species occurs 
 in the hybrid, the two kinds of ids will only be able to co- 
 operate in ontogeny as long as their determinants still corres- 
 pond to one another. As soon as a point is reached in 
 which the ids of N. rustica begin to decline in number, and 
 their last determinants have become disintegrated, the ids of 
 N. paniculata will alone be able to produce series of cells ; but 
 as only half the normal number of these ids are present, the 
 structures arising from them cannot be as complete as they are 
 in the pure ancestral form : and, apart from this, the ids of 
 N. ruslica may possibly not have disappeared entirely, but the 
 succession of the determinants, which are, properly speaking, the 
 final ones, may persist beyond the normal period, and may in 
 this way interfere with the development of the pure characters of 
 N. paniculata. This would, at any rate, render the fact com- 
 prehensible in principle that intermediate forms may also arise 
 in cases in which the struggle of the parental ids does not 
 extend into the final cells of ontogeny, and the characters of 
 the species come into contact at an earlier stage, as in the case 
 of the greater or less degree of ramification in plants. The 
 following considerations will make this more evident. 
 
 In the second kind of combination, either the paternal or the 
 maternal characters predominate in all parts of the hybrid, so 
 that the latter bears a closer resemblance to one parent than 
 to the other : in this case, therefore, the transmission is appar- 
 ently monogonic. 
 
 Several cases of this kind have been recorded, in some 
 of which the paternal, and in others the maternal characters 
 predominate. Instances of both kinds occur in the genus 
 Nicotiana. ' The hybrid Nicotiana paniculata 9 x vincaflora $ 
 bears so close a resemblance to the last-named species that the
 
 268 THE GERM-PLASM. 
 
 zharacter of N. paniculata can hardly be recognised at all.' * In 
 this case, therefore, the paternal plant predominates, while the 
 cross between N. suaveolens ? and N. langsdorfiii 'bears little 
 resemblance' to the latter species. Here, then, the maternal 
 characters predominate, and the hybrid plants are 'extremely 
 similar' to N. suaveolens, and are only to be distinguished from 
 it by a partial separation of the stamens from the corolla-tube, a 
 slight difference in the colour and size of the flowers, the violet 
 or bluish coloration of the antlers, and by complete sterility.' 
 
 On p. 474 of his book, Focke gives further instances. ' In 
 many cases the resemblance of the hybrid to one of the ancestral 
 forms is so close, that it might easily be taken for a minor 
 variety of this form.' Thus in the following instances the hybrid 
 resembles much more closely the parent form mentioned after 
 each : Dianthus armeria x deltoides, D. deltoides; Dianthus 
 caryophyllus x chinensis, D. caryophyllus ; Melandryum rubrum 
 x noctiflorum, M. rubrum; Verbascum blattaria x nigrum, V. 
 nigrum; Digitalis purpurea x lutea, D. lutea. 
 
 These cases seem to me to be of special importance on account 
 of the conclusions which can be drawn from them respecting 
 perfectly similar cases of individual transmission in Man. This 
 1 pseudo-monogonic transmission,' as I shall call it, must be 
 explained in terms of the idioplasm somewhat as follows. 
 
 The predominance of one parent e.g., the mother might be 
 due to the presence of a larger number of idants and ids of the 
 species in question. If, for instance, Digitalis lutea possessed 
 thirty-two idants and D. purpurea sixteen only, the idants in 
 both cases consisting of the same number of ids, and if the 
 controlling force of the ids were the same in both species, the 
 ids of D. lutea would then predominate over the others in 
 every cell during the entire ontogeny ; that is to say, the cha- 
 racter of D. lutea would be impressed more strongly that that 
 of D. purpurea on the cell. The objection might be raised that 
 the cell which is thus produced must possess an intermediate 
 character, and cannot be a pure cell of D. lutea, even though it 
 resembles the latter most closely. As yet, however, we are 
 unable to determine to what extent intermediate forms of indi- 
 vidual cells may occur in individual cases ; and the expression 
 'resultant,' in connection with the unknown forces of the 
 
 * Focke, loc. cit, p. 289.
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 269 
 
 biophors, does not indicate anything beyond a mere abstract 
 idea, which is in any case totally insufficient as an explanation 
 of the phenomena. We must be satisfied with the statement 
 that, when the controlling forces of the two determinants acting 
 together in the cell are very unequal, the effect of the weaker 
 will be extremely small under certain circumstances. A 
 ' struggle betiveen the biophors' takes place, in which we may 
 suppose that the stronger assimilates, grows, and multiplies 
 more quickly, and thus deprives the weaker of room and nourish- 
 ment, prevents its multiplication, entirely destroys it, and even 
 makes use of it as nourishment. Without a very considerable 
 multiplication, the crowd of biophors which migrate from the 
 nucleus into the cell-body cannot, indeed, exert any determining 
 influence on the latter. It therefore seems to me to be conceiv- 
 able that an apparent pseudo-monogonic transmission i.e., 
 a complete suppression of the elements of one parent may take 
 place even when exactly the same number of ids are derived 
 from both parents. 
 
 This is still more likely to be the case if the number of ids 
 derived from one parent is greater than that from the other. 
 We know that the number of idants may vary considerably, 
 even in allied species, and it is therefore not improbable 
 that ' pseudo-monogonic ' heredity is sometimes due to this fact. 
 In many plant-hybrids this assumption can be tested directly by 
 ascertaining the number of idants present. 
 
 The third of the above-mentioned kinds of combination of 
 parental characters seems to me to be theoretically almost the 
 most important of all, for it is most intimately concerned with 
 the ultimate processes which take place in the idioplasm. In 
 this case the parts of the hybrid plant most nearly resemble those 
 either of the paternal or the maternal form. 
 
 Such cases are apparently not often very pronounced, but 
 fluctuations from the paternal to the maternal side occur to a 
 slight extent in almost all those hybrids which are usually 
 described as intermediate forms. The hybrid between N. 
 paniculata $ and N. rustica 9 .which Kollreutter considered to be 
 a pure intermediate form, bears, as mentioned above, a somewhat 
 closer resemblance to the former species as regards the length, 
 and to the latter as regards the diameter, of the corolla-tube. 
 Such slight fluctuations to one side or the other from the pure 
 intermediate form seem to be of frequent occurrence. Cases
 
 270 THE GERM-PLASM. 
 
 are, however, also known in which these fluctuations are so 
 considerable as to become conspicuous. Sometimes, for 
 instance, the hybrid resembles one species as regards its leaves, 
 and the other with regard to its blossoms. Brandza* has 
 recently examined the general, as well as the microscopic 
 structure of certain hybrids, in order to ascertain the parental 
 characteristics exhibited by them, and has succeeded in proving 
 that a fluctuation of this kind exists. Marrubium vaillantii, a 
 cross between Leonurus cardiaca and Marrubium vulgare, for 
 instance, exhibited the winged leaf-stalk characteristic of M. 
 vulgare, while the arrangement of the vascular bundles of the 
 petiole resembled that existing in Leonurus. The upper surface 
 of the petiole bore branched hairs like those of Leonurus^ while 
 on the lower surface the hairs resembled those of Marrubium. 
 
 I shall speak of this fluctuation in the resemblance of the 
 organs to those of the parents as ' the shifting of the hereditary 
 resultants in ontogeny} 
 
 We might imagine, a priori, that such a shifting cannot 
 possibly occur. If, as has been proved, transmission is virtually 
 completed at the time of fertilisation, and if therefore the 
 relative proportion of the two parental idioplasms is also fixed 
 for all the subsequent ontogenetic stages when they have once 
 been combined, we might expect a similar combination of the 
 characters of the parents to appear in all parts of the young 
 plant, so that these would either be exactly intermediate 
 between those of the two parents, or else that the paternal or 
 maternal characters would eveiy where predominate in a similar 
 manner. The fact that this cna\ not be, and in fact is usually 
 not the case, may be accoun'ea for in several ways. 
 
 In the first place, \vt must bear in mind the statement made 
 bse that a combination of two characters in equal proportions 
 need not always give rise to one which is apparently inter- 
 mediate, and that it is impossible to give an exact definition of 
 what is meant by such a character, because we only observe the 
 final effect of forces acting within the cell, and not the actual 
 processes by which this effect is produced. This, however, 
 would not be a true shifting of the hereditary resultants, but 
 only an apparent one. 
 
 Genuine cases of such a shifting, however, undoubtedly occur; 
 
 Brandza, ' Compt. rend.' 1890, T. Ill, p. 317.
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 271 
 
 and I believe that they are principally due to the fact that the 
 number of homodynamous determinants in the idioplasm of a 
 cell may -vary in the course of ontogeny, and that, in fact, it must 
 always do so. In one stage or in one organ the paternal, and 
 in another the maternal ids will contain the majority of homo- 
 dynamous determinants, and the fluctuations in the predominance 
 of the maternal or paternal characters, which is definitely deter- 
 mined in advance, must depend on this fact. 
 
 To make this clear, I must intrench somewhat upon the 
 chapter on Variation. 
 
 b. Intercalary Remarks on Variation. 
 
 Hitherto we have assumed that the germ-plasm of a species is 
 composed of ids, each of which contains all the characters of 
 the species. A brief consideration, however, shows that this 
 cannot be the case. Not only is it conceivable, but it is even 
 necessary to assume that the development of the characters of 
 the species, as well as of those of the individual, is only the 
 expression of the forces to which these processes are due, the 
 ids in which the forces are situated being by no means perfectly 
 similar to one another. A great majority of the ids certainly 
 contain all the determinants of the species, that is, they are 
 capable of giving rise to all the specific characters ; but in a 
 minority of the determinants, to which, indeed, the origin and 
 development of this species was due, will not yet have begun to 
 undergo a phyletic transformation. All the determinants in this 
 minority need not necessarily be similar to one another ; one 
 id, for instance, may contain unmodified determinants of the 
 ancestral species ; while another, although still retaining some of 
 the old determinants, may exhibit a greater resemblance to the 
 pure ids of the existing species, and so on. Such a gradual 
 transformation of the ids affecting the majority of their deter- 
 minants must constitute, indeed, the process of the formation of 
 the species ; and it will be quite in accordance with the principle 
 of variation to assume that modifications must take place in 
 the invisible vital units of a lower order, viz., the ids and 
 biophors, in exactly the same way as they occur in different 
 degrees and directions in different individuals in the visible 
 vital units, the unicellular organisms and persons of stocks. 
 
 Thus the provisional hypothesis put forward above, that the 
 germ-plasm of a species as far as it concerns the specific tha-
 
 272 THE GERM-PLASM. 
 
 racters consists of a number of identical ids, cannot be a strictly 
 correct one: the germ-plasm must, on the contrary, contain a 
 majority of completely modified ids provided with new deter- 
 minants of the species \ and of a minority of ids of the ancestral 
 species which are only slightly or not at all modified. The 
 number of the latter will diminish in the course of time in 
 consequence of selection of the individuals, and the new specific 
 characters will gradually lose their original variability. Owing 
 to natural selection, the germ-plasm will gradually be relieved of 
 those of its ids which have become only slightly or not at all 
 modified in the new direction, for those individuals in which it 
 still contains a large number of unmodified ids are as well 
 adapted as the others for the conditions of existence. As these 
 individuals are gradually eliminated in the struggle for existence, 
 the number of unmodified ids in the subsequent generations 
 must gradually become reduced ; and this process of selection in 
 the germ-plasm will only reach a limit when the number of 
 unmodified or incompletely modified ids has become so small that 
 their influence on the development of the essential characters of 
 the species is inappreciable. 
 
 This process of transformation of the ids of the germ-plasm 
 will, however, reach a limit, as do all processes of selection, when 
 its continuance is no longer of any advantage. All adaptations 
 in an animal remain stationary and are not further perfected, 
 directly further improvement becomes useless ; and in the 
 same way the process of modification and elimination of the 
 ids, which forms the basis of other adaptations, will cease 
 as soon as the completely modified ids are present in such a 
 majority that the others can only exert an inappreciable influ- 
 ence on the nature of the offspring. The useful and adaptable 
 characters will always undergo complete development in the 
 normal process of reproduction, that is to say, in the inter- 
 mingling of individuals of the species, even though some unmo- 
 dified ids may be present in the germ-plasm. Let us take as 
 an example the well-known case of the butterfly Kallimaparal- 
 lecta, which resembles a leaf. The resemblance is very marked, 
 although not a complete one. The form of a leaf, with its 
 midrib and veins, some regions appearing more or less faded, 
 and some dry or wet, and even of the appearance of a devvdrop, 
 is indicated on the folded wings, but<?#/y certain of the secondary 
 veins on the right and left of the middle line are represented
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 273 
 
 This is quite sufficient to deceive the birds which pursue the 
 butterfly, and a more accurate copy of the markings on a leaf 
 would not increase the deception ; for it only needs to be effective 
 at a certain distance, and therefore has not increased in perfec- 
 tion, but has reached its limit at this stage. A precisely similar 
 occurrence must take place as regards the modification of the 
 ids of the germ-plasm if a new adaptation of the species is 
 concerned. In such a case, again, only a relative and not an 
 absolute perfection will be attained. The majority, but not all, 
 of the ids may become modified by selection, while a minority 
 must accompany these through long periods and generations in 
 an unmodified or slightly modified condition. 
 
 I have had to make this digression on the transformation of 
 species in its relation to the germ-plasm, in order that my ex- 
 planation of ' the ontogenetic shifting of the hereditary resultant ' 
 may be comprehensible. It follows from the process of the 
 gradual transformation of the ids of germ-plasm and the trans- 
 ference of unmodified ids from one cell-generation to another, 
 that the germ-plasm of every species must consist of a com- 
 bination of ids of a somewhat different nature. For although 
 specific characters must have appeared simultaneously, and 
 many others must have arisen successively, in the course of 
 many generations, all the characters of the species will not be 
 represented by determinants in the same number of ids. The 
 oldest character, in fact, will be contained in almost all the 
 ids ; those which are somewhat younger, in a considerable 
 majority ; and still younger ones, perhaps also in a slight ma- 
 jority ; while those which have only just begun to be of use 
 to the species will only be present in a minority of the ids of 
 each individual. 
 
 This circumstance is evidently connected with the degree of 
 variability of characters, which may, in fact, be a very different 
 one as regards the different characters of a species. Characters 
 on which selection is only beginning to act, can only be 
 represented by a majority of ids in a minority of individuals, 
 and the less variable characters are those which have been 
 selected for a greater number of generations, and are 
 therefore present in a large number of ids of numerous 
 individuals ; and again, those characters which have long 
 become definitely fixed in all or almost all the individuals of a 
 s
 
 274 THE GERM-PLASM. 
 
 species, must be also represented in the majority of the ids in 
 almost all these individuals. Conversely, those characters which 
 are beginning to become useless to the species must be contained 
 only in a gradually increasing minority of the ids ; the number 
 of the latter must gradually decrease, until it finally becomes so 
 small that it can no longer give rise to the character in question 
 in consequence of an overwhelming majority of the other ids. 
 
 The above conception of the germ-plasm enables us to 
 understand why the force of heredity may vary in the course 
 of ontogeny in the case of crosses between two species, and why 
 the paternal tendency may dominate in one character and the 
 maternal in another. For, quite apart from the possible degree 
 in which the force of heredity may be present in the individual 
 determinants, and assuming it to be equal in the two 
 species, the number of ids which contain homodynamous 
 determinants will nevertheless vary according to the age of 
 the character in question. A greater number of ids containing 
 homodynamous determinants will indicate a greater force of 
 heredity. If the form of the flower in a species A was acquired 
 long ago, while that of the leaf is a new acquisition, and if the 
 reverse is true in the case of a species B, the hybrid which 
 would be obtained by crossing these two species would resemble 
 species A in the form of its flowers and species B in that of its 
 leaves. A larger number of homodynamous determinants of 
 species A, for the rudiments of the flowers, will be opposed to 
 a smaller number of homodynamous determinants of the species 
 B, which, on the other hand, will contain a larger number of 
 homodynamous determinants for the rudiments of the leaves. 
 The importance of the whole principle will be made still more 
 evident in the following section. 
 
 c . The Struggle of Individtial Characters. 
 
 The question of individual characters in the two parents has 
 not been taken into account in our previous investigations : in 
 crosses between different species they may be looked upon as 
 insignificant when compared with the specific characters. We 
 must now consider those cases in which the two parents differ in 
 respect of slight individual characters only, confining ourselves 
 to the human race, the individual characters of which we can 
 recognise most clearly. 
 
 What strikes us most forcibly in connection with the process
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 275 
 
 of reproduction and transmission in man, when compared with 
 that of the formation of hybrids in the vegetable kingdom, is the 
 dissimilarity between children born of the same parents. In the 
 case of plant-hybrids, a striking constancy is observable in the 
 offspring of a cross, and this is true not only of the offspring of 
 the same parents, but also of all the hybrids produced by cross- 
 ing any individuals of the two species in question, if the latter 
 also are constant. 
 
 The dissimilarity between the children of the same parents has 
 been already mentioned, and was explained as resulting from 
 the halving of the germ-plasm in the process of 'reducing 
 division,' which takes place in a different manner each time, 
 and, when a larger number of idants are present, gives rise to 
 a surprising number of combinations. As the idants are very 
 different with regard to the individual primary constituents 
 they contain, new combinations of the latter are thus con- 
 tinually being formed without affecting the characters of the 
 species. 
 
 Three principal kinds of combination have to be considered 
 in any attempt to explain the blending of parental characters in 
 the child, that is to say, to refer it to processes which take 
 place in the idioplasm : these are (i) the characters of the child 
 are intermediate between those of the parents; (2) the child 
 exclusively or principally resembles one parent; and (3) the 
 child resembles the father as regards some characters and the 
 mother in respect of others. 
 
 The first case, if it ever strictly occurs, must be attributed to 
 the presence of a precisely similar controlling force in all the 
 determinants. 
 
 The number of idants derived from each parent must be the 
 same in this case, as the parents belong to the same species, 
 and there will certainly also be very little difference as regards 
 the number of ids ; the number of determinants, moreover, will 
 be the same, or almost the same, in the germ-plasm of the two 
 parental germ-cells. Theoretically, an exactly intermediate 
 form would therefore result if each determinant of the father 
 and mother were homologous to one another, and if the homo- 
 logous determinants were controlled by precisely similar forces, 
 i.e., if they contained the same number of biophors, and the 
 homologous biophors of either side possessed the power of 
 assimilation and reproduction to the same extent. These con-
 
 276 THE GERM-PLASM. 
 
 ditions which we have presupposed will, however, hardly exist 
 all together, but they may concur approximately with regard to 
 a certain number of characters. 
 
 It can hardly be doubted that the second kind of combination 
 occurs, and that in some cases the offspring take after one or 
 other of the parents only, not only in respect of those character- 
 istics with regard to which the term 'resemblance' is generally 
 applied, such as those which concern the form and expression 
 of the face, but equally as regards stature, form of the body, 
 proportions of the limbs, nature of the skin and hair, character, 
 and temperament. 
 
 We are met with a two-fold difficulty in attempting to explain 
 these facts : in the first place, how is it possible for all the 
 primary constituents of one parent e.g., the father to be present 
 in one of the germ-cells of this parent, as the germ-plasm was 
 halved by means of the reducing division before these germ- 
 cells become ripe ? and secondly, how can it happen that the 
 maternal germ-plasm exerts no influence on the formation of 
 the child ? 
 
 Let us first consider the former of these two difficulties : how 
 is it possible for all the characters of the father to be con- 
 tained in one paternal germ-cell in spite of the reducing 
 division ? If the latter process resulted in a quantitative halving 
 of the germ-plasm, no further explanation would be necessary, 
 for the quality of half the mass might be exactly similar to that 
 of the whole. But a reduction of the units of the germ-plasm 
 to half their number occurs in this process ; the number of ids 
 is reduced by one half, and the structure of the offspring results 
 from the combination in the germ-plasm of the ids of both parents, 
 as was shown in the case of plant-hybrids ; it is difficult, therefore, 
 to understand why half the number of ids can nevertheless give 
 rise to all the characters of the parent. Strictly speaking, it is 
 immaterial whether we concern ourselves with all the characters, 
 or with only a single one ; for many characters, in fact, depend 
 on the co-operation of all the ids of the ontogenetic stages in 
 question. 
 
 There is only one way out of this difficulty, we must accept 
 the assumption, which has been confirmed by fact, that the 
 controlling power of the ids of one of the parents may become 
 nullified at every ontogenetic stage. Observations on plant- 
 hybrids are invaluable in this connection, for in them we know
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 277 
 
 for certain that the ids of another species are present, even 
 although they may not produce a perceptible effect. We may 
 conclude from the phenomena of ' pseudo-monogonic ' heredity 
 exhibited by these hybrids, that the ids of one parent may, as 
 has already been explained, prevent those of the other from 
 taking part in the control of the cell, group of cells, or part of 
 the body. 
 
 We may therefore assume that if by means of the reducing 
 division, all the idants which controlled or dominated the 
 ontogeny of the mother, for instance, reach one of the germ-cells 
 produced by this parent, this germ-cell will be capable under 
 certain circumstances of reproducing the maternal ' type ' * in 
 the child. But in order that this may happen, it is necessary for 
 this cell to unite with a sperm-cell, the germ-plasm of which 
 possesses on the whole a weaker controlling power than its own, 
 so that the germ-plasm of the father is dominated by that of 
 the mother. 
 
 As in the case of crosses between species, the controlling 
 power of the idioplasm will not always be dependent on the 
 same cause. 
 
 We cannot enter more deeply into those cases in which a more 
 marked force of heredity of individual characters occurs. Darwin 
 mentions, for instance, that the ' white ' colour, in flowers as well 
 as in animals, is very commonly transmitted to the offspring 
 when white individuals are crossed with dark-coloured ones, the 
 majority of the descendants inheriting the white colour. We 
 can in this case only assume that those biophors which do- 
 minate the cell, and give rise to the white colour, must be 
 'stronger' than those which cause the formation of pigment, 
 and this ' strength ' must, in fact, be attributed to the possession 
 of a greater power of assimilation. 
 
 The case is different in many other instances in which the 
 greater hereditary power is attributable to quantitative differ- 
 ences in the constitution of the paternal and maternal groups of 
 idants. 
 
 The number of ids contained in each idant is certainly 
 constant, or nearly so, in all individuals of the human 
 
 * I shall make use of the term ' type ' (' Bild '), to express the whole aggre- 
 gate of essential characteristics which together constitute the individuality 
 of a human being.
 
 278 THE GERM-PLASM. 
 
 species ;* but a predominance of paternal or maternal ids may 
 nevertheless exist, so that there may be a greater number oj 
 homodynamous determinants in one line of ancestors than in the 
 other, as has already been shown to be the case in hybrids. 
 
 By the term homologous determinants, we understand those 
 elements of the idioplasm which are capable of so controlling 
 homologous parts of the body or determinates ; homodynamous 
 determinants, on the other hand, are those of the homologous 
 determinants which have the special function of impressing a 
 like character on any part of the body, and which, taking an 
 example we have already made use of, serve to produce a 
 particular form and colour in a certain region of a butterfly's 
 wing. That parent in which the ids contain numerous homo- 
 dynamous determinants, or at any rate a larger number than 
 are contained in the ids of the other parent, must undoubtedly 
 exert the greater controlling power. The power of homo- 
 dynamous determinants is simply cumulative, whereas dissimilar 
 or heterodynamous determinants may, in the most favourable 
 cases, co-operate to form a single resultant, but may, under 
 certain circumstances, counteract or even neutralise one another. 
 The larger the number of homodynamous determinants which 
 the entire aggregate of ids of a parent contains at any ontogenetic 
 stage, the greater will be the likelihood that these will pre- 
 dominate in the struggle of the parts which takes place in the 
 cell, and will therefore stamp the latter more or less distinctly. 
 
 The colour of the eyes may be taken as an example. Let us 
 take a case in which those of the mother are blue, and those of 
 the father brown. The number of ids in the idioplasm of the 
 pigment cells of the iris will be the same in both parents, but 
 in the case of the father nine-tenths of them, let us say, are com- 
 posed of ' brown ' t determinants, and only one-tenth of deter- 
 
 * It is conceivable that individual fluctations in the number of ids may 
 occur, although the number of idants always, indeed, remains the same if 
 one can judge by its constancy in many animals and plants. Unfortunately 
 we do not yet know the number of idants in the case of man ; I have, at 
 least, been unable to obtain any information on this point. 
 
 t ' Brown ' and ' blue ' determinants are spoken of in this and subsequent 
 passages simply for the sake of brevity. I am not ignorant of the fact that 
 the blue colour of the iris is not due to blue pigment. The above terms 
 merely indicate that the determinant produces a structure in the iris which 
 causes it to appear blue or brown, as the case may be, quite apart from the 
 histological details on which this depends.
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 279 
 
 minants of other colours ; two-thirds of the ids of the mother 
 on the other hand consist of blue, and one-third of brown deter- 
 minants. The iris of the child will therefore most probably 
 be brown, for in its formation nine-tenths of the determinants 
 of the father will co-operate with one-third of the determinants 
 of the mother, which are homodynamous with these. The 
 predominance of the brown determinants would also, however, still 
 be assured if the maternal ids contain no determinants at all of 
 this colour, but only red or green ones, supposing that red or 
 green pigment cells occurred in the human iris. For in this case 
 nine-tenths of the paternal determinants would be opposed to 
 various small groups of heterodynamous determinants of the 
 mother. The latter might possibly modify the brown colour 
 which would be produced by groups of paternal determinants 
 alone, for they also control part of the cell-body ; but it is quite 
 as conceivable that they might be completely overcome by the 
 paternal determinants, and thus excluded from the control of 
 the cell. We cannot, as already stated, at present judge as to 
 the result of the struggle of the parts in individual cases, but 
 there is no doubt that, provided the controlling power of the 
 determinants is similar, the number of the latter is of the first 
 importance. 
 
 Since, therefore, the nature of the combination in the germ- 
 plasm is different in every individual in consequence of sexual 
 reproduction, the number of homodynamous determinants of 
 any particular characteristic must also be different in every case. 
 It will be shown in the chapter on variation that processes of 
 selection may even bring about an increase or decrease in the 
 number of the homodynamous ids of individual characters, 
 although these are never of sufficient biological importance to 
 give rise to specific characters. 
 
 The same competition of forces must take place in the case 
 of individual as in that of specific characters. A child may inherit 
 the colour of the eyes from its father, and the shape of the 
 mouth from its mother, just as in a plant-hybrid the form of the 
 leaf may resemble that in the paternal, and the flower that in 
 the maternal plant. In both instances the character is inherited 
 from that parent in which the group of idants contains a pre- 
 ponderating majority of homodynamous determinants of this 
 character. When this is the case, this majority preponderates 
 over the scattered minority derived from the other parent.
 
 280 THE GERM-PLASM. 
 
 In neither instance, however, does the preponderance of the 
 father, as regards a single character, necessitate that all the 
 other characteristics will be controlled to the same degree 
 by this parent. The germ-plasm consists of an equal number 
 of paternal and maternal idants and ids, which remains 
 constant throughout ontogeny. We suppose that every id of 
 the germ-plasm contains all the determinants of the species, e.g., 
 the determinant a for the character A, the determinant b for 
 the character B, and so on. But all ids of the germ-plasm need 
 not contain a number of homodynamous determinants only ; for 
 if id i, for instance, contains the determinant a 1 , id ii may per- 
 haps contain the determinant a" for the homologous character, 
 and id iii the determinant a 3 , id iv the determinant a 4 , and so 
 on. There is no reason why id i, in which the determinant a 1 
 represents the character A, should not contain the determinant 
 fi instead of b for the character homologous to B. If we denote 
 the determinants of corresponding characters i.e., those which 
 may become substituted for one another by the same letters, a 
 certain id of the germ-plasm e.g., id i might contain the deter- 
 minants a 1 , &, c 1 , d*, e\f*,g e , h*, i 5 , &c. If in the germ-plasm 
 of the father id i, id ii, id iii, id iv, and so on to the last, which 
 I will call id xx, all contain the determinant a 1 for the character 
 A, and none of the variants a 2 , a 3 , &c., are present in them, 
 this determinant a 1 will be more powerful than any other 
 variant of a derived from the mother, which may exist in the 
 idioplasm of the cell in question, provided that the total number 
 of each of these variants is less than twenty. Hence the char- 
 acter A 1 will be impressed on that particular cell or group of 
 cells, and not A* or A 3 . The case may be entirely different as 
 regards the character Bj the determinant b* or ^ 4 , for instance, 
 may be the dominant one in most of the ids and idants : and in 
 this case, one of the other variants of B, such as 3 or S\ will 
 be produced. 
 
 If we may compare the groups of paternal and maternal 
 idants to two centres of force each of which attempts to obtain 
 control over the cell, each of these two forces will be determined 
 by the individual forces of the idants in which they are con- 
 tained, while the force in each idant will be controlled by the 
 individual forces of the ids of which it is composed. If, for 
 instance, there are two idants derived from each parent, each 
 consisting often ids, the following considerations would enable
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 281 
 
 us to ascertain which variant of the character must actually be 
 produced. The paternal idants P l and P 2 might each consist 
 of ten ids, the same determinant a 5 being contained in six ids in 
 P\ and in eight ids in P*. These two idants would then com- 
 bine in the attempt to give rise to the character A s , with a con- 
 trolling force of 6 + 8= 14 ids. Each of the maternal idants M l 
 and M'* might also contain ten ids, M 1 being composed of two 
 determinants a 1 , four a s , three a 7 , and one a 10 ; this idant will 
 therefore tend to produce the character A 3 with a power of four 
 ids only. If now, the other maternal idant M z , with all its con- 
 tained ids, tended to give rise to the character A 6 , with a power, 
 that is, of ten ids, the group of paternal idants would neverthe- 
 less predominate over that derived from the mother, as fourteen 
 paternal are opposed to ten maternal homodynamous deter- 
 minants. In this case both parental groups might possibly 
 control the character of the child together, but the paternal 
 group would be the stronger of the two. If, however, eighteen 
 homodynamous ids of the father were opposed to four homo- 
 dynamous ids of the mother, the influence of the latter would be 
 entirely suppressed as regards the character A. We must at 
 any rate conclude from the facts of the case, that the characters 
 of one parent may be strictly inherited without any apparent 
 intermingling of the corresponding characters of the other 
 parent. As already mentioned, this very point in the theory 
 seems to me to be the most reliable one, and known facts 
 concerning plant-hybrids compel us to accept this assumption. 
 The controlling force of the groups of paternal or maternal 
 idants may be entirely different in respect of individual char- 
 acters and groups of characters, according to the number of 
 homodynamous determinants by which these are respectively 
 represented. Moreover, this depends not only on the fact as 
 to whether the individual character is derived from the father 
 or the mother, or is a mixed one, but also on the entire number 
 of homodynamous determinants present from each parent. 
 
 Although I have all along spoken of idants, I do not wish 
 to imply that every idant behaves as an independent whole. 
 Neither do I suppose that the resultant of the forces of the 
 whole aggregate of paternal, is opposed to that of the maternal 
 ids. It is quite conceivable that the same homodynamous 
 determinants occur in the ids of both parents, and that their 
 forces are cumulat've, just as they would be if present in one or
 
 282 THE GERM-PLASM. 
 
 the other parent only. Homodynamous ids must, indeed, fre- 
 quently occur in both parents in cases of close interbreeding, as 
 well as in species which exist in comparatively small numbers 
 in small isolated districts. 
 
 It is obvious that this struggle of the parental ids takes place 
 at every stage in ontogeny, and that its result is different 
 according to the state in which the forces exist at this stage. 
 This accounts for the frequent changes as regards the resem- 
 blance to one or other of the parents, and for the combination 
 of parental characters which occurs in different parts of the 
 body. 
 
 The facts, so far as they are known to me, apparently agree 
 very satisfactorily with the above explanation. I have attempted 
 to collect fresh evidence bearing upon this question, which till 
 now has not been closely kept in view. This, however, has 
 unfortunately not been so easy a matter as might have been 
 expected, but I will mention some of the facts relating to this 
 point. 
 
 In some cases a child resembles its father in most respects, 
 and takes after its mother as regards a few minor parts. This 
 would be inexplicable if the whole part were not controlled by 
 the resultant of a determinant other than that for the individual 
 portions of this part. The single determinant and its immediate 
 successors, which control the primary cell of the whole part, 
 determine, in the first place, the rate of the cell-division and the 
 primary form of the entire organ ; but in each subsequent stage, 
 one of the succeeding determinants takes on a controlling 
 unction, and as its influence is always the resultant of the 
 homologous determinants of all the ids of the cell, the successors 
 of this cell may at any time differ from one another with regard 
 to their resemblance to those of the father or mother. 
 
 A person of my acquaintance resembles his father very closely 
 in respect of that portion of the skin which is derived from the 
 external germinal layer. In both the father and son the 
 epidermis is thick, and inclined to be horny. The nails of the 
 hands are much thickened, and the skin on the soles of the feet is 
 especially hard. In this case, therefore, the force- resultant of 
 the paternal determinants must continually have been stronger 
 than that of the maternal ones from the primary ectoderm 
 onwards, through a long series of cells. On the other hand, the 
 resultant of the maternal determinants seems to have pre-
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 283 
 
 dominated at the formation of the brain, which likewise arises 
 from the external germinal layer, for the person in question 
 resembles his mother as regards most of the mental qualities, 
 such as intelligence, talents, and strength of will. This can 
 only be accounted for on the supposition that the determinants 
 of the subsequent descendants of the primary ectoderm-cell were 
 unable to exert any influence Tipon this cell itself; in it they 
 were still latent, and in this condition were merely passed on to 
 succeeding cells. If the ids of subsequent generations of these 
 primary ectoderm-cells which formed the rudiments of the brain, 
 contained more maternal than paternal homodynamous deter- 
 minants, a resemblance to the mother instead of to the father 
 would arise at this point. 
 
 In the above case, the entire brain does not seem to have 
 taken after that of the mother, for marked paternal traits also 
 exist in it. According to our theory, it may well be conceived 
 that even such a repeated alternation of hereditary tendencies as 
 has occurred in this instance is predetermined in the germ, for 
 the power of the paternal, and naturally also of the maternal ids, 
 varies throughout ontogeny at every further stage in the division 
 of the ids, and the relation between the controlling forces of the 
 father and mother may be transposed. In general we might 
 even expect that one or other group would predominate in most 
 cases, and that the child is consequently composed of a com- 
 bination of parental characters which varies in different parts. 
 The parts or organs which resemble those of the same parent 
 may also vary very considerably as regards size : this is 
 possibly true of a single cell, as well as of a whole organ or an 
 entire germinal layer, or even of the entire organism. 
 
 This theoretical deduction is in general supported by facts, 
 for a child is rarely or never an exact repetition of either parent. 
 It is by no means easy, indeed, to form a correct estimate with 
 regard to the resemblance between parent and child, for in order 
 to do so, an exact knowledge of both at the same ages would be 
 necessary, and a detailed comparison is only possible between 
 father and son, or mother and daughter. It would therefore 
 be essential to compare photographs of the father and son, at 
 the same ages ; and this, as far as my knowledge of the observa- 
 tions which have been made on heredity extends, has never yet 
 been done. It would, moreover, be necessary to photograph 
 the whole body, and not merely the face.
 
 284 THE GERM-PLASM. 
 
 As far as we can at present gather from the facts, even 
 in those cases in which the resemblance is a close one, 
 the child always differs from the parent either as regards 
 individual characters, or, as is more frequently the case, 
 in undefinable slightly characteristic details, such as the length 
 of the limbs, colour of the hair or eyes, or the quantity of hair. 
 These parts cannot be said to resemble those of the other 
 parent, but they give the impression that the main direction in 
 which heredity has tended has been slightly changed in an 
 undefinable way. A daughter may resemble her mother, 
 for instance, so closely that she is universally said to be an 
 exact image of her mother ; and yet a close comparison will show 
 that the likeness is by no means an exact one, and although the 
 child may not display a single paternal character, there are 
 nevertheless a number of parts which respectively differ from 
 those of the mother. In the case of identical twins, there can 
 be no doubt that many of the minor differences existing 
 between them are due to differences in the germ-plasm, and not 
 to the diversity of external influences. The germ-plasm of both 
 parents, that is to say, has taken part in determining the differ- 
 ent parts of the child, although in the case of one parent this 
 determination is slight and little marked, and has caused a 
 slight deviation from the maternal characters rather than the 
 development of specifically paternal ones. 
 
 If this view is correct, and the germ-plasm of one of the 
 parents alone never determines the formation of the child, it 
 becomes more obvious than ever that even when heredity tends 
 to follow in the direction of one of the parents in the greatest 
 possible degree, the mother and daughter can never resemble 
 one another so closely as do identical twins. Owing to the slight 
 influence of the germ-plasm of the father, the type of the 
 daughter deviates somewhat from that which would have been 
 produced from the maternal germ-plasm alone; and similarly, 
 if the mother owes her nature to the predominance of the germ- 
 plasm of one parent, a slight deviation must have occurred 
 owing to the weaker influence of the other parent. But the 
 whole of the germ-plasm of both grandparents cannot possibly 
 have been contained in the maternal germ-cell from which the 
 daughter arose, for the reducing division causes the removal of 
 half the germ-plasm from the egg-cell before fertilisation takes 
 place. Even if the idants which materially determine the type
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 285 
 
 of the mother remained in the ovum from which the daughter 
 was developed, the group of idants of the other grandparent, 
 which would modify this type somewhat, must necessarily be 
 absent; the types of the mother and daughter consequently 
 cannot exactly correspond, for the double reason that the 
 influence of one grandparent was wanting at the development 
 of the daughter, while that of the father was present in addition. 
 
 The following examples may serve to show in how many 
 different ways the hereditary tendencies of the parents may, in 
 accordance with our theory, be interchanged in the course of 
 ontogeny. In the bilaterally symmetrical human being, all those 
 parts which are not situated in the median line are paired, and 
 the corresponding organs generally behave nearly, if not quite, 
 similarly as regards heredity. If one hand bears a decided 
 resemblance to that of the mother, the same will, as a rule, be 
 true of the other also ; and if the left leg is intermediate between 
 the character of both parents, the right one will also be so in 
 exactly the same degree. Even such a subtle characteristic as 
 the colour of the eyes usually corresponds in both organs ; and 
 even in those cases in which it is intermediate between that of 
 the two parents, the colour only varies slightly in shade in 
 the two eyes. One might be disposed to conclude from this 
 fact that paired organs are represented by a single primary 
 constituent in the germ-plasm. This, however, would, be 
 an erroneous conclusion ; for, apart from the facts already 
 mentioned which contradict such a view, there are exceptions 
 to the rule that paired organs are similar to one another. One 
 brown and one blue eye sometimes occur in dogs, especially 
 in boar-hounds, and I know of a similar instance in the human 
 subject : the father, a brewer in a small Suabian town, has 
 blue eyes, and the mother brown ones, while a daughter of twelve 
 years of age has one blue and one brown eye. 
 
 In addition to these cases, the frequent inheritance of 
 birthmarks and other minor characters on one side of the body 
 only, necessitates the assumption of double determinants for the 
 corresponding -barts of each half of the body. We must therefore 
 imagine that each id of the germ-plasm of bilateral organisms is 
 primarily bilateral, and that all the determinants, indeed, of the 
 whole body are double, even of course including those for the 
 organs which are apparently situated in the median plane, but 
 which actually consist of corresponding halves. In a large
 
 286 THE GERM-PLASM. 
 
 number of animals the two first segmentation-cells of the egg, or 
 blastomeres, correspond to the future right and left halves of the 
 body. The first division of the nucleus of the fertilised ovum 
 must therefore separate the determinants for the right and left 
 halves of the body ; and the occurrence of this process is ren- 
 dered all the more probable from the fact that a longitudinal 
 division of the idants actually occurs, and results in each of the 
 spherical ids being halved. 
 
 The explanation of the striking correspondence of the homo- 
 logous parts of antimerous or paired organs must be referred to 
 the history of the transformation of species. The permanence 
 of this resemblance during the continued crossing of individuals, 
 is naturally due to this approximate similarity of the correspond- 
 ing determinants for the right and left sides in both parents. 
 It is evident, according to our conception of the structure of the 
 idioplasm, that the resemblance of the corresponding parts of 
 the child originates in the antimerous determinants of both 
 parents, for the latter determine the comparative number of 
 homodynamous ids on the paternal and maternal sides, together 
 with the relative 'controlling power' of the idioplasm of the 
 father and mother in the organ in question. And since this 
 must be the same on both the right and left sides, the organ 
 itself must display the same combination of paternal and 
 maternal characters on either side : that is to say, its two sides 
 must be alike. 
 
 I do not think that these facts can be understood by the aid 
 of any other theoretical assumption with regard to the structure 
 of the hereditary substance. The assumption of the existence 
 of pangenes, for example, might certainly explain the circum- 
 stance that a combination of the paternal and maternal charac- 
 teristics does indeed occur in the organ in question, in the 
 external ear, for instance, but it cannot account for the fact 
 that this combination is similar in the right and left ears. 
 
 These very facts seem to me to furnish a further welcome 
 proof of the correctness of the view arrived at by other methods, 
 that the hereditary combination in each part is predetermined 
 from the germ onwards. The right and left ears could not 
 possibly resemble each other, if the relative strength of the 
 hereditary tendencies on both sides were not predetermined for 
 all parts of the child by the nature of the paternal and maternal 
 idants.
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 287 
 
 There are, however, exceptions to this rule. As already 
 stated, the homologous parts of the two antimeres may diffei 
 horn one another ; and such a difference is even frequent in 
 certain animals, though it only affects characters which are ot 
 minor biological importance. Little attention has hitherto been 
 paid to the fact that many of our domestic animals have lost the 
 original synmTetry of the marking of their coats. Piebald cats, 
 dogs, horses, cows, and guinea-pigs are not uncommon, and 
 show that the symmetry of the markings may become com- 
 pletely lost by domestication. This must be owing to the fact 
 that these originally symmetrical patches of colour are, in conse- 
 quence of domestication, no longer of biological importance. If 
 the determinants for these characters varied in different ways on 
 the right and left sides, and the animal in question thus became 
 spotted, no disadvantage would thereby result, and the animal 
 would nevertheless be able to exist and produce offspring. If 
 two individuals with different piebald markings then paired, the 
 asymmetry in the coloration and markings would be increased ; 
 and as a matter of fact in many of our races of cattle no parts 
 of any two animals are alike in this respect, and the same is 
 true with regard to many dogs and guinea-pigs. We know how 
 important these markings and coloured patterns may be for the 
 preservation of individuals and species in the natural condition, 
 and are therefore justified in attributing the retention of the 
 symmetry to natural selection, and its loss to panmixia. 
 
 Certain facts with regard to the metameres, or successive parts 
 of which the body of a segmented animal is composed, show 
 that the maternal or paternal characters may preponderate in 
 different segments. This naturally cannot be proved in the 
 case of Man, as the metameric segmentation only affects "the 
 bones, muscles, and nerves, which are not externally visible. 
 But I think I have observed that consecutive parts, even when 
 they are homologous, may occasionally exhibit different heredi- 
 tary types. A child may closely resemble its mother as regards 
 the arms and hands, and nevertheless may take after its father 
 in respect of the legs and feet. I have endeavoured to ascer- 
 tain whether any definite rules are followed with regard to 
 certain organs which are closely related to one another in 
 ontogeny, according to which these organs would exhibit 
 a similar combination of parental characters ; but the only 
 rule I could discover is that which relates to the symmetry
 
 288 THE GERM- PLASM. 
 
 of the two halves of the body. Apparently all possible combina- 
 tions may actually occur. The form of the skull may resemble 
 that of the father, and the face that of the mother ; or the form 
 of the entire head and face may be like the mother's, while the 
 eyes may be similar to the father's in every detail. The son 
 may, like his father, possess a dimple on the chin, although he 
 takes much more closely after his mother as regards the shape of 
 the face and nose. That the combination of parental character- 
 istics may even extend into far greater details, is shown especi- 
 ally by the remarkable amalgamation of the mental qualities of 
 the parents which often occurs. The intellect and practical 
 talent may be inherited from the mother, and strength of will 
 and unselfishness from the father ; and all these qualities may 
 be contained in one skull, the form of which essentially re- 
 sembles that of one of the parents only. These combinations of 
 mental characteristics of the parents cannot, however, always be 
 definitely analysed, owing in the first instance to the fact that 
 they are not always sharply contrasted in the parents, but more 
 frequently are only different in degree. We may, however, at 
 any rate consider it certain that the brain rarely resembles that 
 of one parent only in all its parts and as regards the most 
 minute details of its structure ; it usually, on the contrary, exhibits 
 ?. combination or alternation between that of the two parents, 
 ind this combination is of the most varied kind. 
 
 In connection with this statement it may be mentioned that 
 no part of the human body is so important as the brain in the 
 struggle for existence, and its importance extends even to the 
 minutest details ; its parts must therefore be subjected to in- 
 cessant processes of selection. In other words, the number of 
 homodynamous determinants in the various parts of the brain 
 must be extremely different in the individual, and must vary 
 extremely in different individuals. 
 
 These statements with regard to the struggle of the individual 
 characters may perhaps be objected to on the ground that they 
 contradict the suppositions with which we started. It may be 
 contended that an alternation of paternal and maternal hereditary 
 parts is rendered possible on the basis of my theory, because 
 the transmission of a paternal character implies that the whole 
 dominant group of idants of the father passed into the germ- 
 plasm of the child at the reducing division, and the transmission 
 of a maternal character necessitates the presence of the whole
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 289 
 
 of the dominant group of the mother. It might be considered 
 improbable that both of these groups should come together in 
 one germ-cell, or that this can occur as often as must be the 
 case in reality, considering the frequency with which a com- 
 bination of the two parental characters exists. 
 
 In reply to this, it may be stated that the meeting of two 
 germ-cells in the process of fertilisation, one of which contains 
 the dominant group of idants of the mother and the other that 
 of the father, must take place from time to time, for every 
 possible combination will occur at one time or another. It 
 must, moreover, not be forgotten that it is extremely difficult to 
 distinguish between the pure individual characters of a parent 
 and those of one of the immediate ancestors of this parent ; but 
 the production of these characters in the latter does not 
 necessitate the presence of the whole of the dominant group of 
 parental idants, for a portion of them will undoubtedly suffice 
 if the character in question is represented by a majority of 
 homodynamous determinants. The characters which alternate 
 are very often not the specifically paternal or maternal ones, 
 but those which in general characterise the mother's or the 
 father's family. These, however, must be represented in most 
 of the ids of the dominant group of idants, and may therefore 
 become apparent even when the reducing division only causes 
 a certain number of idants, instead of the entire dominant 
 parental group, to pass into the germ-cell of the child. 
 
 On the other hand, the frequency with which a child bears a 
 closer resemblance eitherto its fatheror mother and this in many 
 families is actually the rule can also be easily explained by our 
 theory. In such cases, not only can the whole number of dominant 
 idants produce the type of the parent in question with approxi- 
 mate accuracy, but even a majority of them will be sufficient to 
 do so, provided that a large number of ids with homodynamous 
 determinants are contained in them. Many germ-cells will, 
 therefore, contain a sufficient number of the dominant idants of 
 the parent, while others, although perhaps containing an equally 
 large number of these idants, will consist of various combina- 
 tions of ids enclosing relatively few homodynamous determinants. 
 If, then, an idant of the former kind, derived from the father, 
 should, -in the process of amphimixis, meet with one of the latter 
 kind, derived from the mother, the type of the father would 
 predominate in the child, and vice -versa. If two germ-cells of 
 T
 
 290 THE GERM-PLASM. 
 
 the first kind, derived from the father and mother respectively, 
 came together, a mixture of the characters of the two parents 
 would result ; and if two of the latter kind came into connection, 
 none of the special characteristics either of the father or mother, 
 would be recognisable in the child, which would only posses', 
 such characters as are common to the two families. 
 
 4. The force of heredity. 
 
 We have now seen that ' pseudo-monogonic ' heredity is to be 
 explained in terms of the idioplasm as follows : the dominat- 
 ing group of idants from one parent the mother, let us say- 
 passes over completely into the germ-cell of the offspring, and 
 there meets with a weaker group of idants from the father in 
 the process of amphimixis. Although not all, but only a large 
 number of the determinants in the maternal group of idants 
 predominate, a marked resemblance between the mother and 
 child will result. 
 
 The fact that such cases as this occur at all, was taken above 
 as a proof that the combination of ids in the idants persists 
 during ontogeny, i.e., from germ-cell to germ-cell, and that 
 the idants often, or even generally, remain unchanged in the 
 reducing division. It naturally does not follow, however, that 
 precisely that combination of idants which predominated in the 
 ontogeny of the parent must remain unaltered in the germ-cell 
 of the offspring : it may do so, and such cases will frequently 
 occur amongst the thousands of ova or still more numerous 
 sperm-cells which are produced by a single individual in the 
 course of its life. 
 
 In some families it certainly appears as if the perfect type 
 ('Habitus') of an ancestor had been transmitted with great 
 constancy to the children through a great number of generations, 
 and we must therefore assume that the dominant group of 
 idants in the ancestor reappears very frequently in the germ- 
 cells of the offspring. Thus the high forehead, widely-separated 
 eyes, and small mouth of the imperial family of the Caesars, 
 the large and peculiarly hooked nose of the Bourbons, and the 
 projecting lower lip of the Habsburgs, can all be traced through 
 several generations. It is, however, difficult to say whether 
 such similarities are not accidental, or whether our recognition 
 of them is merely due to an incomplete knowledge of the facts, 
 only those descendants being taken into consideration in whom
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 29! 
 
 these family characters were prominent ; but similar observa- 
 tions with regard to animals lead to the conclusion that they are 
 not all due to chance. 
 
 The phenomenon which breeders describe as ' individual pre- 
 potency' comzs under this category. It seems that a marked 
 tendency occasionally exists in certain individuals to transmit 
 their special individual characters to the majority of the off- 
 spring. It has often been observed that individual horses, 
 cattle, sheep, and other domestic animals possess this capacity 
 in a high degree ; and breeders pay enormous prices for such 
 individuals, which must, it is true, excel not only as regards this 
 supposed special power of transmission, but also in respect of 
 certain particular and desirable characters. It is, however^ 
 believed that similar observations have also been made with 
 regard to plants. Vilmorin,* one of the most eminent raisers 
 of plants, at any rate distinguished in his experiments between 
 individuals possessing the capacity of transmitting their own 
 characters to the offspring in a greater and in a lesser degree. 
 The former he called ' bons ttalons] and made use of them alone 
 for purposes of propagation. He could not, however, find out 
 by a mere examination of the plant whether it belonged to this 
 preferred group. This could only be ascertained by examining 
 the offspring, which therefore served as the guides in the selec- 
 tion of the plants for purposes of propagation. 
 
 Darwin,t Prosper Lucas,} and Settegast give many instances 
 of this kind, one of the best known of which is that of the ' otter 
 sheep.' This race was descended from a ram which was remark- 
 able for'having short, crooked legs and a long body. It trans- 
 mitted this peculiarity to many of its descendants, and so 
 enabled the owner to breed a special race of sheep with crooked 
 legs, the advantage of this peculiarity being that they were 
 unable to leap over fences. Similarly, English thoroughbred 
 
 * Quoted from de Vries. loc. cit. p. 88. L. Leveque de Vilmorin. 
 1 Notices sur 1'amelioration des plantes par le semis,' Nouvelle edition, 
 1886, p. 44. 
 
 t Darwin, ' Animals and Plants under Domestication,' Vol. II., p. 40, 
 et seq., London, 1888. 
 
 t Prosper Lucas, 'Traite philosophique et physiologique de l'heredit 
 naturelle dans les etats de santeS et de maladie du systeme nerveux,' Paris, 
 1850. 
 
 Settegast, ' Die Thierzucht,' Bre&lau, 1878, p. 197.
 
 292 THE GERM-PLASM. 
 
 horses owe their superiority to three, individuals, viz., to the 
 Turkish horse ' Byerley,' and the Arabs ' Darley ' and ' Godol- 
 phin ; ' and the celebrated race of Orlow trotters can be traced 
 back to the stallion ' Bars the First.' 
 
 If these animals really possessed a stronger ' force of heredity' 
 in the sense indicated, it must not be confounded with the 
 property of faithful transmission in a race. This property of 
 'breeding true' must be due to the presence of a large majority 
 of homodynamous determinants in the germ-plasm, or, what 
 amounts to the same thing, to the existence of similar, i.e. of 
 'racial,' determinants for every character in most of the ids. 
 The longer a pure race has been kept up, all the individuals 
 which exhibit variations being carefully eliminated, the greater 
 will be the number of ids containing ' racial,' determinants, and 
 the more rarely will variations appear in individuals. 
 
 At present, however, we are concerned with individual, and 
 not with racial characters. These cannot possibly have been 
 contained in a preponderating majority of the ids of the germ- 
 plasm from which the individual arose, for the germ-plasm is 
 composed of paternal and maternal ids. The transmission of 
 the proper ' type ' can in this case therefore only be due to the 
 fact that the group of idants which preponderated in the deve- 
 lopment of the parent is once more present in the germ -cell. I 
 should consequently prefer to account for the so-called pre- 
 potency in transmission by assuming that in some individuals 
 the reducing division simply occurs in such a manner as to 
 separate the paternal and maternal groups oi idants, while 
 ordinarily it results in combinations of idants of all kinds. 
 It is impossible to say at present on what peculiarity of the 
 idants themselves, or of the apparatus for nuclear division, 
 this must depend ; but it can at any rate be stated that the 
 dominant group of idants cannot possibly be contained in every 
 germ-cell of such an individual, even in the most favourable 
 case. On the contrary, it can only be present in half of them ; 
 for, according to our assumption, the reducing division always 
 causes the dominant group of idants to pass into one of two germ- 
 cells only, the subsidiary group passing into the other. This 
 supposition is in accordance with the facts ; for, so far as I 
 know, it has never been observed that all the offspring resemble 
 the parent which exhibited ' individual prepotency,' but this, on 
 the contrary, was only the case as regards some of them. In
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 293 
 
 fact, it is expressly stated as regards the 'otter sheep,' that 
 the offspring of the first ram 'closely resembled either the 
 mother sheep of the ordinary breed, or the ram ; ' and this 
 statement is in correspondence with the theory. Those sperma- 
 tozoa of the ram which contained the dominant group of idants 
 preponderated over the group of idants of the egg-cell, and an 
 'otter sheep' thus resulted; while those which contained the 
 subsidiary group of idants could only tend to produce an 
 ordinary sheep of the ancestral breed. 
 
 A greater ' force of heredity ' is also spoken of in the sense of 
 the prepotency of one race over another. According to Darwin, 
 the short-horn race of cattle seems to possess a particularly 
 marked power of transmission in contrast to other races ; and 
 this power is more marked in the pouter pigeon than in the fantail, 
 so that when these two races are crossed, the characters of the 
 pouter preponderate in the offspring. This preponderance of 
 one race over the other must be due to the same causes as those 
 which produce a much greater resemblance to one of the parents 
 in the case of plant-hybrids, which were discussed in another 
 section of this chapter. In both cases the preponderance may 
 be due to the presence of a larger number of it'ants, of ids, or 
 possibly even of biophors only, in the individual lieterminants. 
 
 5. SUMMARY OF CHAPTER IX. 
 
 It may be advisable before proceeding further to give a short 
 summary of the results arrived at in the present chapter, and to 
 test the soundness of the assumption on which they are based. 
 
 According to my view, the co-operation of the hereditary 
 substances of the two parents in the fertilised egg depends on 
 the presence in each parental germ-plasm of a large number 
 of units, and not of a single one. These units or ids are, 
 moreover, not all similar to one another in the case of each 
 parent, and although in normal sexual reproduction they all 
 contain homologous determinants, they exhibit slight individual 
 differences. The differences between the ids of the two parents 
 need not in any case be greater than those existing between the 
 ids of the father or the mother alone ; it may, indeed, happen 
 that individual ids derived from both parents may be similar to 
 one another, and this is more likely to be the case the oftener 
 inter-breeding has taken place in previous generations. 
 
 Each id of the germ-plasm passes through all the ontogenetic
 
 294 THE GERM-PLASM. 
 
 stages : that is to say, the number of paternal and maternal ids 
 remains constant throughout ontogeny, and every cell is there- 
 fore controlled by an equal number of ids from both parents ; 
 and, moreover, in normal reproduction between individuals of 
 the same species, the same number of paternal and maternal ids 
 take part in the process. 
 
 The fact that the structure of each cell, organ, or part of the 
 body of the offspring is nevertheless not exactly intermediate 
 between that of the corresponding parts of the parents, must be 
 due to the following causes. 
 
 In the first place, an exactly intermediate structure need 
 not necessarily arise even if all the active paternal and 
 maternal determinants of a cell were exactly alike, or homo- 
 dynamous ; for even in such a case, the ' controlling forces ' of 
 the maternal determinants might preponderate over those of 
 the paternal ones, owing to as uperiority as regards the rate of 
 assimilation and multiplication, or in some other respect. The 
 cell-body would then become filled more rapidly by the biophors of 
 the maternal determinants which pass into it from the nucleus, 
 and the multiplication and distribution of the paternal deter- 
 minants would thus be checked. Hence the controlling force 
 itself may be different with regard to the homologous paternal 
 and maternal determinants. 
 
 The differences are, however, still further increased, owing to 
 the fact that the ids, and more especially the homologous 
 determinants derived from either parent, are not always homo- 
 dynamous, but are, in part at any rate, almost always hetero- 
 dynamous. The controlling force of the homodynamous 
 determinants must, however, necessarily be cumulative, and the 
 inequality in the force of heredity of the father and mother at 
 any particular stage in ontogeny, is essentially due to the fact 
 that although the number of ids is the same, the number of 
 homodynamous determinants i.e., those which have a cumula- 
 tive effect is different. 
 
 It was deduced from the consideration of the phylogeny of 
 variation, that the determinants must have varied independently 
 of each other in the different ids of a germ-plasm, so that the 
 homologous determinants may be present in very different 
 variants in their ids ; and that, moreover, these variants of the 
 determinants in the different onto-idic stages may be combined 
 in a special way in each id. It therefore results that the
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. 295 
 
 number of homodynamous determinants may vary in the 
 different stages of ontogeny, and consequently the paternal and 
 maternal hereditary tendances may preponderate according to 
 the stage or organ under consideration. 
 
 The fact that the offspring may resemble one parent much 
 more closely than the other, is not incompatible with the fact that 
 only half the total number of ids of this parent are present in 
 each of its germ-cells, for all the ids only co-operate when 
 strictly intermediate structures are formed ; but in those cases 
 in which the ids of one parent are overpowered and rendered 
 inactive by a preponderating majority of homodynamous ids of 
 the other parent, the control of the cell is effected by the successful 
 or ' dominant' ids, and the others exert no influence. If, there- 
 fore, the determinants of one parent, in very numerous stages 
 of development, preponderate in this manner over those of 
 the other, an instance of apparently monogonic heredity will 
 result, and the offspring will bear a closer resemblance to this 
 parent ; and if, moreover, the ' reducing division ' should happen 
 to take place in one of the germ-cells of this offspring in such a 
 way as to result in those ids which were 'dominant,' and con- 
 trolled the development of this offspring remaining together in 
 the germ-cell, they might possibly preponderate in the next 
 generation over the ids introduced by another parent in the 
 process of amphimixis. 
 
 The problem concerning the possibility of the offspring bear- 
 ing a much closer resemblance to one parent than to the other 
 in spite of the fact that the hereditary substance of both parents 
 is contained in the fertilised ovum, has already been stated in 
 my essay on 'Amphimixis,' and its solution is to be sought in 
 the struggle of the ids which takes place in every cell in the 
 entire course of ontogeny. This struggle, however, only occurs 
 when the determinants become active, and presumably concerns 
 the biophors which pass into the cell-body, the stronger ones 
 annihilating those with a lesser power of assimilation. It does 
 not concern the determinants which are still ' unalterable,' and 
 are inactive as regards the control of the cell. Moreover, the 
 struggle does not occur between the elements of the ' reserve 
 germ-plasm] which brings about the formation of the germ-cells 
 of the offspring ; and we can therefore understand that the off- 
 spring does not by any means only produce germ-cells con- 
 taining the group of ids which controlled, or was 'dominant'
 
 296 THE GERM-PLASM. 
 
 at, its ontogeny, but many other combinations of ids may be 
 contained in its germ-cells. 
 
 In this connection I should like to call attention to an inter- 
 esting essay which appeared when I had almost finished putting 
 the final touches to my manuscript. It bears the pseudonym 
 ' Josef Miiller'* on the title-page, and contains in particular an 
 attempt to solve the problem discussed above. The ingenious 
 author, who is accurately acquainted with the subject he treats 
 of, doubts my hypothesis of the ids, but endeavours to account 
 for the very remarkable disappearance of the hereditary ten- 
 dencies of one of the parents in ' pseudo-monogonic ' heredity 
 by supposing that the two homologous primary constituents 
 ('Anlage') of the father and mother respectively take part in a 
 struggle ('gamomachia'), which results in the destruction and 
 complete consumption (' gamophagia ') of one of them. In 
 principle this explanation of the problem approaches very 
 closely to the solution I have attempted to give, and though I 
 consider the fundamental idea it contains to be correct, I do 
 not think that we may suppose, as the author does, that this 
 struggle occurs at the beginning of ontogeny. Basing this con- 
 clusion on a statement made by Oscar Hertwig, from which it 
 is conceivable that the homologous 'primary constituents' of 
 the parents unite in the process of fertilisation, he further con- 
 cludes that the struggle takes place during this union, and leads 
 to the destruction of one of them. Apart from the fact that the 
 paternal and maternal idants remain separate during fertilisation, 
 it seems to me that a large number of the phenomena of heredity 
 contradict the idea of such a union and subsequent struggle. 
 The reappearance of the 'destroyed' primary constituent in the 
 germ-cells and consequently in the next generation, the phe- 
 nomena of reversion which show that every primary constituent 
 must be present in more than two variants in the germ-plasm, 
 and, finally, sexual dismorphism, the occurrence of a large 
 number of very different hermaphrodite structures in certain 
 cases, and sexual reversion, all tend to disprove such a 
 hypothesis. Moreover, apart from my theory of the ids, I 
 believe that this struggle of the homologous primary constituents 
 must occur in the individual cells, in which the decision both 
 
 * Josef Miiller, ' Uber Gamophagic, ein Versuch zum weiteren Aufbau 
 der Theorie der Befruchtuntr u. Wrprhunp;.' Stuttgart, 1892.
 
 EFFECTS OF AMPHIMIXIS ON ONTOGENY. ^g^ 
 
 as regards the preponderance or suppression of certain of the 
 primary constituents, and the number derived from each parent 
 which are to become effective, takes place. For it does not 
 appear to me to be essential that any one of them must be 
 entirely suppressed, although this will probably occur in most 
 cases. 
 
 If I am not mistaken in my interpretation of a statement 
 made by de Vries, there is no doubt that the primary constitu- 
 ents from both parents may undergo development in one and 
 the same cell. By crossing a red-flowered with a white-flowered 
 species of bean, this observer obtained a hybrid with pale red 
 blossoms, on which the red colouring matter could be recog- 
 nised in solution in the vacuoles of the cells.* If farts only of 
 the cells were coloured, while other parts were colourless, it 
 proves that at least two different (hcterodynamous) kinds of 
 biophors, derived from both parents, may control the same 
 cell. There is here, however, an extensive field for further 
 investigation. 
 
 My explanation of the process of mingling of the parental 
 characters is based on the assumption of hereditary units or 
 ids, each of which contains the whole of the 'primary con- 
 stituents ' of the species, which are, however, modified in the 
 individual. In this connection it may therefore be as well once 
 more to summarise the reasons which lead to this assumption. 
 
 In the first place, such an assumption naturally follows from 
 the view that the germ-plasm is made up of 'determining parts' 
 or determinants, for the latter necessitate a definite architecture 
 of the germ-plasm. There must therefore be at least one limited 
 unit of the germ-plasm, to which nothing can be added and 
 from which nothing can be removed without producing an altera- 
 tion in its capacity for directing ontogeny. But since the process 
 of amphimixis unites the paternal and maternal germ-plasms, 
 each of which contains all the primary constituents of the species, 
 each being which is produced sexually must contain at least two 
 ids in its germ-plasm. 
 
 The phenomena of reversion, which will be treated of in 
 greater detail in the following chapter, show that there must be 
 several, and indeed many, ids in the germ-plasm of each indi- 
 
 * Cf. de Vries, I.e., pp. 177, 178. The two species referred to are 
 Phaseolus multiflorus and Phaseolus vulgaris nana.
 
 ?9? THE GERM PLASM. 
 
 vidu.il. We know !hat the personal characters of the grand- 
 parents, as well as those of the parents, may reappear in the 
 offspring, and we may therefore conclude that hereditary units 
 or ids derived from the grandparents must be present in the 
 germ-plasm of this offspring, and that it must therefore be 
 composed of more than two ids. 
 
 A similar conclusion is arrived at on other grounds. If the 
 assumption of hereditary units in the form of ids is once made, 
 it follows as a matter of course that their number must be 
 doubled in each process of amphimixis ; and it becomes evident 
 that this number must have increased enormously, in arithmeti- 
 cal progression, if the ' reducing division ' had not intervened 
 and reduced it to the half before each occurrence of amphimixis. 
 This ' reducing division ' must have appeared at a certain stage 
 in the phylogeny of amphimixis. If it arose in the germ-cells 
 of the first animal which was produced sexually, supposing 
 that the germ-plasm of each of the parents previously consisted 
 of only one id, it would always have caused the removal of 
 the id of one parent from each germ-cell of the offspring, and 
 thus no grandchild could ever have inherited characters from 
 both grandparents. According to our theory of the presence 
 of a large number of ids, such a case would seldom occur, 
 although it is apparently not impossible. A further consequence, 
 however, would be seen in an unusually great uniformity in the 
 structure of consecutive generations ; for if only two ids were 
 present, one of which was always removed in the next genera- 
 tion, the same individual ids would pass through a great number 
 of generations, and the diversity of the individual, such as occurs 
 to so great an extent in the human race, would be extremely 
 limited. It is, however, just this extraordinary individual diversity 
 which seems to me to be due to the multiplicity of the ids ; it 
 could not have been produced by only two ids taking part in 
 the process of amphimixis. 
 
 Finally, as soon as we have recognised, on theoretical grounds, 
 the existence of ids at all, the fact that a number of them exist 
 in the cell is supported by direct observation. For whether 
 they correspond to the ' chromosomes' of other writers, which I 
 speak of as idants, or to the ' microsomes,' of which the chromo- 
 somes are composed, as I assume to be the case, a large number 
 of ids may always be observed to be present in the cell.
 
 THE PHENOMENA OF REVERSION. 299 
 
 CHAPTER X. 
 
 THE PHENOMENA OF REVERSION IN THEIR RELATION 
 TO AMPHIMIXIS. 
 
 i. REVERSION TO RACIAL CHARACTERS IN PLANT- HVBRIDS. 
 
 By the term reversion, is meant the appearance of characteris- 
 tics which existed in the more remote ancestors, but were absent 
 in the immediate ancestors ;>., the parents. 
 
 The facts relating to these phenomena are familiar enough, 
 and I shall therefore only refer to as many of them as are 
 necessary for the further development of my theory. 
 
 The simplest case of reversion occurs in hybrids. It occasion- 
 ally happens that hybrids which have been fertilised by their 
 own pollen, produce offspring some of which more or less 
 resemble only one of the two ancestral species. In such cases, 
 therefore, a simple reversion to a grandparent takes place. In- 
 stances of this kind certainly occur, though not in all hybrid- 
 plants ; nor are they often met with even in those species in 
 which they do occur. On this point Darwin quotes two contra- 
 dictory statements made by Wichura and Naudin respectively, 
 the former of whom never observed instances of reversion in his 
 specimens of willow-hybrids, while the latter insisted strongly 
 on the frequent occurrence of reversion in the Cucurbitacece. 
 Darwin thought that this contradiction is explained by Gartner's 
 statement that reversions seldom occur in hybrid-plants raised 
 from wild species, but are of frequent occurrence in those 
 produced from cultivated species. Opinions on this point have 
 since undergone some modification, for Focke states that 
 ' without the influence of the pollen of the parent-species, com- 
 plete reversions to the ancestral form occur practically only in 
 hybrids of closely allied races.' Instances of reversion of this 
 kind do therefore at any rate occur. 
 
 Such cases can easily be explained on the basis of our theory. 
 The germ-mother-cells of the hybrid contain a group of idants 
 derived from the paternal, and another from the maternal
 
 300 THE GERM-PLASM. 
 
 ancestral species. If therefore the ' reducing division ' halves the 
 germ-plasm of these mother-cells in such a manner that idants 
 of the mother alone reach one ripe germ-cell, and those of the 
 father alone are contained in another, it is possible that two 
 such germ-cells may unite when fertilisation takes place between 
 these hybrids. In such a case a plant completely resembling 
 one of the ancestral forms would arise, for it would have been 
 produced from a germ-plasm which contain idants of this species 
 only. As however such cases do not often occur, we may con- 
 clude that the reducing division only rarely effects such a com- 
 plete separation of the paternal and maternal groups of idants, 
 and that, in fact, as a rule, both paternal and maternal idants 
 are distributed to each of the four germ-cells produced by the 
 mother-germ-cell. As this halving of the germ-plasm occurs, as 
 we have seen, in a different manner in different instances, we 
 may presuppose that it will also exhibit differences with regard 
 to the proportion of paternal and maternal idants which come 
 together in each germ-cell in consequence of the reducing 
 division ; and this supposition is most satisfactorily borne out 
 by the facts, for it is well-known that the offspring of hybrid- 
 plants^ produced by fertilisation with their own pollen, become 
 very -variable in the following generation. It is evident, indeed, 
 that they must vary greatly, according to whether each one has 
 received a greater number of maternal or paternal ids, or an 
 equal number of both, from the two germ-cells which combined 
 in the process of fertilisation to produce this particular individual. 
 Thus Focke describes the offspring of hybrid plants of the first 
 or second year as being ' as a rule unusually diverse 'and rich 
 in forms,' and gives as examples the genera Pisum, Phaseolus, 
 Lactuca, Tragopagon, and Datura, mentioning especially in 
 this connection the hybrid of Nicotiana alata and N. langsdorffi. 
 De Vries * also refers to these facts, and describes them very 
 aptly in the following passage :' The hybrids of the first 
 generation have perfectly distinct characteristics in the case ot 
 every pair of species. If a hybrid is produced from two species 
 which have already been crossed successfully by previous 
 experimenters, we may be sure that the description given by 
 them will as a rule apply exactly to the intermediate form in 
 question. If the hybrid is fertile without the help of the parent- 
 
 * Hugo de Vries, ' Intracellulare Pangenesis,' Jena, 1889, p. 25.
 
 THE PHENOMENA OF REVERSION. 301 
 
 forms, and ii in a few generations thousands of specimens of its 
 offspring are raised, it will always be found that scarcely two of 
 them are alike. Some revert to the paternal and some to the 
 maternal form, while others, again, are intermediate between 
 the two. The remainder present the most varied alternation of 
 paternal and maternal characteristics, and show almost every 
 degree of mutual intermingling.' 
 
 De Vries states this merely as a proof of what he calls the 
 'free miscibility of the characters,' without attaching import- 
 ance to the fact that the hybrids of the first generation behave 
 quite differently from those of the second, or attempting to 
 account for this fact theoretically. 
 
 Professor Liebscher* has recently brought forward the 
 following interesting instance, in which the details were very 
 accurately investigated. He crossed two species of barley, 
 Hordeuin steudelii'i and Hordeum trifurcatum d, in the 
 former of which the spikelets are arranged in two rows and are 
 black, while in the latter they are arranged in four rows and are 
 white. The hybrid is as nearly as possible intermediate between 
 the two forms, ' all the ears,' moreover, ' being strikingly uniform J 
 as one would be led to expect theoretically. In all the hybrids 
 the spikelets are arranged in two rows, and in the main spike- 
 lets the tips are black, while in the lateral ones they are white, 
 and the ' Loffel' which are peculiar to Hordeuin trifurcatum 
 are black and white. The offspring produced from these hybrids 
 were exceedingly variable in the Jirst, as well as in the second 
 generation, 
 
 Liebscher has attempted to account for this variability by 
 assuming that a ' loosening (' Lockerung ') of the structure of the 
 germ-plasm,' as well as 're-combination of the individual 
 characters,' is produced by the process of reproduction. The 
 former results in 'a weakening of the power of faithful trans- 
 mission in the generative products,' i.e., 'an inclination to 
 individual variation in the descendants.' This statement indi- 
 cates that Liebscher certainly had some kind of idea of the 
 actual process which occurs in the idioplasm, although it is not 
 made clear in what this 'loosening' consists. 
 
 A definite meaning, however, underlies this expression as 
 soon as it is recognised that the germ-plasm consists of a large 
 
 * Liebscher, ' Vererbung," etc., Jen. Zeitchrift, Bel. 23, 1888.
 
 302 THE GERM-PLASM. 
 
 number of ids. The 'loosening' depends on the repeated 
 removal of half of the ids, which occurs every time the germ-cells 
 are formed. Half the germ-plasm of the hybrid consists of 
 maternal ids of the species A, and the other half of paternal ids 
 of the species B; in the formation of the germ-cells, this per- 
 fectly uniform composition becomes extremely diversified, owing 
 to the fact that the 'reducing division' halves the germ-plasm 
 in different ways. If we suppose that the ids, or even idants, 
 are arranged in a circle, the plane of division will sometimes 
 cut the circle across one diameter, and sometimes across 
 another, and the combination of the germ-plasm in the germ- 
 cells containing ids of A and B will thus be very diversified. 
 If the hybrid is then fertilised by its own pollen, so that amphi- 
 mixis occurs between two of the differently constituted germ- 
 cells, a still greater diversity in the idic combinations will result, 
 and a high degree of variability in the offspring must inevitably 
 ensue. 
 
 The offspring of hybrids are also very variable, even in those 
 cases in which they are produced by a recrossing with one of the 
 ancestral species. ' Fairly dissimilar offspring generally result 
 from the fertilisation of a hybrid with the pollen of an ancestral 
 form ; intermediate forms between the hybrid and the ancestral 
 species in question are as a rule the most numerous ; while a 
 smaller number of examples resemble either the original hybrid 
 or the ancestral species.' * This statement agrees perfectly with 
 our theory, for a consideration of the reducing division renders 
 it evident that in recrossing with one of the ancestral species, 
 a very dissimilar numerical proportion of idants of the two 
 ancestral species must prevail in the fertilised egg-cell of the 
 following generation. Such hybrids are generally spoken of as 
 ' f hybrids,' because in them the force of transmission of one of 
 the ancestral species is assumed to be one-quarter only, and that 
 of the other three-quarters. This term is probably indispens- 
 able in practice, but is obviously totally inaccurate. 'Three- 
 quarter hybrids' do not by any means contain all the hereditary 
 substances of the two ancestral species in the proportion of three 
 to one ; the proportion is in fact a very variable one. A ' f 
 hybrid' is produced, for instance, from two species of pink, 
 Dianthus chinensis and D. barbatus, by fertilising the hybrid 
 
 * Focke, p. 485.
 
 THE PHENOMENA OF REVERSION. 
 
 33 
 
 D. chincnsis ? x barbalus <J by the pollen of D. barbatus. The 
 formula of this hybrid wouldthereforc be Dianthus (chinensis x 
 barbatus)^ x barbatus&, or in more general terms by(A-fB) 
 9 x A <j. 
 
 For the sake of simplicity let us assume that the number of 
 idants and force of transmission is similar in the case of both 
 ancestral species. We will suppose that sixteen idants* are 
 arranged in a circle, as represented in Fig. 22, I. In the 
 production of the \ hybrid, eight idants of A combine with 
 
 FIG. 22. Diagram of the composition of the gcnn-plasnt in hybrids. \. The 
 germ-plasm of the parental species, each composed of sixteen idants ; A, 
 Dianthus chincnsis ; B, Diantltus barbattts. II. Germ-plasm of the hybrid, 
 composed of eight idants from A and eight from B : z, the plasm of section, 
 which may be rotated. III., IV., V. Three of the possible combinations of 
 the germ-plasm which might arise by crossing hybiid II. with the parental 
 species B. III. A true three-quarter hybrid. IV. Contains idants of the 
 parental species B only. V. Contains nine idants from B and seven from A. 
 
 eight idants of B ; the mother germ-cells of this hybrid will 
 therefore always contain 8Ax8B idants (Fig. 22, II.). The 
 reducing division then occurs, and bisects the circle at some 
 point. Fig. 22, II., will make it apparent that by rotating the 
 
 * We might just as well speak of 'ids' as 'idants' in this illustration. 
 I have referred to the idants simply because they are visible units, and not 
 merely hypothetical structures, and also because the number of idants may 
 be assumed to be less than that of the ids, and is thus more easily con- 
 trollable.
 
 304 THE GERM-PLASM. 
 
 plane of section, the circle may be separated into nine dif- 
 ferent combinations of the black idants A of D. chinensis and 
 the white ones B of D. barbatus, viz. : 8A ; ;A+ iB ; 6A + 2B ; 
 5A + 3B ; 4A + 4B ; 3A + $B ; 2A + 6B ; lA + yB ; and 8B. 
 Nine kinds of germ-cells, differing in quality, may therefore 
 be fromed, the egg-cells only being taken into consideration in 
 this connection. 
 
 In the- formation of the so-called f hybrid, one of these egg- 
 cells unites with a germ-cell of the pure ancestral species B. 
 The following combinations of idants may then result : 
 8B x 8B, which would produce the pure ancestral species B ; 
 (7A+iB) 9x8B(J; (6A + 2B) ? x8B <J ; (5A + 3B) ?x8B<j; 
 (4A + 4B) ?x8BcJ; feA + sB) ?x8B<5; (2A + 6B) ?x8B<j; 
 (iA + 7B) 9 x 8B 6 ; and 8A ? x 8B <J. Theoretically, therefore, 
 all stages from the pure ancestral form B (Fig. 22, III.) to 
 the form which is intermediate between the ancestral species 
 may occur, but no individual can arise which inclines more 
 strongly towards A than does the exactly intermediate form. 
 Whether all these are actually produced, and in what relative 
 frequency they occur, can only be decided by further researches 
 carried out from this point of view. Those which have till now 
 been made are insufficient, as the number of seedlings raised 
 has always been too small. We may, however, infer from the 
 facts we already possess, that the different combinations of the 
 two kinds of idants do not occur with equal frequency, and that 
 the intermediate combinations are the most usual. Were this 
 not the case, 'the form which is intermediate between the 
 hybrid and the ancestral species ' could not be ' the most fre- 
 quent.' This is also most probable theoretically, and becomes 
 more so as the number of idants is greater. The cells of many 
 Phanerogams possess far more than sixteen idants, and even if 
 we also assume that the position of the line of section is entirely 
 a matter of chance, the rarest case will always be that in which 
 it accurately separates the idants of A from those of B, and it 
 will happen much more frequently that it divides them in some 
 other plane. This is equivalent to saying that germ-cells of 
 \ hybrids rarely contain idants of A or of B only, and that in 
 most cases there is a combination of the two. 
 
 If the so-called f hybrids are again crossed with the ancestral 
 species B, $ hybrids (the third hybridised generation) are 
 obtained, and these 'as a rule bear a close resemblarre to that
 
 THE PHENOMENA OF REVERSION. 305 
 
 species of which Jths is represented, although the individual 
 specimens still display considerable differences in form.' * The 
 result of these observations also agrees with our theory, for even 
 in the second generation of hybrids, eight was the highest pos- 
 sible number of ids of the ancestral species A in the mature 
 germ-cells of the hybrid, and in the process of fertilisation these 
 met with the same number of idants of the ancestral species B, 
 which was used for recrossing. The germ-mother-cells of the 
 | hybrid cannot contain more than 8Ax8B, and, as a rule, a 
 smaller number of the idants of A will be present. The reducing 
 division then again halves these sixteen idants in various ways ; 
 and in the case which is most favourable for the preservation of 
 the hereditary substance of A, the halving of one of these germ- 
 mother-cells, which was most favourable for A, might again lead 
 to the formation of germ-cells with eight idants of A and eight 
 of B. But in all other f hybrids the germ-mother-cells already 
 contain more idants of B than of A, as is shown in Fig. 22, III., 
 and the reduction therefore results in the germ-cells containing 
 either idants of B only, or, at any rate, some of B in addition 
 to those of A. If these egg-cells are then fertilised by germ- 
 cells of the pure ancestral form B, the largest conceivable 
 number of idants of A which can be contained in the fertilised 
 egg-cell will again be eight. Hence the form exactly inter- 
 mediate between the two ancestors may possibly also appear in 
 the third generation, but this will occur much more rarely than 
 in the second generation. On the other hand, those indivi- 
 duals the idioplasm of which contains only a few idants of A 
 will preponderate ; and in others, even idants of B only may 
 be present, for the idants of B were in the majority in most of 
 the mother germ-cells of the second generation : the halving 
 due to the reduction must therefore have frequently resulted in 
 the distribution of idants of B only to one of the germ- cells, and 
 these then once more combined in the third generation with 
 ,'dants of the pure species B. Theoretically, therefore, the 
 hybrids must in part revert to the ancestral form B. This 
 conclusion is supported by facts, inasmuch as Kolreutter and 
 Gartner, who made a large number of experiments on recross- 
 ing, found 'that three to six generations (four to five as a 
 rule) were required for the complete transformation of the 
 
 * Focke, p. 485. 
 U
 
 306 THE GERM-PLASM. 
 
 hybrid into one of the ancestral forms.' As these investigators 
 were only concerned with obtaining a complete return to the 
 ancestral form, they always selected for purposes of propagation 
 those plants which bore the closest resemblance to the desired 
 parental type. They, nevertheless, still obtained individual 
 plants which bore traces of the other ancestral species, up to 
 the fifth generation, just as would be expected on theoretical 
 grounds. 
 
 The agreement with our theory extends even further than 
 this : individual cases of complete reversion to the ancestral 
 form must theoretically occur in the second generation, and such 
 cases have actually been observed. Godron, indeed, found that 
 ' Melandryum album x rubrum, even when fertilised by its 
 own pollen, reverted in the second generation to the ancestral 
 species.'* This is easily explained on the basis of our theory. 
 If the reducing division took place in certain of the mother- 
 germ-cells of the hybrid in such a manner that each of the two 
 mature germ-cells contained " idantst of M. album or M. rubruvt 
 only, two possible combinations might occur in the fertilisation 
 of these germ-cells : either a germ -cell with idants of M. 
 rubrum would meet with another containing idants of M. album, 
 or else both the conjugating cells would contain idants of the 
 same species. In the former case, the hybrid form would again 
 be produced, while in the latter, the pure ancestral form would 
 result. Such reversions seem to be of rare occurrence, and this 
 may indicate that when they do occur, some unknown conditions 
 favour the halving of the germ-plasm in the mother-germ-cells 
 along a definite plane of section, viz., that which passes between 
 the groups of paternal and maternal idants. 
 
 Although not proved by actual observation, it is apparent 
 from our theory that the hybrid may revert completely to the 
 ancestral form. This would be impossible if the reducing 
 division did not occur, or if the germ-plasm consisted of a 
 homogeneous mass which became completely mixed with that 
 of the other species in the process of hybridisation. The germ- 
 plasm of the one species would then, moreover, never be removed 
 by means of the reducing division : the reduction would be 
 a quantitative, and not a qualitative one. Reversion to the 
 ancestral form could then in no case occur on fertilisation taking 
 
 * Focke, p. 485. 
 
 f n signifies the normal number of idants in the species in question.
 
 THE PHENOMENA OF REVERSION. 307 
 
 place with the pollen of the hybrid ; and even by continued 
 recrossing with one of the ancestral species, the combination of 
 the germ-plasms of the first generation would gradually become 
 weakened, and perfectly pure germ-plasm of the one ancestral 
 species could never be produced. But as soon as we assume 
 that the germ-plasm consists of units which remain separate in 
 the form of idants or ids, it is evident that the whole of the units 
 of one species may be removed from the germ-plasm of the 
 hybrid-offspring, either with or without the occurrence of re- 
 crossing ; and even if recrossing does take place, reversion must 
 occur sooner or later in individual descendants. 
 
 Our theory also enables us to make certain predictions, 
 which, as far as my knowledge extends, have not up to the 
 present been confirmed by facts. If sometimes, though rarely, 
 all the idants of A pass into one germ-cell, and all those of B 
 into the other, in the process of halving of the germ-plasm of 
 the primary hybrid A x B a reversion to both the ancestral 
 forms will occur when the second generation is fertilised with 
 the pollen of the hybrid, as has just been shown : that is to say, 
 such a reversion will take place when a pollen-grain containing 
 idants of A only comes in contact with an egg-cell which also 
 contains idants of A only, or when idants of B meet with 
 others of similar origin. The same must also be possible in the 
 hybrids of the third generation, even when all the complete 
 reversions of the second generation are left out of consideration, 
 simply for the reason that certain individuals of the second 
 generation are produced from the combination of A with B 
 idants, and are therefore exactly intermediate forms like all the 
 hybrids of the first generation. The occurrence, even if only in 
 individual cases, of such intermediate forms, renders it possible 
 that germ-cells may again arise which contain idants of A or B 
 only, so that ultimately reversion to one or other of the two 
 ancestral forms becomes possible. These reversions will cer- 
 tainly be of rare occurrence ; and as it is entirely a matter of 
 chance that germ-cells which contain the most infrequent com- 
 binations of idioplasm should come together in the process of 
 fertilisation, it would be necessary to examine a very large 
 number of seedlings before such cases would be found. 
 
 Before passing on to the consideration of reversion to indi- 
 vidual characters, it should be remarked that the same law of 
 reversion which can be recognised in the case of plant-hybrids,
 
 308 THE GERM-PLASM. 
 
 holds good as regards the crossing of the different human races. 
 According to this law, the intermingling of a racial character 
 is uniform in the first generation, but subsequently becomes 
 quite irregular when recrossing with one of the original races 
 occurs. The skin of mulattoes, which are a cross between 
 white races and negroes, is never quite white, its colour being 
 as a rule an approximate mean between that of the two parents. 
 When mulattoes and white races are recrossed, the skin does 
 not regularly become less black, but the descendants of the 
 third and fourth generations are sometimes white and some- 
 times fairly dark : this fact indicates the absence of uniformity 
 in the process of ' reducing division.' 
 
 2. REVERSION TO INDIVIDUAL CHARACTERS IN MAN. 
 The essential difference between the process of reversion in 
 plant-hybrids and in Man, consists in the fact that the former is 
 concerned with the intermingling and subsequent separation of 
 specific or racial characters, while in the case of the reproduc- 
 tion of human beings of the same race individual characters 
 only are intermingled. As regards hybrid-plants, the idants of 
 each parent might be assumed to be similar, although this may 
 not be strictly true in all instances, as will be seen later on, but 
 in the case of individual differences the idants of each parent 
 cannot be regarded as similar. Each of these idants con- 
 sists of a number of separate ids, which may differ in many 
 respects. In all of them the determinants are as similar as the 
 retention of the specific character renders necessary : that is to 
 say, all the determinants of the same ontogenetic stage are 
 homologous, though at the same time they are never all homo- 
 dynamous, but differ in many respects owing to slight individual 
 deviations. Hence different ids may contain different variations 
 of any particular homologous determinant. In the following 
 examples, each homologous determinant is indicated by a letter, 
 and variants of a determinant are distinguished by dashes after 
 each letter. Thus id i. of the germ-plasm might, for instance, 
 contain the determinants a, b, c, d, e, - - - n ; id ii., the deter- 
 minants a, b', c, d', e, - - - n' ; id iii., a', b", c' d'", e', n, 
 
 and so on. The total effect of the idants is decided by the 
 struggle of the ids ; the laws which regulate this struggle 
 cannot at present be determined more precisely, and until we 
 know more about them, we may suppose that those variants
 
 THE PHENOMENA OF REVERSION. 309 
 
 which are present in the largest numbers always have the best 
 prospect of controlling the cell entirely, or at any rate chiefly ; 
 their effect must be cumulative, and a small minority of homo- 
 dynamous determinants will not be able to take effect against a 
 large majority of some other variant. The control of the cell 
 therefore results from this struggle of the determinants, which 
 must naturally not be conceived to take place in such a manner 
 that the group of paternal ids struggles with the maternal group, 
 but so that all the active determinants which are contained in 
 the idioplasm migrate into the cell body and there strive to 
 obtain the control. If the parents of the organism in question 
 are closely related, the same homodynamous determinants may 
 very possibly be contained in the idants of both parents, and 
 the forces of these will then combine just as would be the case 
 if they had been contained in one idant of the father or mother. 
 It will, however, happen more frequently that homodynamous 
 determinants are present in the ids of each parent respectively, 
 and a majority of homodynamous maternal determinants will 
 then compete with a majority of paternal ones ; they will then 
 either control the cell together, or the prepotency of one of 
 the parents will be so great as to suppress the influence of the 
 other entirely. 
 
 In order to test to what extent these ideas of the co-operatio.i 
 of parental idioplasms may be applied to the phenomena of 
 reversion in Man, we must again consider the simplest of these 
 phenomena, viz., that of reversion to a grandparent. 
 
 It is well known that a child not infrequently resembles its 
 father or mother in a high degree, and it is also assumed that a 
 father may beget a child which does not resemble him, but it 
 and its father's mother are as 'like as two peas.' This pre- 
 supposes that the father himself bore no resemblance to his 
 mother, for otherwise it would not be an instance of reversion 
 of the child to the grandmother. 
 
 This case may be explained theoretically by assuming that 
 the ' reducing division ' of the respective germ-cells of the two 
 generations accurately separated the paternal from the maternal 
 idants, and that, as was shown above, the group of idants of 
 one of the parents may possibly have had no influence on the 
 formation of the child, the other group being the dominant one. 
 The fertilised egg-cell from which the father was developed 
 must therefore have contained the two groups of idants A and
 
 310 THE GERM-PLASM. 
 
 C, A being derived from the grandmother, in the development 
 of whom this group dominated. C was derived from the grand- 
 father, and preponderated to such an extent over A that it alone 
 determined the type of the son. Hence A remained latent 
 in the idioplasm of the father, who therefore resembled the 
 grandfather and not the grandmother. If now, that germ- 
 cell of the father, from which the child of the third generation 
 was developed, just happened in the reducing division to receive 
 the group of idants of A only, which remained without influence 
 on the development of the father, but which was dominant in 
 the grandmother, a reversion to the grandmother would then 
 become possible. It would occur when this sperm-cell unites 
 with an egg-cell in which the group of idants D has a much 
 weaker controlling force than A. The child would then resemble 
 neither the paternal nor the maternal type, but would be like his 
 grandmother, for the group of idants A determined the type of 
 the grandmother. 
 
 I nevertheless consider it doubtful whether reversion of this 
 kind ever occurs so thoroughly and completely as is conceiv- 
 able theoretically. The facts are unfortunately by no means so 
 decisive as one could wish. No one, so far as I know, has 
 hitherto attempted to ascertain whether complete reversion to a 
 grandparent ever occurs, and from a theoretical point of view I 
 should consider this to be improbable. Similarity to a grand- 
 parent may undoubtedly often be observed, but it does not 
 therefore by any means follow that a correspondence exists 
 between all, or at any rate the greater majority of individual 
 characters, such as is seen in ' identical ' twins. All the parts 
 of the child and grandparent have never been carefully compared, 
 not to mention the fact that such a comparison has never been 
 made at corresponding ages. Moreover, to use the same 
 example, only those cases could be utilised in questions of 
 reversion in which the father bears no resemblance to the type 
 of the grandmother. Cases of this kind are, however, not quite 
 reliable, and are certainly not common. 
 
 I am therefore inclined to think that in all these cases we are 
 only concerned with a partial and not a complete reversion 
 to the grandparent, that is, with the reappearance of a more 
 or less extensive aggregate of characters of the grandparent ; 
 and this is certainly sufficient in many cases to produce what 
 appears to be the type of the latter. More or less extensive
 
 THE PHENOMENA OF REVERSION. 311 
 
 aggregates of the characters of the grandparents undoubtedly 
 reappear in the child, and these may be more easily explained 
 theoretically, without the necessity of supposing that chance 
 plays so great a part. For in order that perfect reversion may 
 be possible, numerous most infrequent occurrences must take 
 place together. To use the same example, in each individual of 
 three consecutive generations, only one of the two groups of 
 parental idants could determine the type of the child, and the 
 reducing division, moreover, must affect one of the mother- 
 germ - cells of each of four individuals i.e., the two grand- 
 parents and the two parents in such a manner as to separate 
 the two groups of parental ids. All these possibilities may 
 sometimes concur, but we can only imagine this to be so if 
 it is proved that an exact resemblance exists between the child 
 and grandparent. 
 
 A partial resemblance is theoretically far more likely to occur. 
 Let us suppose, for instance, that the germ-plasm consists. of 
 sixteen idants, eight of which are derived from the mother and 
 eight from the father, and that the reducing division causes six 
 idants of one parent and two of the other to be present in one 
 germ-cell : the former might possibly contain many of those 
 determinants to which the type of this particular parent was due. 
 This need not necessarily be the case, for all the sixteen idants 
 may have had a fairly equal share in the production of this 
 type, and six out of the sixteen idants could not then possibly 
 give rise to the same type even approximately. 
 
 It might, on the other hand, happen that the type of the 
 parent was essentially determined by the group of idants derived 
 from the father or mother of this parent only, and a resemblance 
 of the child to its father could then only occur when the whole 
 or a great part of that group which controlled the development 
 of the father was also present in the germ-cell from which the 
 child was developed. Even then, the production of the type of 
 the father would not absolutely be ensured, for in the process of 
 fertilisation by the other parent a group of idants might be 
 added, in which the controlling force preponderates over those 
 already present, either entirely, or at any rate as regards many 
 or most of the determinants.. 
 
 We may, however, conclude from all these considerations 
 which are unfortunately still very vague, that a large number 
 of idants of the grandfather, for example, may be present
 
 312 THE GERM-PLASM. 
 
 in the germ-cell which will give rise to the father, and still 
 may not result in the production of the type of the grandfather; 
 and that these may then pass into a germ-cell of the next gene- 
 ration, and partially determine the type of the son, provided 
 that they are opposed to a group of idants which has a weaker 
 controlling force. At every ontogenetic stage, then, the struggle 
 of the ids and idants decides which group of the latter is to 
 control the cell. According to the hypothetical principle on 
 which we have supposed this struggle to take place, the majority 
 of homodynamous determinants would always represent the 
 greatest controlling force, so that certain of the more compre- 
 hensive or special characters of the grandparent might very 
 well reappear in the grandchild, even if only six or eight idants 
 which controlled the development of the grandparent were 
 present in the germ-plasm from which the grandchild arises. 
 
 We may even assume that, as regards Man, ontogeny is hardly 
 ever passed through without reversions occurring to one or other 
 of the grandparents; for determinants of one of the grand- 
 parents will almost invariably have been suppressed in the 
 development of the parent by stronger ones derived from the 
 germ-cell of the other grandparent, and will predominate in 
 the formation of the grandchild, because they are here again 
 opposed to other combinations of ids over which they may pre- 
 ponderate under certain circumstances. It may therefore happen 
 that some of the individual characters of the grandparent may 
 reappear in the grandchild, although there may be no general 
 resemblance between the two. 
 
 The answer to the above question relating to the causes on 
 which reversion to a grandparent depend, may be briefly stated 
 as follows : such a reversion is due to the fact that the whole 
 or part of the group of idants which determine the type of the 
 grandparent were present in that germ-cell of the parent from 
 which the grandchild was developed, and that it was there 
 opposed to a weaker group derived from the other parent. 
 
 We do not know what number of the controlling idants of the 
 grandparent must be present in the germ-plasm of the grand- 
 child in order that reversion to the grandparent may occur. 
 Complete reversion can only take place if none of the deter- 
 mining idants are absent ; but, as we have seen, such complete 
 reversion has not by any means been proved to occur in Man. 
 Still less is this the case in the next following generation, viz.,
 
 THE PHENOMENA OF REVERSION. 313 
 
 as regards the great-grandparent. Instances of a descendant 
 of an ancient family reverting to a great-grandparent whose 
 characteristics were not present in the intermediate generations, 
 are certainly occasionally mentioned in novels, but it is only 
 natural to doubt the accuracy of such cases, even when they 
 claim to be true. A great-grandchild certainly often resembles 
 its great-grandparent, but when this is so, this resemblance had 
 not disappeared entirely in the intermediate generations. As 
 has been shown above, it would certainly not be incredible, 
 from a theoretical point of view, that the group of idants which 
 controlled the development of the great-grandparent should 
 remain intact in certain germ-cells of two generations, and 
 should be suppressed by more powerful groups derived from 
 germ-cells of other parents, once more to predominate in the 
 third generation. If reversion of this kind could be proved to 
 occur, it might be explained in this manner, and we should be 
 justified in assuming that in many cases the idants of the two 
 parents may again be separated into their original groups by 
 the reducing division. The great variability of hybrids of the 
 second generation proves that this only occurs very rarely, and 
 in most instances not at all. 
 
 We have seen that the difference between reversions in 
 hybrids and in human beings of the same race is simply due to 
 every idant of one parent, in the case of hybrids, containing 
 specific characters, so that all the idants of the one parent are 
 similar, and all the homologous determinants may also be 
 considered homodynamous. When therefore an ontogeny is 
 directed by the ids of one of the parent species only, the type 
 of this species is produced. The type of a human being, on the 
 other hand, is constituted by ids of very many different kinds, 
 no two of which are exactly alike, for each of them contains 
 determinants of a somewhat different kind from the rest ; and, 
 speaking generally, the type is in all cases only the resultant 
 of all these different components. The same type, or com- 
 bination of characters, can only appear for a second time if the 
 same components are again brought together. Except in the 
 doubling of the fertilised egg in the case of ' identical ' twins, 
 this can, however, never occur, for a new combination of ids and 
 idants which never existed before arises every time fertilisation 
 takes place. Hence cases of complete reversion as regards 
 individual characters to a previously existing form, can never
 
 314 THE GERM-PLASM. 
 
 occur ; and only those instances are possible in which the 
 reversion concerns more or less extensive groups of characters 
 which already existed in an individual of a former generation. 
 
 We have now discussed the question of transmission from the 
 parents, grandparents, and great-grandparents, to the child ; and 
 it will be as well, before going further, to summarise the various 
 possible cases, and to see whether observation and theory 
 contradict one another in any respect. 
 
 The most frequent case seems to be that in which the child 
 is a mixture of the types of the two parents, the characters of 
 both of which are either completely united, or else they alternate 
 with one another in the different cells, parts, and organs, and 
 even in the systems of organs themselves. In all such instances, 
 the group of idants which predominated in the ontogeny of the 
 parent, or rather of both parents, must have been contained in 
 the germ-cell from which the child arose. 
 
 The question might here be asked as to how it can so fre- 
 quently happen that only the dominant group of idants is present 
 in the germ-cell destined to undergo amphimixis. We must, 
 however, remember that a perfectly equal blending of the types 
 of the two parents in the child occurs much more rarely than is 
 generally supposed ; and that it is difficult, and frequently in fact 
 impossible, to say whether the maternal portion of a character 
 is really derived from the type of the mother, and not from 
 that of her father or mother. General characters only, of the 
 mother's family are most usually combined with others derived 
 from the family of the father. But those characters which 
 have been peculiar to many members of a family for several 
 generations are the most likely to occur in numerous ids and 
 idants, and they therefore reach the germ-cells in larger num- 
 bers in many of the modes of ' reducing division.' Theoretically 
 the dominant group of idants of the parent would not be neces- 
 sary for such a transmission of a general family resemblance, 
 but only a majority of the idants of this group. 
 
 The case, however, is different when the resemblance refers 
 mainly to the type of one parent ; and we must then assume 
 that the dominant group of idants of this parent is present, and 
 is opposed to another weaker group derived from the other 
 parent, and provided with a smaller number of homodynamous 
 determinants.
 
 THE PHENOMENA OF REVERSION. 315 
 
 A third case may also occur in which the type of an uncle or 
 aunt is more or less accurately reproduced in the child, or in 
 which this type is blended with the characters of the parent on 
 the other side. I know a man who closely resembles a maternal 
 aunt, but nevertheless possesses many general characters of his 
 father's family This fact may in all probability be explained in 
 terms of the idioplasm as follows : the egg-cell from which 
 this person was developed contained the group of idants which 
 was dominant in the ontogeny of the mother's sister, and not 
 that which predominated in the case of the mother. Theo- 
 retically this might very well be the case. Let us suppose that 
 the primary germ-cells of the maternal grandfather (m p) 
 contained the idants a, b, c, d, e, f, g, h, and those of the grand- 
 mother the idants i, k, 1, m, n, o, p, q ; and also that the fertilised 
 egg-cell from which the mother was developed contained the 
 idants a, b, c, d x i, k, 1, m, and that from which the aunt was 
 developed the idants #, 6, c, f x /, n, o, p. We will further 
 assume that the group of idants which was dominant in the 
 ontogeny of the aunt were those indicated by the letters a, , c, 
 and /, printed in italics. It will then be seen that the same 
 combination a, b, c, I can also be formed from the germ-plasm 
 of the mother by means of the reducing division, for all four of 
 them are present in this germ-plasm (a, , c, d x i, k, /, m). It 
 is doubtful whether such a case ever occurs so accurately, and 
 I know of no instance which renders this hypothesis necessary : 
 the resemblance is always an imperfect one. 
 
 We must also consider a fourth case, in which the child 
 neither specially resembles its father or mother, presents a 
 recognisable combination of the characters of both, nor bears a 
 striking resemblance to one of the four grandparents, but dis- 
 plays an entirely new combination of characters. Such a child 
 would probably always bear some resemblance to both, or at 
 any rate to one, of the parental families, but it would not exhibit 
 certain marked characters of the respective fore-fathers. 
 
 These cases do not contradict our theory, for by means of the 
 reducing division it may possibly happen that none, or only 
 certain of those very idants which were dominant in both 
 parents are present in the germ-cells of the parents which 
 undergo amphimixis. 
 
 If, for instance, the germ-plasm in the ontogeny of the father 
 had the composition a, b, c, d, e,/, g, h, of which the idants a, b,
 
 316 THE GERM-PLASM. 
 
 d, f, were dominant, and the germ-plasm in the ontogeny 
 of the mother consisted of the idants z, k, /, m, ;z, o. p, q, of 
 which i, 1. n, o essentially determined her type. If the re- 
 ducing division then resulted in an egg-cell containing the idants 
 k, m, p, q, and a spermatozoon was formed which contained 
 the idants c, e, g, h, and these came together in the process of 
 amphimixis, the resulting germ-plasm would certainly contain 
 characters of both families, but it could not possibly produce 
 the type either of the father or mother, or a mixture of both. 
 
 I have already discussed the question of reversion of the 
 child to one of its four grandparents in detail, and need not 
 return again to this point. I should, however, like to call 
 attention to the fact that, theoretically, a child can never exhibit 
 a combination of the types of two of its grandparents, simply 
 for the reason that at least half of the idants of a germ-plasm 
 determine the type of the child, the germ-plasm of which can 
 only contain one quarter of the idants of two grandparents. 
 An exception to this rule could only occur in cases of close 
 interbreeding, in which both grandparents in part contained 
 the same idants. 
 
 No case has, however, ever come before my notice in which a 
 child bore no resemblance to its parents and yet exhibited a 
 combination of the types of that of two of the grandparents. 
 
 3. Reversion to the characters of ancestors far removed in 
 animals and plants. 
 
 We will now consider the question of reversion to remote 
 ancestors. Cases of such reversion are so well known through 
 Darwin's researches, that a more detailed description of the 
 mere facts might almost be considered superfluous. But 
 such a description is nevertheless necessary, and I must even 
 express the opinion that the facts have not been so accurately 
 investigated from all points of view as is desirable in order that 
 a theoretical explanation of them may be arrived at. Darwin 
 was the first to point out that in cross-breeding, either of species 
 or of mere varieties, characters not infrequently appear in 
 descendants which were not present in the parents, in some of 
 which cases, indeed, it can be proved, and in others shown to 
 be very probable, that they have been derived from remote 
 ancestors. Mules, for instance, sometimes exhibit distinct 
 stripes, like those of the zebra, on the fore-legs and shoulders,
 
 THE PHENOMENA OF REVERSION. 317 
 
 while such stripes are very seldom present in the horse and ass, 
 and are even then only very faint ; but we must nevertheless 
 suppose them to be derived from the ancestral form of the two 
 species. And, again, when certain races of pigeons are crossed, 
 offspring are produced in which the plumage has the slaty-blue 
 colour of the wild rock pigeon, although the races used for 
 crossing ivere of quite another colour ; in this case, moreover, the 
 descent from the wild rock pigeon is certain. Similar instances 
 also occur in plants. The hybrids of Datura ferox and D. 
 Icevis, in both of which the flowers are white, regularly bear 
 blue (purple ?) flowers, and Darwin * has shown that this is to 
 be looked upon as a reversion to ancestors which possessed blue 
 flowers, as, in fact, is the case at the present day in an entire 
 group of species of Datura. 
 
 I will now attempt to explain these three instances in accord- 
 ance with my theory. In them, as is universally the case, rever- 
 sion must be attributed to the presence of old unmodified 
 determinants in the germ-plasm, which take the place of the 
 younger homologous determinants as regards obtaining the 
 control of the cell or region of cells in question. Similar 
 assumptions must be made in every theory of heredity. In his 
 theory of pangenesis, Darwin makes use of old gemmules for 
 this purpose, while de Vries assumes that reversion is due to the 
 presence of old pangenes. Some unmodified portion or other 
 of the hereditary substance must always fonn the starting-point 
 in attempting to explain the problem ; and the only question is, 
 whether we are to remain satisfied with such a statement, and 
 leave everything else in obscurity, or whether it is possible to 
 obtain a certain insight, in principle at any rate, into the question 
 as to why these minute parts can remain unmodified, and why 
 and under what circumstances they can suddenly obtain control 
 precisely in the region of transformed parts which are homo- 
 logous to them. 
 
 A solution of the first of these problems has already been 
 given above. It has been shown that, according to the principle 
 of selection which controls the whole, a transformation such as is 
 required by the vital conditions of a species never necessitates 
 the transformation of all the determinants relating to the parts 
 to be transformed, but that this process is only necessary in the 
 
 Darwin, ' Animals and Plants under Domestication,' Vol. II., p. 254.
 
 3*8 THE GERM-PLASM. 
 
 case of a majority of them, which is sufficient to ensure the occur- 
 rence of the modification in question in every ontogeny, ic., 
 in every individual of the species. This is all that is required, 
 and consequently the processes of selection cannot accomplish 
 more. After every transformation of the body in the process 
 of development of the species, the germ-plasm will thus con- 
 tain some unmodified determinants in addition to those which 
 have been transformed in any part, and these will only disappear 
 very gradually in the course of the further history of the species. 
 
 The existence of the material by means of which reversions 
 to all specific characters operate, can therefore be proved 
 on the basis of the theory of selection : every germ-plasm must 
 contain a larger or smaller number of old determinants corre- 
 sponding to the characters of the ancestral species. 
 
 The solution of the second problem, as to why these ' ancestral 
 determinants ' always exert their influence at the right spot in 
 the body, naturally follows from our theory, in which the 
 mechanism of ontogeny is referred to the gradual disintegration 
 of the germ-plasm. 
 
 The third problem then only remains : how can the ancestral 
 determinants, which are present in a minority, gain control over 
 the majority of younger ones ? 
 
 We have seen that reversions to the ancestral form occur in 
 the offspring of hybrid plants, even if they are fertilised by 
 their own pollen, when those cells to which the entire group 
 of idants of the ancestral species of the same parent has been 
 distributed at the reducing division happen to come together in 
 the process of fertilisation. Whenever the germ-plasm of the 
 fertilised egg-cell contains idants of the species A alone, an 
 organism of the species A can alone arise. But this occurrence 
 is out of the question in cases of reversion to the characters of 
 remote ancestors. The germ-plasm can then never consist 
 entirely of idants of the ancestral species ; and, in fact, we may 
 doubt whether entire ancestral idants exist at all in the germ- 
 plasm of any individual of a long-established species. For the 
 number of idants (nuclear rods) is not extremely large in any 
 species ; and if, as in the instance given above, we assume this 
 number to average sixteen, even the proportion of one unmodi- 
 fied to fifteen modified idants would be rather large, and would 
 render an occasional reversion to the ancestral form possible 
 in the ordinary reproduction of the species. Fertilisation need
 
 THE PHENOMENA OF REVERSION. 319 
 
 only occur eight times successively, two of the unmodified 
 ancestral idants being added each time to those already pre- 
 sent, in order to produce a germ-plasm consisting of pure 
 ancestral idants, and the organism developed from it would 
 then agree with the ancestor in question in all its characters. 
 Under particularly favourable circumstances the process might 
 even be completed in four generations, as is shown in the 
 following table : 
 
 Generation,. { $ * jj | * <* } '' 
 
 Generation,,. {*;*; | ^^J - 
 
 j a', a', a', a' I b, d, q, s = <J 
 Generation "M^a'.**' I a, ft, 8, f = 9 
 
 Generation 
 
 The letters in the above table indicate the idants, which 
 with the exception of a', which represents the unmodified ances- 
 tral idant, and only occurs once in the first generation are 
 only individually different from one another. The vertical line 
 icpresents the reducing division of the mother-germ-cell, which 
 in the most favourable case we are now considering, always 
 causes this ancestral idant a' to pass into the germ-cell about 
 to undergo amphimixis. For the sake of simplicity, the con- 
 jugating germ-cells are assumed to be similar in respect to 
 their contained ancestral idants. It would thus follow that 
 germ-plasm consisting of ancestral idants only, would be pro- 
 duced in the fourth generation. 
 
 Although we must certainly admit that so favourable a com- 
 bination of events can hardly ever occur, it cannot be doubted 
 that an accumulation of ancestral idants may take place in one 
 germ-cell in the course of a large number of generations, and 
 that a majority of these ids may consequently come together 
 in fertilisation. In this case a more or less complete reversion 
 to the ancestral form must take place. As such reversions to the 
 complete type of the ancestral species cannot be observed to 
 occur in the normal reproduction of pure and long-established 
 species, we must conclude that all the idants have become 
 modified in such old and fiiuc s/'ecz'es, each of which produces
 
 320 THE GERM-PLASM. 
 
 the type of the species, and not that of an ancestor, in case 
 it alone has to control the ontogeny. 
 
 The foregoing considerations seem to me to afford a good 
 explanation of the frequent occurrence of reversion in young 
 species which are not yet definitely established. Cultivated 
 varieties of flowers, such as the Heartsease (Viola tricolor)^ 
 always produce, among a large number of seedlings, certain 
 plants which bear flowers more or less resembling those of the 
 wild species. Evidently only a portion of the idants have become 
 transformed in this modern form, another and smaller portion 
 having remained unmodified. Since the reducing division 
 separates the idants into two groups of all possible combinations, 
 germ-cells, both male and female, containing a preponderance of 
 unmodified ids, must also occur j and if two such germ-cells 
 come together in fertilisation, reversion must result. 
 
 Let us suppose that, of the sixteen idants in the germ-plasm, 
 ten were modified and six unmodified. Under favourable cir- 
 cumstances a case might then occur in which the majority of 
 idants would remain unmodified, without further accumulation 
 taking place in the course of generations />., at every fertilisa- 
 tion. This may be illustrated by the following table, in which 
 the letters with a dash represent the unmodified, and those 
 without one the modified idants, the line dividing the letters 
 indicating the reducing division : 
 
 Maternal germ-plasm f a', b', c, d', e, f , g', h I 5, k, 1' m, n, o, p,q. 
 Paternal t f, r 1 , s, t', u', v', w', x I y, z, a, b', c, d, e, g. 
 
 Germ-plasm of offspring I. : a', b', c, d', e, f , g', h x f, r', s, t', 
 u', v', w', x. 
 
 Owing to the manner in which the reducing division takes 
 place, the maternal and paternal germ-cells each contain five 
 unmodified idants, so that in the germ-plasm of the offspring 
 ten unmodified and six modified idants are present. If, on the 
 other hand, we suppose that the other two cells produced by 
 the reducing division from the same mother-germ-cell undergo 
 fertilisation, the germ-plasm of the offspring II. will have the 
 following composition : 
 
 i, k, 1', m, n, o, p, q, y, z, a, b', c, d, e, g. 
 
 It therefore only contains two unmodified idants, and must 
 consequently give rise to the modified form of the plant.
 
 THE PHENOMENA OF REVERSION. 321 
 
 A large number ot cases of heredity which have not hitherto 
 been understood can thus be explained in a very simple way. 
 Take, for example, the varying degrees of certainty with "which 
 varieties of cultivated plants transmit their peculiarities. The 
 extent to which such peculiarities are transmitted must depend 
 on the number of idants in the germ-plasm which have remained 
 unmodified, and the greater this number, the more easily will 
 reversion occur. 
 
 These theoretical considerations will probably account for the 
 first of the three cases which were mentioned above as types of 
 reversion to more remote ancestors, viz., that of Datura ferox 
 x Icevis. The two species of Datura have white flowers, but 
 produce hybrids in which the flowers are blue ; and this is not 
 only occasionally the case, but occurred in every one of the 205 
 plants which Naudin raised from this cross,* and also in all 
 those cases which had been observed previously by Kolreutter 
 and Giirtner.t If we assume that, in addition to their own 
 specific idants, the two species of Datura contain a certain 
 number of ancestral idants, the latter might be relatively in- 
 creased in individual germ-cells in consequence of the reducing 
 division ; and when these met with germ-cells of the other species, 
 which also contain numerous ancestral idants, a germ-plasm 
 possessing a larger number, and perhaps even a majority, of 
 these idants would be constituted. Individual cases of reversion 
 to the common ancestral form might thus occur. But this 
 assumption is evidently insufficient to account for the facts, for 
 the blue colour of the flower appears in all the hybrids. The 
 reversion in these cases must therefore be independent of 
 a greater accumulation of ancestral idants which may possibly 
 occur in individual germ-cells in consequence of the reducing 
 division. These idants must, on the contrary, come together in 
 each fertilised egg-cell in a sufficient number to preponderate over 
 the modified idants, and to control the ontogeny. They cannot 
 possibly, however, be stronger numerically in every case, and 
 another factor must therefore take part in the process, which 
 causes the ancestral primary constituents to preponderate in 
 every case ; and this is in all probability the specific diversity of 
 the modified idants. We have assumed from the first that 
 
 * Darwin, 'Animals and Plants,' Vol. II., p. 254. 
 t Cf. Focke, ' Pflanzenmischlinge,' p. 269. 
 X
 
 322 THE GERM-PLASM. 
 
 homodynamous determinants have a cumulative effect, while 
 heterodynamous determinants tend to counteract one another. 
 The same must be true of the groups of determinants, the ids 
 and idants ; similar idants must increase the effect produced, 
 while dissimilar ones will interfere with one another to a greater 
 extent the more diversified their composition out of ids and 
 determinants is. Let us confine our attention for the present 
 to the groups of determinants for the flowers. The two species 
 of Datura certainly contain determinants which tend to form 
 white flowers, and we might therefore be disposed to infer that 
 these are homodynamous, and that their effect must be cumu- 
 lative. But this conclusion would be erroneous, for it is quite 
 possible that these determinants only correspond as regards the 
 production of a 'white' colour, and differ widely in respect of 
 many other characteristics of the cells, such as those relating to 
 their size and minute structure. On the other hand, the 'blue' 
 determinants are actually homodynamous, and correspond not 
 only with regard to the colour to which they give rise, but also 
 in respect of all the other characters of the cells of the flowers, 
 for they are derived from the common ancestral species. When 
 the hybrid begins to develop flowers, the structure of the cells 
 in the latter will depend on the determinants of the two white 
 species and of the blue ancestral form. Although the 'blue' 
 determinants are in the minority in the idioplasms of either 
 parent, they may nevertheless, if they all combine, possess a 
 greater power of transmission than the ' white ' ones, if the latter 
 are not homodynamous, i.e., do not possess an exactly similar 
 force of heredity, and consequently cannot produce an inter- 
 mediate effect. They interfere with one another in their action, 
 as they act in different directions to a greater or less extent. 
 Many cases of reversion can be understood even though only 
 in principle by means of this law. They can, moreover, 
 easily be rendered comprehensible in individual cases if we have 
 recourse to figures. 
 
 Let us suppose that the ' blue ' ancestral determinants are not 
 only contained in individual ids, but are present in all the ids of 
 entire idants : a minority of old ancestral idants would then be 
 opposed to a majority of modern ones, half of which, however, 
 would correspond to the type of D. leevis and half to that of D. 
 ferox. We assume that there are in all sixteen idants, six of 
 which are ' blue' and ancestral, and ten ' white.' Since, therefore,
 
 THE PHENOMENA OF REVERSION. 323 
 
 five of the latter belong to D. lavis and five to D.ferox, and these 
 are consequently dissimilar, the six similar ancestral idants, 
 which have a cumulative effect, will preponderate, because the 
 2x5 different idants of D. Icevis and D. ferox do not produce 
 the cumulative effect of 10. 
 
 Hitherto I have assumed that we are in this case dealing 
 with a complete reversion to the ancestral form, and not merely 
 with reversion to individual ancestral characters. I cannot, 
 however, judge with certainty in this respect from the facts 
 known to me concerning these hybrids ; and as this instance 
 was not chosen for its own sake, but merely as an example in 
 which complete reversion to more remote ancestors might be 
 accounted for, it must remain undecided whether it really belongs 
 to the above category, and whether such instances of complete 
 reversion actually occur. The blue colour of the flowers is at 
 any rate not the only apparently new character in these hybrids ; 
 the stem, for instance, is brown, while it is green in the two 
 pure species. In only one of these species (D. ferox) is the stem 
 brown at first, and this colour is only retained as a brown ring 
 around the base of the stem. I must leave botanists to decide 
 whether the shape of the leaves, and the structure of the stem or 
 fruit, afford any reason for considering these parts as intermediate 
 between those of the two parental species, or whether they are 
 to be regarded as deviations from both, and presumably, there- 
 fore, as reversions to the ancestral species. 
 
 It may in all probability be assumed that the process is not a 
 complete one in the above-mentioned cases in which reversion 
 to the wild ancestral form in different races often occurs when 
 the latter are crossed. Darwin certainly gives one instance of a 
 pigeon which ' was hardly distinguishable from the wild Shetland 
 species,' but which was, nevertheless, descended from four 
 grandparents which differed very considerably from the wild 
 species (Colutnba lima}.* This bird, which was blue, and pos- 
 sessed the typical black bars on the wings and tail, was descended 
 from a red spot, a white fantail, and two black barbs. These 
 breeds, as is well known, differ from the wild pigeon in colour 
 as well as in many other details, such as the length of the beak 
 and number of tail feathers ; and it would therefore be interest- 
 ing to ascertain whether these racial characters had all disap- 
 
 * 'Animals and Plants under Domestication,' p. 14.
 
 324 THE GERM-PLASM. 
 
 peared in the grandchild, and had become retransformed into 
 the corresponding characters of the wild species. Were this 
 so, the reversion might be considered complete, and the same 
 theoretical explanation could be given for it as in the case of 
 the Datura hybrid. Unfortunately Darwin leaves this point 
 untouched, as he devoted his attention chiefly to the coloration 
 so characteristic of the species. It seems to me to be very 
 probable, however, from several of his statements, that this was 
 also essentially a mere case of reversion as regards the colora- 
 tion of the plumage. I conclude this principally from the fact 
 that the blue or original coloration of Columba livia occurs in 
 all the principal breeds of pigeons, although these blue sub- 
 varieties are rare in some of them. The other racial charac- 
 teristics do not at any rate exclude the possibility of a blue 
 coloration ; and thus, on the other hand, reversion to the blue 
 colour is not necessarily accompanied by a reversion to all the 
 other characters of the ancestral form. 
 
 It is perfectly certain that in most cases the reversion 
 produced by cross-breeding is not complete, even as regards 
 the coloration, but gradually becomes more marked, so that at 
 first very faint and hardly perceptible indications of the wing- 
 and tail-bars are seen, and these become more pronounced 
 by degrees, so that a partial blue coloration with perfect bars, and 
 finally the perfect slate-colour and complete bars of the ances- 
 tral form are produced. The greater number of reversions in 
 pigeons must consequently be incomplete, z'.e., they must refer 
 to individual characters or groups of characters only, and we are 
 here concerned with the theoretical explanation of such cases as 
 these. 
 
 I take for granted the facts that all valuable races of pigeons 
 breed true, that all the main breeds are characterised by differ- 
 ences in form, and that sub-breeds differ merely as regards 
 colour. In my opinion this implies, in the first place, that the 
 germ-plasm of the main breeds has become essentially modified 
 from that of the rock-pigeon, and that only lesser portions of it 
 correspond to that of the ancestral form ; and also, that all the 
 determinants have not become modified to an equal extent, those 
 for the coloration having undergone the least, and those for 
 the whole body the most, alteration. I therefore suppose that 
 the germ-plasm of one of the main breeds consists of a number 
 of modified idants, none of which any longer correspond exactly
 
 THE PHENOMENA OF REVERSION. 325 
 
 to those of the ancestral form, so that no one of them, did it 
 control the ontogeny, could result in the development of a 
 rock-pigeon. This conclusion depends entirely upon the race 
 breeding true, for a germ-plasm which still contained individual 
 unmodified idants of the ancestral form would also necessarily, 
 or at any rate probably, once in a way, contain a majority of 
 ancestral idants at the same time, in consequence of the 
 incessant recombination of the idants in the reducing division 
 and in fertilisation ; and this must result in reversion to the 
 ancestral form. Such a reversion, however, never occurs in 
 purely bred races, but only when crossing takes place. 
 
 The germ -plasm of a race of pigeons therefore consists, 
 according to my view, of a certain number of idants, each of 
 which represents the type of the race. The majority of the ids 
 of which each idant is composed must consequently virtually 
 contain this type ; or, expressed more accurately, the whole of 
 the racial determinants as compared with those unmodified 
 determinants which may still be present are in the majority in 
 all the ids of every idant. 
 
 The fact that races breed true can in this way be thoroughly 
 explained. 
 
 As regards reversion to the coloration and markings of the 
 wild pigeon, we must suppose that, in the process of artificial 
 selection to which the different races owe their origin, only just 
 as many ids have become completely transformed into racial ids 
 as were required to ensure the desired object, viz., the preserva- 
 tion of the racial characters. A larger or smaller number of 
 determinants in many or perhaps all the ids must have remained 
 unmodified in all, or at any rate in many, of the idants. The 
 determinants on which the coloration depends, must have 
 remained unmodified in larger numbers than did those relating 
 to any other characters, for the coloration is the most liable to 
 revert. 
 
 Reversion in the coloration will therefore occur when the 
 ancestral determinants for any particular region of the bird's 
 plumage gain a predominance over the racial determinants in 
 the course of development ; and in cross-breeds it will take 
 place when the racial determinants are so different that their 
 forces counteract one another instead of being cumulative. 
 Although the ancestral determinants are in the minority in the 
 germ-cells of the two different breeds which unite at fertilisation,
 
 326 THE GERM-PLASM. 
 
 their controlling forces will nevertheless be cumulative, and if 
 they are sufficiently numerous, they will determine the colora- 
 tion in question, and thus produce reversion. 
 
 In this way we may, I think, account not only for the pheno- 
 menon of reversion in general, but also for many special details, 
 and more especially for the different degrees of reversion in 
 different races of pigeons. A sufficient number of experiments 
 has certainly not bean made with regard to this point, but it is 
 nevertheless recognised that reversion occurs more easily and 
 more markedly in some races than in others. Darwin, for 
 instance, by crossing two black barbs with two red spots 
 obtained dark hybrids, of which ' no less than six presented 
 double-wing bars.' * On the other hand, the mongrels derived 
 from two black barbs and two snow-white fantails showed no 
 trace of reversion. This must have been due to the retention 
 of a different number of unmodified specific determinants in 
 the germ-plasm of the two races, as well as to the difference 
 of the modified racial determinants ; for the more marked the 
 difference between the two crossed races, the more easily will 
 the ancestral determinants gain the predominance over them. 
 
 The last-named experiment by Darwin was continued by 
 pairing two of these hybrids, one of which was brown and the 
 other black. The first brood (it is not stated how many there 
 were) displayed wing-bars ' of a darker brown than the rest of 
 the body.' This must have been due to the accumulation of a 
 larger number of unmodified determinants in individual germ- 
 cells in consequence of the reducing division, and to the subse- 
 quent union of two of these cells in the process of fertilisation. 
 We should therefore expect that reversion would not occur in 
 all the offspring of this pair, for the reducing division must also 
 result in certain germ-cells containing a majority of the modified 
 determinants. In the second brood of the same parents, in 
 fact, a brown bird was produced which possessed no trace of 
 wing-bars. 
 
 It is easy on the basis of our theory to account for the fact 
 that a simple crossing of two species did not in many instances 
 produce any traces of reversion, although reversion resulted 
 from the subsequent double crossing. The most complete case 
 of reversion obtained by Darwin was produced as follows. A 
 
 * Darwin, I.e., Vol. I., p. 208.
 
 THE PHENOMENA OF REVERSION. 327 
 
 mongrel female barb-fantail and a mongrel male barb-spot 
 were paired. Neither of these birds ' had the least blue about 
 them, nevertheless the offspring' (the number is not stated) 
 'from the above two mongrels was of exactly the same blue tint 
 as that of the wild rock-pigeon from the Shetland Islands over 
 the whole back and wings ; the double black wing-bars were 
 equally conspicuous.'* In this case the ancestral determinants 
 evidently prevailed all the more strongly because they were 
 opposed by racial determinants of three or four different kinds, 
 the controlling forces of which could not simply be cumulative, 
 like those of the ancestral determinants, but could only partially 
 weaken and neutralise one another. 
 
 The fact which may be deduced from Darwin's observations, 
 viz., that the offspring of simple crosses display practically a 
 very similar tendency to reversion, can also easily be explained 
 theoretically. For the germ-plasm of a well-established race 
 will contain a certain percentage of ancestral determinants, and 
 the germ-cells of an individual will be liable to but few 
 fluctuations in this respect. In such simple cases of crossing, 
 an almost similar number of ancestral determinants, as well 
 as of racial determinants, must therefore come together at each 
 fertilisation ; and the struggle between these different kinds of 
 determinants must always produce approximately the same 
 result. A perfect uniformity in the offspring of the same cross 
 cannot be expected ; but if reversion does occur in some cases, 
 it will be absent in others, and if it occur partially in some, 
 certain parts in others will revert. An instance of the former 
 kind is seen in the above-mentioned cross between a black barb 
 and a white fantail, and of the latter in the case described by Dar- 
 win on p. 207 of his ' Animals and Plants under Domestication,' 
 Vol. I. He crossed a white nun with a red tumbler, and reared 
 five young, all of which presented traces of reversion. One 
 possessed a blue tail ; the second and third ' presented a trace of 
 the bar at the end ' of the blue tail ; the fourth ' was brownish, 
 and the wings showed a trace of the double bar ' ; and the fifth 
 ' was pale blue over the whole breast, back, croup, and tail, but 
 the neck and primary wing-feathers were reddish, and the wing- 
 bars presented two distinct bars of a red colour.' Thus all the 
 five young had reverted some more and some less markedly 
 
 * Loc. cit., Vol. I., p. 209.
 
 328 THE GERM-PLASM. 
 
 in one part or another. The differences between the young 
 support the theoretical assumption that the number of ancestral 
 determinants, which determined the type of the five young ones, 
 was by no means the same in the different germ-cells of the 
 two parents. Their dissimilarity, moreover, admits of a further 
 conclusion, which could likewise be foreseen theoretically, viz., 
 that the determinants for the coloration of the different regions of 
 the body are represented in the different germ-cells in fluctuating 
 numbers. Not only should we naturally suppose this to be the 
 case from the structure of the germ-plasm as here assumed, but 
 we can hardly even imagine it to be otherwise. If the various 
 idants in the germ-plasm of a hybrid together contain twenty 
 ids with ancestral determinants, it does not follow that the latter 
 only are present in each id. There is no reason why in any one 
 id the determinants for the colours of the wings, for instance, 
 should not be ancestral, while those for the colours of the tail 
 are racial, or vice versa. The bird would display reversion in 
 the tail or wings, according to whether the former or the latter 
 kind of determinants is in the majority in the germ-cell : for 
 our theory shows that the struggle of the determinants takes 
 place independently in every cell, and consequently also in 
 every part of the organism ; and the result will therefore be in- 
 dependent in each part, and will depend solely on the combina- 
 tion of homologous determinants which contend with each other 
 in the cell or group of cells under consideration. It can thus 
 easily be explained how it comes about that though all the 
 offspring of a certain cross show reversions, the latter are never- 
 theless combined in very different ways with the racial characters 
 of the parents. 
 
 We must now consider the third type of reversion mentioned 
 at the beginning of this section, viz., that which concerns in- 
 dividual characters of very remote ancestors, and is seen, for 
 instance, in the stripes on the legs of horses and asses, and 
 more especially of mules. 
 
 It will not be necessary to repeat the arguments which Darwin 
 used to show that the horse was primitively dun-coloured and 
 striped. Indication of such a coloration are seen at the present 
 day in horses of different colours in all parts of the world, in the 
 form of a dark stripe down the back, and of transverse markings 
 on the legs and shoulders : such markings, however, are not 
 common, and most frequently occur in dun-coloured horses.
 
 THE PHENOMENA OF REVERSION. 329 
 
 The ass has been proved to be descended from a species pos- 
 sessing stripes on the legs, and corresponding to the existing 
 wild species (Asinus tceniopus) of Abyssinia. The domesticated 
 ass has, as a rule, only retained the crossed stripe on the back 
 and shoulders, but occasionally transverse markings occur on 
 the legs, and this is most often the case in animals of a pure 
 grey ground colour. The stripes on the legs are very rare both 
 in horses and asses, as I know from personal observation, and 
 even when present they are very faint. 
 
 In mules these stripes occur much oftener and much more 
 distinctly particularly in light grey animals, and are some- 
 times seen on the hind-legs as well as on the fore-legs and 
 shoulders. Thus reversion occurs to the ancestral form of both 
 sides. 
 
 The theoretical explanation of this fact must be based on 
 the assumption that a certain number of unmodified ancestral 
 determinants for the hairy coat have been retained in the germ- 
 cells of the two species ; and that these, when they are brought 
 together in the germ-cells of the two parents, may predominate 
 over the modified determinants of the parents. The fact that 
 such reversion does not by any means always occur in mules, 
 points to the conclusion that the number of ancestral deter- 
 minants varies very considerably in the germ-cells of individuals, 
 and that these may even be entirely absent, or only sparsely 
 represented, or, on the other hand, may be present in large 
 numbers. In the latter case, when two of these germ-cells come 
 together in fertilisation, partial reversion will occur, and will be 
 more marked and extensive the greater the number both of the 
 ancestral determinants which meet together and the parts of 
 the body to which these belong. The determinants for the 
 stripes on the hind-legs are evidently much less numerous in 
 the combined germ-plasm of the two existing species than are 
 those for the marks on the fore-legs, as is shown by the manner 
 in which these stripes have been relatively retained by several 
 existing wild species of Equus. All these determinants must, 
 however, be present in very varying numbers in the germ-plasms 
 of different individual horses and asses, for, as already stated, 
 reversion does not by any means always occur in mules ; in Italy, 
 where these animals are used in large numbers, it is not common, 
 and, as far as my experience extends, is perhaps only exhibited 
 in one or two animals out of a hundred. Gosse, however, states
 
 330 THE GERM-PLASM. 
 
 that in the United States of America nine mules out of every 
 ten are striped. 
 
 Before concluding this section, I will analyse a case of 
 reversion to remote ancestors in plants in greater detail. My 
 reason for so doing is not because different principles are 
 necessary for its explanation, but because we possess the results 
 of experiments which render a closer examination of $he theory 
 possible. 
 
 I will select as an example the case of the reversion of 
 irregular or unsymmetrical flowers to a regular or peloric form. 
 Many instances of reversion of this kind have been described, 
 but they are very exceptional : their origin cannot be connected 
 with any external causes, and must evidently be due to purely 
 internal ones, viz., to the composition of the germ-plasm. 
 
 If my opinion concerning the transformation of the germ- 
 plasm in the course of phylogeny is correct, it follows, as has 
 already been shown, that individual unmodified determinants of 
 an old character must always appear here and there in the 
 germ-plasms of the modern species, even after an enormous 
 number of generations. Such ancestral determinants those 
 of the original regular flowers, for instance need not by any 
 means be contained in the germ-plasm of every individual 
 plant ; and the older the modern species is, the fewer will be 
 the number of these determinants, which we will call ' peloric.' 
 They must gradually be displaced by the ' asymmetrical ' deter- 
 minants ; for the latter, being better adapted to the existing 
 conditions, have a better chance in the struggle for existence. 
 A large number of plants, such as Corydalis ttiberosa, for 
 instance, will thus no longer contain any ' peloric ' determinants, 
 and this makes it apparent why reversion so seldom occurs in 
 these cases. The fact that it can occur at all, is to be accounted 
 for by the processes of reducing division and amphimixis in two 
 germ-cells, the latter of which always follows on after the 
 former. For if a small number of ' peloric ' determinants still 
 remained in various idants of individual plants of the species in 
 question, they might occasionally come together in one germ- 
 cell owing to the reducing division ; and if two such germ-cells 
 meet one another in the process of fertilisation, this group of 
 determinants may predominate, and reversion will then occur 
 provided that the combined power of these ' peloric ' deter-
 
 THE PHENOMENA OF REVERSION. 331 
 
 minants is sufficient to predominate over the 'asymmetrical' 
 determinants. 
 
 The experiments which have been made with respect to the 
 transmission of characters in such abnormally peloric flowers, 
 prove that this explanation must be correct in principle. Dar- 
 win crossed the peloric snapdragon (Antirrhinum majus) 
 with its own pollen, and from the seeds thus obtained raised 
 sixteen plants, which ' were all as perfectly peloric as the parent 
 plant.'* We need not be surprised that the .pelorism was 
 inherited, for the ' peloric ' determinants were in the majority in 
 the parental germ-plasm on either side ; it may, however, have 
 been due to chance that all the sixteen plants which were reared 
 proved to be peloric. If a larger number of offspring had 
 been raised, some of them would certainly have produced 
 asymmetrical flowers, for the reducing division would in most 
 cases divide the ' peloric ' determinants unequally amongst the 
 two resulting cells, and consequently two germ-cells containing 
 no, or only a minority of, ' peloric ' determinants might meet 
 together in fertilisation. Reversion to the ordinary form of the 
 flower must then occur. 
 
 The result of Darwin's counter-experiment is particularly 
 interesting. The peloric snapdragon was crossed with the 
 common form, and ' two great beds of seedlings ' t raised, ' not 
 one of which was peloric.' And in ninety plants which were 
 carefully examined there was not a trace of pelorism, ' except 
 that in a few instances the minute rudiment of the fifth stamen, 
 which is always present, was more fully or even completely 
 developed.' Darwin attempted to explain this fact by the 
 assumption that in this case the common form of the flower 
 possessed a ' prepotent force of transmission ; ' but apart from 
 the fact that this statement is merely another formulation of an 
 observed fact, and can hardly be looked upon as a real ex- 
 planation, it does not hold in the case of Darwin's subsequent 
 experiments. For the plants obtained by crossing the common 
 snapdragon with the peloric form, ' which perfectly resembled 
 the common snapdragon, were allowed to sow themselves ; and 
 out of a hundred and twenty-seven seedlings, eighty-eight 
 proved to be common snapdragons, two were in an intermediate 
 
 * ' Animals and Plants,' &c., Vol. II. 1888, p. 46. 
 t Loc. cti., Vol. II., 1888, p. 46.
 
 332 THE GERM-PLASM. 
 
 condition between the peloric and normal state, and thirty-seven 
 were perfectly peloric.'* If now, the character of asymmetry 
 possessed a ' prepotent force of transmission,' we should expect 
 it to become more apparent when the flowers of both parents 
 were asymmetrical, than when this was the case as regards one 
 parent only. In the chapter of the same book which treats of 
 pangenesis, Darwin has attempted to give a special explanation 
 of the fact ' that a character gains strength by the intermission 
 of a generation ' in which this character is not present. 
 
 My own explanation of the above-mentioned facts follows 
 almost directly from what has already been said. The pro- 
 duction of so many individuals of the ordinary kind by the 
 crossing of the common snapdragon with the peloric form, and 
 the fact that a large number of 'perfectly peloric' offspring, 
 as well as of the common form, were produced when those 
 obtained by the first cross were fertilised with their own pollen, 
 is due simply to the reducing division of the germ-mother-cells 
 occurring in different ways, so that sometimes only ' ordinary ' 
 and sometimes only 'peloric' determinants reach the germ- 
 cell, and sometimes, again, a combination of both, in which 
 either the 'peloric' or the 'ordinary 'determinants preponderate. 
 The daughter-plant will produce flowers which are either quite of 
 the common form or more or less peloric, according to whether 
 the determinants which are brought together by the two germ- 
 cells in the process of fertilisation are chiefly ' peloric ' or chiefly 
 of the 'ordinary' kind. It is evident, however, that most of the 
 parents of this generation, which possessed flowers of the ordin- 
 ary form, must have contained 'peloric' as well as 'ordinary' 
 determinants in their germ-plasm before the reducing division, 
 for they were all derived from a peloric father or mother. The 
 predominance of the ordinary form in the next generation may 
 be explained as being due to the peloric grandparents possess- 
 ing only a slight majority of 'peloric' determinants in their 
 germ-plasm, in addition to a considerable number of ' ordinary ' 
 determinants. The whole of the germ-plasm of the germ-cells 
 from which the parent generation arose must have contained 
 very many more ' ordinary ' than ' peloric ' determinants. 
 
 * Lac. eit., Vol. II., p. 46.
 
 THE PHENOMENA OF REVERSION. 333 
 
 4. Reversion to Rudimentary Characters. 
 
 It is well known that organs which have lost their value for 
 the preservation of the species become rudimentary in the course 
 of generations : they diminish in size, become stunted, and 
 ultimately disappear altogether. 
 
 This may be expressed in terms of the idioplasm as follows. 
 The group of determinants in the germ-plasm for the organ in 
 question becomes reduced in one id after the other, first an 
 one determinant and then in another, until finally it disappears 
 completely ; and the process is repeated in an increasing 
 number of ids, until eventually this group of determinants is no 
 longer contained in any of them. It cannot be stated how long 
 a time and how many generations are required for this process, 
 but it may at any rate be asserted, and even proved, that indivi- 
 dual ids still contain determinants for the organ in question long 
 after its disappearance from the mature individual. The fact 
 that the organ occasionally reappears, and that consequently 
 re-version may take place, proves that this is the case. 
 
 The supernumerary nipples -which occasionally occur in human 
 beings are an interesting example of this kind. The two normal 
 nipples generally occur in a rudimentary form in men, but, in 
 addition to these, very diminutive ones are occasionally met with 
 in parts in which they are normally present only in the lower 
 orders of mammals, such as carnivores, rodents, and lemurs : a 
 pair may be present above the normal teats near the axillary 
 region, and two or three others lower down on the abdomen. 
 All of them certainly never occur in the same individual, but 
 usually only a single one or a pair are present : these, however, 
 are found both in men and women. They are undoubtedly 
 to be looked upon as reversions to extremely remote characters 
 possessed by our lower mammalian forefathers. We owe an 
 accurate account of their occurrence in the male sex to the 
 numerous and detailed investigations of Otto Ammon,* who 
 found them in three per cent, of our recruits. 
 
 As Ammon's researches only extend over two, or at most 
 three generations, and only refer to individual cases, we cannot 
 form a precise estimate as to the degree and extent to which 
 these structures are transmitted. We may, however, attribute the 
 
 * I am indebted to Mr Ammon for these details, which are not yet 
 published.
 
 334 THE GERM-PLASM. 
 
 stimuhis to reversion in these instances to interbreeding ; that 
 is to say, to the amphimixis of such germ-cells as contain a 
 certain number of ancestral determinants in their germ-plasm 
 on either side for those regions of the skin in which the nipples 
 were situated in remote ancestors. Merely in consequence of 
 the reducing division, these determinants may possibly accumu- 
 late in one germ-cell in sufficient numbers to produce the 
 character in question. The same may be said with regard to 
 many small individual marks, and such very ancient hereditary 
 parts as supernumerary nipples usually follow the same rule as 
 do individual characters ; they have to a certain extent degene- 
 rated so as to come under the same category, and for a long 
 time past have not been contained in the germ-plasm of every 
 individual ; their determinants, on the contrary, are entirely 
 absent in most cases, and are only found in a certain number 
 of ids in certain individuals. These determinants, like those of 
 individual characters, may be transmitted for several generations 
 without attaining development, and may then suddenly become 
 manifest in consequence of a favourable combination of two 
 conjugating germ-cells. The only difference between these an- 
 cestral characters and ordinary individual peculiarities is, that 
 the determinants of the latter are contained in a larger number of 
 ids, and we must therefore conclude that they become developed 
 far more frequently and regularly. 
 
 Although in the case of the supernumerary nipples in the 
 human race we cannot definitely indicate the ancestor from which 
 they were derived, or the length of time during which they have 
 been transmitted, it is at any rate possible to do so approxi- 
 mately in the case of the supernumerary toes of horses. For, 
 thanks to the excellent researches of Kowalewsky, followed 
 by those of Marsh, we are well acquainted with the phyletic 
 development of the horse : we now know that horses belong- 
 ing to the genera Mesoh'ppus, Miokippus, and Protohippus or 
 Hipparion, which possessed two smaller lateral toes in addition 
 to the large median ones, existed in the middle Tertiary 
 period. When horses are occasionally born at the present 
 day in which one or two such accessory toes are present on two 
 or even on all four feet, we are perfectly right in considering the 
 development of these toes to be due to reversion to an ancestor 
 of the Miocene period. We must therefore assume that in cer- 
 tain series of generations of the existing horse, some idants have
 
 THE PHENOMENA OF REVERSION. 335 
 
 been retained in. which ancestral determinants of the fore-and 
 hind-feet are present, but that these are in the minority, and a 
 large number of them can only accumulate in one germ-cell 
 when a particularly favourable reducing division occurs. This, 
 however, would not in itself be sufficient to produce the character 
 in question : chance must also play a part, in order that such an 
 egg-cell, containing an abundance of ancestral determinants, 
 may be fertilised by a spermatozoon in which a certain number 
 of these determinants are also contained. Then, and then only, 
 will there be a likelihood that the entire number of the latter 
 will be sufficiently large to preponderate over the modern 
 determinants of the foot in the process of ontogeny. 
 
 Cases of reversion of this kind occur very rarely. Marsh has, 
 however, brought forward a small series of such instances, the 
 oldest of which relates to a horse belonging to Julius Caesar, and 
 the most recent was observed by him in a living animal.* 
 
 5. Preliminary Summary of Sections 1-5. 
 All the phenomena of reversion which have so far been 
 considered can be explained on the supposition that every 
 germ-plasm is composed of a large number of equivalent units 
 or ids, each of which possesses all the determinants required for 
 the development of an organism ; every character is therefore 
 produced by the co-operation of many determinants of the 
 same region (homologous determinants). The transformation 
 of a species or race into a new one, moreover, never depends 
 primarily on a simultaneous modification of all the ids and 
 determinants : when the modification begins to take place, even 
 entire groups of ids (idants) may remain unmodified, and this 
 will subsequently be the case as regards a minority of ids, and 
 still later at least with regard to certain of the determinants in 
 individual ids. The characteristic form of every individual cell 
 which takes part in the process of ontogeny is the result of the 
 struggle of the ids which occurs in this cell ; and as some 
 amongst the mass of determinants which constitute every id in 
 the germ-plasm may become modified and others not, and the 
 proportion of modified to unmodified determinants may vary 
 from id to id, we can understand that in crosses between species 
 
 * O. C. Marsh, 'Recent Polydactyle Horses,' American Journal of 
 Science, Vol. xliii., April 1892.
 
 336 THE GERM-PLASM. 
 
 or races, reversion is sometimes more and sometimes less marked 
 in some cases restricted and in others very extensive. In 
 those cases, therefore, in which rudimentary and insignificant 
 characters only are concerned, such as stripes in mules, pelorism 
 in flowers, and rudimentary nipples in the human race, indi- 
 vidual -variations must frequently occur in all degrees, from the 
 complete absence of reversion to the most marked form of it. 
 
 Conversely, it is just as easy to explain why reversion occurs 
 without exception in other cases, such 'as in that of Datura- 
 hybrids, if we assume that entire groups of ids or idants, as well as 
 certain determinants in individual ids, have remained unmodified. 
 For if the former are present in such numbers as to preponderate 
 in a hybrid over the two kinds of modified ids, which counteract 
 one another, they must be contained in nearly equal numbers in 
 the germ-plasm of every individual. 
 
 We can in principle understand the whole series of the 
 phenomena of reversion, if we represent the transformation of 
 the germ-plasm as being due to the majority of the ids in most 
 of the idants becoming modified first, while a minority of them 
 remain unmodified : in the course of generations the number of 
 these unmodified ids then gradually becomes smaller, owing to 
 the action of natural selection, until the ancestral species is only 
 represented by a number of scattered ids ; and finally, by con- 
 tinued selection, these ids also become modified in the same 
 direction, to such an extent that eventually only the determinants 
 of those individual characters escape modification which are less 
 important or quite meaningless for the life of the organism. We 
 know that complete reversion to the ancestral form may occur 
 in young species in the case of favourable crosses between 
 allied species (Datura) and that in the further course of 
 phylogeny, that is, in older species, total reversions can no 
 longer occur, although reversion to single characters, or even to 
 entire groups of characters, may still take place, and does so 
 with certainty under certain conditions as is exemplified by the 
 reversion of certain hybrids between different races of pigeons. 
 In its last stage, then, reversion occurs as an entirely uncertain 
 and apparently capricious re-appearance of an individual an- 
 cestral character, such as, for instance, that of the occasional 
 striping in mules. 
 
 By means of our theory much may also be deduced and ren- 
 dered comprehensible in principle concerning the external causes
 
 THE PHENOMENA OF REVERSION. 337 
 
 of reversion. It can easily be understood that the crossing of 
 different species and races is liable to lead to a preponderance of 
 those ancestral idants, ids, or determinants which are common 
 to the two parents. We might, however, also infer from the 
 theory that certain of the offspring of hybrids are very liable to 
 revert in the next generation to one of the ancestral species ; 
 for the germ-cells of the hybrids are very dissimilar as regards 
 the germ-plasm they contain, in consequence of the reducing 
 division which every germ-cell undergoes at its formation : most 
 of them will contain idants of both parents in every conceivable 
 proportion, while in others, idants of only one of the parents 
 will be present. The ' reducing division ' is therefore one of the 
 most efficacious of the primary causes of reversion, for it renders 
 possible the uneven distribution of the different qualities of the 
 idants which were contained in the germ-plasm of the germ-cells 
 of this parent. This principle applies equally to the cases in 
 which not a single entire idant remains untransformed in the 
 germ-plasm, which then only contains a minority of unmodified 
 ids or even merely of determinants, scattered amongst several 
 idants. 
 
 Reversion in any degree therefore depends, in the first instance, 
 on the process of amphimixis, for without sexual reproduction the 
 reducing division would not have been introduced into the organic 
 world, and the second extremely important factor in reversion 
 viz., the crossing of different germ-cells would also not exist. 
 Reversion is, however, not of necessity connected with actual 
 amphimixis, but may, as I shall show later on, occur in con- 
 nection with parthenogenesis and gemmation ; although this is 
 only true in the case of those organisms in which amphimixis 
 formerly occurred, and the germ-plasm of which may therefore 
 contain ancestral ids or ancestral determinants. It is evident, 
 however, that the chance of reversion occurring must be much 
 greater when amphimixis takes part directly in the process of 
 reproduction ; for the relative proportion of modified and un- 
 modified units of the idioplasm of this order may be quickly 
 or suddenly altered in favour of the unmodified units, and thus 
 the ancestral units which were originally present may undergo 
 accumulation, and predominate over the somewhat dissimilar 
 homologous units of the modified kind. 
 
 Our comprehension of the problem just discussed is certainly 
 increased by the above considerations, and the following passage
 
 338 THE GERM-PLASM. 
 
 from Darwin's 'Animals and Plants under Domestication '* will 
 best show how much our insight has since then advanced : 
 'In purely-bred races, occasional reversion to long lost characters 
 of the ancestors often occurs without our being able to assign 
 any proximate cause.' 
 
 6. REVERSION IN ASEXUAL REPRODUCTION. 
 (a.) Reversion in the Process of Gemmation. 
 
 As already stated, reversion does not entirely depend on am- 
 phimixis, and may, in fact, also occur apart from the crossing 
 of two individuals. The bud-variations of plants form a well 
 known instance of this kind. 
 
 For many years I possessed in my garden a maple (Acer 
 negundo) with variegated leaves which were almost entirely 
 white, and one branch of this tree bore ordinary green leaves, 
 flowers, and seeds. Owing to the greater amount of chlorophyll, 
 this branch grew and bore flowers and seeds far more luxuriantly 
 than the main branch from which it arose. If we look upon the 
 offshoots of a tree as persons, this would be an instance of a 
 person of the plant-stock produced asexually, which rverted 
 to the ancestral form. 
 
 We must go back to the origin of variegated species in 
 order to find a theoretical explanation of this phenomenon. 
 Like most similar varieties, indeed, of our trees and shrubs, 
 this form must have arisen by bud-variation; in other words, 
 a normal maple, from certain unknown causes, gave rise to a 
 branch bearing variegated leaves. The cause of this modification, 
 traced to the idioplasm, must have been due to the determinants 
 of the leaves and other green parts of the shoot becoming 
 modified in such a way as to result in the production of organs 
 deficient in chlorophyll If, however, only the majority, and not 
 all the ids in the apical cells of the first variegated shoot became 
 modified in this manner, a reversion of the variegated variety 
 to the green ancestral species would become possible. 
 
 Another assumption, however, which cannot yet be proved to 
 be true, is required in order to account for the appearance of a 
 green branch upon a variegated tree : we must suppose that 
 even in -ordinary cell and nuclear divisions, the division of the 
 idioplasm may take place in an irregular manner, so that all the 
 
 * Vol. II., p. 25.
 
 THE PHENOMENA OF REVERSION. 339 
 
 ids do not reach both the daughter-nuclei ; but that both the 
 halves of some of them which result from the division pass into 
 the same daughter-nucleus. On the other hand, it might even 
 perhaps be possible for an entire idant to be transmitted to only 
 one of the daughter-nuclei. It must be possible for some such 
 irregularity to occur in the distribution of ids during nuclear 
 division, for otherwise the occurrence of a different combination 
 of the primary constituents in the course of growth such as, 
 indeed, actually occurs in cases of bud-reversion would be 
 inexplicable. 
 
 Let us distinguish between those ids which correspond to 
 the original form, and those which have become modified, by 
 describing them respectively as ' green ' and ' variegated ' ids. 
 The reversion of a shoot must then be due to the unequal 
 distribution of the ids amongst the daughter-nuclei during the 
 divisions of the apical cells of the shoot, so that a majority of 
 'green' ids were distributed to a cambium cell containing 
 ' blastogenic ' germ- plasm, or even also to an apical cell of 
 a young lateral bud. Conversely, a variegated shoot might 
 subsequently originate from a green one, as in fact actually 
 occurs. 
 
 I should be inclined to offer a similar explanation of the re- 
 versions to the ancestral form which so frequently occur in all 
 the numerous varieties of our trees and shrubs, such as, for 
 example, the oak-leaved heterophyllous hornbeam, the fern- 
 leaved oak, the varieties of the maple and birch which possess 
 greatly subdivided leaves, and the copper-beech and copper- 
 hazel. The tendency to revert is very varied even in the 
 different varieties of the same species. The golden-striped 
 variety of Euonymus japonica, for instance, is very liable to 
 revert, while in the silver-striped variety of this plant reversion 
 rarely occurs. 
 
 This difference would simply depend on the relative minority 
 of ancestral ids in the variety in question. Reversion will never, 
 or only rarely, occur when the idioplasm only contains a few 
 ancestral ids ; if, however, there are so many of these that the 
 unmodified ids only form a small majority, reversion can easily 
 take place. 
 
 In this connection I must mention another instance, which 
 has very frequently been discussed since Darwin's time, viz., 
 that of the peculiar ' graft-hybrid ' Cytistts adami. The com-
 
 34 THE GERM-PLASM. 
 
 bination of the characters of the two ancestral species con- 
 tinually varies in this plant, which sometimes bears the yellow 
 clusters of flowers characteristic of the common laburnum 
 (Cytisus laburnum), and sometimes purple flowers like those 
 of Cytisus purpurea, or, again, both these colours may be 
 present in the same flower. I will here quote Darwin's descrip- 
 tion of the plant * : " To behold mingled on the same tree 
 tufts of dingy-red, bright yellow, and purple flowers, borne on 
 branches having widely different leaves and manner of growth, 
 is a surprising sight. The same raceme sometimes bears two 
 kinds of flowers, and I have seen a single flower exactly divided 
 into halves, one side being bright yellow and the other purple, 
 so that one half of the standard-petal was yellow and of larger 
 size, and the other half purple and smaller. In another flower 
 the whole corolla was bright yellow, but exactly half the calyx 
 was purple. In another, one of the dingy- red wing-petals had 
 a narrow bright yellow stripe on it ; and lastly, in another 
 flower, one of the stamens, which had become slightly foliaceous, 
 was half yellow and half purple.' 
 
 The result of the struggle of the parental idants evidently 
 cannot depend in the case of Cytisus adami, as it does in that 
 of the individual characters of the human race, upon the fact 
 that the number of homodynamous determinants varies in the 
 parental idioplasm according to the part concerned ; for were 
 this the case the same parts of the flower could not sometimes 
 be yellow in some instances and red in others, all the flowers, 
 on the contrary, would display the same composition as regards 
 the parental hereditary parts, even though slight variations might 
 occur, such as would be produced by a dissimilarity in the con- 
 ditions of nutrition. As in the case of the hybrids of Oxalis 
 already mentioned, the flowers would at least display a certain 
 combination of parental characteristics which would be uniform 
 in one and the same plant. The fact that this is not the case, 
 seems to me to afford a decisive proof that Cytisus adami is a 
 real graft-hybrid, and not an ordinary seminal hybrid, as in 
 fact was stated to be the case by the nurseryman Adams, who 
 first produced it. I therefore consider the controversy ended 
 as to whether graft-hybrids exist at all, and offer the following 
 explanation of the hereditary phenomena concerned. 
 
 * Loc, cit., Vol. I., p. 414.
 
 THE PHENOMENA OF REVERSION. 34! 
 
 Cytisus adami was obtained from a young bud on a portion 
 of the bark of Cytisus purpurea, which was grafted into the 
 stem of C. laburnum. This bud developed into a shoot which 
 exhibited an intimate combination of the parental characters. 
 The shoot was afterwards propagated, and the plants raised 
 from it exhibited ' reversions to both of the parental forms,' as 
 well as dingy-red, i.e., mixed blossoms, so that the pure characters 
 of the parents were displayed in more or less extensive regions 
 of the hybrid. 
 
 From a theoretical point of view, it can obviously be granted 
 that a mongrel-plant may originate by contact of the living 
 tissues of the parents, only if the transformation of the rudiment 
 of an existing shoot is out of the question, and if the rudiment 
 then appears for the first time. An existing dormant bud, which 
 contains all parts of the shoot, cannot become modified as 
 regards the idioplasm which it contains by the stock of another 
 species which nourishes it : its apical cells, from which further 
 growth proceeds, cannot receive a supply of extraneous idioplasm 
 from the supporting stock ; for the nuclear rods alone contain 
 idioplasm, and this is a solid substance, and can only undergo 
 combination by the fusion of two cells and their nuclei. It is 
 therefore also worthy of notice that Adam did not observe that a 
 hybrid was developed from \hesingle dormant bud which was from 
 the first present on the graft, but that the hybrid arose from one 
 of the later buds which were formed in the second year; moreover, 
 only one, and not all, of these buds produced bud-hybrids. The 
 formation of this single hybrid-bud must have been regulated by 
 an unusual and accidental occurrence, for all efforts to produce the 
 hybrid a second time have so far been in vain. This accidental 
 occurrence must have been that the cambium-cells of the two 
 species came to lie close together, so that they could both enter 
 the same bud arising from the cambium. Botanists must 
 decide whether two cambium-cells, belonging to different species, 
 can conceivably become united into one by a process of con- 
 jugation similar to that which occurs in the union of the male 
 and female cells in fertilisation, and whether the foundation may 
 in this way be laid for a new growing point.* If such a process 
 
 * A fusion of nuclei, apart from that seen in the process of fertilisation, 
 actually occurs in plants in the case of the embryo-sac. Guignard (loc. 
 cit.) describes this process in detail as follows: The 'upper and lower 
 pole-nuclei ' of the embryo-sac approach one another, each accompanied by
 
 342 THE GERM-PLASM. 
 
 occurred in this case, the number ofidanfs ofCytisus adami must 
 be as large as that of the two ancestral species taken together; 
 for, as far as we know, a process of reducing division only 
 occurs in the formation of sexual cells. The correctness of my 
 assumption can therefore be controlled by observation. 
 
 It is hardly conceivable that two young plant-cells could, 
 without fusing, have formed the growing point of the hybrid- 
 shoot ; for probably only one of these cells could have performed 
 the function of an apical cell, and consequently the hereditary 
 influence of the other could not have extended to countless 
 daughter-shoots, as was actually the case. In the course of 
 growth, every cell below the apical cell must necessarily have 
 gradually come to be situated further away from the growing- 
 point. Such an intimate combination of characters as actually 
 occurred could not have been effected in this way. 
 
 I am therefore inclined to suppose that the unusual pheno- 
 mena exhibited by Cytisus adami were due to an abnormal 
 kind of amphimixis, so that the idants of both species were 
 combined in the apical cell of the first shoot ; but that in the 
 subsequent cell-divisions an unequal distribution of the two 
 kinds of parental idants amongst the daughter-nuclei took 
 place, thus producing the variations in the combination of the 
 characters. 
 
 Such an unequal distribution of the superabundant idants 
 might also occasionally occur in an apical cell itself. This pro- 
 cess may, moreover, be connected with the frequent complete 
 reversion of an entire-branch to one of the ancestral species, as 
 well as with the fact that a modification tending to make the 
 parental characters more and more distinct has occurred in the 
 course of time in many examples of hybrids. Shortly after the 
 first appearance of the Cytisus-hybrid, the colour of all the flowers 
 was a dingy-red, that is, an intimate mixture of the two ancestral 
 colours, yellow and purple ; but by degrees this mixture became 
 less perfect, until eventually pure yellow and pure purple flowers, 
 and even entire clusters of flowers and entire branches possessing 
 almost the pure character of one or other of the parent-species, 
 
 its centrosome, which has already become doubled by division. The 
 nuclei then come to be situated close together, the centrosomes uniting in 
 pairs, just as in the ordinary process of amphimixis ; and finally the two 
 nuclei fuse together completely. A spindle of division is then formed, and 
 several divisions follow close after one another.
 
 THE PHENOMENA OF REVERSION. 343 
 
 were formed. It seems only possible to explain this circumstance 
 by supposing that the connection of the two idioplasms was 
 easily severed, and that differential nuclear divisions occurred 
 in such a manner as to cause a larger number of idants of 
 C. purpureus to pass into one daughter-nucleus, and a larger 
 number of those of C. laburnum into the other ; or, at any rate, 
 to cause the one or the other idant to pass completely into one 
 of the daughter-nuclei, instead of dividing longitudinally and 
 one of the resultant halves entering each daughter- nucleus. 
 Though this is certainly only a conjecture, it is, however, not 
 altogether an unjustifiable one, for the apparatus for division in 
 each of the two species is certainly concerned with a smaller 
 number of idants than that which must be present after the 
 fusion of two nuclei ; irregularity might therefore easily occur at 
 the division. It is possible that unknown forces of attraction 
 may also play a part in the process : the idioplasms of the two 
 species do not at any rate exhibit a marked attraction to one 
 another, as might perhaps be conjectured from the negative 
 results obtained by Darwin, Reisseck, and Caspary in ordinary 
 experiments on hybridising. By fertilising C. laburnum with 
 the pollen of C. purpureus, Darwin obtained pods which dropped 
 off ' in sixteen days after the withering of the flowers,' and the 
 reverse cross resulted even less successfully. 
 
 However this may be, and the point can be settled by deter- 
 mining the number of idants, the phenomena of heredity at 
 any rate indicate that the idioplasms of the two parents can 
 easily again become separated in the course of cell-divisions. 
 This separation might perhaps begin with the passing over of 
 one idant only from one side, which would result in the pre- 
 ponderance of one parent in many of the blossoms, &c. This 
 preponderance would have increased considerably in the course 
 of growth, so that far larger groups of cells now frequently con- 
 tain pure idioplasm of C. laburnum or C. purpureus, and new 
 shoots are formed which apparently contain idioplasm of one 
 or the other species only. The fact that the plants raised by 
 Herbert from seeds of pure yellow flowers otCytisus adami, which, 
 although they bore yellow flowers, showed a purple tinge on the 
 flower stalks, proves that even these shoots may still contain 
 some individual idants of the other ancestral species. The 
 idants of C. purpureus seem, however, to have disappeared 
 entirely from certain shoots ; for Darwin raised plants from
 
 344 THE GERM-PLASM. 
 
 the seeds of yellow flowers, which resembled C. laburnum ' in 
 every character, with the exception that some of them had 
 remarkably long racemes.' 
 
 My explanation of the fluctuation in the combination of 
 parental characters in Cytisus adami rests on the fact that we 
 are not here concerned with the subtle differences in the forces 
 which, in the reproduction of the human race, cause either the 
 maternal or the paternal idants to predominate, and which we 
 have attributed to the number of homodynamous determinants, 
 which differs according to the characters concerned : this fluc- 
 tuation depends on the grosser differences, and on the number 
 of idants on the two sides. Sometimes the idants of C. pur- 
 pureus, and at others those of C. laburnum, predominate in the 
 idioplasm of a cell ; and, under certain circumstances, even only 
 the one kind of idant may be represented in a cell, in which 
 case it alone will also be present in all the offspring of this cell. 
 
 My theory is not therefore rendered less likely by the case of 
 Cylisus adami; we can, on the contrary, even explain the most 
 minute details of such phenomena by its means, although it was 
 not formulated for this purpose. 
 
 6. Reversion in Parthenogenesis. 
 
 Reproduction by means of unfertilised ova occurs regularly in 
 some plants and in many animals more especially in crustaceans 
 and insects. We might be inclined to expect, a priori, that no 
 great degree of deviation between the mother and daughter 
 could occur at all in this monogonic form of reproduction, and 
 that at most a reversion to more remote ancestors could take 
 place. 
 
 Such a supposition is not, however, borne out by experience. 
 A series of experiments with parthenogenetic species, which I 
 have made during the course of the last eight years, have shown 
 that although the expected uniformity between parent and off- 
 spring generally results in a very high degree, exceptions occur 
 occasionally, and that these must be regarded as exhibiting 
 reversions to an ancestral form many generations removed. 
 
 The facts are briefly as follows. Two varieties of a small 
 Ostracod (Cypris reptans\ possessing a very marked coloration, 
 occur in certain ponds in the neighbourhood of Freiburg. The 
 colour of one variety A, is light yellow ochre, five green spots 
 being present on either side of the shell ; the other variety B|
 
 THE PHENOMENA OF REVERSION. 
 
 345 
 
 appears dark green, owing to the yellow-ochre ground colour 
 being reduced in extent by the presence of six large green 
 patches. These patches exactly correspond as regards position 
 in the two varieties, and are merely much smaller in A than in 
 B, the sixth spot being wanting in A. Both varieties are repro- 
 duced parthenogenetically in the neighbourhood of Freiburg, 
 and males never appear. 
 
 FIG. 23. The two varieties of Cj-/>rts reptatu. 1-6, indicate the six main patches 
 of colour on the shell. 
 
 My experiments were made by isolating one female of each 
 variety in a small aquarium, feeding them well, and allowing 
 them to multiply until each vessel was filled with their mature 
 descendants, which in their turn produced eggs. The individuals 
 of the colony were then examined, and the greater number 
 killed and preserved, one or more of them being kept alive 
 and placed separately in fresh aquaria for breeding purposes. 
 
 These animals breed very rapidly throughout the year, and 
 thus in the course of the last eight years many thousands of 
 individuals have passed through my hands. 
 
 The first result obtained by these experiments confirmed
 
 346 THE GERM-PLASM. 
 
 my expectations : the descendants of the same mother resembled 
 one another as well as the parent with which tJte experiments 
 began, even as regards minute details of the markings. The 
 differences were mostly as small as those which may be observed 
 in identical human twins : it cannot be stated whether these 
 were due to a want of similarity in the germ-plasm, or whether 
 they were to be accounted for by accidental differences in 
 nutrition. 
 
 Apart from the exceptions about to be mentioned, no modifi- 
 cations occurred even in the course of many generations. I 
 now possess colonies of A and B which cannot be distinguished 
 from their ancestors in the year 1884. Reckoning that from 
 five to six generations were produced each year, about forty 
 generations have been passed through since then. 
 
 In 1887, some individuals of the dark -green variety B appeared 
 for the first time in an aquarium containing the typical yellow- 
 ochre-coloured variety A ; and since then I have twice observed 
 a similar occurrence in other broods of A. In the last of these 
 cases, examined in May 1891, it could be proved that only a 
 single Cypris out of 540 adults contained in one aquarium had 
 changed into the dark variety suddenly, and without apparent 
 cause. In another case, intermediate forms between the two 
 varieties were found, as well as ordinary individuals of the 
 variety B ; and this fact is not only of interest theoretically, 
 but it also removes all doubt as to the trustworthiness of the 
 experiments. 
 
 For a long time I waited in vain for the occurrence of the 
 reverse transformation from the dark variety to the light-coloured 
 one, and was inclined to consider that the former was the ances- 
 tral form. But in the winter of 1890-91, a colony of B appeared 
 in which a few typical individuals of the variety A were found, 
 together with typical specimens of B which had been bred in this 
 aquarium for many years. 
 
 It is out of the question that these sudden transformations 
 were due to external influences, for both forms made their 
 appearance together in the same small aquarium, under precisely 
 the same conditions. We can only suppose that modifications 
 in the composition of the germ-plasm must have taken place, 
 and I think it is possible to prove that this was the case. 
 
 Parthenogenesis, using this term in the strict sense, has in all 
 cases been derived from sexual reproduction, as is proved in
 
 THE PHENOMENA OF REVERSION. 347 
 
 this case by the fact that the unmated females retain the recep- 
 taculum seminis, which is unused and always empty. 
 
 The two varieties must have originated at a time when sexual 
 reproduction occurred at any rate periodically : were this not 
 so, primary constituents of A could not be present in the genii- 
 plasm of B, and vice versa. The co-existence of both kinds of 
 primary constituents in the same animal, can only be understood 
 if we suppose that sexual reproduction had occurred at no very 
 distant period. 
 
 The explanation of the process of reversion naturally follows 
 from the fact that in species in which parthenogenesis regularly 
 occurs, a reducing division nevertheless takes place, but only a 
 single one : one polar body is separated off from the egg, and 
 not two, as in sexual reproduction. This single halving of the 
 idants in the ovum must undoubtedly be preceded by a doubling, 
 just as occurs in sexual reproduction ; for a reduction of the 
 number of idants to one half would otherwise take place from 
 generation to generation, so that ultimately only a single one 
 would be left. If, however, a reducing division preceded by a 
 doubling of the number of idants takes place, reversion becomes 
 possible. 
 
 Let us take a case of the simplest possible kind, and suppose 
 that there are four idants in the germ-plasm, three of which are 
 entirely composed of ids of the type A, and the fourth of ids of 
 the type B. The four idants a, a, a, b, first become doubled, so 
 that eight idants, a, a, a, a, a, a, b, b, result. Let us assume the 
 most favourable case for reversion towards the variety B to 
 occur, the reducing division resulting in the separation of these 
 idants into the groups a, a, a, a, and a, a, b, b, the latter of which 
 forms the nucleus of the egg-cell : the daughter individual arising 
 from this egg would then produce primary ova containing the 
 group of idants a, a, b, b. After the process of doubling in the 
 ripe ovum, this group would then have the composition a, a, a, a, 
 b, b, b, b ; and if the reducing division occurred in such a manner 
 as to result in the combination a, a, a, a, passing into the polar 
 body, and the combination b, b, b, b, remaining in the nucleus 
 of the ovum, an individual of the variety B would undoubtedly 
 arise from the egg, and reversion would ensue. 
 
 These processes would in reality be much less simple, and 
 would take place much more slowly. For the number of idants 
 is doubtless in most cases much larger, and the addition of a
 
 348 THE GERM-PLASM. 
 
 single idant of the other form would result in a much smaller 
 percentage. I have been able to prove the existence of 24-26 
 idants in the case of Artemia salina, a crustacean which multi- 
 plies parthenogenetically. If even several of these belonging to 
 one of the two ancestral varieties were present in a minority, 
 it is questionable whether they would ever become accumulated 
 in one and the same germ-plasm in the course of generations 
 and the corresponding reducing divisions, so as to form a pre- 
 ponderating majority : such an accumulation is, however, con- 
 ceivable. This would partly depend on chance, for the majority 
 of the ova produced by an individual always perish, and rare 
 combinations which would produce reversions are therefore also 
 usually lost. 
 
 We may thus account for the fact that reversion only occurred 
 rarely in my experiments and only in certain individuals of a 
 colony at the same time, as well as for its appearing either 
 suddenly, or else after the production of intermediate forms. 
 The latter case is to be explained theoretically by supposing 
 that a balance of the two kinds of idants was first produced, and 
 that this then in the offspring partially led to the preponderance 
 of the idants on which reversion depended. 
 
 The possibility of reversion in parthenogenesis therefore de- 
 pends upon two factors : firstly, upon the composition of the 
 germ-plasm out of different kinds of ids and idants, i.e., upon 
 the occurrence of sexual reproduction in a previous generation ; 
 and, secondly, upon the 'reducing division' which always takes 
 place at the formation of germ-cells. 
 
 7. Proof that the Determinants become Disintegrated into 
 Biophors. 
 
 I will conclude this chapter with some remarks which might 
 have been more suitably inserted in the chapter on the control 
 of the cell by the idioplasm, but in that place they would have 
 been unintelligible, as they depend upon a knowledge of the 
 phenomena of maturation of the ova and spermatozoa. 
 
 The fact that actual germ-plasm is removed from the animal- 
 ovum by means of the cell-divisions occurring before maturation, 
 that is, in the process of the separation of the two polar 
 bodies, was not by any means easily determined, and was only 
 proved after at least ten years of prolonged and difficult investi- 
 gation and reasoning. I have not thought it advisable to give
 
 THE PHENOMENA OF REVERSION. 349 
 
 an account of the whole history of the development of our 
 knowledge of this subject,* as this is unnecessary for under- 
 standing the phenomena of heredity. I must, however, refer 
 to a period of this history, which is instructive as regards the 
 method by which the cell is controlled by the emigration of 
 nuclear matter into the cell-body as enunciated by de Vries and 
 accepted by myself. 
 
 After the so-called ' extrusion ' of the polar bodies of the egg 
 had been shown to correspond merely to a very unequal cell- 
 division, and convincing proof had been given that the controlling 
 substance must be situated in the chromatin of the nucleus, it 
 followed as a logical postulate that the ovum, like every other 
 cell, must be supposed to contain a special controlling substance, 
 or specific idioplasm, the function of which is to produce the spe- 
 cial histological nature of the cell in question. This conclusion 
 I arrived at, and assumed that the ovum, from its earliest stage 
 to the attainment of its full size and specific nature, is controlled 
 by a special idioplasm, which differs entirely from the idio- 
 plasm which becomes active after the completion of maturation. 
 If the nature of the cell is determined at all by its idioplasm, 
 the ovum, while still growing and undergoing histological de- 
 velopment, cannot possibly be controlled by the same idioplasm 
 as that which serves for embryonic development. I consequently 
 assumed the existence of an 'oogenetic' idioplasm in the egg 
 during the period of its histological differentiation, and also that 
 after maturation, this substance gives up the control of the cell 
 to the germ-plasm. 
 
 The question then arises as to what becomes of the oogenetic 
 idioplasm when this change in the control takes place. 
 
 My answer to this question was, that the oogenetic idioplasm 
 is removed from the ovum by means of the polar divisions, and 
 that it was thus rendered possible for the germ-plasm which was 
 already present in the nucleus of the ovum, and had in the 
 meantime increased considerably in bulk to obtain control of 
 the cell. 
 
 This conjecture has since turned out to be erroneous. Investi- 
 gations which I subsequently made soon showed that at least 
 one of the two polar divisions has a totally different significance, 
 
 * Such an account is given in my essay on ' Amphimixis,' and in still 
 greater detail in the previous 'Essays upon Heredity.'
 
 350 THE GERM-PLASM. 
 
 inasmuch as it results in the halving of the germ-plasm itself. 
 It became apparent subsequently that both divisions serve this 
 purpose, and each of them causes the removal of germ-plasm, 
 and not of oogenetic idioplasm from the ovum. 
 
 My hypothesis must therefore be given up, but I nevertheless 
 believe that the conclusion on which it was based was a correct 
 one, and that it may be further utilised in the light of the theory 
 of heredity here developed. The oogenetic idioplasm must 
 exist, and, using the terminology I have now adopted, it may 
 be spoken of as the oogenetic ' determinant.' This determinant 
 will consequently be the first to become separated from the 
 mass of germ-plasm of the young egg-cell, to disintegrate into 
 its constituent biophors, and to migrate through the nuclear 
 membrane into the cell-body. In this way alone can we account 
 for no trace of it remaining in the nucleus, and for embryonic 
 development not being subsequently impeded by its presence. 
 This determinant is used up, and disappears as such; and the 
 fact that it is not expelled from the egg strongly indicates, if it 
 does not prove, that the control of a cell by a determinant is 
 accompanied by the absorption of the latter, and a further sup- 
 port may thereby be obtained for the hypothesis of emigration. 
 
 A precisely corresponding process must be assumed to occur 
 in the formation of the sperm-cells, in which also the function 
 of the idioplasm during the histological differentiation of the cell 
 differs widely from that of the germ-plasm of the mature sperma- 
 tozoon. The necessity for assuming the existence of 'histogenetic' 
 determinants is perhaps rendered still more evident in the case 
 of the egg-cell, as in some animals two kinds of eggs are pro- 
 duced which are very different as regards size, the relative 
 quantity of food-yolk, colour, and nature of the shell. The 
 assumption of two kinds of oogenetic determinants cannot be 
 avoided in this case, for we cannot suppose that the same germ- 
 plasm can have such different effects on the cell. In the section 
 on alternation of generations, it was shown to be necessary to 
 assume that such species contain two kinds of germ-plasm, 
 containing determinants which are in part similar and in part 
 dissimilar. Thus the germ-plasm from which the winter-eggs 
 of the Daphnidce are developed must contain an oogenetic 
 determinant which is quite different from that of the germ- 
 plasm in the summer-eggs, for these two kinds of ova are 
 entirely dissimilar.
 
 THE PHENOMENA OF REVERSION. 351 
 
 I know of no instance in which there is such a wide difference 
 as regards the activity of the idioplasm in successive cell- 
 generations as is the case in the germ-mother-cells and the 
 mature germ-cells arising from them. If, however, even in this 
 very striking instance of a sudden change of function of the 
 idioplasm, the idioplasm which was active at first is not removed 
 from the cell, such a process cannot occur in any other case ; 
 and we are consequently justified in applying to all other cells 
 the conclusion derived from the behaviour of the germ-cells, 
 and in considering it as proved that the active idioplasm of a 
 cell becomes used up in consequence of its activity.
 
 352 THE GERM-PLASM. 
 
 CHAPTER XI. 
 
 DIMORPHISM AND POLYMORPHISM, 
 i. NORMAL DIMORPHISM. 
 
 The phenomena of reversion discussed in the last chapter 
 depend on the capacity possessed by organisms of conveying 
 in their idioplasm characters which they do not themselves 
 possess in the form of 'latent' primary constituents, and of 
 transmitting these to their descendants, in which they may, 
 under favourable circumstances, undergo development. 
 
 It has hitherto been supposed that all the individuals of a 
 species possess these latent primary constituents in the same 
 degree ; and that, consequently, characters which are capable 
 of becoming occasionally manifest at all in any organic form 
 exist in a latent condition in all individuals of the species, their 
 subsequent appearance or non-appearance depending solely on 
 certain developmental conditions. Even Darwin was of this 
 opinion, as is shown by many passages in his works. He 
 imagined that latent ' gemmules ' existed, for instance, of stripes 
 like those of the zebra in every horse, of the slate-blue colora- 
 tion of the rock-pigeon in every domesticated pigeon, and of 
 the two parental species in every hybrid. In the last chapter 
 I attempted to show that this may be true in many cases, 
 such as, for instance, in the races of pigeons, but that it is by 
 no means necessarily so always, and that in numerous instances 
 certain latent ancestral characters are not present in all, but 
 only in a larger or smaller number of individuals of the form in 
 question. We have seen that the reducing division may, 
 indeed, from one generation to another, divide the germ-plasm 
 of the parent in such a manner that the germ-plasm of some of 
 the offspring receives no portions of the idioplasm of one of the 
 grandparents at all. The most striking example is seen in 
 plant-hybrids, in which reversion to one of the parental forms 
 may even occur amongst the offspring of the hybrid. In spite 
 of the nearness of the generations between such descendants
 
 DIMORPHISM AND POLYMORPHISM. 353 
 
 and the parental forms, none of the characters of one of the 
 parents are present in a latent condition. 
 
 Such a rapid removal of characters from the germ of indivi- 
 dual descendants is, however, only possible if as in the case 
 of these hybrids they belong to one parent only. Thus in 
 the human race, any individual traits of the mother or father 
 may not only be absent in certain of the children, but the 
 corresponding determinants may even be wanting in their 
 germ-plasm, so that these traits cannot reappear in the grand- 
 children or great-grandchildren. The case, however, is very 
 different with regard to those characters which are possessed 
 by both parents. These cannot disappear from the germ- 
 plasm of individual descendants from one generation to the 
 next, for their determinants constitute a majority in the maternal 
 as well as in the paternal half of the germ-plasm. As long as 
 these characters are present in all the individuals of the group 
 of forms in question, the determinants for these characters will 
 predominate in most of the idants, and it will then hardly be 
 possible for them to dwindle into a minority in consequence of 
 successive reducing divisions. But let us take the case of a 
 gradually disappearing character, and select as an example the 
 wing-bars characteristic of the rock-pigeon which are present in 
 the domesticated breeds. The absolute number of the ' wing-bar' 
 determinants in the germ-plasm must have been diminished 
 gradually in the course of the processes of selection which led 
 to the formation of the various races, owing to many of them 
 becoming transformed into differently constituted or 'modern' 
 determinants. The smaller the number of 'ancestral' deter- 
 minants, the more liable are they to be totally eliminated from 
 one half of the germ-plasm in the reducing division. When, 
 however, they are entirely wanting in individual germ-cells, two 
 of these cells may possibly come together in conjugation ; and 
 an animal would then be produced which possessed no ' wing- 
 bar ' determinants, or, in other words, which no longer contained 
 this character in a latent condition. In proportion as such 
 individuals became more frequent, the average number of 'bar' 
 determinants in the germ-plasm of the race must still further 
 decrease, owing to the constant interbreeding of these indivi- 
 duals with others, until finally only a small percentage of 
 individuals would contain such determinants at all. 
 
 I do not think that the above argument can be strictly applied
 
 354 THE GERM-PLASM. 
 
 in the case of this particular character of the pigeon. The 
 frequent appearance of bars in crosses of different races of 
 pigeons indicates, on the contrary, that most individuals still 
 contain a number of these ' bar '-determinants. We may, 
 however, conclude with regard to the occurrence of stripes like 
 those of the zebra on horses and asses, that the infrequency with 
 which reversion occurs, even in the case of crosses, is probably 
 due to the fact that the character in question has long ceased 
 to be present in a ' latent ' condition in all individuals of both 
 species, and that the germ -plasm usually no longer contains 
 ' zebra '-determinants. 
 
 We certainly cannot ascertain whether some few ancestral 
 determinants may not nevertheless still be present occasionally, 
 for even in crosses their cumulative effect might no longer be 
 apparent ; but general considerations compel us to assume that 
 even these sporadic old determinants must ultimately disappear. 
 They will be contained in the germ-plasm of a gradually de- 
 creasing number of individuals, and as these derive no benefit 
 from their presence, they will presumably become less and less 
 frequent. 
 
 We must therefore take into consideration, on the one hand, 
 the dwindling specific or racial characters, which may be pre- 
 sent in a latent condition, though all of them need not by any 
 means necessarily occur in all individuals ; and on the other, 
 the individual characters, which may exist in a latent condition 
 in a varying number of individual descendants. 
 
 The actual specific characters are, however, transmitted to every 
 individual, though they also do not always become manifest, for 
 some regularly remain latent when another opposing group of 
 characters becomes apparent. I am here alluding to primary 
 and secondary sexual characters : but all characters even the 
 non-sexual ones due to dimorphism and polymorphism come 
 under this category. I must now attempt to explain these 
 phenomena in accordance with my theory. 
 
 In considering this problem, we must naturally begin with 
 the simplest case of sexual differentiation, in which the origi- 
 nally monomorphic germ-cells become differentiated into male 
 and female. The question then arises as to the origin of this 
 differentiation in the idioplasm, and how it has arisen phyleti- 
 cally. 
 
 Let us take a definite example. The Volvocinece are lowly
 
 DIMORPHISM AND POLYMORPHISM. 355 
 
 organised multicellular algae of a globular form, which rotate 
 and are propelled through the water by the movement of their 
 cilia. In addition to multiplication by means of asexual germ- 
 cells, several genera of this order, such as Eitdorina and Pan- 
 dorina, exhibit a sexual mode of reproduction, consisting in the 
 conjugation/.^, complete fusion of two germ-cells which are 
 apparently quite similar to one another. 
 
 As long as this is the case in all individuals, we might sup- 
 pose that the reproductive-cells are controlled by the idioplasm 
 which directs the development of the species in general, that 
 is, by the germ-plasm, which is composed of a varied number of 
 similar determinants. This will, however, no longer be the case 
 as soon as the conjugating germ-cells become differentiated 
 into male and female, as has occurred in the allied genus Vol- 
 vox. The utilitarian motive for this differentiation is not far to 
 seek, for it must be advantageous for the germ-cells to contain 
 the greatest possible accumulation of nutritive material ; and 
 this could only occur to a very slight extent as long as the two 
 germ-cells destined to undergo amphimixis retained a marked 
 power of movement, as is the case in Pandorina. They con- 
 sequently become differentiated into the stationary egg-cell, 
 containing a large quantity of nutritive material, and the small 
 motile sperm-cell, in which very little food-material is present. 
 On what process in the idioplasm, then, does this differentiation 
 depend ? 
 
 The idioplasms of the egg- and sperm-cells evidently cannot 
 be precisely similar. They cannot simply consist of germ- 
 plasm ; but the egg-cell must contain a determinant which gives 
 it the histological character by which it is distinguished from 
 the sperm-cell, which latter must also contain a determinant 
 controlling its histological development. The germ-plasm of 
 Volvox must therefore contain spermatogenetic and oogenetic 
 determinants besides those for the ciliated somatic cells, only one 
 or the other of which, however, becomes active, and impresses 
 the male or female character on the germ-cell. I imagine that 
 these sexual determinants are double, each of the two parts always 
 occurring together and being closely united, but so regulated that 
 only one of them becomes active at a time. We might repre- 
 sent this figuratively by supposing that each iJ of the germ-cells 
 consists of a central globular mass of germ-plasm, surrounded 
 by a layer composed of this sexual double-deterip-'-'ant. either
 
 35 6 THE GERM-PLASM. 
 
 the male or female part of which may be external, and conse- 
 quently dominant. 
 
 This is only a metaphor, and is not intended to represent the 
 actual occurrences. We are ignorant of the forces and sub- 
 stances which are here concerned, but we at any rate know that 
 the idioplasm of the primary germ-cells in the higher animals is 
 still capable, in by far the most cases, of giving rise to either 
 kind of germ-cells, and that the decision as to whether the 
 germ-cells will develop into ova or spermatozoa occurs at some 
 early stage in embryogeny : in the eggs of the bee it takes place 
 at the beginning of embryonic development, long before the 
 first primary germ-cell is differentiated, while in other animals 
 it perhaps occurs at a later stage. The well-known researches 
 of Siebold and Leuckart, prove that at any rate in the case of 
 bees, this decision undoubtedly rests upon the occurrence or 
 omission of fertilisation, it occurs, that is, at the time when the 
 germ-plasm which controls the new organism is constituted ; and 
 this fact seems to me to be of great importance. If fertilisation 
 takes places, the organism develops into a female, and if not, 
 a male will result. This at least proves that the decision may 
 take place at such an early stage, and I doubt whether it can in 
 any case occur later : in some animals, at any rate, it occurs 
 still earlier, during the period of maturation of the egg. The 
 Phylloxera lays large eggs which produce females, and small 
 ones from which males arise. Both become fertilised, so that 
 in this case fertilisation takes no part in the determination 
 of sex. 
 
 These questions cannot be discussed more fully here. We 
 are only concerned in making it clear that sexual determinants 
 in the sense indicated must be assumed to exist, and that both 
 kinds are contained in the primary germ-cells. The following 
 considerations will render it apparent why we have assumed 
 these determinants to be double, i.e., to consist of two groups of 
 biophors of a common origin lying close to one another. Even 
 apart from such low organisms as Volvox, it is well known that 
 the male and female individuals only differ from one another as 
 regards the kind of sexual cells they produce in many of the 
 lower Metazoa viz., the sponges and Hydromedusae. In these 
 forms the sexual determinants alone are double. In most other 
 animals, however, the difference of sex is not confined to the germ- 
 cells, but affects the soma itself to a greater or less extent. Hence
 
 DIMORPHISM AND POLYMORPHISM. 357 
 
 in all sexually dimorphic organisms the germ-plasm must contain 
 a varied number of double primary constituents of somatic char- 
 acters, namely, those which represent characters which differ 
 in male and female individuals. These in the first instance con- 
 cern the organs in which the sexual cells are developed, nourished, 
 stored, and removed, that is to say, the so-called sexual glands 
 and their ducts ; then follow the active and passive copulatory 
 organs, and the structures connected with oviposition ; and, 
 finally, special sexual characters arise with regard to the organs 
 for supporting and nourishing the offspring such as mammary 
 glands, teats, and uterus, or they may refer to the instinct 
 of carrying the eggs in the mouth, as in the male of a tropical 
 species of frog, or to that in the female butterfly, which deposits 
 its eggs in a definite manner on a certain plant. In the last two 
 instances, the structure of the body and the nerve-centres must 
 also be different in the two sexes, and the male and female type 
 of these parts must exist in a latent condition in every germ- 
 plasm. Under this head, moreover, ' secondary ' sexual charac- 
 ters must be included, such as the various tracking and alluring 
 organs of the males, the gorgeous colours of male birds and 
 butterflies, the scent-producing organs of the latter, which exhale 
 perfume, and the song of male birds and insects. 
 
 In the human race we know that all the secondary sexual 
 characters are transmitted by individuals of the opposite, as well 
 as of the corresponding sex. A fine soprano voice, for instance, 
 may be transmitted from mother to granddaughter through a 
 son, and a black beard from the father to the grandson through 
 a daughter. And in other animals, the sexual characters of both 
 sides must be present in every sexually differentiated organism, 
 some of them becoming manifest and others remaining latent. 
 This fact can only be proved in certain cases, for we seldom 
 notice the individual differences of these characters with suffi- 
 cient accuracy ; it can, however, be shown to be true, even in 
 tolerably simply organised species, and we must therefore sup- 
 pose that latent characters belonging to the other sex are always 
 present in each sexually differentiated organism. In bees, the 
 males developed from unfertilised eggs possess the secondary 
 sexual characters of the grandfather ; and in the water-fleas, in 
 which several generations of females arise from one another, 
 the last of these generations produces males with the secondary 
 sexual characters of the species, which must consequently have
 
 3$ THE GERM-PLASM. 
 
 been present in a latent condition in an entire series of female 
 generations. 
 
 The germ-plasm of the fertilised egg-cell must therefore 
 contain the primary constituents of all the secondary male and 
 female sexual characters, as well as those of the male and female 
 germ-cells. We might, then, suppose that this would be 
 accounted for by the assumption that the determinants of both 
 kinds of characters are contained in the germ-plasm, and that 
 the decision as to sex is not only determined with regard to the 
 sexual determinants, so that the germ-cells take on a male or a 
 female character, but also as regards the somatic determinants, 
 in order that the secondary sexual character of the male or of 
 the female may take the lead in the building-up of the body. 
 
 This assumption is certainly indispensable, and it suffices as 
 far as the latent transference of secondary characters of both 
 kinds from the germ-plasm of one generation to that of the 
 next is concerned, but it still requires an essential addition. 
 A number of facts indicate that the latent primary constituents 
 of secondary characters of both kinds are not only present in 
 the germ-plasm from which the organism arises, but also in the 
 fully developed body of the organism. The fact that the char- 
 acters of both sexes can be transmitted to descendants, proves 
 that the germ plasm must contain the corresponding primary 
 constituents of both ; and in the sixth chapter it has already 
 been shown how this transmission may be accounted for by a 
 continuity of the germ-plasm from one generation of germ-cells 
 to the next. We are now concerned with the fact that mature 
 individuals may also contain the primary constituents of se- 
 condary sexual characters of both kinds, and that those of the 
 female may be present in the body as well as in the germ-cells 
 of a male organism, and conversely those of the male may exist 
 in the female. The well-known facts to which I refer have 
 been carefully collected and fully discussed by Darwin, and are 
 briefly as follows: the secondary sexual characters of one 
 sex may, under special circumstances, become developed sub- 
 sequently in fully-developed individuals. This results in both 
 sexes, especially in the case of castration. The removal of the 
 sexual 'glands' from young mammals and birds prevents the 
 development of secondary male sexual characters. Castrated 
 cocks, for example, retain the appearance of hens, and do not 
 develop the beautiful tail or the large comb and spurs of the
 
 DIMORPHISM AND POLYMORPHISM. 359 
 
 male bird, nor do they crow ; and conversely, when hens 
 become sterile from age, or if their ovaries become degenerated, 
 they take on the external sexual characters of cocks. I possess 
 a duck which no longer lays eggs, and has assumed the colora- 
 tion of the drake. Men who have been castrated in their youth 
 retain a high voice like that of the other sex, and the beard does 
 not become developed. 
 
 These facts obviously compel us to assume that the capacity 
 for the development of secondary female characters exists in a 
 latent condition in the body of the male, and vice versd, and 
 that these are ready to undergo development under certain con- 
 ditions. Darwin also arrived at the same conclusion. The only 
 argument which might be advanced against its correctness is 
 that a change of secondary sexual characters in any particular 
 individual has only been observed in rare instances, and in very 
 few species of the higher animals, such as birds and mammals. 
 It might, therefore, be doubted whether it is possible to draw 
 a general conclusion from such isolated observations. These 
 instances nevertheless remain to be accounted for, and we must 
 attempt to explain them in accordance with our theory. 
 
 Let us take a very simple example. In many butterflies of 
 the family Lycanidce, the upper surface of the wings is brown in 
 the females, and blue in the males ; and it seems very probable 
 that brown was the original colour, as species of Lyccena exist 
 at the present day in which both sexes are of this colour. 
 The process which took place in the idioplasm in order that 
 the change of colour may have occurred, must have consisted 
 in the primary determinants of those cells which decide the 
 colour of the wings which we will speak of as ' brown ' deter- 
 minants becoming transformed into 'blue' determinants in the 
 germ-plasm, this change only occurring after they had become 
 doubled and in such a manner that only one of the twin-deter- 
 minants in each case remained brown, an arrangement also taking 
 place which only allowed each to become active alternately. 
 We thus arrive at the assumption of double determinants, just 
 as in the case of the determination of sex in the germ-cells. 
 I at first believed that it was indispensable to assume the 
 existence of determinants with different halves, merely because 
 the presence of inactive determinants in the corresponding 
 part of the body could not be otherwise explained. As the 
 adoption of the characters of the opposite sex cannot possibly
 
 360 THE GERM-PLASM. 
 
 be of any use to the species, natural selection can have taken no 
 part in the addition of male determinants to the somatic cells 
 of the female body, or vice versd. Such a transference must 
 therefore depend on an unintentional secondary effect of existing 
 arrangements and forces. I soon, however, recognised that 
 such arrangements actually exist, and that the assumption of 
 mechanically unseparable double-determinants is not necessary 
 in order to explain the presence of the two sexual determinants 
 in the region where one of them undergoes development. I 
 therefore attach no value to the idea of the material connection of 
 the two dimorphic halves of the primary determinants in question. 
 In fact I shall have occasion to show presently that these halves 
 must, in any case, sooner or later become separated as inde- 
 pendent determinants in the course of phyletic development. 
 
 The reason that such double determinants must, however, 
 always remain close together, even after their separation, results 
 simply from the mechanism for the ontogeny of the idioplasm, 
 which we suppose to consist in the gradual disintegration of 
 the mass of determinants of the germ-plasm into smaller groups. 
 They divide according to a definite law into smaller and smaller 
 groups in the course of the embryonic cell-divisions. None of 
 them remain unused or undergo destruction, but each passes 
 through a definitely prescribed course, and the determinants 
 for any particular part or region of the body must necessarily 
 remain together, even when they are not inseparably connected 
 mechanically. In a. physiological sense, therefore, they are still 
 double determinants, i.e., each half controls the same region, 
 and in this sense I shall now use the term. 
 
 A transfer of the secondary sexual characters, such as occurs 
 in birds, cannot take place in the Lyccsnida;, because the wing- 
 scales are never formed more than once in the course of life, 
 and consequently we have no means of proving the presence 
 of double determinants in the cells of the wing. Other ob- 
 servations on insects indicate, however, that their idioplasm is 
 nevertheless capable of producing such a sexual transference. 
 
 This is especially shown in the case of the occasional herma- 
 phrodite forms of insects, the most instructive instance which 
 has been very accurately investigated by Leuckart* and von 
 
 * R. Leuckart, ' Sitzungsberichte der deutschen Naturforschei ver- 
 sammlung,' 1864.
 
 DIMORPHISM AND POLYMORPHISM. 361 
 
 Siebold,* and more recently by Kraepel in being furnished 
 by bees. The male and female secondary sexual characters are 
 combined in the most wonderful manner in these hermaphrodite 
 forms : in some bees, the right side was female and the left male; 
 in others, the anterior hnlf of the body was male and the poste- 
 rior female ; while in others, again, the entire trunk was male 
 and one side of the head female. As Leuckart remarks, ' the 
 male and female characters' in these hermaphrodite bees 'have 
 been intermingled in the most varied and unsystematic manner, 
 so that it is difficult to discover two individuals with perfectly 
 similar characteristics.' 
 
 We are indebted to Kraepelin for an excellent account of 
 the external sexual parts in these hermaphrodite bees,t in- 
 cluding the copulatory apparatus. His description shows that 
 the blending of the male and female characters even affects 
 very small parts. It not only often happens, for instance, 
 that half the entire stinging apparatus of the left side is 
 female, while an intromittent organ is developed on the right, 
 but certain chitinous plates on the ventral side of the last 
 abdominal segment, which is almost male in character, also 
 display a distinct tendency to take on the form of the corre- 
 sponding plates of the female stinging apparatus ; in other 
 words, these chitinous plates are intermediate in form between 
 those of the male and female. Their formation must therefore 
 have been controlled by a combination of ' male ' and ' female ' 
 determinants. It would be incredible that these harmonising 
 determinants could have met together at the right point in the 
 extremely complex combination of determinants in the germ- 
 plasm if they had not been arranged together from the first, and 
 if an arrangement of male and female double determinants had 
 not previously existed in every such region of the germ-plasm, 
 so that they reached the corresponding part of the body together 
 in the course of ontogeny, either the male or the female half 
 then becoming active. 
 
 In the determination of sex in the normal development of bees, 
 all these somatic double determinants must be correspondingly 
 determined. We do not know to what factor the prevention of 
 the determination of sex in a similar manner in the formation 
 
 * C. Th. von Siebold, 'Zeitschrift fiir wissenchaftliche Zoologie,' Bd. 
 xiv., 1864, p. 73. 
 + Kraepelin, ' Zeitschr. f. wiss. Zoologie,' Bd. xxiii.. 1873, p. 326.
 
 362 THE GERM-PLASM. 
 
 of the hermaphrodites is due. Siebold attempted to explain 
 this difficulty by supposing that the eggs which produced 
 hermaphrodites were imperfectly fertilised. He found that in 
 the queen, after producing hermaphrodites, the receptaculum 
 was almost empty ; and as drones arise from unfertilised, and 
 females from fertilised eggs, the view that imperfect fertilisa- 
 tion must produce hermaphrodites appeared to be a plausible 
 one. It was not known at that time that a single spermatozoon 
 suffices for fertilisation, and at the present day we are no longer 
 justified in using such an expression as ' imperfect fertilisation.' 
 Whenever a living spermatozoon enters an egg, the latter 
 becomes fertilised ; and an imperfect fertilisation could only be 
 supposed to occur if the spermatozoon is abnormal if, for 
 instance, it contains too few idants. But even if such a case 
 occurred, it would be of very little value theoretically, and we 
 could only state that the determination of sex did not take place 
 in all the double determinants at once, but only in certain larger 
 or smaller groups, in the case of the germ-cells as well as of the 
 dimorphic pans of the body. Besides the ordinary case, in 
 which the sexual gland of an individual was developed on the 
 female type on the right side and the male type on the left, 
 other instances occurred in which female and male germ-follicles 
 were formed, on the same side, seminal tubules and ovarioles 
 being present close together. With reference to this fact, von 
 Siebold remarks that 'the hermaphroditism of the sexual appa- 
 ratus hardly ever corresponded to that observable in the external 
 form;' and this seems to me to be of special theoretical interest, 
 as it allows us to conclude with certainty that the harmony 
 of the normal condition is due to a simultaneous decision 
 respecting the double determinants of the germ- cells and those 
 of the body, and not to a primary determination of sex in the 
 sexual glands, from which the somatic male or female sexual 
 characters would only be determined secondarily. The existence 
 of double determinants in the germ-plasm can be actually proved 
 in the case of bees. For if any egg can develop into a male 
 or female according to whether it is fertilised or not, it must 
 contain both kinds of determinants. 
 
 Although this assumption is undoubtedly a correct one, it 
 alone is insufficient, because the secondary differences of sex 
 do not always only concern single cells or groups of cells which 
 correspond exactly in the two sexes, such as, for example, the
 
 DIMORPHISM AND POLYMORPHISM. 363 
 
 brown and blue scales of the Lycanida ; and because in many, 
 and perhaps in most cases, the dimorphic parts correspond only 
 partially or not at all. 
 
 The degree of sexual differentiation is very different in the 
 various groups of the animal kingdom. In the lower and 
 higher Crustacea the males commonly possess more ' olfactory 
 setae' on their antennas than do the females. In the large 
 water-flea, Leptodora hyalina, for instance, the anterior antennas 
 are represented in the female by short stumps provided with 
 five olfactory setae, while the male has long rod-like feelers, 
 which bear about eighty setae. This difference evidently cannot 
 be referred to one double determinant. Each olfactory seta 
 must be represented by a special determinant in the germ- 
 plasm ; even if the first five corresponded, and the differences 
 between them could be attributed to the double determinants, 
 more than seventy determinants for these setae which occur in 
 the male sex only would still be left, quite apart from those for 
 the feeler itself. These seventy determinants are not double, 
 because the corresponding parts are absent in the female, and 
 consequently two groups of determinants must exist side by side 
 in the germ-plasm, those, namely, for the antennas in the male 
 and in the female, only one of which becomes active in each 
 case. We might imagine that the two groups pass, in close 
 proximity to one another, through the cell-series of embryogeny 
 up to the formation of the rudiments of the antennas, and then 
 become separated, the inactive group remaining in an ' undif- 
 ferentiated ' cell at the base of the feeler, while the other causes 
 the development of the antennas of the sex in question by 
 continued cell-division. 
 
 The same must be true in numerous other and much more 
 complicated cases. If a tail feather in a male humming- 
 bird, for instance, is six times as long as the corresponding 
 feather in the female, the colour of the former being of a 
 brilliant ultramarine, and that of the latter greyish, we must 
 assume that two groups of determinants are present, which 
 differ in number and in nature. The two groups are situated 
 close together in the germ-plasm, pass through the same cell- 
 series in ontogeny, and ultimately reach the same part of the 
 skin covering the last caudal vertebra. Here, however, one of 
 them remains inactive, the other alone causing further cell-divi- 
 sions and the consequent development of a feather to take place.
 
 364 THE GERM-PLASM. 
 
 The greater the extent to which sexual dimorphism occurs, 
 and the larger the parts which it affects, the larger the two 
 groups of determinants must be, and the earlier in ontogeny 
 will one of them remain inactive in a cell and cease to undergo 
 further division, while the other gives rise to further cell- 
 divisions. 
 
 Sexual dimorphism largely consists in the arrest of develop- 
 ment in an organ in one sex. In many female butterflies, for 
 instance, wings are wanting. This must be due to the group of 
 determinants for the wings, which existed in a double condition, 
 i.e., were -male and female in the earlier phyletic stages, be- 
 coming arrested as regards their male portion. Such females 
 commonly possess rudiments of the wings, and in this case the 
 two groups of wing-determinants must pass through ontogeny 
 together up to the stage in the caterpillar in which the forma- 
 tion of the imaginal disc of the wing arises from a cell of the 
 hypodermis. If the animal is a female, the arrested group, 
 and if a male, the perfect group of wing-determinants will 
 then become active. It is, however, also conceivable that the 
 development of the group of determinants in the female might 
 continue to be arrested until they disappeared completely, as in 
 the case of female PsychidcE, in which the wings are altogether 
 wanting. 
 
 But the highest degree of sexual dimorphism is not reached 
 even when certain parts disappear completely. In various 
 groups of the animal kingdom species exist in which the males 
 differ from the females in nearly all their characters. In many 
 Rotifers the males are very much smaller than the females, and 
 exhibit an entirely different structure ; the alimentary canal, 
 moreover, is entirely wanting. In Bonellia viridis, a marine 
 gephyrean worm, the male differs so much from the female that 
 one might be tempted to class it with an entirely different group 
 the Turbellaria. The difference in the sizes also of the two 
 sexes is still more marked in this instance, the length of the male 
 being 1-2 mm., and that of the female 150 mm.; the former, 
 moreover, lives as a parasite within the latter. In this case the 
 eggs which give rise to males cannot be distinguished from 
 those which develop into females, even in size ; and the rela- 
 tively enormous bulk of the female simply results from subse- 
 quent growth. Even the young male and female larvae cannot 
 be distinguished from one another, and their development only
 
 DIMORPHISM AND POLYMORPHISM. 365 
 
 begins to differ when metamorphosis of the larva into the sexu- 
 ally mature animal takes places. The larva is of an elongated 
 oval shape, is furnished with two bands of cilia by means of 
 which it swims about, and possesses an alimentary canal with 
 a mouth and anus. The transformation into the definitive form 
 of the species begins with the loss of the posterior band of cilia, 
 
 FIG. z^.Bonellia viridis. (After Huxley, from R. Hertwig's "Zoology.") On 
 the left hand the male is shown, considerably magnified ; and on the right the 
 female, about natural size, d, Rudimentary intestine of the male ; vd, Sperm 
 duct ; ', Intestine of the female. 
 
 and from this point onwards the mode of development is different 
 in the two sexes. The females grow rapidly, develop a ' pro- 
 boscis' at the anterior end, and the intestine increases in 
 length ; the males, on the other hand, become entirely covered 
 with cilia, and the mouth and anus, as well as the fore- and 
 hind-gut disappear, the mid-gut, filled with yolk-granules, alone 
 being retained. Notwithstanding their great diversity, the two 
 sexes are formed on the same plan ; the male, however, may be 
 said in general to remain stationary at a certain stage of organi-
 
 366 THE GERM-PLASM. 
 
 sation, while the female continues to develop, and reaches a 
 much higher stage of organisation, at any rate as regards many 
 organs, such as the nervous and vascular systems, which are 
 altogether wanting in the male. But neither is this the full 
 extent of the difference between them, for the testes, as well as 
 the skin and certain hooked organs of attachment, are only 
 developed in the male at a later stage in a peculiar manner. 
 A certain correspondence still remains in the most essential 
 points of structure of the body, in spite of the great differ- 
 ence between the adult sexual animals ; so that, as Sprengel 
 says, the male is also 'a Gephyrean possessing all the known 
 structural characteristics of the group.' 
 
 In terms of the idioplasm, this course of development may be 
 described somewhat as follows : the determinants which direct 
 the development of the larvae are single, and consequently 
 mo now orphic larvae are produced. The idioplasm of all or most 
 of the cells, however, which constitute the organs of these larvae, 
 contain double determinants or double groups of determinants, 
 of which those for the female are, in most cases, far the larger : 
 in fact, the group for the female will usually not be opposed 
 by any for the male at all, in the case, that is, of such organs 
 as the long proboscis, of which there is no homologue in the 
 male. It is certainly very remarkable that these groups of 
 determinants, although present in the male and unopposed by 
 others, do not become active ; but, even although we do not in 
 the least understand how it comes about that this inactivity is 
 enforced, the case is not more surprising than that of the deter- 
 mination of sex as a whole, and the inactivity of existing indi- 
 vidual primary constituents. Why do the wings first appear at 
 the pupal stage of the caterpillar, and not long previously, since 
 they must be present all along in rudiment i.e., in the form of a 
 group of determinants in certain cells of the hypodermis ? Or 
 why does a boy not grow a beard, seeing that the necessary 
 determinants must be contained in certain cells of the skin ? 
 We can no more account for all these cases than for the in- 
 activity of sexually differentiated determinants. All that can be 
 said is, that these determinants have the peculiarity of only 
 becoming active at a certain ontogenetic stage ; but this scarcely 
 gives any further insight into the matter than does the statement 
 that sexually differentiated determinants become active or remain 
 inactive, according to the sex of the organism in question.
 
 DIMORPHISM AND POLYMORPHISM. 367 
 
 The ids of the germ-plasm must contain more determinants 
 in sexually dimorphic than in monomorphic species, the number 
 increasing in proportion to the increase of difference between 
 the sexes. They must also increase in size ; and the question 
 therefore arises as to whether dimorphism may not perhaps 
 increase to such an extent and finally involve all parts of the 
 body, that double ids arise ; that is to say, each id of the germ- 
 plasm comes to consist of a male and a female half, in which 
 all the determinants are different. This seems to be practically 
 the case in some animals : in the Rotifera, for instance, the 
 males commonly differ so much from the females that they, 
 like the sexual form of Phylloxera, arise from special eggs, 
 smaller than those which develop into females. But it would 
 nevertheless be incorrect to suppose that each of these two 
 kinds of eggs contained either male or female ids only. The 
 number of common determinants present must certainly be 
 small, but even here the germ-plasm of each egg must never- 
 theless contain all the male and female determinants. This is 
 proved by the interpolation of generations consisting of females 
 only in the cycle of generations which is passed through each 
 year, the parthenogenetic females eventually producing males. 
 
 We must now return to the question of sexual reversion if I 
 may so call it which has already been referred to ; that is to 
 say, the appearance of characters of the opposite sex after cas- 
 tration or degeneration of the sexual glands. Hitherto this has 
 always been considered a universal phenomenon, but I do not 
 think that such a conclusion is justified. As already mentioned, 
 observations with regard to such cases of 'sexual reversion' 
 have practically been confined to birds and mammals, and even 
 in these do not always refer to all the parts which are sexually 
 dimorphic in the species in question. Cases have certainly been 
 observed in which, for instance, an old ' hen which had ceased 
 laying, assumed the plumage, voice, spurs, and warlike dis- 
 position of the cock.'* This proves that in these birds all the 
 secondary sexual characters of the male are present in a latent con- 
 dition in the soma of the female. We might, however, suppose 
 that this form of reversion only takes place when the characters 
 concerned are completely homologous in the two sexes ; that is 
 
 * Darwin, 'Animals and Phnts tndcr Domestication,' Vol. II., p. 26.
 
 368 THE GERM-PLASM. 
 
 to say, when they exactly correspond with regard to the time 
 and place of their ontogenetic origin and the number of their 
 determinants : but if this is not the case, such a 'reversion' to 
 the characters of the other sex can hardly be possible, because 
 the foundation is wanting from which the reverted organ could 
 arise. Let us suppose that the same conditions as apply to 
 fowls hold good in the case of butterflies : that is to say, that the 
 secondary sexual characters are present in a latent condition in 
 the somaof the other sex, and undergo development on removal 
 of the sexual glands. The male of Lyccena alexis, for example, 
 which has blue wings, would then develop brown ones on 
 being castrated. This would take place by the shedding of old 
 scales and the giowth of new ones ; or if castration had been 
 effected in the caterpillar stage, the scales in the growing wing 
 would be. brown from the first. The scales are homologous 
 st.'uetures in the male and female, and each of them is controlled 
 by a single determinant. If, therefore, a cell containing the 
 determinant for a brown scale jf the female is situated at the 
 base of the already devebptd scale of the male, a reversion 
 to the colour of the scales in the female might occur under such 
 circumstances, which are of course purely imaginary. 
 
 The matter would, however, be entirely different if the female 
 Lycana had no wings at all, as is the case in females of some 
 BombycidcE. The character of the blue scales in the male 
 would then have no homologue in the female, and an inactive 
 cell, with the determinant of a brown scale, could not possibly 
 be situated at the base of the blue scales in the male. This 
 may be expressed in general terms as follows : double-deter- 
 minants, possessing a definite male and female character, can 
 only be present up to the phase and point in ontogeny in which 
 the development of the two sexes is exactly homologous. We can 
 therefore only expect a reversion to the secondary characters of 
 the other sex to occur when this point remains permanent. In 
 Lycana the divergence would occur at the formation of the wing- 
 scales ; in Psyche (the female of which is wingless), in a certain 
 group of cells in the hypodermis of the thorax ; in Bonellia, in 
 all the cells of the larva ; and in the Rotifera, in the egg itself. 
 If our view is a correct one, a female Bonellia would conse- 
 quently be incapable of developing male characters in conse- 
 quence of castration, because it has long since passed that stage 
 of optogeoy in which the divergence into a male or female
 
 DIMORPHISM AND POLYMORPHISM. 369 
 
 occurs ; and in the case of the Rotifera, it is not to be expected 
 that any influence could cause a male to produce female char- 
 acters. 
 
 In the part treating of sexual reproduction including the 
 section on the struggle of the paternal and maternal hereditary 
 tendencies which takes place during the development of the 
 offspring no mention was made of a very general, and, in my 
 opinion, erroneous conception of sex ; and it will be as well to 
 explain this omission before proceeding further. 
 
 The transference of sex has hitherto usually been looked upon 
 as an act of transmission. This cannot be the case, inasmuch 
 as every germ-plasm contains the primary constituents for both 
 sexes, and the process of transmission itself has evidently nothing 
 to do with the determination of sex. As already mentioned, 
 it does not by any means follow that because a child is a femal", 
 its secondary or primary sexual characters will resemble those 
 of its mother. This, indeed, has long been known, but has not 
 led to a general recognition of the fact that sex is not trans- 
 mitted at all ; and that, on the contrary, the primary consti- 
 tuents of both sexes are always passed on from both sides : the 
 decision as to which of them are to become active depends on 
 secondary factors, which have not yet been clearly recognised 
 in any case. The male halves of the sexual double-determinants 
 of the mother are just as capable of undergoing development as 
 are the female halves, and vice versa: the Maw of sexual trans- 
 mission,' * which was propounded by Haeckel some time ago, is 
 not tenable. Expressed in a purely empirical manner, the facts 
 have been more correctly formulated by Dejerine t according 
 to Darwin's (?) views, in his valuable work on the heredity of 
 nervous complaints : ' the prepotency of one of the parents in 
 transmission may be direct, and follow the sex, or may cross 
 over, and become manifest in the opposite sex.' 
 
 For this reason the so-called 'transmission of sex' was entire. y 
 left aside in the section on the struggle of the parental char- 
 acters during the formation of the child ; transmission of the 
 primary and secondary sexual characters occurs, but sex itself 
 cannot be transmitted. 
 
 * Ernst Haeckel. ' Generelle Morphologic der Organismen,' Bd. II., 
 Berlin, 1866, p, 183. 
 
 t J. Deje ine, ' L'Heredite dans les maladies du systeme nerveux,' Paris, 
 t886, p. 17. 
 
 2 A
 
 37 THE GERM-PLASM. 
 
 2. PATHOLOGICAL DIMORPHISM : HAEMOPHILIA. 
 In connection with the attempt to trace sexual dimorphism 
 to its origin in the idioplasm, I will now make a few remarks 
 with regard to a certain disease or rather structural anomaly 
 which affects the human race, and which, I believe, will be 
 better understood from this point of view ; the analysis of it 
 may, moreover, possibly throw a new light upon the causes 
 of sexual dimorphism. 
 
 This anomaly is known as hcemophilia, and although of rare 
 occurrence, it has been very accurately observed in ar.umber of 
 cases, and is known to be transmitted in a marked degree and 
 in a very peculiar manner. It only occurs in the male members 
 of a family, but is transmitted by the female members, ai\d in 
 this respect resembles a secondary sexual character. 
 
 The disease, however, is apparently not connected with the 
 sexual organs, or with those parts which differ in the two 
 sexes. It consists in an abnormal flaccidity of the walls of the 
 blood-vessels, in consequence of which slight injuries cause 
 serious haemorrhage, which cannot be easily stopped. As the 
 blood-vessels are developed from certain cells of the so-called 
 mesoblast or ' parablast,' haemophilia is described in many 
 text-books on pathological anatomy as an anomaly of the 
 ' parablast ' or ' Bindegewebekeim.' It must certainly be assumed 
 that this diastrous variation in the blood-vessels or rather of the 
 cells from which the blood-vessels are formed is due to some 
 variation in certain mesoblast cells which cannot be more defi- 
 nitely defined at present. The determinants for the cells of the 
 blood-vessels must in this case have varied in some way or other 
 in the germ-plasm of the individuals affected, and the anomaly 
 must originally Jiave occurred in a male individual. There is 
 apparently no reason why a similar variation of the determinants 
 should not take place in a female, and cases of women exhibit- 
 ing the hasmorrhagic diathesis may perhaps still be ascertained. 
 The exclusive transmission of this anomalous condition to the 
 male sex seems to me, however, to indicate that the deter- 
 minants for the blood-vessels differ in men and women, in spite 
 of the apparent similaiity of the vessels themselves -.these 
 determinants must be double. 
 
 On this assumption, we can easily account for the otherw.se 
 mysterious phenomena of heredity observed in these cases. As
 
 DIMORPHISM AND POLYMORPHISM. 371 
 
 this disease only appears in men, the pathological variation 
 must affect the ' male' half of the determinants for the cells of 
 the blood-vessels of the person affected, and we may compare 
 it with the variation which occurs in the cells constituting 
 the larynx, the determinants of which must certainly be re- 
 garded as being double, and as undergoing variation in the 
 ' male ' half. The decision as to which halves of the double- 
 determinants in the idioplasm are to be active during embryogeny 
 and which passive, takes place simultaneously with that as to the 
 sex of the embryo, as is proved by the case of hermaphrodite 
 bees. It is therefore self-evident that this disease must remain 
 latent i.e., no diseased formation of the tissues whatever can 
 be produced in the case of every female descendant of a 
 ' bleeder,' for in them the ' female' untransformed halves of the 
 determinants for the cells of the blood-vessels become active. 
 If, however, the offspring develops into a male, the pathologi- 
 cally transformed ' male ' halves of these determinants become 
 active, and the disease can develop, provided that a stronger 
 hereditary influence is not exerted in the formation of the blood- 
 vessels of the healthy maternal side, so that the tendency to 
 disease, which has been derived from the father, is overcome by 
 the healthy tendency inherited from the mother. This was the 
 case in four generations of a family of ' bleeders,' observed 
 by Chelius, Mutzenbecher, and Lossen, * the sons were not 
 affected. In another case described by Thulasius-Grandidier, on 
 the other hand, the disease was transmitted from the father to the 
 male members of three generations. We can understand both 
 cases from our point of view, for in no instance is an individual 
 variation due to the variation of the corresponding determinants 
 in all the ids of the germ-plasm, but only in the majority of them. 
 But this majority may become reduced to a minority at every 
 'reducing division' and every time amphimixis occurs, the varia- 
 tion thus ceasing to manifest itself. As soon therefore as only 
 a small majority of the ids contain ' haemorrhagic determinants,' 
 a considerable number and hereditary force of the healthy 
 maternal determinants for the blood-vessels would preponderate 
 over the morbid paternal ones, and consequently the male 
 descendants would not inherit the disease. If, however, a con- 
 siderable majority of ' haemorrhagic determinants' were present 
 
 * Klebs, ' Lehrbuch der pathologichen Anatomic.'
 
 372 THE GERM-PLASM. 
 
 in the germ-plasm of the father, a favourable reducing division 
 is necessary if the son is to remain free from the disease. We 
 can, moreover, even account for those cases in which several 
 female members of a hasmorrhagic family in which the fathers 
 were healthy, bear sons all of whom suffer from the disease. 
 For the 'male' halves of the double-determinants in almost all 
 the ids might have undergone a pathological change in the 
 germ-plasm of the mothers, without producing any apparent 
 result in them ; in the sons, on the other hand, this would 
 lead to the development of the disease, unless an unusually 
 favourable reducing division had counteracted the marked pre- 
 ponderance of the morbid determinants. Haemophilia, which 
 remains latent in the mother, is just as liable to be transmitted 
 by the mother to her male descendants as is any other mascu- 
 line characteristic of the grandfather, such as the colour of his 
 beard, or the quality of voice. 
 
 It seems to me that we cannot overlook the indication afforded 
 by this agreement between the mode of transmission of ordinary 
 sexual characters and of haemophilia that all, or nearly all, the 
 determinants in the human germ are double, half being 'male' 
 and half ' female,' so that a determinant for any particular part 
 may cause the development of the male or female type of the 
 corresponding character. 
 
 The facts which Prosper Lucas * brought forward, and illus- 
 trated by numerous examples, concerning the occasional trans- 
 mission of new characters to one sex only, even when they 
 have nothing to do with secondary sexual characters in the strict 
 sense, may be understood by this assumption of a wide distri- 
 bution of double-determinants in the germ. The modification 
 affects only the ' male ' or the ' female ' halves of these deter- 
 minants of the germ-plasm in such instances. This is true 
 as regards the disease we have just considered, inasmuch 
 as it must have made its first appearance at some time or 
 other, as well as in numerous instances of colour-blindness, of 
 the possession of supernumerary fingers, of the absence of certain 
 ringers or of segments of fingers, and so on. Even the peculiar 
 nature of the epidermis in the well-known case of Lambert, the 
 ' porcupine-man,' was only transmitted to the male descendants. 
 
 * ' Traite philosophique et physiologique de I'h^redite naturelle,' Tom. 
 II., Paris, 1850, p. 137.
 
 DIMORPHISM AND POLYMORPHISM. 373 
 
 In certain instances, polydactylism is known to be transmitted 
 to the male members of a family only, while in others it passes 
 from the mother to the daughters exclusively. 
 
 It appears, however, that such modifications of the one half 
 in the double-determinants may in the course of time be trans- 
 ferred to the other half, even though, in the first instance, this 
 only occurs to a slight extent ; for cases are known in which 
 an abnormality first arose (?) in the male sex, and afterwards 
 passed over to certain individual female descendants. These 
 cases have certainly not been followed out with sufficient accu- 
 racy to enable us definitely to deny that a modification of both 
 halves of the double-determinants in question might possibly 
 have taken place from the first, and that it only affected those 
 in the one half (the homologous determinants) in a smaller 
 number of ids. 
 
 Numerous instances in which an abnormality appears, some- 
 times in the male, and sometimes in the female members of a 
 family, prove that both halves may become modified at the same 
 time. Prosper Lucas mentions several instances of this kind, such 
 as that of the family Ruhe: in the first generation observed, 
 the mother transmitted her polydactylism to the daughter, and 
 in the second this peculiarity was passed on from the mother to 
 the son, while in the third it was transmitted from the father to 
 the son. Numerous facts in zoology indicate the correctness of 
 the assumption that modifications in one half of a double-deter- 
 minant exert an influence on the other half, so as to result in a 
 similar transformation. It is well known that many secondary 
 sexual chaiacters of the male in birds and insects appear in a 
 slighter degree in the female. Amongst the Lyccenida which 
 are called 'blues,' on account of the preponderance of this 
 colour in the members of the family some species exist in 
 which both sexes are brown, while in most the male is blue and 
 the female brown, and in a small number of southern species, 
 again, both sexes are blue. There can be no doubt that brown 
 was the original colour of these species, and that the blue tint first 
 appeared in the males, while the females remained brown ; and 
 that, finally, in certain species, the females also became blue, 
 although not so markedly so as the corresponding males. The 
 males therefore preceded the females as regards the change of 
 colour ; and if, with Darwin, we attribute the impulse to this 
 change to sexual selection, it follows that the blue colour of the
 
 374 THE GERM-PLASM. 
 
 females must have been introduced mechanically, owing to its 
 previous existence in the males, and that this secondary sexual 
 character affected the other sex in the course of a great number 
 of generations. According to our theory, this must have 
 been due to the modification of the 'male' halves of the 
 determinants having gradually caused a similar, if less marked, 
 modification of the ' female ' halves. We can thus also to some 
 extent understand how it has been possible for certain females 
 to precede the others as regards this modification, as the 
 influence which produced it would not take effect to the 
 same extent and at a similar rate in all individuals. In 
 many species of Lyccenidcs in which the females are brown, 
 individuals of this sex occur more or less frequently which are 
 clouded with blue, or even exhibit this colour in a marked 
 degree. 
 
 3. POLYMORPHISM. 
 
 Sexual trimorphism is of frequent occurrence in the animal 
 kingdom. I will here refer to certain instances amongst butter- 
 flies, which were first discovered by Alfred Russel Wallace, and 
 will begin with a case in which apparently the first step towards 
 polymorphism has been taken. 
 
 The male of a common North American butterfly, Papilio 
 turnus, resembles the ordinary ' swallow-tail,' having yellow 
 wings ornamented with black transverse stripes ; while the 
 females sometimes resemble the males, and are sometimes quite 
 black, and may thus differ markedly from one another. The 
 yellow females occur in the eastern and northern parts of the 
 United States, and the black ones in the west and south ; we 
 must therefore suppose that two local varieties of this butterfly 
 exist, in the northern of which the two sexes have a similar 
 coloration, while the southern form is dimorphic. This indi- 
 cates, in terms of the idioplasm, that the determinants for the 
 wing-scales are single in the northern, and double in the southern 
 variety. Describing these determinants according to the colour 
 which they produce, we may say that the last-named variety 
 possesses double-determinants, the 'male' half of which is 
 'yellow' and the 'female' half 'black'; while the single-deter- 
 minants of the northern form are ' yellow.' This species is 
 properly speaking, therefore, not trimorphic, but includes two 
 local varieties, one of which is dimorphic. If the two varieties
 
 DIMORPHISM AND POLYMORPHISM. 375 
 
 interbreed, as is actually the case at the junction of the dis- 
 tricts in which the two forms are respectively distributed, the 
 double-determinants of the southern form will meet with single- 
 determinants of the northern form in the germ -plasm of the 
 offspring. The male descendants of such a cross would remain 
 unmodified, but the females would be either black or yellow 
 according to the power of transmission of the 'female' halves of 
 the determinants, or as was observed by Edwards * a com- 
 bination of both these colours. Such combinations might either 
 arise from the cross between a yellow female and a yellow male 
 of the dimorphic variety, or from that between a black female 
 and a yellow male of the monomorphic form ; for in the 
 dimorphic variety the germ-plasm contains double-determinants 
 in the males as well as in the females. If we suppose that 
 these crosses occur frequently, the number of females of an 
 intermediate form in the borderland of the two districts would 
 gradually increase, and might ultimately result in the production 
 of a constant intermediate female form. But if the males ex- 
 hibited a preference for the females corresponding to them, the 
 female forms would remain essentially distinct from one 
 another. This seems to be the case in Papilio lurntts, at least 
 Edwards states that the intermediate forms are rare. 
 
 The dimorphic and polymorphic females in many butterflies 
 may perhaps be looked upon as belonging to sexually dimorphic 
 local forms which have subsequently spread and occasionally 
 crossed. In places where the varieties merely exist side by side 
 without interbreeding, each of them also contains either single- 
 or double -determinants, according to whether it is sexually 
 monomorphic or dimorphic ; but when interbreeding occurs, 
 the determinants of the two races come together, and then 
 several homologous double-determinants may even meet in the 
 same germ-plasm, some in certain ids and some in others. 
 
 In Papilio lurnus the case is not quite so simple as I have 
 stated it : as a matter of fact, this species exhibits a double 
 dimorphism, for the yellow females do not exactly resemble the 
 males, but differ considerably from them as regards the shade 
 of yellow and the pattern on the wings : thus they possess an 
 oiange-coloured eye-spot on the posterior wings, which is absent 
 in the male. We must therefore assume that double-deter- 
 
 W. H. Edwards, ' Butterflies of North America.'
 
 376 THE GERM-PLASM. 
 
 minants are present in the yellow variety also. If we imagine 
 that the two sexes were identical as regards the wing-marking 
 in the immediate ancestors of Papilio turnus, as they are in the 
 closely allied European species Papilio machaon, and that this 
 monomorphic ancestral form had persisted in California, let 
 us say, we should have an instance of that kind of poly- 
 morphism which Wallace has described in the case of Papilio 
 memnon, in which there are one male and three female forms. 
 In this case we must suppose that the first and oldest form 
 possessed single-determinants for the wings ; while in the 
 second and third forms the determinants were double, and their 
 'male' halves retained their original nature, the female halves 
 becoming modified in two different directions. 
 
 It is therefore not necessary, as might have been supposed, 
 to assume the existence of triple determinants from the fact of 
 the trimorphism of a species alone, or of quadruple or quintuple 
 ones in the case of polymorphism. 
 
 The polymorphism of animal and plant stocks rests on a 
 different basis, as it concerns the physiologically dissimilar 
 members of a higher stage of individuality that of the stock. 
 This kind of polymorphism has already been treated of as a 
 phenomenon of development in connection with alternation of 
 generations. We must, however, take the closely allied poly- 
 morphism of animal communities into consideration. 
 
 The differences between the male and female individuals in 
 bees has already been referred to the existence of double- 
 determinants. But a third form of individual, the worker, 
 occurs in the honey-bees. These workers differ from the 
 females in the slight development of the ovaries, the ovarioles 
 not only being fewer in number than in the ' queen,' but even 
 frequently containing no eggs at all, and at most only very few. 
 The receptaculum seminis is also more or less reduced, and the 
 abdomen is much shorter and thinner than that of the queen 
 bee. If these were the only differences between the two forms, 
 there would scarcely be any need to assume the existence of 
 special determinants for these parts in the germ-plasm of the 
 workers : we might imagine that the determinants of the 
 ovarioles, for example, were so constituted as to become active 
 in consequence of abundant nourishment, and to cause the 
 development of ovarioles ; while a smaller supply of nutriment 
 would not always be sufficient for this purpose, and thus the
 
 DIMORPHISM AND POLYMORPHISM. 377 
 
 complete formation of the sexual organs would be prevented. 
 We know, indeed, that a fertilised egg may develop into a queen 
 or a worker, according to whether the larva arising from it is 
 fed on royal diet or with the less nutritious food supplied to 
 the workers. 
 
 This explanation, however, even if correct as regards the 
 degenerated parts of the workers, does not sufficiently account 
 for the other differences between the two kinds of females. 
 For the workers are not inferior to the queen bee in all respects : 
 on the contrary, the worker's sting is straighter, longer, and 
 stronger, and is provided with more teeth, than the queen's ; 
 the wings, moreover, are longer, the tarsal segment of the 
 hind-limb is provided with the well-known brush, and the tibia 
 has a depression known as the pocket, for carrying the masses 
 of pollen which the insect collects. These two characteristic 
 parts are wanting in the queen. Important differences must 
 also exist as regards the minute structure of the brain, for the 
 instincts of the queen are very different from those of the 
 workers. After fertilisation has taken place, the queen lays 
 eggs, but she neither gathers honey from flowers, excretes wax, 
 nor makes the honeycomb. It is therefore incredible that the 
 queen and workers should be formed by the agency of similar 
 determinants. The germ-plasm must contain double-deter- 
 minants for certain parts of the body of the queen and workers 
 respectively. But as we have already assumed the existence 
 of double-determinants for the formation of male and female 
 bees, or at any rate for the development of those parts which 
 differ in the two sexes, we can only make the further assumption 
 that the ' female ' halves of the double-determinants may them- 
 selves consist of two halves, corresponding to the queen and 
 worker respectively, and that each of these halves must naturally 
 be looked upon as a complete determinant as regards size 
 and structure. It is of no consequence whether they are re- 
 garded as being closely connected together, or as independent 
 structures in close proximity with one another : in either case 
 they must have arisen by the doubling or tripling of a single 
 ancestral determinant. The terms 'double-determinant* and 
 1 half-determinant ' are simply used for the sake of simplicity. 
 Their relation to one another is similar to that existing between 
 the homologous but heterodynamous determinants of different 
 ids.
 
 378 THE GERM-PLASM. 
 
 In the case of bees, the factor that determines which of the 
 two halves of the ' female ' determinants is to become active, 
 seems to be the quality of the food supplied to the larva, 
 so that the critical moment only arrives long after the termina- 
 tion of embryogeny, and before the chrysalis stage is reached. 
 It is well known that when the queen is lost, another one is 
 produced by feeding a larval worker with royal diet. Thus the 
 sex depends on the occurrence or omission of fertilisation, but 
 the modification into a queen or worker takes place much later, 
 when the animal has reached the larval stage. The idea of the 
 trimorphism of certain determinants thus becomes much less 
 difficult to realise. We must look upon them as double-deter- 
 minants contained in the ids of the germ-plasm, the female 
 hemisphere of which is again composed of two dissimilar 
 quarter-spheres. If the egg becomes fertilised the male half 
 becomes inactive, and we have already represented this figura- 
 tively as taking place by the ' female ' hemisphere extending 
 over the ' male ' hemisphere, and enveloping it like a mantle. 
 This ' female ' ' determinant mantle ' consists of two halves, re- 
 presenting the queen and worker respectively, and we may 
 suppose that the subsequent determination during the larval 
 stage as to which half is to control the cell, takes place in such 
 a manner that the ' worker ' half extends over the other when 
 the nourishment is poor, while with more abundant food the 
 'queen' half grows more rapidly, and prevents the 'worker' 
 half from exerting any influence on the cell. I naturally do not 
 in the least suppose that this figurative representation of the 
 process represents the actual facts of the case, but it at any rate 
 shows that the existence of trimorphic determinants or more 
 accurately, of double determinants each possessing a dimorphic 
 half is conceivable. 
 
 We might, however, also assume the existence of three in- 
 dependent determinants side by side, so arranged that they 
 become active under other definite influences ; and this con- 
 ception would better agree with the unavoidable assumption 
 that the three determinants which act vicariously are of a 
 similar size. 
 
 The differentiation of the determinants into several equivalent 
 parts, each of which prevents the others from becoming active, 
 may take place to a still further degree by the ' male ' half oj the 
 double-determinants becoming differentiated into two dissimilar
 
 DIMORPHISM AND POLYMORPHISM. 379 
 
 halves. The Termites, in addition to the workers or stunted 
 females, possess ' soldiers ' or males, in which the sexual organs 
 are stunted, which possess very strong mandibles, and differ in 
 other important structural details from the ordinary males. In 
 this case, therefore, four determinants must be present, each 
 capable of being substituted for another, and only one of which 
 can be active at a time. 
 
 Apart from local dimorphism, a temporary dimorphism occa- 
 sionally occurs, and is especially well known amongst butterflies 
 as seasonal dimorphism. In this case the individuals of the same 
 generation, hatched at the same time of year, are alike, but the 
 summer and spring generations differ from one another. 
 
 In the European species Vanessa levana-prorsa, the indi- 
 viduals of the spring generation are characterised by a yellow 
 and black pattern on the upper side of the wings ; while the 
 summer form {prorsa) has black wings, with a broad white 
 transverse band, and delicate yellow lines running parallel to the 
 margins. Were we to superpose these two patterns, it would 
 be seen that the black parts in prorsa do not correspond to 
 the yellow ones in levana, and that the white band in the 
 former does not correspond to a yellow or black part in the 
 latter. This band is, on the contrary, entirely wanting in 
 levana, and is represented by both black and yellow regions. 
 
 These cases of dimorphism can also, it seems to me, only 
 be accounted for in terms of the idioplasm by the assumption 
 of double-determinants, which, however, are concerned in this 
 case merely with the wing-scales, and essentially with those 
 on the upper side of the wings only ; for the lower surface, 
 though not precisely similar, differs far less in the two forms 
 than does the upper side. We will speak of the halves of these 
 double determinants as 'winter' and ' summer' determinants, 
 and may suppose that the influences of temperature which affect 
 them at the beginning of the pupal stage determine which of 
 the two halves is to predominate over the other. Nearly twenty 
 years ago I showed that it is possible to compel the pupas of the 
 summer-generation to assume the winter form by exposing them 
 to a low temperature, so that the butterfly emerged as a levana 
 instead of a prorsa. The converse experiment was also occa- 
 sionally successful, the pupae of the winter-generation being 
 forced to assume the summer form by the influence of a higher 
 temperature during, or shortly after, pupation. We may per-
 
 380 THE GERM-PLASM. 
 
 haps therefore suppose that an increase of temperature prevents 
 the 'winter' halves of the double-determinants in question from 
 developing, while it is beneficial to the 'summer' halves; and 
 that conversely, the development of the 'summer' halves 
 remains stationary when the temperature is lowered, while the 
 'winter' halves continue to develop. This may be illustrated, 
 as in the case of ordinary sexual dimorphism, by supposing a 
 determinant of the germ-plasm to be spherical, and to consist 
 of a ' summer ' and a ' winter ' half. This determinant would 
 remain unchanged throughout embryogeny, and even during 
 the entire caterpillar stage ; and the increased or diminished 
 temperature would only determine which half should outgrow 
 the other and prevent it from controlling the cell, at the 
 beginning of pupation, when the wings are formed. 
 
 In Vanessa levana the males and females resemble each 
 other so closely in the pattern of the wings that they cannot be 
 distinguished from one another with any degree of certainty ; 
 but in many other seasonally dimorphic butterflies, sexual dimor- 
 phism is exhibited in the pattern and coloration of the wings, 
 and we must therefore assume that they possess double-deter- 
 minants consisting of 'male' and 'female' halves, each of 
 which is again subdivided into a ' summer ' and a ' winter ' half. 
 We do not know what factors determine the sex of butterflies, 
 but in many cases the determination is effected early, for ovaries 
 and spermaries can be distinguished from one another in the 
 full-grown caterpillar. Thus, as in the case of Termites, the 
 decision concerning the subdivisions of the double-determinants 
 takes place subsequently to that with regard to the primary 
 halves. 
 
 4. DlCHOGENY IN PLANTS. 
 
 De Vries has made use of the term ' dichogeny ' to describe 
 that form of dimorphism which becomes manifest when a 
 young vegetable tissue, under normal conditions, is capable of 
 developing in different ways according to the external influences 
 to which it is exposed. Shoots of ivy bear leaves on the side 
 which is exposed to the light, and roots on the opposite side ; 
 but if the plant is rotated, the same shoot will grow leaves on 
 the side which previously bore roots, and vice versa. The 
 stimulus due to light therefore apparently causes a group of 
 cells, which would have formed roots if they had been in the 
 shade, to give rise to leaves.
 
 DIMORPHISM AND POLYMORPHISM. 381 
 
 The assumption, which is in accordance with de Vries's ideas, 
 that all the hereditary tendencies of the species are contained 
 in every cell of the ivy-shoot, and that those which concern the 
 leaves only undergo development in response to the stimulus 
 caused by the light, and those corresponding to roots only to 
 that produced by the shade, does not materially help us in the 
 solution of the problem. In point of fact, the same cells are 
 not capable of forming roots and leaves ; the leaves are much 
 less numerous than the short and closely aggregated roots, and 
 therefore a large number of cells, or groups of cells, which give 
 rise to roots when shaded, do not develop leaves when exposed 
 to the light ; they consequently contain no 'leaf-determinants.' 
 Hence the same idioplasm cannot produce roots when shaded 
 from, and leaves when exposed to, the light ; but the deter- 
 minants for the roots or leaves respectively must be distributed 
 very differently in the cells. 
 
 The predisposition to form either of these two structures is 
 obviously determined in the growing point or apex of the shoot. 
 The cells which are continually being produced by the apical 
 cells are destined at a very early stage to form the rudiments of 
 roots or leaves ; only a certain number of cells on the illumi- 
 nated side are provided with leaf-determinants, while much 
 more numerous cells on the shaded side are furnished with 
 root-determinants. The determination therefore takes place at 
 a very early stage, when the shoot is actually in an embryonic 
 condition ; and the degree of illumination determines which side 
 is to be provided with leaf- and which with root-determinants, 
 each kind being distributed in accordance with a different 
 law, as well as the side to which the groups of each kind of 
 determinants are to pass during the nuclear divisions in the 
 cells arising from the apical cells. This case is analogous to 
 that of the inversion of the viscera in man, except that we do 
 not know the cause of this change of position. Some influence, 
 however, must in this case also be exerted during the early 
 embryonic stages, and cause the liver to take up a position on 
 the left side, and the spleen and heart on the right, long before 
 these parts are actually formed. No subsequent influences 
 could cause the liver to become shifted from the right to the 
 left side, or could transform the liver into the spleen ; just 
 as in the ivy-shoot no influences can lead to the formation of 
 leaves on the shaded side when it is once covered with roots.
 
 32 THE GERM-PLASM. 
 
 It therefore seems to me that the same cells cannot give rise 
 to two kinds of structures in this case, but that some groups 
 form roots, and others leaves ; and that, moreover, the idio- 
 plasms of these groups of cells differ from, and cannot be trans- 
 formed into, one another, but that the sides of the shoot which 
 produce roots and leaves respectively, are determined by the 
 influence of the light as soon as the first embryonic rudiments of 
 the shoot is formed. 
 
 The fact that rudiments of roots can actually be proved by 
 the aid of the microscope to exist in the tissue of the stem in some 
 plants, seems to me substantially to support this view. This is 
 the case in the willow, in which the power of producing roots 
 from cuttings exists in a high degree. But numerous other 
 plants are also capable of multiplying by means of cuttings, and 
 we may suppose that in them, in spite of the absence of visible 
 root-germs, the determinants for the tissue of the roots are 
 present, and are ready to develop into germs whenever external 
 influences leading to the formation of roots come into play. 
 
 All cases of dichogeny, however, cannot be explained in this 
 manner. In some instances the same cells may actually develop 
 in different ways, and the idioplasm may therefore become 
 transformed by external influences. A transformation as re- 
 gards the dorso-ventral arrangement of the parts in a young 
 shoot of Thuja takes place when it is turned round so that the 
 relative positions of the upper and under surfaces are reversed.* 
 The cells which, under ordinary conditions, take on the form 
 of palisade cells, then assume the structure of the cells on the 
 lower surface, and -vice versa. 
 
 It appears to me that this fact is to be explained by suppos- 
 ing that the determinants of these two forms of cells are both 
 present in each cell, but that only one or the other of them 
 becomes active, according to the degree of illumination. I can, 
 however, form no idea as to why such an arrangement is met 
 with in this case. 
 
 * Compare the statements made on this subject by Detmer, Biolog. 
 Centialblatt. Vol. VII., No. 23.
 
 DOUBTFUL PHENOMENA OF HEREDITY. 383 
 
 CHAPTER XII. 
 DOUBTFUL PHENOMENA OF HEREDITY. 
 
 i. 'XENIA' AND TELEGONV. 
 
 ALTHOUGH it is certainly unnecessary in a theory of heredity 
 to discuss all the possible kinds of phenomena which are with 
 doubtful justice included under this head, I should not like to 
 pass over in silence the consideration of certain presumptive 
 observations, as they have so often Leen discussed, and were 
 considered worthy of notice by so eminent an authority as 
 Darwin. These refer in the first place to the so-called 'jr/wVi,' 
 and to the phenomenon generally known as ' infection o) the 
 genii,' which, in case it really exists, 1 should prefer to speak 
 of as telcgony* 
 
 Focke has used the term ' xenia ' to describe those cases 
 in which 'hereditary characters are supposed to have been 
 transmitted by the pollen to the tissues of the fruit as well as 
 to the fertilised egg-cell and the embryo arising from it.' 
 
 Darwin mentioned many instances of this kind, and attempted 
 to account for them by supposing that an emigration of ' gem- 
 mules ' takes place from the sperm-cells (pollen-tubes) to the 
 surrounding tissue of the fruit. Focke has collected all the known 
 cases, and on reading them, one receives the impression that 
 they may very likely be deceptive. Blue grains occasionally 
 occur amongst the yellow ones in cobs of the yellow-grained 
 maize (Zea) after fertilisation with the pollen of a blue- 
 grained species. It is possible that previous crossings of the 
 two species may have produced this result, which might wrongly 
 be ascribed to the immediate influence of the pollen of another 
 species on the fruit. ]. Anderson Henry even thought he had 
 observed that all the flowers in an inflorescence of a white 
 Calceolaria were reddened by the influence of the pollen from 
 a red kind on a single flower of this inflorescence ! 
 
 * From TT}\e at a distance, and y6i>os oftspi ing.
 
 384 THE GERM-PLASM. 
 
 As such eminent botanists as Focke,* and more recently de 
 Vries,t have expressed much doubt with regard to these obser- 
 vations or rather interpretations, we must wait until these 
 cases have been critically reinvestigated before attempting to 
 account for them theoretically. The chief difficulty we should 
 meet with in any such explanation would be due to the fact that 
 we are here concerned with the influence of the germ-plasm of 
 the sperm-cell on a tissue of another plant which only con- 
 stitutes a part of this plant. It would thus be necessary to 
 assume that all the determinants of this germ-plasm are not 
 active, and that only those take effect which determine the 
 nature of the fruit. 
 
 The uncertainty of the observations is still greater in the in- 
 stances of so-called infection of the germ. If the case recorded by 
 Darwin but not observed by him personally is reliable, and 
 has been accurately described, all doubts must be set aside. A 
 mare belonging to Lord Morton ' bore a hybrid to a quagga,' 
 and subsequently ' produced two colts by a black Arabian horse; 
 these colts were partially dun-coloured, and were striped on 
 the legs more plainly than the real hybrid, or even than the 
 quagga. One of the two colts had its neck and some other 
 parts of the body plainly marked with stripes,' and the hair of 
 the mane is said to have resembled the short stiff and upright 
 mane of the quagga, instead of that of the horse.! Similar cases 
 of the influence of a previous fertilisation on the structure of 
 subsequent offspring are related of several domestic animals, 
 viz., of cows, sheep, pigs, dogs, and pigeons, as well as of human 
 beings when crosses occur between white races and negroes. 
 
 Up to the present time no experiments have been made with 
 this special object, and it would be necessary to use every con- 
 ceivable precaution in conducting such experiments, or they 
 would be valueless ; they could, therefore, be best made in 
 zoological gardens, not only because of the undoubtedly pure 
 material which might be used for the purpose, but also because 
 
 * Focke, ' Die Pflanzen-Mischlingc,' Berlin, 1881, p. 510, ct seq. 
 
 t Hugo de Vries, ' Intracellulare Pangenesis,' Jena, 1889, p. 206. 
 
 % I have quoted this case from Darwin's ' Variation of Animals and 
 Plants under Domestication,' and ed., Vol. I., p. 435. Darwin does not 
 seem to have known of the drawings of these colts which will be mentioned 
 in a subsequent paragraph. I have not seen them, and only learnt of 
 their existence from Settegast's book.
 
 DOUBTFUL PHENOMENA OF HEREDITY. 385 
 
 it would be possible to isolate the animals, and for the keepers 
 to exert a strict control over them for considerable periods of 
 time. 
 
 The philosopher Carneri mentions a case which came under 
 his own notice. He kept a herd of cattle of the dark grey 
 Miirzthal breed. On one occasion he put one of the cows to a 
 'light-coloured Pinzgau ' bull instead of to one of the same 
 breed. The cow threw a calf with the characteristic brown and 
 white patches of the Pinzgau breed, as well as with distinct 
 traces of the ' dark grey Miirzthal cross.' The cow was sub- 
 sequently covered by a Miirzthal bull, and, contrary to expecta- 
 tion, the second calf was also a ' hybrid,' being for the most 
 part grey, ' but possessing large brown spots like those of the 
 Pinzgau breed.' 
 
 Both the above mentioned cases are not so conclusive as they 
 appear to be at first sight. A drawing by Agasse of the foal 
 possessing the characters of the quagga is to be seen at the 
 Royal College of Surgeons in London, and shows indistinct 
 dark stripes on the neck, withers, and legs. Similar stripes are, 
 however, not very uncommon on purely bred foals, and ordinarily 
 disappear as the animal grows older. No further resemblance 
 to the quagga can, however, be detected in these pictures.* 
 
 I must not omit to mention that before having heard of the 
 hypothesis of ' infection,' Carneri accounted for the case of the 
 two breeds of cattle described above by supposing that ' a drop 
 of Pinzgau blood' must have previously got into the Miirzthal 
 herd without his being aware of it. 
 
 Thus even the best of these ' cases ' are not reliable and 
 actually convincing. We may, however, at any rate suppose 
 that this so-called ' infection,' if not altogether deceptive, only 
 occurs in rare instances, and by no means regularly, or at most 
 only in some cases. Experienced breeders, like Settegast and 
 Kiihn of Halle, do not believe in it ; for although they have 
 frequently crossed various domestic animals, they have never 
 observed an instance of it. Such cases could only be accounted 
 for from our point of view by supposing that spermatozoa had 
 reached the ovary after the first sexual union had occurred, and 
 had penetrated into certain ova which were still immature. The 
 
 * According to Settegast ('Thierzucht,' Breslau, Bd. I., 1878, pp. 
 223-234). 
 
 2 B
 
 386 THE GERM-PLASM. 
 
 immediate fertilisation of the latter is rendered inconceivable 
 by the fact of their immaturity, and the sperm-cell must have 
 remained in the body of the ovum until the maturation of the 
 latter, with the nucleus of which it then united in the process of 
 amphimixis. If this occurred sometime after the first of the 
 offspring was born, it might easily have coincided approximately 
 with the second coitus, from which the fertilisation would then 
 apparently be due. If the ' infection ' were proved beyond a 
 doubt, a supplementary fertilisation of an egg-cell in this 
 manner must be considered possible ; we certainly might then 
 reasonably ask why mares, cows, or sheep, should not occa- 
 sionally become pregnant without being covered a second time. 
 Rut this has never yet been known to occur, and I incline to 
 Settegast's view that there is no such thing as an ' infection ' of 
 this kind, and that all the instances which have been recorded 
 and discussed critically by him are based upon a misconception. 
 
 2. THE INFLUENCE OF TEMPORARY ABNORMAL CONDITIONS 
 OF THE PARENTS ON THE CHILD. 
 
 Although I do not consider that the cases which come under 
 the above heading have anything to do with heredity, I should 
 not like to leave them entirely on one side. 
 
 It has often been supposed that drunkenness of the parents at 
 the time of conception may have harmful effect on the nature of 
 the offspring. The child is said to be born in a weak bodily 
 and mental condition, and inclined to idiocy, or even to mad- 
 ness, &c., although the parents may be quite normal both 
 physically and mentally. 
 
 Cases certainly exist in which drunken parents have given 
 rise to a completely normal child, although this is not a con- 
 vincing proof against the above-named view ; and in spite of 
 the fact that most, or perhaps even all, the statements with 
 regard to the injurious effects on the offspring will not bear a 
 very close criticism, I am unwilling to entirely deny the possi- 
 bility that a harmful influence may be exerted in such cases. 
 These, however, have nothing to do with heredity, but are 
 concerned with an affection of the germ by means of an external 
 influence. 
 
 The experiments of the brothers Hertwigshowthat the develop- 
 ment of the fertilised egg in lower animals may be considerably
 
 DOUBTFUL PHENOMENA OF HEREDITY. 387 
 
 retarded by the action of various chemical substances, such as 
 chloral, quinine, and morphia ; and we also know that the ova of 
 sea-urchins, if kept too long in the sea-water before being fer- 
 tilised, tend to lose their vital energy, and consequently many 
 spermatozoa, instead of a single one, are likely to enter each of 
 them. A similar result may follow from the effects of the above- 
 mentioned chemical reagents, and in both cases an abnormal 
 development of the egg, such as a duplication of parts, may be 
 the consequence. 
 
 It does not appear to me impossible that an intermixture of 
 alcohol with the blood of the parents may produce similar 
 effects on the ovum and sperm-cell. According to the relative 
 quantity of alcohol, either an exciting or a depressing influence 
 might be exerted, either of which would lead to abnormal de- 
 velopment. A depressing influence exerted on both germ-cells 
 would certainly retard, or even quite prevent, the process of 
 fertilisation ; while if the egg-cell were alone affected, super- 
 fertilisation (polyspermy) might result ; and the same might 
 occur by an excitation of the sperm-cells alone. The entrance 
 of several spermatozoa into the small human ovum, which con- 
 tains only a small amount of yolk, might produce an abnormal 
 development just as much as in the case of the eggs of the star- 
 fish or sea-urchin. A high degree of excitation in both germ- 
 cells might, on the other hand, cause the complicated processes 
 of the increase of the germ-plasm in the ovum and the subse- 
 quent conjugation of the two germ-plasms to take place in an 
 inexact manner, owing to their being passed through too quickly, 
 and would then produce an irregular development. 
 
 New predispositions can certainly never arise owing to such 
 deviations from the normal course of development, and therefore 
 a modification of the process of heredity itself is out of the 
 question. It is, however, conceivable that more or less con- 
 siderable abnormalities may affect the course of development, 
 and either cause the death of the embryo, or else produce more 
 or less marked deformities. The question as to whether such 
 deformities really result in consequence of the drunken con- 
 dition of the parents can only be decided by observation. 
 
 3. THE SUPPOSED TRANSMISSION OF DISEASES. 
 There is no doubt that some diseases are passed on from one 
 generation to another. All such cases are not, however, con-
 
 388 THE GERM-PLASM. 
 
 nected with heredity, and many of them are in all probability 
 to be explained as the result of infection of the parental 
 germ-cell with microscopic parasites, and ought consequently 
 to be described as infections of the germ. 
 
 In man such a transference of disease has only definitely 
 been proved to occur in the case of syphilis.* The father, as 
 well as the mother, is capable of transmitting this disease to the 
 embryo, and the only possible explanation of this fact is, there- 
 fore, that the specific bacteria of syphilis can be transmitted 
 by the spermatozoon. Amongst the lower animals the 'peb- 
 rine' of the silkworm is an example, which has been well 
 known for several decades, of the transference of a fatal 
 disease from one generation to another through the egg : the 
 germs of the fungus which produces the disease penetrate into 
 the yolk. It is not known why these germs do not develop 
 and multiply within the egg, and thus destroy it, but this is, 
 however, the case. The fungi only begin to multiply in the 
 young caterpillar t when it is half- or full-grown, or the disease 
 may, again, only be fatal in the butterfly stage. 
 
 As we now know that many diseases of man and other 
 mammals are due to such low forms of parasites, it is natural 
 to suppose that the transmission of such diseases results from 
 infection of the germ-cell with microbes, and not from inherit- 
 ance in the true sense of the word that is, from the trans- 
 mission of an anomalous state of the germ-plasm itself. 
 
 I have elsewhere attempted to trace the 'heredity' of 'epilepsy' 
 produced artificially in guinea-pigs, by supposing that in this 
 case a similar process occurs. The slow development of 
 this form of ' epilepsy,' resulting from an injury to the spinal 
 cord or one of the larger nerves, seems to me, indeed, to support 
 the conclusion that its symptoms, which resemble those of true 
 epilepsy, are due to the migration of microbes, which advance 
 from the injured part along the nerves in a centripetal direction 
 until they reach the brain, where they set up the state of irrita- 
 tion characteristic of the disease. The great inconstancy of 
 the symptoms, and the variety of forms of nervous diseases 
 which the offspring exhibit, also indicate that a true heredity is 
 
 * Cf., e.g., Dohrn, ' Zur Frage der hereditaren Infection,' Deutsche 
 med. Wochenschrift, Sept. 15, 1892. 
 
 t Cf., F, Haberlandt, 'Der Seidenspinner des Maulbeerbaums u. seine 
 Krankheiten,' Wien, 1871,
 
 DOUBTFUL PHENOMENA OF HEREDITY. 389 
 
 not concerned in the process, and that the transmission is in this 
 case due to infection of the germ with the microbes by which 
 the disease is induced. * 
 
 The ' transmission ' of carcinoma might be accounted for in 
 a similar way, if, as has recently been supposed, this disease 
 is really due to microbes. 
 
 It is, however, also conceivable that both causes the trans- 
 mission of abnormal predispositions, and infection of the 
 germ might combine to bring about the transference of a 
 disease from one generation to another. Without desiring to 
 encroach upon the domain of pathology, I am inclined to sup- 
 pose that this is the case as regards 'hereditary' tuberculosis : 
 there is no doubt about the occurrence of a 'tuberculous 
 habit,' that is, a certain complication of structural peculiarities 
 which is commonly connected with the disease, such as a 
 narrowness of the chest, for instance. These peculiarities 
 must result from the structure of the germ-plasm, in which a 
 definite variation of certain determinants and groups of deter- 
 minants must have taken place, and they are therefore certainly 
 transmissible. The disease itself, however, is not due to this 
 ' habit,' but is caused by the presence of specific parasites, the 
 tubercle-bacilli, which have a harmful effect upon the various 
 living tissues. They may be introduced artificially into the blood, 
 and then produce the disease even in perfectly normal individuals. 
 They may, moreover, enter the body ' spontaneously,' e.g., by 
 some natural means, and will then also give rise to the disease. 
 But in the latter case the probability of infection seems largely 
 to depend upon the susceptibility or power of resistance of the 
 individual, and at the present day pathologists are of opinion 
 that persons exhibiting the ' tuberculous habit ' already referred 
 to have a much slighter power of resistance to the parasites 
 which have passed into the body than strongly-built people. 
 The inheritance of the disease would accordingly depend on the 
 transmission of a constitution very liable to infection. 
 
 Without wishing to deny the existence of such a predisposi- 
 tion to infection, I do not believe that the transmission of 
 tuberculosis is due merely to the inheritance of a greater degree 
 
 * A more detailed account and proof of this view concerning the infec- 
 tious nature of traumatic epilepsy in guinea-pigs is contained in my essay 
 on ' The Significance of Sexual Reproduction,' Appendix iv.
 
 39 THE GERM-PLASM. 
 
 of susceptibility. A large number of facts seem to me, on the 
 contrary, to support the view that infection of the germ plays 
 the chief part in the process. It would be out of place to enter 
 into particulars and attempt to prove this view here the question 
 belongs to the province of the pathologist : I merely wished to 
 point out in this connection that a combination of hereditary 
 transmission and infection of the germ is perfectly conceivable. 
 T\i& phyletic origin of such constitutional diseases is presumably 
 to be explained as being due to the occurrence of certain in- 
 dividuals possessing constitutions which were abnormally sus- 
 ceptible to a certain kind of microbe. Such persons would be 
 more readily attacked from without by this particular disease. 
 If, however, it once attacked them, and were it of such a kind 
 as to cause death only after some time, a further and much 
 surer opportunity was offered to the microbes for transferring 
 themselves to other hosts than was previously the case when 
 they passed into the body from without : they settled in the 
 germ-cells of the individual affected, and were thus transferred 
 to the descendants of this individual. Although the presence 
 of parasites in the germ-cells has not yet actually been proved 
 in the case of tuberculosis, in my opinion it by no means follows 
 that such an infection does not nevertheless take place : we do 
 not even know whether such microbes are of the ordinary 
 form and size. In any case they must possess different vital 
 qualities ; for did they multiply in the egg- or sperm-cell in 
 the same manner as in the tissues in which they are known 
 to occur, the germ-cells would soon be destroyed. Numerous 
 adaptations to the host may have occurred in this case as in 
 that of other parasites ; and, moreover, latent periods of develop- 
 ment may have arisen during which the parasite does not 
 undergo multiplication. It seems improbable that such arrange- 
 ments should not be met with, and that the parasite should not 
 make use of the favourable opportunity of becoming distributed 
 with the greatest certainty. Latent periods very commonly 
 occur in the germs of animals and plants whenever they are 
 useful, and hence this arrangement must come about without any 
 great difficulty. 
 
 Even although our most eminent pathologists, such as Ernst 
 Ziegler, are now of opinion that tuberculosis is not transmitted 
 by infection of the germ, because such a transmission has not 
 been directly proved, and because, on the other hand, an in-
 
 DOUBTFUL PHENOMENA OF HEREDITY. 391 
 
 fection from without cannot be conclusively disproved in any 
 individual instance, I am inclined to believe that they have been 
 too cautious in their conclusions, of which only a negative proof 
 is furnished by either factor. For neither of these in the least 
 proves that infection of the germ does not take place : from a 
 more general, biological point of view, indeed, it seems to be far 
 more probable that it does. 
 
 It will, I think, at any rate be conceded that a 'constitu- 
 tional ' disease cannot be taken as a proof that the processes of 
 heredity are therein concerned until we can determine whether 
 we are actually dealing with heredity, i.e., the transmission of 
 a constitution, and not only with a transference of microbes ; 
 and the main object of this section was to make this clear. 
 But at the same time I have stated my reasons for using so 
 few facts from the domain of pathology in support of my 
 theory.
 
 PART IV. 
 
 THE TRANSFORMATION OF SPECIES: ITS 
 ORIGIN IN THE IDIOPLASM. 
 
 CHAPTER XIII. 
 
 THE SUPPOSED TRANSMISSION OF ACQUIRED 
 CHARACTERS. 
 
 i. DIFFICULTIES IN THE WAY OF A THEORETICAL BASIS 
 
 FOR THIS ASSUMPTION. 
 
 BY acquired characters I mean those which are not preformed 
 in the germ, but which arise only through special influences 
 affecting the body or individual parts of it. They are due to 
 the reaction of these parts to any external influences apart from 
 the necessary conditions for development. I have called them 
 ' somatogenic' 1 characters, because they are produced by the 
 reaction of the body or soma, and I contrast them with the 
 *blastogemc* characters of an individual, or those which origi- 
 nate solely in the primary constituents of the germ (' Keimesan- 
 lagen '). It is an inevitable consequence of the theory of the 
 germ-plasm, and of its present elaboration and extension so 
 as to include the doctrine of determinants, that somatogenic 
 variations are not transmissible, and that consequently every 
 permanent variation proceeds from the germ, in which it must 
 be represented by a modification of the primary constituents. 
 
 I will first attempt to show how this conclusion is arrived at 
 theoretically, and will then proceed to test it by ascertaining 
 how far it is in agreement with actual observation, and whether 
 the theory can be justified by facts.
 
 SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS. 393 
 
 Somatogenic variations may be classified according to their 
 origin into three categories, viz., injuries, functional variations, 
 and variations depending on the so-called ' influences of environ- 
 ment] which include mainly climatic variations. 
 
 The hereditary transmission of any of these three kinds of 
 somatogenic variations could be accounted for theoretically only 
 by the assumption that that part of the soma which had been 
 changed by external influences, could modify the germ-plasm 
 contained in the germ-cells of the same individual, so that its 
 offspring would, from the germ onwards, undergo similar varia- 
 tions to those which had been acquired by the action of external 
 influences on the parental part in question. 
 
 As far as I can see, there are only two ways in which such a 
 variation could conceivably occur in the germ-plasm in con- 
 sequence of a corresponding somatic variation. We should 
 either have to assume the presence in all parts of the body of 
 definite tracks along which each somatogenic variation might 
 be transferred to the germ-cells, in the germ-plasm of which it 
 would produce a corresponding change ; or else that gemmules, 
 such as Darwin supposed to exist, are given off from every 
 somatic cell and are conveyed to the germ-cells, either 
 through the vascular system, when one exists, or by some 
 other means, and that they must then penetrate into these 
 cells, and become incorporated in their germ-plasm. Thus 
 either the presence of hypothetical tracks along which a 
 modifying, though totally inconceivable, influence might be 
 transferred to the germ-cells, or else the discharge of material 
 particles from the modified organ, must take part in the for- 
 mation of the germ-plasm : there is no third way out of the 
 difficulty. 
 
 Both these hypotheses have already been used to explain 
 the supposed transmission of somatogenic variations, the 
 former, it is true, only in vague allusions hinting at 'nerve- 
 influences,' which are supposed to start from the modified part, 
 and to produce a corresponding alteration in the hereditary sub- 
 stance of the germ-cells. But no one has yet ventured to state 
 more precisely how nerve-excitation can modify the germ-plasm 
 materially, and in accordance with the somatic variation. It 
 would probably be useless even to expect an answer to the 
 question as to how a part, such as a muscle, enlarged by functional 
 hypertrophy, is capable of producing a specific nervous current
 
 394 THE GERM-PLASM. 
 
 signifying ' enlargement.' If such an explanation were at- 
 tempted, we should be compelled to imagine that every cell in 
 the body was placed in communication with every germ-cell of 
 the ovary or spermarium by means of a large number of nerve- 
 tracks, and was capable of continually sending information to 
 the germ-cells of what was occurring in its own substance, and 
 of the manner in which it was influenced, and also of giving 
 instructions how each of the millions of units in the germ-plasm 
 should behave. I believe that it would be impossible to avoid 
 absurdities in explanations of this kind, and consider the whole 
 idea inadmissible. 
 
 The second possible explanation appears to me to be less 
 acceptable at the present day than when it was put forward 
 by Darwin in the form of a hypothesis of pangenesis. And, as 
 already stated in earlier essays, I believe that the talented 
 author of this hypothesis of heredity did not look upon it as 
 a well-grounded assumption, but considered it merely as a 
 working hypothesis, only intended to lead to a better insight. 
 Meanwhile, many changes have been made, and we have 
 become acquainted with facts which compel us to reject the 
 idea of a ' circulation of gemmules,' and I am surprised that 
 this has not hitherto been done. This hypothesis is rendered 
 inadmissible, not merely because we must imagine that the 
 gemmules are given off", and then circulate through the body, 
 but principally on account of the implied addition of gemmules 
 i.e. t of primary constituents to the germ-plasm of the germ- 
 cells ! 
 
 According to Darwin's idea, there must be a constant addition 
 of ' primary constituents ' or gemmules to the germ-plasm 
 already present in the germ-cells, unless, indeed, it is assumed 
 that the entire nuclear substance in the germ-cells is formed by 
 gemmules which migrate into them. Such an assumption is, 
 however, contradicted by the fact that the hereditary substance 
 of the germ-cells^ which we observe in the form of nuclear rods or 
 idants, receives no addition to its organised bodies, the primary 
 constituents. I have come to this conclusion, not from the fact 
 that we have never observed an addition of this kind, but from 
 the way in which the hereditary substance has been shown to 
 behave during its multipliation. We know that the cell contains 
 a most wonderful mechanism which apparently has the sole 
 function of distributing the idants quantitatively and qualitatively,
 
 SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS. 395 
 
 according to the primary constituents which they contain ; and 
 this is done as equally as possible, or, at any rate, in a definitely 
 prescribed manner. Why should the centrosomes and spindle- 
 threads be present, and the longitudinal fission of the idants 
 occur, if myriads of primary constituents of the germ-plasm 
 circulate separately through the body, and are capable of enter- 
 ing the germ- and other cells from without, as well as of 
 becoming properly arranged in them in the order in which they 
 subsequently undergo development ? Why should nature be 
 so scrupulously careful to divide the idants as accurately as 
 possible, if their composition were open to alteration at any 
 moment by the entrance of new primary constituents or gem- 
 mules ? The process of the fission of the idioplasm in nuclear 
 and cell-division seems to me directly and conclusively to refute 
 the -whole idea of the circulation of gemtnules. For the very 
 reason that these nuclear rods or idants can never receive an 
 addition of new primary constituents from without, the most 
 extreme care is required, during their multiplication by division, 
 to prevent the different qualities of the mother-cell from being 
 distributed improperly amongst the daughter-cells, and causing 
 an irreparable loss of certain primary constituents to one of the 
 latter and its descendants. 
 
 It is impossible to assume the transmission of somatogenic 
 variations in any theory which accepts the nuclear substance 
 of the germ-cells as germ-plasm or 'hereditary substance' ; for 
 it is theoretically impossible to account for these variations, no 
 matter how ingeniously the theory is constructed. 
 
 At the present day I can therefore state my conviction still 
 more decidedly than formerly, that all permanent i.e., heredi- 
 f ar y variations of the body proceed from primary modifications 
 of the primary constituents of the germ; and that neither injuries, 
 functional hypertrophy and atrophy, structural variations due to 
 the effect of temperature or nutrition, nor any other influence of 
 environment on the body, can be communicated to the germ- 
 cells, and so become transmissible. 
 
 This statement naturally implies the rejection of Lamarck's 
 principle of variation ; for those factors which this talented 
 philosopher and investigator believed to be all-important in the 
 modification of species, viz., the use and disuse of parts, can 
 have had no direct share in the process. I am by no means the 
 only one to hold this view at the present day ; and although
 
 39^ THE GERM-PLASM. 
 
 the truth cannot be decided by a consensus of opinion, it is 
 nevertheless a significant fact that the views of such naturalists 
 as Ray Lankester,* Thiselton Dyer, Brooks, Meynert,t van 
 Bemmelen,! and others, coincide with my own. 
 
 The fact, however, that we deny the transmission of the effects 
 of use and disuse, does not imply that these factors are of no im- 
 portance ; and I have already attempted to show in former essays 
 that both use and disuse may lead indirectly to variations, the 
 former wherever an increase as regards the character concerned 
 is useful, and the latter in all cases in which an organ is no 
 longer of any importance in the preservation of the species, and 
 in which, so far as the disused organ is concerned, ' panmixia ' 
 occurs. 
 
 Want of space prevents me from discussing these questions 
 in detail ; their consideration belongs rather to a work on the 
 theory of descent than to one on that of heredity, and I need 
 only refer to my former essays, in which, I think, sufficient 
 proof is given to show that the gradual degeneration of organs 
 which are no longer of use does not require the assumption of 
 the transmission of somatogenic variations, and that conse- 
 quently the facts do not compel us to adopt a hypothesis which 
 we seem unable to accept theoretically. 
 
 It therefore remains to be seen whether we are not acquainted 
 with other facts which are explicable only on such a hypothesis : 
 one side of this question will now be treated of in particular. 
 
 2. THE HYPOTHESIS TESTED BY FACTS. 
 
 A few words will suffice concerning the hypothesis of the 
 transmission of injuries and mutilations, which has been accepted 
 for so long a time, and is obstinately defended even at the 
 present day ; for since the appearance of my essay on ' The 
 
 * Ray Lankester, 'The History and Scope of Zoology,' ' Enc. Brit.,' 
 Vol. xxiv. 
 
 t Meynert, ' Mechanik der Physionomik,' a Lecture held at the Meeting 
 of German Naturalists at Wiesbaden, 1887. 
 
 J J. F. van Bemmelen. ' De Erfelijkheid van verwooven Eigenschappen,' 
 s'Gravenhage, 1890. 
 
 C/., 'Uber die Vererbiing,' Jena, 1883, and 'Die Continuitat des 
 Keimplasma's als Grundlage einer Theorie der Vererbung,' Jena, 1885. 
 English translation, ' Essays upon Heredity,' pp. 71 and 165.
 
 UPPOSED TRANSMISSION OF ACQUIRED CHARACTERS. 397 
 
 Supposed Transmission of Mutilations,' * no new observations 
 on this point have appeared.t The old arguments, on the 
 scientific worthlessness of which I then expressed my opinion, 
 are constantly being brought forward, in part altered, and in 
 part with a new interpretation. It is now even less necessary 
 than ever to return to the matter, as even among those observers 
 who supported the view of the transmission of functional varia- 
 tions, a few agree with me in denying the transmission of 
 mutilations. As an instance, I may mention Osborn, who, 
 however, goes a little too far when he compares the contest of 
 the old view of the transmission of mutilations with Don 
 Quixote's celebrated fight with the windmills. { Only a few 
 years ago, at a meeting of the German association of naturalists, 
 two ' tailless ' cats were exhibited, in which the absence of the 
 tail was supposed to be due to their mother having accidentally 
 lost hers; and biologists of such eminence as Ernst Haeckel|| 
 have accounted for similar cases in the same way. Since such 
 men still regard the inheritance of mutilations as possible, the 
 exposition of the subject has not been a superfluous task.1I 
 
 There is, however, a third kind of somatogenic -variation, 
 produced by the influence of environment, the mode of nutrition, 
 the climate, and so on ; this often appears to be transmissible, 
 and consequently capable of becoming increased in the course 
 of generations. I myself called attention to this fact a number 
 of years ago in the following words : ' I only know of one class 
 
 * Jena, 1889, ' Essays upon Heredity,' Oxford, 1889, p. 421. 
 
 f None, that is to say, which are in opposition to my views. My experi- 
 ments with mice have been confirmed by Ritzema Bos and by Rosenthal. 
 I have now continued these experiments to the nineteenth generation 
 always with the same negative results ; cutting off the tails has no influence 
 on the tails of the descendants. A similar result was obtained by both the 
 above-mentioned observers from experiments on rats (cf, ' Biolog. Central- 
 blatt,' Vol. xi., 1891, p. 734, &c.). 
 
 J Osborn, ' Are Acquired Variations Inherited ? ' Boston, 1890, p. 3. 
 
 At the meeting at Wiesbaden, zoth September 1887. 
 
 || Haeckel, 'Natiirliehe Schbpfungsgeschichte,' 3rd ed., 1889, p. 194. 
 English edition, 'The History of Creation,' London, 1876, Vol. i., p. 214. 
 
 IT See the Section on the Transmission of Mutilations in Eimer's book 
 entitled 'Die Entstehung der Arten,' &c., Jena, 1888. (Cf. 'Organic 
 Evolution as the Result of the Inheritance of Acquired Characters,' &c., 
 translated by J. T. Cunningham, London, 1890.) A whole collection" of 
 ' proofs' arc there given.
 
 398 THE GERM-PLASM. 
 
 of changes in the organism which is with difficulty explained 
 by the supposition of changes in the germ ; these consist 
 in modifications which appear as the direct consequence oj 
 some alteration in the surroundings. But our knowledge on 
 this subject is still very defective, and we do not know the 
 facts with sufficient precision to enable us to pronounce a final 
 verdict as to the cause of such changes.' Mention was then 
 made of a few of the large number of cases which have been 
 repeatedly quoted, and I attempted to show that none of them 
 stood criticism, that they could not be explained in the way some 
 investigators supposed, and that somatogenic variations are 
 only apparently hereditary ; for in reality a change must first be 
 brought about in the germ-plasm by the influence of the surround- 
 ings before such a variation can be produced. I then continued : 
 ' It must be admitted that there are cases, such as the climatic 
 varieties of certain butterflies, which raise some difficulties against 
 this explanation. I myself, some years ago, experimentally 
 investigated one such case, and even now I cannot explain the 
 facts otherwise than by supposing,' as I did then, that somatic 
 variations were transmissible. ' It must be remembered, how- 
 ever, that my experiments,' which have been repeated upon 
 several American species by H. W. Edwards, ' were not under- 
 taken with the object of investigating the question from this 
 point of view alone. New experiments, under varying con- 
 ditions, will be necessary to afford a true explanation of this 
 aspect of the question.' * 
 
 Since 1883 I have waited in vain for some skilled entomolo- 
 gist or for one of the numerous advocates of the transmission 
 of acquired characters, to carry out the proposed experiments. 
 In the meantime, as far as the time and material at my disposal 
 permitted, I have myself made a start on this line of research, 
 and now possess the results of a series of new experiments, 
 which, though not so numerous, complete, or exhaustive as I 
 could have wished, are nevertheless sufficient to form a more 
 trustworthy basis for a theory dealing with variations of this 
 kind. Some of these are described in the following section, a 
 more detailed account of them being left for another occasion. 
 
 * Cf. my essay, ' Uber Vererbung.'Jena, 1883. English edition, Oxford, 
 1889, pp. 98-99.
 
 SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS. 399 
 
 3. CLIMATIC VARIATION IN BUTTERFLIES. 
 
 Polyommatus phlceas, a butterfly belonging to the family 
 Lycanida, is distributed over the whole of the temperate and 
 colder parts of Europe and Asia. It also occurs on the shores 
 of the Mediterranean, in Madeira, the Canaries, and in part of 
 North America. Before the glacial epoch this species must 
 have inhabited the more northern circumpolar regions, and 
 have been driven southward during that epoch ; subsequently 
 it must have again migrated towards the north. In our lati- 
 tudes the upper surface of the wings of this form is of a beautiful 
 reddish-gold colour, and hence it has received the popular name 
 ' Feuerfalter ' (fire butterfly). Further south, the reddish-gold 
 colour is more or less thickly dusted with black, and specimens 
 from Sicily, Greece, or Japan often display only a few reddish- 
 gold scales, the general appearance being almost black. In 
 Germany this butterfly is double-brooded, and the two genera- 
 tions are similar ; but in certain districts of Southern Europe, 
 such as the Riviera di Levante, the first generation is 
 reddish-gold, the second, which flies in midsummer, and is 
 known as the variety eleus, having the wings well dusted with 
 black. As in Germany, during exceptionally hot summers, 
 individuals with a blackish tint have repeatedly been caught 
 together with the ordinary form, and as, moreover, in the extreme 
 southern limit of their range so far as my experience extends 
 both generations have a blackish colour, it would appear at 
 first sight that the modifications are merely due to the effect of 
 heat ; the butterfly becomes red when exposed to a moderate 
 temperature, and black when the heat is greater. 
 
 The following experiments, however, prove that this conclusion 
 cannot be a correct one. Caterpillars were raised from the eggs 
 of the German form of P. phlceas, and the pupas were then 
 exposed to a much higher temperature till the emergence of the 
 butterfly. The result was that many of the butterflies were 
 slightly dusted with black, but none of (hem resembled the 
 darkest forms of the southern variety eleus. I then made the 
 reverse experiment, subjecting caterpillars which had just en- 
 tered the pupal stage, and had been raised from the spring 
 generation of the Neapolitan form, to a very low temperature.* 
 
 * I must take this opportunity of expressing my warmest thanks to Dr 
 Schiemenz, of the Zoological Station at Naples, for the kind and generous
 
 400 THE GERM-PT.ASM. 
 
 Many butterflies were thus obtained which were not so black as 
 those which had emerged from pupae kept at a higher tempera- 
 ture, but none were so light-coloured as the ordinary German 
 form. The difference between the Neapolitan specimens which 
 had become light-coloured from exposure to cold, and the normal 
 German form, on the one hand; and that between the German 
 specimens artificially darkened by warmth, and the normal 
 Neapolitan form, on the other, is too great to be attributable to 
 the incompleteness of the experiments. The German and the 
 Neapolitan forms are therefore constitutionally distinct, the 
 former tending much more strongly towards a pure reddish- 
 gold, and the latter towards a black coloration. 
 
 Both experiments, however, prove the correctness of the old 
 assumption of Lepidopterists that the action of heat on a single 
 generation is capable of giving the German form a blackish tint ; 
 and since, moreover, it is clear that the development of a single 
 generation at a lower temperature can render the colour of the 
 Neapolitan butterfly less black, it appears that the two varieties 
 may have originated owing to a gradual cumulative influence of 
 the climate, the slight effects of one summer or winter having 
 been transmitted and added 'to from generation to generation. 
 This would then seem to be an instance of the transmission of 
 acquired characters. 
 
 I do not believe, however, that this is the correct interpreta- 
 tion of the facts. If it were, there could be no region in which 
 the species is seasonally dimorphic, as I have myself ascertained 
 it to be on the Ligurian coast. The germ-plasm would then 
 contain either the primary constituents of the red variety, if 
 the colony had been exposed for many generations to a low 
 temperature ; or those of the black one, if a high temperature 
 had influenced it for the same length of time. It would then 
 make no difference to what degree of temperature a single 
 generation were exposed at the present day in artificial breeding, 
 for the colour would have already been determined in the 
 germ-plasm, which would contain, to use my own phraseology, 
 either ' reddish-gold ' or ' black ' determinants for the wing- 
 scales in question. Hence it would be quite impossible for the 
 spring generation to develop reddish-gold, and the summer one 
 
 assistance he has given me in my efforts. Without his help I should have 
 been unable to obtain the necessary living specimens.
 
 SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS. 461! 
 
 black scales, for the germ-plasm would only contain either ' red ' 
 or ' black ' determinants for a certain spot on the wing. 
 
 The theory of determinants will, I believe, supply a very 
 simple explanation of this apparently complicated case, which 
 I consider of great value, because it confirms this theory. 
 Instead of supporting the doctrine of the transmission of 
 somatogenic characters, this example shows how such a process 
 may apparently be brought about, and on what it depends. 
 A somatogenic character is not in this case inherited, but 
 the modifying influence the temperature affects the primary 
 constituents of the wings in each individual, i.e., a part of 
 the soma, as well as the germ-plasm contained in the germ- 
 cells of the animal. It modifies the same determinants in 
 the rudiments of the wings of the young chrysalis as in the 
 germ-cells, namely, those of the wing-scales. The variation 
 cannot be transmitted from the wings to the germ-cells, but only 
 affects the coloration of these organs of the individual in ques- 
 tion ; whereas it is transmitted from the germ-cells to successive 
 generations, and consequently controls the coloration of their 
 wings in so far as this is not again modified by subsequent influ- 
 ences of temperature ; for the same determinants which are now 
 present in the germ-cells of generation I are afterwards passed 
 into the rudiments of the wings in the caterpillar and chrysalis 
 of generation II, and the change which they underwent while 
 lying in generation I may be increased or weakened by the 
 influence of the temperature to which they are exposed after 
 entering into generation II. 
 
 Since warmth affects the whole body, it is not surprising that 
 the determinants which are modified by it should undergo these 
 modifications, whether they are contained in the germ-plasm of a 
 young egg or sperm-cell of the caterpillar, chrysalis, or butterfly, 
 or in certain cells in the rudiments of the wings in the chrysalis 
 or caterpillar. This, however, does not imply that they must 
 undergo the same amount of variation in both places, for they 
 have not by any means the same environment in the two situa- 
 tions. In the germ-plasm they are grouped amongst thousands 
 of determinants of the species, all of which constitute the germ- 
 plasm ; while in the rudiments of the wings, they are associated 
 with only a few other kinds of determinants, and a time must 
 come when each of them controls a cell by itself, and transforms 
 it into a red or a black wing-scale. 
 2 C
 
 402 THE GERM-PLASM. 
 
 We know, however, of a fact which definitely proves that the 
 susceptibility of the scale-determinants to the influence of 
 temperature is greatest at a certain stage in the development 
 of the butterfly much greater than either before or afterwards. 
 I have frequently noticed in seasonally dimorphic species like 
 Vanessa prorsa-levana, that the modifying influence of heat 
 or cold only acts at the beginning of the pupal stage. 
 Although I have not yet been able to ascertain the time at 
 which this occurs more precisely, it can be definitely stated that 
 the winter pupae of Vanessa levana, for instance, which have 
 been exposed to a high temperature even only a month after 
 entering the pupal stage, are never transformed into theprorsa 
 fonn ; they all emerge as V. levana. 
 
 This is not due to the fact that the colour of the wings is 
 already deposited a month after the insect has entered the pupal 
 stage, for at this time there is no trace of colour whatever. 
 There must, consequently, be a period in the disintegration of 
 the determinants when they are most susceptible to the influences 
 of temperature : subsequently this is no longer the case, and 
 although they are susceptible before this period, I nevertheless 
 venture to suppose that they were so to a. far slighter extent. 
 This may be due to their connection with other determinants, 
 or to other causes which we are not yet able to discover. 
 
 If, then, the determinants for the scales are only influenced 
 very slightly by the temperature as long as they are situated in 
 the germ-plasm, and are subsequently greatly affected by it at 
 a certain period in the development of the wings, the above- 
 mentioned phenomena admit of a simple explanation. The 
 germ-plasm of the southern colony of P. phlceas must contain 
 many determinants among those for the wings, which, in con- 
 sequence of the exposure of thousands of generations to heat, 
 have been adapted for the production of black scales, together 
 with a large number of others which only require a small 
 increase of temperature during pupation in order to give rise 
 to a black colour. These latter kind cause such fluctuations 
 in the coloration as occurred in my experiments ; while the 
 former produce the black coloration of the wings, which has 
 become fixed in the constitution of the southern colony, and 
 can no longer be removed by the action of cold on the young 
 chrysalis. 
 
 In this case it is taken for granted that the ancestral form
 
 SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS. 403 
 
 possessed pure reddish-gold wings, and that it inhabited high 
 northern latitudes, an assumption which alone enables us to 
 understand the present distribution of the species, and which 
 has been adopted by Hofmann * in his splendid researches on 
 the origin of European butterflies. This, however, is of no 
 great importance in the present question, but we must assume 
 that either the reddish-golden tint, or the deep black dusting is 
 the primary colour. The seasonal dimorphism and the occur- 
 rence of blackish specimens in Germany in hot summers are 
 easily accounted for on the former assumption. 
 
 In consequence of the increase in temperature of the habitat 
 of the species, many scale-determinants in the germ-plasm 
 would gradually become so modified that the action of only a 
 slight further increase on the rudiments of the pupal wings 
 would lead to the production of black scales. In Germany the 
 species has attained this point in its phyletic modification ; and 
 if the weather happens to be hot when the second annual brood 
 enter upon the pupal stage, some butterflies of a blackish tint 
 will be produced. This will be more likely to happen as the 
 internal transformation of the determinants in question advances 
 further, and the blackish tint will become more conspicuous as 
 the scale-determinants which have reached this stage of modi- 
 fication in the germ-plasm become more numerous. These 
 two conditions will obtain most often in districts where the 
 summer is usually tolerably warm ; and the fact is thus 
 accounted for that dark specimens of P. phlceas are rarely 
 caught in northern Germany, and in the far north not at all, 
 although very dark forms occur comparatively often in the warm 
 valleys of Valais. 
 
 In still warmer districts, like the Riviera, the summer brood 
 of P. phlcEos is almost always exposed to a high temperature, 
 and hence the transformation of the determinants for the scales 
 has become so great, that with the help of the usual summer 
 heat at the time when the caterpillar enters the pupal stage, the 
 variety eleus has been produced. This variety does not appear 
 in the spring brood, because the additional heat required for the 
 complete transformation of the determinants for the scales is 
 absent during the pupal stage. 
 
 * Ernst Hofmann, ' Isoporien der europsischen Tagfalter,' Stuttgart, 
 1873.
 
 404 THE GERM-PLASM. 
 
 If the area of distribution of the species extended unin- 
 terruptedly from the Polar regions to South Italy or North 
 Africa, all the intermediate forms would occur, from the pure 
 reddish-gold single-brooded form in Lapland to the black double- 
 brooded variety eleus : there would thus be first, two similar 
 reddish-gold broods ; then similar ones, those butterflies which 
 are exposed to a higher temperature during the pupal stage 
 having a tendency to develop a black tint ; and then seasonal 
 dimorphic forms, the butterflies being black in the summer and 
 reddish-gold in spring, as is actually the case in Genoa. A 
 still longer action of a higher temperature would at first change 
 a small and then an increasing number of determinants for the 
 scales into the 'black' variety, so that finally two broods, both 
 consisting of the black form eleus, would occur. The case of the 
 Neapolitan colony is somewhat similar to the last-mentioned 
 one, for many black specimens certainly occur amongst the 
 spring brood, although there are also many light-coloured ones ; 
 none, however, are as light as the northern reddish-gold form of 
 P.phlaas. I do not know whether a complete change of colour 
 has been attained by both annual broods in any locality ; but if 
 it has, I should expect it to be seen in Southern Japan rather 
 than elsewhere, for the butterflies which I possess from the 
 neighbourhood of Tokio display an unusually dark colour. 
 
 This case has been discussed at length because it appears to 
 me to be especially significant, not only in the explanation of the 
 climatic varieties of butterflies, but also as regards the theory of 
 heredity, and the assumption of material determinants which 
 exist in the germ-plasm and are passed on from one genera- 
 tion to another. The facts are so evidently in favour of this 
 assumption that no other explanation seems possible. It must, 
 however, be remembered that the artificial modification of the 
 colour on the wings does not take place if the change in tem- 
 perature occurs only when the scales begin to become coloured. 
 The colouring matter is consequently not produced by the direct 
 influence of chemical transformations, but by an indirect influ- 
 ence, which we may suppose to be due to a mutual disarrange- 
 ment and rearrangement of the ' biophor-material ' of which the 
 determinants consist, by the co-operation of which latter the 
 chemical process forming the colour is derived. 
 
 The occurrence of seasonal dimorphism alone, shows with 
 certainty that the determinants for the scales are influenced by
 
 SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS. 405 
 
 temperature in the germ-plasm to a much slighter extent than 
 in the rudiments of the wings. If the modifying influence had the 
 same effect in both cases, the germ-plasm in the germ-cells of 
 a butterfly of the summer generation would be modified as much 
 as the wings of the same individual ; and consequently the off- 
 spring, even if exposed to a low temperature, would necessarily 
 display a greater tendency towards the summer coloration, 
 because the latter was already potentially contained in the germ. 
 This, then, would only be the case if the influence of the cold 
 were stronger than that of the heat. In any case, however, a 
 coloration intermediate between that produced by cold and by 
 heat respectively would result, and would be transmitted to both 
 generations, even when the two influences were equally strong. 
 If we indicate the winter and summer colorations respectively 
 by A and B, the coloration of each generation would then be 
 i A + I B. It is only when the germ-plasm is modified to a much 
 smaller extent than the determinants which have already entered 
 the rudiments of the wings, that an alteration of coloration can 
 become permanent. 
 
 In many other animals and plants influences of temperature 
 and environment may very possibly produce permanent here- 
 ditary variations in a similar manner ; but it is difficult in fact 
 almost impossible to identify such cases with anything like 
 certainty from the observations which have hitherto been made. 
 Thus we find it stated that 'in Cashmere dogs soon become 
 covered with a woolly hair ; ' * but we do not know who observed 
 this, or who ascertained that such a change if it really does 
 occur is transmitted. ' Merino sheep lose their fine wool when 
 they are transported to a tropical climate ; ' but I have not been 
 able to discover whether this loss occurs in the first, or in the 
 course of several, generations. We are thus left in uncertainty 
 as to the possibility of a direct climatic variation of a somatic 
 part having taken place in these instances, which would again 
 disappear in the next generation provided that the descendants 
 were placed under the original climatic conditions. The same 
 applies to the races of naked dogs from the tropics, such as the 
 
 * These statements are quoted from an essay by Giard, who takes 
 them as a proof of the transmission ot somatogenic modifications. Cf. 
 4 L'Her4dit6 des modifications somatiques,' Revue Scientifique, December 
 6th, 1890.
 
 406 THE GERM-PLASM. 
 
 Guinea dog, for ' they do not become covered with hair when 
 transported to a temperate climate.'* 
 
 Many climatic varieties of plants may also be due wholly 
 or in part to the simultaneous variation of corresponding de- 
 terminants in some part of the soma and in the germ-plasm of 
 the reproductive cells, and these variations must of necessity be 
 hereditary. Temperature, and nutrition in its widest sense, 
 affect the whole body of the plant, the somatic -cells as well as 
 the germ-cells. It cannot, however, at present be stated whether 
 the determinants in the soma are in this case influenced more 
 strongly than those which are still in the germ-plasm. It is 
 conceivable, and, I am inclined to think, even most usual, that 
 certain determinants are affected to the same extent whether 
 the influence of the environment happens to act on them in the 
 germ-plasm, or in any stage of somatic transformation. In this 
 case the change may have been perhaps scarcely or not at all 
 noticeable in the first generation, and may gradually have become 
 apparent, and also transmissible, in the course of subsequent 
 generations. On the other hand, there are probably many 
 influences of environment which produce a considerable change 
 in the body of the plant, without, however, modifying the cor- 
 responding determinants in the germ-plasm. The experiments 
 made by Nageli and many others on the genus Hieracium at 
 any rate support this view, though they have hardly been carried 
 on long enough to exclude the possibility of a very faint and 
 gradual alteration occurring in the germ-plasm. 
 
 The question as to which influences are capable of simul- 
 taneously modifying the developing and growing soma and the 
 corresponding determinants in the germ-plasm, even in a very 
 different degree, can only be solved by future experiments. The 
 cases of an apparent inheritance of somatogenic variations are 
 due to this coincidence ; no others are, it seems to me, con- 
 ceivable. All those influences, however, such as the use and 
 disuse of a part, which can only affect this part itself in a specific 
 manner, are incapable of producing a corresponding change in 
 the respective determinants of the germ-cells, and consequently 
 cannot lead to hereditary modifications. In such cases the 
 
 * Compare also the cases of degeneration in the descendants of the 
 European dog in India, which have been carefully collected by Darwin in 
 his ' Variation of Animals and Plants under Domestication,' Vol. I., p. 45.
 
 SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS. 407 
 
 external influence affects only the fully-formed organ, such as 
 a muscle which has become enlarged by exercise ; for the influ- 
 ence consists in the increased activity of the organ, which takes 
 place within it alone ; the germ-plasm of the germ-cells, and 
 even the determinant in the germ-plasm for the muscle in ques- 
 tion, are not thereby affected. In all cases of functional hyper- 
 trophy or atrophy, the external influence affects none of the 
 determinants, but only the fully-formed organs, *>., groups of 
 specific cells produced from determinants. In my opinion it is 
 very probable that such twofold modifying influences of environ- 
 ment as we meet with in P. phlceas, can only occur when the 
 determinants which have not yet been transformed into the 
 organ, as well as the germ-cells, have been affected by the modi- 
 fying influence. And this will be most likely to happen in those 
 structures which, like the scales on the wings of butterflies, 
 are formed at a later stage of the animal's existence, and the 
 determinants of -which are consequently stored in an undeveloped 
 condition in the idants of certain somatic cells during a great part 
 of the ontogeny. The wings of the butterfly arise as outgrowths 
 from the hypodermis of the caterpillar. Before these outgrowths 
 can be formed, the determinants for the wing- scales must be 
 contained in the idioplasm of some of the cells of the hypo- 
 dermis, but after their appearance they would be found in some 
 of the cells in the rudiments of the wings. The wings at first 
 are small, and contain by no means so large a number of cells 
 as when full-grown, so that inactive determinants for several 
 wing-scales must be contained in the idants of one nucleus. At 
 a certain period in the course of further growth, however, the 
 number of cells increases to such an extent that each determinant 
 constitutes the idioplasm of a particular cell, and the modifying 
 external influences seem then to have the greatest effect on these 
 determinants. By means of experiments it may be possible to 
 ascertain exactly when this occurs. 
 
 It might have been expected that in this section I should enter 
 into the whole question of the possibility of the transmission of 
 acquired variations, about which there has been so much dispute 
 of late years, and that all the arguments and facts which have 
 been put forward in favour of the theory should be discussed. 
 But, as I have already remarked, this seems to be out of place 
 in a theory of heredity, the object of which is to show whether 
 this form of transmission is or is not possible from a theoretical
 
 40S THE GERM-PLASM. 
 
 point of view, and to ascertain further whether in the latter case 
 such an apparent transmission might not possibly occur under 
 certain circumstances, and to account for this theoretically. I 
 have always emphasised the fact that it is easier to explain the 
 transformation of species on Lamarck's principle ; but this is no 
 reason for the retention of a theory which cannot be accepted 
 on theoretical grounds, unless no other explanation can be 
 given for the facts. So far, my opponents have been unable to 
 prove that this is the case. 
 
 The above explanation of the causes of the climatic variations 
 of butterflies may perhaps convince some of those who have 
 till now opposed my views that we are here not dealing blindly 
 with mere principles, but with inductive methods. The view of 
 the non-inheritance of acquired modifications has been espe- 
 cially opposed in America, principally by the palaeontologists. 
 It can certainly not be denied that certain facts in palaeontology, 
 such as the development of the feet and teeth in Ungulates, 
 furnish us with an extremely fine and uninterrupted series of 
 forms which may apparently be very easily explained on the 
 assumption of the inheritance of acquired modifications. But is 
 not this exactly what would be expected, in case phylogeny essen- 
 tially depends on selection that is, on an increasingly complete 
 adaptation to certain external conditions of life of a purely 
 general nature ? It appears to me that neither the complete- 
 ness of the developmental series, nor the close relation of the 
 nature of the modifications to function, give any clue to the 
 causes which have produced these series. They may quite as 
 well have originated by continued selection alone, as by continued 
 transmission of functional variations. 
 
 The eminent American naturalist, Lester Ward,* is, however, 
 in error if he supposes that the proof that climatic influences 
 are capable of modifying the germ-plasm, contains all that is 
 required by the neo-Lamarckian school. Further details will be 
 given in the next chapter as to the manner in which I now 
 suppose variation has originated : but quite apart from this, the 
 supposition that climatic influences can produce modifications of 
 the germ-plasm, has certainly nothing to do with the view that 
 
 * Cf. the essay directed against my views by this author ('Neo- 
 Darwinism and Neo-Lamarckism,' Washington, 1891), which is written 
 from the thoroughly objective and truly scientific point of view.
 
 SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS. 409 
 
 functional modifications of any particular organ can cause a 
 corresponding change in the germ-plasm. I believe I have 
 here furnished a proof that the former supposition is a correct 
 one : the onus of proof of the latter lies with the neo- 
 Lamarckians.
 
 410 THE GERM-PLASM. 
 
 CHAPTER XIV. 
 VARIATION. 
 
 I. NORMAL INDIVIDUAL VARIATION. 
 
 HEREDITY is the transmission of the physical nature of the 
 parent to the offspring. We have seen that this transmission 
 affects the whole organism, and extends to the most trifling 
 details ; and we also know that it is never complete, and that 
 the offspring and parent are never identical, but that the former 
 always differs more or less from the latter. These differences 
 give rise to the phenomenon of variation, which thus forms 
 an integral part of heredity, for the latter always includes the 
 former. 
 
 A theory of heredity must therefore include a theoretical 
 substantiation of variation, such as I will now attempt. Why 
 is the offspring never an exact duplicate of its parent, even 
 when it possesses but one parent, as is the case in partheno- 
 genesis and reproduction by budding ? And what forms the 
 basis of the constant ' individual variations ' which, after the 
 precedent of Darwin and Wallace, we regard as the foundation 
 of all processes of natural selection, and as the means which 
 rendered possible such a rich development of organic forms of 
 the most diverse kinds on the face of the earth ? 
 
 Darwin himself considered the difference between parent 
 and offspring as due to the diversity of external influences, 
 and I was essentially of the same opinion formerly, and stated 
 that 'all dissimilarities of organisms must depend upon the 
 individuals having been affected by dissimilar external influences 
 during the course of the development of organic nature.' * At 
 that time I attributed to the organism the virtual 'power of 
 
 * ' Studien zur Descendenztheorie,' II., Leipzig, 1876, p. 304. English 
 edition, ' Studies in the Theory of Descent,' translated and edited by R. 
 Meldola, with a prefatory note by Charles Darwin, London, 1882, Vol. II., 
 p. 677.
 
 VARIATION. 411 
 
 giving rise by multiplication only to exact copies of itself ; but 
 this capacity, in my opinion, did not produce accurate results, 
 because the organism is also capable of reacting to external 
 influences, and may therefore deviate from the inherited tend- 
 ency in one or another direction, according to the nature of 
 these influences. 
 
 Variation would consequently not depend upon a special 
 force existing in the organism, but would simply be the result 
 of external influences, which, either directly or indirectly, are 
 capable of preventing the organism from keeping strictly to the 
 inherited course of development. 
 
 Although I still consider this view to be in general correct, 
 the origin of individual variation, on which the transformation 
 of species is based, cannot be deduced so easily from the action 
 of dissimilar external influences as seemed possible when I wrote 
 the passage quoted above. I have discussed this question in 
 full on a previous occasion, and will here only briefly refer 
 to it* 
 
 At that time, no one supposed that any difference existed 
 between the modifications which may be brought about in the 
 soma and those which proceed from the germ-plasm. Since 
 then, however, we have been compelled at least in my opinion 
 to consider that only those variations which are ' blastogenic] 
 and not those which are ' somatogcnic' can be transmitted. We 
 can no longer regard the direct influence of external impressions 
 on the soma as a means of producing hereditary individual 
 variations. It therefore remains to be seen what is the origin 
 of these variations, upon the existence of which we imagine the 
 entire development of organic nature to depend. 
 
 This development could be accounted for most easily on 
 Nageli's hypothesis, according to which the idioplasm is so 
 constituted, that in the course of generations it could exert a 
 definite and regular transforming influence upon itself, and by 
 this means could convert one species into another. Many 
 reasons may, however, be urged against this hypothesis. In 
 the first place, development by internal forces only is contra 
 dictory to the close adaptations of organisms to their conditions 
 of life ; and secondly, we should not make use of unknown 
 
 * Cf. ' Die Bedeutung der sexuellen Fortpflanzung fur die Selections- 
 theorie,' Jena, 1886. English Edition, Oxford, 1889, p. 255.
 
 412 THE GERM-PLASM. 
 
 forces for the explanation of natural phenomena until we have 
 proved that the known ones are insufficient. 
 
 Such a phyletic principle of development would naturally 
 not produce ordinary individual variations, but would lead to 
 modifications which affect all the individuals of a species in the 
 same manner. All the individual modifications which actually 
 exist would, in that case, have to be considered insignificant 
 in phyletic development ; and in plants Nageli actually 
 looked upon them to be transitory, and not hereditary habitat- 
 modifications ('Standorts-Modificationen')- But this view is 
 opposed by the transmission of an immense number of indi- 
 vidual differences in man and animals. 
 
 If we recognise that the processes of selection are the chief 
 factors in organic development, we must attach the greatest 
 importance to these hereditary differences in individuals, and 
 must try to discover their origin. 
 
 Brooks accepts this view, and has propounded a theory of 
 heredity which is based on Darwin's hypothesis of pangenesis, 
 and according to which variation is in the main dependent on 
 sexual reproduction.* Variability, in his opinion, is caused by 
 every ' gemmule ' of the spermatozoon uniting with ' that particle 
 of the ovum which is destined to give rise in the offspring to the 
 cell which corresponds to the one which produced the germ or 
 gemmule. . . . When this cell becomes developed in the 
 body of the offspring it will be a hybrid, and it will therefore 
 tend to vary.' Brooks, moreover, assigns different functions 
 to the two kinds of germ-cells, and represents them as being 
 charged or filled to different degrees with 'gemmules,' the 
 egg-cell containing a much smaller number than the sperm-cell. 
 In his opinion the egg-cell is the conservative principle which 
 controls the transmission of purely racial or specific characters, 
 whereas the sperm-cell is the progressive element which causes 
 variation. 
 
 Brooks has ingeniously advanced every argument which could 
 be brought forward in favour of his theory, but I doubt whether 
 he still holds to it, for many new discoveries have since been 
 made which contradict it. Although the view that ' acquired ' 
 characters are not hereditary is not universally admitted, it has 
 
 * W. A. Brooks, The Law of Heredity : A Study of the Cause of 
 Variation and the Origin of Living Organisms,' Baltimore, 1883.
 
 VARIATION. 413 
 
 been accepted by Brooks ; and the assumption of a diversity of 
 the hereditary substances contained in the male and female 
 germ-cells, on which his theory is based, is rendered untenable 
 if it is recognised that the idants of both cells remain precisely 
 the same as regards number and quality during the process of 
 amphimixis. It will not, therefore, be rash to conclude that the 
 few observations which seem to be in favour of the dissimilar 
 effect of male and female germ -cells are not convincing, 
 although we may not be able at present to explain them fully. 
 To these belong certain rare cases, which have, perhaps, not 
 been very accurately observed, and possibly form exceptions to 
 the rule that hybrids of two species are identical, whether the 
 father or mother belonged to the species A or B. 
 
 If we are forced to reject the assumption of the dissimilar 
 action of the two germ-cells, together with that of an internal 
 force of transformation, we can only refer hereditary individual 
 variability to the inequality of external influences ; and it then 
 remains to be seen how such influences can produce hereditary 
 differences if somatogenic modifications are not hereditary : for 
 external influences act directly, and often exclusively, upon the 
 body, and not on the germ-cells. 
 
 In a former essay I have already attempted to show that the 
 constant occurrence of individual variability and the continual 
 transformation in the intermixture required by selection is 
 brought about by amphimixis, although it is not the primary 
 cause of this variability ; and that the accomplishment of 
 sexual reproduction is even based in almost all the known 
 organic forms on this necessity of preserving and continually 
 remodelling the hereditary variability of individuals. I am 
 convinced that the two forms of amphimixis namely, the 
 conjugation of unicellular, and the sexual reproduction of 
 multicellular organisms are means of producing -variation. 
 The process furnishes an inexhaustible supply of fresh com- 
 binations of individual variations which are indispensable to the 
 process of selection. 
 
 Hatschek* has contested this view of the significance of sexual 
 reproduction, and states that ' transformations of species occur 
 far too rarely to admit of their explaining such an uninterruptedly 
 active process as sexual reproduction.' It seems to me, how- 
 
 B. Hatschek, ' Lehrbuch der Zoologie,' Jena, 1888, p. 10.
 
 414 THE GERM-PLASM. 
 
 ever, that he has overlooked the fact that the transformation of 
 a species, as well as ^^preservation of its constancy, are based 
 upon natural selection, and that this is incessantly at work, 
 never ceasing for a moment. 
 
 From what was said in the chapters on the struggle of the 
 determinants of the two parents in ontogeny, and on the 're- 
 ducing-division ' of the germ-plasm which is indispensable in 
 amphimixis, it follows that by means of the latter process fresh 
 combinations of the possible variations in a species must con- 
 stantly be produced. On the one hand, the germ-plasm of a 
 new individual produced by amphimixis never receives more 
 than half the ids of each parent and these are differently 
 selected and arranged in each case ; and on the other, the co- 
 operation of the ids of both sides would not always strike an 
 average in all parts of the new organism, but each part would 
 resemble that either of the father or the mother in proportion to 
 the number and controlling force of the individual homologous 
 determinants ; the resultant of the co-operating forces may be 
 different in different parts. 
 
 Although the process of amphimixis is an essential condition 
 for the further development of the species, and for its adaptation 
 to new conditions of existence amongst the higher and more 
 complicated organisms, // is not the primary cause oj hereditary 
 variation. By its means those specific variations which already 
 exist in a species may continually be blended in a fresh manner, 
 but it is incapable of giving rise to new variations, even though 
 it often appears to do so. 
 
 When it first occurred to me that sexual reproduction was 
 necessary to produce the variations required for the action of 
 selection, I imagined that its influence upon the germ-plasm was 
 still more powerful. Since all differences even the qualitative 
 ones are ultimately of a quantitative nature, and as the union 
 of the primary constituents of the parents may either strengthen 
 or weaken a 'character,' I imagined that the combination of very 
 strong primary constituents for the same part in both parents 
 would not only cause the part to reappear especially markedly in 
 the child, but would also double the strength of the primary 
 constituents of the part in the germ-cells of the child ; and thus 
 the continued intercrossing of offspring in which this part is 
 strongly developed, might cause it to be increased more and 
 more, so that it exhibited far more than an ordinary individual
 
 VARIATION. 415 
 
 difference. If we suppose this process to go on in the various 
 parts of the body, the transformation of a species would thus be 
 accounted for. * 
 
 The production of races by artificial seleclion appears, in fact, 
 to be in part due to such an accumulation of parental ' charac- 
 ters ' ; but I shall show later on that it is not accompanied by 
 an actual "variation of the determinants, which alone could 
 gradually lead to a transformation of the species. We know that 
 the paternal and maternal idants do not fuse in the process of 
 amphimixis, and the immense number of cases of perfect trans- 
 mission proves that the determinants of both sides undergo no 
 alteration by being brought together. The modification of the 
 determinants is a process which is not directly connected with 
 sexual intermingling, but follows its own course, and must be 
 due to special causes. 
 
 This is made still clearer if we reflect that the lower organisms 
 e.g., sponges and polypes must possess a very small number 
 of determinants compared to those in higher forms, such as 
 birds and mammals. The number of determinants in an id 
 of germ-plasm has therefore increased considerably, and even 
 enormously, in the course of phyletic development. A single 
 peacock's feather may possibly be controlled by as many 
 determinants as an entire polype. Amphimixis alone, how- 
 ever, could never produce a multiplication of the determinants. 
 
 The cause of hereditary variation must lie deeper than this ; 
 it must be due to the direct effect of external influences on the 
 biophors and determinants, which I imagine to take place in the 
 following way. 
 
 The entire substance of the earliest organisms must have 
 consisted of equivalent biophors, the nucleus and cell-body not 
 having yet become differentiated. In these lowest forms, whether 
 they exist or not at the present day, the perfect constancy of the 
 composition of the body may occasionally have been disturbed 
 by external influences of different kinds, and these modifications 
 must have been preserved, as they persisted in the two parts 
 resulting from reproduction by binary fission. 
 
 When the morphoplasm and idioplasm subsequently became 
 differentiated, and the latter was enclosed in the nucleus as the 
 
 * Compare my essay ' Die Bedeutung dei sexuellcn Fortpflanzung,' 
 Jena. 1866, p. 40. English edition, Oxford, 1889, p. 279.
 
 41 6 THE GERM-PLASM. 
 
 hereditary substance and controlled the body of the cell, varia- 
 tions which had been produced solely by the direct action of 
 external influences on the body of the cell, could no longer be 
 transmitted to the offspring as an inheritance, for they were 
 dependent on the part of the morphoplasm in which they origi- 
 nated, and did not pass into the idioplasm, which comprises 
 all the primary constituents of the species. Every hereditary 
 variation must therefore have originated in the nucleus, even in 
 unicellular organisms which in this respect therefore resemble 
 the Metazoa and Metaphyta, the sole difference being that in the 
 unicellular form we are concerned with the characters of one 
 cell and not of many. In many unicellular organisms e.g., the 
 higher Infusoria the cell-body is very highly differentiated : it 
 may possess a complex arrangement of cilia, undulating mem- 
 branes, trichocysts, and flagella, each having a definite position 
 in the body, as well as a protective case of a definite form, 
 with a lid capable of being closed ; the animal, moreover, is 
 capable of reconstructing all these parts should any of them 
 become injured. We are therefore forced to admit that this 
 minute organism must have a centre in which the latent germs 
 for all these structures slumber, and from which reproduction 
 may take place. This centre is the nucleus, and modifications 
 of the nuclear matter can alone give rise to transformations of 
 a hereditary nature in the cell-body. 
 
 The fact that species have remained sharply defined as long 
 as we have known them, proves that these transformations occur 
 neither easily nor rapidly. 
 
 The germ-plasm of multicellular forms is obviously also very 
 constant, and the biophors constituting it are capable of nourish- 
 ing themselves and of growing, so as to furnish exact duplicates 
 of themselves when they divide. We could not otherwise under- 
 stand how it would be possible, in spite of the enormous growth 
 of the germ-plasm from one generation to another, for the specific 
 characters, and even the most minute individual characteristics, 
 to be preserved through so many generations. 
 
 The difficulty with which the germ-plasm becomes changed 
 is not so clearly proved by the instances of constancy 
 displayed by some species of ancient Egyptian animals the 
 ibis and crocodile through thousands of years, which were 
 formerly advanced by Nageli and myself for this purpose. It 
 may be objected that these species were always subject to the
 
 VARIATION. 417 
 
 control of natura selection, which eliminated every case of 
 deviation from the perfectly adapted form. If insignificant 
 individual characteristics which are of no use to the species can, 
 however, be preserved in the human race for several generations, 
 this must be owing to the fact that the corresponding de- 
 terminants have very little tendency during their growth and 
 multiplication to suffer any marked variation, and that, on the 
 contrary, they reproduce exact duplicates of themselves. I was 
 therefore quite justified in supposing that the germ- plasm pos- 
 sessed a great power of remaining constant.* 
 
 We can none the less avoid assuming that the elements of the 
 germ-plasm />., the biophors and determinants are subject to 
 continual changes of composition during their almost uninter- 
 rupted growth, and that these very minute fluctuations , which 
 are imperceptible to us, are the primary cause of the greater 
 deviations in the determinants, which we finally observe in the 
 form of individual variations. 
 
 The assumption that such very minute fluctuations occur, 
 naturally follows from the impossibility of a complete uniformity 
 as regards nutrition existing during growth ; and in fact, though 
 underestimating its importance,! I formerly made this assump- 
 tion, in correctly supposing that the influences producing these 
 fluctuations 'are mostly changeable, and occur sometimes in 
 one and sometimes in another direction.' I had not then taken 
 into consideration the fact that the fluctuations accumulate 
 in consequence of the process of amphimixis. If a single 
 determinant increases to a hundred thousand during the multi- 
 plication of the germ-cells of an individual, it is not likely that 
 the nutrition of all these determinants during the process will 
 be absolutely the same in strength and kind. If this is not the 
 case, minute differences could not fail to appear in the subsequent 
 determinants. These minute fluctuations may undoubtedly 
 again disappear, as I formerly assumed, provided that the modi- 
 fied determinant is exposed to counteracting influences, and alone 
 they are quite incapable of producing an individual variation of 
 any perceptible character; but they may become cumulative. 
 For the germ-plasm always consists of a large number of ids, 
 each of which contains one of the homologous determinants in 
 
 * Cf. my essay 'Die Bedeutung der sexucllen Fortpflanzung,' Jena, 
 1886, p. 28. English edition. Oxford, 1889, p. 271. 
 
 t Loc. cit,, p. 272. 
 
 2 D
 
 418 THE GERM-PLASM. 
 
 question, and the co-operation of all of these determines the 
 character. A hereditary individual variation will therefore 
 arise if many of the homologous determinants vary in the same 
 "way. 
 
 We can thus understand the process of doubling of the 
 determinants, which must have occurred repeatedly, as more 
 complicated structures arose in phylogeny. More abundant 
 nourishment would cause a determinant to grow and multiply 
 more rapidly, and if the first multiplication occurs before the 
 ' reserve germ-plasm ' for the next generation has become 
 separated off, this double determinant will be permanently 
 retained by the species. The corresponding part of the body, 
 however, will only display a perceptible variation when the 
 majority of the homologous determinants have become doubled. 
 
 Minute fluctuations must thus continually occur in the com- 
 position of the biophors and determinants. Their variability 
 depends on the same principle as the systematic disintegration 
 of the determinants in the germ-plasm, and is due to the 
 dissimilar composition of the elements of the growing substance. 
 If the determinants consisted of masses which were all exactly 
 alike, inequality of nutrition could never transform determinant 
 A into A, : it could only alter its rate of growth. They are, 
 however, composed of biophors of different kinds, which react 
 dissimilarly under different conditions of growth. This renders 
 possible a disarrangement of the proportional numbers of the 
 different biophors in a determinant, and consequently also the 
 variation of the latter. It is therefore quite conceivable that all 
 the qualities of a cell are not affected by these influences, but 
 that only certain of them vary, and that only a few amongst a 
 large number of similar determinants need become modified. 
 
 The facts already mentioned with regard to the climatic 
 variation of butterflies, prove that such an alteration can actually 
 occur. In these insects the determinants of certain coloured 
 wing-scales are slowly changed in the course of generations, in 
 consequence of the rise in temperature of the climate, and thus 
 the colour of the scales becomes considerably modified. Such 
 cases of conspicuous variation are not often met with ; all species 
 of butterflies, at any rate, are not affected in this way by changes 
 of temperature, and those that are thus influenced do not display 
 the variation in all the different kinds of scales. This indicates 
 that the determinants have a strong tendency to remain constant,
 
 VARIATION. 419 
 
 and that the deviations to which they are subject on account of 
 inequalities of nutrition are as a rule so infinitesimal that their 
 effect is unnoticeable. 
 
 These deviations are nevertheless of great significance, for 
 they form the material from which the visible individual 
 variations are produced by means of amphimixis together with 
 selection; and new species arise by the increase and combination 
 of these variations. 
 
 The hypothesis that the germ-plasm consists of ids is quite 
 indispensable in this case. Every determinant is represented 
 in the germ-plasm as many times as there are ids in the latter, 
 for every id contains all the kinds of determinants. The deter- 
 minant N, for example, is represented a hundred times if the 
 germ-plasm consists of a hundred ids. Most of these ids must 
 differ slightly from one another, for in the course of generations 
 they are continually brought together in new combinations by 
 the process of amphimixis. On the occurrence of this process, 
 however, the diversity of the ids persists, even if we go back to 
 the origin of the multicellular forms, or of the unicellular ones 
 from the primordial organism. A condition in which all the ids 
 could be considered similar is never found ; in fact, as already 
 stated, the dissimilarity of individuals must be traced to the 
 primordial organism, at a period when neither the process of 
 amphimixis nor idioplasm had come into existence, and in which 
 every individual organism derived its individuality directly from 
 the dissimilarity of external influences. From these organisms 
 the dissimilarity was transmitted to the unicellular forms, which 
 cannot all have originated from one primordial organism, but 
 each species must have arisen polyphyletically from a large 
 number of similarly modified organisms. This point has often 
 been misunderstood, and I have been asked to explain, for 
 example, how the adaptations of flowers, fruits, and seeds in 
 Phanerogams, could have been derived from a combination of 
 characters acquired by the shapeless primordial ancestors. The 
 characters were not inherited from the primordial beings, but 
 variability, or the dissimilarity of individuals. 
 
 We might, however, be inclined to believe that external influ- 
 ences must affect all homologous determinants of a genii-plasm 
 in the same way, and must cause them to vary ; but this would 
 be erroneous. 
 
 Since reproduction is connected with amphimixis in the
 
 420 THE GERM-PLASM. 
 
 multicellular organisms, and the latter process is never entirely 
 wanting in any species, the germ-plasm of these forms consists 
 of many ids from different sources, one half of them being 
 derived from the father, and one half from the mother, each 
 half, again, containing ids from the grandparents in varying 
 proportions ; the ids of the grandparents, moreover, are derived 
 from one, two, or even three great-grandparents, and so on. 
 As I have shown above, the proportions in which the indivi- 
 dual ancestors may be represented by ids may vary very much ; 
 and consequently the germ-plasm of different individuals, even 
 when they are closely related, must always differ. 
 
 Each id contains all the determining elements of the species, 
 but in a manner peculiar to the individual. If a determinant N 
 differs slightly in every id, it will also vary a little during growth 
 if exposed to modifying influences ; so that, for example, the 
 determinant N may remain unaltered in id A, while N 1 varies 
 in B. On the other hand, the modifying influence of nutrition 
 may very likely be slightly different in A and in B, and may 
 produce a variation in N, while N 1 remains unaltered. Sufficient 
 factors would thus be present to cause a variation of one or 
 more homologous determinants in certain, but not in all ids. In 
 the only carefully observed cases of blastogenic variation, due 
 to the direct influence of external conditions, viz., those of the 
 climatic variations of Polyommatus fihlceas, it is evident that 
 the influence of temperature was not quite uniform. Some of 
 the captive Neapolitan specimens of this species are darker 
 and others lighter, although they were raised in a room under 
 conditions as nearly the same as possible ; and in those 
 caught in the neighbourhood of Naples, the amount of black- 
 dusting the character modified by the climate varied con- 
 siderably. The same influences, even when they act during 
 many successive generations, do not necessarily cause the indivi- 
 duals of a species to vary to the same extent ; and I account for 
 this by the fact that the ids of every animal contain different 
 variants of a particular determinant N, some of which are more 
 susceptible to heat than others. The germ-plasm, as a whole, 
 will vary to a greater or less extent in proportion to the number 
 of the determinants which are more or less variable. 
 
 Many enigmatical points, however, still remain. Thus the 
 transformation by heat of many of the original reddish-gold 
 wing-scales of P. phlceas into black ones does not occur evenly,
 
 VARIATION. 421 
 
 so as gradually to change the entire reddish-gold upper surface 
 of the wing into black ; but certain parts first become darker, 
 and then other adjacent ones, the whole surface being blackened 
 only in the very darkest specimens. The margin and base of the 
 wing first turn black, and the change then spreads slowly towards 
 the middle, which, however, remains unaltered in most indivi- 
 duals. Since we must suppose that scales of the same colour 
 arise from similar determinants, why are they affected to such an 
 unequal extent by the modifying influence of heat ? 
 
 The explanation previously used can, however, be applied in 
 this case also. 
 
 It was shown in the chapter on reversion that although new 
 specific characters are produced by the modification of certain 
 determinants or groups of determinants, this modification never 
 affects the homologous determinants in all the ids of the germ- 
 plasm simultaneously. It must, on the contrary, be assumed that 
 variation affects only a small majority of modified determinants 
 at first, but increases by the selection and preference of the 
 most modified individuals, until finally a predominant majority 
 of all the ids contain modified determinants. 
 
 This evidently implies that new and old specific characters 
 are respectively represented by a small and a large majority of 
 modified determinants. If this statement be applied to the case 
 of P. phlczas, we obtain a principle by the aid of which the 
 dissimilar effect of heat on the determinants for the middle and 
 the margin of the wing respectively may be understood. It is 
 very improbable that the surface of the wing of the brown ances- 
 tral form of P. phlceas has changed uniformly into reddish-gold, 
 and it is much more likely that a lighter spot in the middle 
 became modified first and took on a reddish-gold colour, in con- 
 sequence of sexual selection, and that this then gradually 
 extended towards the margin. If this were so, the reddish-gold 
 scales of the centre must be represented in the germ-plasm by a 
 greater majority of homodynamous determinants than are those 
 at the margin : and in this way we can understand why the 
 black-dusting of the wings affects the sides first, and the middle 
 last of all. This must be so if the old ' brown ' determinants 
 under the influence of heat give rise to black scales more easily 
 than do the 'reddish-gold' ones. Whether this explanation be 
 correct in this particular case or not, it is nevertheless true that 
 the diversity of the extent to which the determinants for the same
 
 422 THE GERM-PLASM. 
 
 character are represented in different regions of the body, affords 
 us a principle by which ive can understand the unequal effect 
 produced by similar modifying influences upon various regions 
 of the body. 
 
 Even though it can no longer be doubted that climatic and 
 other external influences are capable of producing permanent 
 variations in a species, owing to the fact that, after acting uni- 
 formly for a long period, they cause the first slight modifications 
 of certain determinants to increase, and gradually affect the less 
 changeable variants of the determinants also, the countless 
 majority of modifications is not due to this cause, but to the 
 processes of selection. The question then arises as to the origin 
 of variations which are sufficiently considerable for natural selec- 
 tion to act upon them. External influences at first produce only 
 very slight fluctuations in the determinants presumably not 
 only in some, but in all ; a continual supply of the most minute 
 variations of the different determinants ivill thus always be 
 present. 
 
 I do not, however, believe that variations, as we perceive 
 them, are the direct result of these minute fluctuations on the 
 part of individual determinants : they can only be produced by 
 the accumulation of a large number of fluctuations of tJiis kind. 
 This is an immediate consequence of the theory. Since the germ- 
 plasm consists of many ids, each of which contains the same 
 number of homologous determinants, and as, moreover, any 
 character is the result of the interaction of all its homologous 
 determinants, the variation of a single determinant would be 
 imperceptible ; a character can only be modified to an appreci- 
 able extent when a majority of the determinants are equivalent, 
 or at any rate are similarly modified. 
 
 In my opinion, a variation of this kind is produced by 
 solitary homodynamous determinants in different ids and 
 individuals being brought together in one germ-plasm by means 
 of the processes of ' reducing division ' and amphimixis, so that 
 they can thus form a majority. 
 
 I will illustrate this by a simple example. A small brown 
 European butterfly, Lyccena agestis, Hb., has a small black 
 spot formed by a few scales in the centre of the wing. Let us 
 suppose that this spot is controlled by a single determinant F, 
 and that the germ-plasm in this species contains a hundred ids, 
 and consequently a hundred determinants F ; and that, in
 
 VARIATION. 423 
 
 consequence of a change in nutrition, some of these deter- 
 minants F in different individuals constantly vary in such 
 a manner that if they were in the majority in the germ- 
 plasm they would give rise to a white spot instead of a 
 black one. It will sometimes happen that they are in the 
 majority ; for, in consequence of the process of amphimixis, 
 the modified determinants F may become accumulated in the 
 course of generations, so as to exceed fifty in one or more 
 individuals. The black spot will then turn white, and among 
 thousands of individuals belonging to this species some will 
 exhibit this variation. 
 
 The subsequent course of the phyletic development of this 
 white spot depends upon its physiological value to the species. 
 Even if it is only of slight importance, it will gradually come to 
 be possessed by an increasing number of individuals, and will 
 ultimately be transmitted to all of them : i.e., it will become a 
 specific character. This extension could hardly occur without 
 amphimixis ; for by its means any minority of determinants 
 F 1 , wherever produced, may accumulate so as to constitute a 
 majority ; and otherwise they could not have been effective, 
 since a minority could never have produced a white spot. 
 
 This example was selected because it is based on facts. A 
 variety of Lyccena agestis actually exists which possesses a 
 milky-white spot on each of the four wings in the place of the 
 black one ; it is known as the variety artaxerxes, and occurs 
 in the north of England. It is immaterial whether sexual selec- 
 tion or a protective resemblance has caused the dominance of 
 this modification. 
 
 Many variations of one species or another are merely due to 
 the modification of a few or a large number of determinants ; 
 changes in the colour of individual parts or of the whole body 
 may occur without an accompanying increase in the total number 
 of determinants of the germ-plasm, but, on the other hand, as 
 was shown above, many modifications do result from an increase 
 in their total number. We have already seen that the doubling of 
 a determinant of the germ-plasm may be referred to the influences 
 of nutrition, and no difficulties therefore arise in the application 
 of the principle just propounded to the multiplication of the 
 determinants. The important modifications in species, includ- 
 ing all enlargement of parts and higher differentiation of organs, 
 must be connected with this increase : and the accumulation
 
 424 THE GERM-PLASM. 
 
 of double determinants of single ids, as well as their purely 
 qualitative modifications, may be increased by means of the 
 ' reducing division ' and amphimixis, till the variation becomes 
 perceptible, and natural selection comes into play. 
 
 The ''increase* of a character derived from two parents which 
 merely possessed it to a slight extent, is an entirely different 
 matter. A fusion of the primary constituents of a character 
 common to both parents, and a consequent increase of these 
 primary constituents, such as was hitherto supposed to take 
 place, evidently do not occur : such an assumption is contra- 
 dicted by ordinary experience ; for if the primary constituents of 
 the parents could accumulate in this way, all parts of the offspring 
 must be twice as large, or at any rate larger than, those of the 
 parent, and this is not the case. If, however, it were main- 
 tained that it is merely a question of the differences in the 
 primary constituents, and that the offspring receives half the 
 sum of the characters of the two parents, it could be replied that 
 this might be approximately true in some cases, but no expla- 
 nation would be given as to how an increase of a characteristic 
 can occur, and may even be produced artificially by pairing 
 animals which exhibit a certain tendency to vary in the desired 
 direction. If two animals are paired which possess the cha- 
 racter a in the degree 2a, the offspring would contain -- a -~ 
 
 i.e., 2<z, and no increase would be produced. Moreover, the 
 real meaning of the term ' halving' would then remain perfectly 
 vague. Hensen * thought that 'the effect of transmission was to 
 halve the characters of the parents,' since ' a similar whole can 
 only result from two equal half-transmissions.' I believe that this 
 is to a certain extent true, but not in the sense which would entail 
 the halving of an indivisible primary constituent. 
 
 The solution of the problem is to be found in the multiplicity 
 of the ids and determinants, and in distinguishing carefully 
 between the vague idea of the ' character^ and the definite one of 
 the determinate or hereditary part. Each determinate is con- 
 trolled by as many determinants as there are ids in the germ- 
 plasm ; but half the ids of each parent, together with half the 
 determinants of every kind, are removed from the germ-plasm by 
 the reducing division of the germ-cell. Half the primary consti- 
 
 * V. Hensen, ' Physiologic der Zeugung. '
 
 VARIATION. 425 
 
 tuents are, however, not removed in this process ; on the con- 
 trary, the occurrence of ' pseudo-monogonic heredity' proves 
 that every parent-organism transmits the whole of them to the 
 offspring ; every primary constituent is, however, represented by 
 only half the number of determinants. 
 
 From this theoretical basis, the results arrived at by breeders 
 are easy to understand. ' Like begets like] is their chief maxim, 
 and this is true whenever two individuals are coupled which 
 possess a certain character as an inheritance transmitted 
 through a long series of generations. For such a character 
 must be represented by homodynamous determinants in the 
 great majority of ids in the germ-plasm ; and as it consequently 
 will not be entirely removed from either half of the latter 
 by the reducing division, it will in most cases be represented in 
 a majority of ids in the offspring. 
 
 The idea that the 'increase* of a 'character' can be produced 
 simply by crossing, is due to an inaccuracy of expression. ' Like 
 begets like,' but not something else ; and in this respect theory 
 and practice agree. It is theoretically inconceivable that pre- 
 cisely the same part that is to say, the same determinate or 
 group of determinates may be increased merely by the pairing 
 of parents possessing it. If, for instance, two individuals of 
 Lyccena agestis pair, and each of them possesses a white spot 
 instead of a black one, in the centre of the wing, none of the 
 offspring could exhibit a spot twice as large as, or even any 
 larger than, that of the parents. For the spot is controlled by 
 one or more homologous determinants, and if those of the white 
 variety are in the majority, the spot will be of this colour ; but 
 the adjacent determinants cannot thereby become modified. 
 The spot can at any rate only become pure white in case it was 
 previously merely grey, owing to the relatively larger number of 
 ' black ' determinants which took part in the control of the spot 
 in the parents having now been completely excluded from the 
 control of the cells by a preponderating majority of ' white ' 
 determinants. 
 
 The term 'quality' or 'character' (' Eigenschaft') is the real 
 cause of confusion in this instance. In the Introduction to this 
 book I pointed out that this term may have several different 
 meanings with regard to heredity. Whiteness in plumage, for 
 example, is a ' quality ' which a breeder tries to obtain by always 
 selecting the whitest bird for breeding purposes. From a breed
 
 426 THE GERM-PLASM. 
 
 blue pigeons, for instance, a bird with a white head is paired 
 with another possessing a white tail, and in this way some 
 young may be obtained with white heads and tails. The 
 ' qualities ' of the white head and white tail are here, therefore, 
 combined. The feathers of both parts of the body, however, 
 have their own determinants ; and in this cross the process 
 which took place in the idioplasm was not the accumulation of 
 homologous determinants, but the acquisition of a majority by 
 the 'white' determinants for the feathers of the head and tail 
 over the ' blue ' ones. This is, therefore, not a case of the 
 summation of like with like, but a victory of similar deter- 
 minants in different parts of the body. 
 
 Surprise has often been expressed at the case, mentioned by 
 Darwin, of two crested canaries which produced young none of 
 which possessed larger crests, while many, at any rate, were 
 bald. As Darwin himself pointed out, the crest in birds is due 
 to a sparser covering of feathers on the head, and this peculiarity 
 may be increased so as to result in baldness in the offspring. 
 This is also an instance of the increase of a ' quality,' but only 
 that of baldness, and not of the crest which is valued by the 
 breeder. In terms of the idioplasm, this may be explained in 
 the same way as was done with regard to the increase of the 
 character of whiteness in pigeons. It is due to the arrangement 
 of 'bald' determinants, if I may use such an expression, 
 some of which are derived from the father and control the 
 region a, the others from the mother controlling the region b. 
 In the father the region &, and in the mother the region a, 
 still possessed a ' feather ' determinant ; in the offspring the 
 'bald' determinants for both regions were accidentally brought 
 together owing to the reducing division and the subsequent 
 amphimixis. It is shown by our theory that bald-headed 
 offspring need not invariably result in such a case ; and, as 
 a matter of fact, all the young birds did not exhibit this 
 peculiarity. 
 
 The increase in the number of feathers in any particular part, 
 such as has occurred, for instance, in a fantail pigeon, is another 
 case of this kind. The tail of this bird consists of about forty 
 quill-feathers, instead of twelve, as in the original form ; and the 
 breed has undoubtedly been produced by artificial selection, 
 those pigeons always being chosen for breeding purposes which 
 possessed an extra feather in the tail. The young frequently,
 
 VARIATION. 427 
 
 or at any rate in some cases, would thus exhibit more tail- 
 feathers than either of the parents. But this would not by any 
 means be an ' increase in the force' of a ' character,' but simply 
 an increase in the number of new feathers in the individual. Let 
 us suppose that the male bird possessed two extra feathers 
 which were situated at a 1 and d l between the normal feathers a 
 and b and c and d, and that the mother had two supernumerary 
 feathers in the positions f 1 and X 1 : a majority of the deter- 
 minants for the extra feathers of the two parents might then be 
 united in the germ-plasm of one of the offspring, so that the 
 latter would possess all the four new feathers. This ' increase ' 
 in the character of additional feathers in the tail, therefore, 
 depends upon the constitution of the germ-plasm, and can 
 consequently be transmitted to the next generation. 
 
 This example clearly shows that all really new structures are 
 not merely the result of transmission, but are due to the variation 
 and frequent multiplication of the determinants. The mere 
 extension of a 'character' over larger regions or the whole of 
 the body, even if we choose to speak of it as an ' increase,' may 
 be produced by pairing individuals which possess the desired 
 ' character ' in different parts. But an increase which is con- 
 nected with the formation of new structures, and consequently 
 with the multiplication of the determinants in the germ-plasm, 
 can never be produced by such means alone. When this 
 results, the cause of the modification must be the variation of 
 the determinants themselves. 
 
 Thus in the case of the fantail, a new feather can never be 
 produced by transmission alone : the offspring can merely 
 possess new combinations of such feathers as were present in 
 the parents. All really new structures can only originate in a 
 previous modification of the germ-plasm. 
 
 Let us now take an example from among sexually dimorphic 
 forms, in which we are quite certain of the phyletic modification, 
 apart from the complication arising from the existence of 
 sexual double-determinants. The long tail-feathers of male 
 humming birds have arisen by a gradual lengthening of the 
 ordinary tail-feathers, such as are possessed by the females 
 at the present day. As already remarked, this lengthening is 
 the result of a considerable multiplication of the determinants 
 which give rise to the feather : the process of lengthening implies 
 that variations in the ids which possessed a larger number of
 
 428 THE GERM-PLASM. 
 
 determinants than those of the original feather were exposed to 
 selection. It was stated above that fluctuations in the structure 
 of the determinants, caused by inequalities of nutrition, may 
 also be produced in consequence of their more rapid growth and 
 earlier division. If, then, certain determinants for the feather 
 of the original form underwent division at an early stage in 
 individual males, so that their number became doubled, a 
 lengthening of the feather must have resulted as soon as the 
 doubling occurred in the majority of the ids. The majority 
 need not have appeared in one individual from the first, but, 
 like every other simple variation of a determinant, it may have 
 originated sporadically in separate ids of different individuals, 
 and have become increased by the repetition of the process 
 of amphimixis in every generation. The modification would 
 become apparent and the processes of selection could take place, 
 as soon as these ids had attained a majority in any individual. 
 
 2. Pathological Variation. 
 
 The above theoretical explanation of the 'increase' of a 
 character possesses the important advantage of accounting for 
 the sudden appearance of more extensive variations. If more 
 abundant nourishment can cause the doubling of a determinant 
 in the germ-plasm, it is possible, and even probable, that many 
 or all the contiguous determinants for the same feather will 
 become doubled. The feather must consequently at once 
 increase to twice the size. Doubts have often been justly raised 
 as to whether the process of sexual selection at first produces 
 very slight variations, which would scarcely be noticed and 
 preferred by the selecting sex. We learn from the doctrine of 
 determinants that it is unnecessary to take such minute varia- 
 tions into account, and that more extensive ones may suddenly 
 arise directly from the germ-plasm. 
 
 The doubling by division, not only of biophors and deter- 
 minants, ids and idants, but even of individual groups of 
 congruent determinants, such as must be contained in the 
 germ-plasm in the form of primary constituents of any organ 
 e.g., a feather is theoretically possible. But this is one of the 
 more special questions, the details of which may be reserved 
 for subsequent investigations. I think it is highly probable 
 that many congenital deformities, such as the occasional 
 doubling of the tarsus in the hind-limbs of beetles and other
 
 VARIATION. 429 
 
 insects, are due to the doubling of a group of determinants, 
 and perhaps the much-discussed and debated problem con- 
 cerning supernumerary fingers and toes in human beings may 
 be explained in a similar way. There is nothing impossible in 
 the assumption that the latter phenomenon is due to reversion 
 'to an extremely remote, lowly-organised, and many-fingered 
 ancestor,' for we know of other cases of reversion to very distant 
 ancestral forms. But in none of the reliable instances does rever- 
 sion to ancestral characters extend through such an enormous 
 lapse of time or immense number of generations as must be 
 assumed in this case. The striping on mules points back to an 
 early equine ancestor, and we are led to the conclusion that at 
 the present day the germ-plasm of horses and asses still contains 
 solitary ' zebra ' determinants. Reversion may occur to yet more 
 remote ancestors of the modern horse, even to those possessing 
 three toes ; but cases of reversion to still earlier ancestors can 
 hardly be proved with any degree of certainty ; nor is it probable, 
 from a theoretical point of view, that any groups of determinants 
 of such extremely remote primitive mammals should have been 
 preserved in the germ-plasm of human beings. Moreover, it is 
 not at all certain that the primitive mammals possessed more 
 than five fingers and toes ; and it would be necessary to go back 
 to much more remote ancestors before obtaining any support for 
 the explanation of human polydactylism given by Darwin, and 
 formerly accepted by Bardeleben, Wiedersheim, and others. 
 
 Not only is there no firm foundation for this latter assumption, 
 but there appear to me to be very weighty reasons against it. 
 We must not, in the first place, overlook the fact that these primi- 
 tive ancestors did not possess 'human' fingers ; supernumerary 
 fingers are, nevertheless, real fingers, and though they are not 
 always perfect, they are furnished with the form of nail typical 
 of the human finger, and not with claws. In my opinion, we are 
 not justified in assuming that such a supernumerary finger is 
 represented in the germ-plasm by a group of determinants de- 
 rived from the primitive ancestor, and that this group has in the 
 meantime become transformed into the type of the human finger. 
 
 Apart from polydactylism, cases of the doubling of the limbs 
 are known, which, from their nature, cannot be looked upon as 
 atavistic : insects, for example, have never possessed a double 
 tarsus. There must, therefore, be another way in which this 
 doubling might originate.
 
 430 THE GERM-PLASM. 
 
 The sudden appearance of polydactylism and its great ten- 
 dency to transmissibility can, moreover, be easily understood if 
 we suppose that excessive local nutrition has caused the group 
 of determinants in question to become doubled.* For when 
 this doubling has once occurred in several ids in the germ-plasm, 
 it must be capable of being transmitted on account of the 
 continuity of the germ-plasm ; and the degree of certainty with 
 which this will take place will increase as the number of ids in 
 which it has occurred becomes greater. I am entirely of Ernst 
 Ziegler's opinion that polydactylism is due to a germ-variation : 
 this must be so whenever it is hereditary, for it would not other- 
 wise be transmissible. 
 
 In this way we can also understand why polydactylism, after 
 it has once arisen and has been transmitted through several 
 generations, may finally disappear ; for at every fresh ' reducing 
 division ' the number of abnormal ids is increased or decreased, 
 and in the latter case their effect may be entirely obliterated, in 
 consequence of their meeting with perfectly normal ids during 
 the process of amphimixis ; and in the next generation they 
 may be for ever eliminated. Like all individual variations, 
 they may be absent from one generation and appear again in 
 the next ; but in case of continual crossing in normal human 
 beings, theoretically they may be expected once more to dis- 
 appear completely. This agrees with fact, for supernumerary 
 fingers have never been observed in more than five consecutive 
 generations. 
 
 We know that variation consists not only in the addition of 
 parts, but also in their disappearance. The process of degene- 
 ration of parts must be attributed to the disappearance of the 
 respective determinants from the germ-plasm. In the chapter 
 on reversion, I have already attempted to show that regressive 
 transformations need not occur in all the ids of the germ-plasm 
 at the same time, and that reversion to long-lost ancestral 
 characters may be ascribed to the preservation of a minority 
 
 * Dr R. Zander has recently declared himself in favour of the view that 
 supernumerary fingers, &c., are formed by the mechanical constriction 
 of the rudiments of the embryonic fingers by amniotic threads. But if 
 this is true, they would not be transmissible, and another explanation 
 must be made for the doubling of the tarsus in beetles. (C'f. ' 1st die 
 Polydactylie als theromorphe Varietal oder als Missbildung anzusehen? 1 
 Virch. Arch., Bd. ias, 1891, p. 453.
 
 VARIATION. 431 
 
 of the determinants in question, which gradually decreases in 
 course of time. The cause of the regression of a determinant 
 is to be looked for in insufficient nutrition, which condition 
 may occur in a determinant quite as likely as that of more 
 abundant nutrition. If this occurs in the majority of the ids 
 either directly, or in consequence of the accumulation produced 
 by amphimixis, the character controlled by these determinants 
 becomes regressive in that particular individual. If, however, it 
 no longer has a physiological value, it becomes slowly but surely 
 suppressed by panmixia in an ever-increasing number of indivi- 
 duals until it disappears. Specific characters which have long 
 been unrecognisable externally, instead of disappearing entirely, 
 may still be retained in individual ids in the form of incompletely- 
 degenerated determinants ; and, as already mentioned, these 
 may cause the reappearance of a character under particularly 
 favourable circumstances. 
 
 3. Summary of Sections i and 2, and Conclusions. 
 The above remarks may be briefly summarised as follows : 
 The origin of a variation is equally independent of selection 
 and of amphimixis, and is due to the constant recurrence of 
 slight inequalities of nutrition in the germ-plasm which affect 
 every determinant in one way or another, and differ even in 
 the same germ-plasm, not only in different individuals but 
 also in different regions. These variations are at first infini- 
 tesimal, but may accumulate ; and, in fact, they must do so 
 when the modified conditions of nutrition which gave rise to 
 them have lasted for several generations. In this way deviations 
 may occur in the structure of single determinants or of groups of 
 them, never, perhaps, in all ids at once, but at any rate in 
 several or even many of them. A doubling of certain determin- 
 ants of the germ-plasm may originate in the same way. The pro- 
 cess of amphimixis has an important share in the accumulation 
 of these modified determinants, for it may raise the minority pre- 
 viously existing in the two parents to a majority by combining 
 their halved germ-plasms. Then, and then only, does selection 
 begin to take place. 
 
 The extreme importance of sexual reproduction in processes of 
 transformation only becomes evident, however, when we realise 
 that adaptations are usually concerned with several variations 
 at the same time, and rarely or never arise in connection
 
 432 THE GERM-PLASM. 
 
 with a single one. The process of amphimixis alone rendered 
 it possible for such manifold combinations of characters to be 
 offered to selection, so that the proper choice could be made. If 
 the view which I have long held is correct, a single character 
 is never alone acted upon by natural selection, but the whole 
 aggregate of specific characters is incessantly exposed to this 
 process. The constancy as well as the transformation of the 
 existing specific characters, the removal of superfluous ones, and 
 the development of new characters, is due to the incessant and 
 uninterrupted control of selection. This is only rendered 
 possible by the continual intermingling of all the existing 
 varieties of characters, which can only be effected by amphi- 
 mixis. Hence, although the latter process is not the primary 
 cause of individual variation, it is nevertheless an indispensable 
 factor in selection, for by its means alone can the material from 
 which variations arise be so arranged that selection can operate. 
 
 The theory of variation here propounded also affords a more 
 satisfactory explanation of a further difficulty than can be 
 obtained by any other. In considering the unlimited number of 
 adaptations of organisms to the conditions of existence, we 
 must be surprised at the wonderful plasticity of the species. It 
 gives us the impression that every variation, however unexpected, 
 might be produced by a species as soon as the species has 
 use for it. On reflecting how certain animals and plants, or 
 parts of plants, are imitated in colour, form, and marking by 
 other animals, we might be inclined to suppose that every part 
 of an animal may assume any required form, colour, or marking, 
 according to requirement. 
 
 This must not, however, be taken literally ; an organism 
 cannot assume every form, though it may become adapted in so 
 many ways that we cannot possibly attribute its immense 
 number of adaptations to rare, fortuitous variations, occurring 
 only once. The necessary -variations from which transformations 
 arise by means of selection, must in all cases be exhibited over 
 and over again by many individuals. 
 
 The presence of such an ever active material for primary varia- 
 tions is a direct consequence of the theory here propounded, 
 according to which every part or ' determinate ' of a species must 
 present every possible variant in different individuals in the course 
 of generations, and will sometimes be represented in a larger, 
 and sometimes in a smaller majority of modified ids. Since an
 
 VARIATION. 433 
 
 absolute equality of nutrition in homologous determinants, either 
 in different individuals or in the different ids of the same germ- 
 plasm, is inconceivable, and as every minute variation of a 
 determinant does not disappear of itself with the individual in 
 which it is present, but is transmitted directly to the germ-plasm 
 of the next generation, there can be no scarcity of variations of 
 every determinant, and the presence of the material required 
 for all the possible variations of all parts seems to be supported 
 theoretically. 
 
 Before considering the modifications which the germ-plasm 
 must undergo as a whole during the transformation of species, 
 I should like to meet an objection which might be raised. If 
 all determinants are incessantly exposed to slight inequalities of 
 nutrition and consequently suffer slight variations, what is the 
 cause of the extraordinary pertinacity by means of which the 
 species is preserved without the type undergoing variation, 
 what is the cause of the constancy of species ? We might expect 
 that all organic forms must be in a constant state of transition, 
 and that no form, and no organ, could be retained for any length 
 of time. 
 
 Several points are disregarded in such a question. In the 
 first place, every species is under the uninterrupted control of 
 natural selection, as is clearly shown by the degeneration of 
 parts which have become useless. And since the old hypothesis 
 of the transmission of somatic variations must, it appears to me, 
 be definitely rejected, this process of degeneration can only be 
 explained as the result of panmixia, i.e., the cessation of the 
 control of natural selection over that part which is no longer 
 of use. We may, however, conclude from the fact that such 
 degeneration is universal, that the determinants are in a con- 
 stant state of fluctuation ; and as degeneration takes place very 
 slowly in all cases, I further infer that, in spite of the frequency 
 with which these fluctuations occur, they only increase very 
 gradually so as to give rise to perceptible variations. 
 
 As at first stated, the individual fluctuations of the deter- 
 minants must be regarded as excessively small. Natural 
 selection could produce no direct result from an individual 
 variation, for it could not produce a cumulative effect ; an 
 accumulation can only be produced by amphimixis, and I am 
 inclined to assume that much of the importance of the latter 
 process is due to this fact. It can cause minorities of modified 
 2
 
 434 THE GERM-PLASM. 
 
 determinants to accumulate into majorities by mingling the 
 halved germ-plasms of two individuals. By the aid of the 
 ' reducing division ' it can also level and equalise matters by 
 fortuitously dispersing the homologously modified determinants 
 of an individual. 
 
 It must not be forgotten that slight primary variations of a 
 determinant need not always continue in the same direction : 
 influences of nutrition in the reverse direction will frequently 
 cause them once more to disappear. Only after a determinant has 
 been modified to a considerable degree by the action of a uniform 
 influence during a long period, and the determinants of many 
 ids have become similarly and simultaneously modified, can a 
 variation become visible after being first accumulated by 
 amphimixis. And even then it by no means forms a permanent 
 specific character, for the question as to whether it will or will 
 not give rise to one is decided by natural selection. 
 
 Thus several powerful influences prevent the constant varia- 
 tion of the specific type. 
 
 An answer to the question as to ivhat -variations the idio- 
 plasm undergoes in the transformation of species will be found 
 in the chapter on reversion, and it will here only be necessary to 
 summarise what I have already said. 
 
 The transformation of species is due to the variation of some, 
 and frequently even of most, of the determinants. Many species 
 do not possess a single character which resembles that of an 
 allied species, and in this case all the determinants must be 
 different. But this only implies that all the determinants a x 
 have been modified in the majority of ids; a minority of the 
 latter will contain unmodified ancestral determinants. As the 
 transformation of a species proceeds, the number of modified 
 determinants increases together with the number of ids in 
 which they occur. Nevertheless the dominating principle of 
 selection only permits the transformation of all the ids to occur 
 very gradually, so that the germ-plasm of a young species may 
 often contain completely unmodified ids of the ancestral species ; 
 and even older species may contain solitary groups of unmodi- 
 fied determinants in many of their ids. This, and this alone, 
 renders reversion possible. 
 
 It has been recently maintained that, as a consequence of 
 my theory, I must adopt one of two alternatives, and assume 
 either that the germ-plasm of the higher animals consists of ids
 
 VARIATION. 435 
 
 of the primitive protozoan ancestors,* or that every id is con- 
 structed in accordance with the existing character of the species : 
 my real view, however, is intermediate between these two. I 
 believe that the germ-plasm of a species always consists to a 
 great extent of specific ids, amongst which, however, some more 
 or less unmodified ancestral ones are present, the number being 
 largest when the species is young. The germ-plasm varies in 
 different species, and must differ very much in the higher and 
 lower forms ; but its transformation does not go on at the same 
 rate in all the ids, for some are modified only gradually, or are 
 transmitted unaltered through long lapses of time till they are 
 at last casually removed by a ' reducing division.' 
 
 This might be regarded as a defect in the process of the 
 transformation of species, for the possibility of reversion, as well 
 as the retention of inactive ancestral ids in the germ-plasm, can 
 scarcely be considered useful to the species. But in nature no 
 contrivance is absolutely perfect, not even the marvellously 
 developed human eye : all structures are only as perfect as 
 possible, that is to say, as perfect as they need be in order to 
 perform their required functions. This statement also applies 
 to the mechanism for the transformation of species : it 
 approaches perfection as nearly as is necessary for the per- 
 formance of its function. 
 
 4. VARIATIONS ON A LARGER SCALE. 
 
 a. The Origin of these Variations, 
 
 We have till now chiefly confined our attention to the question 
 of general individual variation, as exemplified by those minor 
 hereditary differences which distinguish individuals from one 
 another. But there can be no question that variations some- 
 times occur on a larger scale, and these usually appear 
 suddenly, are met with only in single individuals, and are as a 
 rule hereditary. Darwin has given a large number of instances 
 of this kind. Although the special peculiarity of the black- 
 shouldered peacock, for example, may be due to reve'rsion to an 
 unknown ancestral form, there are many well attested cases in 
 
 * Compare Marcus Hartog, ' Nature,' Vol. 44, December 1891. The 
 deductions made by this author from my former views are logically correct, 
 but are no longer justifiable, since in the meantime I myself have gained 
 further insight into the problems concerned.
 
 436 THE GERM-PLASM. 
 
 which a given structure in a species has suddenly become 
 considerably modified. How far such modifications have to 
 do with the formation of new species need not concern us at 
 present : we have only to consider the causes and method of 
 origin of these modifications. 
 
 A large number of cases of this kind have been observed, 
 especially amongst plants. Not only fruits, but leaves, blossoms, 
 and entire shoots have been found to vary suddenly, and in 
 a striking manner. Several varieties of fruits must be included 
 in this category such as the variety of peach known as the 
 nectarine, as well as the moss-rose, copper-beech, copper-hazel, 
 the varieties of the beech, hornbeam, and oak, the fern-leaved 
 variety of the maple, and numerous other plants cultivated in 
 our gardens. 
 
 These varieties in some cases first appeared as seedlings, i.e., 
 as entire plants, and in others as simple branches or shoots, in 
 which latter case they are usually known as bud variations. 
 
 Let us first take into consideration the varieties ivhich 
 have originated from seeds. These occur most frequently in 
 cultivated plants, i.e., in species which have existed for some 
 length of time under conditions which differ more or less from 
 i he natural ones. We are therefore undoubtedly justified in 
 attributing the cause of the variation to the influence of changed 
 external surroundings. But a wild plant, which is transplanted 
 into a garden soil, does not always begin to vary at once : it 
 has, in fact, been shown by Hoffmann's experiments, of which 
 we have already given an account, that many generations often 
 elapse before conspicuous variations occur. And even then they 
 do not appear in all the seedlings, occurring perhaps only in one 
 among several hundreds or thousands. 
 
 As in the case of ordinary individual variations of a minor kind, 
 the modification begins a long time before it becomes apparent. 
 A few determinants are first changed, and then a gradually 
 increasing number, until at last, by means of the ' reducing divi- 
 sion ' and amphimixis, they occur in such numbers in certain 
 germ-cells that they form the majority. The fact that these 
 variations occur on a larger scale than the ordinary ones is due 
 to the permanent action of uniform changes in nutrition, which 
 give a constant direction to the modification of susceptible 
 determinants, so that an increase is effected. This is wanting 
 in the case of the ordinary and constantly varying nutrition.
 
 VARIATION. 437 
 
 ' The accumulative action of changed conditions of life' sug- 
 gested by Darwin,* is consequently theoretically supported to 
 a certain extent by the theory of the continuity of the germ- 
 plasm : those determinants which varied in the first generation 
 continue to do so in a similar direction in the second and third. 
 
 Professor Hoffmann has for many years been making very 
 interesting experiments in the Botanical Garden at Giessen 
 which bear on this point, some of which I will now describe. 
 
 Various plants, bearing flowers of the normal structure, were 
 exposed during a number of generations to greatly modified 
 conditions of life ; they were, for example, grown crowded 
 together in small pots, so that each plant restricted the amount 
 of food obtainable by the others, and were thus scantily 
 nourished. Under this treatment some species such as 
 Nigella damascena, Papaver alpinum, and Tagetes patula 
 bore a number of non-typical double flowers more or less fre- 
 quently. The fact that these deviations from the ordinary 
 type in no case appeared in the first generation, proves that 
 they were due to an influence exerted upon the germ-plasm, 
 and not to the direct influence of the abnormal conditions of 
 nutrition upon the soma of the plant. Seeds of normal wild 
 flowering plants of different species, when grown in cultivated 
 soil or even when thickly sown in pots, never produced plants 
 possessing even one double flower. Only in the course of 
 several, and often many generations, did any of these wild 
 plants exhibit a greater or less number of double flowers, or 
 occasional modifications in the leaves or in the colour of the 
 flowers. It seems to me that only one explanation can be given 
 of this fact, viz., that the altered conditions at first only pro- 
 duced imperceptible variations in the germ-plasm of an individual 
 plant, such, for instance, as alterations in the determinants for 
 the leaves or flowers in individual ids, but not in all of them 
 at once. These modified determinants were transmitted to the 
 next generation in consequence of the continuity of the germ- 
 plasm ; but since the causes of variation continued to operate, 
 the homologous determinants in several other ids also became 
 modified, and thus the number of modified determinants for 
 the leaves and flowers continued slowly to increase, until finally 
 they exceeded the normal determinants in number, and the 
 
 * Darwin, 'Animals and Plants under Domestication,' Vol. II., p. 249,
 
 43^ THE GERM-PLASM. 
 
 abnormality became apparent as a variation in the flower or in 
 the leaf. 
 
 The reason why I lay special stress on these cases is, that 
 natural selection plays no part in them, for we are here dealing 
 with artificial, and not with the natural conditions of life. The 
 imperceptible variations of the germ-plasm naturally did not 
 increase continuously, for the processes of ' reducing division ' 
 and amphimixis exerted their influence on every fresh generation, 
 and helped to bring about either a marked decrease of the modi- 
 fied determinants, or their sudden increase a doubling, or a 
 still greater augmentation. In this way it is easy to understand 
 all the particulars connected with the occurrence of the abnormal 
 flowers and leaves. As a rule, the number of these abnormali- 
 ties increased in the course of generations with a fairly constant 
 regularity. The following results, for example, were obtained 
 from four generations of Nigella dainascena, when sown close 
 together : 
 
 1883. No double flowers. 
 
 1884. 
 
 1885. Six double and twenty-three typical flowers, or 24 per 
 cent. 
 
 1886. One double and ten typical flowers, or 10 per cent. 
 The number of double flowers was, however, not always 
 
 constant, and in some cases they again disappeared entirely. 
 Papaver alpinum, for instance, which Hoffmann had cultivated 
 uninterruptedly since 1862, displayed 'a trace of variability in 
 the form of the leaves, and a more decided variation in the 
 colour of the flowers in 1882.' These experiments were con- 
 tinued from 1882 to 1886, and yielded the following proportion of 
 double and normal flowers : 
 
 In 1 88 1 the proportion was 40^0 per cent. 
 In 1882 ., 4'o 
 
 In 1883 5-3 
 
 In 1884 13-0 
 
 In 1885 o 
 
 In 1886 o 
 
 The complete disappearance of abnormal flowers in the two 
 last of these years seems inexplicable at first sight ; but it is 
 easily accounted for if we bear in mind the fact that natural, 
 and not artificial fertilisation occurred, i.e., the plants were 
 crossed at random, and that every reducing division and subse-
 
 VARIATION. 439 
 
 quent amphimixis were capable of reducing the modified deter- 
 minants to a minority, and even of removing them completely 
 from the germ-plasm. The modification of the determinants 
 manifestly occurs very gradually, as the small number of modi- 
 fied flowers in most of the years shows ; and the chances must 
 have been greatly in favour of the union in amphimixis of 
 germ-cells which contained few or no modified determinants. 
 
 Had fertilisation been produced artificially, and abnormal 
 flowers always employed for the purpose, it would have been 
 easy in the course of a moderate number of generations to 
 modify the species entirely, and these cases would then have 
 served as an illustration of the process of natural selection. It 
 would have been still easier to produce the contrary effect by 
 selection, that is to say, to keep the species constant, and 
 suppress any subsequent variation in this direction. For in the 
 present instance, variation evidently occurred slowly, and most 
 of the determinants were not easily affected by it ; and these 
 experiments furnish additional proof of the truth of the statement 
 propounded above that the elements of the germ-plasm only 
 change slowly, and with difficulty : they merely fluctuate to a very 
 slight degree, and only undergo an important change of any 
 duration when uniform influences continue to act on them in one 
 direction for a longer time. 
 
 It by no means follows from what has just been said that 
 influences of environment and nutrition exist, which, when they 
 have acted for a long time, are able to modify the majority 
 of the determinants for certain parts of the body, and thus to 
 produce purely climatic variations, in the origin of which natural 
 selection has no share. Many -perhaps even most of the 
 ' climatic ' varieties are rightly so called. 
 
 Such sudden variants, however, appear not only in entire plants 
 which have been produced sexually, but also in the individual 
 shoots of a plant. These bud-variations are rarely met with ; 
 but in cases in which they occur, they can be propagated by 
 cuttings or grafting, and often even by seeds. 
 
 When I ventured some years ago to suggest that sexual 
 reproduction has come into force in organic nature in order to 
 preserve the variability which had existed since the time of 
 the primordial beings, facts concerning bud-variation were put 
 forward by several persons to prove that variability may occur 
 in the absence of sexual reproduction. At that time I certainly
 
 440 THE GERM-PLASM. 
 
 did not attach sufficient importance to the variation of the 
 germ-plasm in consequence of influences acting directly ; but 
 the existence of bud-variations does not prove that variation 
 occurs without amphimixis. For all those plants in which bud- 
 variation has been observed are reproduced sexually, and their 
 idioplasm therefore contains ids and determinants which differ 
 individually : the different intermingling and behaviour of 
 these in the process of growth would alone form a basis for 
 variations. 
 
 In my opinion, indeed, this heterogeneous composition of the 
 germ-plasm produced by amphimixis is an essential factor in 
 bud-variation, notwithstanding the fact that it may not in this 
 case give the first stimulus to variation, any more than it does 
 in ordinary individual variation. Plants which have for a long 
 time been propagated by means of buds and shoots, like the 
 potato and sugar-cane, must possess a germ-plasm consisting 
 of different kinds of ids ; for they were formerly reproduced 
 sexually, and the complex intermingling of their determinants 
 thus produced, cannot have undergone an appreciable modifica- 
 tion during the period in which they have multiplied asexually. 
 Their germ-plasm must therefore present far more favourable 
 conditions for variation than would one composed of identical 
 ids or of one kind of id only, did such a germ-plasm exist. 
 
 The primary cause of bud-variation must be the same as 
 that of variation from seeds, and must be due to inequality of 
 nutrition in the germ-plasm ; the term 'nutrition' being used 
 in its widest sense, so as to include differences in tempera- 
 ture, &c. This view not only receives support from theoretical 
 considerations, for theoretically it could only be replaced by 
 the assumption of an internal phyletic developmental force, 
 but it is also supported by observation. For all recorded ob- 
 servations go to prove that bud-variations are most likely to 
 occur when the plant is placed under abnormal conditions, and 
 especially when it is cultivated. As the direct modification of 
 the soma caused by these conditions is not hereditary (Ntigeli), 
 and cannot be so, for somatic variations are only hereditary 
 when they proceed from the germ, we are obliged to assume 
 that the modification of several or many determinants in the 
 germ-plasm is due to inequality of nutrition. 
 
 That bud-variations are produced by the same causes as 
 those which occur in reproduction by seeds, is borne out by
 
 VARIATION. 441 
 
 the fact that the former occur most frequently in those species 
 which have already varied greatly in multiplying by seeds.* 
 Expressing this in terms of the idioplasm, we may say that 
 it occurs oftenest in those species in which the homologous 
 determinants already exhibit considerable differences. 
 
 We attribute these variations to influences of nutrition, which 
 at first bring about slight, and then more marked deviations in 
 certain determinants of the germ-plasm during the course of their 
 growth and multiplication, if these influences continue ; but this 
 alone does not fully account for the process. The question then 
 arises as to how the modifying influences can cause a particular 
 bud to undergo variation while all the rest remain unchanged, 
 although they are exposed to the same influence. Some other 
 influence is therefore required before a modification of this 
 kind can appear. 
 
 If we remember that bud-variations sometimes occur in wild 
 plants, or in those which, like the forest-trees of our parks, 
 exist under practically the same conditions as many wild ones, 
 it will appear still more probable that the inequalities of nutri- 
 tion, while constituting the primary cause of bud-variation, 
 cannot alone bring it about. 
 
 My own conception of the process is as follows. Just as 
 in the case of ordinary individual variation, bud-variation is 
 primarily due to those slight, fluctuating, structural changes 
 which all determinants undergo in consequence of minute and 
 inevitable fluctuations in nutrition. As in the former case, the 
 homologous determinants of the various ids are not all affected 
 to the same extent ; some become greatly modified, and others 
 little or not at all. A difference is, however, seen in the fact 
 that in this case the same influence of change e.g., generally 
 improved nutrition occurs for a considerable time, and through- 
 out several generations. As in the case of variations in plants 
 raised from seeds, a modification of greater extent can thus be 
 produced in these determinants. 
 
 Up to this point the process is quite similar to that of the 
 spasmodic variation of seedlings. A difference, however, results 
 owing to the non-occurrence of amphimixis in the case of gem- 
 mation, for the variable bud does not arise from the germ-plasm 
 of a seed, but from ' blastogenic' germ-plasm (' Knospen-Keim- 
 
 * Darwin, loc. cit.
 
 44 2 THE GERM-PLASM. 
 
 plasma '). The latter is derived directly from the former, from 
 which the plant in question or one of its ancestors, if the 
 plant itself was raised from a cutting arose. If, during the 
 growth of a tree, one of the determinants, N, varied in the same 
 way in certain ids, and tended, for example, to produce red 
 leaves instead of green ones, red leaves would nevertheless not 
 be produced till all, or at any rate a majority, of the determin- 
 ants N had become transformed into the red variety. If a 
 ' reducing division ' intervened between the germ-plasm of the 
 bud and the growth of the latter into a shoot, a minority of ' red ' 
 determinants might give rise to a majority in one of the two 
 daughter-cells ; but this reducing division is exactly what does 
 not take place in ordinary cell-multiplication. 
 
 Since bud-variations are of very rare occurrence, and only 
 one of many thousand buds on the same plant varies so as to 
 produce red leaves, for instance, while the altered conditions 
 simultaneously affect all the buds, I conclude that the modified 
 determinants which the tree contains may sometimes attain a 
 majority in consequence of an abnormal differential nuclear 
 division. Should this take place in the apical cell or cells 
 of a bud, the resulting shoot might, to return to our former 
 example, bear both green and red leaves ; for, according to our 
 pre-supposition, the 'green' and 'red' determinants of the 
 apical cells would have been separated during the cell-divisions 
 in such a manner that in some leaves the green determin- 
 ants, and in others the red ones, might be in the majority. 
 If the separation occurs at an earlier stage, before the apical 
 cells are formed, that is to say, in the cambium, a shoot 
 bearing red leaves alone might arise from the cell which 
 receives the 'red' group at the differential division of the 
 nucleus. 
 
 There is nothing impossible about this assumption ; for during 
 the process of mitotic nuclear division, irregularities might occur 
 in the complex apparatus by which this process is effected, 
 and individual cases of such irregularity have actually been 
 observed : even the possibility of a direct nuclear division 
 cannot be entirely overlooked. I am, however, far from con- 
 sidering this hypothesis as established, and merely offer it as a 
 suggestion. 
 
 Nageli was of the opinion that all variations are slowly 
 prepared in the idioplasm in the course of generations before
 
 VARIATION. 443 
 
 they become apparent, and in stating this view he also specially 
 referred to bud-variations. I fully agree with him in this 
 respect, and in the course of this book have repeatedly shown 
 how these gradual modifications (' Umstimmungen ') of the 
 germ-plasm, or of individual parts of it, are to a certain extent 
 the natural result of its assumed structure. In the case of bud- 
 variations these invisible modifications may occur in a much 
 earlier generation than that in which they appear ; and hence 
 it is easy to understand why this form of variation mostly 
 occurs in those plants which, like the rose and Azalea, have 
 already varied in reproduction by seed. For modified deter- 
 minants are more readily accumulated by amphimixis ; and a 
 germ-plasm which has inherited such determinants from its 
 ancestors may, after these have been still further modified, 
 give rise to a bud in which, by a fortuitous differential nuclear 
 division, the modified determinants are in the majority, and can 
 thus become effective. 
 
 Unfortunately it has not been observed whether complicated 
 modifications, like that of the moss-rose, owe their origin to bud- 
 variation. This is quite possible theoretically, for the invisible 
 preliminary to variation the modification of certain determin- 
 ants is just as likely to affect a single determinant as a whole 
 group ; even the formation of two or more new determinants, 
 by the multiplication of a primary one, is quite as possible here 
 as in the case of the ordinary transformation of species. The 
 sudden appearance of such modified groups of determinants is 
 due therefore to fortuitous differential nuclear division. Such 
 cases prove that the preparation for the modification is a slow 
 process, for it appears impossible to conceive of any cause pro- 
 ducing a sudden variation of an entire group of determinants by 
 any method. 
 
 The extent of any spasmodic variation will depend on the 
 extent to which the various groups of determinants have been 
 permanently exposed to abnormal nutrition. It would doubt- 
 less be as yet premature to bring their qualitative characteristics 
 into any causal relation with definite influences producing 
 variation. We can only state the necessity of assuming, a 
 priori, that the extraordinarily complex germ-plasm is provided 
 with special means for the transmission of nutrient fluid, 
 the increase or diminution of which must produce purely local 
 differences of nutrition ; and that, on the other hand, the vital
 
 444 THE GERM-PLASM. 
 
 units must, owing to slight changes in their structure, modify 
 the characteristics of the part of the body which they represent, 
 in some inexplicable way. It is not yet possible, for instance, to 
 give an exact account of the changes in the group of determinants 
 for the legs which led to the sudden production of a bow-legged 
 'otter '-sheep, or of the modifications of certain determinants in 
 the rudiments of the leaves which caused the formation of the 
 notches characteristic of a certain variety of birch. 
 
 b. The Transmission of these Variations, 
 The transmission oi 'sports' in plants has hitherto been a 
 very obscure problem. Seed variations are often propagated by 
 means of seeds, but cannot always be thus reproduced, or at 
 least only in individual cases. Bud-variations can usually only 
 be propagated by cuttings or grafting, but some instances are 
 known in which they have also been reproduced by seeds, 
 though only in a certain percentage of cases. The origin of 
 these irregularities was unknown, and no previous theory of 
 heredity could offer a reason for them ; all that could be said 
 was, that in these instances transmission was very capricious. 
 The theory of the germ-plasm, however, offers a very simple 
 explanation of them. We will first consider certain facts 
 recorded by Darwin, which are very valuable in respect of this 
 theory. 
 
 Darwin states that 'when a new peculiarity first appears, 
 we can never predict whether it will be inherited.'* If both 
 parents exhibit the variation, ' the probability is strong that it 
 will be transmitted to at least some of their offspring.' Bud- 
 variations are propagated to a much smaller extent than seed- 
 variations, but the power of transmission often appears to be 
 very capricious, inasmuch as one and the same modification 
 on a single plant is transmitted by seed in one case and not in 
 another. Thus vain attempts were for a long time made to pro- 
 pagate the weeping ash by seeds. Over twenty thousand seeds 
 developed into trees of the ordinary form, but offspring with 
 hanging boughs were finally raised from the seeds of another 
 specimen of this variety. The same ash-tree, however, did not 
 transmit this character to all its offspring, but only to a certain 
 
 * 'The Variation of Animals and Plants under Domestication,' Vol. I., 
 London, 1888, p. 460.
 
 VARIATION. 445 
 
 percentage of them ; and Darwin mentions a famous weeping 
 oak at Moccas Court which transmitted its special character to 
 all its seedlings, though in varying degrees. 
 
 According to our theory, the transmission of a variation by 
 seeds depends on the presence of a corresponding modification 
 in the majority of the determinants in the germ- plasm of the 
 seeds. If the germ-plasm contains a hundred ids, the con- 
 trolling forces of which are equal, more than fifty determinants 
 N must be transformed into N 1 before the modification would 
 be perceptible in the seedling. Since therefore, as was re- 
 marked above, new variations probably never appear simultane- 
 ously in all the determinants, but only in a varying percentage 
 of the ids, the chances are greatly against all the seedlings 
 produced by the transformed plant exhibiting this modification. 
 For every germ-cell has undergone a ' reducing division," and 
 hence many of them will always only possess a minority of the 
 modified determinants N 1 , and this may even be a very small 
 one if the majority in the germ-plasm of the parent-plant was 
 small. When two such cells unite in the process of amphimixis, 
 the resulting germ-plasm contains only a small minority of 
 modified determinants N 1 , and the modification is inappreciable. 
 This accounts for the fact that the seedlings of a variety hardly 
 ever reproduce the variety in all cases ; and that in rarer in- 
 stances, such as that of the weeping oak already mentioned, 
 all the seedlings may exhibit the corresponding variation, al- 
 though in varying degrees. For the composition of the germ- 
 plasm must differ in each of them in consequence of the 
 processes of ' reducing division' and amphimixis, even when the 
 parent-plant contains only a small but ever varying minority of 
 ancestral determinants. 
 
 We can, moreover, easily account for the fact that seedlings 
 of a variety, such as the balsamine, may resemble the parent 
 plant, without transmitting the character of the variety to their 
 offspring. In this case all the daughter-plants of the variety 
 in question must retain a majority of the modified determinants, 
 but in very varying degrees. In those which contain a very 
 large majority in their germ-plasm, it necessarily follows that the 
 larger number of the germ-cells produced must contain a majo- 
 rity of modified determinants ; but in the case of those in which 
 the proportions are more equal, the chances are in favour of the 
 seedlings containing only minorities of these determinants.
 
 446 THE GERM-PLASM. 
 
 Darwin mentions that only thirty per cent, of the seedlings 
 of the wild variegated variety of Ballota nigra possessed the 
 variegated leaves of the parent plant ; but sixty per cent, in 
 the second generation were variegated. This is also quite in 
 accordance with our theory ; for the determinants for the leaves 
 in the mother-plant cannot all have been modified, but only the 
 majority ; these would then become differently grouped by means 
 of the reducing divisions of the germ-cells. A seed reproduces 
 either the variety or original form according to whether modified 
 determinants, which were brought together in fertilisation, con- 
 stituted the majority or minority. As in the instance first quoted, 
 variegated plants were alone retained for cultivation in the 
 second generation, and consequently a greater number of the 
 determinants of the variety were brought together at fertilisation : 
 the percentage of the variety was therefore bound to rise in the 
 third generation. This proportion would have increased still 
 further if a fourth and fifth generation had been raised in the 
 same way ; for in those seeds which yield variegated plants 
 the germ-plasm must of necessity contain more modified deter- 
 minants than in the case of those producing the original form. 
 Hence with every generation the chances of an increased 
 majority of modified determinants become greater ; and I have 
 no doubt that in this instance, by the constant selection of the 
 most variegated plants for further cultivation, a ' pure ' variegated 
 race might eventually have been obtained, which would have 
 transmitted its character to the great majority of its offspring 
 or as we usually express it 'to all its offspring.' 
 
 The fact that many weeping ashes transmit their special 
 character to many, but not all, the seedlings is also attributable 
 to the diverse effects of the 'reducing division' and amphi- 
 mixis, the former of which- causes the introduction into the 
 germ-plasm of a large majority of modified determinants, and 
 the latter of only a slight one. The manner in which the 
 'variety arises must here, however, be taken into account. Bud- 
 variations are much more rarely reproduced by seeds than are 
 -variations arising from seeds ; but, on the other hand, they can 
 almost always be propagated by grafting, budding, or by cuttings. 
 Hence the capacity of a certain individual for transmitting the 
 variation to its seeds may be owing to its having arisen from 
 a seed, while in another case the same variation arose in a bud, 
 and it could therefore rarely or never be transmitted by seeds.
 
 VARIATION. 447 
 
 The most difficult point to explain theoretically, is why bud- 
 variation is not generally, but only occasionally, transmitted by 
 seeds. Even this may to a certain extent be accounted for by 
 the present theory. 
 
 A bud is a growing point, enveloped by scales. It arises 
 from the apical cells, which produce the other cells of the shoot 
 by means of continual division, and these form the interfoliar 
 parts, leaves, and flower-stocks. In accordance with the 
 principle of the continuity of the germ-plasm, a part of the 
 ' blastogenic ' germ-plasm of the apical cells must be transmitted 
 in an ' unalterable ' (' gebundenen ') condition to certain cellr 
 of the shoot as accessory idioplasm (' Neben-Idioplasma') or 
 ' reserve germ-plasm' From these cells it is passed on to the 
 sexual organs, where it is used for the formation of germ-cells. 
 This reserve germ-plasm remains undisintegrated, and is 
 perfectly distinct from, and independent of, the 'blastogenic' 
 germ-plasm, which is gradually distributed during the ontogeny 
 of the shoot into groups of determinants. 
 
 The fact that bud-variations are so rarely transmitted by 
 seeds seems to me to be owing to the cause just mentioned ; 
 for the majority of modified determinants required to make a 
 modification apparent may evidently be present in the 'blasto- 
 genic' germ-plasm, though absent in the reserve germ-plasm. 
 If we remember that these variations have been prepared long 
 beforehand in the germ-plasm, and that at first a few, and then 
 gradually a larger number of determinants become modified in 
 a similar way, and that finally a fortuitous differential nuclear 
 division must, on our assumption, intervene before the ' blasto- 
 genic' germ-plasm of a certain apical cell can contain a majority 
 of modified determinants, it becomes comprehensible why the 
 reserve germ-plasm contained in this cell may behave differ- 
 ently, and contain only a few or none of the detenninants 
 in question in a modified condition. We must not forget that 
 the influences which produce the variation are not those of the 
 nutrition of the bud in which the modification appears, but 
 those which affected the determinants during their long course 
 from the ancestral plant of a past generation to the bud in 
 which they now appear. 
 
 This explanation seems to me to sufficiently account for the 
 fact that the seeds which give rise to 'sports' indirectly, need 
 not necessarily transmit the modification.
 
 44^ THE GERM-PLASM. 
 
 The question as to why this transmission is of such rare 
 occurrence must, however, be considered more closely. I am 
 inclined to seek the causes of this fact in the processes of 
 ' reducing division ' and amphimixis, to which the reserve germ- 
 plasm, but not the ' blastogenic ' germ-plasm, is subjected. Let 
 us suppose that the two were precisely similar at first with 
 respect to their contained modified determinants N 1 , each pos- 
 sessing a small majority of them : the shoot must then exhibit 
 the variation, but only certain germ-cells would contain a 
 majority of N 1 , the rest containing a minority of these deter- 
 minants in consequence of the diversity resulting from the 
 reducing division. It is true that two germ-cells containing 
 majorities of N 1 might unite in fertilisation ; but this extremely 
 favourable case would only occur very rarely, when, as assumed, 
 the majority in the reserve germ-plasm was only a slight one ; 
 while the other cases, in which amphimixis leads to N 1 being in 
 the minority, would take place much more frequently. 
 
 It is, however, by no means certain that reserve germ-plasm 
 and ' blastogenic ' germ-plasm must contain a similar percentage 
 of N 1 . The two may very well differ in this respect, and it is 
 extremely probable that a larger percentage of N 1 in the reserve 
 germ-plasm leads to the formation of seeds which give rise to 
 seedlings exhibiting variation. We may therefore suppose that 
 in the one case, when the 'blastogenic' germ-plasm contains 
 even a small majority of N 1 , the modification of the shoot will 
 actually occur ; and that in the alternative case, in which the 
 reserve germ-plasm alone contains a majority of N 1 , variation 
 will take place in a larger or smaller number of seedlings in the 
 following generations. When both contain a majority of N 1 , 
 the shoot must vary, and some of the seeds arising from it 
 must transmit the variation. 
 
 This explanation is not so hypothetical as it might appear. 
 Certain facts point with certainty to the conclusion that these 
 ' sports ' in many cases really contain only a very slight majority 
 of modified determinants. In most bud-variations reversions 
 to the original form are of frequent occurrence, both in the 
 buds themselves and in the generation derived from them either 
 by seeds or buds. This has already been mentioned in the 
 chapter on reversion, and is so well known that a brief state- 
 ment of a few cases will suffice. In the botanical gardens at 
 Bonn, Professor Strasburger showed me an immense horn-
 
 VARIATION. 449 
 
 beam with deeply notched leaves, like those of the oak, a large 
 branch of which had reverted completely, and bore leaves of 
 the ordinary form. I have in my garden a 'fern-leaved' beech, 
 some of the leaves of which have the usual form ; Darwin 
 states, in fact, that some shoots of this variety may produce 
 fern-like leaves, normal leaves, and various intermediate forms. 
 Slight inequalities in the nuclear divisions may in this case 
 displace the controlling resultant of the determinants N and 
 N 1 , supposing that the majority of N 1 is only a small one. 
 
 2 F
 
 SUMMARY AND CONCLUSION. 
 
 ALL the phenomena of heredity depend on minute vital units 
 which we have called ' biophors,' and of which living matter is 
 composed : these are capable of assimilation, growth, and mul- 
 tiplication by division. We are unacquainted with the lowest 
 conceivable organisms, and do not even know if they still exist. 
 But they must at any rate have done so at some time or other, in 
 the form of single biophors, in which multiplication and trans- 
 mission occurred together, no special mechanism for the purposes 
 of heredity being present. A higher order of beings would then 
 have been constituted by those organisms which were composed 
 of a large number of similar biophors. Of these also we have no 
 actual knowledge based on observation, but must suppose that 
 they too required no special apparatus for the processes of 
 transmission ; for a reproduction by binary fission must result 
 in two perfectly corresponding halves, each containing similar 
 biophors, and each of which, simply by the multiplication of 
 these units, is able to give rise to a complete organism exactly 
 like the parent. 
 
 This simple form of transmission must have become modified 
 when the biophors underwent differentiation in connection with 
 a division of labour, and became combined in various ways to 
 form the body of the organism. These two kinds of hypothetical 
 beings might be respectively distinguished as homo-biophorids 
 and hetero-biophorids. Not only might a firmer cortex and 
 softer internal substance be present in the latter, but a differen- 
 tiation into anterior, posterior, dorsal, and ventral regions might 
 occur ; several layers of the body substance, differing structurally 
 and functionally from one another, might also be developed, to- 
 gether with motile and non-motile processes such as flagella, 
 ci!ia ; spines, and hooks, like those present amongst the Infu-
 
 SMMMARY AND CONCLUSION. 451 
 
 soria ; and there might, moreover, be a permanent aperture in 
 the firmer outer layer through which solid food passed into the 
 interior, and so on. 
 
 When the body thus became constructed, in a more or less 
 complex manner, of various kinds of biophors arranged in a 
 definite manner, simple binary fission no longer sufficed for the 
 transmission of the characters of the parent to the offspring. 
 If the parts situated in the anterior, posterior, right, left, 
 dorsal, and ventral regions differed from one another, all the 
 elements/.*?., all the kinds and groups of biophors could not, 
 by any method of halving, be transmitted to both the offspring 
 resulting by division so that they could develop by mere growth 
 into an organism resembling the parent. Special means must 
 then have been adopted to render such a completion and conse- 
 quent perfect transmission possible ; and this was attained 
 by the formation of a nucleus. 
 
 We may, with de Vries, regard the cell-nucleus as having 
 originally served merely for the storage of reserve biophors, 
 which were destined to become doubled on the division of the 
 organism, each half rendering the completion to an entire 
 individual possible when those kinds of biophors which were 
 wanting were transferred to it. Subsequently that is, in the 
 multicellular organs possessing highly differentiated cells the 
 nucleus took on other functions, which regulated the specific 
 activity of the cell, though it still retained biophors capable of 
 supplying the characters of the cells which were still wanting, 
 and therefore still served as the bearer of the biophors con- 
 trolling the character of the cell. 
 
 If, therefore, a special apparatus for transmission became 
 necessary in the hetero-biophorids or unicellular organisms, and 
 appeared in the 'cell' in the form of a 'nucleus] it must 
 have become still more complex on the introduction of the 
 remarkable process. of amphimixis, which, in its simplest and 
 original form, consists in the complete fusion of two organ- 
 isms in such a manner that nucleus unites with nucleus and 
 cell-body with cell-body. In the higher unicellular organisms 
 this process is in most cases restricted to the fusion of the 
 nuclei, half the nucleus of one animal uniting with half that of 
 another. The process of division shows that the nucleus has a 
 structure precisely analogous to that of the nucleus in multi- 
 cellular organisms ; we may therefore assume that the hereditary
 
 452 THE GERM-PLASM. 
 
 substance here likewise consists of several equivalent groups of 
 biophors, constituting ' nuclear rods ' or ' idants,' each of which 
 contains all the kinds of biophors of the organism, though they 
 deviate slightly from one another in their composition, as they 
 correspond to individual variations. Half the idants of two 
 individuals become united in the process of amphimixis, and 
 thus a fresh intermixture of individual characters results. 
 
 The apparatus for transmission in those multicellular organ- 
 isms in which the cells have undergone a division of labour, 
 is essentially similar to that seen in unicellular beings ; although, 
 in correspondence with the greater complexity of their structure, 
 it is more complicated. As the process of amphimixis occurs 
 in them also, and the fusion of highly-differentiated multicellular 
 individuals seems only to be possible by a temporary return to 
 the unicellular condition, we find that the so-called ' sexual 
 reproduction,' which is of general occurrence amongst them, 
 consists in all the primary constituents (' Anlagen ') of the entire 
 organism being collected together in the nuclear matter of a 
 single reproductive cell. Two kinds of such cells, which are 
 differently equipped, and mutually attract one another, then 
 unite in the process of amphimixis, and constitute what we are 
 accustomed to call the ' fertilised egg-cell,' which contains the 
 combined hereditary substances of two individuals. 
 
 According to our view, this hereditary substance of the multi- 
 cellular organisms consists of three orders of vital units, the 
 lowest of which is constituted by the biophors. In the uni- 
 cellular forms a more or less polymorphic mass of biophors, 
 having a definite arrangment, constitutes the individual nuclear 
 rods or idants, several of these making up the hereditary sub- 
 stance of the nucleus which controls the cells ; and similarly 
 in these higher forms, groups of biophors, arranged in a certain 
 order, constitute the primary constituents of the individual cells 
 of the body, and together form the second order of vital units, 
 the cell-determinants, or simply, the ' determinants} 
 
 The histological character of every cell in a. multicellular 
 organism, including its rate and mode of division, is controlled 
 by such a determinant. The germ-plasm does not, however, 
 contain a special determinant for every cell ; but cells of a simi- 
 lar kind, when, like the blood-cells, they are not localised, may 
 be represented by a single determinant in the germ-plasm. On 
 the other hand, every cell, or group of cells, which is to remain
 
 SUMMARY AND CONCLUSION. 453 
 
 independently variable, must be represented in the germ-plasm 
 by a special determinant. Were this not the case, the cell in 
 question could only vary in common with other cells which are 
 controlled by the same determinant. 
 
 7*he germ-cell of a species must contain as many determinants 
 as the organism has cells or groups of cells which are inde- 
 pendently -variable from the germ onwards, and these de- 
 terminants must have a definite mutual arrangement in the 
 germ-plasm, and must therefore constitute a definitely limited 
 aggregate, or higher vital unit, the '/#.' 
 
 From the facts of sexual reproduction and heredity we must 
 conclude that the germ-plasm contains many ids, and not a 
 single one only. The formation of hybrids proves that the two 
 parents together transmit all their specific characters, so that in 
 the process of fertilisation each contributes a hereditary sub- 
 stance which contains the primary constituents of all parts of 
 the organism, that is, all the determinants required for build- 
 ing up a new individual. The hereditary substance becomes 
 halved at the final stage of development of the germ-cells, and 
 consequently all the determinants must previously have been 
 grouped into at least two ids. But it is very probable that 
 many more ids are usually present, and that in many cases their 
 number far exceeds a hundred. 
 
 It cannot be stated with certainty which portions of the ele- 
 ments of the germ-plasm observable in the nucleus of the ovum 
 correspond to ids, though it is probable that only parts of, and 
 not the entire 'chromosomes,' are to be regarded as such. 
 Until this point can be definitely decided, our further detailed 
 deductions will be based on the view that the nuclear rods 
 (chromosomes) are aggregates of ids, which we speak of as 
 l tdants.' In a certain sense, the latter are also vital units, for 
 they grow and multiply by division ; and the combination of ids 
 contained in them, although not a permanent one, persists for 
 some time. 
 
 The l germ-plasm^ or hereditary substance of the Metazoa 
 and Metaphyta, therefore, consists of a larger or smaller number 
 of idants, which in turn are composed of ids ; each id has 
 a definite and special architecture, as it is composed of de- 
 terminants, each of which plays a perfectly definite part in 
 development. 
 
 The development of the primary constituents, contained
 
 454 THE GERM-PLASM. 
 
 in the germ-plaun of the reproductive cell, takes place in 
 the course of the cell-divisions to which the ontogeny of a 
 multicellular organism is due, in which process all the ids 
 behave in an exactly similar manner. In the first cell-division 
 every id divides into two halves, each of which contains only 
 half the entire number of determinants ; and this process of 
 disintegration is repeated at every subsequent cell-division, so 
 that the ids of the following ontogenetic stages gradually become 
 poorer as regards the diversity of their determinants, until they 
 finally contain only a single kind. 
 
 Each cell in every stage is in all cases controlled by only one 
 kind of determinant, but several of the same kind may be con- 
 tained in the id ; and the ' control ' of the cell is effected by the 
 disintegration of the determinants into biophors, which pene- 
 trate through the nuclear membrane into the cell-body ; and 
 there, according to definite forces and laws of which we are 
 ignorant, bring about the histological differentiation of the cell, 
 by multiplying more rapidly at the expense of those biophors 
 already forming the cell-body. Each determinant must become 
 ' ripe,' and undergo disintegration into its biophors, at a definite 
 time or at a certain stage of ontogeny. The rest of the deter- 
 minants in the id of a cell, which are destined for subsequent 
 stages, remain intact, and have therefore no effect on the control 
 of the cell ; but the mode of their arrangement in the id, and the 
 special rate of multiplication of each kind, determine the nature 
 of the next nuclear division that is, as to which determinants 
 are to be distributed to one daughter-cell, and which to the 
 other. The histological nature of these two cells, as well as the 
 control of their successors, is determined by this division ; and 
 thus the distribution of the primary constituents contained in the 
 germ-plasm is effected by the architecture of the id, which is at 
 first of a definite kind, but afterwards undergoes continual and 
 systematic changes in consequence of the uneven rate of multi- 
 plication and gradual disintegration of the ids. 
 
 The apparatus for cell-division is only of secondary import- 
 ance in the process ; its chief part, the ' centrosome,' like the 
 hereditary substance, is derived from the parental germ-cell or 
 cells, but only constitutes the mechanism for the division of the 
 nucleus and cell, and contains no ' primary constituents.' The 
 rate of the cell-divisions cannot, moreover, be determined by 
 the centrosome, although it produces the required stimulus : the
 
 SUMMARY AND CONCLUSION. 455 
 
 apparatus for division is set in motion by the cell, which is con- 
 trolled by the idioplasm. Were this not the case, the nuclear 
 matter could not be the hereditary substance, for most of the 
 hereditary characters of a species are due in a less degree to the 
 differentiation of individual cells than to the number and group- 
 ing of the cells of which a certain organ or entire part of the 
 body consists ; these, however, again depend on the mode and 
 rate of cell-division. 
 
 The processes occurring in the idioplasm which direct the 
 development of the organism from the ovum or to speak in 
 more general terms, from one cell, the germ-cell, do not 
 in themselves furnish an explanation of a series of phenomena 
 which are in part directly connected with the ontogeny, or else 
 result from it sooner or later : the phenomena of regeneration, 
 gemination, and fission, and the formation of new germ-cells, 
 all require special supplementary hypotheses. 
 
 The simplest cases of regeneration are due to the fully 
 formed tissue, consisting of similar cells, always containing 
 a reserve of young cells, which are capable of replacing a 
 normal or abnormal loss. This, however, is insufficient in 
 the more complex cases, in which entire parts of the body, 
 such as the tail or the limbs, are regenerated when they 
 have been forcibly removed. We must here assume that the 
 cells of the parts which are capable of regeneration contain 
 ' supplementary determinants ' in addition to those which 
 control them, and that these are the primary constituents 
 of the parts which are to be formed anew in the process 
 of regeneration. They are supplied to certain parts of the 
 body at an earlier ontogenetic stage in the form of 'inactive 
 accessory idioplasm," and only become active when the opposi- 
 tion to growth has been removed in consequence of the loss 
 of the part in question. The equipment of a cell of any 
 part with supplementary determinants presupposes a greater 
 complexity in their distribution, in correspondence with the 
 greater complexity in structure of the part ; and thus the 
 capacity for regeneration is limited, for a part can no longer 
 be provided with an apparatus for regeneration when its 
 structure is too complicated. The ordinary assumption that 
 the regenerative 'force ' decreases as the complexity in structure 
 increases, is therefore to a certain extent true, but not if it 
 implies the existence of a special force which provides for
 
 456 THE GERM-PLASM. 
 
 regeneration, and which always diminishes in correspondence 
 with the degree of organisation. Even if we imagine this 
 'force' to be a mechanico-physiological one, it could not be 
 considered as a primary quality of the organism, and to 
 some extent the inevitable result of life itself, but must be 
 looked upon as an adaptation. 
 
 Reproduction by fission is closely connected with regeneration ; 
 it presupposes the existence of a similar apparatus in the idio 
 plasm, which, however, has in most cases reached a higher 
 stage of development ; fission must have arisen phyletically from 
 regeneration. 
 
 The origin of multiplication by gemmation, and the pheno- 
 mena exhibited by this form of reproduction, are different 
 from those concerned in fission. In plants and Ccelenterates, 
 gemmation originates in one cell, which must consequently 
 contain a combination of all the determinants of the species 
 closely resembling that existing in the fertilised ovum. In 
 the Polyzoa, however, this process does not originate in one 
 cell, but in at least two, and probably more, belonging to 
 two different layers of cells (germinal layers) of the body; 
 and in Tunicata, again, the material for the bud is produced 
 from all three germinal layers. 
 
 The first of these forms of budding must be primarily due to 
 the admixture of ' unalterable ' (' gebundenem ') germ-plasm to 
 certain series of cells in ontogeny in the form of inactive ' accessory 
 idioplasm] or l blastogenic' idioplasm ('Knospungs-Idioplasma')- 
 In plants this is contained in the apical cells ; and in hydroid 
 polypes, in the cells of the ectoderm. 
 
 In the second group of animals mentioned above, we must 
 assume that the ' blastogenic ' germ-plasm becomes disintegrated 
 into two groups of determinants at an early ontogenetic stage, 
 and that each of these is passed on in an ' unalterable ' condition, 
 through various generations of cells, until the time and place of 
 its activity are reached. 
 
 In the third group, the inactive ' blastogenic ' idioplasm 
 divides into three groups of determinants, one of which passes 
 into the ectoderm, the second into certain cell-series of the 
 mesoderm, and the third into others in the endoderm, until 
 they reach the part in which they have to become active. 
 
 Gemmation must have originated phyletically by a doubling 
 of the germ-plasm taking place in the fertilised egg, so that
 
 SUMMARY AND CONCLUSION. 457 
 
 one half remained inactive, and was then either passed on 
 as inactive ' blastogenic ' germ-plasm, or else became divided 
 up in the course of ontogeny into groups, which were passed 
 separately to the same region, viz., that of the bud. 
 
 In all cases in which the power of budding was permanently 
 retained by the species, the occurrence of this process of 
 doubling of the germ-plasm seems to have persisted through 
 the ontogenetic stages from an early period ; for we find that 
 the individuals arising by gemmation very frequently vary in- 
 dependently of one another, and often even to a great extent. 
 But independent variation from the germ onwards implies 
 the existence of special determinants in the 'blastogenic' 
 germ-plasm. Medusae could never have been produced from 
 polypes by budding if independently variable determinants 
 of the buds had not been present in the germ of the fertilised 
 ovum. We therefore assume that two kinds of germ-plasm 
 exist in those species in which alternation of generations occurs, 
 both of which are present in the egg-cell as well as in the 
 bud, though only one of them is active at a time and controls 
 ontogeny, while the other remains inactive. The alternating 
 activity of these two germ-plasms causes the alternation of 
 generations. 
 
 The formation of germ-cells is brought about by the occur- 
 rence of similar processes in the idioplasm to those which cause 
 gemination. One part of the germ-plasm contained in the ferti- 
 lised egg-cell remains inactive and ' unalterable,' that is, it does 
 not immediately become disintegrated into groups, but is passed 
 on in the form of accessory idioplasm to certain series of cells 
 in ontogeny, and thus reaches the parts in which germ-cells are 
 to be formed. Thus the whole of the parental germ-plasm, 
 with all its determinants, forms the foundation of the germ-cells 
 which will give rise to the next generation, and the extremely 
 accurate apd detailed transmission of parental characters to the 
 offspring is thereby rendered comprehensible. 
 
 In multicellular plants and animals, the germ-plasm becomes 
 more complex in consequence of sexual reproduction, in which 
 process the ids of two different individuals, the parents, are 
 accumulated in the fertilised egg-cell every time amphimixis oc- 
 curs. This has caused the occurrence of the ' reducing division,' 
 which accompanies the formation of male and female germ-cells, 
 and results in the number of ids and idants being reduced to
 
 45^ THE GERM-PLASM. 
 
 the half. The reduction is important in elucidating the pheno- 
 mena of heredity in forms which are reproduced sexually, for the 
 ids of a germ-plasm are not by any means all alike, but differ 
 to the same relative extent as do the corresponding individuals. 
 As the reduction does not always occur in the same way, and 
 the resulting halves contain different idants on different occa- 
 sions, and these fall to the share of individual germ-cells, it is 
 possible for the germ-cells of one individual to contain very 
 different combinations of idants. This results in the dissimi- 
 larity between the offspring of the same parents, or, to express 
 it in more general terms, in the extreme diversity as regards the 
 intermixture of individual differences. 
 
 During the development of a new individual from the fertilised 
 egg-cell, the ontogeny is directed by the ids of the two parents 
 which constitute the germ-plasm. Structures intermediate 
 between those of the parents thus frequently arise but only 
 when perfectly homologous ids are opposed to one another, 
 and have a similar 'controlling force.' This force depends 
 not only on the similar rate of multiplication of the biophors 
 transmitted by the controlling determinants into the cell-body, 
 and on the suppression of those already present, but also on the 
 number of precisely similar determinants derived from each 
 parent. The larger the number of ' homodynamous ' deter- 
 minants, the greater is their controlling effect on the cell ; and 
 if a larger number of homodynamous determinants are opposed 
 to fewer heterodynamous determinants of the other parent, the 
 former gain the victory. The preponderance of one of the 
 parents in transmission is thus rendered comprehensible, whether 
 it concerns individual parts or the entire organism. 
 
 The type of the child is determined by the paternal and 
 maternal ids contained in the corresponding germ-cells meeting 
 together in the process of fertilisation, and the blending of 
 parental and ancestral characters is thus predetermined, and can- 
 not become essentially modified by subsequent influences. The 
 facts relating to identical twins and to plant-hybrids prove that 
 this is so. In the latter, the individuals produced by crossing two 
 constant species display as constant an intermixture of charac- 
 ters as would be the case if they constituted a natural species. 
 The ids of each species must be looked upon as perfectly 
 homodynamous as regards the specific characters ; two distinct 
 groups of homodynamous ids are opposed to one another, and
 
 SUMMARY AND CONCLUSION. 459 
 
 the preponderance of one or other parental group in any parti- 
 cular part of the plant depends on the presence of a larger 
 number of homodynamous determinants representing the part in 
 question, and on their possession of a greater controlling force. 
 
 Reversion to grandparents and great-grandparents, or to . 
 uncles and aunts, may be accounted for by the fact that, in the 
 first place, the idants and ids are not formed anew in the germ- 
 plasm of the parents, but are derived from the grandparents ; 
 and, secondly, that the combination of ids contained in the 
 individual germ-cells of the parent becomes very diversified in 
 consequence of the ' reducing division.' The usually accepted 
 assumption of breeders that one-fourth of the 'blood' of the 
 grandchild is derived from each of the four grandparents, and 
 one-eighth from each of the eight great -grandparents, is there- 
 fore inaccurate. The number of ids of any particular ancestor 
 which are contained in the germ-plasm of a ripe germ-cell 
 depends entirely on the manner in which the reducing division 
 occurs ; and, under certain circumstances, a germ-cell might 
 presumably contain half the entire number of ids of one grand- 
 parent, and none of those of the other three. The larger the 
 number of ids derived from an ancestor, the greater is the pro- 
 bability that some of the characters of this ancestor will appear 
 in the descendant ; but this depends on the force of the ids of 
 the other parent, which comes into play when amphimixis takes 
 place, and also on whether the ids derived from this ancestor are 
 the dominant ones which determined his ' type ' (' Bild '). 
 
 Reversion to an ancestor must consequently always occur 
 when, in consequence of the 'reducing division,' the ids deter- 
 mining the type of this ancestor reach a particular germ-cell 
 of the individual in question, if they are not opposed by a 
 stronger group of ids derived from the other parent in the 
 process of amphimixis. This holds good for each individual 
 part of the offspring, as well as for the entire organism, for the 
 number of homodynamous determinants may be, and generally 
 is, different in the various parts, at any rate in the case of the 
 individual differences between human beings. 
 
 From this theory, it could be predicted that hybrid-plants 
 fertilised with their own pollen must produce very variable 
 offspring, and that individuals of these hybrids must, moreover, 
 revert to one or other of the ancestral species : both these state- 
 ments are borne out by fact.
 
 460 THE GERM-PLASM. 
 
 Although reversion to more remote ancestors is also brought 
 about by the same factors, viz., the 'reducing division' and 
 amphimixis, it requires further elucidation. The theory of 
 selection requires that only a majority, and not all, the deter- 
 minants of a part which is to be modified shall undergo a 
 corresponding change. Old unmodified determinants of various 
 parts are therefore retained in the germ-plasm of a species, and 
 can only be removed from it very gradually by fortuitous 
 'reducing divisions.' This renders reversion to the characters 
 of very remote ancestors possible ; its occurrence, however, 
 depends upon the reducing division and amphimixis taking 
 place in a favourable manner. If the reduction causes similar 
 groups of ancestral determinants to be brought together in 
 several ids, and this germ-plasm, in the process of amphimixis, 
 unites with that of another germ-cell, which also contains 
 similar ancestral determinants in several ids, these may gain 
 the victory over the modern determinants in the struggle of 
 the ids during ontogeny. This, however, will chiefly depend on 
 the kind and strength of the modern determinants which are 
 opposed to them ; and thus reversion to ancestral characters 
 occurs very frequently in crosses between races (pigeons) and 
 species (mules), in which the modern determinants are heterody- 
 namous ; they do not co-operate, and their forces counteract 
 one another, while the ancestral determinants are similar and 
 their forces cumulative. 
 
 Numerous phenomena of reversion in plants and animals 
 may be explained in a very simple manner on these principles, 
 and from this point of view it is also possible to understand 
 that form of reversion which occurs in gemmation and partheno- 
 genesis. The more remote the ancestors to the characters of 
 which reversion occurs, the more rarely will it take place. 
 Reversion to the three-toed ancestors of the horse, for instance, 
 is of extremely rare occurrence, for it is due to a retention of 
 the ancestral determinants in question which have certainly 
 disappeared from all the ids in the germ-plasm of most existing 
 horses in single individuals of certain series of generations, 
 and to the chance of the coming together of two germ-cells 
 containing such ancestral determinants. 
 
 The remarkable phenomenon of dimorphism, which has been 
 introduced so extensively more especially into the animal king- 
 domby means of sexual reproduction, must be due to the
 
 SUMMARY AND CONCLUSION. 461 
 
 presence in the idioplasm of double determinants for all those 
 cells, groups of cells, and entire organisms, which are capable 
 of taking on a male and female form. But only one half of such 
 a double determinant remains inactive, while the other becomes 
 active. The sexual differentiation of the germ-cells must thus be 
 due to the presence of spermatogenetic and oogenetic double 
 determinants ; and even all the secondary sexual characters must 
 be traced to a similar origin in the idioplasm. The corresponding 
 double determinants are contained not only in the germ-plasm, 
 but are passed on through the cell-stages of ontogeny to that 
 part of the body in which the two characters become separated 
 from one another. One of the determinants then becomes 
 active, its twin half remaining in an inactive condition in the 
 nucleus of a somatic cell, and under certain circumstances 
 becoming active subsequently. This, however, only occurs 
 exceptionally, in such cases as that in which a female animal 
 (e.g., a hen or duck) developes male characters in consequence 
 of castration. Hermaphrodite bees, in which the whole body 
 consists of the most wonderful intermixture of male and female 
 parts, furnishes an instructive proof of the presence of both 
 kinds of characters in all parts of the body, and consequently 
 of the truth of the assumption of double determinants. 
 
 Double determinants not only occur individually, but entire 
 groups of male and female determinants are opposed to one 
 another, and these are just as dependent on one another as are 
 the two halves of the individual double determinant, one of 
 which always remains inactive when the other becomes active. 
 These groups may be very dissimilar ; in many cases (e.g., the 
 olfactory organs of male crustaceans and the ornamental 
 feathers of male birds) the male group contains many more 
 individual determinants than the female. One half of the 
 double group may also become degenerated, so that the corre- 
 sponding organ (e.g., the wing in many female butterflies) 
 disappears in one sex. 
 
 The number of double determinants reaches its highest limit 
 when the two sexes differ completely from one another in all 
 their parts, as is the case in Bonellia viridis; even then, how- 
 ever, a number of single determinants may still be present, if, as 
 in this case, the larval stage is similar in both sexes. 
 
 The assumption of double determinants is also able to throw 
 some light upon certain enigmatical phenomena of heredity
 
 462 THE GERM-PLASM. 
 
 exhibited by human beings. It has long been known that 
 hcemophilia occurs in men only, but is transmitted by women. 
 If we assume that the visible sexual differences, as well as those 
 existing in the system which we are unable actually to recognise, 
 are due to the presence of double determinants, the peculiar 
 limitation of this uncommon disease to one sex is explained. 
 The disease, like a secondary sexual character, is only trans- 
 mitted to the sex in which it first appeared, for this half of 
 the double determinants of the ' mesoblast germ ' has alone been 
 modified by the disease. 
 
 The sexual polymorphism exhibited by certain butterflies may 
 also be explained by assuming the presence of double deter- 
 minants of several local varieties of the same species which 
 interbreed with one another. The polymorphism of bees and 
 other animals which form communities, requires, however, the 
 assumption of triple or quadruple determinants. In these 
 animals the female half of the double determinant again becomes 
 doubled, and this may also be the case as regards the male half 
 (Termites). 
 
 Lastly, the assumption of double determinants in the idio- 
 plasm accounts for temporary dimorphism, such as seasonal 
 dimorphism. 
 
 The occurrence of dimorphism is in all cases attributable to 
 the presence of two kinds of determinants ; but the causes which 
 determine which of the two is to become active, are extremely 
 varied, and cannot in many cases be accurately indicated. 
 The determining influences, however, are always external ones 
 such as fertilisation, nutrition, and the effect of light in cases 
 of dichogeny in plants. 
 
 It is self-evident from the theory of heredity here propounded, 
 that only those characters are transmissible which have been 
 controlled i.e., producedby determinants of the germ, and 
 that consequently only those variations are hereditary which 
 result from the modification of several or many determinants 
 in the germ-plasm, and not those which have arisen subse- 
 quently in consequence of some influence exerted upon the cells 
 of the body. In other words, it follows from this theory that 
 somatogenic or acquired characters cannot be transmitted. 
 
 This, however, does not imply that external influences are 
 incapable of producing hereditary variations ; on the contrary, 
 they always give rise to such variations when they are capable
 
 SUMMARY AND CONCLUSION. 463 
 
 of modifying the determinants of the germ-plasm. Climatic 
 influences, for example, may very well produce permanent 
 variations, by slowly causing gradually increasing alterations 
 to occur in certain determinants in the course of generations. 
 An apparent transmission of somatogenic modifications may 
 even take place under certain circumstances, by the climatic 
 influence affecting certain determinants of the germ-plasm at 
 the same time, and when they are about to pass to that part of 
 the body which they have to control. This is indicated by the 
 climatic variations of the butterfly Polyommatus phlaas. 
 
 The primary cause of variation is always the effect of 
 external influences. Were it possible for growth to take 
 place under absolutely constant external influences, variation 
 would not occur ; but as this is impossible, all growth is con- 
 nected with smaller or greater deviations from the inherited 
 developmental tendency. 
 
 When these deviations only affect the soma, they give rise to 
 temporary non-hereditary variations ; but when they occur in 
 the germ-plasm, they are transmitted to the next generation and 
 cause corresponding hereditary variations in the body. 
 
 Since the germ-plasm undergoes a very considerable growth 
 from the fertilised egg-cell to the germ-cells of the offspring, 
 minute fluctuations continually take place in the composition 
 of its vital units, the biophors and determinants. If permanent 
 and constant influences, such as those of climate, act upon them, 
 these minute fluctuations will become accumulated in the course 
 of time and generations, and may thus give rise to appreciable 
 individual variations, and then gradually to racial, and even 
 perhaps to specific characters. If an influence acts in a certain 
 direction for a short time only, it alone may or may not give 
 rise to an individual variation in the soma, according to the 
 number of ids of the germ-plasm affected by it. Whenever 
 a majority of ids become modified, a corresponding variation 
 must appear in the soma. As, however, an intermingling of 
 the ids takes place twice, owing to the successive processes 
 of ' reducing division ' and amphimixis, minorities of modified 
 ids may be increased to majorities ; and sexual reproduction 
 may then cause the fluctuating material for invisible variations 
 in the determinants to give rise to perceptible somatic variations, 
 and these are made use of by natural selection, aided by con- 
 stantly recurring amphimixis. The latter process gives natural
 
 464 THE GERM-PLASM. 
 
 selection a choice of innumerable combinations of the most 
 diversified variations, resulting from the constant minute fluctua- 
 tions of all the units in the germ-plasm. 
 
 Strictly speaking, the process of amphimixis alone cannot 
 bring about an increase or decrease in the development of a 
 character; though it may, indeed, establish it more firmly in 
 the germ-plasm by causing an increase in the number of ids, 
 the determinants of which produce the character. An increased 
 development, in the ordinary sense of the term, may, it is true, 
 take place by the extension of a variation over larger areas of 
 the body ; the multiplicity of the ids, and the possibility of the 
 constant production of new idic combinations by the process 
 of amphimixis, accounts for the statement of breeders that the 
 constancy of a character, as well as its increased development, 
 may be affected by selection. The so-called ' individual potency ' 
 must, moreover, be due to the presence of a large number of 
 homodynamous determinants for all the more important char- 
 acters, and it probably results, not only from breeding a race 
 true for a long time, although this is of course necessary, but 
 also from favourable combinations of ids being produced by the 
 processes of reducing division and amphimixis. 
 
 Variations do not, however, depend merely on modifications 
 in the composition of a determinant or group of determinants, 
 but frequently result from a doubling or further multiplication 
 of the latter ; and this must also depend primarily on modified 
 external influences, such as those produced by changes in the 
 nutrition of a part of the germ-plasm. The apparently sudden 
 appearance of parts such as feathers and other epidermic 
 structures, as well as of certain pathological structures, such 
 as the supernumerary fingers and toes of human beings may 
 be explained in this manner. All such variations do not, indeed, 
 actually arise suddenly, but take place gradually in some of 
 the ids, and only suddenly become apparent when they have 
 accumulated to form a majority. 
 
 The suddenness with which variations appear is, in all pro- 
 bability, only apparent in most cases, as is well shown by 
 Hoffmann's experiments on wild plants, in which variations 
 were produced by abnormal conditions of life. The degenera- 
 tion of parts which are no longer required, or have simply 
 become useless, is due to the reduction and final disappearance 
 of the corresponding determinants from the germ-plasm. But
 
 SUMMARY AND CONCLUSION. 465 
 
 as this again depends on the fluctuating variations of these 
 determinants in the different ids, it will not occur to the same 
 extent in all the ids at the same time ; and thus the remains of 
 these reduced determinants are often preserved in individual 
 ids through countless generations, and may occasionally cause 
 reversion to take place when they have accumulated in conse- 
 quence of the 'reducing division' and amphimixis. 
 
 Sudden variations of buds have only been observed in plants 
 which also are, or have been, propagated sexually, and in 
 which the structure of the germ-plasm is therefore just as 
 complex as in species in which sexual reproduction alone 
 occurs. These variations are also due to the effects of dis- 
 similar modifying external influences on the determinants 
 contained in the ' blastogenic ' germ-plasm, which is passed on 
 from cell to cell during the process of growth. It would, how- 
 ever, be impossible to understand why only a single bud out 
 of millions should undergo transformation unless some other 
 cause were also at work. This may be due to occasional 
 irregular nuclear divisions, which would give rise to a similar 
 result to that produced by amphimixis in reproduction by 
 seeds, the modified determinants of individual ids occasionally 
 becoming accumulated and then taking effect. 
 
 The power of transmission of ''sudden 1 variations in plants, 
 which is apparently very capricious, may be easily understood 
 in principle. As the modification never occurs in all the ids 
 of the germ-plasm, but only in many of them, and as this 
 majority may be a slight or a considerable one, the trans- 
 mission of the variation will depend on whether the majority 
 is often obtained and even increased, or whether it becomes 
 diminished, or even entirely lost, during the reducing divisions 
 and amphimixis, when the plant is reproduced by seeds. In 
 the case of the first alternative, the ' sport ' will be transmitted ; 
 while in that of the second, transmission will only occur rarely 
 or not at all. Even details of the apparently enigmatical 
 phenomena of heredity of known ' sport ' varieties such as 
 those of the balsamines, weeping ashes, the variegated variety 
 of Ballota nigra, and others thus receive a very simple 
 explanation. 
 
 The capricious transmission of bud-variations by seeds, 
 which only occurs in the smaller proportion of cases, cannot 
 be explained so easily. It is due to the 'blastogenic' germ- 
 2 G
 
 466 THE GERM-PLASM. 
 
 plasm and the ' reserve germ-plasm ' destined to form germ- 
 cells following different courses, and consequently they will not 
 always contain the same number of modified ids. The in- 
 frequency of the transmission of bud-variations by seeds may 
 depend on the production of a new combination of ids of the 
 'reserve germ-plasm' from which the germ-cells are formed, 
 whenever this formation occurs, owing to the 'reducing divi- 
 sion' and amphimixis. No such occurrence takes place in 
 the ' blastogenic ' germ-plasm as long as merely asexual repro- 
 duction continues. 
 
 According to our view, the power of transmission which is 
 possessed by all organisms, and on which the development of 
 the higher organic forms is based therefore depends on simple 
 growth merely in the case of the very lowest conceivable 
 organisms with which we are not acquainted ; while in all forms 
 which have already undergone differentiation, it results from 
 the possession of a special apparatus for transmission. 
 
 This apparatus first occurs in the unicellular organisms, in 
 which it consists of a substance composed of the different kinds 
 of vital units or biophors, which occur in the substance of the 
 organism, and presumably in a similar proportion ; there are, 
 at any rate, numerous individual biophors of every kind, all of 
 which are arranged together on a definite plan. This substance 
 is surrounded by a membrane, the nuclear membrane, pro- 
 vided with pores, through which the biophors of the nucleus 
 can pass into the cell-body, there to multiply at the expense 
 of the nutritive materials to which the vital particles of the 
 cell-body themselves may become reduced under certain cir- 
 cumstances and to become arranged in virtue of the forces 
 dwelling within them. 
 
 To these processes is due the power possessed by the 
 organism of giving rise by division to two complete individuals 
 of a similiar nature. 
 
 Even at this stage of differentiation, the hereditary substance 
 is rendered more complicated by the process of amphimixis, 
 or mingling of individual differences, in which this substance 
 periodically becomes halved, and is then again completed by the 
 hereditary substance of another individual. The result is, that 
 every part of the organism is represented in the hereditary 
 substance by different varieties of the same kind of biophor,
 
 SUMMARY AND CONCLUSION. 467 
 
 and that, consequently, the individuals subsequently arising by 
 division will be approximately intermediate in structure between 
 the two parents. 
 
 In the multicellular forms exhibiting cell-differentiation, the 
 apparatus for transmission becomes more complicated the more 
 numerous and diversified are the kinds of cells composing the 
 organism. Multiplication can only take place if each individual 
 originates in, and again returns to, the unicellular condition. 
 The division of the entire organism would only result in the 
 production of two unequal halves, which could not of them- 
 selves undergo completion, and would require a special apparatus 
 for the purpose. In order that this may arise, the previous 
 production of an apparatus jor transmission, adapted for 
 reproduction by unicellular germs, is indispensable. 
 
 The production of this apparatus results from the formation 
 of a germ-plasm, i.e., a nuclear substance which contains 
 reserve biophors for the construction of the corresponding 
 cell-body, as well as for the formation of all the cell-bodies 
 of the entire organism ; and all of which are connected together 
 into a definitely arranged structure, in such a manner that the 
 constituent parts share regularly and successively, and not 
 simultaneously, in the control of the cell-body. In order that 
 this result may be produced, the smaller vital units or biophors 
 are combined to form those of the next higher order the deter- 
 minants, each of which controls one kind of cell, and conse- 
 quently includes all the biophors required for the determination 
 of this particular kind of cell. The germ-cell contains at least 
 as many determinants as there are different cells or groups of 
 cells in the fully-formed organism which are capable of being 
 individually determined from the germ onwards. 
 
 Since the process of amphimixis or ' intermingling of indi- 
 vidual differences ' is also retained in multicellular forms, the 
 individual germ-cells must for this reason alone contain a mass 
 of germ-plasm, each unit of which contains all the kinds of 
 determinants of the species in close combination. The heredi- 
 tary substance of the germ-cell thus came to be composed of 
 ids and idants. 
 
 The multiplication of multicellular organisms by fission and 
 gemmation results from a considerable increase in the com- 
 plexity of the apparatus for transmission, in which process not 
 only the determinants required for the control of their own
 
 468 THE GERM-PLASM. 
 
 nature, but also ids of germ-plasm in an unalterable condition 
 in which they are at the time incapable of undergoing 
 disintegration were distributed to certain cells of the body. 
 This addition of latent germ-plasm to certain series of somatic 
 cells also results in the formation of germ-cells in most multi- 
 cellular forms ; while the power of regeneration depends on a 
 systematic addition of certain inactive determinants, or groups 
 of determinants, to certain cells of the body. 
 
 Further complications of the germ-plasm produce the pheno- 
 mena of alternation of generations and the polymorphism often 
 connected with it, as well as the sexual dimorphism always oc- 
 curring in a greater or a less degree in connection with ' sexual 
 reproduction.' All determinants and groups of determinants 
 which exist in two or more forms, must be present in a double 
 or multiple condition in the germ-plasm, and be so arranged 
 that each constituent part only becomes active in turn. This 
 assumption is, however, insufficient in the case of alternation 
 of generations, in which several kinds of germ-plasm must be 
 present and become active in turn. 
 
 Thus an ever increasing complexity of the substance which 
 renders the repetition of the organism possible is gradually 
 produced in the phylogeny of living beings, and eventually 
 reaches so high a degree that it is difficult to believe that 
 such an infinite complexity of structure can actually exist in 
 particles so minute. The more deeply, however, we penetrate 
 into the phenomena of heredity, the more firmly are we con- 
 vinced that something of the kind does exist, for it is impossible 
 to explain the observed phenomena by means of much simpler 
 assumptions. We are thus reminded afresh that we have to 
 deal not only with the infinitely great, but also with the infinitely 
 small ; the idea of size is a purely relative one, and on either 
 hand extends infinity.
 
 INDEX. 
 
 negundo, reversion in, 338 
 
 Actiniae, regeneration in, 127 
 
 Adler, on galls, 219 
 
 Adventitious buds, Sachs's view of, 
 211 ; formation by means of 
 accessory idioplasm, 212, 218 
 
 Alternation of generations, in its re- 
 lation to the idioplasm, 173 ; 
 hereditary variability of individual 
 generations, 174 ; in Daphnidae, 
 175 ; in Aphis, 178 ; in Medusae, 
 179 ; caused by two kinds of 
 germ-plasm, 182, 457 
 
 Amphimixis, or the intermingling 
 of individuals, 20, 24; its relation 
 to reproduction, 232 ; resulting in 
 complications in the composition 
 of the germ-plasm, 235 ; the 
 cause of all reversion, 337 ; the 
 means of producingvariation, 413 ; 
 its relation to bud- variations, 439 ; 
 the cause of an increase or de- 
 crease in the development of a 
 character, 464 
 
 Ancestral germ -plasms not ana- 
 logous to Spencer's ' physiological 
 units,' II 
 
 Antirrhinum majus, Darwin's ex- 
 periments with the peloric form 
 
 of, 331 
 
 Aphis, alternation of generations, 
 178 
 
 Ascaris megalocephala, the chromo- 
 somes of7 86 ; diagram, fig. 18, 
 page 233 
 
 Asciflians, regeneration, 139; eggs 
 contrasted with those of frogs, 
 141 ; gemmation, 160 ; com- 
 parison of gemmation with em- 
 bryogeny, 162 
 
 Auerbach, on nuclear division, 23 
 
 BALFOUR, phylogeny of gemmation, 
 169 
 
 Balsamine, transmission of varia- 
 tions of, 445 
 
 Baumann, chemical constitution of 
 protoplasm, 40 
 
 Bees, polymorphism in, 376 
 
 Begonia, adventitious buds, 211 
 
 Beneden, Ed. van, observations on 
 fertilisation of egg in Ascaris, 23 j 
 function of the centrosomes, 25 ; 
 view concerning idants, 244 
 
 Biophors, or bearers o( vitality, the 
 smallest vital units, 40 ; a group 
 of molecules on which the pheno- 
 mena of life depend, 40 ; resem- 
 blance to Spencer's ' physiological 
 units,' 40; difference bet ween them 
 and Nageli's ' micellae," 41, de 
 Vries's ' pangenes/and Wiesner's 
 ' plasomes,' 42 ; bearers of cell- 
 qualities, 42 ; number of kinds 
 unlimited, 44 ; all protoplasm 
 constituted by, 45 ; their size and 
 number, 85 ; combined anew in 
 the formation of galls, 221, 448 
 
 Beyerinck on galls, 219 ; gall of 
 Nematus, 222 
 
 Blending, of parental characters not 
 a fundamental phenomenon of 
 heredity, 8 ; of paternal and ma- 
 ternal idioplasm in ontogeny, 253 ; 
 of parental characters in the off- 
 spring, 261, 266, 297 
 
 Blood -corpuscles controlled by de- 
 terminants, 57 
 
 Blumenbach's ' nisus formativus,' 
 103, 104 
 
 Concilia viridis, its dimorphism, 364 
 
 Bonnet, on the regeneration of eye 
 of Triton, 125 ; on worms, 131
 
 47O IN] 
 
 Born, development of eggs in a 
 fixed position, 135 
 
 Boveri, observations on fertilisation 
 of egg in Ascaris, 23 ; function 
 of the centrosomes, 25 ; observa- 
 tions on the egg of the sea-urchin 
 artificially deprived of its nucleus, 
 28 ; chromosomes of Ascaris 
 megalocephala, 86 ; differentia- 
 tion of somatic cells of Ascaris 
 megalocephala, 191 ; number of 
 nuclear rods in two species of 
 Ascaris, 241 ; view concerning 
 idants, 242 
 
 Brandza, examination of hybrids 
 with regard to parental characters, 
 270 
 
 Brooks, W. K., on the laws of 
 heredity, 9; on variation, 412; 
 on the non-transmission of ac- 
 quired characters, 396 
 
 Briicke, Ernst, on ' Elementar 
 organismen," 2 ; on ultimate vital 
 particles, 20 ; on albumen, 38 : 
 on the organic nature of proto- 
 plasm, 39 
 
 Bryophyllum, budding in, 211 
 
 Bud -variation, 436 ; theoretical ex- 
 planation of, 441 
 
 Biitschli, investigations on the pro- 
 cesses of nuclear division, 23 ; 
 the polar bodies correspond to 
 aborted ova, 251 
 
 Butterflies, the markings on the 
 wings of, as confirming the theory 
 of determinants, 54, 174, 264, 
 265 ; Kallima parallecta as an 
 example of relative perfection, 
 272 ; the origin of climatic varia- 
 tions in the idioplasm, 379, 399, 
 407, 418 
 
 CARDAMINE pratensis, budding in, 
 211 
 
 Carneri, observations on telegony, 
 385 
 
 Caspary, experiments in crossing 
 Cytisus laburnum and C. pur- 
 pureus, 343 
 
 Castration, as a cause of sexual re- 
 version, 358 
 
 Cecidomyia pose, formation of galls, 
 221 
 
 Centrosome, with its sphere of attrac- 
 tion, is an apparatus for the 
 division of the cell and nucleus, 
 23 ; continuity of the centrosome, 
 
 Chabry, experiments on eggs of 
 Ascidians, 136 
 
 Chromatin, the hereditary substance, 
 24, 26 ; must be different in each 
 kind of cell, 32 
 
 Chromosomes correspond to the 
 idioplasm, IO ; their form, 24 ; 
 spoken of as idants, 67 
 
 Continuity of the germ-plasm, his- 
 torical account of, 198 
 
 Control of the cell, 45 
 
 Cuttings, development and origin 
 in the idioplasm of, 212 
 
 Cyclops, observations on segment- 
 ing ova, 191 
 
 Cymathoidae, temporary sexual di- 
 morphism, in 
 
 Cypris reptans, markings on the 
 shell in their relation to the theory 
 of determinants, 88 ; reversion 
 in, 344 ; theoretical explanation 
 of reversion in, 347 
 
 Cytisus adami, 339 ; theoretical ex- 
 planation of the variation in, 344 
 
 DAPHNIA pulex, alternation of 
 generations in, 175 
 
 Daphnidae, embryogeny of, 184 ; 
 secondary nature of the differen- 
 tiation of germ-cells of, 186 ; 
 germ -track of, 192 
 
 Darwin, Charles, ' pangenesis,' 2 ; 
 ' gemmules,' 3 ; comparison of 
 the ' gemmules ' with Spencer's 
 ' physiological units,' 6 ; on cor- 
 related variations, 84 ; onthewhite 
 coloration in animals and plants, 
 277 ; on breeding, 291 ; on the 
 force of heredity in cattle, 293 ; 
 on reversion in plant-hybrids, 299 ; 
 on reversion to remote ancestors, 
 316 ; on reversion in pigeons, 
 323 ; in mules, 328 ; in Antir- 
 rhinum majus, 331 ; experiments 
 on crossing Cytisus laburnum and
 
 C. purpurea, 340 ; on latent 
 primary constituents, 352 ; on 
 transmission of acquired char- 
 acters, 384, 394 ; on variation, 
 406, 410 ; on baldness in birds, 
 426 ; on the transmission of bud- 
 variations, 441 ; on the trans- 
 mission of a variation in Ballota 
 nigra, 446 
 
 Datura ferox and D. laevis, reversion 
 of hybrids to an unknown an- 
 cestral form, 317 
 
 Davenport, on budding in Polyzoa, 
 159 
 
 Degeneration in its relation to the 
 theory of determinants, 83 
 
 Dejerine, on the transmission of 
 nervous complaints, 369 
 
 Determinants or determining parts, 
 57 ; control groups of cells, 57 ; 
 are a group of biophors, 59 ; 
 possess special qualities, 60 ; are 
 definitely localised in the idio- 
 plasm, 61 ; constitute the ids, 62 ; 
 their forces of attraction, 66 ; 
 their behaviour in the course of 
 ontogeny, 69 ; their disintegration 
 into biophors, 69 ; their growth 
 and multiplication, 71 ; their 
 number, 89 ; supplementary de- 
 terminants, 103, 112, 127, 131, 
 149 ; the separation of a group 
 of, 153 ; their disintegration in 
 gemmation, 160 ; their separation | 
 into groups in the development of ' 
 somatic cells, 209 ; their modi- ' 
 fication in the formation of galls, 
 222 ; control of the cell by the 
 combined influence of paternal 
 and maternal determinants, 261 ; 
 homologous and heterologous 
 determinants, 264 ; their con- 
 trolling forces, 269 ; number 
 of homodynamous determinants 
 varies in ontogeny, 271 ; homo- 
 dynamous and heterodynamous 
 determinants, 264, 278 ; proof of 
 their disintegration into biophors, 
 348 ; in cases of dimorphism, 
 355 ; variations due to changes 
 in their composition, 406, 418, 
 448 
 
 EX. 471 
 
 Determinants, the theory of, 53 ; 
 summary, 225 ; reversion ex- 
 plained by, 335 ; applied to sexual 
 dimorphism, 366 
 
 Determinates or hereditary parts, 57 
 
 Dianthus chinensis and D. barbatus, 
 crosses between, 302 
 
 Dichogeny in plants, 1 14, 380, 462 
 
 Dicyemidae, distribution of here- 
 ditary parts, 59 
 
 Digitalis lutea and D. purpurea, 
 crosses between, 255 
 
 Dimorphism, normal, 352 ; its basis 
 in the idioplasm, 354 ; patho- 
 logical, 370 ; double, in Papilio 
 turnus, 375 ; seasonal, 379, 462 
 
 Diptera, course of germ-track, 192 ; 
 renewal of the alimentary epi- 
 thelium, 57 
 
 Disarticulation of the limb of Triton, 
 in connection with the hypothesis 
 of supplementary determinants, 
 119 
 
 Diseases, transmission of, 387 ; 
 haemophilia, 370 
 
 Doubling of limbs in insects, 429 ; 
 of whole groups of determinants, 
 428 
 
 Driesch, experiments with eggs of 
 sea-urchins, 137 ; on the blasto- 
 meres of the frog, 141 
 
 ELSBERG, Louis, 41 
 Embryonic cells, 33 
 Eudendrium, budding of, 155, 156, 
 fie. 6 
 
 FERNS, regeneration and gemma- 
 tion, 215 
 
 Fertilisation, essential part of the 
 process, 232 
 
 Fission in Naidae, 146 ; in Micro- 
 stomidae, 149, 456 
 
 Flemming, on nuclear division, 23 ; 
 on the splitting of the chromo- 
 somes in nuclear division, 25, 68 
 
 1'ocke, on plant-hybrids, 261 ; on 
 hybrids of Nicotiana, 267 ; on 
 reversion in hybrids, 299 ; on re- 
 crossing, 302 ; on ' xenia,' 383
 
 472 
 
 INDEX. 
 
 Fol, on the transference of the 
 
 ' blastogenic ' idioplasm, t66 ; its 
 
 centrosome into the ovum during 
 
 regular division, 171 ', accessory 
 
 fertilisation, 29 
 
 germ-plasm, 174; its continuity, 
 
 Fraisse,on regeneration in Amphibia, 
 
 183 ; its composition, 37, 77, 186 ; 
 
 96 ; in Salamandra, 99 ; in 
 
 its modification caused by am- 
 
 lizards, no, 116 
 
 phimixis, 235 ; its composition 
 
 Fucoidse, polar bodies of, 251 
 
 out of paternal and maternal 
 
 
 idants, 254 ; its partial variations, 
 
 GALLS, 218 ; definite new forma- 
 
 249, 271 ; reversion due to its 
 
 tions composed of modified cells, 
 
 ancestral determinants, 336 ; 
 
 221 
 
 transformation and the gradual 
 
 Gallon, Francis, acceptation of the 
 
 production of reversion, 336 ; 
 
 'gemmule' hypothesis, but rejection 
 of that of their free circulation, 7 ; 
 
 ' blastogenic ' germ-plasm, 441 ; 
 reserve germ-plasm, 447, 453 
 
 theory of the blending of charac- 
 
 Germ-tracks, course taken by the 
 
 ters of parents in the children, 7> 
 257 ; germ-substance composed 
 of homologous gemmules, 72 ; 
 
 germ-plasm from the ovum to 
 the reproductive cell, 184 ; their 
 course in the Metazoa, 192 ; their 
 
 supposed priority as regards the 
 
 cells alone capable of giving rise 
 
 assumption of the continuity of 
 
 to primary germ-cells, 194 ; each 
 
 the germ-plasm, 198 
 
 cell in them contains perfectly 
 
 Gartner, on plant-hybrids, 299 ; re- 
 
 definite somatic elements, 210 ; 
 
 crossing of hybrids, 305 
 
 brief summary, 228 
 
 Gemmation in Coelerentata, 154 ; 
 
 Giard, view that the polar bodies 
 
 'blastogenic' idioplasm, 157; in 
 Polyzoa, 158; in Tunicata, 160; 
 
 correspond to aborted ova, 251 
 Godron, reversion in Melandryum 
 
 in plants, 163 ; comparison of the 
 
 album and M. rubritm, 306 
 
 process in plants and animals, 
 
 Gotte, regeneration in Salamandra, 
 
 1 66 ; its phylogeny, 168 ; result- 
 
 99 
 
 ing from adaptation in polypes, 
 
 Gruber, August, artificial division 
 
 217 ; brief summary, 225, 456 
 
 of Infusoria, 52 
 
 Germ-cells, their formation, 183 ; 
 
 Guignard, on the reduction of idants 
 
 shifting of place of origin in the 
 
 in the germ-cells of plants, 250 ; 
 
 course of phylogeny, 186 ; only 
 
 on the union of nuclei, 30 
 
 certain series of cells contain the 
 
 
 primary constituents of germ-cells, 
 
 HABERLANDT, on the relations be- 
 
 189 ; distinction between germ- 
 
 tween the functions and position 
 
 and somatic -cells, 208 ; the com- 
 
 of the nucleus, 46 
 
 bination of ids in germ-cells, 245, 
 
 Hackel, Ernst, on the ' perigenesis 
 
 247 ; only half the number of 
 
 of the plastidule,' 41 ; on the law 
 
 parental ids contained in them, 
 
 of sexual transmission, 369 
 
 257, 457 
 
 Hacker, Valentin, on the phenomena 
 
 Germ-plasm constituting the im- 
 
 of reduction in the germ-cells of 
 
 mortal reproductive substance, 9 ; 
 
 Arthropods, 250 
 
 is the first ontogenetic stage of 
 
 Haemophilia, 370 
 
 the idioplasm, 35 ; its funda- 
 
 Hallez, on the embryology of 
 
 mental units, 37 ; its composition 
 
 Nernatodes, 137 
 
 out of biophors, 40, 48 ; its fixed 
 
 Hatschek, 53 ; on variation and 
 
 architecture, 61 ; it forms a com- 
 
 sexual reproduction, 413 
 
 plete unit by itself, 62 ; summary 
 
 Henking, on the phenomena of 
 
 relating to its structure, 75 ; 
 
 reduction in the germ - cells of 
 
 magnitude of its constituents, 85 ; 
 
 Arthropods, 250
 
 INDEX. 
 
 473 
 
 Heasen, Victor, on amphigonic 
 heredity, 253 ; on the increase of 
 a character, 424 
 
 Herbert, experiments with Cytisus \ 
 adami, 343 
 
 Hereditary substance, not contained 
 in the body of the cell, but in its 
 nucleus, 10 ; composed of primary 
 constituents, 15 ; forms but a 
 small part of the substance of the 
 egg, 22 ; composed of different 
 qualities, 26 ; the extreme com- 
 plexity of its structure, 29 ; its 
 growth, 31 ; of unicellular organ- 
 isms, 451 ; of multicellular organ- 
 isms, 452 
 
 Heredity, its fundamental pheno- 
 mena, 20 ; in unicellular organ- 
 isms, 52 ; in reference to the 
 structure of the germ-plasm, 69 ; 
 homotopic and homochronic 
 forms, 75 ; in monogonic repro- 
 duction, 92 ; in sexual reproduc- 
 tion, 230, 253 ; explanation of 
 apparently monogonic heredity in 
 plant-hybrids, 259 ; in Man, 259 ; 
 as regards the colour of the eyes, 
 278; example of apparently mono- 
 gonic heredity, 267, 280 ; force 
 of, 290 ; as regards acquired 
 characters, 392 ; mutilations, 396 ; 
 ' sports, ' 444 ; the apparatus for 
 transmission, 466 
 
 Hertwig, Oscar, view of fertilisation 
 as a conjugation of nuclei, 23 ; 
 on the germinal layers of the 
 Metazoa, 113; on the nuclear 
 rods in two varieties of Ascaris 
 megalocephala, 241 
 
 Heterobiophorids, hypothetical pri- 
 mitive organisms, 450 
 
 Heterokinesis, nuclear division re- 
 sulting in parts which are dis- 
 similar as regards their hereditary 
 tendencies, 34 
 Hildebrandt, hybrids of two species 
 
 of Oxalis, 255 
 
 His, theory of special regions in the 
 germ giving rise to special organs, 
 
 134 
 
 Hoffmann, experiments with Papa- 
 ver alfinnm, 437 
 
 Homobiophorids, hypothetical pri- 
 mitive organisms, 450 
 
 Homoeokinesis, nuclear division 
 resulting in parts which are simi- 
 lar as regards their hereditary 
 tendencies, 34 
 
 Hoppe-Seyler, chemical constitu- 
 tion of protoplasm, 40 
 
 Humming - birds, sexual dimor- 
 phism of, 427 
 
 Hybrids of various species of plants, 
 255 ; the theory of idants, 259 ; 
 plant - hybrids, 260 ; apparent 
 monogonic transmission in, 302 
 
 Hydra, regeneration, 127 ; gem- 
 mation, 155 
 
 Hydractinia echinata, formation of 
 germ-cells, 189 
 
 Hydroids, formation of the bud 
 from a single cell, 166 ; embryo- 
 geny, 184 : shifting of place of 
 origin of germ-cells, 186; germ- 
 tracks of, 207 
 
 ID, the, in ontogeny, 60 ; the num- 
 ber contained in the individual 
 idant, 241 
 
 Idants, constituting the hereditary 
 substance in sexual reproduction, 
 234 ; composed of dissimilar ids, 
 238 ; constituting groups of ids, 
 241 ; their number, 245 ; become 
 doubled by division, 246 ; com- 
 bination of, 247 ; of ancestors 
 contained in the germ-plasm, 
 257 ; their combination in plant- 
 hybrids, 297, 302 ; intermingling 
 of, in transmission, 312 
 
 Idioplasm, Nageli's conception of 
 the, 10; my view of the, 33 ; con- 
 trasted with morphoplasm, 38; 
 composed of ids, 63 ; phyletic 
 variation of, in the differentiation 
 of species, 79, 434 
 
 Ids or ancestral germ-plasms com- 
 posed of determinants, 62 ; their 
 individual difference, 236 ; homo- 
 logous and heterologous, 264 ; 
 gradual transformation of, 271 ; 
 numl>er variable according to age 
 v of characters, 273; struggle of 
 parental ids, 285
 
 474 
 
 Increase of a character by crossing, 
 
 424 
 
 Individual prepotency, 291 
 Ivy, in ; dichogeny in, 380 
 
 JAGER, Gustav, supposed priority as 
 regards the hypothesis of the 
 continuity of the germ -plasm, 200 
 
 KENNEL, regeneration of beak of 
 
 stork, 125 ; division, 150 
 Kiwi (Apteryx), disappearance of 
 
 wings, 82 
 Kleinenberg, development of Lum- 
 
 bricus trapezoides, 170 
 Kolreutter, crossing of Nicotiana 
 
 rustica and N. paniculata, 261, 
 
 269 ; on recrossing, 305 
 Kowalewsky, on supernumerary toes 
 
 of horses, 334 
 
 Kraepelin, investigations on her- 
 maphrodite bees, 361 
 
 LAMARCK, on the transmission of 
 acquired characters, 395 
 
 Lang, A., the regenerative faculty 
 as an arrangement for protec- 
 tion, 93 ; budding in hydroids, 
 
 X 55 
 
 Leptodora hyalina, development of, | 
 
 176; sexual characters of, and i 
 
 the theory of determinants, 363 
 Leuckart, Rudolph, discovery of 
 
 parthenogenesis in bees, 356 ; on 
 
 the combination of secondary 
 
 sexual characters in hermaphro- j 
 
 dite bees, 360 
 Leydig, regeneration inHizards, I IO, j 
 
 116 
 Liebscher, crossing of Hordeum 
 
 steudelii and H. trifurcatum, 301 
 Loeb, gemmation in Tubularia 
 
 mesembryantkemum, 216 
 Lumbriculus, regeneration in, 126, 
 
 151 
 Lucas, Prosper, on transmission of 
 
 polydactylism, 373 
 Lycsena, as illustration of the theory 
 
 of determinants, 58, 62 ; L. adonis 
 and L. agestis, 87 ; colquring 
 
 and dimorphism of, 359, 373 ; j 
 
 variation in, 422 
 
 MARRUBiUMvaillantii, 270 
 
 Marsh, on supernumerary toes in 
 horses, 334 
 
 MeduSce, formation of germ -cells, 
 187 ; degeneration into a mere 
 gonophore, 187 
 
 Mesohippus, 334 
 
 Meyer, O. E. , on molecules, 86 
 
 Micellar theory, 41 
 
 Miescher-Rusch, on nuclein, 50 
 
 Morphoplasm, 38 
 
 Mules, transverse stripes of, indi- 
 cating reversion, 316; theoretical 
 explanation of stripes, 329; patho- 
 logical variation, 429 
 
 Muller, Josef, ' gamomachia ' and 
 ' gamophagia,' 296 
 
 NAGELI, Carl von, physiological 
 theory of descent, 9 ; the heredi- 
 tary substance constitutes only 
 an infinitesimal part of the egg- 
 cell, 22; the 'micella,' 41; 
 on variation, 41 1 ; the ibis and 
 crocodile in connection with varia- 
 tion, 416 ; bud-variation, 442 
 
 Nais, regeneration of, 126 
 
 Naudin, experiments with Datura, 
 321 
 
 Nicotiana rustica and N. paniculata, 
 crosses between, 261 ; difference 
 in the flowers of, 267 ; N. alata 
 and N. langsdorffii, 300 
 
 Nipples, supernumerary, in human 
 beings as examples of reversion, 
 
 Nitsche, O., budding in Polyzoa, 
 158 
 
 Nussbaum, artificial division in In- 
 fusoria, 52 ; idea of the continuity 
 of germ-cells, 201 
 
 OKA, budding in Polyzoa, 159 
 Ontogeny, or development of the 
 individual, 32 ; depends upon 
 gradual changes in the germ- 
 plasm, 32 ; part played by deter- 
 minants in the course of, 80 ; can 
 only be explained by 'evolution,' 
 and not: by epigenesis, 138 ; 
 resulting from the union of the 
 germ-plasm of two parents, 253
 
 INDEX. 475 
 
 Otter-sheep, 291 
 Owen, Sir Richard, on cells, 198 
 Oxalis, crosses between different 
 species of, 255 
 
 PANGENES, the, of de Vries as 
 bearers of constituent qualities, 
 15 ; their miscibility, 15 ; com- 
 pared with biophors, 42 
 
 Papilio turnus, 375 
 
 Pfliiger, the hereditary substance 
 only an infinitesimal part of the 
 egg-cell, 22 ; influence of gravity 
 on the development of frogs^ eggs, 
 
 135 
 
 Phanerogams, budding in, 2l6 
 Philippeaux on regeneration in 
 
 Tritons, 118 
 
 Phylogeny, 77 ; parallelism be- 
 tween phylogeny and ontogeny, 
 80 ; of the process of fission in 
 the Metazoa, 151 ; of regenera- 
 tion, 114; of gemmation, 168; 
 of the'japparatus for transmission, 
 
 45 1 
 
 Pigeons, reversion to the rock- 
 pigeon, 323, 352 ; increase of a 
 character by selection, 426, 427 
 
 Plasomes, the, of Wiesner, 20 
 
 Plastidules, the, of Haeckel, 41 
 
 Plumularia, budding in, 155 
 
 Podocoryne cornea, 1 58 
 
 Polydactylism, the transmission of, 
 373, 429 
 
 Polymorphism, 374 ; of animal and 
 plant stocks, 376, 462 
 
 Polyommatus phlseas, 399, 420 
 
 Polyzoa, budding in, 158 ; forma- 
 tion of buds from different cells, 
 165; ' blastogenic ' germ-plasm, 
 218 
 
 Primula acaulis, reversion to long- 
 stalked variety in, 16 
 
 Protohippus, 334 
 
 RATH, O. VOM, on threads of 
 4 linin ' between the idants, 244 ; 
 on the phenomena of reduction in 
 the germ -cells of Arthropods,*25O 
 
 Rauber, supposed priority as re- 
 gards the view of the continuity of 
 the germ-plasm, 200 
 
 Ray Lankester on the non-transmis- 
 sion of acquired characters, 396 
 
 Reducing division of the germ- 
 plasm, 1 1, 22, 235 ; consists in the 
 extrusion of ids, 240 ; influence 
 on the composition of the germ- 
 plasm, 242 ; cause of difference 
 between children of the same 
 parents, 257; in its relation to 
 reversion, 306 
 
 Regeneration, its cause and origin 
 in the idioplasm, 93 ; constitutes 
 an arrangement for protection, 
 93 ; physiological, 94 ; of the 
 epidermis, 95 ; palingenetic, 105 : 
 ccenogenetic, 108 ; of the caudal 
 region of the vertebral column, 
 109 ; phylogeny of, 1 14 ; physio- 
 logical and pathological, 119; 
 depends on adaptation, 119; in 
 fishes, birds, and mammals, 120 ; 
 difference of the capacity for in 
 lower and higher animals, 124 ; 
 acquired by selection, 125 ; facul- 
 tative or polygenetic, 126; in 
 plants, 132 ; formation of callus, 
 133; in animal embryos, 134; of 
 segmentation - cells, 139 ; sum- 
 mary, 225, 455 
 
 Reisseck, on crosses between Cytisus 
 laburnum and C. purfureus, 343 
 
 Reproduction, sexual, 230, 457 ; its 
 effect, 240 ; in Man, and trans- 
 mission of parental characters, 
 257 ; by fission, 456 
 
 Reversion in plant-hybrids, 299 ; to 
 the pure ancestral form, 305 ; in 
 Man, and the crossing of different 
 races, 308 ; to a grandparent, 
 309 ; to characters of remote an- 
 cestors, 316 ; in Datura, 317, 321 ; 
 to ancestral forms of flowers in 
 plants, 305 ; to rudimentary char- 
 acters, 333 ; in gemmation, 338 ; 
 in parthenogenesis, 344 ; sexual, 
 367 ; in bud-variations, 447, 458 
 
 Rhabditis nigrovenosa, segmenta- 
 tion of ovum, 195, fig. 15; germ- 
 track of, 196 
 
 Roux, W., on the purpose of the 
 apparatus for division of the nu- 
 cleus, 26; on the struggle of
 
 476 
 
 parts, 107 ; on gravity producing 
 no effect in the differentiation of 
 the egg into the embryo, 135 ; 
 experiments on frogs' eggs, 1 36 ; 
 on post-generation, 142 
 Riickert, J., behaviour of chromo- 
 somes during maturation in the 
 ovum of the dog-fish, 5 7 1 ! 
 247 
 
 SALHS, on the formation of callus, 
 133 ; on the growth of Chara, 
 164 ; on adventitious buds, 211 
 
 Sagitta, embryogeny of, 185 ; course 
 of the germ-track of, 192 
 
 Salamandra, regeneration of limbs, 
 
 Salpae, budding in, 162 ; alternation 
 
 of generations in, 180 
 Schreiber, on the regenerative power 
 
 in Triton marmoratus, 115 
 Seeliger, O., on budding in Poly- 
 
 zoa, 158; in Clavelina, 160 ; in 
 
 Salpse, 162; in Pedicellina, 218 
 Self-differentiation of cells, 136 
 Semper, Carl, on the process of 
 
 fission in Nais, 147 
 Settegast, on infection of the germ, 
 
 385 
 
 Siebold, von, on the determination 
 of sex in bees, 356; on her- 
 maphrodite bees, 361 
 
 Siren lacertina, regenerative power 
 in, 115 
 
 Spallanzani, on the capacity for re- 
 generation in different organs, 
 117; inJTriton, 120 ; in the jaw of 
 Triton, 125 
 
 Spencer, Herbert, ' physiological 
 units,' 51 ; their relation to Dar- 
 win's 'gemmules,'6 ; on heredity, 
 7 ; on regeneration, 104 ; regene- 
 ration compared with crystallisa- 
 tion, 128 
 
 Stimuli considered as causing an 
 incitement to growth, 129 
 
 Strasburger, on fertilisation in 
 Phanerogams, 23 ; on the essen- 
 tial similarity of male and female 
 nuclei, 23 ; on the dynamical effect 
 of nuclear matter, 45 
 
 Struggle of individual characters, 
 274 ; of the ids in ontogeny, 260 
 
 Supplementary determinants, 103 5 
 their change in the course of 
 phylogeny, 112; in processes of 
 regeneration in Hydra, 127 ; an- 
 timeral, 131 ; the origin of, 149 
 
 Syphilis, the transmission of, 388 
 
 TARDIGRADA, extent of the here- 
 ditary parts, 59 
 Telegony or infection of the germ, 
 
 383. 
 
 Termites, polymorphism in, 379 
 
 Thuja, dichogeny in, 382 
 
 Transmission (see Heredity). 
 
 Triton, too ; diagram of regenera- 
 tion of fore-limb in, 102, 115; 
 experiment on regeneration oi 
 lung in, 117 
 
 Tuberculosis, and infection of the 
 germ, 389 
 
 Tubulariamesembryanthemum, bud- 
 ding in, 216 
 
 Tunicata, budding in, 160 
 
 Twins, 140 ; identical and dissimi- 
 lar, 254 
 
 Type in a technial sense, 313 
 
 VANESSA, levana and prorsa, 379 
 
 Variation, intercalary remarks on, 
 271 ; theoretical demonstration of, 
 410 ; normal individual varia- 
 tion, 410; cause of hereditary 
 variation, 415; pathological va- 
 riation, 428 ; summary of the 
 present view with regard to 
 variation, 431 ; variations on a 
 larger scale in plants, 435 
 
 Verworn, M., view that the heredi- 
 tary substance is also contained 
 in the cell-body, 28 
 
 Vilmorin, on individual potency in 
 plants, 291 
 
 Vines, view that the assumption of 
 a special reproductive substance 
 is unnecessary, 202 ; on embry- 
 onic substance, 204 
 
 Viola tricolor, reversion to the an- 
 cestral form, 320 
 
 Vochting, H., on ' transplantatioi 
 " in the plant-body,' 129
 
 Volvox, proof of that a difference 
 exists between the somatic- and 
 the germ-cells, 213 ; as an exam- 
 ple of sexual dimorphism, 355 
 
 Vnes, Hugo de, objection to the 
 theory of ancestral germ -plasms, 
 12 ; ' intracellular pangenesis ' 
 compared with Darwin's ' pan- 
 genesis,' 12 ; objection to the view 
 that the hereditary substance is 
 contained in the nucleus only, 
 26 ; on the structure of proto- 
 plasm, 38 ; on Haeckel's ' plas- 
 tidules,' 41 ; objection to Stras- 
 burger's and Haberlandt's views, 
 46 ; the ' pangenes ' as bearers of 
 cell-qualities, 42 ; on the con- 
 tinuity of cellular organisms, 56 ; 
 on primary and accessory germ- 
 tracks, 206, 21 1; on the con- 
 tinuity of the germ-plasm, 207 ; 
 on galls as contradicting the 
 latter, 218 ; galls of Cecidomyia 
 poa, 221 ; on the crosses between 
 two species of beans, 297 ; on 
 plant-hybrids, 300; on dicho- 
 geny, 380, 451 
 
 477 
 
 WAGNER, F. von, on gemmation and 
 fission, 145 ; on fission in Micros- 
 torn a lineare, 149 ; on gemma- 
 tion, 154 
 
 Wallace, Alfred Russel, on Papilio 
 mtmnon, 376; on variation, 
 410 
 
 Ward, Lester, Neo-Darwinism and 
 Neo-Lamarckism, 408 
 
 Weeping Ash, 444, 446 
 
 Wiesner, Julius, on the elementary 
 structure and growth of living 
 substance, 19; on the constitu- 
 tion of protoplasm, 39 ; on the 
 continuity of the constituent parts 
 of the cell, 48 
 
 Willow, rudiments of roots in the 
 shoots of the, 382 
 
 ZANDER, R., on polydactylism, 
 
 430 
 
 Ziegler, 'Ernst, on the law of the 
 specific character of the tissues, 
 112; on tuberculosis, 390; on 
 polydactylism regarded as a germ- 
 variation, 430 
 
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 to Dickens and his works . . . we should, until we came across this volume, 
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 novelist as being really satisfactory. The difficulty is removed by Mr. 
 Marzials's little book."-Athe>i(eum. 
 
 LIFE OF DANTE GABRIEL RO.-iSETTI. By J. KNIGHT. 
 
 "Mr. Knight's picture of the great poet and painter is the fullest and 
 best yet presented to the public." The Graphic. 
 
 LIFE OF SAMUEL JOHNSON. By COLONEL F. GRANT. 
 
 " Colonel Grant has performed his task with diligence, sound judgment, 
 good taste, and accuracy." Illustrated London Xeivs. 
 
 LIFE OF DARWIN. By G. T. BETTANY. 
 
 " Mr. Q. T. Bettany's Life of Darwin is a sound and conscientious work." 
 Saturday Review. 
 
 LIFE OF CHARLOTTE BRONT0. By A. BIRRELL. 
 
 "Those who know much of Charlotte Bronte will learn more, and those 
 who know nothing about her will find all that is best worth learning in Mr. 
 Birrell's pleasant book." St. James' Gazette. 
 
 LIFE OF THOMAS CARLYLE. By R. GARNETT, LL.D. 
 
 "This is an admirable book. Nothing could be more felicitous and 
 fairer than the way in which he takes us through Carlyle's life and 
 works." Pall Mall Gazette. 
 
 London: WALTKB, SCOTT, LIMITED, 24 Warwick Lane.
 
 GREAT WRITERS-continued. 
 LIFE OF ADAM SMITH. By R. B. HALDANB, M.P. 
 
 "Written with a perspicuity seldom exemplified when dealing with 
 economic science." Scotsman. 
 
 LIFE OF KEATS. By W. M. ROSSKTTI. 
 
 "Valuable for the ample information which it contain*." Cambridot 
 Independent. 
 
 LIFE OF SHELLEY. By WILLIAM SHARP. 
 
 "The criticisms . . . entitle thin capital monograph to be ranked with 
 the best biographies of Shelley." tt'ettmimter Review. 
 
 LIFE OF SMOLLETT. By DAVID HANNAY. 
 
 "A capaWe record of a writer who still remains one of the great masters 
 of the English novel." Saturday Reviev. 
 
 LIFE OF GOLDSMITH. By AUSTIN DOBS' N. 
 
 "The story of his literary and social life in London, with all its humorous 
 and pathetic vicissitudes, is here retold, as none could tell it better."-i;atiy 
 tiewt. 
 
 LIFE OF SCOTT. By PROFKSSOR YONOE. 
 
 "This is a most enjoyable book." Aberdeen Free rreti. 
 
 LIFE OF BURNS. By PROFKSSOR BLACKIB. 
 
 " The editor certainly made a hit when he persuaded Blackie to write 
 about Burns." Pall Mall Gazette. 
 
 LIFE OF VICTOR HUGO. By FRANK T. MARZIALS. 
 
 "Mr. Marzials's volume presents to us, in a more handy form than any 
 English or even French handbook gives, the summary of what is known 
 about the life of the great poet." Saturday Review. 
 
 LIFE OF EMERSON. By RICHARD GARNBTT, LL.D. 
 
 " No record of Emerson's life could be more desirable." Saturday Review. 
 
 LIFE OF GOETHE. By JAMES SIME. 
 
 "Mr. James simc's competence as a biographer of Goethe is beyond 
 question." Manchester Guardian. 
 
 LIFE OF CONGREVE. By EDMUND GOSSK. 
 
 "Mr. Gosse has written an admirable biography." Acadrmy. 
 
 LIFE OF BUNYAN. By CANON VENABLBS. 
 
 " A most intelligent, appreciative, and valuable memoir." Scotsman. 
 
 LIFE OF CRABBE. By T. E. KEBBEL. 
 
 "No English poet since Shakespeare has observed certain aspects of 
 nature and of human life more closely." Athenaeum. 
 
 LIFE OF HEINE. By WILLIAM SHARP. 
 
 " An adi 
 recent kno 
 
 " An admirable monograph . . . more fully written up to the level of 
 wledge and criticism than any other English work." Scottinan. 
 
 London: WALTER SCOTT, LIMITED, 24 Warwick Lane.
 
 GREAT WRITERS continued. 
 LIFE OP MILL. By W. L. COURTNEY. 
 
 "A most sympathetic and discriminating memoir." Glasjow Herala. 
 LIFE OF SCHILLER. By HENRY W. NEVINSON. 
 
 " Presents the poet's life in a neatly rounded picture." Scotsman. 
 LIFE OF CAPTAIN MARRYAT. By DAVID HANNAY. 
 
 "We have nothing but praise for the manner in which Mr. Hannay has 
 done justice to him." Saturday lieoiew. 
 
 LIFE OF LESSING. By T. W. ROI.LESTON. 
 
 " One of the best books of the series." Manchester Guardian. 
 LIFE OF MILTON. By RICHARD OARNETT, LL.D. 
 
 " Has never been more charmingly or adequately told." Scottish Leader. 
 LIFE OF BALZAC. By FREDERICK WEDMORK. 
 
 "Mr. Wedmore's monograph on the greatest of French writers of fiction, 
 whose greatness is to be measured by comparison with his successors, is a 
 piece of careful and critical composition, neat and nice in style." Daily 
 News. 
 
 LIFE OF GEORGE ELIOT. By OSCAR BROWNING. 
 
 "A book of the character of Mr. Browning's, to stand midway be- 
 tween the bulky work of Mr. Cross and the very slight sketch of Miss 
 Blind, was much to be desired, and Mr. Browning has done his work with 
 vivacity, and not without skill." Manchester Guardian. 
 
 LIFE OF JANE AUSTEN. By GOLDWIN SMITH. 
 
 "Mr. Goldwin Smith has added another to the not inconsiderable roll 
 of eminent men who have found their delight in Miss Austen. . . . His 
 little book upon her, just published by Walter Scott, is certainly a fas- 
 cinating book to those who already know her and love her well ; and we 
 have little doubt that it will prove also a fascinating book to those who 
 have still to make her acquaintance." Spectator. 
 
 LIFE OF BROWNING. By WILLIAM SHARP. 
 
 "This little volume is a model of excellent English, and in every respect 
 it seems to us what a biography should be." Public Opinion. 
 
 LIFE OF BYRON. By HON. RODEN NOEL. 
 
 " The Hon. Itoden Noel's volume on Byron is decidedly one of the most 
 readable in the excellent 'Great Writers' series." Scottish Leader. 
 
 LIFE OF HAWTHORNE. By MOXCURE CONWAY. 
 
 " It is a delightful caugerie pleasant, genial talk about a most interest- 
 ing man. Easy and conversational as the tone is throughout, no important 
 fact is omitted, no valueless fact is recalled ; and it is entirely exempt from 
 platitude and conventionality." The Speaker. 
 
 LIFE OF SCHOPENHAUER. By PROFESSOR WALLACE. 
 
 "We can speak very highly of this little book of Mr. Wallace's. It 
 is, perhaps, excessively lenient in dealing with the man, and it cannot 
 be said to be at all ferociously critical in dealing with the philosophy." 
 Saturday Review. 
 
 London : WALTER SCOTT, LIMITED, 24 Warwick Lane.
 
 GREAT WRITERS -continued. 
 LIFE OF SHERIDAN. By LLOYD SANDERS. 
 
 "To iay that Mr. Lloyd Sanders, in tlii* little volume, has produced UM 
 best existing memoir of Sheridan, is really to award much fainter prals* 
 than the work deserves." ManehetUr Examiner. 
 
 LIFE OF THACKERAY. By HERMAN MERIVALB and F. T. MARZIAIA 
 "The monograph just published is well worth reading, . . . and the book, 
 with its excellent bibliography, is one which neither the student nor the 
 general reader can well afford to miss ." Pall Mall Gazette. 
 
 LIFE OF CERVANTES. By W. E. WATTS. 
 
 " We can commend this book as a worthy addition to the useful series 
 to which it belones."-/.ondn Daily ChronicU 
 
 LIFE OF VOLTAIRE. By FBANCIS ESPINABSB. 
 
 George Saintslmry, in The Illustrated London Newt, says: "In this 
 little volume the wayfaring man who has no time to devour libraries will 
 find most things that it concerns him to know about Voltaire's actual life 
 and work put very clearly, sufficiently, and accurately for the most part." 
 
 LIBRARY EDITION OF "GREAT WRITERS," Demy 8vo, 2s. 6d. 
 
 London . WALTER SCOTT, LIMITED, 24 Warwick Lane.
 
 AUTHORISED VERSION. 
 Crown 8vo, Cloth, Price 6s. 
 
 PEER GYNT: A Dramatic Poem, 
 
 BY HENRIK IBSEN. 
 
 TRANSLATED BY 
 
 WILLIAM AND CHARLES ARCHER. 
 
 This Translation, though unrhytned, preserves throughout the 
 various rhythms of the original. 
 
 " In Brand the hero is an embodied protest against the poverty of 
 spirit and half-heartedness that Ibsen rebelled against in his country- 
 men. In Peer Gynt the hero is himself the embodiment of that spirit. 
 In Brand the fundamental antithesis, upon which, as its central theme, 
 the drama is constructed, is the contrast between the spirit of com- 
 promise on the one hand, and the motto ' everything or nothing ' on 
 the other. And Peer Gynt is the very incarnation of a compromising 
 dread of decisive committal to any one course. In Brand the problem 
 of self-realisation and the relation of the individual to his surroundings 
 is obscurely struggling for recognition, and in Peer Gynt it becomes the 
 formal theme upon which all the fantastic variations of the drama are 
 built up. In both plays alike the problems of heredity and the influence 
 of early surroundings are more than touched upon ; and both alike 
 culminate in the doctrine that the only redeeming power on earth or in 
 heaven is the power of love." Mr. P. H. WICKSTEED. 
 
 London : WALTER SCOTT, LIMITED, 24 Warwick Lane. 
 
 W"
 
 38444 
 
 UNIVERSITY OF CALIFORNIA LIBRARY 
 
 Los Angeles 
 This book is DUE on the last date stamped below. 
 
 'AN 09 1992
 
 ill 
 
 000 678 106 6