’t 
 
 THE UNIVERSITY 
 OF ILLINOIS 
 LIBRARY 
 
 575 
 
 L76r5 
 
 I 
 
Alfred Russel Wallace, in the Contemporary Review ol 
 August, 1908, p. 140, in an article on ‘The Present Posi- 
 tion of Darwinism,' writes as follows : 
 
 * In conclusion, I would suggest to those of my readers 
 who are interested in the great questions associated with the 
 name of Darwin, but who have not had the means of study- 
 ing the facts either in the field or the library, that in order 
 to obtain some real comprehension of the issue involved in 
 the controversy now going on they should read at least one 
 book on each side. The first I would recommend is a 
 volume by Mr. R. H. Lock on “Variation, Heredity and 
 Evolution” (1906) as the only recent book giving an account 
 of the whole subject from the point of view of the Mendelians 
 and Mutationists. When they have mastered this, I ask 
 them to read my own book on “ Darwinism” (1901), which, 
 though published before Mendelism became prominent, 
 gives some idea in popular language of the vast range of 
 subjects which Darwinism explains, and adduces a sufficient 
 body of facts to show the inadequacy of the whole series of 
 phenomena yet made public. 
 
 ‘ Having read these two works and again considered the 
 _ arguments adduced in this article, I leave them to form 
 their conclusions as to whether Darwinism is or is not an 
 “ unsuccessful hypothesis.” ' 
 
 ‘Alfred R. Wallace.’ 
 
Digitized by the Internet Archive 
 in 2017 with funding from 
 
 University of Illinois Urbana-Champaign Alternates 
 
 https://archive.org/details/recentprogressinOOIock 
 
VARIATION, HEREDITY, AND 
 EVOLUTION 
 
f 
 
 
 
 \ 
 
THE LIBRARY 
 OF THE 
 
 UHIYERSITY OF ILLINOIS 
 

 {Frontispiece, 
 
RECENT PROGRESS IN 
 THE STUDY OF VARIATION, 
 HEREDITY, AND EVOLUTION 
 
 BY 
 
 ROBERT HEATH LOCK, M.A., Sc.D. 
 
 SOMETIME FELLOW OF GONVILLE AND CAIUS COLLEGE, CAMBRIDGE ; 
 
 LATE ASSISTANT DIRECTOR OF THE ROYAL BOTANIC 
 GARDENS, CEYLON 
 
 REVISED BY 
 
 LEONARD DONCASTER, Sc.D., F.R.S. 
 
 PROFESSOR OF ZOOLOGY, UNIVERSITY OF LIVERPOO!. 
 
 BIOGRAPHICAL NOTE BY BELLA SIDNEY WOOLF 
 
 (MRS. R. H, LOCK) 
 
 NEW YORK 
 
 E. P. DUTTON AND COMPANY 
 1920 
 
Imr 3 T Kdition, December^ 1006 
 Keprinied, April y 1907 
 
 ,, Marchy 1909 
 
 Second Edition, Jmtey 1909 
 Third Edition, September y 1911 
 Fourth Edition, Jidy ^ 1916 
 Fifth Edition, ApnU 1Q20 
 
 Printed in Great Britain 
 

 0 
 
 \ 
 
 5n( 
 
 PREFACE TO THE FOURTH EDITION 
 
 
 
 A CERTAIN number of additions have been made since 
 the first edition was published ; these were enumerated 
 by my husband in the prefaces to the second and third 
 editions of the book, but it is not necessary to re- 
 capitulate them. 
 
 In revising the book for a fourth edition, the object 
 has been to make the fewest and smallest changes 
 consistent with giving a true idea of the present state 
 of our knowledge. In many cases progress has been 
 made since the author revised the book for the third 
 edition, but except when this progress has rendered 
 the text in any way misleading, it has been thought 
 best to make no change, and merely to add references, 
 either in footnotes or at the ends of the chapters, to 
 books or papers in which the later work is described. 
 Material alterations or additions, and footnotes other 
 than simple references, are enclosed in square brackets. 
 The only considerable alterations to the text are on 
 pp. 200-202, 269-271, and 275-279. 
 
 I wish to express my warmest thanks to Dr. Leonard 
 Doncaster, F.R.S., for his great kindness in revising 
 
 vu 
 
 
 611889 
 
PREFACE TO THE FOURTH EDITION 
 
 the fourth edition and for making those alterations 
 and valuable additions which the lapse of time rendered 
 necessary. I cannot put into adequate words my 
 appreciation of this proof of the friendship which 
 existed between Dr. Doncaster and my husband, j 
 am also indebted to Professor Bateson, F.R.S., for his 
 valued advice on Millardet’s hybrids. 
 
 B. L. 
 
 NOTE TO FIFTH EDITION 
 
 In revising the book for a fifth edition, I have made 
 only slight alterations. A few sentences and footnotes, 
 referring to work published since 1916, have been 
 added ; as in the fourth edition, these are enclosed 
 in square brackets. The only other alteration that 
 requires mention is the omission, in several places, of 
 the words ‘ recent,’ ‘ recently published,’ or others of 
 similar import, when they referred to events or ideas 
 now some fifteen or more years old, and of expressions 
 of doubt about facts which have subsequently been 
 definitely established. With the exception of these 
 slight textual alterations, made advisable by the lapse 
 of time, and of the passages enclosed in brackets, the 
 book remains substantially as it was left by the author. 
 
 L. D. 
 
PREFACE TO THE FIRST EDITION 
 
 The idea of writing this little book occurred to me 
 whilst reading Mr. W. C. D. Whetham’s volume on 
 ‘ The Recent Development of Physical Science.’ I 
 found the story of the modern progress of physics so 
 interesting as to encourage the belief that a similar 
 account of the subjects with which I was myself more 
 particularly familiar might prove of a like interest to 
 other people. I did not, indeed, suppose for a moment 
 that I could vie with Mr. Whetham in the power ol 
 literary expression which renders his book so eminently 
 readable. I rather hoped that the peculiar interest 
 and importance of the theme might outweigh the 
 present author’s deficiencies in this respect. 
 
 For the group of subjects of which I intended to give 
 a brief account Mr. W. Bateson has recently proposed 
 the term ‘ genetics,’ an expression which sufficiently 
 indicates their scope to the initiated. Since, however, 
 the meaning of the word ‘ genetics ’ is not yet clearly 
 understood by everybody, it seemed better to adopt 
 in the present instance a somewhat more descriptive 
 title. 
 
 b 
 
 IX 
 
PREFACE TO THE FIRST EDITION 
 
 The rediscovery of Mendel’s law some seven years 
 ago led to a complete change in our attitude towards 
 the problems of variation, heredity, and evolution ; 
 and the new method of study thus introduced has 
 rendered possible a renewal of the attack upon these 
 problems with increased vigour and with remarkable 
 results. At the present time this activity may be 
 said to have its centre in the school of genetic research 
 founded at Cambridge by the independent energy of 
 Mr. Bateson. So far-reaching are the results already 
 arrived at by Mr. Bateson and others, both in their 
 scientific interest and in their probable influence upon 
 human affairs, that it seemed desirable to give an 
 immediate account of these and of kindred fines of 
 recent study, even though the rapid progress which is 
 a characteristic manifestation of this department of 
 science must render any such attempt a more or less 
 transitory one. ^ 
 
 Whilst I was still engaged upon my task, the first 
 volume of Dr. Lotsy’s admirable ' Vorlesungen fiber 
 Descendenztheorien ’ made its appearance. But for 
 the fact that most of the following pages had then 
 already been written, I might have hesitated to pursue 
 my project, since a book not altogether unlike the 
 present might be produced by the comparatively simple 
 process of making a series of judicious extracts from 
 Dr. Lotsy’s work. The latter is, however, in the 
 German language, and on a considerable scale, so that 
 there seemed still to be room for an introduction to 
 
PREFACE TO THE FIRST EDITION 
 
 the science of genetics of the more modest dimensions 
 which I had contemplated. I should wish, however, 
 particularly to recommend Dr. Lotsy’s lectures to any 
 reader who wishes to go further into these matters. 
 
 I am indebted to several friends for assistance during 
 the course of my work. Mr. R. P. Gregory kindly 
 read through the proof of the chapter on cytology ; and 
 I wish here to record my thanks to Mr. J. Stanley 
 Gardiner, to Mr. C. T. Regan, to Mr. W. S. Perrin, and 
 to Mr. R. C. Punnett for information on special points. 
 To the last-named I owe the photograph which appears 
 as Fig. 15. I am particularly grateful to Mr. R. H. 
 Biffen and to Mr. G. Udny Yule for access to work 
 which has not hitherto appeared in print. 
 
 Adequately to acknowledge Mr. Bateson’s influence 
 upon these pages is a more difflcult matter, and not 
 the less so because I have deliberately refrained as 
 far as possible from consulting him whilst the book 
 was in course of preparation, in order that it might 
 retain if possible some traces of individuality. It is 
 therefore clear that he is in no way responsible for its 
 deficiencies. But, apart from the fact that I am 
 conscious of having quoted his ideas at more points 
 than could possibly be acknowledged seriatim, I owe 
 to Mr. Bateson both my first introduction to the science 
 of genetics, and a continual fund of encouragement in 
 the prosecution of studies connected with it. 
 
 I have to thank Mr. Francis Darwin for kind per- 
 mission to reproduce a portrait of his father ; Professor 
 
 XI 
 
PREFACE TO THE FIRST EDITION 
 
 de Vries for the present of an excellent portrait ; and 
 Mr. Francis Galton for the loan of a photograph well 
 known from earlier reproductions. The portrait of 
 Mendel is reproduced from the frontispiece to Mr. 
 Bateson’s ' Defence,' by the permission of the Syndics 
 of the Cambridge University Press. Messrs. Macmillan 
 and Co. have kindly allowed the reproduction of the 
 diagram which occupies p. 87, and of the table and 
 figure on pp. 89 and 90. The figures facing pp. 143 
 and 150 are from de Vries’ ‘ Mutationstheorie,’ pub- 
 lished by Messrs. Veit. 
 
 The attempt has been made to render the following 
 pages intelligible to the general reader, as well as to 
 the more scientific public, to which they are primarily 
 addressed. A short glossary has been added, which 
 may be found useful by those who have no previous 
 acquaintance with biological terms. 
 
 Cambridge, 
 
 October 23, 1906. 
 
THE LIBRARY 
 Of THE 
 
 UNIVERSITY Of ILLIftOil 
 
\To face p. xiii. 
 
BIOGRAPHICAL NOTE 
 
 ‘ We live in deeds, not years ; in thoughts, not breaths | 
 
 In feelings, not in figures on a dial. 
 
 We should count time by heart-throbs. He most lives 
 Who thinks most, feels the noblest, acts the best.^ 
 
 Philip J. Bailey. 
 
 It has been suggested that a few details of the 
 author’s life might fittingly preface this fourth 
 edition, which he was not fated to see through the 
 press. 
 
 Robert Heath Lock was the third son of the Rev. 
 John Bascombe Lock and Emily, daughter of Edwin 
 Baily of Cirencester. He was born on January 19, 
 1879, at Eton, where his father was at that time a 
 master. ‘ R. H.,’ as he was known to many of his 
 friends, was educated at Mr. Stone’s school at Broad- 
 stairs, and at Charterhouse. He was a quiet, sensitive 
 child and boy, finding from an early age great pleasure 
 in books. Still, he was not too much of a bookworm to 
 revel in the world of sport and games. All his life he 
 enjoyed outdoor pastimes with the keenness and zest 
 that he threw into everjdhing he did. At Charter- 
 house he was head monitor of Hodgsonites and got his 
 second eleven colours for football. But it was as a 
 
 xiii 
 
BIOGRAPHICAL NOTE 
 
 shot that he scored his great success. He fired the 
 winning shot for the Ashburton Shield won by Charter- 
 house at Bisley in 1893, 
 
 Most boys find considerable difficulty in choosing 
 a career, and pass through various phases when soldier- 
 ing, sailoring, or engine-driving seem an ideal existence, 
 and many of them eventually are ‘ put into something ’ 
 by parents or friends. But while still at school 
 R. H. Lock’s bent for science was already very marked, 
 and there was no question of any other career for him. 
 
 R. H. Lock came up to Caius as an Exhibitioner 
 for Natural Science in 1898, and entered on a very 
 happy period of college life. He took a First Class 
 % the Part I. of the Natural Science Tripos in 1900, 
 thereby becoming a Scholar of the College, and a 
 First Class in Part II. (Botany) in 1902. He was 
 awarded a Frank Smart Studentship, and was able to 
 gratify the ‘ wander-spirit ’ which was strong in him 
 by going to Ceylon for research work. 
 
 He worked at Peradeniya under Dr. Willis, Director 
 of the Royal Botanic Gardens, and the result of his 
 experiments, especially in the cross-breeding of maize, 
 won him in 1902 a Fellowship at Caius, where his father 
 was also a Fellow. He wandered home via China 
 Japan, and America, enjoying the trip in his own 
 quiet, observant way. The next four years were spent 
 at Cambridge, working in the Botanical Laboratory 
 (where he was appointed Curator of the Herbarium 
 on material he had brought back from Ceylon. He also 
 wrote at this time the book ‘ Variation, Heredity, and 
 Evolution,’ to which these words are prefaced. 
 
 XIV 
 
BIOGRAPHICAL NOTE 
 
 In 1908 he returned to Ceylon at the request of 
 Dr. Willis, as Assistant Director of the Royal Botanic 
 Gardens. Dr. Willis first formed the idea of collecting 
 at Peradeniya a staff dealing with entomology, plant- 
 breeding, mycology, etc., ready to experiment for, 
 and give advice to, the planter and the native culti- 
 vator — an idea which has been successfully imitated 
 in India. 
 
 In 1910 R. H. Lock returned to England on short 
 leave, when he married Bella Sidney Woolf, eldest 
 daughter of the late Sidney Woolf, Q.C. He also 
 took his degree of Sc.D. He returned to Ceylon to 
 take up residence in a charming bungalow that 
 Government had built for him in the Peradeniya 
 Gardens — an ideal spot to live in. From 1908 to 
 1912 his work was very strenuous. For a considerable 
 period he was Acting-Director of the Gardens in the 
 absence of the Director on leave. At the same time 
 he conducted extensive experiments in rubber-tapping 
 and paddy-breeding. 
 
 The result of the latter research work was a new strain 
 of rice, known as ‘ Lock’s paddy.’ This variety is now 
 displacing other varieties throughout the island. He 
 is the only official connected with the Botanic Gardens 
 throughout the hundred years and more of their 
 existence who has given his name to any product. 
 
 But changes were taking place which led eventually 
 to R. H. Lock’s resignation of his post. Government 
 decided to create a Department of Agriculture, and 
 to place the Gardens under its administration. Dr. 
 Willis retired, and R. H. Lock was offered the post of 
 
BIOGRAPHICAL NOTE 
 
 Superintendent of the Gardens. He felt that this was 
 a post out of his "line ’’ and special interests, and one 
 distinctly fitted for a systematic botanist. He there- 
 fore refused it. 
 
 He returned to England and spent a few months in 
 writing a book on ‘ Rubber and Rubber Planting ’ 
 (Camb. Univ. Press) — the result of his extensive 
 experiments and study of rubber during his time in 
 Ceylon. He then accepted an Inspectorship under the 
 Board of Agriculture and Fisheries. He was given 
 charge of Birmingham and district, Shropshire, Stafford- 
 shire, and the fruit- and hop-growing counties of 
 Worcestershire, Herefordshire, and Gloucestershire. 
 Pie threw himself into his work with his usual ardour. 
 An Inspector under the Board of Agriculture has to 
 travel continuously through his extensive district, 
 with the object of persuading farmers and fruit and 
 hop growers to check the spread of plant diseases. 
 R. PI. Lock was a keen and expert motorist, and he 
 travelled thousands of miles in his two-seater car, 
 enjoying the beauties of the English countryside after 
 his sojourn in the Tropics. He always exerted in his 
 work a very powerful influence upon those with whom 
 he came in contact, and as an Inspector this was 
 particularly noticeable. A friend of his — a well-known 
 Professor of Agriculture — wrote : 
 
 ‘ In the summer of 1913 I spent a day with him 
 visiting Birmingham, and was much interested in 
 observing the quiet and forceful manner in which he 
 got his ovm way.’ 
 
 As he had now little time or opportunity for sport. 
 
BIOGRAPHICAL NOTE 
 
 he followed the advice of his favourite Candide, and 
 cultivated his garden in his leisure. He put his 
 characteristic energy and thoroughness into this hobby, 
 and obtained correspondingly good results. 
 
 Wlien war broke out, R. H. Lock was one of the 
 first to feel he must join the army. He had shot for 
 his College when he was a member of the University 
 Volunteers, and he had been a member of the Ceylon 
 Mounted Rifles. But Government requested him to 
 undertake special war work. He was appointed 
 Chairman of a Vegetable Drying and Fruit Preserving 
 Committee, and he threw himself heart and soul into 
 the immense amount of work that arose. Two 
 factories were started by the Committee with a view 
 to experimenting on the drying and preserving of 
 fruit and vegetables, thus conserving and increasing 
 the national food-supply, and providing attractive 
 and nutritive food for the troops. The work interested 
 him keenly, for he united in a remarkable degree 
 scientific and business abilities. He was intensely 
 practical in everything he did. Naturally the lion’s 
 share of work and responsibility devolved on the 
 chairman. In addition, he had his own work of 
 Inspector rendered still heavier by the shrinkage of 
 the staff of the Board, many of whom were with the 
 colours. 
 
 In February, 1915, at the most interesting and 
 crucial stage in the development of the factories, 
 R. H. Lock was struck down by a severe attack of 
 influenza, but he refused to give in for more than a few 
 days. He made an incomplete recovery, and in 
 
 xvii 
 
BIOGRAPHICAL NOTE 
 
 March was again attacked by the same illness accom- 
 panied by tonsilitis. Even then he refused, in the 
 face of the doctor’s serious warnings, to give in or 
 to take a holiday. He felt that things were shaping 
 well and he could not abandon them just then. With 
 extraordinary fortitude he struggled through the illness 
 and carried on his work till June, when certain dis- 
 quieting symptoms of breathlessness showed them- 
 selves. Several specialists were consulted, but none of 
 them could actually point to the real cause of the 
 trouble. The opinions inclined to overstrain of nerves, 
 and a holiday was insisted on. In the middle of June 
 he went to Eastbourne. He enjoyed the change, but 
 the alarming symptoms did not abate. Suddenly, 
 without a moment’s warning, he died of heart failure 
 on June 26, 1915- 
 
 Kis devotion to duty cost him his life. As one of 
 his chiefs at the Board wrote : 
 
 ‘ He gave his life for his country as much as if he 
 had fallen on the fields of France.’ 
 
 It was only given to a few to realize the extraordinary 
 depths and beauty of his nature. He was shy and 
 reserved in society, and yet in his own four walls he 
 was full of whimsical humour. He could not suffer 
 fools gladly, and his hatred of humbugs both great 
 and small was as fiery as Huxley’s. An old college 
 friend, writing about the photograph which prefaces 
 this note, said : 
 
 ‘ The photograph is quite the best of R. H. that I’ve 
 ever seen. I can’t tell you how glad I am to have it. 
 He looks as though he were imploring me not to be 
 
 xviii 
 
BIOGRAPHICAL NOTE 
 
 merely specious in my argument — a thing which in 
 the old days often happened ! I am so glad the 
 photo has this characteristic expression. I think, 
 in a very charming way, it reflects the outstanding 
 feature of his moral and mental personality. He 
 liked cleverness, but wouldn’t put up with it for a 
 moment if it were superficial. His love of truth was 
 something more than the habit of a trained scientist.’ 
 
 He would never go out of his way by a hair’s-breadth 
 to conciliate or pander to ‘ useful ’ people whom he 
 disliked in order to advance his interests. He was 
 tittiid socially, but he was absolutely fearless where 
 the rights and wrongs of life were concerned. 
 
 He would not ‘ strike leagues of friendship with 
 cheap persons, where no friendship can be,’ but he 
 had a few friends to whom he stood in the closest 
 relation. His best friend wrote : ‘ He was more to 
 me than a brother.’ 
 
 From those over whom he was set in authority he 
 won the most loyal and devoted service. This applied 
 to the native staff at Peradeniya no less than to his 
 Sub-Inspectors of the Board of Agriculture. He was 
 always considerate, just, and above all anxious to give 
 every man credit for work well done. His staff knew 
 that he never spared himself, and they had a great 
 respect for his untiring energy and his thoroughness. 
 
 The following beautiful tribute came to me from one 
 of his Sub-Inspectors : 
 
 ‘ Although it is only a little over a year since I first 
 knew him, yet I feel that I have lost a valuable friend. 
 He was more like a friend than a supervisor, for he 
 
 XIX 
 
BIOGRAPHICAL NOTE 
 
 was always most considerate; and the year I spent 
 under his supervision I shall always look back upon 
 as one of the happiest years of my life, and I am sure 
 the influence and knowledge gained will be a valuable 
 asset to me in the future. 
 
 ‘ He has also left behind him many friends amongst 
 the market gardeners, etc., here, for on all hands I 
 hear genuine expressions of regret at his untimely 
 death.’ 
 
 Pie was entirely unselfish in daily life, and tender 
 and humane towards all living things. Although he 
 was a good shot at the ranges, during all the years in 
 Ceylon he never ‘ went out to kill something.’ Killing 
 for sport had no attraction for him. He preferred 
 golf, and tennis, and football — in all these games 
 he was proficient above the average. 
 
 In his thirty-six years he had achieved much. 
 Men of mark looked upon him as one of the leading 
 younger men in the world of agricultural science. 
 The Board of Agriculture destined him for the impor- 
 tant post of British Representative at Rome on the 
 Council of the International Agricultural Institute. 
 
 To England the loss of such sons is great ; he gave up 
 his whole life to the service of his country — and laid it 
 down for her. No man ever had a stronger sense of 
 duty. To those who knew him well he was a source 
 of inspiration and strength. He never preached or 
 posed. He was just himself, incapable of affectation or 
 pretence. 
 
 B. L. 
 
 June, 1916 . 
 
 XX 
 
RECENT PROGRESS 
 IN THE STUDY OF VARIATION, 
 HEREDITY, AND EVOLUTION 
 
 CHAPTER I 
 
 INTRODUCTION 
 
 The present volume deals with variation and inheii- 
 tance in plants and animals, especially in so far as 
 those subjects bear upon the problem of the origin of 
 species. By inheritance we mean those methods and 
 processes by which the constitution and characteristics 
 of an animal or plant are handed on to its offspring, 
 this transmission of characters being, of course, asso- 
 ciated with the fact that the offspring is developed by 
 the processes of growth out of a small fragment de- 
 tached from the parent organism. The term ‘ varia- 
 tion,’ on the other hand, includes a number of different 
 phenomena which will be described at greater length 
 as the work proceeds ; but, broadly speaking, we may 
 say that the study of variation is concerned with the 
 circumstance that members of the same species are 
 not aU alikei and more particularly with the fact that 
 
 I 
 
 
2 
 
 INTRODUCTION 
 
 differences are to be found between different members 
 of the same family. Some of these differences arise 
 comparatively late in life, and may be the result of 
 circumstances or of education. It is the first duty of 
 the student of variation to distinguish as far as may 
 be possible between differences of this kind on the one 
 hand, and those differences on the other which depend 
 upon the fact that the different detached fragments, 
 \f as we have termed them, of the parent organism — its 
 germ-cells, in fact — show greater or smaller differences 
 among themselves. 
 
 The facts of variation have this very special impor- 
 tance, that the whole theory of organic evolution is 
 based upon them. The fact that members of the same 
 species are not all alike, depending upon the further 
 fact that offspring may differ from their parents, makes 
 it possible in the course of generations for progressive 
 changes to take place, so that from the offspring of 
 different members of the same species different new 
 species may arise. But for this fact of variation it 
 would have been quite impossible for Darwin'^to have 
 ( overthrown the former crude belief in a fecial crea- 
 ^ tion of each separate species, since there would have 
 been no material for his great factor — natural selection 
 — ^to work upon. It is with variation, then, and with 
 the manner in which characters appear in the succes- 
 sive generations of living things, that we are here 
 concerned. 
 
 Ever since the publication of Darwin’s ‘Origin of 
 Species ’ in 1859, these subjects, and especially the 
 theoretical aspects of them, have been received even 
 
THE STUDY OF EVOLUTION 
 
 3 
 
 by the general public with all the signs of a genuine 
 enthusiasm ; and nonej moreovetj can be more fas- 
 cinating to the professional naturalist. But since the 
 time of Darwin the more popular accounts have dealt 
 almost exclusively with theoretical considerations and 
 with matters of opinion. Highly abstruse contro- 
 versies have raged freely between Neo-Lamarckians 
 and Neo-Darwinians^ and these have found a place 
 in the pages of works ostensibly intended for the in- 
 struction of all and sundry ; whilst only a bare re- 
 siduum of actual matters of fact has seen the light of 
 popular publication. If the truth must be told^ the 
 experimental method was given up for a long time by 
 the majority of specialists themselves in favour of the 
 controversial, and, indeed, this tendency has by no 
 means yet died out from among the habits of some 
 professed evolutionists. On the other hand, during 
 the last fifteen to twenty years, a few scattered workers 
 have diligently applied themselves to a study of the 
 facts of variation and inheritance, with results which 
 already more than justify the anticipation in which 
 their work was begun — namely, that by such methods 
 alone can any real progress in our knowledge of the 
 processes of evolution be brought about. 
 
 The science of organic evolution is by no means the 
 simple and isolated study it might be supposed to be 
 from a perusal of some of the more popular accounts. 
 Its footing rests immediately upon the widest founda- 
 tions which zoology, botany, and physiology can afford ; 
 and these in their turn are ultimately based upon the 
 results of chemical and physical science. But some 
 
 I — 3 
 
4 
 
 INTRODUCTION 
 
 of the most fundamental parts of physical science, as 
 I think we may fairly call the branches of electricity 
 and molecular physics, seem at present to be under- 
 going modifications which bid fair to bring about a 
 complete revolution in current ideas upon these sub- 
 jects. It is highly probable that these results will 
 ultimately lead to a considerable modification in pre- 
 vailing notions about living things ; but the new 
 developments have yet to reach biology through the 
 channels of organic chemistry^ physiology^ cytology, 
 and the like, and at present we do not know what the 
 result of this influx is likely to be. These considera- 
 tions need not, however, detain us, for the new know- 
 ledge of variation and inheritance, of which it is pro- 
 posed to give some account, is largely concerned with 
 the grosser characters of organisms, so that ultraminute 
 structures may be left alone for the present until the 
 stream of physical knowledge stirs them into greater 
 prominence. So much is this the case with the study 
 of variation and inheritance by experimental methods 
 at the present day, that this science is treated by some 
 with a fine contempt, because its tools are those of the 
 breeder and gardener, and because the assistance of 
 the compound microscope may often be laid aside for 
 days together. Yet this applies only to one aspect of 
 the subject, and the microscopic study of the embryonic 
 rudiments of organisms, going hand in hand with the 
 experimental observation of adult structures, is rapidly 
 leading to a clearer understanding of the processes of 
 heredity. 
 
 The problem which those who are engaged in this 
 
THE PROBLEM OF SPECIES 
 
 5 
 
 kind of work have set themselves for solution is that 
 of the nature and method of origin of the existing 
 differences between certain groups of organic beings 
 — namely, species. • Basing their studies on the doc- 
 trine that the present species have arisen through the 
 modification of pre-existing species, they endeavour 
 to observe how modifications of existing species do 
 actually arise in Nature, as well as under domestica- 
 tion ; and they watch the hereditary transmission of 
 the modified forms when like is bred with like, and 
 when different types are crossed together.- For the 
 theory of uniformity, now universally accepted, teaches 
 us that the organisms with which we are now familiar 
 owe their present characteristics to the accumulation 
 of a series of changes similar to those which are still 
 in progress. It has, thereforej appeared likely to a 
 few that a further understanding of the processes of 
 evolution might best be obtained by a closer study< 
 firstly, of variationj or the ways in which offspring 
 differ from their parents ; and, secondly, of inheritance, 
 or the ways in which the resemblances between parents 
 and their offspring are perpetuated from one genera- 
 tion to another. 
 
 It may be well to point out at once that the further 
 study of the method of origin of new species, admitting, 
 as it does, that this process is not yet by any means 
 fully understood, does not for this reason imply that 
 the theory of organic evolution itself is open to criti- 
 cism. The evidence that new species arise by the 
 modification of pre-existing species is quite indepen- 
 dent of the evidence that this process invariably occurs 
 
6 
 
 INTRODUCTION 
 
 by the action of natural selection upon minute differ- 
 ences, in the manner which Darwin described, and 
 which has been claimed by others as the sole means 
 by which the origin of new forms takes place.* The 
 evidences of evolution are much more numerous and 
 more weighty than the evidences of the survival, of 
 the fittest. ' The theory of evolution, as opposed to 
 the creation hypothesis, is supported by innumerable 
 facts of classification, of morphology, and of embryo- 
 logy, by the geographical distribution of animals and 
 plants, and by their succession in the geological strata, 
 as weU as by direct observation of the actual occurrence 
 of changes in the case of domestic productions as well 
 as under Nature, and many of these facts have no 
 direct bearing upon the theory of natural selection. 
 
 Before discussing the problem of the origin of species, 
 it is necessary to arrive at some idea as to what the 
 term ‘ species ’ means. And this is not altogether an 
 easy matter, since a precise definition has not been, 
 and cannot be, agreed upon. The idea of species is, 
 indeed, of great antiquity and very gradual growth. 
 Primitive men doubtless recognised certain plants or 
 animals as being like one another, and different from 
 others, and they gradually came to distinguish such 
 forms by giving a different name to each. The names 
 first used must have applied as a rule to genera rather 
 than to species. Thus, such common names of plants 
 as rose, bramble, vetch, nettle, dock, crowfoot, are 
 names of genera— groups of greater extent than species^ 
 and often more easily defined than the latter. Later 
 
LINNiEAN SPECIES 
 
 7 
 
 on civilized men paid closer attention to tne' different 
 kinds of plants', and the old herbalists discovered and 
 described a number of different sorts of roses, of butter- 
 cups, and of other plants, and distinguished each by a 
 descriptive sentence. 
 
 As more and more species came to be described, this 
 method of designation became very cumbersome, until 
 Linnaeus , about the middle of the eighteenth century, 
 adopted the idea of a binomial nomenclature (originally 
 suggested by Bachmann), in which every species of 
 each known genus received a separate name of its own 
 to distinguish it, so that the different kinds of butter- 
 cups were now known as Ranunculus acris, R. bulbosus, 
 R. sceleratus, and so on. 
 
 Linnaeus himself appears to have had a very definite 
 idea of what constituted a species, and in accordance 
 with the view then current,' he defined a species as 
 being a group of organisms which owed its origin to a 
 separate act of creation. ' From the nature of the case 
 this definition could be of little use in practice. Prac- 
 tically, then, species were defined as groups of animals 
 or plants, the members of which resembled one another 
 in definite morphological characteristics — that is to 
 say, in constant features of form and structure. This 
 definition has survived the downfall of the dogma of 
 the constancy of species, and at the present day species 
 as defined by Linnaeus are found to be groups of much 
 merit both for naturalness and for convenience — at 
 any rate in the case of plants. The fact that inter- 
 mediate forms and minor groups do sometimes and to 
 some extent bridge over the gap which separates a 
 
8 
 
 INTRODUCTION 
 
 pair of species thus defined seems to have caused dis- 
 quiet in the mind of Linnaeus himself, and he recom- 
 mended his disciples to have no dealings with these 
 inferior varieties, as being beneath the dignity of 4 
 botanist to notice. Of late years these minor species 
 have excited much attention, and it is to a study of ^ 
 this kind of species in particular that the m u tati on 
 theory of de Vries owes its origin, as will be told in a 
 later chapter. 
 
 Such minor groups, occurring within the limits of a 
 single Linnsean species, and subdividing it into smaller 
 collections of individuals, were made the object of 
 special study in the case of plants by the French 
 botanist, Jordan ; and for this reason they are some- 
 \ times referred to as Jordan’s species. Jordan, for 
 example — though the example is indeed an extreme 
 one — described more than two hundred different types, 
 all of which would formerly have been included in 
 the single Linnasan species, Draha verna. To take a 
 more familiar instance. We find in the ‘ British 
 Flora ’ of Bentham and Hooker the primrose, the 
 cowslip, and the true oxlip, all described as varieties 
 of one and the same species ; yet these three kinds of 
 plants are now almost universally recognised to be 
 as good species as any in nature.* ' In a similar way, 
 on closer investigation, it has been found necessary to 
 split up a considerable number of Linnaean species, 
 and to subdivide each into several species of smaller 
 range. 
 
 * A contrary opinion is, however, expressed in the Journal 
 of Botany for July, 1906. 
 
JORDAN’S SPECIES 
 
 9 
 
 It has already been pointed out that Linnseus him- 
 self distinctly deprecated this process of splitting. 
 ‘ Varietates levissimas non curat botanicus,’ said 
 Linnaeus. Jordan, however, applied the method of 
 experiment to many of the species of his own defini- 
 tion, and having transplanted them from a variety 
 of localities to the uniform soil of a garden, found that 
 they preserved their distinctive characters and came 
 perfectly true to seed. 
 
 It appears then that Jordan’s species are just such 
 true and constant groups as those of Linnaeus. They 
 are separated from one another by definite features of 
 form and structure, only these differences are not so 
 wide as those which separate Linnaean species. The 
 latter are, indeed, to be looked upon as more or less 
 artificial groups or aggregates of these physiological 
 species, as Jordan’s species have also been called. The 
 problem of the origin of the smaller groups is clearly 
 to be placed before that of the origin of the larger 
 species. 
 
 It is true that in the case of certain groups of 
 animals and plants there would appear to be no possi- 
 bility of drawing hard and fast lines between the 
 species, which thus seem to shade gradually one into 
 the other. There is, however, a great difference 
 between the admission that certain nearly related 
 species are difficult or impossible to separate definitely, 
 and the statement that there is no true distinction 
 between them, and the latter statement is one which 
 few are bold enough to make. The case stands thus. 
 We know that great numbers of large groups (classes 
 
10 
 
 INTRODUCTION 
 
 and families) of animals and plants exist, in which 
 the most nearly related species are quite definitely 
 distinct from one another. In other classes systema- 
 tists have so far found great difficulty in framing 
 definitions of specific groups. We shall see later on, 
 though at first sight it may appear almost paradoxical, 
 that it is quite possible for groups to be perfectly 
 distinct, although individual members of them may 
 have deviated so far, each from its proper type, as to 
 render impossible the task of deciding from their 
 appearance which group any of these individuals 
 belong to. 
 
 Let us next consider a particular example of a class 
 of animals in which the discrimination of species is 
 difficult or impossible. This is said to be the fact 
 with the majority of sessile animals — such animals 
 as resemble plants in their stationary habit, and in 
 no case are the problems of species separation more 
 difficult than in the class of the stony corals. Now, 
 attempts to determine the species of corals have so 
 far been made almost entirely from a study of what 
 may be called vegetative characters — ^usually from 
 details of the shape and structure of the stony skeleton 
 of the animals. How far these features may be affected 
 by external circumstances has not been determined, 
 but it must be noted that the so-called skeleton is 
 entirely external to the living organism. Now we 
 know that in the case of many of the higher plants 
 vegetative characters are extremely liable to become 
 modified owing to the action of the environment. 
 Differences of moisture, light, soil, climate, and alti- 
 
DISCONTINUITY OF SPECIES 
 
 II 
 
 tude, are aU capable of changing the general appear- 
 ance of a plant so as to render it scarcely recognisable. 
 Fortunately, in the case of the higher plants, the 
 floral organs, which are the ones chiefly made use of 
 for purposes of specific discrimination, are very little 
 liable to modification by external conditions ; but 
 in the corals a similarly stable set of organs does 
 not appear to have been discovered. It seems, there- 
 fore, hardly fair to regard the example of the corals 
 as affording an established exception to what we 
 must look upon as the general rule — namely, that 
 species are on the whole definite and discontinuous 
 groups. 
 
 As a rule, then, the species riddle presents itself 
 definitely as the problem of the existence of a series 
 of discontinuous groups of creatures, sharply marked 
 off the one from the other, and often, too, existing 
 among surroundings which afford no corresponding 
 discontinuity, though each is well enough fitted for 
 the life which it has to lead. 
 
 The problem which we have to face has been 
 enunciated by Bateson in the form of the two following 
 propositions ; 
 
 ‘ I. The forms of living things are various, and on 
 the whole are discontinuous or specific. 
 
 ‘ 2. The specific forms on the whole fit the places ^ 
 they have to live in. 
 
 ‘ How,’ he continues, ‘ have these discontinuous 
 forms been brought into existence, and how is it they 
 are thus adapted ? This is the question the naturalist 
 is to answer. To answer it completely he must find 
 
12 
 
 INTRODUCTION 
 
 (i) the modes and {2) the causes by which these things 
 have come to pass.’ 
 
 The differences between existing species are open 
 to study in more than one way. By way of limiting 
 the discussion for the present, we shall consider the 
 case of plant species only ; but the methods of study 
 which are applicable to animal species are of quite 
 similar kinds. 
 
 Four methods at least are available. Firstly, that 
 of comparison ; secondly, the method of statistical 
 examination ; thirdly, the method of cultural experi- 
 ment ; and lastly, that of cross-breeding. 
 
 The method of comparison is the one to which the 
 ordinary worker in descriptive botany is almost of 
 necessity confined. In this way plants which closely 
 resemble one another are grouped together as belong- 
 ing to the same species, and separated from others, 
 the appearance of which is different. By appearance 
 is not meant simply the general habit of the plants ; 
 all morphological features whatever may be used for 
 purposes of comparison, and the most minute are 
 often of the greatest importance. But the systema- 
 tist who works only in this way knows nothing of the 
 real relationships between the plants with which he 
 is dealing. 
 
 ■^Tien a sufficient number of specimens is available, 
 the methods of statistics can be applied. These 
 involve the making of a series of accurate measure- 
 ments or countings of the parts upon which depend 
 the supposed differences or resemblances of the plants 
 
METHODS OF STUDY 
 
 13 
 
 under consideration. The resulting numbers are then 
 ranged in order so that a precise view of the numerical 
 characters of a large number of specimens can be 
 readily obtained. By the use of such methods valu- 
 able information is often to be arrived at. But the 
 same limitation affects them as in the preceding case. 
 
 So that the only way in which we can come to a 
 definite decision as to whether a given putative 
 species does or does not represent a definite and con- 
 stant type is by resorting to our third method, that 
 of sowing its seeds and actually rearing its progeny. 
 And this is not so simple a matter as might appear 
 at first sight, for a great many precautions have to 
 be taken. Thus we must separately sow the seeds of 
 many different individuals of the species which we 
 are examining, so as not to base our conclusions upon 
 a few experiments only. But in many cases, even 
 when this has been done, we should only know one of 
 the parents of our seedlings — that is to say, in cases 
 where the pollen for fertilization may possibly have 
 been conveyed by natural agencies from a different 
 plant. In such a case we must either ensure self- 
 polhnation by isolating our plants, or we must arti- 
 ficially provide pollen from a separate known parent. 
 If under these circumstances a particular group of 
 plants preserves the characteristic differences which 
 distinguish it from another group which has also been 
 grown for a number of generations under the same 
 conditions, we have at last reasonable grounds upon 
 which to base the opinion that we are dealing with 
 two distinct physiological species, even though the 
 
14 
 
 INTRODUCTION 
 
 visible differences between them may seem very small 
 to an untrained eye. 
 
 Lastly, evidence of a confirmatory nature may be 
 obtained by observing the results of cross-fertilization 
 between a pair of closely allied species. Much, too, 
 may be made out from the failure of such experimentsj 
 since the refusal of two plants to breed together is 
 generally regarded as clear evidence of their specific 
 distinctness. But for this reason the method of cross- 
 breeding is more particularly adapted for the examina- 
 tion of forms somewhat nearly related to one another 
 — for example, different members of the same species. 
 
 As the result of the methods presently to be 
 described, the fact has been established that two 
 entirely distinct sorts of divergencies may appear 
 among members of a single family. Variations, that 
 is to say, may be of at least ,two different kinds. In 
 the first place we have those slight differences which 
 invariably distinguish all the members of every family 
 — individual variations which affect every part and 
 every character. Such differences are known as 
 fluctuating, normal, or continuous variations. As an 
 example we may cite the variations in size or stature 
 shown by the various members of any purely-bred race. 
 When a large number of individuals are compared in 
 respect of a character of this kind, they are found to 
 fall into a continuous series ranging from a certain 
 extreme of shortness on the one hand to an extreme 
 of tallness on the other. Individuals of a medium 
 height , however, are usually more numerous than either 
 
VARIATION 
 
 X5 
 
 of the extreme forms. Some further account of the 
 study of continuous variations will be given in 
 Chapter IV. 
 
 A second kind of variation is variously known as 
 abnormal, definite, and discontinuous variation, and 
 includes what are known as sports and mutations. 
 Such variations, as is indicated by the terms applied 
 to them,; involve definite differences usually of con- 
 siderable amplitude. A good example of a discon- 
 tinuous variation would be afforded by the appear- 
 ance of a child having six fingers in a family in which 
 this abnormality was not previously known to occur. 
 We shall pursue the discussion of discontinuous varia- 
 tion and of the methods of perpetuation of the types 
 which thus arise in Chapters V. and VII. 
 
 A short account of the historical development of the 
 theory of organic evolution is given in Chapter II. As 
 alternatives to the theory of the special creation of 
 each individual species, a number of more or less crude 
 speculations were indulged in by the philosophers of 
 ancient Greece. Passing to more recent times, three 
 distinct accounts of the method of origin of specific 
 differences have been proposed almost within the last 
 century, and each of these theories still finds a number 
 of supporters. 
 
 I. The view of Lamarck, published first in i8oi, and 
 in an enlarged form in 1809, was briefly as follows 5 
 Noticing that the organs of men and other animals are 
 increased and strengthened by use, and particularly 
 by conscious use, Lamarck assumed that this effect 
 
i6 
 
 INTRODUCTION 
 
 could be passed on by inheritance from parent to off- 
 springs and so accumulated from generation to genera- 
 tion. In the case of animals Lamarck conceived the 
 prodiiction of a new specific form to take place in the 
 following way : Owing to some change of external con- 
 ditionss the desire to perform some new kind of action 
 was set up in the parent species, and by the hereditary 
 effect of the striving occasioned by this desire a modi- 
 fication of the organs affected into forms better fitted 
 to carry out the new function was gradually achieved. 
 
 Thus Lamarck supposed that snakes were evolved 
 from a pre-existing type of animal which was of a much 
 less attenuated shape, and which possessed two pairs 
 of limbs like any other vertebrates. And he supposed 
 this evolution to have taken place owing to the con- 
 stant striving of these animals to pass through narrow 
 crevices ; the effect of such striving being inherited, and 
 so accumulated from one generation to another. 
 
 In the case of plants, in which conscious effort is 
 precluded, a similar result was supposed to have been 
 attained by an hereditary accumulation of the effects 
 of the environment. 
 
 2. The explanation of Darwin, or at least the Neo- 
 Darwinian form of it, as interpreted by Wallace, 
 Weismann, and others, and as opposed to and exclud- 
 ing the view of Lamarck, was as follows : Two separate 
 factors are primarily concerned : (i) the fact of fluc- 
 tuating variation — ^the fact that no two members of 
 the same family ever resemble one another exactly ; 
 and ( 2 ) the occurrence of a struggle for existence 
 between organisms owing to the geometric rate of 
 
THEORIES OF THE ORIGIN OF SPECIES 17 
 
 increase of living things. From these two facts it 
 follows that when a change of environment takes - 
 place, certain members of an existing species will be 
 somewhat better adapted than others to withstand 
 the new conditions, and the former will tend to survive 
 to the exclusion of the latter. It is assumed that 
 during a long series of generations this process will 
 cause a steady change in the character of the species in 
 the direction of better adaptation to the new conditions. 
 
 Thus we might suppose that among the ancestors of 
 the snakes those which happened to possess the longest 
 and thinnest bodies and the smallest limbs had the 
 advantage over their fellows that they were able to 
 crawl through narrower holes, and that for this reason 
 a greater number of them survived to produce off- 
 spring. Here we have a better basis for reasoning 
 than the supporters of Lamarck’s doctrine, because 
 we actually know that longer parents^ in whom this 
 quality was apparently not the result of taking thought j 
 do tend to produce on the average longer offspring. 
 
 3. The view of the mutationists, already fore- 
 shadowed by Aristotle, and in recent years especially 
 associated with the names of Bateson and de Vries, 
 expresses the conclusion that the evolution of new 
 species has taken place principally by the help of 
 variations of the discontinuous kind. By this process 
 there can arise at a single step new forms which have 
 already the complete and definite character usually 
 associated with a species specially adapted to particular 
 conditions. Of these new forms, those which happen 
 to be fitted for their surroundings as well as or better 
 
 2 
 
i8 
 
 INTRODUCTION 
 
 than their predecessors will survive, whilst those which 
 are worse will be destroyed by the action of natural 
 selection. 
 
 Thus it would be an appropriate use of this concep- 
 tion to seek in a mutation the explanation of the final 
 loss of the much reduced limbs presumably exhibited 
 by comparatively I'ecent ancestors of the family of 
 snakes. This final loss is especially difficult to under- 
 stand on the Darwinian theory. Moreover, changes of 
 a closely similar nature are not hypothetical, but have 
 actually been observed to take place. At the same 
 time it must not be supposed that mutations are con- 
 fined to the loss of pre-existing organs ; indeed, the 
 origin of a totally new organ is quite inexplicable on 
 either of the two preceding theories. The very first 
 inception of such an organ must* it would seem, of 
 necessity be sudden. 
 
 After giving some account of the earlier theories of 
 evolution, we shall next proceed to treat of those sub- 
 jects with which we are more properly concerned — 
 that is to say, the recent experimental observations 
 on variation and natural inheritancej together with 
 their bearing on the theories of evolution. And in the 
 first place we shall describe some recent studies which 
 are not strictly experimental, but which nevertheless 
 deal to some extent with actual facts — namely, the 
 statistical study of variations, particularly of con- 
 tinuous variations. This subject has been dignified 
 by a special name, and is now described as the science 
 of biometry. 
 
MENDEL’S METHOD 
 
 19 
 
 Of even greater interest, however, are the more 
 strictly experimental researches which have been 
 published during the present century. In the first 
 place, we have the observations of de Vries, who has 
 introduced a new method of study — that of cultivating 
 great numbers of seedling plants with the object of 
 discovering definite new forms or mutations among 
 their number. Lastly, and in its results much 
 the most important of all, we have the method of 
 Mendel, published half a century ago, but only re- 
 cently brought into prominence owing to its redis- 
 covery and confirmation by three independent workers 
 — Correns, Tschermak, and de Vries. This method 
 consists in the cross breeding of strains of plants or 
 animals which differ in definite characters, and in the 
 statistical examination of the proportions in which 
 these characters appear among the offspring obtained 
 from the crosses. 
 
 Further experiments on the lines which Mendel in- 
 dicated bid fair to revolutionize within a few years the 
 arts of the breeders of plants and animals. This is 
 due to the fact that such experiments are leading to 
 the introduction into these pursuits of a degree of 
 scientific exactness which was previously altogether 
 unforeseen. The change in our ideas regarding the 
 method of hereditary transmission of characters, which 
 has resulted from these experiments, has been aptly 
 compared with the change brought about in men’s 
 understanding of the science of chemistry by 
 Dalton’s conception of the atom. For the rest the 
 new experiments tend on the whole to confirm the 
 
 3 — 3 
 
20 
 
 INTRODUCTION 
 
 experience of practical breeders ; only the elucida- 
 tion of one simple rule of inheritance has brought into 
 order a host of phenomena, which were previously quite 
 incapable of a coherent explanation. 
 
 The experimental results with which it is the pro- 
 vince of this book to deal are, then, firstly those of 
 biometry* or the statistical study of variations, and 
 particularly of continuous variations ; secondly, the 
 results of direct observations bearing upon the origin 
 of species by the discontinuous method ; and thirdly, 
 the results of experimental observations on heredity 
 by the methods of scientific breeding. By these 
 methods results of the utmost moment to mankind 
 have been, and are being, arrived at, quite apart from 
 their interest as bearing upon the problems of evolu- 
 tion. From a biologist’s point of view, however, the 
 latter is, of course, paramount. And so it has been 
 thought fitting to begin with a brief discussion of the 
 problems of evolution, and of the various solutions of 
 them which have been from time to time suggested. 
 
 In a later chapter some of the more prominent recent 
 results of the kindred science of C3d:ology — the micro- 
 scopic study of the minute constituent parts of organ- 
 isms — ^will be briefly described, on account of the very 
 close connection which recent progress in this subject 
 bears to the experimental study of the inheritance of 
 the grosser characters. 
 
 Finally, we have essayed a brief account of the 
 science to which the name of ‘ Eugenics ' has recently 
 been applied by Sir Francis Galton. In the chapter 
 dealing with Eugenics an attempt is made to show 
 
EUGENICS 
 
 21 
 
 how far our recently acquired knowledge of variation 
 and heredity can be applied to mankind, and to ' the 
 study of agencies under social control that may im- 
 prove or impair the racial qualities of future genera- 
 tions/ 
 
 BIBLIOGRAPHY. 
 
 The two following works will be found specially useful on 
 account of the extensive bibliographies which they contain : 
 
 Morgan, T. H. : Experimental Zoology, 1907. 
 
 Thomson, J. A. : Heredity, revised edition, 1919. 
 
 These may be consulted for further references beyond those 
 given at the end of each of the following chapters. 
 
 As an introduction to the whole subject of Heredity : 
 
 Doncaster, L. : Heredity in the Light of Recent 
 Research, 1911. 
 
CHAPTER II 
 
 EVOLUTION 
 
 Evolution may be defined as progress involving dif- 
 ferentiation, an ever-growing complication of things 
 which accompanies almost aU the operations of Nature. 
 The idea of a differentiation of this kind may be en- 
 forced by a homely and quite imaginary illustration 
 of such a process. Imagine the proper ingredients of 
 a plum cake to be very finely minced and intimately 
 mixed together, so as to form a more or less homo- 
 geneous material. Then, if by any means the separate 
 particles of currants, raisins, peel, and so forth, could 
 be made to segregate out in such a way as to give rise 
 to the ordinary structure of this pleasant confection, 
 we should have arrived at the structure of a plum cake 
 by a process of evolution involving considerable dif- 
 ferentiation. 
 
 The progressive increase in complexity which is 
 Characteristic of so many natural processes is in great 
 part occasioned by the fact that a single ‘ cause ’ is 
 followed as a general rule by more than one ‘ effect.’ 
 This apparently simple circumstance was pointed out 
 
 22 
 
EVOLUTION OF MATTER 
 
 23 
 
 by Herbert Spencer,* who has perhaps done more than 
 any other to establish and emphasize the general ap- 
 plicability of the evolution idea. For the law of origin 
 by evolution is by no means exclusively confined to 
 the method of coming into existence of the species of 
 animals and plants. On the contrary, it was equally 
 well applied by Spencer himself to describe the manner 
 in which are supposed to have arisen the stars and 
 other heavenly bodies, the geological strata and geo- 
 graphical configuration of the earth, and the various 
 gradations of human society. 
 
 The discovery that certain chemical elements exist 
 which are themselves not immutable has been made 
 since Spencer’s time. Quite recently ‘ the phenomena 
 of radio-activity have forced us to believe that radium 
 is passing continuously into helium,’! and something 
 more than a suspicion has been aroused that radium 
 is itself derived from uranium. Thus the dreams of 
 the alchemists are shown to have been not wholly 
 without foundation, for the probability is strong 
 
 * Spencer gives the following illustration : Regarding the 
 striking together of two bodies as a ' cause/ he points to the 
 following possible * effects * : A sound ; other vibrations or 
 movements in the surrounding air ; a disarrangement of the 
 particles of the two bodies in the neighbourhood of the point 
 of collision ; the production of heat, and possibly of a spark — 
 i,e,, of light. 
 
 Two words in this sentence are placed between inverted 
 commas, to indicate that they are used in a strictly popular 
 sense. The use of the words ' cause ' and * effect,' though 
 seldom striciiy scientific, is often convenient, and if used with 
 caution, there is no reason why they should lead to misunder- 
 standing. See Whetham, ‘ The Recent Development of 
 Phyrical Science/ chapter i, 
 
 t W'hctham. 
 
24 
 
 EVOLUTION 
 
 that under suitable conditions other matter may be 
 observed to behave in the same way as radium. More 
 than this. Professor Sir J, J. Thomson has been able to 
 describe the atoms of the elements as different aggrega- 
 tions of a single kind of corpuscles, and to show that 
 a progressive change in the number of corpuscles 
 making up the atom is accompanied by a progressive 
 alteration in the properties of the atom itself, so that 
 it has now become possible to establish a theory of 
 the evolution of the chemical elements themselves. 
 
 Passing from the almost immeasurably small to the 
 almost immeasurably great, we may briefly consider 
 the probable mode of origin of the solar system from 
 an extremely diffuse cloud of material substance, ac- 
 cording to the famous nebular hypothesis of Laplace. 
 By a long-continued process of contraction under the 
 influence of gravity the nebular substance came to be 
 of varying density, and acquired a rotary movement 
 in one plane. As the mass continued to contract 
 owing to the mutual attraction of its particles, the 
 velocity of rotation increased, until at last the increas- 
 ingly rapid motion of the outermost ring of the now 
 lens-shaped nebula gave rise to a centrifugal force great 
 enough, to counteract the tendency to contraction, and 
 in the further condensation of the mass this ring was 
 left behind. The ring next broke down at one point, 
 and contracting on itself gave rise to a single spheroidal 
 body which acquired a movement of rotation in the 
 same direction as that of the parent nebula. This 
 body was the outermost planet Neptune, and the rest 
 of the planets were produced in a similar manneri 
 
EVOLUTION IN GEOLOGY 
 
 25 
 
 until at last a central mass was left, and this became 
 the sun. Satellites were thrown off from several of 
 the planets just in the same way as the planets them- 
 selves arose from the original nebula, and Saturn’s 
 rings are pointed to as showing this process even now 
 in course of operation. 
 
 Such a description as this may appear fanciful at 
 first sight, but it was worked out quantitatively as 
 well as qualitatively by its author, and was shown to 
 explain in detail a multitude of phenomena. Spencer 
 points out that when we have, worked out by one of 
 the first of mathematicians, a definite theory of plane- 
 tary evolution based on established mechanical lawsi 
 and one which accounts in a satisfactory way for all 
 the known phenomena, the conclusion that the solar 
 system really did arise by a process of evolution is, to 
 say the least, difficult to avoid. 
 
 The establishment and propagation of the idea that 
 the present condition of the earth’s surface arose 
 through a course of gradual evolution, by the agency 
 of such processes only as are known to be in operation 
 at the present day, is the great contribution of Sir 
 Charles LyeU to the science of geology. We may 
 briefly trace the evolution of the idea itself, beginning 
 with the speculations of Werner, who, from observa- 
 tions of the geological formations of a limited tract of 
 country, came to the conclusion that the successive 
 strata were precipitated one by one from an universal 
 ocean. Here we see the first germ of the idea of 
 evolution embodied in the notion that the stratified 
 
26 
 
 EVOLUTION 
 
 rocks came into existence gradually and through the 
 operation of a supposed natural cause. 
 
 A great advance upon Werner’s theory was made 
 by Hutton, who, observing the formation of strata 
 at the present day from the sediment washed down 
 by rivers, concluded that the ancient strata were 
 deposited in the same manner. Since, by the long 
 continuation of this process the continents must 
 gradually become reduced to the level of the sea, 
 Hutton supposed that at long intervals of time the 
 action of subterranean heat came into play, and 
 fresh continents were upheaved, a process accom- 
 panied by the outpouring of the igneous rocks, 
 the true origin of which he had duly recognised. 
 In this theory a hypothetical cause stUl survives, since 
 we have no actual experience of vast upheavals of the 
 kind which Hutton supposed to have taken place. 
 Lyell showed that such slight changes of level as are 
 known to be in progress at the present day, especially 
 in association with the phenomena of earthquakes, 
 might, if continued over a long series of ages, give 
 rise to the necessary amounts of elevation. Lyell also 
 pointed out a number of subsidiary causes of dis- 
 integration and deposition of strata of the kind which 
 can stUl be seen in operation at different parts of the 
 earth’s surface. At the present time it is sometimes 
 thought that Lyell went a little too far in his cham- 
 pionship of the cause of uniformity. LyeU supposed 
 that the agencies which may now be everywhere ob- 
 served in operation, such as rain and rivers, the sea, 
 volcanoes and earthquakes, were sufficient to account 
 
AGE OF THE EARTH 
 
 27 
 
 for all the phenomena which the crust of the earth 
 exhibits. It is now more generally supposed that in 
 very early times forces similar in kind to those in 
 action at the present day may have exhibited con- 
 siderably greater violence. 
 
 To produce the present condition of the surface of 
 the earth by the action, gradually accumulated, of 
 such processes of denudation and upheaval as are now 
 going on around us, vast periods of time are clearly 
 necessary. The early evolutionists, having once got 
 rid of the idea that the date given by Bishop Usher 
 as that of the creation of the world is a necessary and 
 integral part of religion, immediately allowed their 
 imaginations to run riot with regard to the amount 
 of time at their disposal. Since this question of the 
 extent of geological time has an important bearing on 
 the problem of organic as well as upon that of inorganic 
 evolution, it will be well to pay some attention to 
 more recent views upon the subject. 
 
 Some years ago the generous ideas of biologists as 
 well as of geologists were to a great extent shattered 
 by the calculations of Lord Kelvin. These were based 
 upon three separate sets of data, which we may enume- 
 rate without entering into a lengthy explanation of 
 the calculations involved. The evidence made use of 
 consisted of (i) the rate of the earth’s rotation, as 
 affected by tidal retardation ; (2) the rate of secular 
 cooling of the earth, as deduced from the rate at which 
 the temperature of the earth’s crust rises on passing 
 inward from the surface ; and (3) the rate of cooling of 
 the sun by radiation. The three calculations were 
 
28 
 
 EVOLUTION 
 
 found to show a very fair measure of agreements and 
 they led to the conclusion that considerably less than 
 a hundred million years has elapsed since the first for- 
 mation of seas upon this planet, an event which must 
 have preceded the possibility of aqueous geological 
 action and the existence of living organisms 
 
 Allowing for the circumstance that geological pro- 
 cesses may have gone forward with considerably 
 greater rapidity during the earlier periods of the 
 earth’s history than is the case at the present day, the 
 time thus allowed by the physicist is generally regarded 
 by geologists as too little. Reckoning from the known 
 rate of denudation, which is, of course, the same as 
 the rate at which the same material is deposited 
 beneath the sea, Geikie, who admitted, however, that 
 such data are only of a very rough description, 
 concluded that the space of a hundred nuUion years 
 would afiord sufficient time for the laying down 
 of the known aqueous strata. But there can be 
 little doubt that the lower metamorphosed rocks 
 represent a much longer period of time than 
 the primary, secondary, and tertiary epochs added 
 together ; consequently, the respective estimates of 
 Lord Kelvin and the geologists appear to be contra- 
 dictory. The recent discovery of the enormous quan- 
 tities of energy stored up in radio-active substances 
 introduces a serious modification into the mathematical 
 argument from astronomical data, and Sir George 
 Darwin ‘ sees no reason for doubting the possibility 
 of augmenting the estimates of solar heat, as derived 
 from the theory of gravitation, by some such factor 
 
ORGANIC EVOLUTION 
 
 29 
 
 as ten or twenty,’ on the supposition that a consider- 
 able proportion of the sun’s substance was made up of 
 radio-active material. 
 
 The above remarks may serve to illustrate the im- 
 portance of the theory of evolution as applied to the 
 two sciences of astronomy and geology. We pass next 
 to a brief historical consideration of the development 
 of the evolution theory as a method of describing the 
 origin of the species of animals and plants. 
 
 The views of the ancient Greeks cannot be said to 
 have much more than a purely speculative interest. 
 Some rudiments of the idea of evolution have been 
 attributed to Empedocles as well as to several other 
 early writers, and in the writings of Aristotle, for 
 whom the too great faith of his successors for many 
 ages has been followed by a somewhat unmerited 
 degree of contempt in modern times, we find that the 
 evolution idea had reached quite a respectable degree 
 of development. 
 
 In the Middle Ages the adoption of the Jewish cos- 
 mogony by the Christian Churches effectually annihi- 
 lated all useful thought upon the subject of speciesj 
 since the hypothesis of separate creation affords no 
 scope for further speculation or experiment, and it is 
 not until the end of the seventeenth century that we 
 find thoughtful men beginning to struggle against the 
 ecclesiastical bondage. Thus Erasmus Darwin de- 
 rived the idea of generation rather than creation of 
 the world from David Hume, and himself waxes 
 enthusiastic over the thought : 
 
 ‘ That is, it (the world) might have been gradually 
 
30 
 
 EVOLUTION 
 
 produced from very small beginnings, increasing by 
 the activity of its inherent principles, rather than by 
 a sudden evolution of the whole by the almighty fire. 
 What a magnificent idea of the infinite power of The 
 Great Architect ! The Cause of causes. Parent of 
 parents. Ens entium.’’ 
 
 De Maillet, writing in 1735, showed a definite idea 
 of the production of existing species by the modifica- 
 tion of their predecessors. At the beginning of the 
 nineteenth century similar speculations were published 
 by Goethe and by Treviranus, and the latter was the 
 first to apply the term ‘ biology ’ to the science of the 
 phenomena of life. Lamarck about the same time pro- 
 vided a definite theory as to the method by which the 
 modification of species takes place. 
 
 Before discussing Lamarck’s hypothesis and the 
 alternative theories more recently proposed, it will be 
 well to pass in review the evidence upon which is 
 based our belief that the species of animals and plants 
 have arisen through the modification of pre-existing 
 species^ and to show that the greater part of this evi- 
 dence is quite independent of any views which we may 
 adopt as to the actual method by which a particular 
 species came into existence. And in the first place we 
 may point out the entire absence of any evidence, direct 
 or indirect, in favoiir of the alternative supposition of 
 a special creation of each separate species. 
 
 The evidence for evolution falls naturally into a 
 number of fairly well defined sections ; these have 
 been so admirably summarized by Huxley in his 
 essay on ‘ Evolution in Biology,’ that we cannot well 
 
EVIDENCES OF ORGANIC EVOLUTION 31 
 
 do better than recapitulate, with only slight modifi- 
 cations, the arguments there given : 
 
 1. The Gradation of Organisms. — Both in the 
 animal and vegetable kingdoms we may trace, in spite 
 of certain gaps, a long series of gradations in com- 
 plexity of structure, so that between the simplest and 
 the most complicated of living things a great number 
 of intermediate stages are to be found. When we 
 pass to the lower end of the scale in either case, we 
 come upon a group of creatures of comparatively 
 simple organization. Among them we find members 
 with regard to which we cannot definitely say that 
 they are either animals or plants. Moreover, these 
 unicellular organisms resemble in many ways the 
 egg-cell from which every individual among the higher 
 animals and plants originates. 
 
 It is true that we now know it to be quite impossible 
 to dispose all the members of the animal kingdom in 
 a single linear series, such as was formerly suggested, 
 passing in orderly sequence from the amoeba up to 
 man. ‘ Instead of regarding living things as capable 
 of arrangement in one series, like the steps of a ladder, 
 the results of modern investigation compel us to dis- 
 pose them as if they were the twigs and branches of a 
 tree. The ends of the twigs represent individuals, 
 the smallest groups of twigs species, larger groups 
 genera, until we arrive at the source of all these 
 ramifications of the main branch, which is represented 
 by a common plan of structure.’ 
 
 2. Embryology. — All the members of a particular 
 group of animals or plants as a rule resemble one 
 
32 
 
 EVOLUTION 
 
 another more closely in the early stages of their indi- 
 vidual development than they do in the adult condi- 
 tion, and in the earliest stages of all they are often 
 indistinguishable. These facts are explained if we 
 suppose that such individuals have a common origin, 
 that they are descended from a common ancestor, and 
 that traces of their pedigree are still to be observed in 
 the developmental stages through which each one 
 passes. We do not find a complete parallelism be- 
 tween the development of the individual and the 
 history of the race, nor should we expect to do so, 
 since embryonic as well as adult stages may be modified 
 in the course of evolution ; what we should expect is 
 a more or less vague historical sketch, and this is what 
 is usually found remaining. 
 
 ‘ It is not true, for example, that a fish is a reptile 
 arrested in its development, or that a reptile was ever 
 a fish ; but it is true that the reptile embryo, at one 
 stage of its development, is an organism which, if it 
 had an independent existence, must be classified 
 among fishes ; and all the organs of the reptile pass, in 
 the course of their development, through conditions 
 which are closely analogous to those which are per- 
 manent in some fishes.’ 
 
 3.. Morphology. — On comparing together the dif- 
 ferent members of one of the great groups or classes 
 of animals or plants, we find the same fundamental 
 plan of organization running through all of them. 
 Series of corresponding organs are often to be made 
 out which are built upon the same general scheme^ 
 although their functions may be quite dissimilar ; so 
 
EVIDENCES OF ORGANIC EVOLUTION 33 
 
 that, for instance, in the hand of a man^ the paw of a 
 dog, the wing of a bat, and the paddle of a whale, 
 almost identically the same series of bones can be 
 traced. An obvious explanation is to be found in the 
 supposition that these parts have arisen by the 
 divergent modification of parts which were originally 
 identical. 
 
 4. Geographical Distribution. — Observation 
 shows that groups of closely allied creatures are often 
 found living in neighbouring districts, and that when 
 such a barrier as an ocean or a range of lofty moun- 
 tains is passed an entirely new fauna and flora are 
 usually to be met with. These facts may be explained 
 by the hypothesis that allied groups of species origi- 
 nated by a process of descent in the same countries 
 which they now inhabit, and they can be explained 
 by no other known hypothesis. The alternative sup- 
 position that each species was specially created and 
 placed in the locality in which it was best adapted 
 to dwell is singularly in disagreement with the well- 
 known facts that animals and plants transported into 
 entirely new regions often thrive better than in their 
 original homes. The examples of rabbits in Australia 
 and of cardoons and thistles on the Pampas of La 
 Plata are familiar to all from the writings of Darwin. 
 
 5. The Geological Succession of Organisms. — 
 The general facts regarding the distribution of allied 
 species of animals and plants in time point in pre- 
 cisely the same direction as those relating to their 
 distribution in space. In a few cases, notably in that 
 of the extinct horse of North America, a long chain of 
 
 3 
 
34 
 
 EVOLUTION 
 
 possibly ancestral types has been found leading back 
 to a remote and very different progenitor. This sup- 
 posed ancestor of the horse was a creature little larger 
 than a moderate-sized dog. It had four separate toes 
 to each fore-limb, and three to each hind-limb, and its 
 teeth were much simpler and less specialized than 
 those of existing horses. We say possibly ancestral, 
 advisedly. Geological evidence is naturally insufficient 
 to establish the actual relationship of the series of 
 types which has been described, and Professor 
 Sedgwick has criticized the view that this series 
 of forms constitutes a demonstrative historical proof 
 of the doctrine of organic evolution. It is, therefore, 
 preferable to claim this group of fossils as an illustra- 
 tion of the possible geological ancestry of an existing 
 species rather than as affording concrete proof of an 
 actual pedigree. Even with this reservation, we claim 
 that such a series constitutes a valuable collateral proof 
 in favour of evolution. The general distribution of 
 organisms throughout the geological strata agrees 
 moreover, in a remarkable way with what is to be 
 expected on the evolution theory. 
 
 6. Changes under Domestication. — Among do- 
 mesticated animals and plants we know of numerous 
 cases in which the actual origin of new forms has been 
 observed. These have often differed from their pre- 
 decessors by amounts quite comparable with the dif- 
 ferences by which natural species or even genera are 
 separated. A notable example of this process is afforded 
 by the numerous breeds of pigeons known to have arisen 
 under domestication from a single wild species. We 
 
EVIDENCES OF ORGANIC EVOLUTION 35 
 
 have no reason whatever for supposing that domesti- 
 cated species are more mutable than wild species, and 
 there is consequently every reason to beheve that 
 changes of a similar character take place in Nature. 
 
 Mr. Alfred Russel Wallace has quoted with approval 
 Sir W. Thiselton Dyer’s criticism that if there is an 
 equal chance of the occurrence or origin of new forms 
 in nature and under cultivation, then their appearance 
 — i.e., their survival until a stage at which they can 
 be readily recognized as distinct from the original type 
 — should be more frequent in nature than in cultiva- 
 tion, because the former has a larger population to 
 work with. The reply to this argument is obvious. 
 In the first place, the much greater facilities for obser- 
 vation under cultivation may fairly be set against the 
 greater numbers stated to exist in nature ; but, in the 
 second place, Mr. Wallace may well be challenged to 
 cite a natural species of which a larger number of 
 individuals has passed under man’s observation than 
 is the case with cultivated wheat, for example. But a 
 third line of argument is much more conclusive than 
 either of these. The modifications which occur under 
 cultivation are in most cases decidedly weakly as com- 
 pared with the original forms, as every gardener knows 
 to his cost. They are only enabled to survive to a 
 recognizable stage, because cultivation consists in the 
 removal of competition ; all are given an equal chance. 
 This is not so in nature. There, competition (according 
 to the exponents of the WaUacian doctrine) is so intense 
 that even very slight variations may determine success 
 or failure. According to the doctrine of natural selec- 
 
 3— a 
 
36 
 
 EVOLUTION 
 
 tion, then, decidedly weakly specimens, if they occurred 
 in nature, would have practically no chance of sur- 
 vival, and would consequently never be seen. This 
 attempt to undermine one of the strongest evidences 
 of organic evolution, therefore, falls to the ground. 
 
 7. The Observed Facts of Mutation. — As a 
 matter of fact, novel types are seen in nature not in- 
 frequently, and are specially common in some groups 
 of plants, as Mr. C. T. Druery has sho\vn for the case 
 of the British ferns ; and isolated specimens of par- 
 ticular wild species belonging to other families are fre- 
 quently found, which, if they had occurred as con- 
 stant features of a considerable group of individuals, 
 would afford a basis for the description of a new species. 
 The study of mutation will, however, require a special 
 chapter of its own. 
 
 We see, therefore, that the evidence in favour of the 
 existing species of animals and plants, having arisen 
 by a process of evolution, is of a most ample and con- 
 vincing kind. The theory of organic evolution is, 
 however, incomplete until we have arrived at a true 
 account of the method or methods by which new 
 species arise from old ones. The earliest definite 
 explanation, as already stated, was that given by 
 Lamarck, and we may next proceed to consider the 
 Lamarckian theory of the origin of species. 
 
 Earlier writers had already supposed that species 
 became modified through the action of the external 
 conditions to which they were exposed. Lamarck laid 
 special stress upon the observed facts that the organs 
 of individuals become increased and developed through 
 
THE THEORY OF LAMARCK 
 
 37 
 
 use, and that disuse is followed by a dwindling and 
 loss of the power of action. By the inherited effects 
 of use and disuse, and of modifications caused by ex- 
 ternal conditions, Lamarck supposed all evolution of 
 species to have come about. 
 
 Reference has already been made to Lamarck’s de- 
 scription of the method of origin of the characteristic 
 form of snakes, owing to the endeavours of the snakes’ 
 ancestors to creep through narrow passages. Lamarck 
 was quite consistent inasmuch as he explained the 
 different types which have arisen among domesticated 
 species by the same theory as he applied to the origin 
 of species in a state of nature. Thus he supposed the 
 differences between race-horses and heavy cart-horses 
 to be the direct result of the different kinds of enforced 
 exercise to which the ancestors of these races were 
 respectively subjected. Similarly, all the different 
 breeds of dogs were supposed to have arisen owing to 
 the different habits which the various successors of the 
 first domesticated dogs acquired, small changes being 
 accumulated by inheritance in each successive genera- 
 tion. 
 
 Turning now to species in a state of nature, the case 
 of the giraffe is one of those most often quoted. 
 Lamarck supposed a comparatively short - necked 
 ancestor of the giraffes to have taken up the habit of 
 browsing upon the leaves of trees, owing to the diffi- 
 culty of obtaining other food in an arid region. In 
 order to obtain their new food the animals were obliged 
 to be continually stretching upward, and the effort to 
 elongate their necks was attended with some small 
 
38 
 
 EVOLUTION 
 
 measure of success in each individual. This increase, 
 being accumulated by inheritance in every succeeding 
 generation, ultimately led to the great stature exhibited 
 by the giraffes of the present day. 
 
 The stilt-like legs of many wading birds were ascribed 
 by Lamarck to the result of the continued attempts of 
 ancestors which had shorter extremities to obtain their 
 food in shallow water without wetting their feathers. 
 The long-continued endeavours of these birds to stretch 
 and elongate their legs had the same effect as the 
 similar efforts made by the ancestors of the giraffes. 
 It has been suggested, however, by a critic of Lamarck's 
 position that such birds would be likely to eschew fish 
 dinners long before any notable increase in the length 
 of their legs was arrived at. 
 
 If some of the above cases appear a little ludicrous, 
 there are other instances in which the Lamarckian 
 hypothesis seems to afford a perfectly adequate and 
 natural explanation. Such a case is afforded by the 
 family of the flat fisheSj including such well-known 
 species as the sole and plaice. In the adult condition 
 these fishes lie flat on one side ; and during their 
 development from the young condition, that eye which, 
 if it remained in its original position, would look 
 directly downwards travels round the head until it 
 comes to lie quite upon the upper surface. As Darwin 
 pointed out, agreeing in this with Mivart, a sudden 
 spontaneous transformation in the position of the eye 
 is hardly conceivable, and it is equally impossible to 
 explain the origin of this remarkable feature by the 
 action of natural selection, because a slight change 
 
THE THEORY OF LAMARCK 
 
 39 
 
 in the position of the eye could be of no advantage 
 so long as this organ remained upon the under surface. 
 The very young fish, whilst still symmetrical, are 
 known sometimes to fall upon one side, and when in 
 this position to twist the lower eye forcibly upwards. 
 Darwin himself therefore supposed that the origin of 
 the adult structure is to be attributed to the inherited 
 effect of efforts of this kind. 
 
 The interest of the last case lies in the fact that it 
 relates to a structure, the origin of which does not 
 appear explicable on the theory of natural selection ; 
 its bearing will therefore be better understood when 
 we come to discuss that theory in the next chapter. 
 
 The inherited effects of voluntary striving can clearly 
 have no application to the case of plants. Lamarck 
 therefore supposed that evolution in the vegetable 
 kingdom had taken place entirely through the action of 
 external agencies upon plants. The soil, for examplej 
 in which a plant grows has a direct influence upon its 
 form. Altitude, moisture, heat, and light are other 
 important factors^ and the effect of their influence upon 
 the plant was supposed by Lamarck to be inherited. 
 The shape of irregular flowers was regarded as having 
 been directly caused by the strains and pressures 
 occasioned by bees and other insects whilst making 
 their visits in search of honey or pollen. 
 
 Lamarck’s theory turns entirely upon the question 
 whether acquired characters are inherited,* and if soi 
 to what extent; siifee, if such inheritance is shown to 
 be extremely slight, the cause, though a true one, 
 may be insufficient to explain the effects attributed 
 
40 
 
 EVOLUTION 
 
 to it. Now, theories of heredity apart, and leaving 
 aside the results of minute observations which had 
 not been made in Lamarck’s time, the natural supposi- 
 tion undoubtedly is that acquired characters are 
 inherited just as much as any others. Given the ob- 
 served fact that offspring resemble their parents more 
 closely than they do other members of the same species,' 
 it is natural to believe that the child will take after 
 the forms exhibited by its parents at the time of its 
 conception rather than after those shown by them at 
 any previous period of their lives. This seems to be 
 the natural view in the absence of any other evidence 
 for or against, and so accurate a thinker as Herbert 
 Spencer, writing before the publication of the ‘ Origin 
 of Species,’ regarded the term inheritance as neces- 
 sarily implying inheritance of this particular kind. 
 For this rfeason it has sometimes been thought that 
 Darwin scarcely accorded to Lamarck the appreciation 
 which he deserved ; and yet Darwin himself fell back 
 upon the Lamarckian explanation on the few occasions 
 when natural selection seemed to have failed him. 
 
 When, however, we come to know more of the actual 
 facts of sexual generation, we find that it is very 
 difficult j if not impossible, to imagine any kind of 
 mechanism by which this supposed transmission of 
 acquired modifications can take place. We shall defer 
 the further discussion of this subject, as well as the 
 question of the existence of direct and other evidence 
 of use inheritance, until the latter half of the next 
 chapter,; where we shall refer briefly to the contro- 
 
THE THEORY OF LAMARCK 41 
 
 versy upon these subjects which followed the estab- 
 lishment of the principle of natural selection. 
 
 BIBLIOGRAPHY. 
 
 Whetham, W. C. D. : The Recent Development of Physical 
 Science, 1904. 
 
 Spencer, Herbert : Essays — Scientific, Political, and Specu- 
 lative, 1868. 
 
 Huxley, T. H. : Collected Essays, vol. ii., 1899. 
 
 Romanes, G. J. : Darwin and After Darwin, vol. i., 1897. 
 Lamarck, J. B. : Philosophie Zoologique, 1809. 
 
 Lyell, Sir Charles, Principles of Geology, eleventh edition, 
 1872. 
 
 Butler, S. : Evolution Old and New, 1879. 
 
CHAPTER III 
 
 THE THEORY OF NATURAL SELECTION 
 
 In 1813 a communication was read before the Royal 
 Society by Dr. W. C. Wells upon the differentiation 
 which exists between certain races of mankind. In 
 Dr. Wells’s paper this differentiation was explained 
 from the facts thats since no two individuals are alike, 
 some would be better fitted than others to resist the 
 diseases proper to a particular country, and would 
 consequently tend to survive, whilst their less fortunate 
 neighbours would perish in greater numbers. Wells 
 supposed the dark races of mankind to be better 
 adapted to warm climates than white races are, and 
 he thus applied to the particular case of the human 
 species the true Darwinian principle of a gradual 
 evolution through the survival of the fittest. 
 
 A similar view was applied to the origin of species 
 in general by Patrick Matthews in a book on naval 
 timber and arboriculture published in 1831. 
 
 Both these works were unknown to Darwin at the 
 time of the first publication of the ‘ Origin of Species,’ 
 and it is quite unnecessary to point out that their 
 existence does not in the least prejudice the value or 
 originality of that great work. Their interest at the 
 
 42 
 
HISTORICAL 
 
 43 
 
 present time is merely historicaiii as showing the 
 direction in which thought was tending in the earlier 
 half of the nineteenth century. 
 
 Before the ‘ Origin of Species ’ was published, 
 
 A. R. Wallace communicated to Darwin a paper in 
 which the bearing of the same idea was worked out at 
 some length,, and this paper was read, together with 
 an abstract of Darwin’s own views, at a meeting of 
 the Linnean Society in July, 1858. 
 
 With this notice of other claimants to the idea of . 
 natural selection we may proceed to give an account 
 of the theory as it is developed in the earlier chapters 
 of the ‘ Origin of Species.’ 
 
 We must first glance at Darwin’s method of using 
 the term variation. Darwin applied this term to every 
 kind of difference which is found to occur between 
 parents and their offspring, or between members of 
 the same family, no matter whether these differences 
 were great or small. It has since been shown that a 
 number of quite distinct phenomena were in this way 
 regarded from a single standpoint, without a proper 
 discrimination being made between them. But the 
 differences between continuous and discontinuous 
 variation, quantitative and qualitative variation, and 
 the rest, were not pointed out until long subsequent to 
 1859. Thus, beyond recognising a distinction between 
 sports and individual differences, and attaching greater 
 weight to the latter kind of changes, as being those 
 which chiefly led to the origin of new species, Darwin 
 made no further analysis of the facts of variation, but 
 accepted all sorts of differences between individuals as 
 
44 THE THEORY OF NATURAL SELECTION 
 
 affording the material upon which natural selection 
 might be expected to operate. 
 
 The idea that a selective influence exists in Nature 
 arose from a study of the remarkable effects produced 
 in the case of domestic animals and plants by the action 
 of artificial selection. Darwin seems, however, to have 
 been a little credulous in accepting the statements of 
 certain breeders with regard to their power of producing 
 any desired new type to order. Now that scientific 
 men are themselves beginning to make experiments 
 in breeding, with the check of exact records to act 
 as a drag upon the exuberance of the imagination, 
 they are becoming somewhat sceptical as to the mystic 
 and almost miraculous powers attributed to the old- 
 fashioned breeders, though, indeed, Mr. Liither Bur- 
 bank would seem to be a survival from the period 
 we speak of, if the statements of his recent enthusiastic 
 biographer are to be credited.* Less gifted but more 
 methodical observers find that they have no creative 
 powers worth speaking of, and that all they can do is 
 to keep a sharp look-out for the novelties which Nature 
 may send them. 
 
 Selection, whether natural or artificial, can indeed 
 of itself have no power in the direction of creating 
 anything new ; its influence is destructive or preserva- 
 tive, but nothing more than this. The breeder keeps 
 the new forms which take his fancy, and destroys the 
 rest ; that is the whole story. 
 
 ♦ Harwood, ‘ New Creations in Plant Life.' Mr. Burbank 
 certainly seems to have a really wonderful instinct for the dis- 
 covery of curious and useful novelties. 
 
ARTIFICIAL SELECTION 
 
 45 
 
 Yet a remarkable number of new kinds of creatures 
 are known to have arisen in this way, and their diver- 
 sity is no less astonishing, as a visit to any great show 
 of domestic plants or animals will at once demonstrate. 
 Here may be seen varieties of pigeons, for example, 
 like the carrier, pouter, fantail, and tumbler, which, 
 if they were found existing in a wild condition, would 
 be placed in separate genera by any ornithologist. 
 The domestic races of fowls, dogs, horses, sheep, and 
 cattle show scarcely less divergence, and modifications 
 no less remarkable have been perpetuated in the case 
 of many cultivated species of plants. Whilst these 
 types have survived, being deliberately preserved on 
 account of their use or beauty or curious appearance, a 
 still greater number have doubtless been exterminated, 
 either because they did not attract the breeder’s 
 favourable attention, or on account of their having 
 passed out of fashion. 
 
 Darwin sought in Nature a substitute for the baleful 
 judgment of the breeder, and found it in an extension 
 of the Malthusian doctrine to organic beings in general. 
 The idea which is identified with this expression 
 did not, however, originate with Malthus, nor does 
 that author claim it as his own, as the following extract 
 from the first chapter of the ‘ Essay on Population ' 
 will show : 
 
 ‘ It is observed by Dr. Franklin that there is no bound 
 to the prolific nature of animals and plants but what 
 is made by their crowding and interfering with each 
 other’s means of subsistence. Were the face of the 
 earth, he says, vacant of other plants, it might be 
 
46 THE THEORY OF NATURAL SELECTION 
 
 gradually sowed and overspread with one kind only, 
 as, for instance, with fennel ; and were it empty of other 
 inhabitants, it might in a few ages be replenished from 
 one nation only, as, for instance, with Englishmen.’ 
 
 Malthus’ ‘ Essay ’ was first published in 1798, and 
 was subsequently much enlarged. Its author proved 
 incontrovertibly, by a survey of facts gathered from 
 almost all the countries of the world, that human popu- 
 lation tends to increase in a geometrical ratio, and 
 that, consequently, after a time, the less gifted classes 
 of any community are bound to suffer from a stress of 
 poverty, only partly relieved by a high infant mortality, 
 periodic famines, and similar factors, or in less civilized 
 countries by infanticide and other artificial checks. 
 
 Among animals and plants in a state of nature the 
 rate of increase is often very much greater than in the 
 case of the human family, and even where it is not so, 
 unchecked breeding would in a comparatively few years 
 lead to the overpeopling of the earth with the de- 
 scendants of a single pair. As an example of the rate 
 of increase shown by a wild species, we may consider 
 the case of the elephant, instanced by Darwin himself, 
 since this is regarded as being one of the slowest 
 breeders among all known animals. Darwin assumes 
 that the elephant begins breeding at thirty years, and 
 continues to do so until it reaches the age of ninety, 
 bringing forth six young in the interval, and surviving 
 to the age of a hundred. Then, if there were no 
 casualties, he calculates that after from 740 to 750 
 years there would be nearly nineteen million elephants 
 alive descended from the first pair. 
 
RATE OF INCREASE 
 
 47 
 
 Let us also consider the case of a minute rapidly 
 breeding animal of a typical kind. My friend Mr. 
 R. C. Punnett has recently been engaged upon an 
 experiment which involved the breeding of rotifers, 
 a kind of animal barely visible to the naked eye. 
 They were bred for sixty-seven generations, and 
 each individual produced on the average thirty eggs. 
 The whole experiment occupied less than a year, yet 
 Mr. Punnett calculated that if he had been able to 
 rear all the animals which* at this rate of breeding, for 
 this number of generations, were theoretically obtain- 
 able, he would have become the possessor of a solid 
 sphere of organic material with a radius greater than 
 the probable limits of the known universe. 
 
 This geometrical rate of increase is common in a 
 greater or less degree to all living organisms. Since 
 the space and food-supply available for the support of 
 any species has no corresponding tendency to in- 
 crease, it follows that a large proportion of the 
 individuals born must perish before they reach the 
 adult state, or at least without producing offspring. 
 Darwin’s contention is that there will be a strong 
 tendency for those individuals which show slight 
 modifications in the direction of a better adaptation 
 to their environment to survive at the expense of 
 those of their brethren which do not exhibit similar 
 modifications. This is the principle called natural 
 selection by Darwin, and by Herbert Spencer the 
 survival of the fittest. Let us quote Darwin’s own 
 summary of the process : 
 
 ‘ If under changing conditions of life organic beings 
 
48 THE THEORY OF NATURAL SELECTION 
 
 present individual differences in almost every part of 
 their structure, and this cannot be disputed ; if there 
 be, owing to their geometrical rate of increase, a severe 
 struggle for life at some age, season, or year, and this 
 certainly cannot be disputed ; then, considering the 
 infinite complexity of the relations of all organic 
 beings to each other and to their conditions of life, 
 causing an infinite diversity in structure, constitu- 
 tion, and habits, to be advantageous to them, it would 
 be a most extraordinary fact if no variations had ever 
 occurred useful to each being’s own welfare, in the same 
 manner as so many variations have occurred useful to 
 man. But if variations useful to any organic being ever 
 do occur, assuredly individuals thus characterized will 
 have the best chance of being preserved in the struggle 
 for life ; and from the strong principle of inheritance, 
 these will tend to produce offspring similarly charac- 
 terized. This principle of preservation, or the survival 
 of the fittest, I have called Natural Selection. It leads 
 to the improvement of each creature in relation to its 
 organic and inorganic conditions of life, and, conse- 
 quently, in most cases, to what must be regarded as an 
 advance in organisation. Nevertheless, low and 
 simple forms will long endure, if well fitted for their 
 simple conditions of life.’* 
 
 We have here a very definite and concise statement 
 of the way in which Darwin believed the principle of 
 natural selection to take effect in the production of new 
 kinds of organisms. It will be our business in this and 
 in succeeding chapters to show how far the modern 
 * ‘ Origin of Species,’ sixth edition, p, 96. 
 
NATURAL SELECTION 
 
 49 
 
 study of the nature of individual differences and of 
 other kinds of variations, as well as of the manner of 
 operation of ‘ the strong principle of inheritance,’ has 
 confirmed this view as to the method of origin of 
 species, or has led to the introduction of modifications. 
 
 Let it be remembered that this suggestion of a 
 natural means of modification had, within a few years, 
 the effect of convincing practically the whole thinking 
 world of the truth of the theory of organic evolution — 
 an effect which all the other arguments recited in the 
 last chapter were quite unable to produce, so strong 
 was the then existing prejudice in favour of the doc- 
 trine of special creation. 
 
 The truth of the general principle of the survival of 
 the fittest is quite untouched by recent criticism ; 
 but a great deal of argument has been expended over 
 the questions : (i) how much fitness is sufficient to 
 lead to survival, and (2) whether very small advan- 
 tages in the way of fitness, even if they lead to the sur- 
 vival of the individuals which exhibit them, will be 
 followed to an indefinite extent in succeeding genera- 
 tions by further improvements in the same direction. 
 We shall find that a good deal of evidence has accu- 
 mulated tending to show that the second of these 
 questions must be answered in the negative, although 
 the point is not yet settled to the satisfaction of 
 everyone. The remainder of the present chapter will 
 be occupied in discussing some of the arguments which 
 bear upon this question. 
 
 An important factor which must probably be added 
 to the factors of variation, selection, and heredity, in 
 
 4 
 
50 THE THEORY OF NATURAL SELECTION 
 
 order to complete the Darwinian account of the origin 
 of species, is that of segregation or isolation. If the 
 selection of minute fortuitous variations in different 
 directions is capable of breaking up a species into a 
 number of new species, it seems clear that this can 
 only happen when the members of the different branches 
 are prevented from interbreeding ; since otherwise the 
 effect of selection would be counteracted by the 
 mingling or blending of characters which may be sup- 
 posed to result from free intercrossing. Further, many 
 zoologists, and more especially the systematists among 
 them, believe that isolation in itself has a most im- 
 portant function in modifying species. This isolation 
 may be either geographical, as when distance or some 
 physical barrier separates different members of the 
 same species ; or it may be physiological, as when 
 structural or temperamental differences, or mutual 
 distaste, prevent the mating of certain individuals. 
 
 The researches of Gulick upon the species of snails 
 found in the Haiwaian Islands showed that the differ- 
 ences between the species correspond in amount with 
 their degree of separation in space — i.e., with their 
 isolation. The characters which separate these species 
 could not be shown to have any relation to differences 
 in the environment, since adjoining valleys, which 
 differed considerably in vegetation and rainfall, pos- 
 sessed closely related species ; whilst in valleys further 
 apart, but more similar in the environment offered 
 to the snails, the characteristic species showed much 
 greater differences. The case of the Haiwaian snails, 
 therefore, appears to afford an exception to the prin 
 ciple next to be described. 
 
NATURAL SELECTION 
 
 51 
 
 The fact that organic beings on the whole are, as a 
 general rule, very closely fitted for the conditions in 
 which they have to pass their lives is clearly shown 
 by the study of adaptations. This is a subject which 
 those followers of Darwin who believe in the all- 
 sufEiciency of natural selection have brought into con- 
 siderable prominence. For a full account of many 
 supposed beautiful adaptations, from the point of view 
 of the most prominent member of the school in ques- 
 tion, reference may be made to Weismann’s book, 
 
 ‘ The Evolution Theory.’ 
 
 On the theory of natural selection in its extreme 
 form, all the parts of an animal or plant — or, at any rate, 
 all the points in which one species differs from another 
 nearly related species — are supposed to have arisen on 
 account of their usefulness to the creatures possessing 
 them. Every detail of structure is thus regarded as 
 being more or less closely adapted to the circum- 
 stances which attend the life of the animal or plant in 
 question. This adaptation is never, indeed, regarded 
 as perfect, because natural selection is always in 
 progress, and its work is never absolutely done ; 
 but the point is that the features of every part are 
 aimed at some useful purpose ; or, if they are not, 
 then they have been useful in former times and under 
 different circumstances, and are now undergoing a 
 process of gradual removal, because the individuals in 
 which the useless structure is least developed will now 
 have the best chance of surviving. That the form and 
 structure of an animal or plant is in general closely 
 fitted to its environment is of course true ; otherwise 
 
 4—2 
 
52 THE THEORY OF NATURAL SELECTION 
 
 the creature would very soon cease to cumber the earth. 
 But the student of adaptation goes into details, and 
 endeavours to find a use for every minute point of 
 structure, on the assumption, which we shall presently 
 see to be open to criticism, that but for their useful- 
 ness these details would not exist. We may proceed 
 to glance at one or two examples of the kind of thing 
 which is meant when it is said that an animal or plant 
 exhibits very marked adaptative features. 
 
 The order Cetacea belongs to the class of mammals 
 of which the more typical members are land animals 
 possessing four legs, and having their bodies covered 
 with hair. The true whalebone whales, a sub-order 
 which includes the Greenland whale, are in many ways 
 the most specialized members of the group. 
 
 The Greenland whale* has a spindle-shaped body 
 like that of a fish, and its fore limbs are modified into 
 flippers resembling the pectoral fins of fishes. The 
 hind legs are only represented by a few rudimentary 
 bones, which are completely hidden within the body 
 wall, and the function of propulsion, which is performed 
 by the hind legs in such less completely aquatic animals 
 as seals, is here taken over by a great tail-fin which 
 resembles that of a fish, except that it is placed hori- 
 zontally. Hair is absent, but under the skin a thick 
 layer of blubber is developed, which prevents a too 
 rapid loss of heat, and at the same time adjusts the 
 specific gravity of the body to that of the surrounding 
 water. External ears are entirdy wanting, and the 
 
 * Weismann, ‘ The Evolution Theory,’ English edition, 
 
 ii. 313. 
 
ADAPTATION 
 
 53 
 
 waves of sound are apparently transmitted to the 
 drum of the ear directly through the bones of the head. 
 The external openings of the nostrils are placed quite 
 on the upper surface of the head, so that the animal 
 can breathe whilst almost completely submerged ; and 
 the larynx is so modified that the function of swallowing 
 does not interfere with that of breathing. Perhaps the 
 most remarkable feature of all is the enormous develop- 
 ment of the head, and especially of the mouth. The 
 huge jaws, in combination with the extraordinary plates 
 of whalebone which fringe the edges of the mouth and 
 act as a sieve, enable the animal to get its nutriment 
 from the minute free-swimming creatures with which 
 the surface waters of the ocean abound. Associated 
 with this special method of feeding is the fact that 
 teeth are only to be recognised in the embryo, and 
 afterwards entirely disappear. 
 
 The whales differ in all these points from any other 
 mammals, and failing almost any of these differences, 
 would not be able to live in the special conditions in 
 which they find themselves. It must therefore be 
 admitted that we have here a case of very close adapta- 
 tion of an animal to its natural surroundings, and one 
 which extends to almost every detail of its structure. 
 Darwin himself, moreover, has been at special pains to 
 show how some of the most remarkable of these 
 structural adaptations may possibly have arisen 
 through natural selection. 
 
 One of the most remarkable cases of mutual adapta- 
 tion, in which an animal and a plant are associated 
 together, is shown by the method of fertilization 
 
54 THE THEORY OF NATURAL SELECTION 
 
 observed to take place in the flowers of the Yucca 
 plant of the Southern United States. The act of 
 pollination is performed by a moth — Pronub a — ^which 
 possesses special organs particularly adapted for this 
 purpose, in the shape of peculiar maxillary tentacles 
 which are found in no other kind of, moth. The female 
 has also a long ovipositor with which she can pierce 
 the tissues of the ovary of the plant, and so lay her 
 eggs within it. With the aid of her peculiar tentacles the 
 female moth collects from several flowers a ball of pollen 
 of considerable size, which she kneads into a firm pellet. 
 She then carries this to a different flower^ and after 
 depositing a few eggs in the ovary she climbs to the 
 top of the style and presses the ball of pollen into the 
 stigma. Thus the ovules of the flower are fertilized^ 
 and whilst some are eaten by the larvae of the moth, 
 others develop into seeds and reproduce the plant. 
 
 The foregoing are perhaps two of the most remarkable 
 cases known of animals having peculiar habits, and 
 possessing at the same time special organs which render 
 them well fitted for these habits and no others ; but 
 many other cases of scarcely less wonderful adapta- 
 tions have been pointed out. 
 
 Darwin himself indicated the direction in which the 
 study of adaptation was to proceed, and his books on 
 ‘Insectivorous Plants’ and on the ‘Fertilization of 
 Orchids ’ afford us a delightful insight into a number of 
 adaptive contrivances which are to be seen in plants. 
 Another very interesting series of adaptive characters 
 are those which have been gathered together under 
 the heads of Protective Resemblance and Mimicry, and 
 
MIMICRY 
 
 55 
 
 these have a special interest for us, because they illus- 
 trate the way in which the zeal of the seeker after adap- 
 tive contrivances may run away with him if not kept 
 well in hand. For there is scarcely any limit to the num- 
 ber of problematical cases which have been described 
 as adaptive resemblances, and so explained as having 
 arisen through natural selection, whilst the evidence 
 in favour of such a supposition is in many cases highly 
 questionable. On the other hand, in a number of 
 well-marked instances, the theory of mimicry certainly 
 seems to afford an adequate explanation of the way 
 in which many curious characters and structures may 
 possibly have come into existence. 
 
 The families of the Mantidcs and Phasmidce, includ- 
 ing the walking-stick and leaf insects, afford many 
 examples of animals which both in their colour and 
 configuration show a very close resemblance to sur- 
 rounding inanimate objects. This resemblance must 
 have the effect of concealing them from their enemies, 
 and more particularly from their prey, as, indeed, a 
 study of their habits indicates quite clearly. 
 
 Phyllopteryx, an Australian fish allied to the well- 
 known sea-horse {Hij>pocampus), is provided with a 
 number of irregular appendages of ragged skin 
 resembling the seaweed amongst which this animal 
 is found. In this way the characteristic symmetrical 
 appearance of a live animal is got rid of, and the 
 creature is rendered extremely difficult of observation. 
 Here, again, the concealment afforded is probably 
 useful in leading to the deception of the smaller 
 organisms upon which the creature feeds. 
 
56 THE THEORY OF NATURAL SELECTION 
 
 Examples of this kind in which the shape of an 
 animal leads to its concealment are less numerous 
 than those in which protection is afforded by an obscure 
 pattern or inconspicuous colour. Indeed, some re- 
 semblance between the colour of an animal and its 
 surroundings is to be traced in the majority of the 
 members of many groups. Familiar examples are 
 afforded by the white colour of animals which live in 
 snow, the tawny grey colour of most desert species, 
 the green of grass-frequenting animals, and so on. It 
 is perhaps not quite certain that in some of these cases 
 the peculiar colour is not evoked by the direct action 
 of some cause which affects different species in the same 
 way ; but such a cause awaits discovery, and in the 
 meantime natural selection has certainly a strong 
 claim to be regarded as the proper explanation. 
 
 A more strict use of the term mimicry, however, is 
 to restrict it to cases where one species apes the colour 
 pattern or other external character proper to another 
 species which inhabits the same region ; and the idea 
 of mimicry has been put forward as especially appro- 
 priate in cases where the mimicked species is common, 
 and can be thought to possess some special means of 
 protection. Numerous supposed examples of this 
 phenomenon have been described among insects, espe- 
 cially in the case of various butterflies from Africa, 
 Malaya, and South America. It would be beyond the 
 scope of this work to do more than call attention to the 
 fascinating subject, the literature of which includes a 
 large number of papers to be found in the Proceedings 
 of the Linnaean Society and elsewhere. For a general 
 
MIMICRY 
 
 57 
 
 account of this work, the reader is referred to Professor 
 Poulton’s recent ‘ Essays on Evolution,’ and to Pro- 
 fessor R. C. Punnett’s ‘ Mimicry in Butterflies.’ 
 
 Manystudents of evolution in its more recent develop- 
 ments are disposed to attach greater importance than 
 does Professor Poulton to the difficulties which beset 
 the theory of mimicry, in so far as the theory consists 
 in explaining these resemblances by natural selection 
 accumulating minute variations in the proper direction. 
 Indeed, the power of this evolutionary factor seems 
 here to be stretched to its utmost limits of tension. 
 The independent evolution of a similar external appear- 
 ance has certainly taken place in some cases in which 
 any suggestion of mimicry is excluded, and there is 
 nothing to prove that colour-patterns of the same type 
 may not have arisen from the same causes in widely 
 different groups. In cases where the environment to 
 which the different forms were exposed was similar — 
 as would be the case especially in any single locality — 
 such a process of parallel evolution might be thought 
 to be all the more likely. 
 
 It is not to be supposed that we intend for a moment 
 to impugn the reality of these marvellous resem- 
 blances. The smallest acquaintance with the facts 
 must show the absurdity of any such suggestion, just 
 as the multiplicity of the cases described renders any 
 suggestion of coincidence ridiculous. It is only the 
 current explanation of these resemblances to which we 
 take exception, for the brain reels before the task of 
 picturing the gradual building up of such a resemblance 
 by the successive additions of small differences, each 
 one useful to the possessor of it. 
 
58 THE THEORY OF NATURAL SELECTION 
 
 Resemblances can only be properly explained as 
 representing cases of mimicry when both the species 
 concerned — the mimic and the mimicked — inhabit the 
 same locality ; but plenty of cases of matching between 
 the colour patterns of insects which live in quite 
 different parts of the world could also be pointed out. 
 Let us take a concrete example. Everyone is familiar 
 with the flower-frequenting flies {Syrphidce)yfh.ich. are to 
 be seen hovering about plants in sunny weather. These 
 insects closely mimic the appearance of various small 
 bees and wasps, the habits of which are similar. Here, 
 then, is surely a case where the deceptive resemblance 
 to an animal well armed in its sting must cause pro- 
 spective enemies to let these flies alone. In Southern 
 Japan, as Dr. Andrese pointed out to me, flies of this 
 kind are surprisingly numerous, and their resemblance 
 to bees particularly noticeable. So abundant are they 
 that, from the point of view of the flowers which they 
 visit, these flies doubtless provide an efflcient substitute 
 for the bees of other countries, which are here con- 
 spicuous by their absence. But if real stinging insects 
 are wanting, or even very scarce, the supposed enemies 
 of the flies can have no experience of the ill-effects 
 produced by catching them. How, then, can these 
 flies benefit from their resemblance to bees ? 
 
 This kind of thing must make us somewhat sus- 
 picious of supposed cases of mimicry even between 
 species possessing the same range. 
 
 When the ideas of mimicry and protective resem- 
 blance are carried into the vegetable kingdom, as they 
 have been by some writers, absurdities are soon found 
 
PARALLEL EVOLUTION 
 
 59 
 
 to arise. Thus it has been suggested that the leaves 
 of dead nettles resemble those of the common nettle 
 for the sake of the protection so afforded, and that the 
 mottled stems of certain tropical herbaceous plants 
 gain a similar immunity on account of their resemblance 
 to snakes. 
 
 In plants a great number of fanciful resemblances 
 between different species can be detected, and some 
 between plants and animals, very few of which can be 
 supposed to be of any possible utility to the species 
 which exhibit them. They must be regarded as cases 
 of parallel evolution, the causes of which are quite 
 unknown. Such resemblances as that between the 
 shoots of Casuarina indica and those of the common 
 horse-tail, between Saxifraga hypnoides and certain 
 mosses, between the horse- and Spanish-chestnut, be- 
 tween the seed of a pine and the fruit of an ash-tree, 
 are so frequent in the vegetable kingdom as to be the 
 delight of malicious examiners in elementary botany. 
 It is impossible to believe that in such cases the 
 resemblance is in itself of any value to either species, 
 and few people wiU be found to maintain that the 
 likeness of a bee- or spider-orchis to an insect is of any 
 utility to either animal or plant. 
 
 But if resemblances can arise which are useless, and 
 which, consequently, cannot be explained through 
 natural selection, it becomes uncertain whether this 
 principle can hold good as the true description of the 
 origin of any sort of resemblance. On the other hand, 
 resemblances which are useful will tend to survive 
 through natural selection in whatever way they may 
 
6o THE THEORY OF NATURAL SELECTION 
 
 have arisen. This last consideration will account for 
 the frequency with which apparently adaptive like- 
 nesses are to be found in nature, even if we suppose 
 that their origin was ‘ accidental,’ or simply due to 
 the operation of similar external causes. The same 
 criticism applies to all cases of adaptation of whatever 
 kind, so far as concerns their supposed method of origin 
 by the action of natural selection upon individual 
 differences. 
 
 Perhaps a still more serious criticism of the methods 
 of those who spend their time in seeking out or devising 
 cases of adaptation has been made by Bateson, who 
 points out the logical difficulty that we can never 
 make any quantitative estimate of the amount of 
 benefit or the reverse which any particular structure 
 may afford to its possessor. It is easy enough to 
 imagine particular circumstances in which an organ 
 developed in a particular way may be of undoubted 
 service, but whether the net amount of such service 
 throughout the life of the creature considered is 
 greater or less than the strain upon its resources caused 
 by the development of such an organ is quite beyond 
 our powers of determination. 
 
 ‘ The students of adaptation forget that even on the 
 strictest application of the theory of selection it is 
 unnecessary to suppose that every part an animal has, 
 and everything which it does, is useful and for its good. 
 We, animals, live not only by virtue of, but also in 
 spite of what we are. It is obvious from inspection 
 that any instinct or organ may be of use ; the real 
 question we have to consider is how much use it is. 
 
REGENERATION 
 
 6i 
 
 To know that the presence of a certain organ may lead 
 to the preservation of a race is useless if we cannot tell 
 how much preservation it can effect, how many indi- 
 viduals it can save that would otherwise be lost ; unless 
 we know also the degree to which its presence is 
 harmful ; unless, in fact, we know how its presence 
 affects the profit and loss account of the organism.’* 
 
 A great many other criticisms and objections have 
 been brought at various times against the theory of 
 the origin of adaptations by the action of natural 
 selection, and many of these were considered and 
 replied to by Darwin in the later editions of the ‘ Origin 
 of Species.’ We shall only consider here a few which 
 have been put forward more or less recently. Before 
 doing so it will be well to point out once more that no 
 one questions the validity of natural selection as a means 
 of exterminating types which are unfitted for their 
 environment — there is clearly a tendency for the 
 fittest types to survive once they have come into 
 existence. Nor can there be any doubt that species 
 in general are well adapted to the conditions which 
 their environments present. But when this is admitted 
 it does not necessarily follow that natural selection, 
 directing accumulation of minute differences, has 
 been the method by which these adapted forms have 
 originated. ' 
 
 The power of regenerating a lost part must clearly 
 often be of service to the creatures which possess it. 
 Such a power may in many cases be considered to be 
 a weU-marked adaptation. But, as Morgan has well 
 • W. Bateson, ‘ Materials for the Study of Variation,’ p. 12, 
 
62 THE THEORY OF NATURAL SELECTION 
 
 pointed out, there are insuperable difficulties in the 
 way of adopting the belief that such a power can have 
 been acquired through the action of natural selection. 
 Many crustaceans, when they have lost a claw or limb, 
 proceed straightway to grow a new one. To account 
 for this power by natural selection we should have to 
 suppose, firstly, that every stage in the growth of a 
 partly regenerated claw, even its first small rudiment, 
 was useful to the animal ; and, secondly, that there 
 was so much competition between lobsters which had 
 lost their claws, that those which could regenerate 
 them a little better would survive rather than the 
 others. The first of these suppositions as to the utility 
 of a partly regenerated claw is in the highest degree 
 improbable ; but against the second there is an 
 entirely fatal argument, since, if the lobsters which 
 regenerated badly were exterminated owing to com- 
 petition with those which had better powers of re- 
 generation, much more would all the injured lobsters 
 be exterminated in competition with those which had 
 escaped injury. 
 
 The theory of sexual selection constitutes an im- 
 portant branch of the Darwinian account of the origin 
 of specific structures. We are here concerned with 
 this hypothesis only in so far as it leads to a criticism 
 of the efficacy of natural selection from another point 
 of view . By the theory of sexual selection Darwin at- 
 tempted to explain the origin of two sorts of characters 
 in particular, one or other of which frequently appears 
 in the male sex only of many of the higher animals. 
 
SEXUAL SELECTION 
 
 63 
 
 In the first place, we have to notice the presence of 
 special weapons, such as horns or tusks, developed 
 exclusively or to a special extent in the males of 
 those species in which it is the habit of the members 
 of this sex to strive together for the possession of the 
 females. In such cases the stronger and better-armed 
 males are supposed to survive, and to leave a greater 
 number of offspring than their weaker rivals, so that 
 this form of competition is regarded as acting in 
 quite a similar way to natural selection. 
 
 In a second set of cases, of which many remarkable 
 instances are to be seen among birds, the males are 
 found to exhibit brilliant and varied colours, or to 
 possess special decorations in the form of plumes or 
 other appendages, or to be gifted with the power of 
 song. It is to cases such as these that the term sexual 
 selection more properly applies, since the females are 
 supposed to bestow their favours upon the most 
 beautiful males, and to reject the advances of those 
 among their suitors which are less lavishly provided 
 with ornament. 
 
 In these cases, where the development of brilliant 
 colours or other ornamental arrangements is believed 
 to have taken place owing to the choice of the females 
 — particularly in such a case as is represented by the 
 peacock’s tail or the wings of the Argus pheasant — 
 the supposed change must have come about in direct 
 opposition to the action of natural selection, since the 
 latter would favour a production of colours resembling 
 those of the natural environment for the sake of con- 
 cealment, and would hinder the formation of such 
 
64 the theory of NATURAL SELECTION 
 
 exaggerated appendages on account of the loss of 
 activity which they must entail. We are, therefore^ 
 obliged to conclude that natural selection is much less 
 rigorous in its action than some people have supposed,; 
 for if this principle is inadequate to prevent such an 
 exuberance of form and colour in these particular 
 instances, its action becomes open to question in 
 other cases as well. 
 
 Similarly, Morgan finds a difficulty in understanding 
 why natural selection has not led to the extermination 
 of species which are handicapped by the existence of 
 internecine strife between the males, in favour of other 
 species which faced the battle of life with united 
 strength. But in this argument it seems to be for- 
 gotten that examples of the kind of strife in question 
 are most frequent among herbivorous animals, where 
 the struggle for existence must be chiefly determined 
 by the quantity of vegetable food which the individuals 
 can obtain, so that the loss of the weaker males may 
 not be a disadvantage. Moreover, Darwin’s conclusion 
 that natural selection is most rigorous between members 
 of the same species is left out of account. 
 
 The preceding arguments seem to show that in par- 
 ticular cases certain structures and phenomena asso- 
 ciated with species cannot be explained as having 
 arisen through the unaided action of natural selection. 
 When weighed against the great mass of evidence 
 which Darwin accumulated in support of his theory, 
 these few considerations cannot be said to be in any 
 way fatal to the belief that natural selection of minute 
 differences has played an important part in the origin 
 
INHERITANCE OF ACQUIRED CHARACTERS 65 
 
 of species. Still, they add in some measure to the 
 weight of recent evidence which points to the con- 
 clusion that many specific structures have had a 
 different method of origin. We have already pointed 
 out that there are two alternative methods, each of 
 which has its adherents. Before passing to a con- 
 sideration of the now prevalent view of mutationt 
 something still remains to be said with regard to the 
 remaining theory — the theory of Lamarck. 
 
 Darwin himself, as we have seen, admitted the 
 minor importance of the inheritance of acquired 
 ^iharacters, as well as that of the phenomenon of 
 sporting, regarding both these processes as causes of 
 the origin of new species subsidiary to the action of 
 natural selection upon individual differences, whilst he 
 looked upon the latter as the main process in organic 
 evolution. 
 
 Later writers, however, have asserted that natural 
 selection is the sole cause of the origin of species, and 
 in particular they have denied any effect to the in- 
 heritance of acquired characters — the Lamarckian 
 factor — asserting that there is not, and cannot be, 
 any such inheritance. Among the most distinguished 
 opponents of the theory of use-inheritance were A. R. 
 Wallace, the co-discoverer of natural selection ; and 
 Professor Weismann, who has argued the case with 
 particular ability. Much the most able defender of 
 the principle of use-inheritance was Herbert Spenceri 
 who was one of the few who had thoroughly convinced 
 themselves of the truth of the theory of evolution 
 y^ars before the ‘ Origin of Species ’ made its appear- 
 
 5 
 
66 THE THEORY OF NATURAL SELECTION 
 
 ance. Since all arguments in favour of the evolution 
 of species were incomplete unless some means by 
 which such an evolution could take place had been 
 suggested, Spencer adopted the Lamarckian theory of 
 modification, and to this he always firmly adheredi 
 though admitting the validity of natural selection as 
 an additional factor in the process. Some of Spencer’s 
 arguments in favour of a belief in the inheritance of 
 acquired characters are well worth repetition, since 
 they have never been altogether refuted. 
 
 Herbert Spencer’s argument consisted mainly in the 
 enumeration of structures the origin of which cannot 
 be explained by natural selection. On the other hand, 
 the inheritance of acquired characters, if this form of 
 inheritance could be proved to have a real existence 
 — as Spencer believed it could — ^was shown to be a 
 perfectly adequate explanation of the origin of the 
 structures in question. In 1893, when Spencer up- 
 held his opinion for the last time, Bateson had not yet 
 pointed out that the facts of definite and discontinuous 
 variation afford an alternative way out of some of 
 these difficulties. In the absence, therefore, of any 
 other effective cause, the result of the argument 
 pointed strongly to the conclusion that the inheritance 
 of acquired characters must be a reality. 
 
 The first of Spencer’s arguments was based upon 
 the different powers of tactual discrimination which 
 are to be found in different parts of the human body- 
 The degree of this sensitiveness may be estimated by 
 the use of a pair of compassesi the points of which can 
 be set at different distances apart. It is then found 
 
DISCRIMINATION BY TOUCH 
 
 67 
 
 that with the tip of the forefinger the points can be 
 distinctly recognised as two when they are separated 
 by no more than inch. When applied to the middle 
 of the back, on the other hand, the points must be 
 opened to a distance of 2J inches before the sensation 
 of a single touch becomes resolved into two distinct 
 sensations. 
 
 The distribution of this power of discrimination over 
 the surface of the body is approximately as follows ] 
 
 Tip of tongue 
 
 Tip of finger . . 
 
 Inner surface of second joint of 
 finger . . 
 
 Tip of nose . . 
 
 Cheek, palm of hand, and end of 
 great-toe 
 
 Forehead 
 
 Back of hand, crown of head 
 
 Breast 
 
 Middle of back, middle of thigh, 
 middle of forearm 
 
 inch. 
 
 1 
 
 Tir 
 
 i a 
 i 
 
 ,1 
 
 xi 
 
 Now, it is out of the question to suppose that natural 
 selection can account for aU these differences. An 
 increased sensitiveness of the tips of the fingers might, 
 indeed, be of so much use as to give the individual 
 possessing it a definitely increased chance of survival. 
 But it is hard to believe that it can be important for 
 a man to have the tip of his tongue twice as sensitive 
 as the tips of his fingers. And why should the tip of 
 the nose be more sensitive than the cheek, or the 
 cheek than the top of the head, or the breast than 
 the back ? In the last case it might even be suggested 
 
 5—2 
 
68 THE THEORY OF NATURAL SELECTION 
 
 that in a savage, since the sense of touch is the only 
 one with which his back is provided, it might be 
 useful for that surface to have acquired a more deli- 
 cate sense of touch than the anterior surface, which 
 is guarded by the power of vision, as well as being 
 more readily explored by the sensitive finger-tips. 
 If such an argument is regarded as far-fetched, so in 
 an equal degree must be any attempt to explain the 
 actually observed distribution through the action of 
 natural selection. 
 
 On the other hand, Spencer points out that the 
 series of parts enumerated in the above table stands 
 in almost exactly the order of the frequency with 
 which the members composing it are actually exposed 
 to tactual experience. 
 
 The tongue is perpetually in contact with the minute 
 unevennesses afforded by the surfaces of the teeth. 
 
 The palm of the hand and the lower joints of the 
 fingers are used chiefly in grasping, and not in the 
 more minute manipulations for which the finger-tips 
 are employed. And the experience of the back of the 
 hand in coming into contact with various irregular 
 bodies is not to be compared with that of the palmar 
 surface, yet it is very much greater than that of so 
 unexposed a part as the middle of the forearm. 
 
 For the carrying on of his argument, Herbert Spencer 
 has shown that increased use of the power of dis- 
 criminating small objects by touch is accompanied by 
 an increased degree of sensitiveness in individuals. 
 Blind people use their finger-tips in this way to a much 
 greater extent than those whose sight is unimpaired. 
 
CO-ORDINATED STRUCTURES 
 
 69 
 
 Two blind boys examined by Spencer were both found 
 to be able to distinguish with the tips of their fingers 
 points separated by only inch. And two skilled 
 compositors could both distinguish in this way points 
 placed no more than xV inch apart, so that a person 
 with a trained sense of touch acquires a considerably 
 finer development of this faculty than an ordinary 
 individual. 
 
 If, then, acquired characters of this kind are in- 
 herited, even to an extremely minute extent, such as 
 would be scarcely perceptible in a single generation, 
 the account of the origin of the observed phenomena 
 would be complete. 
 
 As a second argument, Herbert Spencer points out 
 the difficulty of accounting for the development of 
 co-ordinated sets of structures by the action of natural 
 selection upon separate minute variations of the several 
 parts concerned. 
 
 The enormous horns of the ancient Irish elk, weigh- 
 ing in some cases over a hundredweight, required 
 specially strong neck muscles, bones, and ligaments, 
 and strong fore legs for their support. But an increase 
 in the strength of a single muscle following increased 
 weight of the horns would be useless if unaccompanied 
 by a corresponding increase in many other structures, 
 and, if useless, could not be selected. The chance of 
 all the parts concerned varying simultaneously in a 
 corresponding direction is very small if these variations 
 are really independent, and the chance of their doing 
 so repeatedly is in such a case infinitesimal. 
 
 Let us take another case of a similar nature. The 
 
70 THE THEORY OF NATURAL SELECTION 
 
 hind legs of such an animal as a cat are admirably 
 adapted for the purpose of making a spring. In order 
 to arrive at such a structure by the modification of 
 limbs previously adapted only for running, changes 
 must occur in almost all the bones, muscles, and 
 ligaments of the limbs, and these changes must keep 
 pace with one another so that one part may not grow 
 out of proportion with the rest. It is quite impossible 
 to suppose that this can be effected by the natural 
 selection of minute fortuitous variations of the various 
 parts, each occurring independently. But simultane- 
 ously with these changes the fore legs have become 
 modified in a totally opposite direction. They have 
 become straight, firm, and pillar-like for receiving the 
 weight of the body in the downward leap. Compare, 
 says Herbert Spencer, the silence of a cat’s leap up 
 on to a table with the thud made by the fore legs as it 
 jumps down upon the floor. 
 
 Modification of the fore legs and of the hind must 
 thus have proceeded in almost exactly opposite direc- 
 tions in the two cases, and in each a great number of 
 parts are separately co-ordinated. For natural selec- 
 tion to have had any effect, all the co-ordinated parts 
 of one pair of legs must have varied in one direction, 
 whilst similar parts in the other pair of legs varied 
 simultaneously in another direction. It is out of the 
 question to suppose that this could have happened 
 simply by chance. 
 
 ‘ What, then, is the only defensible interpretation ? 
 If such modifications of structure produced by modifi- 
 cations of function as we see take place in each indi- 
 
INHERITANCE OF ACQUIRED CHARACTERS 71 
 
 vidual are in any measure transmissible to descendants^ 
 then all these co-adaptations, from the simplest up to 
 the most complex, are accounted for. In some cases 
 this inheritance of acquired characters suffices by itself 
 to explain the facts ; and in other cases it suffices when 
 taken in combination with the selection of favourable 
 variations. An example of the first class is afforded 
 by the change just considered ; and an example of the 
 second class is furnished by the case, before named, 
 of development in a deer’s horns. If, by some extra 
 massiveness spontaneously arising, or by the formation 
 of an additional “ point,” an advantage is gained either 
 for attack or defence, then, if the increased muscularity 
 and strengthened character of the neck and thorax, 
 which wielding of these somewhat heavier horns pro- 
 duces, are in a greater or less degree inherited, and in 
 several successive generations are by this process 
 brought up to the required extra strength, it becomes 
 possible and advantageous for a further increase in the 
 horns to take place, and a further increase in the 
 apparatus for wielding them, and so on continuously. 
 By such processes only in which each part gains 
 strength in proportion to function can co-operative 
 parts be kept in adjustment, and be re-adjusted to meet 
 new requirements. Close contemplation of the facts 
 impresses me more strongly than ever with the two 
 alternatives — either there has been inheritance of 
 acquired characters, or there has been no evolution.’* 
 
 As we pointed out in the last chapter, there seems at 
 
 * Herbert Spencer, ' The Inadequacy of Natural Selection,’ 
 p . 20 
 
72 THE THEORY OF NATURAL SELECTION 
 
 first sight to be no inherent difficulty in the way of 
 acquired characters being inherited. Weismann has, 
 however, pointed out a very serious difficulty, which 
 is brought into prominence on making a study of the 
 minute anatomy of the cells of organisms during the 
 earlier stages of their development. 
 
 In the ordinary course of events every one of the 
 higher animals and plants begins its existence in the 
 form of a single minute cell — the fertilized ovum or 
 egg. This cell exhibits no trace of the complicated 
 series of organs which will develop from it when it is 
 subjected to the proper conditions. When the egg 
 is placed in favourable circumstances with regard 
 to warmth, moisture, food-supply, and the like, it 
 first divides into two equal portions ; and microscopic 
 study shows that elaborate precautions are taken to 
 insure the equal bipartition of its minute constituent 
 parts. Each of the two cells thus formed divides 
 again into two further cells, and by a series of repeated 
 bipartitions of this kind the cells which constitute the 
 adult body are at last brought into existence. Since 
 the body soon becomes differentiated into a number of 
 unlike organs, it is clear that at certain stages of the 
 process the two cells arising from a division must come 
 to differ slightly from one another ; and the cells ulti- 
 mately produced show very considerable differences 
 of form, structure, and size. Among all the cells which 
 finally arise those which have undergone the least 
 modification from their original condition are those 
 from which are developed the sexual reproductive cells, 
 or germ-cells, of the organism. Indeed, Weismann con- 
 
PURITY OF THE GERM-CELLS 
 
 73 
 
 eludes that there is no reason for supposing that these 
 have undergone any modification at all. 
 
 If we consider the cells which build up an adult organ, 
 and for the moment regard each separate cell as an 
 individual, we see that each of these individuals 
 possesses an ancestry of cells stretching right back to 
 the fertilized ovum — the single cell in which the whole 
 organism originated. So far as the later cell divisions 
 are concerned, the cell-lineage of a particular organ is 
 separate and distinct from that of the cells of any 
 other organ. At a certain distance back in the history 
 of the organism we shall come across a common cell- 
 ancestor for the cells belonging to a pair of neighbour- 
 ing organs, and the more widely separate the parts to 
 which the cells we are considering belong the further 
 back must we go before we find their ancestry merging 
 in a single cell. In a similar way as with other organs, 
 so it is found that the sexual cells or germ-cells of an 
 adult organism have a history quite distinct from that 
 of the cells of any other part of the body ; and these 
 cells are the only ones which are concerned in the 
 formation of the offspring. Thus we see that the 
 particular cell-lineage leading up to the germ-cells is 
 the only one which is continued into another genera- 
 tion; all the others terminate with the death of the 
 individual creature of which they form a part. From 
 this point of view we may consider the nature of a 
 given series of animals as being determined only by 
 the particular series of cells which constitute the 
 direct ancestry of the germ-cells in each individual ; 
 the cells which make up the bodily structure are the 
 
74 THE THEORY OF NATURAL SELECTION 
 
 result of so many offshoots which come to an end at 
 the death of the organism, and leave no progeny of 
 their own. 
 
 Wilson has expressed this view of Weismann’s very 
 clearly ; ‘ It is a reversal of the true point of view to 
 regard inheritance as taking place 
 from the body of the parent to that 
 of the child. The child inherits 
 from the parent germ-cell , not from 
 the parent body, and the germ-cell 
 owes its characteristics not to the 
 body which bears it but to its 
 descent from a pre-existing germ- 
 cell of the same kind- Thus the 
 body is as it were an offshoot from 
 the germ-cell. As far as inheri- 
 tance is concerned the body is 
 merely the carrier of the germ-cells 
 which are held in trust for coming 
 generations.’ (The diagram illus- 
 trating Weismann’s theory of in- 
 heritance is a modification of that 
 given by Wilson.*) 
 
 In the light of this conception 
 it may be seen that the idea of the 
 inheritance of a modification ac- 
 quired by an adult bodily organ 
 is comparable with the supposition that if a man 
 develops his muscles by exercise his brother’s children 
 will be thereby modified. 
 
 ♦ ‘ The Cell in Development and Inheritance,’ p. 1 3. 
 
 G, Germ cells ; S, Somatic 
 cells. 
 
PURITY OF THE GERM CELLS 
 
 75 
 
 The minute study of the germ-cells, taken in connec- 
 tion with modern experimental work on the methods 
 by which inheritance takes place, shows a strong 
 tendency to confirm Weismann’s view, so far as the case 
 of distinct and definite characters is concerned. But 
 if we regard such definite characters as having arisen 
 by definite steps or mutations according to the view 
 now gaining ground, the study of them will have no 
 bearing upon the question of use-inheritance, since 
 use does not lead to large and definite changes in the 
 individual, but to comparatively small changes of a 
 quantitative kind. 
 
 There are some, including de Vries, who regard all 
 fluctuating variations (individual differences) as being 
 of the nature of acquired characters, and as being at 
 the same time capable of hereditary transmission, 
 although de Vries believes the amount of progress 
 possible in this way to be strictly limited. Let us see 
 if there is any way in which a transmission of such 
 characters can be conceived of. 
 
 It must be pointed out that the cells which make 
 up an organism are not completely marked off and 
 separated from one another ; on the contrary, it seems 
 impossible to doubt that reactions may take place 
 between them long after their first formation. Indeed, 
 Sedgwick has shown that in a number of diverse kinds 
 of animals there is never any sharply limiting barrier 
 between cells at all, and this writer has gone so far as 
 to speak of animals in general as being built up of a 
 continuous network of protoplasm with nulcei at the 
 nodes. In plants, too, though at first sight their con- 
 
76 THE THEORY OF NATURAL SELECTION 
 
 stituent cells seem to be cut off from one another like 
 so many closed boxes< it has been shown that there is 
 almost universal communication between the proto- 
 plasmic masses so enclosed, in the shape of minute 
 fibrils of living substance which traverse the interven- 
 ing walls. 
 
 It would thus seem possible for liquid or easily 
 soluble substances to pass freely from one part of the 
 body of an organism to another. It is possible, for 
 example, supposing the enlargement and strengthening 
 due to the exercise of a particular muscle to be associ- 
 ated with an increased production of some definite 
 chemical substance, to imagine that an increased 
 amount of the same substance might become enclosed in 
 the germ-cells, so that this substance would be present 
 in the offspring in greater abundance than would have 
 been the case if the muscle of the parent had not been 
 exercised. And this might facilitate a further develop- 
 ment of the same muscle by exercise in the next 
 generation. In a similiar way increased bulk following 
 upon better nutrition might be inherited, and this 
 de Vries seems to have succeeded in showing to be 
 actually the case in plants. Such changes might 
 normally be so slight as to be almost imperceptible in 
 a few generations, and yet after marry generations 
 might accumulate to an important extent. It would 
 be impossible in practice to distiirguish changes of this 
 kind from what are known as accidental individual 
 differences, and, indeed, there is no evidence at hand 
 to disprove de Vries’ assertion that all continuous 
 variations are of the nature of acquired characters — 
 
INHERITANCE OF ACQUIRED CHARACTERS 77 
 
 and we know that continuous variations are in- 
 herited. 
 
 On the other hand, several lines of inquiry have 
 separately led to the conclusion that a great number 
 of the visible characteristics of organisms are of a 
 definite kind, and are inherited definitely, their appear- 
 ance being determined by the presence of definite 
 structures or substances in the germ-cells. The evi- 
 dence, as we shall see later on, points to the conclusion 
 that such characters have arisen suddenly at a single 
 step, and we must conclude that in such a case a definite 
 change in the germinal structure has been followed by 
 a definite alteration in the character of the organism 
 arising from the germ ; since no one can suppose that a 
 large and definite structural alteration can be first 
 acquired by the adult organism and then inherited by 
 its offspring — such a process is unthinkable. 
 
 Thus we see that the inheritance of acquired char- 
 acters, if such inheritance really takes place at all, 
 must be confined to the transmission of changes of an 
 indefinite and quantitative kind — to the case, in fact, of 
 continuous variations or individual differences. More- 
 over, there is nothing to show that all continuous 
 variations are not of the nature of acquired characters.* 
 
 ♦ We know, at any rate, that continuous variations are 'not 
 invariably due to the cause which Weismann supposed — na:feely, 
 to the mingling together of characters derived from the twO 
 parents — a supposition which is of fundamental importance 
 to his theory — because, as Karl Pearson has pointed' out in 
 this connection, parthenogeneticaJly reproduced organisms, 
 in which no such mingling has taken place, may be just as 
 variable as those which owe their origin to the process of 
 sexual generation. 
 
78 THE THEORY OF NATURAL SELECTION 
 
 It is possible that variations of this nature may gradu- 
 ally lead to important and even to specific changes, 
 but whether this is the case still remains to be proved. 
 On the other hand, we shall see that specific differences 
 do sometimes arise at a single step, and there is strong 
 but indirect evidence to show that this is the way in 
 which a very great number of specific differences have 
 actually arisen. Indeed, some have contended that 
 this is the universal process by which such differences 
 originate, but this again is not proved, nor is it 
 altogether likely. In any case the inheritance of 
 acquired characters can have nothing to do with that 
 of definite and discontinuous differences. 
 
 This is a problem to which we shall return in the 
 concluding chapter, in the light of further evidence con- 
 cerning continuous and discontinuous variations and 
 their manner of inheritance, which will be by that 
 time available. 
 
 BIBLIOGRAPHY. 
 
 Darwin, Charles : The Origin of Species, sixth edition, 1872. 
 „ „ The Variation of Animals and Plants 
 
 under Domestication, 1868. 
 
 ,, „ The Descent of Man, 1871. 
 
 ,, „ The Various Contrivances by which 
 
 Orchids are Fertilized by Insects. 
 
 „ „ Different Forms of Flowers. 
 
 „ „ The Movements and Habits of Climbing 
 
 Plants. 
 
 ,, „ Insectivorous Plants. 
 
 „ „ Life and Letters of Charles Darwin. 
 
 „ ,, More Letters of Charles Darwin. 
 
 Malthus, T. R. : An Essay on Population. 
 
 Wallace, A. R. j Darwinism. 
 
BIBLIOGRAPHY 
 
 79 
 
 Weismann, a. : Essays upon Heredity, 1889. 
 
 ,, The Evolution Theory, 1906. 
 
 Spencer, Herbert i The Inadequacy of Natural Selection, 
 
 1893. 
 
 „ „ A Rejoinder to Professor Weismann, 
 
 1893 - 
 
 Cope, E. D. : The Primary Factors of Organic Evolution, 1896. 
 Morgan, T. H. : Evolution and Adaptation, 1903. 
 
 „ „ A Critique of the Theory of Evolution, 1916. 
 
 Romanes, G. J. : Darwin and After Darwin, vols. ii. and iii, 
 PouLTON, E. B. : Essays on Evolution, 1908. 
 
 Punnett, R. C. : Mimicry in Butterflies, 1916. 
 
 Darwin, F.: Presidential Address to the British Association, 
 1908. 
 
 Butler, S.: Life and Habit, 1877. 
 
 ,, Unconscious Memory, 1880. 
 
 „ Luck or Cunning, 1886. 
 
 Willey, A.: Convergence in Evolution, iqir. 
 
CHAPTER IV 
 
 BIOMETRY 
 
 In the present chapter we have to consider in some 
 detail the manner in which purely statistical methods 
 have been applied to certain biological data, a proceed- 
 ing to which the term biometry has been attached 
 by Professor Karl Pearson. Before concluding our 
 account we shall give a brief sketch of some of the 
 more important evidence bearing upon the problems 
 of evolution which has been brought to light by the 
 methods of biometrical science. 
 
 The first investigator to apply the methods of 
 statistics to the solution of biological problems was the 
 Belgian astronomer, Quetelet. In 1845, in the form 
 of a series of letters addressed to the Grand Duke of 
 Saxe-Coburg and Gotha, Quetelet published an admir- 
 able account of the theory of probability and its 
 relation to human affairs, and one in which the use of 
 advanced mathematics was avoided. The pioneer of 
 biometry in this country is Francis Galton, whose book 
 on ‘ Natural Inheritance ’ embodies an extremely 
 lucid introduction to the statistical study of variation 
 and inheritance. From these two works are derived 
 most of the ideas submitted in the present chapter. 
 The more recent advances in biometry are mostly 
 
 80 
 
BIOLOGY AND STATISTICS 
 
 8i 
 
 the result of work published by Professor Karl Pearson ; 
 they consist largely in the elaboration of mathematical 
 methods of dealing with statistical problems, and as 
 such it would be inappropriate to give any further 
 account of them here. 
 
 The mention of the word • statistics ’ at once raises a 
 certain prejudice in the ordinary mind ; in common 
 parlancej the unreliability of arguments based upon 
 statistics is sometimes treated as proverbial, and as 
 used in biology they have, as a matter of fact, one very 
 serious danger at least. Statistics deal with groups 
 and not with individuals, and there is a real difficulty 
 involved in the fact that the average of a group may 
 represent something quite different from any individual 
 which the group contains, whilst at the same time a 
 group may include individuals of very diverse natures. 
 Nevertheless, when used without prejudice to the future 
 examination of individual inheritance by more detailed 
 investigations, the methods of biometry have im- 
 doubtedly yielded information of great value to the 
 evolutionist, particularly in the case of such material 
 as that afforded by the human race, since the appUca- 
 tion of precise experiments to this particular species is 
 at present out of the question. 
 
 Some students of biometry, however, would go very 
 much further than this, for it is their professed opinion 
 that their own form of study is the only method by 
 which any real advance in our understanding of the 
 processes of evolution can be brought about.* This 
 opinion is based upon the assumption, of which proof 
 *[ This statement is perhaps no longer true. — Ed.] 
 
 6 
 
82 
 
 BIOMETRY 
 
 is wanting, that new species have arisen exclusively 
 through the accumulation by natural selection of 
 variations of a strictly indefinite, fluctuating, or normal 
 kind. We have already seen reasons for believing that 
 this is very far from being the case, and future chapters 
 will be found to add considerably to the force and 
 quantity of the evidence already adduced. 
 
 Normal variations, strictly speaking, are individual 
 differences which can be supposed to depend upon a 
 large number of small factors or causes — factors so 
 numerous and so minute that the numerical distribu- 
 tion of the individuals examined, when ranged in order 
 according to the feature chosen for examination, is 
 found to conform closely to that which would be 
 expected on the mathematical theory of chance. 
 Such a distribution will only result when the differences 
 considered can be strictly regarded as lying upon a 
 linear scale, and when they are also evenly distributed 
 along that scale. That is to say, the biometrician 
 deals with continuous variations of a quantitative kind. 
 It is to be hoped that these somewhat obscure sayings 
 will be more easily understood in the light of what 
 follows. 
 
 The facts of variation have not been found readily 
 amenable to precise definition, but we shall endeavour 
 to make plain by the aid of a few examples what kinds 
 of variations do and what kinds do not appear to be 
 legitimate objects for the application of biometrical 
 methods. Thus it may be thought that the biome- 
 trician is outrunning his license when he ranks the 
 colours shown by the iris of the human eye in a con- 
 
DEFINITE DIFFERENCES 
 
 83 
 
 tinuous series of eight shades, because in doing so he 
 groups together a number of probably definite factors 
 with others which are of an indefinite kind. When the 
 colours of the human eye come to be studied in greater 
 detail, there can be little doubt that they will be found 
 to depend upon some such factors (among others) as 
 the following ; 
 
 1, («) Definite differences in structure, and (b) the 
 definite presence and absence of pigment in certain 
 definite positions ; as well as — 
 
 2. (a) Indefinite variations (individual differences) in 
 structure, and (b) in quantity of pigment — if, indeed, 
 the quantitative differences are not found to be also 
 definite. 
 
 In the above example a suitable and legitimate 
 object for biometrical investigation would be the 
 differences in amount of a particular pigment. 
 
 But definite differences may also exist in the case of 
 an apparently simple quantitative character. The 
 accompanying figure (Fig. 2) shows the variations in 
 length of the fruits of three different but closely allied 
 species of evening primrose, as measured by de Vries. 
 In this diagram the vertical distances are in each case 
 proportional to the number of individuals having 
 particular lengths of fruit, and the actual length of the 
 fruit is in each case proportional to the horizontal 
 distance from an imaginary vertical line some way to 
 the left of the figure ; the points thus plotted are 
 joined by straight lines, so that a polygonal figure is 
 obtained representing the nature of the variation in 
 each particular case. The diagram shows at once that 
 
 6—2 
 
84 
 
 BIOMETRY 
 
 the species A and C have each a characteristic mean 
 size of fruit, and the existence of this definite mean 
 is not affected by the fact that the range of variation 
 
 overlaps in all three cases. Species B, on the other 
 hand, seems to show signs of division into at least two 
 separate groups. 
 
 Differences of a similar kind are sometimes to be 
 found among the progeny of the same individuals. 
 Races of garden peas may be selected which, amongst 
 other differences, are characterized by the presence of 
 large and of small seeds respectively. In each case 
 there is variation of a normal kind about a mean value, 
 but in each case the mean is quite distinct. There is 
 evidence that if a race of large-seeded peas is crossed 
 with a small-seeded variety, and the resulting cross-bred 
 plants are self-fertilized, their progeny in the second 
 generation will be separable into different groups, and 
 some of these will show almost exactly the same size 
 
DEFINITE DIFFERENCES 
 
 85 
 
 characteristics as those which were exhibited by the 
 two original parental strains. The only difficulty in the 
 way of invariably distinguishing the two original kinds, 
 after their segregation in the offspring of the cross, lies 
 in the fact that the smallest seeds of the large type 
 may be smaller than the largest seeds of the smaller 
 strain, and this is a difficulty which applies equally 
 to the original strains before crossing, as well as to the 
 case of the evening primrose fruits just mentioned. 
 
 Now, it is clear that if we mixed together the seeds 
 of several different races of peas in the proper pro- 
 portions, the result might lead to a normal distribution 
 of the kind presently to be described. The several 
 races, however, would none the less be perfectly distinct, 
 even though we could not separate the individual 
 seeds belonging to each by any direct method.* Such a 
 mixture of races would constitute a decided pitfall for 
 the unwary statistician, and it is well to remember 
 thatj after even the most elaborate mathematical 
 analysis, the final result cannot be clothed with any 
 greater amount of certainty than the facts from which 
 the calculation set out. Those who have made a large 
 expenditure of intellectual effort in such processes have, 
 unfortunately, a natural tendency to overlook this 
 elementary fact. 
 
 Prior to the application of statistical methods to a 
 particular case of normal variation a number of pre- 
 liminary processes have to be gone through. 
 
 * It would generally be possible to decide which strain a 
 particular seed belonged to by sowing it and observing the 
 variation of its offspring. 
 
86 
 
 BIOMETRY 
 
 Having selected a particular character for investiga- 
 tion, we must make a quantitative estimate of its 
 development in each member of a fair sample of 
 individuals which show the character in question. 
 What is to be understood as a fair sample was well 
 expressed by Quetelet when he wrote that statistics 
 must be collected without any preconceived ideas, 
 and without neglecting any numbers. We shall find 
 that in this point the biometrical method differs 
 from the method introduced by Mendel, since in the 
 latter careful discrimination of data is an essential 
 feature. 
 
 The quantitative determination of a character may 
 be made either by counting or by measurement. 
 That is to say, we must proceed by measurement if 
 the character we are dealing with is one of size or weight, 
 and by counting if the character shows a series of 
 numerical values of its own — e.g., if it is such a 
 character as the number of veins in a leaf or the 
 number of stigmatic bands on a poppy capsule. 
 Before we make any determinations we ought to be 
 quite certain that we are dealing with the same 
 character in each individual, and that the individuals 
 themselves are truly comparable with one another. 
 Thus we might make a series of measurements of a 
 particular bone in a particular limb of a particular 
 race of human beings with some assurance that we 
 should be dealing with homogeneous material. 
 
 Our • measurements or countings will fall either 
 naturally or artificially into groups. In the case of 
 countings the groups are naturally limited by the 
 
BIOMETRICAL METHODS 
 
 87 
 
 1 5 10 15 SO S5 80 
 
88 
 
 BIOMETRY 
 
 numbers which represent the character of each indi- 
 vidual, whilst measurements are artificially limited 
 through the fact that they have to be made in units 
 of some kind— ij.g., to the nearest inch or some other 
 value. Such groups, characterized by equality of 
 range — each, that is to say, covering an equal number 
 of units — are technically known as classes. 
 
 Thus if we are dealing with human stature, and if our 
 measurements were made only to the nearest inch, all 
 the individuals of 6 feet in height would fall into one 
 class, those of 6 feet i inch into another class, and so 
 on. If, on the other hand, we were engaged in count- 
 ing the number of ray florets in the heads of daisies* 
 a class would include all those heads which possessed 
 a particular number of rays. 
 
 Without division into classes, however, a survey of 
 a comparatively small number of measurements may 
 be facilitated by ranging the values in some kind of 
 order. This is done, for example, in the accompanying 
 figure (Fig. 3) for the measurements to hundredths of 
 an inch of the lengths of the body, wing, and tail of 
 thirty-one specimens of a North American bird. The 
 diagram is taken from A. R. Wallace’s ‘ Darwinism.’ 
 
 Even with this small number of measurements the 
 diagram brings out two points very clearly. In the 
 first place, there is no close correspondence between the 
 variations in length of body, wing, and tail. Secondly, 
 in the case of body-length, in respect of which the 
 specimens are ranged in order, the number of indi- 
 viduals of a medium size is seen to be greater than the 
 number of those which show extreme values. This 
 
NORMAL VARIABILITY 
 
 89 
 
 excess of mediocre individuals comes much more 
 prominently into view as soon as a larger number of 
 measurements can be considered, and the results 
 arranged in a different way. 
 
 Fig. 4. 
 
 The above diagram is constructed from the entries in 
 the third column of the accompanying table, which is 
 taken from Gabon’s ‘ Natural Inheritance.’ It repre- 
 sents the variations in the strength of pull (as exerted 
 by an archer in drawing a bow) shown by 519 men 
 as recorded at the International Health Exhibition in 
 1884. Here equal distances measured off along the 
 base line represent equal increments in the strength 
 of pull of the right hand, and the vertical heights of the 
 rectangles erected upon these bases represent the 
 percentage numbers of the men examined which 
 exhibited each value of the character under con- 
 sideration. In this example it is easy to see that the 
 central class is the largest, whilst the extreme classes 
 contain a comparatively small number of individuals. 
 
90 
 
 BIOMETRY 
 
 TABLE I. (FROM Galton). 
 
 STRENGTH OF PULL (519 MALES, AGED 23-26). 
 
 From Records made at the International Health Exhibition 
 in 1884. 
 
 Strength of Pull. 
 
 Number of Cases 
 observed. 
 
 Percentages. 
 
 Number of Cases 
 observed. 
 
 Sums from 
 beginning. 
 
 Under 50 lbs. 
 
 10 
 
 2 
 
 2 
 
 
 42 
 
 8 
 
 10 
 
 70 » 
 
 J40 
 
 27 
 
 37 
 
 » 80 „ 
 
 168 
 
 33 
 
 70 
 
 )) 90 » 
 
 II3 
 
 21 
 
 91 
 
 „ TOO „ 
 
 22 
 
 4 
 
 95 
 
 Above TOO „ 
 
 24 
 
 5 
 
 TOO 
 
 Total 
 
 519 
 
 TOO 
 
 
 Finally, we may display in a somewhat more detailed 
 fashion the result of a still larger number of measure- 
 
 ments. Fig. 5 shows the variation in stature of a large 
 number of members of Cambridge University of British 
 
NORMAL VARIABILITY 
 
 91 
 
 extraction, and exhibits in a concise form the result of 
 4,426 measurements recorded by the Cambridge Anthro- 
 pometric Society. In this figure the stature in inches is 
 indicated on the base line, whilst the perpendicular 
 distances indicate the number of cases in which each 
 particular height was recorded. The separate classes 
 in this case include those who were found to fall within 
 the limits of ^ inch on either side of each consecutive 
 integral inch of stature, measurements which fell 
 exactly half-way between two classes — e.g., one of 
 6 gi inches — being reckoned as a half to each of the 
 classes in question. The continuous line in the diagram 
 represents the form of the ‘ normal curve ’ which 
 approximates most nearly to the line obtained by 
 joining together the points actually plotted. 
 
 There seems to be good evidence that in such a case 
 as that of human stature the figure obtained in this way 
 will approximate more and more closely to the shape 
 of what is known as a normal curve, according as the 
 number of individuals measured and the accuracy of the 
 measurements increase. 
 
 In order to arrive at a proper understanding of this 
 fact, we must consider the derivation of the ‘ normal ’ 
 curve from another point of view — namely, from the 
 point of view of the mathematical theory of proba- 
 bility, which it will be our endeavour to present in as 
 simple a manner as possible. 
 
 Let us consider the result of tossing up a number 
 of similar coins simultaneously. If we toss up two 
 coins only we may get any of the following results : 
 (ij Head head, (2) head tail, (3) tail head, (4) tail tail. 
 
92 
 
 BIOMETRY 
 
 And It is clear that any one of these combinations is 
 equally likely to appear on any given occasion, if the 
 coins are supposed to be strictly symmetrical, and are 
 tossed up entirely at random. Now, the second and 
 third results are the same unless the two coins are indi- 
 vidually distinguishable. So we may write the most 
 
 ikely result of tossing up two pennies four times in the 
 following way : 
 
 1HH+2HT-1-1TT. 
 
 And in a similar way we may discover that the most 
 ikely result of tossing up three coins eight times is : 
 
 I HHH-i- 3 HHT-f-3 HTT + iTTT. 
 
 ^ the first case H T is twice as likely to appear as 
 H H at any single throw, and in the second case H H T 
 IS three times as likely as H H H in any single toss. 
 
 It IS possible to work out the most probable relative 
 frequency of the various possible combinations in the 
 case of any number of coins. Thus for ten coins the 
 sequence of numbers runs : 
 
 TABLE ll. 
 
 Heads. 
 
 Tails. 
 
 Relative 
 
 — 
 
 
 Probability. 
 
 10 
 
 0 
 
 I 
 
 9 
 
 8 
 
 7 
 
 1 
 
 2 
 
 3 
 
 10 
 
 45 
 
 120 
 
 0 
 
 5 
 
 4 
 
 3 
 
 4 
 
 210 
 
 5 
 
 6 
 
 252 
 
 210 
 
 7 
 
 120 
 
 2 
 
 j 
 
 8 
 
 45 
 
 
 9 
 
 10 
 
 0 
 
 
 I 
 
NORMAL VARIABILITY 
 
 93 
 
 These values are plotted in the accompanying 
 diagram (Fig. 6) as vertical distances above a base 
 line. The figure obtained by joining together the 
 points thus arrived at may be observed to show some 
 resemblance to the previous Figs. 4 and 5. 
 
 The three series of numbers already given are those 
 which are obtained on expanding the expressions 
 (i + i)*, (1 + 1)3, (i + i)“. In general the probabilities 
 of the various possible combinations when n coins are 
 tossed simultaneously are given by the expanded value 
 of 
 
 Quetelet has worked out the relative probabilities of 
 the most frequent combinations in the case of 999 coins 
 simultaneously tossed — i.e., the expanded value of 
 (i + i)®®®. A few of these values are given in the 
 following table : 
 
 TABLE III. 
 
 Heads. 
 
 Tails. 
 
 Relative 
 
 Probability. 
 
 500 
 
 499 
 
 1*0 
 
 501 
 
 498 
 
 0*996 
 
 502 
 
 497 
 
 0*988 
 
 503 
 
 496 
 
 0*976 
 
 504 
 
 495 
 
 0*961 
 
 505 
 
 494 
 
 0*942 
 
 510 
 
 489 
 
 0*803 
 
 520 
 
 479 
 
 0*432 
 
 530 
 
 469 
 
 0*155 
 
 540 
 
 459 
 
 0*037 
 
 550 
 
 449 
 
 0*006 
 
 560 
 
 439 
 
 0*0006 
 
 It may thus be seen that the likelihood that a result 
 appearing in any given throw will show a stdi greater 
 
94 
 
 BIOMETRY 
 
 difference in the relative number of heads and tails 
 than 560 1 439 becomes very small indeed. Although 
 
 a throw of all heads or all tails is possible, the odds 
 against such a result being ever actually seen are 
 
 Fig. 7. 
 
 almost inconceivably great. In Fig. 7 the middle 
 values for (i + i)®®® are plotted — those combinations 
 
THE NORMAL CURVE 
 
 95 
 
 being included which lie between 550 : 449 and 449 : 550. 
 The points thus obtained are so close together that the 
 eye can scarcely distinguish whether they are joined by 
 straight or curved lines. We have, in fact, arrived at 
 a close approximation to the normal curve. 
 
 The curve thus approximately indicated may be 
 seen to be closely similar to the one shown in Fig. 5 ; 
 in fact, the two curves are of such a kind that by 
 altering the vertical and horizontal scales in one of 
 the figures in a suitable ratio their form could be made 
 practically identical. 
 
 The figure arrived at in this way approximates to a 
 mathematical curve which is intelligible to the mathe- 
 matician from the formula y = e-^. The theoretical 
 curve is really arrived at by supposing n in the ex- 
 pression (i +1)" to become indefinitely great. Prac- 
 tically, by making n very large we can get as near an 
 approximation as we may wish to the normal curve of 
 theory. Even in the case of relatively small values of 
 n the approximation to the normal curve is fairly close, 
 as may be seen by comparing together Figs. 6 and 7. 
 
 The example of tossing up coins was only taken 
 as a means of illustrating the more general assumption 
 of an event or a magnitude depending upon a number 
 of causes of equal strength, which in the long-run act 
 with equal frequency in two opposite directions. We 
 can understand that human stature may afford a 
 comparable case, when we consider the large number 
 of bones and cartilages the lengths of which must be 
 added together in order to make up the total stature 
 of any individual, and that the separate length of each 
 
96 
 
 BIOMETRY 
 
 one of these elements depends upon factors which we 
 have no means of classifying exactly. 
 
 It now becomes necessary to mention one or two 
 technical terms which are used in connection with the 
 normal curve. The mode of such a curve is the 
 longest perpendicular which can be drawn from the 
 
 base-line to meet the curve itself (M in the above 
 figure). The curve is symmetrical on either side of the 
 mode — that is to say, any two perpendiculars drawn 
 from the base to the curve on either side of the mode 
 and at the same distance from it will be equal in 
 length. 
 
 When dealing with a symmetrical curve the position 
 of the mode is identical with that of the median — the 
 perpendicular line which divides the area of the 
 curve into two equal halves, and the foot of this 
 perpendicular also represents the mean or average of 
 
THE NORMAL CURVE 
 
 97 
 
 all the values from which the curve is constructed. In 
 any actual case obtained by practical methods the 
 position of the mode, the median, and the mean will 
 only be approximately the same, because such a curve 
 is never perfectly symmetrical. 
 
 The same curve can always be reconstructed if the 
 position and magnitude of the mode are known, and, 
 in addition, any one other point on the curve itself. 
 A convenient point to take for this purpose is the point 
 at which the curve is met by a straight line erected 
 perpendicular to the base at such a distance from the 
 median that it divides the area enclosed by the median, 
 the base, and half the curve into two equal parts. 
 The distance of such a perpendicular from the median 
 is known as the quartile. Any given curve will have 
 two quartiles one on either side of the median ; they 
 are shown at Q and Q' in Fig. 8 . 
 
 In practice an approximation to the normal curve of 
 variability is constructed by plotting the values of a 
 number of separate measurements or other determina- 
 tions made upon different individuals. A variate is 
 one of the separate numerical values from which a 
 curve of variability can be constructed ; the biome- 
 trician usually deals with some such number as 
 1,000 variates. The total number of variates is 
 represented by the area enclosed by the curve, and 
 it will be seen that half the total number of variates 
 falls between the two quartiles and half outside 
 them. 
 
 A class (cf. p. 88 ) may be defined as j. group of 
 variates all of which show a particular value or a value 
 
 7 
 
98 
 
 BIOMETRY 
 
 falling between certain limits. The frequency of a class 
 is the number of variates which it contains. 
 
 The amount of variation shown by a particular 
 group of variates is measured by the degree of slope 
 of the curve. A flat curve indicates greater variability 
 and a steep curve denotes less variability. The flatter 
 the curve — supposing the area (the number of 
 variates) to remain the same — ^the further from the 
 mode will be the position of the quartile, so that the 
 distance of the quartile from the mode may be taken 
 as a convenient measure of variability. In a theoreti- 
 cally perfect curve the distance of Q and Q' from M 
 is equal. A curve obtained from an actual series of 
 variates is never perfectly symmetrical, so that in 
 practice the distance of Q and Q' from M may not be 
 quite the same. In such a case the average of the 
 two distances is taken as the measure of the variability 
 of the material in question, and this value may be 
 briefly denoted by the letter q. 
 
 In the example of variability of stature represented 
 by Fig. 5, q is equal to i’6 inches. This amount of 
 variability can therefore be compared with other 
 values representing the variability in statiire and in 
 other characters shown by various other groups of 
 individuals. This, then, is the first important biome- 
 trical result which we have arrived at — the determina- 
 tion of a numerical value representing the amount of 
 normal variability in any given case. 
 
 A measure of variability more often used than the 
 quartile, especially in recent work, is what is known 
 as the standard deviation of a normal curve, and may 
 
PROBABLE ERROR 
 
 99 
 
 be expressed shortly as <j. <r represents a distance from 
 the mode equal to 0-6745. Thus if a is known, q 
 can be readily determined, and vice versa. The reason 
 for the more frequent use of a- is that it happens to be 
 determinable with greater accuracy from an actual 
 series of variates.* 
 
 We have still to find a measure which will enable us 
 to compare the variability of parts or organisms so 
 different that they require to be expressed in units of 
 quite different magnitudes. For this purpose what is 
 known as the coefficient of variability is used. This is 
 a purely abstract number obtained by dividing the 
 standard deviation by the magnitude of the mean in 
 any particular case, and multiplying the result by 100. 
 In this way a measure of variability is arrived at inde- 
 pendent of the particular kind of units of measurement 
 which were employed in obtaining it, and variabilities 
 previously expressed in terms of different units can 
 thus be compared together. 
 
 The circumstance that half the total number of 
 variates lies outside the limits of the quartiles and half 
 within leads us to the consideration of what is known 
 as the probable error. The probable error of any 
 statistical determination is obtained by finding a pair 
 
 * o- is found by multiplying the square of the deviation of each 
 class from the mean (or mode) by the frequency of the class, 
 adding together the series of products so obtained, dividing 
 this number by the total number of variates, extracting the 
 square root of the result, and multiplying by the number of 
 units in the class range (this last number is very often unity). 
 For further details with regard to the properties of the normal 
 curve Daven4?ort’s ‘ Statistical Methods ’ may be consulted 
 
 7—2 
 
100 
 
 BIOMETRY 
 
 of values lying one above and one below the true value 
 required — e.g., the average stature of the whole of a 
 race — such that it is an even chance that the value 
 actually found will lie between them. Or the same 
 thing may be expressed in another way. . If we plot in 
 the form of a curve a long series of actual determina- 
 tions of a particular value, the probable error of a 
 single determination will be nearly equal to the 
 quartile of the curve so obtained. We may illustrate 
 this state of things from our example of tossing coins, 
 or still better by the essentially similar case of draw- 
 ing balls out of a bag which contains a very large 
 number of balls — black and white in equal numbers. 
 Here the value to be determined experimentally is 
 the relative number of black balls to white, which we 
 know as a matter of fact to be equality; and our 
 single determination may consist in drawing out a 
 hundred balls, which are afterwards returned to the 
 bag. If we do this i,ooo times, and plot the number 
 of black balls drawn each time, we shall arrive approxi- 
 mately at a curve having its mode at 50, and possessing 
 a standard deviation which it is possible to determine 
 from the instructions given in the footnote to p. 99. 
 Multiplying a by 0-6745 gives us the quartile, which 
 represents the probable error of a single determination. 
 That is to say, it is an even chance whether any single 
 determination differs from 50 by more or less than q. 
 In this particular example the quartiles would be found 
 to lie very nearly at 46-6 and 53'4> so that the value of 
 the probable error is 3-4. 
 
 The properties of the normal curve tell us a number 
 
PROBABLE ERROR 
 
 lOI 
 
 of useful things about t he pro bable error. In the first 
 place its value varies inversely as the square root of 
 the number of variates — that is to say, that in such a 
 case as we have just described the probable error varies 
 inversely as the square root of the number of balls 
 drawn each time. We can realize this point more 
 clearly when we remember that the linear dimensions 
 of a curve vary with the square root of its area (the 
 number of variates) ; the accuracy of our determination 
 varies in fact with the quartile, which is the linear 
 distance from the mode of a certain perpendicular. 
 
 We have seen that it is an even chance whether a 
 single determination differs from the proper value by 
 more or less than the amount of the probable error, 
 an amount which we may denote by the letter e. 
 The chance that any particular determination differs 
 from the true value by more than twice the probable 
 error is 4-5 to i against. 
 
 The chance that it differs by more than 3^ is 21:1 against, 
 w w » 4*^ 142 : I ,, 
 
 » )) M » 5^ >> ^>3^0 : I ,, 
 
 This is clearly very valuable information to possess 
 when we are dealing with any kind of statistics. 
 
 We must now pass on to consider what methods are 
 available to the biometrician for deahng with the 
 problems of heredity. His way is to take a large 
 number of pairs of relations, each pair consisting, say, 
 of a father and a son, and to find out how much more 
 like the members of such a pair are to one another on 
 the average than the members of similar pairs of 
 individuals would be, if taken at random and without 
 
TABLE IV. 
 
 103 
 
 BIOMETRY 
 
 JO siCvjJv JO 
 sapo]p^ 
 
 uniy^u^iriLnii-ivnu^ 
 
 ijn vO vb r-. K OO 00 ON On 0 O >-1 M 
 
 vOnOvOvOvOnOnOnOvO 
 
 On 
 
 NO 
 
 •;qSi9H 
 
 qoB9 JO suog joj 
 saaqiv^ l«I0X 
 
 cn O tv, Ln i-i tv^oo VO ONVO N O vn 
 
 H-( VO 00 00 vn uM-ooo •-< -^00 00 vn i-i 
 
 t-H cn vr»vO t^.'O vn CO *-< 
 
 CO 
 
 0 ON 
 
 VO \0 
 
 Heights of Fathers 
 (I n inches). 
 
 CO 
 
 1 1 1 ! 1 1 1 
 
 VO 
 
 H. M 
 
 tv. 
 
 lO 
 
 t- 
 
 CO 
 
 fc- 
 
 j j j j j 
 
 CN< 
 
 VO tv^ 
 
 1 1 j j N tv. On tv. On OnvO N 
 
 Hi 
 
 00 ^ 
 tv^ 
 
 1 1 1 M N O >-< nO t^ On vooO N O 
 
 j j j HHI-(l-(COCOCOt-<H-ll-l 
 
 HI 
 
 S' 
 
 C<J 
 
 , , , cOvntv.OOvOtV'VTM^OOOO^ 
 
 j j >HMCSTtvnvjn'^cot-i>-i 
 
 rf VO 
 
 CO Vv 
 
 ^ R 
 
 rH 
 
 C* 
 
 1 WMOOOOi-^ONCOvot^vr^GNONN 
 j (-< t-H vooo 0 00 vn ^ i-t 
 
 • 1-1 
 
 ^ R 
 
 VO 
 
 o 
 
 l> 
 
 , Hi CO ''t CO tv,00 ONvO 00 VO Hi 
 
 i-icovnOC>CSONvocS 
 
 • »-( l-H 
 
 CO VO 
 
 - ^ 
 
 05 
 
 CO 
 
 1 C<vO'^vr>cO(N)ONOf^OO V0\0 M , 
 
 H. Tt-00 CO tv. COOO Tt Hi 
 
 ■ hH H-( ^ > 
 
 5 ON 
 
 00 
 
 o 
 
 HiVOOM'-^C'It^vr^ vo\0 Hi O *-1 1 
 
 •1 covO O n CO O vi^ CO Hi 1 
 
 O VO 
 
 l^* 
 
 CO 
 
 NOO(^0OvnHii^C>M0OOC< I 1 
 
 Hi rt voOO O 00 VO Hi 
 
 Hi • ' 
 
 5?> S 
 
 VO ^ 
 
 CO 
 
 CO 
 
 rf vr\vO Hi ON tv. VO u^vO 00 N M W 1 I 
 
 Htt-icO'^vnvo'^cOHii-i 1 1 
 
 VO 
 
 lO 
 
 CO 
 
 »jnHiC^tv.coc)OtvC^O'TM| j 1 
 
 t-iHIHiCO’^COHiHiHI 1 1 1 
 
 R 
 
 CO 
 
 '^tvo C^HiOOOVOCOMHij 1 1 1 
 
 Hi M Hi H. I I i 1 
 
 00 NO 
 
 VO 
 
 CO 
 
 CO 
 
 |«'^0'OOncocommW| I 1 j 
 
 Hi vO 
 
 VO vO 
 
 cq 
 
 CO 
 
 1 1 1 1 1 1 1 1 
 
 F' 
 
 Hi VO 
 
 VO 
 
 
 CqCO-^iOCDt-OOOoOp^OaCOTiiiOCO 
 
 COCOCOCOCOCOCDCDt>»l>l>-Dv.c^£^l:^ 
 
 
 
 m 
 
 c . 
 
 o 'S 
 
 ui Ci 
 
 j 
 
 ffi a 
 o tu, 
 
 S 
 
 X 
 
 Total sons for fathers! 
 
 of each height ...J 
 Modes of arrays ofl 
 sons J 
 
CORRELATION 
 
 103 
 
 regard to relationships from among the general 
 population to which these fathers and sons belonged. 
 
 Now we shall see later on that this is not the only 
 way of looking at the phenomenon of heredity, nor is 
 it the way which is most familiar to biologists. But 
 
 62 64 - 66 68 70 72 74 - 76 
 
 Fig. 9.— Diagram of Correlation. 
 
 it is important to remember that what the biometrician 
 means by amount of inheritance is a numerical value 
 which expresses the average degree of likeness between 
 a particular pair of relatives — for example, fathers and 
 
 sons. 
 
104 
 
 BIOMETRY 
 
 In the * correlation table ’ given on p. 102 — a 
 purely imaginary illustration — there are tabulated the 
 statures of 4,503 fathers, and those of one son of 
 each of them. Thus 14 fathers, each 62 inches high, 
 are supposed to have had 14 sons, whose heights are 
 given in the first column. The series of heights of sons 
 corresponding to a particular class of fathers is known 
 as an array. Thus each column of the table represents 
 an array of sons, and similarly each line represents an 
 array of fathers. The mode of each array of sons is 
 given in the bottom line of the table. 
 
 Now if sons were on the average exactly the same 
 height as their fathers, the modal value of each array 
 of sons would be the same as the height of the corre- 
 sponding class of fathers. If, on the other hand, there 
 were no correlation between the heights of sons and 
 those of their fathers the mode of every array of sons 
 would be the same, and this value would be identical 
 with the mode of the heights of all the sons taken 
 at once. The actual result is found to be intermediate 
 between these two possible extremes. Thus we see 
 that sons tend to be like their fathers in respect of 
 stature, but not exactly like, cind if the example given 
 were a real one the fundamental fact of a positive 
 resemblance or correlation between the statures of 
 fathers and sons would at once be clearly established. 
 
 The way in which a numerical value is attached to 
 this correlation can be shown graphically. 
 
 In the diagram shown in Fig. 9, tfih dots indicate 
 the values of the modes of the several arrays of sons 
 as read off on the vertical scale to the left of the figure. 
 
CORRELATION 
 
 105 
 
 the heights of the corresponding classes of fathers 
 being read off on the horizontal scale. It will be seen 
 that this series of dots lies nearly in a straight line 
 which is inclined at a certain angle to the horizontal. 
 
 Now if there were perfect correlation between the 
 heights of fathers and sons, and no tendency existed 
 for sons to be more like the general mode of the popula- 
 tion than their fathers are, the inclination of the line 
 obtained in the above manner would be one of 45 
 degrees, as in the case of the line CD which passes 
 through the points at which the values as read off 
 in the vertical and horizontal scales are identical. If, 
 on the other hand, there were no correlation the line 
 would be horizontal, as EF. 
 
 The value taken to represent the amount of correla- 
 tion is the degree of slope of the line AB. This is 
 expressed mathematically as tan a, a being the angle 
 which the line in question makes with the horizontal. 
 
 When there is positive correlation this angle falls 
 between o and 45 degrees, and tan a between o and i. 
 In the present instance tan a is 0-5. This value is 
 known as the coefficient of correlation, and affords 
 the basis of a numerical comparison with other similar 
 coefficients obtained for other characters besides 
 stature, and in the case of other pairs of relatives 
 besides fathers and sons. 
 
 It ought now to be clearly understood that a com- 
 plete resemblance between each class of fathers of a 
 particular stature and the average stature of the 
 corresponding array of sons would be indicated by 
 the close approximation of our plotted points to a 
 
io6 
 
 BIOMETRY 
 
 straight line making an angle of 45 degrees with the 
 base line — a line, that is to say, having a slope of 
 I in I, or unity ; whilst the entire absence of correlation 
 would be represented by a line having no slope — that 
 is to say, a horizontal line. The actual result in the 
 example given is represented by a line having a slope 
 of nearly i in 2, or 0-5.* 
 
 In the following table there are set down the corre- 
 lation coefficients for stature in the case of seven pairs 
 of relations, as obtained from actual data of a similar 
 character to that already given by way of illustration. 
 
 TABLE V. (from Pearson). 
 
 CORRELATION COEFFICIENTS FOR HUMAN STATURE. 
 
 Father and son 
 
 ... 0-514 
 
 Father and daughter ••• 
 
 ... 0-510 
 
 Mother and son 
 
 ... 0-494 
 
 Mother and daughter... 
 
 ... 0-507 
 
 Brother and brother ... 
 
 ... 0-5 II 
 
 Sister and sister 
 
 ... 0-537 
 
 Brother and sister 
 
 - 0-553 
 
 Of the above, the first four values, representing 
 correlation between parents and children, are seriously 
 
 * Correlated Variability , — A precisely similar method is 
 employed to measure the corrdation of two parts or organs of 
 the same individual. For example, the lengths of the right 
 and left arms of men are very closely correlated. In order to 
 attach a numerical value to this correlation the lengths of 
 the right arms of a number of men are treated in the same 
 way as the statures of fathers in the example given, and the 
 lengths of their left arms in the same way as the statures of 
 sons. The proper correlation coefficient can then be found 
 by plotting the result ; or the labour of plotting may be 
 obviated by a process of calculation. 
 
CORRELATION 
 
 107 
 
 affected, as regards their value for representing quanti- 
 tatively a hereditary relationship between two indi- 
 viduals, by the fact that the statistics from which they 
 are derived show the existence of a marked correlation 
 between husbands and wives in point of stature, 
 amounting, indeed, to as much as 0-28 — the result 
 of what is technically described as selective mating. 
 In the absence of such a relation between the statures of 
 the parents, the correlation between parent and child 
 might be expected to be distinctly less than that 
 between pairs of brothers or sisters. 
 
 The term correlation replaces to some extent the 
 older term regression employed by Galton. WTien 
 speaking of regression the facts already described are 
 regarded from a slightly different point of view. It 
 is sometimes found convenient to speak of the regres- 
 sion of the mean stature of an array of sons toward 
 the mean of the general population, instead of speaking 
 of the correlation between the filial mean and the 
 value of the parental class. 
 
 Regression represents the extent to which the 
 average son is more like the mean of the general 
 population than his father is. Correlation, on the 
 other hand, indicates the amount by which the son is 
 more like his parent than he is to the average of the 
 general population. Thus, instead of being exactly 
 like their parents, children are said to show regression 
 towards the mean of the general population to which 
 both parents and children belong. 
 
 In a case where the mean height of the fathers is 
 identical with the mean height of the sons examined. 
 
io8 
 
 BIOMETRY 
 
 and both are the same as the mean height of the general 
 population, the coefficient of regression is simply equal 
 to the reciprocal of the correlation coefficient between 
 fathers and sons. In actual practice this condition is 
 seldom realized, and it is then necessary to use a more 
 elaborate method in order to determine the value of 
 the regression coefficient. 
 
 j Professor Pearson has extended the idea of correla- 
 tion to the case of characters which are not capable of 
 exact quantitative measurement. This extension is 
 based upon the assumption that such characters foUow 
 a normal law of distribution in their variation, just in 
 the same way as such a character as human stature was 
 found to do. There is considerable doubt as to how 
 far this assumption is justified, so that at the outset 
 we may feel disposed to attach less importance to the 
 actual values arrived at in this way than we should in 
 the case of characters which can be shown to vary 
 normally. The method of calculation actually em- 
 ployed involves somewhat comphcated mathematical 
 processes, but on Professor Pearson’s authority we 
 may assume both the validity of the method and the 
 accuracy of the results obtained — ^so far as the actual 
 process of computation is concerned. For the purpose 
 of making the necessary calculations the data were 
 arranged in such a form as in the table on p. 109. 
 
 By the suitable treatment of these figures the 
 value 0-45 was obtained as representing the coefficient 
 of correlation between sire and filly. 
 
 The amount of reliance which is to be placed in the 
 
CORRELATION 
 
 109 
 
 above method of determining the value of a correlation 
 coefficient was tested by arranging in a similar manner 
 data with regard to stature which had already been 
 treated in the form of a complete correlation table. 
 The whole number of fathers was divided into two 
 groups containing the individuals above and below a 
 certain stature, and the same was done in the case 
 of the sons. And the separation into two groups was 
 made in several different ways by taking the dividing 
 
 PARENTAL CORRELATION OF COAT COLOURS IN HORSES. 
 
 Fillies. 
 
 Sires. 
 
 1 
 
 > 
 
 Total. 
 
 1 
 
 Colour. 
 
 Bay 
 
 and Darker. 
 
 Chestnut and 
 Lighter 
 
 Bay and darker 
 
 631 
 
 125 
 
 . 1 
 
 756 
 
 Chestnut and lighter 
 
 147 
 
 147 
 
 294 
 
 Total 
 
 778 
 
 272 
 
 
 line between the groups at various heights. By 
 applying to the statistics disposed in these various 
 arrangements the same method as w'as applied to the 
 statistics of horse colour already referred to, values 
 varying between 0-52 and o-6 were obtained for 
 parental correlation ; whereas the value arrived at by 
 the more usual and reliable method was 0‘5i4. It 
 would therefore appear that there is with this method 
 a tendency to obtain too high a figure, as compared with 
 that derived from the method of the complete correla- 
 tion table. When this source of inaccuracy is taken 
 
110 
 
 BIOMETRY 
 
 into consideration, in combination with the doubtful- 
 ness of the assumption upon which the method is 
 based, it seems clear that its use will only give us a 
 roughly approximate view of the correlation actually 
 existing in the cases to which it is applied. Having 
 made this reservation, we may compare the values 
 given in the following table with those which appeared 
 in Table V. : 
 
 TABLE VI. (from Pearson). 
 
 AVERAGE PARENTAL CORRELATION. 
 
 Human eye colour ... ... ... 0*495 
 
 Horse, coat colour ... ... ... 0*522 
 
 Basset hound, coat colour ... ... 0*524 
 
 Greyhound, coat colour ... ... 0*507 
 
 AVERAGE FRATERNAL CORRELATION. 
 
 Human eye colour ... ... ... 0*475 
 
 Horse, coat colour ... «.. ... 0*633 
 
 Basset hound, coat colour ... ... 0*524 
 
 Greyhound, red in coat ... ... 0*700 
 
 Greyhound, black in coat ... ... 0*660 
 
 Thus if we use the term inheritance at present simply 
 to express the fact that a more or less definite 
 numerical value can be attached to the average amount 
 of resemblance between any specified pair of relatives, 
 we see that a considerable number of physical characters 
 appear to be inherited at approximately the same rate 
 in men and in animals. 
 
 More than this, Professor Pearson has shown, by 
 the use of the same method as was applied to the case 
 of physical characters not quantitatively measurable, 
 that the average resemblance in mental characteristics 
 
MENTAL CHARACTERS 
 
 III 
 
 between pairs of brothers, pairs of sisters, and pairs 
 made up of a brother and a sister, can be expressed 
 by the values given in the following table : 
 
 TABLE VII. (from Pearson). 
 
 Character. 
 
 Brothers. 
 
 Sisters. 
 
 Brother and 
 Sister. 
 
 Vivacity 
 
 0*47 
 
 0*43 
 
 0*49 
 
 Assertiveness 
 
 0*53 
 
 0*44 
 
 0*52 
 
 Introspection 
 
 0*59 
 
 0*47 
 
 0*63 
 
 Popularity 
 
 o‘ 5 o 
 
 0*47 
 
 0*49 
 
 Conscientiousness ... 
 
 0*59 
 
 0*64 
 
 0-63 
 
 Temper 
 
 0*51 
 
 0*49 
 
 0-51 
 
 Ability 
 
 0*46 
 
 0*47 
 
 0*44 
 
 Handwriting 
 
 0*53 
 
 0*56 
 
 048 
 
 Mean 
 
 0*52 
 
 0*51 
 
 0*52 
 
 A sample of the collected facts from which this 
 information is arrived at is given in the following 
 table : 
 
 CONSCIENTIOUSNESS : BROTHER— BROTHER. 
 
 
 First Brother. 
 
 
 Second Brother. 
 
 
 
 Total. 
 
 Keen. 
 
 Dull. 
 
 
 
 Keen 
 
 970 
 
 2i6’5 
 
 i,i86'5 
 
 Dull 
 
 2i6*5 
 
 287 
 
 5o3'5 
 
 Total 
 
 i,i86’5 
 
 5o3’5 
 
 1,690 
 
 Every child was classified in this way as being either 
 above or below an average standard in respect of each 
 
II2 
 
 BIOMETRY 
 
 character. The estimations were made by teachers 
 having at least six months’ experience of the children 
 in question. 
 
 The method of statistical treatment was, as we have 
 said, the same as that employed in the case of physical 
 characters not capable of quantitative measurement, 
 and there is little doubt that it is equally valid in the 
 present case.' We may well feel, however, some 
 hesitation in accepting as sound the data to which the 
 method is applied. At the best this data can only be 
 of a roughly approximate kind. The evidence is, 
 however, undoubtedly sufficient to establish the con- 
 clusion that mental characters are inherited in man, 
 and that they are probably inherited at a rate not 
 greatly different from that at which physical characters 
 are inherited. For it will be observed that the values 
 given in Table VII. are in close agreement with one 
 another, and that they also agree with the average 
 value of fraternal correlation as found for a variety of 
 physical characters both in men and in other animals. 
 Assuming — and the assumption seems to be a reasonable 
 one — that equal fraternal correlations indicate the exist- 
 ence of equal correlations between parents and children, 
 we arrive at the conclusion that the resemblance 
 between parents and their offspring is of much the 
 same kind and amount in the case of mental as it is in 
 the case of bodily characteristics. 
 
 What we may perhaps describe as the main general- 
 ization so far arrived at by biometricians is known 
 as the Law of Ancestral Heredity. This hypothesis 
 supposes, or at least in its original form supposed, that^ 
 
LAW OF ANCESTRAL HEREDITY 
 
 113 
 
 every ancestor of a particular individual contributes 
 its quota to the heritable qualities displayed by that 
 individual. The law also states that the average 
 amount of resemblance between an individual and any 
 particular ancestor is capable of definite numerical 
 expression. Thus the mean amount of correlation 
 between (i) the two parents and the offspring, (2) the 
 four grandparents and the offspring, (3) the eight 
 great-grandparents and the offspring, and so on, is 
 believed to diminish in a geometrical series, which is 
 the same for all organisms and for all characters. The 
 actual amounts of these correlations were expressed by 
 Galton in the form of the series 0-50, 0-25, 0‘i25, etc. 
 Pearson regards them as being more nearly represented 
 by the more rapidly diminishing series 0-6244, O'IQSS, 
 0-0630, etc. 
 
 Now, there can be no doubt that the law as stated 
 above has been disproved in specific instances, and was 
 indeed disproved by the work of Gregor Mendel before 
 ever it was enunciated, although Mendel’s work was 
 not generally known until later. According to Mendel’s 
 theory of inheritance, certain ancestors contribute 
 nothing to the constitution of certain offspring in respect 
 of certain characters. Furthermore, the modification of 
 the law of ancestral heredity which apphed to alterna- 
 tive inheritance, and which was assumed in working 
 out the inheritance of coat colour in thoroughbred 
 horses, has since been shown not to apply to that 
 particular case. 
 
 Unfortunately, most of the further biometrical 
 generalizations which concern students of heredity 
 
 8 
 
BIOMETRY 
 
 114 
 
 are based upon the assumption that the law of ancestral 
 heredity is strictly true. So that whilst we have 
 spent some time in considering the facts of normal 
 variability and of correlation between relatives, 
 because these facts are quite independent of any 
 theoretical assumption, the remainder of our review 
 must be passed over at a more rapid rate. Until 
 the theoretical conclusions now to be described have 
 been revised by their authors in the light of recent 
 knowledge, it is difficult to say how much reliance is to 
 be laid upon them, but it seems quite likely that they 
 will hold good as approximations. Indeed, though 
 not applying to individual cases, the law of ancestral 
 heredity does seem to hold good as a statistical state- 
 ment of general results, so that there would be no objec- 
 tion to it on either theoretical or practical grounds if 
 only it had been enunciated in some such terms as ‘ a 
 law of average ancestral resemblance.’ Thus it is quite 
 possible that the total contribution of the eight great- 
 grandparents of an individual may be on the average 
 correctly represented by Pearson’s fraction, even 
 though their individual contributions are not always 
 the same. 
 
 Let us, then, briefly examine some of the further 
 conclusions which have been drawn from the data of 
 the biometricians. 
 
 Assuming the law of ancestral heredity, Pearson has 
 arrived at very interesting conclusions with regard to 
 the effects of artificial selection when the correlation 
 coefficients have those values which have been actually 
 found for them in the case of the human race. In the 
 
EFFECT OF SELECTION 
 
 115 
 
 statement which follows, ancestors are supposed to 
 have been selected showing in each generation a devia- 
 tion h from the general mean of the population. Thus, 
 suppose the character selected to be stature j suppose 
 the mean height of the population to be 6 feet, and 
 the selected individuals to be 6 feet 6 inches high ; h is 
 then 6 inches, and only individuals of a height of 
 6 feet 6 inches would be selected as parents in each 
 generation, so that after three generations of selection 
 we should be dealing with children whose parents, 
 grandparents, and great-grandparents were all of this 
 particular height. 
 
 Pearson calculates that after one generation of 
 selection the immediate offspring will show 0'62 of 
 the character selected (0-62 h). After two generations 
 they will show 0-82 h, after three 0-89 h, and after a 
 great number of generations 0 92 h. Thus in a com- 
 paratively small number of generations the development 
 of a character may be raised to within 90 per cent, 
 of the value selected, but, after this, further selection has 
 very little effect. If selection is stopped after one 
 generation, and the selected stock is then inbred, it 
 was calculated that the first generation of inbred stock 
 would show 0-59 h, the second 0 56 h, the third 0^52 h, 
 and the tenth 0-35 h. If, on the other hand, in- 
 breeding w'as started after the selection had continued 
 for a large number of generations, the first generation 
 of inbred stock will show o-86 h, the second o-8i h, 
 the third 077 h, and the tenth 0-51 A. So that in- 
 breeding of a selected stock is followed by a very 
 
 8 — 2 
 
ii6 
 
 BIOMETRY 
 
 gradual return towards the mean character of the 
 original race.* 
 
 It must be remembered that in the calculation which 
 led to this result perfect normal variabihty was assumed, 
 and the contribution of every ancestor of the same 
 degree to the hereditary endowment of the offspring 
 was supposed to be exactly equal. Since both these 
 assumptions are very unlikely to be realized in any 
 actual case, the statement here given must only be 
 regarded as an approximate indication of what is 
 likely to take place. 
 
 Some remarkable observations have been published 
 by Professor Johannsen, of Copenhagen, and from 
 them are drawn conclusions which seem likely to lead 
 to a distinct advance in our understanding of the pro- 
 cess of so-called continuous variation, and of the way 
 in which such variations are transmitted. Johannsen’s 
 conclusions have more recently received remarkable 
 confirmation from the work of Jennings on protozoa, 
 so that we are now justified in accepting the theory 
 of the pure line as a well-established working hypo- 
 thesis. We shall confine our present account to 
 Johannsen’s now classical observations. 
 
 The experiments in question were made upon plants 
 which could be self-fertilized for a series of generations. 
 In this way many complications were avoided which 
 are inevitably introduced in the case of biparental 
 
 * From this it seems necessajrily to follow that it is impos- 
 sible to establish a permanent breed simply by a process of 
 selection. Professor Pearson, however, avoids tMs conclusion. 
 
THEORY OF THE PURE LINE 
 
 117 
 
 inheritance. Barley and kidney beans were among 
 the plants examined, and the simplest character con- 
 sidered was the size of the seeds of the latter as 
 measured by weighing. In this particular series of 
 experiments each plant was regarded as being 
 characterized by the average weight of the seeds 
 which it produced. 
 
 AU the descendants arising from a single plant by self- 
 fertilization are spoken of by J ohannsen as making up 
 a ‘ pure line.’ And the members of such a line showed, 
 in respect of the weight of their seeds, normal varia- 
 bility about a mean or type value. The general 
 population of bean plants, made up of a great number 
 of such pure lines, also exhibited a normal curve when 
 the weights of the seeds were plotted. The pure 
 lines composing such a population showed various 
 types, some of them elose to the modal value of the 
 population, but others differing widely from it. If 
 now a somewhat widely deviating member of a par- 
 ticular line was selected for propagation, its off- 
 spring showed regression to the t3^pe of this par- 
 ticular line, and not to the mean value of the general 
 population. 
 
 The case is indeed precisely similar to the supposed 
 example of a mixture of races of peas, which was made 
 use of as an illustration at the beginning of the present 
 chapter. In other words,{_a pure line consists of a 
 group of individuals which has a normal variability of 
 its own, and the offspring of which by self-fertilization 
 breed true to the type of their own particular group. 
 
ii8 
 
 BIOMETRY 
 
 and show no regression towards the type of the general 
 population to which the group belongs. 
 
 If we were to carry on this conception to the case of 
 bisexual inheritance, we should find that the different 
 pure lines would become crossed and confused together 
 in a way which would be very difficult to disentangle. 
 There is no reason to doubt that statistical treatment 
 of such a population would yield similar results to 
 those actually obtained by biometricians from the 
 data at their disposal ; and we may notice that a for- 
 tuitous mixture of a considerable number of pure 
 lines, having shghtly different types, would admirably 
 fulfil the conditions we have seen to be necessary in 
 the case of material, to which methods based upon 
 the theory of chance are to be applied. The phe- 
 nomena which follow upon the crossing together of 
 two or more pure lines have been found, in the majority 
 of cases so far studied, to conform to those laws of 
 heredity associated with the name of Mendel which 
 are explained in Chapter VII. This being the case, 
 there appears to be every probability that the theory 
 of pure lines, in combination with the method of in- 
 heritance referred to, may adequately serve to describe 
 those phenomena to account for which the law of 
 ancestral inheritance was called into existence. 
 
 The conclusions to which Professor Johannsen’s 
 experiments lead him may be summed up as follows : 
 Individuals which differ (in size, for example) from 
 the mean of a population give rise to offspring which 
 differ from that mean value in the same direction 
 but to a smaller extent. Selection, therefore, will 
 
THEORY OF THE PURE LINE 
 
 119 
 
 produce a change in the average character of a popula- 
 tion taken as a whole. Selection within a pure line 
 produces no effect of this kind. The average character 
 of the offspring of typical members of the hne is the 
 same as that of the offspring of members which show 
 the widest deviations from the type. 
 
 Selection in a population consists in the partial 
 separation of those lines the types of which differ in 
 the required direction from the average character of 
 the population. This effect must of necessity come to 
 an end when the most eccentric line is completely 
 isolated. The great comphcations introduced when 
 the lines are intermingled through mixed breeding may 
 make this process of isolation a very tedious one. 
 
 It will be seen that the values calculated by Pearson 
 to represent the result of selection in a population 
 agree quite well with Johannsen’s explanation of the 
 constitution of such a population out of a number 
 of pure lines. The result of Professor Johannsen’s 
 further experiments will therefore be awaited with 
 great interest by biologists and biometricians alike. 
 
 On the theory of pure Knes it is to be noticed that 
 the personal character of a particular ancestor has 
 no influence upon his descendants ; it is only the 
 type of the line to which he belongs which influences 
 the offspring, so that this theory is in perfect agreement 
 with Weismann’s theory of inheritance as described 
 on p. 74. 
 
 It is also to be observed that the principle of the 
 pure line appUes only to quantitative characters — 
 such characters of size, or of weight, or of proportion. 
 
120 
 
 BIOMETRY 
 
 as are very seldom made use of by systematists for 
 the distinction of natural species. 
 
 BIBLIOGRAPHY. 
 
 Galton, F. : Natural Inheritance, 1889. 
 
 Pearson, K. : The Grammar of Science, second edition, 1900. 
 Vernon, H. M. : Variation in Animals and Plants, 1903. 
 Davenport, C. B. : Statistical Methods, 1904. 
 
 Biometrika, 1901 — 
 
 JoHANNSEN, W. I Ueber Erblichkeit in Populationen und in 
 reinen Linien, 1903. 
 
 ,, Elemente der Exakten Erbhchkeitslehre, 
 
 1909. 
 
THE LIBRARY 
 OF THE 
 
 UfJIVERSlTY OF ILLINOIS 
 
Hugo de Vries. 
 
 \To Jace p. 121. 
 
CHAPTER V 
 
 THE THEORY OF MUTATION 
 
 Mutation is the term applied by de Vries to express 
 the process of origination of a new species, or of a new 
 specific character, when this takes place by the dis- 
 continuous method at a single step — a process which 
 he regards as the most important if not the sole 
 method by which new species or specific characters 
 arise. We shall see that although de Vries has 
 recently done much to forward the propagation of 
 this idea, the belief that such a discontinuous process 
 is the normal method by which new species come into 
 existence has been developing for a considerable time. 
 
 We have seen that those who accept the idea of 
 evolution by the action of natural selection upon a 
 series of minute and almost imperceptible variations 
 are confronted with the difficulty of explaining how 
 by this method there could arise a number of different 
 structures or parts so co-ordinated as to share in a 
 common function. Moreover, a closer examination 
 of the actual processes of variation and inheritance 
 render it doubtful whether the selection of continuous 
 variations of even a simple characteristic can ever lead 
 to the development of a permanent new race. The 
 
 I2I 
 
X22 
 
 THE THEORY OF MUTATION 
 
 result of Pearson’s calculations, described in the 
 preceding chapter, seems to indicate that the selection 
 of a certain value of a particular character for many 
 generations will never lead to the formation of a race 
 in which the mean value of the character is as high 
 as the selected value. But, says the selectionist, it 
 will happen in Nat^re that as the standard of the 
 race is raised by selection, the value selected will be 
 still further raised, and so on, and in this way an 
 indefinite amount of improvement is rendered possible. 
 If Johannsen’s conclusions are well founded, this is 
 clearly not the case ; on the contrary, there is a 
 perfectly definite limit to the effect which selection 
 can produce. 
 
 The question whether or not a gradual method of 
 evolution is possible has not yet been absolutely 
 decided for any single species or character, but it cer- 
 tainly seems that now for the first time the possibility 
 of a definite decision is within sight. At the same 
 time it is impossible to prove a universal negative. If 
 we look at the other side of the problem we shall find 
 that the evidence in favour of an alternative process 
 has multiplied even faster than the evidence against 
 the continuous accumulation of minute differences ; 
 an d the present tendency is certainly to look for other 
 sources of specific distinctness than that which is 
 offered by the natural selection of . continuous varia- 
 tions. 
 
 Even before the new evidence which we have briefly 
 outlined was available, Herbert Spencer found the 
 difi&culties in the way of accepting the purely 
 
COORDINATED STRUCTURES 
 
 123 
 
 Darwinian explanation to be so great, that he adopted 
 the hypothesis of the inheritance of acquired char- 
 acters, as being the only adequate explanation of the 
 phenomena which was in his time available. 
 
 Unfortunately, satisfactory evidence that such a 
 form of inheritance ever actually takes place has never 
 been forthcoming in sufficient amount to lead to 
 universal conviction. Indeed, at the present day the 
 consensus of opinion among experts is undoubtedly 
 to the effect that acquired characters are not inherited 
 at all, except in so far as better nutrition of the parent 
 may lead to the production of more vigorous off- 
 spring. And it seems clear that such an effect as the 
 latter cannot go on accumulating for more than a few 
 generations. 
 
 Thus we see that in the purely Darwinian view there is 
 something wanting, whilst the Lamarckian explanation 
 is ruled out of court for the present for lack of direct 
 evidence. If, at this point, we find that in Nature a 
 co-ordinated set of structures can and does arise in 
 an already perfected condition at a single step, and 
 that such phenomena take place with sufficient fre- 
 quency to give ample opportunities for the survival 
 of the new type so arising, we have at once discovered 
 an alternative way out of the difficulty. Such a 
 discovery must throw abundant light on the obscurity 
 overshadowing the methods by which evolution has 
 taken place, even though we may not yet have arrived 
 at any kind of explanation of the cause of this phe- 
 nomenon of co-ordinated and definite variability. 
 
 The actual observation of variations of this kind is 
 
124 
 
 THE THEORY OF MUTATION 
 
 of quite recent date, and their recognition is largely 
 due to the exertions of Bateson. But the idea that 
 this is the way in which evolution takes place is very 
 ancient, and seems to have been guessed at by Aristotle. 
 Perhaps the earliest use of the actual word ‘ muta- 
 tion ’ in this sense is to be found in ‘ Pseudodoxia 
 Epidemica,' by Sir. Thomas Browne. I quote from 
 Book VI., chapter x., ‘Of the Blackness of Negroes ’* 
 (second edition, 1650) : ' We may say that men became 
 black in the same manner that some Foxes, Squirrels, 
 Lions, first turned of this complection, whereof there 
 are a constant sort in diverse Countries ; that some 
 Chaughes came to have red legges and bills, that Crows 
 became pyed ; All which mutations, however they 
 began, depend upon durable foundations, and such 
 as may continue for ever.’ 
 
 The experiments upon cross-breeding, which are 
 described in a later chapter, will be found fully to bear 
 out the idea that ‘ mutations,’ or definite character- 
 istics which have arisen in a definite way, do depend 
 upon durable foundations. 
 
 The late Professor Huxley’s emphatic approval of 
 the ‘ Origin of Species,’ as signalized in his reviews of 
 the first edition of that work, was tempered by the 
 following mild criticism : ‘ Mr. Darwin’s position 
 might, we think, have been even stronger than it is 
 if he had not embarrassed himself with the aphorism 
 “Natura non facit saltum,” which turns up so often 
 in his pages. We believe . . . that Nature does make 
 jumps now and then, and a recognition of the fact is 
 
 * I am Indebted to my friend Mr. R. C. Punnett for this 
 reference. 
 
ORGANIC STABILITY 
 
 125 
 
 of no small importance in disposing of many minor 
 objections to the doctrine of transmutation.’* 
 
 The first person to formulate a more or less precise 
 view upon the subject of definite variation was Francis 
 Galton, although this author never entered into the 
 question at any great length. Galton’s attitude 
 towards the problem in its early stages may be gathered 
 from the following quotation from his ‘ Natural In- 
 heritance ‘ The theory of natural selection might 
 dispense with a restriction for which it is difficult to 
 see either the need or the justification — namely, that 
 the course of evolution always proceeds by steps that 
 are severally minute, and that become effective only 
 through accumulation. That the steps may be small, 
 and that they must be small, are very different views ; 
 it is only to the latter that I object, and only when the 
 indefinite word “ small ” is used in the sense of “ barely 
 discernible,” or as small compared with such large sports 
 as are known to have been the origins of new races. ’f 
 But more than this, the idea of the existence of 
 stable forms, such as may be supposed to have arisen 
 by large and sudden variations, is very well expressed 
 by Galton in his division of varieties into the three 
 groups of primary types, subordinate types, and mere 
 deviations from the latter. A most luminous analogy 
 is affprded by the three t3q)es of public vehicles which 
 at the end of the nineteenth century were character- 
 istic of the streets of London ; and it is impossible to 
 resist quoting Galton’s account of them. These three 
 
 * ‘ Collected Essays,’ vol. ii., p. 77. 
 
 t ‘ Natural Inheritance,’ p. 3a 
 
126 
 
 THE THEORY OF MUTATION 
 
 kinds of carriages, ‘ namely, omnibuses, hansoms, 
 and four-wheelers, are specific and excellent illustra- 
 tions of what I wish to express by mechanical types 
 as distinguished from subtypes. Attempted im- 
 provements in each of them are yearly seen, but none 
 have as yet superseded the old familiar patterns, 
 which cannot, as it thus far appears, be changed with 
 advantage, taking the circumstances of London as 
 they are. Yet there have been numerous subsidiary 
 and patented contrivances, each a distinct step in 
 the improvement of one or other of the three primary 
 
 types, and there are or may be an indefinite number of 
 varieties in details, too unimportant to be subjects of 
 patent rights.’* 
 
 More recently Galton might have pointed out the 
 introduction of motor traffic as illustrating a distinct 
 mutation. 
 
 The distinction between primary and subordinate 
 positions of stability is further excellently illustrated 
 by the model which is here represented, and which 
 is known as Galton’s polygon (Fig. lo). 
 
 ♦ ‘Natural Inheritance,’ p. 26 . 
 
A PHYSICAL ANALOGY 
 
 lay 
 
 The first position of the model, resting upon the side 
 A B, may be taken to represent the condition of a 
 type or stable form. A comparatively small push 
 (variation) will lead to the production of the subtype 
 illustrated by the position B C. When in this new 
 position, it is easier to cause the model to return to its 
 original position A B than it is to make it pass on to the 
 new and more modified position resting upon the side 
 CD. A strong push (mutation) may force the model 
 to pass through the position C D until it comes to rest on 
 the side opposite to A B. This fresh position represents 
 a new stable form, and it is now once more surrounded 
 by positions of subordinate stability — subtypes. 
 
 One more analogy before we pass on to consider the 
 more recent observations upon discontinuous varia- 
 tions or mutations. We may compare the difference 
 which exists between deviations and stable forms, 
 arising by fluctuating and by definite variation respec- 
 tively, with the behaviour of the atoms of chemistry, 
 as expressed in the account of their structure recently 
 given by Professor Sir J. J. Thomson. Such an atom 
 is regarded as being made up of a number of electrons 
 or corpuscles bearing definite relations to one another 
 in space. In certain circumstances it seems that it 
 may be possible to remove a series of these corpuscles 
 from the atom one at a time, in which case every such 
 successive removal would be accompanied by a com- 
 paratively gradual and progressive change in the 
 properties of the atom so modified. But after a 
 certain time a point would be reached at which the 
 removal of one more electron would necessitate a 
 
128 
 
 THE THEORY OF MUTATION 
 
 complete rearrangement of the remaining corpuscles 
 in order to arrive at a new position of equilibrium, 
 and this change would be accompanied by a marked 
 alteration in the chemical properties of the atom itself. 
 In like manner the chemical composition of the living 
 substance of a race of organisms may be conceived to 
 alter step by step, every such step being accompanied 
 by comparatively unimportant changes in its visible 
 characters, until the time arrives when any further 
 alteration must be associated with a deep-seated revo- 
 lution in the constitution of the living substance, and 
 with a corresponding marked mutation in the external 
 features of the members of the race. 
 
 The first really definite attempt to collect and co- 
 ordinate the facts of discontinuous variation was made 
 by Bateson in his book entitled ‘ Materials for the 
 Study of Variation,’ published in 1894. The intro- 
 duction and concluding remarks at least of this volume 
 ought to be read by everyone who is interested in 
 these subjects. The bulk of the book contains a mass 
 of material of great value to specialists. 
 
 After pointing out the difiiculties which prevent his 
 acceptance of the orthodox belief in the origin of dis- 
 continuous and apparently adaptative types of animals 
 and plants through the action of natural selection on 
 minute variations, difficulties to which we have 
 already paid some attention, Bateson records his con- 
 viction that the facts of discontinuous variation afford 
 a way out of the difficulty. He shows (i) that differ- 
 ences of the kind which are generally used to dis- 
 tinguish separate species may arise as single variations ; 
 
SYMMETRY 
 
 129 
 
 (2) that such a form of variation is by no means so 
 uncommon a phenomenon as was formerly supposed ; 
 and (3) that variations of this kind may occur in 
 every description of organ and part in a number of 
 different plants and animals. The facts with which 
 the main bulk of the book is concerned have reference 
 to the animal kingdom. 
 
 We shall find it profitable to consider the views 
 expressed in this book a little more closely, though it 
 would occupy too much space to give even a brief 
 summary of the facts upon which they are based, and 
 for which reference must be made to the original. 
 
 In the first place Bateson calls attention to the 
 phenomenon of symmetry as being a characteristic 
 feature common to almost all organisms. This sym- 
 metry may manifest itself in a number of different 
 ways. In bilateral and radial symmetry the parts 
 symmetrically disposed are related to one another in 
 the same kind of way as are an object and its image 
 reflected in a plane mirror. Such symmetry, as, 
 indeed, every kind of symmetry, is usually associated 
 with a repetition of parts. In the present instances 
 the parts are repeated in pairs, as with the two eyes 
 in the human face ; or in a radial series, like the arms 
 of a star-fish, or the petals of a buttercup. To this 
 phenomenon of the repetition of parts, generally 
 occurring in such a way as to produce a symmetry or 
 pattern, the term merism is applied. 
 
 Symmetry may affect the proportions and shape 
 of the body of an animal or plant as a whole, or, on 
 the other hand, separate parts or organs may show a 
 
 9 
 
130 
 
 THE THEORY OF MUTATION 
 
 separate symmetry of their own. For the phenomena 
 thus distinguishable separate terms are proposed. A 
 major symmetry is a form of pattern which includes the 
 body as a whole, as in the case of most animals where 
 the two sides of the body closely resemble one another. 
 A minor symmetry is a pattern completed in a separate 
 organ or part — for instance, in the flower of a plant or 
 the limb of an animal. 
 
 Once more we may lay stress upon the universal 
 existence of pattern among living things. Bateson 
 points out that in collecting any kind of living creature 
 it is the symmetry of it which, as a general rule, first 
 catches the eye and distinguishes the organized body 
 from surrounding inanimate objects. 
 
 The phenomenon of merism or repetition of parts 
 being understood, we are in a position to consider the 
 subdivision of variations into meristic variations and 
 substantive variations respectively. 
 
 Meristic variations are variations in symmetry and 
 in the number of repeated parts. A change in the 
 number of organs in a series may conceivably take 
 place gradually by the addition or subtraction of suc- 
 cessive fractions of a part. But, as a matter of fact, 
 this is very seldom the case. The increase or decrease 
 usually involves one whole member at a time and some- 
 times more, so that this kind of variation is, as a rule, 
 discontinuous. Abundant illustrations of this fact are 
 to be found in the case of changes in the number of 
 such parts as the teeth or vertebrae of mammals ; and 
 a particulaily good instance is afforded by the variations 
 which take place in the number of ray florets in various 
 
MERISTIC VARIATIONS 
 
 131 
 
 composite plants — e.g., the daisy and chrysanthemum. 
 It is suggested that meristic variations are connected 
 with definite changes in the mechanical relations of 
 dividing parts, and that it is in the mechanics of cell- 
 division that the explanation of their discontinuous 
 appearance is to be sought for. 
 
 Thus when, for example, a tulip-flower appears 
 having its parts perfectly developed in sets of four 
 instead of in sets of three, it is suggested that the 
 arrangement in fours, like the arrangement in threes, 
 fulfils certain conditions of equilibrium among the 
 forces which affect the cell-divisions in the rudiment 
 of the flower, and that these conditions of stability 
 would not be equally well provided for by any inter- 
 mediate arrangement. 
 
 Substantive variations are changes in the actual 
 constitution or substance of the parts themselves. 
 For example, a plant with coloured flowers may give 
 rise to offspring the flowers of which are white. There 
 seems to be no mechanical necessity for such varia- 
 tions to be discontinuous rather than continuous ; it 
 is quite possible to imagine a gradual dilution of colour 
 taking place throughout a long series of generations. 
 Discontinuous substantive variations are, however, not 
 infrequent, and in such cases it is suggested that they 
 may be associated with definite changes in chemical 
 composition. Thus, for example, definite alterations 
 in the colour of offspring as compared with their 
 parents are almost necessarily of a chemical nature. 
 
 The further evidence contained in the book we are 
 considering refers entirely to meristic variation. 
 
 9—2 
 
132 
 
 THE THEORY OF MUTATION 
 
 An important point with regard to repeated parts 
 is to be observed in the fact that in a pair of allied 
 species, in which a series of repeated organs in the one 
 is clearly comparable with a similar series in the other, 
 aU the parts in one form may differ from those in the 
 second by the same kind of distinction, whether this 
 be qualitative or numerical. The facts suggest strongly 
 that such cases are to be accounted lor by all the 
 parts in question in one or both species having varied 
 in a similar way at the same time rather than in suc- 
 cession. The occurrence of such a similar and simul- 
 taneous process of variation of repeated parts clearly 
 simplifies in a marked degree the process of evolution, 
 and greatly reduces the time which would be required 
 for this process, if similar changes in repeated parts 
 always took place successively. If we take an ex- 
 treme case the latter supposition becomes absurd. In 
 the albino or pure white types which occur as varia- 
 tions in many species of birds and mammals it is 
 obvious that every hair or feather has taken on the 
 white colour at the same time and for the same reason, 
 whatever that reason may have been. Hairs or 
 feathers are very good examples of repeated parts of 
 the kind of which we have been speaking. It appears, 
 too, that colour patterns may originate and change in 
 a similar manner. In the case of such a bird as the 
 peacock we should expect on this view that the pattern 
 varied in all the tail feathers simultaneously, nor is it 
 necessary to suppose that even this process took place 
 by a very long series of minute steps. If we find that 
 the splendid coloration of the peacock’s tail arose 
 
HOMCEOSIS 
 
 133 
 
 by a few marked variations, each of which occurred 
 simultaneously in all the feathers at once, several 
 serious difficulties are avoided, and on the analogy of 
 similar known cases we have every reason to believe 
 that this was so. And similar changes may take place 
 in cases where the pattern depends on the coloration 
 of a group of feathers or hairs. Indeed, if we con- 
 sider, we shall find it very difficult to picture such a 
 process as taking place in any other way. We can 
 scarcely suppose the spots of the leopard, for instance, 
 to have arisen one at a time. 
 
 An important kind of discontinuous variation is 
 that to which Bateson has applied the term homceosis. 
 The same sort of change had previously been described 
 by Masters in the case of plants under the name 
 ‘ metamorphy,’ but the latter expression has also 
 been employed in other senses. Homceosis consists 
 in the assumption by one member of a meristic series 
 of the form or character proper to another member 
 of the same series ; for example, the modification of 
 the petal of a flower into a stamen, or of the eye of a 
 crab into an antenna- like organ. 
 
 ‘In these cases a limb, a floral segment,, or some 
 other member, though itself a group of miscellaneous 
 tissues, may suddenly appear in the likeness of some 
 other member of the series, assuming at one step the 
 condition to which the member copied attained pre- 
 sumably by a long course of evolution.’ * 
 
 The phenomenon of homceosis is frequently to be 
 seen among the parts of flowers. Double flowers in 
 * ‘Materials for the Study of Variation,’ p. 570. 
 
134 
 
 THE THEORY OF MUTATION 
 
 many cases — for instance, in the case of the rose — arise 
 by the development of petal-like organs in the position 
 which would properly be occupied by stamens. A 
 parallel process is to be seen in the heads of com- 
 posite flowers, such as the chrysanthemum. In a 
 double chrysanthemum the florets of the disc develop 
 in the likeness of ray florets. Both these cases would 
 be classed as examples of outward homceosis, because 
 the parts concerned resemble organs normally de- 
 veloped in a whorl exterior to themselves. A case of 
 inward homceosis, on the other hand, is afforded by 
 the appearance of a petaloid calyx — ^for example, in 
 a tobacco-plant — the outermost whorl of the flower 
 taking on the appearance of a whorl internal to itself. 
 
 In cases such as these we observe once more the 
 occurrence of a marked and definite change, which, 
 though at first sight quite distinct from the method of 
 similar and simultaneous variation, yet bears a certain 
 resemblance to that process in the fact that the direc- 
 tion in which a particular part varies is not wholly 
 unrelated to the behaviour of other parts of the same 
 organism. The process thus, briefly described seems 
 likely to have had considerable importance in evolu- 
 tion, notably in the origin of differences in the numerical 
 relations of the bones in various parts of the spinal 
 column in different vertebrate animals. 
 
 The preceding account of the conclusions drawn 
 from Bateson’s laborious study of variation has in- 
 volved a good deal of technicality, but this is, un- 
 fortunately, unavoidable. The point chiefly to be 
 emphasized is the frequent occurrence in Nature of 
 
EFFECT OF SELECTION 
 
 135 
 
 variations of a definite or discontinuous type — the 
 fact that differences of the kind "which are constantly 
 used to distinguish natural species can and do arise 
 in Nature at a single step, so that it is not necessary 
 for such differences to be built up gradually by the 
 action of natural selection. 
 
 De Vries, in his ‘ Mutations Theorie,’ goes further 
 than this, and attacks the position held by those who 
 accept the doctrine that natural selection of individual 
 differences can ever lead to definite and permanent, or 
 specific, distinctions. Indeed, one of the chief contri- 
 butions of this author to the species controversy is to 
 point out that the belief that artificial selection acts 
 in this way upon domestic plants is based upon a mis- 
 apprehension. De Vries himself has carried out a 
 number of experiihents in selection, and he comes to 
 the conclusion that selection of ordinary individual 
 differences has no permanent effect at all.* The actual 
 effect of this kind of selection is well illustrated by the 
 results of the processes employed in the sugar-beet 
 industry, in which elaborate care is taken to select 
 those roots which contain the highest percentage of 
 sugar for the purpose of propagation. This process 
 was followed at first by a rapid improvement, but the 
 rate at which the percentage of sugar increased soon 
 fell off, until at the present day all that selection can 
 effect is to keep up the standard of excellence already 
 attained. Moreover, that this process of improvement 
 
 * Compare, however, Johanssen’s more recent conclusions 
 see p. 1 1 8). 
 
136 THE THEORY OF MUTATION 
 
 was a very gradual one is to be accounted for in part, 
 at least, from the fact that the methods of selection 
 themselves gradually improved from year to year. 
 There is no reason to doubt that a thoroughly efficient 
 method of selection would have worked its full effect 
 in a few generations. A similar state of things is said 
 to be the case with the cereals, such as wheat and 
 barley, which have been selected largely for the size 
 of the grains. From his own experiments, de Vries 
 has come to the conclusion that, when selection is 
 really efficient, the full possible effect of this process 
 is exhausted in quite a small number of generations, 
 and that then the only further effect of selection is to 
 keep up the standard already arrived at. 
 
 We have seen that the theoretical conclusions of the 
 biometricians are in agreement with the opinions here 
 expressed, so long as selection is understood to be con- 
 fined to the choosing out of parents which show a 
 definite standard value of the character under con- 
 sideration, this value being the same in each genera- 
 tion. Under these circumstances. Professor Pearson 
 concludes that in the first two or three generations 
 a marked advance in the desired direction will take 
 place, but that further selection (in this sense) will 
 have comparatively little effect. But the believer 
 in continuous evolution maintains in addition that 
 selection will be foHow'ed to an indefinite extent by 
 further variations in the direction of selection, since 
 otherwise selection could never lead to important 
 changes in organization. In the face of the strong 
 contrary evidence, and of the fact that alternative 
 
EFFECT OF SELECTION 
 
 137 
 
 methods of evolution are now known to be available, 
 the burden of proof of this proposition seems to 
 lie with those who maintain the all-important influ- 
 ence of continuous variation and selection. At present 
 we are free to reply in the words of Malthus, who long 
 ago protested against the extravagant powers which 
 were ascribed to the selection of small differences. 
 
 ‘ I have been told,’ Malthus writes, ‘ that it is a 
 maxim among some of the improvers of cattle that 
 you may breed to any degree of nicety you please, 
 and they found this maxim upon another, which is, 
 that some of the offspring will possess the desirable 
 qualities of the parents in a greater degree. In the 
 famous Leicestershire breed of sheep, the object is to 
 procure them with small heads and small legs. Pro- 
 ceeding upon these breeding maxims, it is evident that 
 we might go on until the heads and legs were evan- 
 escent quantities ; but this is so palpable an absurdity 
 that we may be quite sure the premises are not just, 
 and that there really is a limit, though we cannot see 
 it or say exactly where it is.’ * 
 
 The only recorded example I am aware of in the 
 case of animals, which shows the result of long-con- 
 tinued selection acting upon a quantitative character, 
 is afforded by the case of the American trotting-horse. 
 In this case it appears highly probable that we are 
 dealing with a character which varies in a strictly 
 continuous fashion. In his book upon ‘ The Trotting 
 and Pacing Horse in America,’ Hamilton Busbey 
 gives a table from which the diagram on p. 138 
 * ‘ Essay on Population,’ 6th ed., vol. ii., p. ii. 
 
138 
 
 THE THEORY OF MUTATION 
 
 is constructed. The entries in this table show the 
 fastest times recorded for the feat of trotting a measured 
 mile in various years beginning with i8i8. The ver- 
 tical scale contains the times, which vary from three 
 minutes down to one minute fifty-six seconds, and the 
 horizontal scale shows the year in which the record was 
 
 2 , 00 - 
 
 2,10 - 
 
 2.20 
 
 2,30 
 
 2,40 
 
 2,60 
 
 3.00 
 
 Year !C20 ‘30 ’4-0 ’so '60 ’70 ’80 ’90 1900 
 
 Fig. II. 
 
 The figures to the left of the diagram are to be read as minutes and 
 
 seconds. 
 
 made. Some part of the improvement shown is clearly 
 to be associated with better tracks, improved methods 
 of training, etc., but these will scarcely affect the 
 general character of the improvement due to selec- 
 tion. As may readily be seen from the diagram, the 
 improvement is at first rapid, but afterwards becomes 
 
 ar ~!C20 ^30 ^0 ^0 '60 ’70 ’80 ’90 1900 
 
EFFECT OF SELECTION 
 
 139 
 
 gradually slower and slower. At the end of the series 
 two sudden steps upward break the general regularity 
 of the series of records. But on examination of the 
 evidence it is found that these are associated with 
 special conditions, and are not really exceptional. The 
 first of these breaks — that which occurs in 1892 — is 
 coincident with the introduction of a new type of sulky, 
 having ball bearings and other improvements ; whilst 
 the record of 1903 was accomplished behind a pace- 
 maker carrying a wind-shield. Neither of these records, 
 therefore, is strictly comparable with the rest of the 
 series. 
 
 The observations in this case do not, indeed, seem 
 to be sufiicient to afford the basis for a final decision 
 against the theory of the indeterminate power of selec- 
 tion. Yet Malthus’ criticism clearly applies very 
 definitely to such a case — i.e., there must be a limit 
 beyond which the speed of the trotting-horse will 
 never improve without a fundamental change taking 
 place in his organization. It seems, therefore, safe to 
 conclude that the curve to which the series of records 
 approaches is of the character of a parabola — i.e., one 
 which continually becomes more and more nearly 
 horizontal — as the speed of the horse gradually ap- 
 proaches its highest possible limit. 
 
 De Vries, then, contends that all new domestic breeds 
 have arisen by the discontinuous method as definite 
 novelties. Darwin himself was perfectly aware that 
 this is usually the case, but the conclusion which he 
 drew from the fact was a different one, as the following 
 passage shows ; 
 
140 
 
 THE THEORY OF MUTATION 
 
 ‘ He (man) often begins his selection by some half- 
 monstrous form, or at least by some modification pro- 
 minent enough to catch the eye or to be plainly useful 
 to him.’ But he goes on ; ‘ Under Nature, the slightest 
 differences of structure or constitution may well turn 
 the nicely-balanced scale in the struggle for life, and 
 so be preserved.’ * 
 
 Of the origin of a new type of plant in this definite 
 and sudden fashion, the Shirley poppies afford an 
 excellent example. These originated in a mutation 
 of the common wild field-poppy {Pap aver rhceas). In 
 1880 the Rev. W. Wilks, Vicar of Shirley, near Croydon, 
 noticed among a patch of this plant growing in a waste 
 corner of his garden a solitary flower, the petals of 
 which showed a very narrow border of white. The 
 seeds which this flower produced were sown, and next 
 year, out of about two hundred plants, there were four 
 or five upon which all the flowers showed the same 
 modification. From these, by further horticultural 
 processes, the strain of Shirley poppies originated. 
 We may point out in passing that if the original plant 
 had been self-pollinated, a much larger proportion of 
 the new type might have been expected to appear in 
 the next generation. 
 
 In the course of his own experiments, de Vries has 
 obtained quite a number of new types of plants by 
 methods like the above. It is to be observed that the 
 novelty in these cases usually shows a considerable 
 range of normal variability of its own, and that its 
 first appearance is generally in the form of an extreme 
 ♦ ‘ Origin of Species,’ 6th. ed., p. 60. 
 
EFFECT OF SELECTION 
 
 141 
 
 negative variation* from the proper type of the new 
 variety. In this way the novelty may not appear to be 
 very far removed from a high normal variation of the 
 original type. The behaviour of the progeny of the two 
 types, however — types which might thus in themselves 
 be readily confused — is entirely different, and a ready 
 means of distinguishing them is thereby provided. 
 Each set of offspring shows regression to its own proper 
 modal value ; so that the offspring of the novelty show 
 a further marked development of the new features, 
 whilst the offspring of an extreme normal variation 
 resemble the type of the original form more closely 
 than they do their own immediate progenitor. 
 
 If new types are not produced among domesticated 
 productions by the action of artificial selection, and all 
 that selection can effect is to pick out definite novelties 
 when they occur, the analogy between natural selec- 
 tion and artificial selection breaks down, and a large 
 and important section of the evidence in favour of the 
 production of natural species by the action of natural 
 selection is destroyed. In the place of this explana- 
 tion dearies would put the theory of mutation, ac- 
 cording to which new species arise by single steps as 
 definite novelties, just in the same way as we find that 
 domestic varieties are produced. More than this, de 
 Vries believes that he has discovered a set of new 
 species in the very act of originating from an old 
 one in this way, a discovery which affords the basis 
 
 * I.e., a variant belonging to a class situated some dis- 
 tance from the mode of normal variability of the novelty, 
 and on the side of it nearest to the mode of the original type. 
 
142 
 
 THE THEORY OF MUTATION 
 
 and groundwork of the views which he puts for- 
 ward. 
 
 The plant which afforded the material for this dis- 
 covery is known as (Enothera Lamarckiana — that is to 
 say, this is the name of the old species from which the 
 new species were found to be arising. O. Lamarckiana 
 is an American plant, but the specimens which de Vries 
 found to be in a state of mutation had made their 
 escape from a garden, and were running wild over a 
 disused potato-field near a town called Hilversum, in 
 Holland. On examining these plants, de Vries found 
 two distinct new forms, which were quite unlike the 
 remainder. Each kind occurred in an isolated patch, 
 as if it had arisen from the seed of a single plant. 
 
 No description of either of these forms was to be 
 found in botanical literature, nor were there specimens 
 of them in any of the great herbaria. But when de 
 Vries took seeds from some of the plants and sowed 
 them in his garden, he found that the new forms came 
 true to type — the plants produced resembled the 
 parents from which the seeds were taken, and not the 
 normal form of 0. Lamarckiana. 
 
 Here, then, we have a case in which two new species 
 had originated from an old one in a state of nature. 
 But de Vries went further than this, and took measures 
 for observing the actual origin of new forms in the 
 cultivated offspring of the semi-wild (Enothera. 
 
 For this purpose he transplanted a number of roots 
 from the field where they were growing, and also took 
 seed from a number of other plants, and from these he 
 cultivated large numbers of seedlings for a series of 
 
THE LIBRARY 
 OF THE 
 
 UNIVERSITY OF ILLINOIS 
 
Fig. 12. — Mutation in CEnothera. 
 
 {From de Vries.) 
 
 Top row . . 
 
 .. Lam. 
 
 lata 
 
 Lam, 
 
 Second row 
 
 . . siihovata 
 
 alhida 
 
 Lam. 
 
 Third row . . 
 
 , . alhida 
 
 albtda 
 
 lata 
 
 Fourth row 
 
 . , oblonga 
 
 lata 
 
 Lam. 
 
 Fifth row . . 
 
 . . Lam. 
 
 Lain. 
 
 nibrinevvis. 
 
 [To face p » 143. 
 
MUTATION IN (ENOTHERA 
 
 143 
 
 generations. The net result of his experiments was 
 this : out of about 50,000 individuals which were grown 
 to a recognisable stage, more than 800 showed muta- 
 tion — that is to say, they differed specifically from 
 the parent 0 . Lamarckiana. The 800 individuals 
 belonged to about fifteen new kinds, most of which 
 appeared repeatedly, though some were more frequent 
 than others. The process of mutation had, therefore, 
 taken place in about per cent, of the seedlings which 
 were grown, and owing to various reasons this estimate 
 is probably considerably too low. For example, many 
 of the new forms were very weakly, and often died 
 before it was possible to distinguish them. Others, 
 again, could not be recognised until an advanced stage 
 of their growth had been reached, whereas only a small 
 proportion of the seedlings raised could be grown after 
 they had reached any considerable size, owing to con- 
 siderations of space. 
 
 We cannot now follow de Vries veiy far into his 
 elaborate account of his new species and of the way 
 in which they originated ; a few general remarks only 
 must suffice. Many of the new forms were recognis- 
 able as quite young seedlings, notably 0 . albida, 
 others not until a much later period of their growth. 
 0 . gigas was the finest and strongest of the new forms, 
 but only made its appearance on two occasions. 
 0 . lata also appeared to be as strong as the parental 
 type, whilst two other forms were able to survive in 
 nature in competition with the original species, as 
 has been already described. Other forms which were 
 grown and flowered were plainly less well fitted for 
 
144 
 
 THE THEORY OF MUTATION 
 
 the battle of life than 0. Lamarckiana, and only reached 
 the flowering stage by the help of careful cultivation, 
 and others, again, were never got to flower at all. 
 Some of the latter, however, were readily distinguish- 
 able by the strikingly original types of radical leaves 
 which they exhibited. 
 
 When they had once made their appearance, the 
 majority of the new tji'pes came true to seed. Some- 
 times new mutations appeared among their offspring, 
 but these always appeared in smaller numbers than 
 among the offspring of the parent 0. Lamarckiana, and 
 some of the commoner mutations were usually omitted, 
 so that it appeared as if the process of mutation was 
 accompanied by a tendency towards a fresh stability. 
 Some of the most marked new forms came quite true 
 so far as the observations were carried. 
 
 Speaking generally, the nature of the differences 
 which distinguished the new forms from the parental 
 species was just of the same type as that of those which 
 distinguish Jordan’s species when found in nature. 
 The differences were not, as a rule, of the sort shown 
 when new garden varieties arise as sports. An example 
 of this latter kind occurred, however, in the case of the 
 new form 0. nanella, which was a dwarf or permanently 
 stunted form, but in other respects closely resembled 
 the parent type. Apart from this, the new forms 
 appeared to be given off quite at random, without 
 showing any definite tendency towards progress in a 
 particular direction. One of the new species was 
 almost sterile as far as its ovules were concerned, 
 though producing good pollen, whilst in another the 
 
MUTATION IN (ENOTHERA 
 
 T45 
 
 formation of the pollen was very defective. None of 
 the others was lacking in either of these respects. Each 
 new form was distinguished by certain definite features 
 which affected almost all its parts, not by one new 
 character only ; and these features were never separable, 
 but always appeared in common on the same plant. 
 
 The new species, of course, showed normal fluctuat- 
 ing variability, and, as an extreme result of this varia- 
 bility, forms occasionally appeared midway between 
 one of the new species and the parental type. In such 
 cases, when the self-fertilized seed of the plant showing 
 such an intermediate character was sown, the offspring 
 were found to group themselves round the normal 
 form of the new species or round that of the parent 
 Lamarckiana, thus affording evidence as to the true 
 nature of their parent. 
 
 Whether or not we are prepared to accept the whole 
 of de Vries’ conclusions from his experiments, we can 
 see at least that from one point of view they are of the 
 very greatest importance. For before de Vries pub- 
 lished this work it had been supposed to be quite im- 
 possible to make direct observations upon the manner 
 of origin of new species in Nature. De Vries has now 
 shown that such observations can be made, and this is 
 in itself a most valuable piece of information. He has 
 introduced an entirely new method into the domain of 
 species research, and one by the use of which it is to be 
 hoped that before long a definite answer will be obtained 
 to the question whether species in general arise by defi- 
 nite steps, or with an imperceptible degree of slowness. 
 
 When results of the novelty and importance of those 
 
 10 
 
t46 the theory OF MUTATION 
 
 which have been published by de Vries are brought 
 to our notice, we are naturally disposed to reserve our 
 acceptance of the conclusions which they seem to indi- 
 cate until observations have been made in confirma- 
 tion of them by some competent observer. This has 
 now been done by Professor MacDougal at the New 
 York Botanic Garden.* MacDougal has carried out 
 observations similar to those above described upon 
 the offspring of seeds sent by de Vries from Holland, 
 and with closely similar results. Thus he has observed 
 all the new forms which de Vries described, as well as 
 some additional ones ; and he has obtained an even 
 higher percentage of ‘ mutants ’ than de Vries him- 
 self — namely, about 3 per cent, of the total number 
 of seedlings grown. This last result is probably only 
 due to the application of more thorough methods of 
 investigation, and to a smaller mortality of the weakest 
 plants, arrived at by greater care, and rendered pos- 
 sible by the warmer summer climate and by American 
 efficiency in method. De Vries himself, in one of his 
 later generations, when particular care was applied to 
 the methods of cultivation, obtained nearly 3 per cent, 
 of new forms. MacDougal also states that he has 
 observed undoubted cases of mutation taking place 
 in other species besides (Enothera Lamar ckiana. 
 
 It appears, then, that there can be no doubt about 
 the genuineness of the phenomenon described by de 
 Vries. But it is, of course, quite a different thing to 
 assert that all natural species arise in this fashion, and 
 this is what de Vries’ theory, as distinguished from 
 
 * Reference should also be made to the work of R. R. Gates ; 
 see his ‘ Mutation Factor in Evolution,’ 1915. 
 
MUTATION IN OENOTHERA 
 
 147 
 
 his facts, amounts to. De Vries made observations 
 upon a large proportion of the plants of his district 
 by the method of growing great numbers of their 
 seedlings, but he failed to find the same phenomenon 
 going on in any of them. He therefore supposes that 
 species are subject to comparatively short periods of 
 mutability which recur at relatively long intervals, 
 and that all the species he examined except the 
 (Enothera were in this intermediate stable period of 
 their existence^ Direct proof of this suggestion is 
 naturally out of the question. 
 
 It will be well to summarize briefly the conclusions 
 at which de Vries has arrived, as the result of his 
 observations upon (Enothera. 
 
 The following are the points to which he attaches 
 chief importance : 
 
 1. The new species arise suddenly at a single step, 
 without transitional forms. 
 
 2. They are usually fully constant from the first 
 moment of their origin. 
 
 3. The distinctive characters of the new forms agree 
 in kind with those which distinguish from one another 
 such old and established species allied to (Enothera 
 Laniarckiana as O. biennis and 0 . muricata. Only one 
 of the new forms — namely, 0. nanella, a dwarf type — 
 is analogous with any ordinary kind of variety of 
 garden origin. 
 
 4. A considerable number of individuals of the same 
 sort usually make their appearance at the same period. 
 
 5. Although the new types vary in a normal fashion, 
 and frequently transgress the limits dividing them 
 
 10 — 2 
 
148 
 
 THE THEORY OF MUTATION 
 
 from the parental type, yet their first appearance has 
 nothing to do with normal or continuous variability. 
 
 6. The mutations take place indefinitely, showing 
 no special tendency in any particular direction. 
 
 7. The tendency to mutate recurs periodically. But, 
 as was previously stated, there is no direct evidence 
 of this last supposition. 
 
 In addition to what has already been said with 
 reference to the method of origin of garden varieties in 
 general, de Vries has described a number of special 
 phenomena regarding the behaviour of garden varieties 
 of plants, some of which are of considerable interest. 
 Taken together, the facts substantiate to a great extent 
 the view that selection does not of itself lead to the 
 production of specific characters. But de Vries also 
 introduces certain new conceptions which require to 
 be briefly described on account of their great general 
 interest to practical breeders and gardeners. They 
 consist in the idea of races existing intermediate be- 
 tween a species and a complete variety or sub-type of 
 it. Such between-races are of two kinds, of which it is 
 unusual to find both in the case of the same species ; 
 moreover, either of them may occur even when the 
 complete variety is quite unknown. In the case of a 
 half-race a small percentage only of seedlings is found 
 to produce plants which show the racial character, the 
 remainder being of the original specific type ; and even 
 if the racial type is selected for several generations, the 
 percentage of plants of this type which is produced 
 does not notably increase. A mid-race, on the other 
 hand, can readily be improved by selection, and when 
 
HALF RACES 
 
 149 
 
 best developed as a rule either shows the racial char- 
 acter in about half of the seedlings produced, or else 
 exhibits in the great majority of its members a com- 
 bination of the character of the species with that of 
 the race. As an example, we may take the case of 
 variegated plants, in which the leaves show streaks 
 or patches of a yellow colour owing to the want of 
 development of the proper green tint. An ordinary 
 variegated plant, then, is looked upon as showing a 
 combination of the green type with the yellow char- 
 acter of a completely modified race — the aurea variety, 
 although the latter exists as such only in a few rare 
 cases, in which the plants bear leaves showing no 
 green pigment at all. On the other hand, many 
 species of plants produce a small proportion of varie- 
 gated individuals at each sowing, as is often the case, 
 for example, with Indian corn ; and this circumstance, 
 according to de Vries, indicates the existence of the 
 corresponding half-race. 
 
 The relative development of the two coexisting 
 characters in such cases is highly variable, as anyone 
 may observe for himself in variegated grasses and 
 similar plants. 
 
 It might be supposed that it would be possible to 
 pass from the species to the half-race, thence to the 
 mid-race, and so on to the complete race simply by 
 selection. De Vries shows that this is very rarely, if 
 ever, the case. He regards the passage from a half- 
 race to a mid-race, for example, as a mutation, and his 
 observations seem to show that this transition is not 
 more frequent than any other mutations. 
 
150 
 
 THE THEORY OF MUTATION 
 
 As a further illustration of what is meant by a 
 between-race, mention may be made of the five-leaved 
 race of purple clover {TrifoUum fratense) obtained by 
 de Vries, and developed by a process of selection. It 
 would appear that the plants occasionally found grow- 
 ing wild, which bear a single four-lobed leaf, usually 
 belong only to a half-race. De Vries was fortunate 
 enough to find two plants upon each of which several 
 of the leaves showed this anomaly, and from these, by 
 an elaborate process of selection extending over several 
 years, a race was obtained, the leaves of which in the 
 majority of cases showed five lobes, whilst some had 
 six or seven. Since, however, it appeared impossible 
 to get rid of a certain proportion of three-lobed leaves, 
 and equally so, on the other hand, to obtain leaves 
 with more than seven lobes, de Vries concluded that 
 his experiment exemplified the development of a mid- 
 race, and not that of a constant race or true variety. 
 
 The phenomena described by de Vries under the 
 head of ‘ Between -Races ’ are difficult to understand 
 until the reader has arrived at some appreciation of 
 the very wide differences which even slight changes in 
 external conditions may make in the growth and 
 habit of plants. We may briefly describe one very 
 striking example. 
 
 The little water ranunculus, which often covers areas 
 of stagnant water in the spring vdth a sheet of white 
 blossom, possesses two kinds of leaves, the appearance 
 of which entirely depends upon the stimulus of external 
 circumstances. In the young state the whole plant is 
 submerged beneath the surface of the water, and bears 
 
Fig. 13. — Trifolium Pratexse Quixquefolium. 
 {From de Vries.) 
 
 [ 7 'o /ace p. 150 . 
 

BETWEEN RACES 
 
 151 
 
 leaves so finely divided or dissected into minute seg- 
 ments as to resemble a camel’s-hair pencil when re- 
 moved from the water. Sooner or later the growing 
 terminal bud reaches the surface, and rises above it 
 into the air. As soon as this happens, the rudimentary 
 leaves just beginning to swell within the bud entirely 
 change their course of development. They grow now 
 into flat-lobed blades, which float upon the surface of 
 the water. The change of environment from water to 
 air has worked such an alteration in form that no one 
 who was not in the secret would suppose that these 
 two kinds of leaves could possibly have been borne 
 upon the same plant. 
 
 De Vries would say that the tendency to produce 
 the floating kind of leaf was latent in the submerged 
 plant. In other words, the appearance of any given 
 plant, or that of any given part of it, depends partly 
 upon its hereditary qualities and partly to the external 
 circumstances to which it is submitted. Many other 
 examples of similar changes could be alluded to, and 
 one recently described is of rather special interest to 
 students of genetics. This relates to a variety of 
 Primula sinensis, which, if kept at a temperature of 
 30° C., in a moist greenhouse, produces red flowers 
 only ; whilst under similar conditions, but at a tem- 
 perature of 20° C., bears only pure white blossoms. 
 Another variety of the same species exists which only 
 produces white blooms when exposed to any condi- 
 tions under which it will flower at all. In describing 
 the hereditary difference between these two varieties, 
 we cannot say that it consists in the former having red 
 
152 
 
 THE THEORY OF MUTATION 
 
 flowers and the latter having white. The difference is 
 that the former variety has the power of producing red 
 flowers under certain circumstances which can be more 
 or less rigidly defined, whilst the contrasted type has 
 no such power. Thus, a few degrees difference in 
 temperature may determine a marked change in the 
 colour of a flower. 
 
 It is not absolutely certain what it is that deter- 
 mines the difference between the successive leaves on 
 a plant of the five-leaved clover, but a strong hint is 
 afforded by the fact that the leaves with the maximum 
 number of lobes only appear when the plant is at the 
 height of its vegetative activity. Early in the season, 
 and again towards the close of the growing period, 
 leaves with fewer lobes are produced. It would there- 
 fore seem as if the change in the number of lobes were 
 intimately connected with changes in the vigour and 
 rate of growth of the plant. 
 
 In the present chapter we are dealing particularly 
 with the variations of plants, and, indeed, de Vries 
 himself has never applied his views to the case of the 
 animal kingdom. Although animals show some direct 
 response in structure and functions to changes in their 
 surroundings, these are not usually nearly so extensive 
 or definite as the changes which we have just described. 
 The statement has even been made that one of the 
 fundamental differences between plants and the higher 
 animals lies in the much greater susceptibility of the 
 former to environmental changes. 
 
 The views of de Vries with regard to the actual 
 origin of new species may be summed up as follows : 
 
THE THEORY OF MUTATION 
 
 153 
 
 Bioadly speaking, species arise by mutation, by a 
 sudden step in which either a single character or a 
 whole set of characters together become changed. In 
 the former case a new variety in the strict sense of the 
 word is the result ; in the latter a new species (accord- 
 ing to Jordan’s definition) is produced. 
 
 But mutation may be of several kinds. In the first 
 place, an entirely new character or set of characters 
 may make its appearance. To such a phenomenon 
 de Vries applies the term of progressive mutation, and 
 it is by steps of this kind that he believes the main 
 divisions of the vegetable kingdom to have been built 
 up. In the case of such mutations the new character 
 is supposed to come into existence first in a latent or 
 hidden condition, and it may be only after many 
 generations that it makes its appearance visibly. On 
 this view the period of mutation is preceded by a 
 premutation period, during which the appearance of 
 the new character is being prepared for. 
 
 A second method of species formation, entitled 
 by de Vries degressive mutation, is indicated when a 
 change takes place in the partial latency of a character. 
 A completely latent character is, indeed, unrecognisable 
 as such. But characters may also be only partially 
 latent, and in these cases they exhibit themselves from 
 time to time in rare individuals in the form of sports 
 or abnormalities — a phenomenon which we have already 
 seen to be characteristic of half-races ; indeed, a half- 
 race might have been defined as a strain in which the 
 character of the complete race is usually latent, and 
 only rarely appears. An active character, on the other 
 
154 
 
 THE THEORY OF MUTATION 
 
 hand, is apparent in the great majority of the indi- 
 viduals of a race. If, now, a change fi'om latency to 
 activity occurs suddenly, this is a form of mutation. 
 The reverse case, too, may occur — a character pre- 
 viously active may become latent ; the character then 
 appears to be lost, and the mutation is said to be 
 retrogressive. De Vries regards the great variety of 
 allied species which is to be found in many groups as 
 being to a large extent the result of retrogressive muta- 
 tion. This type of mutation is also frequent among 
 cultivated plants. Thus, the appearance of a white 
 variety of a species previously only known to produce 
 coloured flowers may constitute a good example of a 
 retrogressive change. Many of the phenomena some- 
 what vaguely described by de Vries as cases of latency 
 have now received a more precise interpretation in 
 terms of the interaction of invisible factors in perfectly 
 ■definite ways ; these are more fully described in the 
 chapters on Mendelism. Finally, new and distinct 
 types may arise by the intercrossing of separate 
 species, but this is not regarded by de Vries as being 
 an important source of permanent new forms. 
 
 Without following de Vries into all the niceties of 
 his theory as to the particular kinds and methods of 
 mutations, we must admit that his experiments go 
 far to establish the doctrine, in support of which a 
 considerable amount of evidence had previously been 
 accumulated, especially by Bateson, that the origin of 
 species in Nature is generally a definite process, and 
 takes place by steps of considerable amplitude. What, 
 then, is the meaning of individual differences, of that 
 
THE THEORY OF MUTATION 
 
 155 
 
 continuous variability which is often so considerable, 
 and of the inheritance of this kind of differences which 
 the biometricians have been at so much pains to prove ? 
 De Vries points out that for no two plants are the con- 
 ditions of life exactly the same ; a considerable degree 
 of diversity among the plants themselves is therefore 
 advantageous, even when these belong to the same 
 specific type. Upon continuous variability depend 
 local races, forms adapted to wetter and drier situa- 
 tions, highland and lowland races, and the like, but 
 none of these are permanent. As regards the cause 
 of this variability, apart from the effect of sexual 
 reproduction, which combines the tendency to vary 
 of two separate parents, de Vries believes that indi- 
 vidual variability depends entirely upon nutrition ; 
 but under this head he includes practically the whole 
 etivironment of plants — light, space, soil, moisture, 
 and the like. Characters acquired in a similar way by 
 previous generations are inherited, and the effect of 
 conditions upon the developing seed whilst still borne 
 upon the parent plant may be considerable. Thus 
 easily does de Vries dispose of the puzzling question 
 of the inheritance or non-inheritance of acquired 
 characters. Acquired characters are inherited ; they 
 are not of any importance in the origin of species. 
 
 With regard to the causes of mutations, little is 
 known. Still, it is no longer incumbent upon us, as it 
 was a few years ago, to admit that we know nothing at 
 all about the means by which this form of variation 
 can be produced. W. L. Tower, in his ‘ Evolution in 
 Chrysomelid Beetles,’ has shown that variations due 
 
156 THE THEORY OF MUTATION 
 
 to the influence of the environment upon the larval 
 stages of the beetle are not inherited ; but if the female 
 is subjected to abnormal conditions for a few days at 
 the time when the eggs are maturing, the eggs and 
 larvae being afterwards allowed to develop in the 
 normal environment, a greatly increased number of 
 mutations is obtained, the majority of which are the 
 same as those found much more rarely in Nature. 
 
 MacDougal, too, has met with some success in the 
 attempt to produce mutations artificially in plants. 
 In one or two cases, after injecting weak solutions of 
 different chemical substances into the young ovaries 
 of Raimannia and CEnofhera, seedlings were obtained 
 which differed from anything previously seen. Up to 
 the present time these successes seem to be too few in 
 number to allow of any definite conclusions being based 
 upon them. 
 
 Blaringhem has also recently published observations 
 which seem to show that in the maize-plant injuries to 
 the parent occurring previously to the differentiation 
 of the germ cells may lead to permanent modifications 
 in the offspring. In neither of these three sets of 
 experiments did the modification produced in the off- 
 spring show any trace of an adaptive relation to the 
 exciting cause which operated upon the parent. 
 
 So much may be stated in order to indicate the 
 direction in which research is proceeding. In the 
 course of another decade we may hope perhaps to 
 find out something more about the natural and artificial 
 production of mutations. 
 
THE THEORY OF MUTATION 
 
 157 
 
 According to the view upheld by Wallace, Weismann, 
 and others, the actual origin of specific distinctions 
 takes place by natural selection acting upon individual 
 differences ; and in this case it is to be observed that 
 it is the struggle between individuals of the same species 
 which is of primary importance. On the mutation 
 theory it is only the competition between allied species 
 which interests us from the point of view of evolution. 
 Natural selection is thus regarded as having no influ- 
 ence in the formation of species themselves. On the 
 other hand, the gaps existing between genera and still 
 larger groups, such as families and classes, are still sup- 
 posed to be due to the destructive action of natural 
 selection determining the survival of the fittest species, 
 so that this principle is by no means ousted from its 
 prominent position in the philosophy of evolution even 
 when the latter is expounded by the mutationist. 
 
 One further point. On the theory of mutation the 
 survival of useless structures becomes readily com- 
 prehensible. Indeed, a structure which is actually of 
 the nature of a handicap to its possessor may fail to 
 cause extinction if it is either combined with a vigorous 
 constitution, or correlated with other characteristics 
 which are sufficiently useful to make up for the dis- 
 advantages entailed. The survival of many apparently 
 useless and some apparently harmful structures is very 
 difficult to understand on the hypothesis of a con- 
 tinuous evolution by the survival of the fittest indi- 
 viduals. This is an argument upon which de Vries 
 lays considerable stress, although it may be pointed 
 
158 
 
 THE THEORY OF MUTATION 
 
 out that it is usually very difficult to form a judgment 
 as to the real usefulness or otherwise of organs. 
 
 [Note. — A most remarkable series of mutations has 
 been obtained by Prof. T. H. Morgan, of New York, in 
 the little Fruit Fly Drosophila. A description of many 
 of them, and of the manner of their inheritance, will be 
 found in his books ^ A Critique of the Theory of Evolu- 
 tion,' Princeton University Press, 1916, and ' The 
 Physical Basis of Heredity,' 1919.] 
 
 BIBLIOGRAPHY. 
 
 Bateson, W. : Materials for the Study of Variation, 1894. 
 Vries, H. de : Die Mutationstheorie, 1901. 
 
 ,, Species and Varieties, their Origin by Mutation, 
 
 1905. 
 
 MacDougal, D. T. : Various papers published by Carnegie 
 Institution. 
 
 Tower, W. L. : An Investigation of Evolution in Chrysomelid 
 Beetles of the Genus Leptinotarsa, 1906. 
 
 Gates, R. R. : The Mutation Factor in Evolution, 1915* 
 
THE LIBRARY 
 Of THE 
 
 UNIVERSITY OF ILLINOIS 
 
Joseph Gottlieb Kolreuter, 1733-1806. 
 {Ajtev an engraving by J. Cederquist. ) 
 
 \To face p. 159. 
 
CHAPTER VI 
 
 THE OLDER HYBRIDISTS 
 
 There is one side of the practical study of heredity 
 which dates back to the middle of the seventeenth 
 century — namely, that branch of the subject which is 
 concerned with the hybridizing or artificial cross- 
 breeding of different species and varieties of plants. 
 Quite recently the great importance which attaches 
 to this method of study has been realized once more, 
 and the interest thus awakened has led to a closer 
 examination of the accounts of experiments under- 
 taken a century or more ago, with the result of showing 
 that much of the work then carried out in this direc- 
 tion had attained to quite an astonishing degree of 
 excellence. In the brief sketch of the history of 
 hybridizing work here following, account will be taken 
 almost exclusively of experiments of which the in- 
 terest is not historical only, but which possess an 
 actual scientific value. Amongst other matters of 
 interest, it will be found that more than one observer 
 came very near to anticipating Mendel’s epoch- 
 making discovery, and thus arriving at the clue which 
 should unravel almost all the complex problems which 
 beset the early hybridizers. 
 
 159 
 
i6o 
 
 THE OLDER HYBRIDISTS 
 
 Following the modern usage, we shall apply the 
 term ‘ hybrid ’ to all individuals arising from a cross 
 between parents which belong to distinct groups, no 
 matter whether these groups are separated as distinct 
 genera or species, or whether they are regarded as 
 representing only different races or varieties. This 
 wide interpretation of the term hybrid has only re- 
 cently been reintroduced. The use to which it has 
 returned is, indeed, the original one ; but many inter- 
 mediate writers, including Darwin, confined the em- 
 ployment of this expression to cases of crossing between 
 species, and applied the word ‘ mongrel ’ to the off- 
 spring of crosses between races or varieties of the same 
 species. Darwin, however, did not regard species as 
 differing in kind from varieties, and he even particu- 
 larly emphasized the smallness of the distinction which 
 can be drawn between the behaviour and properties 
 of hybrids and mongrels respectively. Indeed, he 
 came to the highly important conclusion that the laws 
 of resemblance between parents and their children are 
 the same, whatever may be the amount of difference 
 between the parents in question — whether, that is to 
 say, they are distinguished only by individual differ- 
 ences, or whether they belong to separate varieties or 
 even species. We have already seen that the more 
 recent facts of biometry point strongly towards the 
 conclusion that individual and race differences are 
 inherited at approximately the same rate. It seems, 
 however, to be at present somewhat doubtful whether 
 all sorts of specific differences follow the sar e law of 
 propagation on cross-breeding. 
 
KOLREUTER 
 
 i6i 
 
 Between 1760 and 1766 Joseph Gottlieb Kolreuter 
 carried out the first series of systematic experiments 
 in plant hybridization which had ever been under- 
 taken.' These experiments not only established with 
 certainty for the first time the fact that the seeds of 
 plants are produced by a sexual process comparable 
 with that known to occur in animals, but also led to a 
 knowledge of the general behaviour of hybrid plants, 
 which was scarcely bettered until Mendel made his 
 observations a century afterwards. 
 
 Kolreuter found that the hybrid offspring of tw'o 
 different plants generally took as closely after the 
 plant which yielded the pollen as after that upon 
 which the actual hybrid seed was borne. Indeed, he 
 found that it made little or no difference to the ap- 
 pearance of the hybrid which of the parental species 
 was the pollen-parent (male), and which the seed- 
 parent (female) — that is to say, in the case of plants 
 the result of reciprocal crosses is usually identical. 
 Thus, for the first time it was definitely shown that 
 the pollen-grain plays just as important a part in 
 determining the characters of the offspring as does the 
 ovule which the pollen-grain fertilizes. This was a 
 wholly novel idea in Kolreuter’s time, and the fact was 
 scarcely credited by his contemporaries. 
 
 Kolreuter had no means of discovering that the 
 contents of a single pollen-grain unite with the con- 
 tents of a single ovule in fertilization. But he ascer- 
 tained by experiments that more than thirty seeds 
 might be made to ripen by the application of between 
 
 fifty and sixty pollen-grains to the stigma of a par- 
 
 II 
 
i 62 
 
 THE OLDER HYBRIDISTS 
 
 ticular flower, so that, if he had had any hint of the 
 actual microscopic processes of fertilization, he w'ould 
 have been quite prepared for the more fundamental 
 discovery. 
 
 Kolreuter, indeed, believed that the act of fertiliza- 
 tion consisted in the intimate mingling together of two 
 fluids, the one contained in the pollen-grain, and the 
 other secreted by the stigma of the plant. The mingled 
 fluids, he supposed, next passed down the style into 
 the ovary of the plant, and arriving at the unripe 
 ovules, initiated in them those processes which led to 
 the formation of seeds. In this belief Kolreuter simply 
 followed the animal physiologists of his time, who 
 looked upon the process of fertilization in animals as 
 taking place by a similar mingling together of two 
 fluids. Now that we know that fertilization consists 
 essentially in the intimate union of the nuclei of two 
 cells, one of which, in the case of plants, is the ovum 
 contained within the ovule, whilst the other is repre- 
 sented by one of a few cells into which the contents 
 of the pollen-grain divide, we can understand more 
 clearly the bearing of Kolreuter’s observation. And 
 it is greatly to this eminent naturalist’s credit that he 
 succeeded in carrying out his observations with so 
 much accuracy, when the full meaning of those 
 observations was of necessity hidden from his com- 
 prehension. 
 
 Kolreuter was the first to observe accurately the 
 different ways in which pollen can be naturally con- 
 veyed to the stigma of a flower. This may take place 
 either by the pollen-grains falling directly upon the 
 
KOLREUTER 
 
 163 
 
 stigma, or by the agency of the wind, or, lastly, the 
 pollen may be carried by insects visiting the flowers. 
 And he recognised many features characteristic of 
 flowers apt to be fertilized in one or other of these 
 ways in particular. Thus he was aware, for example, 
 of the nature and use of the nectar which so many 
 flowers produce — namely, that it is the substance from 
 which the bees — by far the most diligent visitors of 
 flowers— obtain their honey. 
 
 Curiously enough, Kolreuter was not aware of the 
 existence of any natural wild hybrid plants. But he 
 was quite right in contending that supposed examples 
 of such hybrids required for their substantiation the 
 experimental proof, which could only be afforded by 
 making actual artificial crosses between the putative 
 parent species. 
 
 The first hybrid made artificially by Kolreuter was 
 obtained in 1760 by applying the pollen of Nicotiana 
 paniculata to the stigma of Nicotiana rustica. The 
 hybrid offspring of this cross showed a character inter- 
 mediate between those of the two parent species in 
 almost every measurable or recognisable feature, with 
 a single notable exception. This exception was 
 afforded by the condition of the stamens and of the 
 pollen grains produced by the hybrids. These organs 
 were so badly developed that in all the earlier experi- 
 ments self-fertilization of the hybrid plants yielded no 
 good seed at all, nor were the pollen grains of the 
 hybrid any more effective when applied to the stigmas 
 of either of the parent species. On the other hand, 
 when pollen from either parent was a.pplied to the 
 
 II — 2 
 
164 
 
 THE OLDER HYBRIDISTS 
 
 stigmas of the hybrid plants, a certain number of seeds 
 capable of germination was obtained, although this 
 number was much smaller than in the case of normal 
 fertilization of either parent species. This partial 
 sterility, affecting in particular the stamens and the 
 pollen which they produce, is a feature common to the 
 majority of hj^brids between different natural species. 
 Many such hybrids, indeed, are altogether sterile, so 
 that a further generation cannot in any way be ob- 
 tained from them. On the other hand, the members 
 of different strains or varieties which have arisen under 
 cultivation yield, as a rule, when crossed together off- 
 spring which are perfectly fertile. 
 
 In subsequent years Kolreuter w'as able to obtain a 
 very few self-fertilized offspring from hybrids of the 
 same origin as the above. The resulting plants were 
 described as resembling their hybrid parent so closely 
 as to be practically indistinguishable from it. 
 
 The offspring obtained by crossing the hybrid plants 
 with pollen from either parent showed in each case a 
 form more or less intermediate between that of the 
 original hybrid and that of the parent species from 
 which the pollen was derived. But the plants were 
 not all alike in this respect, some of them being much 
 more like the parent species than others, and some, 
 again, varying in other directions. There were also 
 considerable differences between the different indi- 
 viduals in respect of fertility, so that some of the 
 plants were more and some less sterile than the original 
 hybrids. Also, there was some tendency to the produc- 
 tion of malformations of the flowers and other parts. 
 
KOLREUTER 
 
 165 
 
 One of the most noted of Kolreuter’s experiments 
 was that which consisted in repeatedly recrossing a 
 hybrid plant with one of the parent species from which 
 the hybrid was derived. By continuing to pollinate 
 the members of one generation after another with the 
 pollen of the same parent species, plants were at last 
 arrived at which were indistinguishable from the parent 
 in question. We shall return to this fact later on, 
 when the reader will be in a position to appreciate its 
 importance more fully. 
 
 Kolreuter found that the result of reciprocal crosses 
 is usually identical — that is to say, the offspring ob- 
 tained by fertilizing a plant A with pollen from a 
 plant B are not to be distinguished from those ob- 
 tained when B is fertilized with the pollen of A. But 
 the two opposite processes of fertilization are not 
 always equally easy to carry out. An extreme instance 
 of this circumstance was met with in the case of the 
 genus Mirabilis. Mirabilis jalapa was easily fertilized 
 with pollen from M. longiflora. During eight years 
 Kolreuter made rhore than two hundred attempts to 
 effect the reverse cross, but without success. 
 
 It was shown by Kolreuter that hybrids between 
 different races or varieties of the same species are 
 usually much more fertile than hj^brids obtained by 
 crossing distinct species. Indeed, he believed that 
 varieties of a single species were in all cases perfectly 
 fertile together, whilst hybrids between species always 
 showed some degree of sterility. But in this case Kol- 
 reuter based his definition of a species upon the very 
 point at issue, and when he found forms, which other 
 
i66 
 
 THE OLDER HYBRIDISTS 
 
 botanists regarded as good species, to be perfectly 
 fertile together, he immediately regarded them as 
 being only varieties of a single species. 
 
 One curious point is worth quoting in this connec- 
 tion. Five varieties of Nicotiana tabacum were found 
 to be perfectly fertile with one another, but when 
 crossed with Nicotiana glutinosa one of them was 
 found to be distinctly less sterile than the rest. 
 
 Another interesting point observed by Kolreuter 
 was the fact that hybrid plants often exceed their 
 parents in luxuriance of growth. Upon this fact, as 
 we shall see later on, Knight and afterwards Darwin 
 based theoretical conclusions of considerable impor- 
 tance in connection whh the problem of sex. 
 
 Thomas Andrew Knight, who was also a botanist of 
 high reputation in other fields, was the earliest observer 
 to lay stress upon the practical aspect of the study of 
 hybrids, and he occupied himself to a considerable 
 extent with the improvement of useful races of plants 
 by cross-breeding. Breeders of animals had already 
 made important improvements by the method of inter- 
 crossing different races, and selecting the most notable 
 types which made their appearance in consequence, 
 when Knight bethought him of applying the same 
 principles to the improvement of plants, and particu- 
 larly of fruit-trees. 
 
 Knight also carried out a series of experiments with 
 domestic peas, the results of which were published in 
 1779. These experiments have a particular interest 
 from the historical point of view, since it was by dint 
 of similar experiments upon the same kind of plants 
 
KNIGHT 
 
 167 
 
 that Mendel’s law was afterwards discovered. This 
 very discovery might even have befti made by Knight 
 himself, if he had only realized the importance of ascer- 
 taining on a large scale the numerical proportions in 
 which the different kinds of plants, arising in the 
 second generation from the crosses, made their appear- 
 ance. Unfortunately, this particular form of inquiry 
 never seems to have occurred to him. 
 
 Knight’s experiments were made with a different 
 object in view — namely, that of discovering whether 
 a cross with a distinct race would provide the stimulus 
 necessary to restore its lost vigour to a strain of plants 
 which was supposed to have become debilitated, owing 
 to its members having been bred exclusively by self- 
 pollination for a long series of generations. 
 
 The result of the experiments undoubtedly estab- 
 lished the fact that in some cases the hybrid offspring 
 of two distinct races shows a more vigorous habit of 
 growth than either of the parental types. The follow- 
 ing extract from Knight’s own account will indicate 
 the nature of the experiments upon which his con- 
 clusions rest : 
 
 ‘ By introducing the farina of the largest and most 
 luxuriant kinds into the blossoms of the most diminu- 
 tive, and by reversing this process, I found that the 
 powers of the male and female, in their effects on the 
 offspring, are exactly equal. The vigour of the growth, 
 the size of the seeds produced, and the season of 
 maturity, were the same though the one was a very 
 early, the other a very late variety. I had in this 
 experiment a striking instance of the stimulative effects 
 
i68 
 
 THE OLDER HYBRIDISTS 
 
 of crossing the breeds, for the smallest variety, whose 
 height rarely exceeded two feet, was increased to six 
 feet, whilst the height of the large and luxuriant kind 
 was very little diminished.’ 
 
 We shall see, however, that the phenomenon last 
 alluded to admits of a different interpretation. 
 
 It was upon the somewhat slender basis afforded 
 by this experiment that the generalization known as 
 the Knight -Darwin law was originally established. 
 Knight’s own expression of this idea was to the effect 
 that ‘ Nature intended that a sexual intercourse should 
 take place between neighbouring plants of the same 
 species.’ And the same conclusion was expressed still 
 more forcibly by Darwin in the aphorism : ‘ Nature 
 abhors perpetual self-fertilization.’ But although it 
 may be true that in a considerable number of cases 
 advantages are gained from the process of cross- 
 fertilization between different members of the same 
 species, which do not accrue when self-fertilization 
 takes place, yet several cases are now known in which 
 self-fertilization really does seem to be indefinitely 
 continued, 
 
 Knight crossed a pea having white flowers and seed- 
 coats, and green stems, with one in which the flowers 
 and stems were coloured purple, and the seeds grey. 
 The seeds immediately resulting from the cross were 
 unchanged in appearance, but the plants arising from 
 these took closely after their coloured male parent. 
 On crossing the cross-bred plants once more with a 
 white strain a certain proportion of white plants was 
 again obtained, though what that proportion was 
 
KNIGHT 
 
 169 
 
 Knight failed to notice. He observed, however, that 
 white crossed by a purple strain invariably gave purple, 
 whilst the cross-bred purples, when crossed again with 
 white, yielded some white and some purple plants. 
 
 In 1822 John Goss recorded the fact that a ‘ blue ’ 
 pea crossed with a ‘ white ’ yielded from the crossed 
 flowers pods with white seeds only, the seeds contained 
 in other pods upon the same plant being, of course, 
 blue. The plants produced from the white seeds bore 
 some pods with all blue, some with all white, and many 
 pods with both white seeds and blue ones ; and a 
 coloured plate is given which shows one of the latter 
 pods together with its contents. The blue seeds, when 
 sown separately, yielded plants which produced blue 
 seeds only, but plants arising from the white seeds 
 yielded a mixture of blue and white seeds. 
 
 Knight pointed out quite correctly that the colours 
 of the seeds which are here referred to are occasioned 
 by the colour of the cotyledons or seed-leaves of the 
 pea, which are visible through the semitransparent 
 seed-coat. Green cotyledons give rise in this way to 
 a bluish appearance, whilst, when the cotyledons are 
 yellow, the resulting appearance of the seed is described 
 as whitish. 
 
 The Hon. and Rev. W. Herbert was another observer 
 who made many important experiments in hybridiza- 
 tion towards the beginning of the nineteenth century. 
 These led him to the conclusion that Kolreuter and 
 Knight were wrong in their assertion that hybrids 
 between distinct species were always sterile. Herbert 
 considered that only generic or family types were 
 
170 
 
 THE OLDER HYBRIDISTS 
 
 constantly sterile, and this led him to the further 
 conclusion, now believed to be erroneous, that the 
 separate genera or families were those which were 
 originally created, whilst he believed that the separate 
 species of the same genus arose from a single original 
 type by a genuine process of evolution. 
 
 The most prolific in work of all the hybridists, how- 
 ever, was undoubtedly Carl Friedrich v. Gaertner 
 (1772 - 1850). Gaertner made a great number of 
 crosses between species belonging to all sections of 
 the natural system, and his book, published in 1849, 
 contains a great mass of valuable information. Gaert- 
 ner’s theoretical conclusions, for the most part, only 
 amplify and confirm those of Kolreuter, upon whom 
 in this direction he made but little advance. 
 
 C. Naudin’s essay, entitled ‘ New Researches on 
 Hybridity in Plants,’ made its appearance in 1862. 
 The author pointed out that the facts of the return 
 of hybrids to the specific forms of their parents, when 
 repeatedly crossed with the latter, are naturally ex- 
 plained by the hypothesis of the disjunction of the 
 two specific essences in the pollen grains and ovules 
 of the hybrid. The idea may, perhaps, be made some- 
 what clearer as follows : Let us consider the case of a 
 species A crossed with another species B. Naudin 
 supposes that some of the pollen grains and ovules of 
 the hybrid plant will be potentially* of the exact 
 
 * When it is said that a pollen grain or ovule potentially 
 resembles the species A, it is meant that the germ-cell in 
 question is of such a kind that, when united with one derived 
 from an ovule or pollen grain of similar constitution, it would 
 give rise to a plant exactly resembling A. 
 
NAUDIN 
 
 171 
 
 character of one species (A), whilst others will bear 
 no potential resemblance to A, but will be precisely 
 similar in nature to the ovules and pollen grains of 
 the pure species B. In cases where this separation of 
 the materials representing the two types in a potential 
 condition is complete, forms exactly resembling the 
 parents might be obtained. As we shall see, this 
 hypothesis makes a remarkably near approach to that 
 of Mendel ; and the importance of the fact that the 
 first hybrid generation is generally uniform, as con- 
 trasted with the diversity of types often appearing 
 in the second generation, is clearly recognised by 
 Naudin. This observer considered the hybrid in the 
 adult state to consist of an aggregate of particles, 
 homogeneous and characteristic of a single species 
 when taken separately, but mingled in various pro- 
 portions in the organs of the hybrid, which is thus 
 looked upon as a kind of living mosaic. 
 
 Great numbers of observations upon the character- 
 istics and behaviour of hybrid plants and animals have 
 been from time to time recorded, and the preceding 
 pages contain only a brief selection of such facts as 
 are most necessary for a proper understanding of 
 modem work in hybridization. Until quite recently 
 the laws of transmission of characters in hybrids were 
 still completely hidden. The facts were wonderful 
 enough, but they showed no signs of falling into an 
 orderly arrangement. In the next chapter it will be 
 our business to describe the remarkable discovery 
 which has introduced order into this previously chaotic 
 region, and which has enabled a few workers to estab- 
 
172 
 
 THE OLDER HYBRIDISTS 
 
 lish in half a dozen years the foundations of a great 
 science, the importance of which is now becoming 
 generally realized. 
 
 [Note. — In the earlier editions mention was made 
 of Millardet’s ‘ false hybrids ’ between species of 
 strawberries (1894), in which it appeared that which- 
 ever way the cross was made, the offspring resembled 
 the maternal species and were said to breed true. 
 Subsequent work by other investigators has not com- 
 pletely confirmed Millardet’s observations, and as the 
 facts are still doubtful, it has been thought better to 
 omit the passage from the text. For somewhat 
 analogous phenomena in Tripsacum and Euchlcsna 
 (genera allied to maize), see Collins and Kempton, 
 Journal of Heredity, 1916, p. 106.] 
 
THE OLDER HYBRIDISTS 
 
 X73 
 
 BIBLIOCxRAPHY. 
 
 Kolreuter, J. G. : Vorlaufige Nachricht von einigen das 
 Geschlecht der Pfianzen betreffenden Versuchen und 
 Beobachtungen, 1761. 
 
 Gaertner, C. F. von : Bastarderzengung, 1849. 
 
 Focke, W. O. : Die Pflanzen-Mischlinge, 1881. 
 
 Wichura, M. : Die Bastardbefruchtiing im Pflanzenreiche, 
 1865. 
 
 Darwin, C. ; The Effects of Cross and Self Fertilization. 
 
CHAPTER VII 
 
 MENDELISM 
 
 We have already had occasion to point out how im 
 port ant it is, when engaged upon questions of heredity, 
 not to treat whole animals or plants as units, but to 
 deal with their separate characters one at a time. In 
 the course of the present chapter the reason for pro- 
 ceeding in this way will appear more clearly, and we 
 shall find that the adoption of this method is fully 
 justified by the results which it enables us to obtain, 
 and which could not have been arrived at in any other 
 way. We shall also find reasons for believing that 
 this method is the correct one from a theoretical point 
 of view. 
 
 Naturally, considerable care is necessary in deter- 
 mining what are and what are not separable characters. 
 At the outset it is not always possible to make this 
 discrimination with certainty, but during the course 
 of the experiments which follow it is almost always 
 possible to arrive at a clear definition of each character, 
 and in many cases the distinction of characters is quite 
 obvious from the beginning. 
 
 Up to the present time the experimental study of 
 heredity by the methods of definite breeding has yielded 
 
 174 
 
Gregor Johann Mendel. 
 
 [To /ace p. 174. 
 
the 
 
 Of THE 
 
 UHWERSaT Of lUlHOlS 
 
MENDELISM 
 
 175 
 
 clear and definite information only when applied to 
 cases where clearly definable characters have distin- 
 guished the parental forms examined. This, however, 
 is in great part due to the fact that the experimental 
 method has hitherto been little used in dealing with 
 characters of a less definite nature. The science is 
 still in its infancy, and attention has naturally been 
 first paid to the simpler problems which it affords. 
 The difficulties of treatment which confront those who 
 would deal with highly variable characters and those 
 of a ‘ more or less ’ nature are considerable, although 
 there is no reason for supposing that such problems are 
 insuperable. As we have seen, however, the majority 
 of characters which distinguish species or races from 
 one another appear to be of a perfectly definite descrip- 
 tion, so that the limitation just referred to is not so 
 serious as might appear at first sight. The recent 
 cevival of work upon the subject of inheritance by 
 the use of breeding methods has, as a matter of fact, 
 already been rewarded with results as valuable and 
 as clear as could possibly have been anticipated — 
 results which are sufficient in themselves to show that 
 the discovery made by Mendel was of an importance 
 little inferior to those of a Newton or a Dalton. 
 
 It is important to remember that every animal or 
 plant, which has come into existence in the ordinary 
 way through sexual generation, owes its individuality 
 to the mingled natures of two separate parents. The 
 following lines, in which the poet Goethe speaks of 
 his own hereditary endowment, have been quoted 
 more than once in this connection ; 
 
176 
 
 MENDELISM 
 
 ‘ Vom Vater hab’ ich die Statur, 
 
 Des Lebens emstes Filhren, 
 
 Vom Mbtterchen die Frohnatur 
 Und Lust zu fabulieren.’ 
 
 In such a case we must always look upon the corre- 
 sponding character of the second parent as existing 
 in the offspring side by side with the character which 
 finds expression, only the former is overpowered by 
 the latter, and forced to remain invisible. That the 
 hidden character is nevertheless actually present is 
 shown by the fact that a feature characteristic of a 
 particular grandparent, which did not appear in the 
 parent, may reappear in the child. For instance, a 
 characteristic masculine feature of the maternal grand- 
 father may thus make its appearance in the son. 
 
 It is found that any individual may be looked upon 
 as being to a large extent an aggregation of separate 
 characteristics. In a pair of allied races a great 
 number of the separate characters are the same in 
 the two cases, the distinction between the two forms 
 depending upon a few definite features only. The 
 majority of salient characteristics are identical in such 
 a pair, but some of the corresponding characters are 
 opposed. Thus in different races of mankind com- 
 plexions may be dark or fair, eyes blue or brown, 
 hair straight or curly, and the like. Now the off- 
 spring of parents which had smooth and curly hair 
 respectively might exhibit smooth or curly or inter- 
 mediate (wavy) hair, according as one or the other 
 character, or both in combination, made its presence 
 obvious ; for in the simplest cases both will necessarily 
 
UNIT CHARACTERS 
 
 177 
 
 be present, though one may be hidden. What will 
 happen in the grandchildren ? 
 
 The manner in which characters comparable with 
 the above are actually transmitted has been worked 
 out in the case of many races of animals and plants, 
 and in cases where experimental matings can be 
 readily carried out, and a large number of offspring 
 reared, it is found that a simple rule applies which 
 holds good in every example thoroughly examined 
 hitherto. This law was discovered by Mendel about 
 the year 1865, and has since been called by his name. 
 Before enunciating it we shall consider the informa- 
 tion afforded by the case of a single pair of simple 
 characters. Afterwards we shall endeavour to show 
 the application of the law to the more complex cases 
 in which combinations of characters are concerned. 
 
 A grain of Indian corn or maize contains a germ or 
 embryo, which under suitable conditions will give rise 
 to the future plant. The embryo is surrounded by a 
 certain amount of reserve food material constituting 
 the endosperm — a store which is made use of by the 
 young plant during its germination. The embryo 
 arises as the result of a process of fertilization which 
 takes place in the following manner : The ovum, or 
 female cell hidden in a flower, contains a nucleus, and 
 this on fusion with one of the nuclei derived from a 
 grain of pollen initiates the vital processes which lead 
 to the development of an embryo plant. 
 
 Nuclei are the central and, from the point of view of 
 
 heredity, the most important parts of cells — ^the con- 
 
 12 
 
178 
 
 MENDELISM 
 
 stituent units of the plant body. The cells which, 
 together with their nuclei, take part in the process of 
 fertilization are known as gametes, or germ-cells—malQ 
 and female respectively, the latter being the ovum. 
 
 It is less generally known that the endosperm of a 
 grain of Indian corn arises by a very similar process 
 to the one which gives rise to the embryo itself. A 
 second nucleus derived from the same pollen grain 
 fuses with a nucleus situated near the ovum, and to 
 the product of this fusion the endosperm owes its 
 origin. It is further found, so far at least as those 
 characters are concerned to which we shall at present 
 confine our attention, that these two important nuclei 
 hidden in the same female flower are exactly alike in 
 hereditary constitution, and so are the two generative 
 nuclei derived from a single pollen grain. In conse- 
 quence of this fact, the observed character of the endo- 
 sperm may be regarded as a true guide to the nature 
 of the plant into which the associated embryo will 
 afterwards develop. The hereditary qualities of the 
 two are exactly the same. 
 
 It is not difficult to find a variety of Indian corn in 
 which the endosperm is yellow, and another in which 
 the colour of this tissue is white, owing to the absence 
 of any visible yellow pigment. If a female flower of a 
 white variety is fertilized with pollen taken from a 
 yellow variety, the resulting grain shows its hybrid 
 nature by the presence of the yellow colour in its endo- 
 sperm. This is found to be a regular rule. Grains 
 upon a plant of a white strain which has been pollinated 
 with ‘ white pollen ’ are white, but if pollinated from 
 
INHERITANCE IN MAIZE 
 
 179 
 
 a yellow strain the grains are yellow. On the other 
 hand, the grains upon a plant belonging to a yellow 
 strain retain their yellow colour even if the flowers 
 which produce them have been pollinated from a white 
 variety. 
 
 These facts are expressed in technical language by 
 saying that yellowness is dominant over whiteness, and 
 the latter is said to be recessive. 
 
 Let us now suppose that we have sown a number 
 of the yellow grains derived from the cross yellow x 
 white* or white x yellow, and that we have exposed 
 the female flowers of the resulting plants at the proper 
 stage of their existence to the influence of pollen 
 derived from a pure white strain, taking care that 
 none of their own hybrid pollen falls upon them at 
 the same time. The result of this experiment takes 
 us at once to the very heart of the Mendelian theory. 
 Half the total number of grains obtained in this way 
 — from the cross {white x yellow) x white — are white, 
 and half are yellow. 
 
 Thus in an experiment carried out in the manner 
 described there were obtained upon ninety-five plants : 
 
 Yellow grains 26,792, or 50'03 per cent. 
 
 White grains 26,751, „ 49-97 „ 
 
 But we must go further than this. On sowing the 
 white grains obtained in this second generation (F2), 
 and allowing the plants obtained from them mutually 
 to pollinate one another, cobs were obtained bearing 
 exclusively white grains without any trace of yellow- 
 ness. 
 
 ♦ X is to be read ‘ fertilized with pollen from.’ 
 
 12 — 2 
 
i8o 
 
 MENDELISM 
 
 Half the grains, then, of the parentage {white x 
 yellow) X white are pure white in colour, and not to be 
 distinguished from grains of the parentage white x white 
 even after an extensive examination of their offspring, 
 which is the most rigorous test we are able to apply. 
 
 The yellow grains born upon the same hybrid plants 
 (F2)* had clearly each of them one white parent — 
 namely, the plant from which the white pollen was 
 derived. On sowing these yellow grains and once 
 more pollinating by pure white, a precisely similar 
 result was observed to that obtained in the preceding 
 generation — that is to say, these plants, derived from 
 yellow grains, produced once more 50 per cent, of 
 white grains and 50 per cent, of yellow. We are, 
 therefore, led to suppose that the yellow grains born 
 upon the hybrid plants are of precisely the same nature 
 as the original yellow hybrid grains {white x yellow), 
 since their behaviour when pollinated from the same 
 white strain is identical. We may express the result 
 so far obtained in the form of the following diagram : 
 P* white X yellow 
 
 Fi yellow x white 
 
 F2 white X yellow (50 %) (50 %) white x white 
 
 Fg white (50 %) yellow (50 %) white (exclusively). 
 
 * The following shorthand expressions are adopted to denote 
 the different generations in cross-breeding experiments : P is 
 the generation of the original parents ; is the first genera- 
 tion of offspring — the cross-bred seeds and the plants to 
 which they give rise. To the Fg generation belong the seeds 
 produced upon the F^ plants, and the plants to which they 
 give rise, and so on. 
 
INHERITANCE IN MAIZE i8i 
 
 The pollen of the Fj plants {i.e., those plants which 
 were derived from the yellow cross-bred grains) — when 
 applied to the female flowers of the same pure white 
 strain of maize, caused in like manner the appearance 
 of white and yellow grains in equal numbers. This 
 result is equally well expressed by the above diagram 
 on simply regarding the yellow in as the male parent 
 (pollen-parent) instead of as the female parent (seed- 
 parent) of Fg. 
 
 What is, then, the meaning of these results ? The 
 case is really very simple. The germ-cells (ova and 
 pollen-nuclei) of the cross-bred plants {white x yellow) 
 must be potentially either pure white or pure yellow, 
 with no blending of these characters. Further, the 
 two kinds (yellow and white) of male germ-cells or 
 pollen-nuclei must arise in equal numbers, and the 
 same must be true of the female germ-cells or ova. 
 By this supposition only can the observed facts be 
 explained. If the supposition is true, then, when the 
 cross-bred plant (Fj) is crossed again with the pure 
 white form, its white germ-cells give rise to white 
 grains which are of the nature {white x white), and 
 are therefore pure. Its yellow germ-cells give rise to 
 yellow grains which are of the nature {yellow x white). 
 And, since the number of yellow- and white-bearing 
 germ-cells is equal, the number of yellow and of white 
 grains produced in this way is approximately the same. 
 The yellow grains are of the same composition as the 
 original cross-bred grains obtained by crossing pure 
 white with pure yellow, and we have seen that they 
 behave in exactly the same way on further cross- 
 
i 82 
 
 MENDELISM 
 
 breeding. This conclusion is at least so far firmly 
 established that no alternative hypothesis has been 
 put forward which will explain the facts. 
 
 We have next to consider what will be the result of 
 crossing our cross-bred plants with one another instead 
 of with the pure white form. The following possi- 
 bilities present themselves : 
 
 A yellow female gamete may pair with a yellow male gamete. 
 
 )> )> >» »i >» white I, 
 
 A white „ „ „ „ yellow „ 
 
 )) » >» n » white „ 
 
 All these combinations are equally likely to occur, 
 because in each plant there are the same number of 
 yellow and white female gametes as well as of yellow 
 and white male gametes. In the long-run, therefore, 
 each of the above pairings will be found to have taken 
 place in an equal number of cases. The grains which 
 we shall obtain, then, will be yellow and white in colour, 
 and the two kinds will occur in the following propor- 
 tions : I pure white ; 2 white x yellow or yellow x 
 white, which, as we have already seen, will be yellow 
 in appearance ; and i pure yellow. Altogether, we 
 shall expect a ratio of 3 yellow grains to i white. 
 
 In an actual experiment the following result was 
 obtained : 
 
 Yellow grains 16,592, or 74‘5 per cent. 
 tYhite „ 5 j68i, „ 25 5 ,, 
 
 — ^that is to say, a ratio of 2’9 yellow to i white. 
 
 The expression lA : 2 Aa : la, in which A represents 
 the dominant character (yellow) and a the recessive 
 character (white), may be spoken of as a Mendelian 
 
MONO-HYBRID CROSSES 
 
 183 
 
 formula. It indicates the proportion in which the 
 two pure types and their hybrid brethren will appear, 
 on breeding together the offspring of a simple or 
 mono-hybrid cross — i.e., one in which attention is 
 paid to the behaviour of a single pair of characters only. 
 
 So far we have been dealing with a pair of characters 
 consisting in the presence and absence respectively of 
 a particular pigment. Precisely similar results are to 
 be obtained in the case of a pair of structural char- 
 acters. The endosperm, or reserve substance, of cer- 
 tain varieties of Indian corn shows a smooth surface, 
 and contains an essentially starchy reserve material, 
 whilst in other races the surface of the endosperm is 
 wrinkled and the reserve product is of a sugary nature. 
 This sugary endosperm is characteristic of the kinds 
 of corn largely used in the United States of America 
 as a table vegetable. 
 
 On crossing together a variety with smooth starchy 
 grains and one with wrinkled sugary grains, the grains 
 immediately resulting are smooth and starchy, no 
 matter whether the starchy strain is used as the seed- 
 parent or as the pollen-parent — that is to say, thestarchy 
 character is dominant, a dominant character being one 
 which appears in Fj to the complete or almost com- 
 plete exclusion of the corresponding character ex- 
 hibited by the other parent, which is spoken of as 
 recessive. In the present case the sugary character is 
 recessive. 
 
 The further behaviour of the cross between smooth 
 and wrinkled is precisely the same as that of yellow 
 
i84 
 
 MENDELISM 
 
 crossed with white. Thus, if the hybrid plants are bred 
 together or self-fertilized, the resulting cobs will exhibit 
 a proportion of three smooth grains to one wrinkled 
 grain. In an actual example there were obtained 5,310 
 smooth grains and 1,765 wrinkled, or 75-06 per cent, 
 of the former and 24-94 cent, of the latter. 
 
 In a further generation the wrinkled grains breed 
 true. On the average one out of every three smooth 
 grains does the like. The remaining two smooth grains 
 are of hybrid nature, and on self-fertilization yield 
 again the same proportion of three smooth to one 
 wrinkled. Such hybrid grains and the plants into 
 which they develop are spoken of as heterozygotes. 
 
 Thus, if we write B for smooth and b for wrinkled, 
 the following scheme will express the result of crossing 
 together plants which bear these characters, and after- 
 wards self-fertilizing the offspring obtained j 
 
 7 P (dominant character) B x b (recessive character) 
 
 Bb 
 
 (heterozygote looking like B) 
 on self-fertilization yields 
 
 trT Fj BB 
 
 Bb bB 
 
 bb 
 
 (extracted 
 
 recessive) 
 
 (extracted 
 
 dominant) 
 
 (heterozygotes looking like BB) 
 on self-fertilization each gives 
 
 BB BB ' Bb bB 
 
 I civf v'o ^ 
 
 (extracted ^ 
 
 dominant) and so on 
 
 (extracted 
 
 recessive) 
 
 bb bb 
 
 
 and so on, and so on, 
 breeding breeding 
 true. true. 
 
 BB BB 
 
 bb bb 
 
 and so on, and so on, 
 breeding breeding 
 true. true. 
 
DI-HYBRID CROSSES 
 
 185 
 
 So far we have seen that both a pair of structural 
 characters and a pair of colour characters can 
 ‘Mendehze,’ according to the phrase coined by the 
 Germans — that is to say, the germinal representa- 
 tives of^Juch pairs of characters remain perfectly 
 distinct in the hybrid plant, and separate completely 
 at the formation of its gametes, in such a way that 
 an equal number of gametes arises containing either 
 character^ 
 
 The members of a pair of characters which behave 
 in this way on crossing are called allelomorphs. When 
 a pair of gametes fuse together in the process of fer- 
 tilization the resulting cell is known as a zygote. A 
 zygote formed by the conjunction of two like gametes 
 is called a homozygote. When the gametes contain 
 opposite members of a pair of allelomorphs the result 
 is called a heterozygote. The same terms may also be 
 applied to the adult multicellular organisms into which 
 these fertilized egg-cells develop. 
 
 We have still to consider what happens when parents 
 are crossed which differ in more than one pair of allelo- 
 morphs. The actual result is as follows : 
 
 Suppose a smooth yellow tj^pe of maize to be crossed 
 with a wrinkled white variety, both smoothness and ' 
 yellowness being dominant. The grains produced in 
 Fj are therefore yellow and smooth. Let the Fj plants, 
 arising from the smooth yellow heterozygote grains, 
 be crossed with the wrinkled white parent, which 
 is recessive in respect of both these characters. 
 In this way the true nature of every germ cell 
 produced by the heterozygote will be able to manifest 
 
i86 
 
 MENDELISM 
 
 itself in the visible character of the grain produced 
 from it. 
 
 The following result was actually obtained in this 
 way ; 
 
 Smooth yellow grains 2,869, or 25*3 per cent. 
 
 Smooth white grains 2,933, or 257 „ 
 
 Wrinkled yellow grains 2,798, or 24*5 „ 
 
 Wrinkled white grains 2,803, or 24-5 „ 
 
 Thus we see that a nearly equal number of the germ 
 cells of the double heterozygote bears each of the four 
 possible combinations of characters — that is to say, it 
 is an even chance whether a particular gamete, which 
 bears the allelomorph yellowness, bears also smoothness 
 or wrinkledness. In other words, the two pairs of 
 allelomorphs segregate in entire independence the one 
 of the other. It is particularly to be noticed that we 
 arrive in this way at two perfectly new combinations 
 of characters, which were not shown by the original 
 parent strains. We have synthesized two new sorts 
 of maize with smooth white and wrinkled yellow grains 
 respectively. In a precisely similar way, if the cross 
 is made between strams of which the grains are re- 
 spectively smooth white and wrinkled yellow, we 
 should obtain in F2 the new combinations smooth 
 yellow and wrinkled white. 
 
 The result obtained on self-fertilizing the hybrid 
 plant is somewhat more complicated. 
 
 If we write A for yellowness, a for whiteness, B for 
 smoothness, and h for wrinkledness as before, AB x ab 
 gives the heterozygote AJB&b- Equal numbers of the 
 germ cells of the heterozygote will be of the composi- 
 tions AB, Ah, aB, and ah. 
 
Fig. 14. — Mendeltan Proportions in Maize. 
 
 Cobs born by heterozygote plants pollinated with the recessive, showing 
 equality of smooth and wrinkled and of coloured and white grains. 
 
 \To face p. 18 $. 
 
THE LIBRARY 
 Of THE 
 
 UNIVERSITY OF ILLINOIS 
 
DI-HYBRTD CROSSES 187 
 
 AU the following zygotic combinations are, then, 
 equally likely 5 
 
 ABAB 
 
 ABAb 
 
 ABaB 
 
 A Bab 
 
 AbAB 
 
 AbAb 
 
 AbaB 
 
 A bob 
 
 aBAB 
 
 aBAb 
 
 aBaB 
 
 aBab 
 
 abAB 
 
 abAb 
 
 abaB 
 
 abab 
 
 Altogether there are sixteen combinations. The 
 result can be expressed more shortly in the form 
 (A + 2 Aa + a) (B + 2 Bh + h),* which will be found to give 
 the above terms when expanded. Thus the combina- 
 tion of the Mendelian formulae for Fg when each of 
 the pairs of allelomorphs is considered separately, 
 gives us the formula for the two pairs of allelomorphs 
 considered simultaneously. 
 
 The same result may also be written in the form ; 
 
 4^ ] , . 1 
 
 r iB 
 
 2 Bb 
 
 I b 
 
 8 Aa r combined with i 
 
 2 B 
 
 4Bb 
 
 2 b 
 
 4 a J ' 
 
 1 
 
 2 Bb 
 
 I b 
 
 or 
 
 AB 2 ABb Ab 
 
 2 A ab 4 AaBb 2 Aab 
 
 aB 2 aBb ab* 
 
 * It is customary to condense these expressions as far as 
 possible by never repeating the same letter more than once 
 in each term. Thus, A stands for AA, B for BB, and so on. 
 On expansion, i,e,y multiplying together the contents of the two 
 brackets, ^ xi5 gives A BAB, AxBb gives ABAb, and so on 
 for all the other terms of the expression. 
 
i88 
 
 MENDELISM 
 
 Let us consider the external appearance of these 
 various types in the particular example before us. 
 
 Nine of the above sixteen terms include A and B, 
 and' are therefore smooth yellow in appearance. (We 
 need not stop to consider whether a or 6 or both are 
 present in addition, since these are recessive.) 
 
 Three terms include A and b, B being absent. These, 
 therefore, appear wrinkled yellow. 
 
 Three include a and B, A being absent. These, 
 therefore, appear smooth white. 
 
 One contains a and b only, and is, therefore, wrinkled 
 white. 
 
 With regard to internal constitution : 
 
 The nine individuals of appearance AB include the 
 following types : 
 
 One pure, ABAB, breeding true to the smooth 
 yellow type on self-fertilization. 
 
 Two ABAb, heterozygous in respect of the 
 pair B-b, but pure yellow. 
 
 Two ABaB, heterozygous in respect of A-a, 
 but pure smooth. 
 
 Four A Bab, heterozygous in respect of both 
 pairs of characters. 
 
 The three individuals of appearance Ab include the 
 following types : 
 
 One pure, AbAb, breeding true to the (new) 
 wrinkled yellow type. 
 
 Two Abab, giving both wrinkled yellow and 
 wrinkled white. 
 
DI-HYBRID CROSSES 189 
 
 The three individuals of appearance aB include the 
 following types : 
 
 One pure, aBaB, breeding true to the (new) 
 smooth white type. 
 
 Two aBab, giving both smooth white and 
 wrinkled white. 
 
 The remaining individual is ah in appearance and 
 abab in constitution, and breeds true to the wrinkled 
 white type. 
 
 The expected behaviour of all these different types 
 can be followed out by the aid of suitable breeding 
 experiments, and not only has this been done in the 
 case of the cross which we have been considering, but 
 precisely similar phenomena have been shown to be 
 taking place for a large number of other characters in 
 many different species of plants and in a good many 
 animals as well. 
 
 We are now in a position to state the important 
 proposition known as Mendel’s law, which is to the 
 following effect : 
 
 The gametes of a heterozygote bear the pure parental 
 allelomorphs completely separated from one another, 
 and the numerical distribution of the separate allelo- 
 morphs in the gametes is such that all possible com- 
 binations of them are presfent in approximately equal 
 numbers. (Note that it is impossible for more than 
 one member of the same pair of allelomorphs to occur 
 together in the same gamete.) 
 
 This is the essence of the great discovery made by 
 Gregor Mendel, Abbot of Brunn, and published by 
 
MENDELISM 
 
 190 
 
 him in the Transactions of the Brunn Natural History 
 Society in 1866. By some extraordinary chance 
 Mendel’s paper was entirely lost sight of until the 
 same facts were independently rediscovered in 1899 
 by de Vries working in Holland, by Correns in Germany, 
 and by Tschermak in Austria. 
 
 Gregor Johann M6ndel was born on July 22, 1822, 
 at Heinzendorf, near Odrau, in Austrian Silesia. In 
 1843 he entered as a novice the Augustine Convent at 
 Altbrunn, and was ordained priest in 1847. 
 
 Mendel was a teacher of natural science in the Brunn 
 Realschule from 1853 to 1868, when he was appointed 
 Abbot of his monastery. During this time he was 
 largely occupied with experiments in cross-breeding a 
 great variety of plants, and some idea of his activity 
 in this line of scientific work is to be gathered from a 
 perusal of his letters to the German biologist Nageli, 
 a correspondence which has recently been published 
 by Professor Correns. Mendel himself only published 
 the result of his work with peas, and that of a few of 
 his experiments with Hieracium. 
 
 After 1873 the cares associated v/ith the position of 
 Abbot of Brunn appear to have prevented further 
 biological work. His death took place in 1884, two 
 years after that of Charles Darwin, to whom Mendel 
 was thirteen years junior. 
 
 Mendel’s own experiments — that is to say, the 
 chief ones published by him — ^were made with peas, a 
 kind of plants which were found to be remarkably 
 well suited to this kind of work. Seven pairs of 
 characters in these plants were found to behave in 
 
GREGOR MENDEL 
 
 191 
 
 precisely the same manner as those characters of the 
 maize-plant which have been already described, and 
 in all of them the phenomenon of dominance also 
 appeared. The characters dealt with by Mendel were 
 as follows, the dominant member of the pair being in 
 each case placed first : 
 
 Smooth seeds, and wrinkled seeds. 
 
 Yellow, and green reserve material — t.e., cotyledons. 
 
 Deeply coloured (grey), and nearly colourless test as 
 or seed-coats. 
 
 Inflated or stiff, and wrinkled or soft pods. 
 
 Green, and yellow pods. 
 
 Flowers scattered up the stem, and flowers in a 
 terminal bunch or umbel. 
 
 Tall, and dwarf stems. 
 
 As the result of these experiments Mendel came to 
 the conclusion with which his name is now closely 
 associated — ^that the male and female germ-cells of 
 hybrid plants contain each of them one or the other 
 member only of any pair of differentiating characters 
 exhibited by the parents, and that each member of 
 such a pair of characters is represented in an equal 
 number of germ-cells of both sexes. Furthermore, 
 separate pairs of differentiating characters (allelo- 
 morphs) conform to this law in complete independence 
 of one another. 
 
 Although in Mendel’s own experiments one member 
 of each pair of differentiating characters was always 
 dominant, dominance is by no means an universal 
 phenomenon when different varieties of plants are 
 crossed together. In a considerable number of in- 
 
193 
 
 MENDELISxM 
 
 stances the heterozygote is found to exhibit an appear- 
 ance which is more or less intermediate between the 
 types of character shown by the parents. It may be 
 almost exactly intermediate, or the appearance of the 
 cross-bred form may be nearer to that of one parent 
 than to that of the other. Dominance is clearly only 
 an extreme case of this latter phenomenon. The 
 term ‘ dominance ’ is applied to those cases in which 
 the appearance of the hybrid offspring is so near to 
 that of one parent as to be no longer clearly distin- 
 guishable from it. 
 
 In other cases, still of a simple Mendelian nature, 
 the appearance of the heterozygote may be quite 
 different from that of either parent homozygote. 
 An excellent example of this nature is afforded by the 
 Andalusian fowls studied by Messrs. Bateson and 
 Punnett. And this will also serve as our first illus- 
 tration of the application of these principles to animals 
 as well as to plants. The facts of the case are as 
 follows : 
 
 The ‘ blue ’ type of Andalusian appears to be a 
 heterozygote form which has never been got to breed 
 true. When a pair of these birds are mated together 
 only about half their offspring are like themselves, the 
 remainder being entirely different. Half these re- 
 maining ‘ wasters ’ are black, and half are nearly 
 white, showing only a few black ‘ splashes.’ If, now, 
 a pair of the black wasters are mated together, they 
 breed perfectly true, yielding only black offspring like 
 themselves. Similarly the splashed whites mated 
 together give rise to splashed white, and nothing else. 
 
HETEROZYGOTE FORMS 
 
 193 
 
 Both these forms, then, the black and the splashed 
 white, are clearly pure homozygotes. On mating a 
 black and a splashed white together, black-bearing 
 gametes and white-bearing gametes will meet together 
 in fertilization. In every case in which this form of 
 mating was carried out the resulting chicks were 
 invariably blue. 
 
 The gametes of the blue type of Andalusians, then, 
 according to our supposition, do not bear the blue 
 character at aU, Half of them contain the black and 
 half of them the splashed white allelomorph. Such 
 gametes, meeting by chance when a pair of blue An- 
 dalusians are mated together, give rise to the zygotes 
 — one black-black, two black-white, one white-white — 
 the black-whites being, of course, blue in appearance 
 as before. 
 
 Now, we may put this explanation to the test by a 
 very simple experiment — namely, by mating the sup- 
 posed heterozygote blues with the black and with 
 the splashed white types respectively. Both these 
 forms of mating were examined by Bateson and 
 Punnett, and the results were as follows : It was found 
 that blues crossed with blacks gave rise to equal 
 numbers of blue and of black offspring, whilst when 
 blues were crossed with splashed whites there ap- 
 peared blue and splashed white chicks in equal numbers. 
 And by a repetition of the process it could be shown 
 that the blues so obtained were heterozygotes as before. 
 Here, then, we have clear evidence that equal numbers 
 of the germ-cells produced by the blue birds bear the 
 pure black allelomorph and the pure splashed white 
 
 13 
 
194 
 
 MENDELISM 
 
 allelomorph respectively, since half the offspring ob- 
 tained on mating the blue birds with black are black, 
 and half the offspring obtained on mating them with 
 splashed white are splashed white. The following 
 scheme of inheritance illustrates the phenomena 
 described s 
 
 I (zygote) 
 
 
 blue heterozygote 
 
 
 I (gametes) 
 
 
 black and white 
 
 1 
 
 
 2 (zygote) 
 
 1 1 
 
 1 black-black 2 black-white 
 
 1 
 
 I white-white 
 
 1 
 
 (blue heterozygote) 
 
 1 
 
 3 (gametes) 
 
 black 
 
 black 
 
 white 
 
 black 
 
 1 
 
 white 
 
 1 
 
 white 
 
 3 (zygote) 
 
 1 
 
 black 
 
 1 1 1. I 
 
 I black 2 blue i white white 
 
 3 (gametes) 
 
 black 
 
 1 
 
 / white 
 
 black 
 
 ’ 1 
 
 \white 
 
 4 (zygote) 
 
 
 1 
 
 all blue heterozygotes 
 
 
 A case which is closely similar to that of the An- 
 dalusian fowl is afforded by the cross between Primula 
 sinensis and Primula stellata. 
 
 P. sinensis crossed with P. stellata gives rise to a 
 type which is different from either parent, being in 
 some respects intermediate between the two. The 
 hybrid is so distinct that a special name has been given 
 to it, and the new type is known as P. fyramiialis. 
 So far it has been found impossible to obtain a strain 
 of P. Pyramidalis which will breed true. Oh self- 
 fertilization the offspring are found to show the types 
 
Fig. 15. — Primula Sinensis crossed with P. Stellata. 
 
 Above, the parents. In the middle, the heterozygote offspring 
 — P. pyramidalis. Below, the result of self-pollinating P. pyramidalis : 
 I. P. sinensis ; 2, P. pyramidalis : i. P, stellata. 
 
 [To face p. 194* 
 
THE LIBRARY 
 OF THE 
 
 UHIYERSITY OF ILLIHOIS 
 
MENDELISM 
 
 195 
 
 of P. sinensis, P. fyramidalis, and P. stellata in the 
 ratio of I : 2 : I. 
 
 Cases like the above illustrate the essential part of 
 Mendel’s law even better than those in which domi- 
 nance is present, the characteristic proportion of one 
 of each homozygote type to two .of the heterozygote 
 being at once recognisable in such a case without the 
 necessity for further breeding ; whereas, in cases where 
 there is dominance, further study is necessary in order 
 to distinguish, among the individuals of dominant 
 appearance, those which are pure dominant and those 
 which are heterozygous in constitution. 
 
 In concluding our account of the simpler forms of 
 Mendelian phenomena we may consider one further 
 point with regard to the nature of the two allelomorphs 
 making up any particular pair. In what is probably 
 a majority of the cases hitherto examined the dominant 
 and recessive allelomorph seem to represent respec- 
 tively the presence and absence of something. Thus 
 the dominance of colour to absence of colour, or white- 
 ness, is a very frequent phenomenon. And in some of 
 the more complex cases to be described in the next 
 chapter we shall find the presence and absence of a 
 particular factor very often behaving as a pair of Men- 
 delian allelomorphs. The question arises as to how 
 far this conception should be extended. It seems, for 
 instance, somewhat far-fetched to speak of dwarfness 
 as being simply determined by the absence of the factor 
 for tallness, though it is not impossible that this may 
 be the correct way of looking at the facts. Be this as 
 it may, it is to be remembered that a Mendelian pair 
 
 13—2 
 
196 
 
 MENDELISM 
 
 often represents the presence and absence respectively 
 of a particular feature. 
 
 BIBLIOGRAPHY. 
 
 Mendel, G. J. : Versuche ueber Pflanzenhybriden, 1865. 
 
 „ Ueber einige aus kunstlicher Befruchtung 
 
 gewonnen Hieracium-Bastarde, 1869. 
 PuNNETT, R. C.: Mendelism, fifth edition, 1919. 
 
 Bateson, W. : Mendel’s Principles of Heredity, 1902 and 1909. 
 „ Address to the Zoological Section, British 
 
 Association, 1904. 
 
 Lock, R. H. : In Annate of the Royal Botanic Gardens 
 Peradeniya, 1904. 
 
CHAPTER VIII 
 
 MENDELISM {continued) 
 
 Mendel’s law, as stated in the preceding chapter, 
 has already been found to hold good in a very large 
 number of cases — cases in which all kinds of characters 
 are concerned, belonging to many different species of 
 animals and plants. In certain instances, however, 
 complications arise, and these may be treated of in 
 two main sections. 
 
 The first kind of complication arises from the pheno- 
 menon known as coupling. The essence of this pheno- 
 menon consists in the existence of some kind of affinity 
 occurring in the same individual between allelomorphs 
 which belong to distinct pairs. In consequence of such 
 an affinity exceptions are found to the rule that sepa- 
 rate pairs of allelomorphs segregate independently. 
 
 The closeness of the connection between the char- 
 acters concerned shows a series of gradations in dif- 
 ferent cases. In the simplest cases of all, what are 
 loosely spoken of as separate characters are found on 
 closer examination to be only different aspects of one 
 and the same characteristic feature. These cases, then, 
 offer no real exception to the rule, for only one pair of 
 allelomorphs is actually concerned. As an example, we 
 may take the case of the wrinkled sugary type of maize 
 already contrasted with the smooth starchy variety. 
 The essential difference between the two kinds depends 
 
 upon the fact that in the former the reserve product 
 
 197 
 
MENDELISM 
 
 198 
 
 laid down in the endosperm is different, being largely 
 of a sugary nature instead of being starchy. With 
 this circumstance is associated the presence of a larger 
 proportion of water in the unripe grain. And the 
 result of this is that, when the grain dries, its surface 
 falls into folds. The sugary nature of the grains also 
 causes them to take on a more hyaline or semi- 
 transparent appearance than the grains of the starchy 
 variety. All these characters, if they can be so called, 
 behave on crossing as a single Mendelian allelomorph, 
 and are doubtless represented in the germ cells by a 
 single substantive representative. 
 
 A simple example of what may probably be regarded 
 as a real case of coupling is afforded by certain colour 
 characters exhibited by pea-plants. In these plants 
 coloured flowers, a red or purple colouration in the 
 axils of the leaves, and a marked pigmentation of the 
 testas, or seed-coats, are always associated together on 
 the same plants ; so that, if we find a plant which has 
 green leaf axils, we may be sure that its flowers will 
 be white, and the testas of its seeds only slightly pig- 
 mented. On crossing plants bearing coloured axils, 
 coloured flowers, and pigmented testas, on the one 
 hand, with plants bearing green axils, white flowers, 
 and unpigmented seed-coats, on the other, the two sets 
 of characters are found to behave as a simple pair of 
 allelomorphs, and the simultaneous appearance of 
 colour in these different situations doubtless depends 
 upon the presence of a particular pigment in the plant 
 which exhibits it. Nevertheless, we can scarcely 
 fail to look upon these three separate manifestations 
 of the pigment as representing distinct characters, and 
 
COUPLING 
 
 199 
 
 this being so, we suppose their germinal representa- 
 tives to be coupled together in such a way that they 
 remain associated at the time when, during the forma- 
 tion of the germ-cells of the heterozygote, other allelo- 
 morphs become independently segregated. 
 
 And this way of looking at the facts is further 
 justified by the behaviour of the characters in ques- 
 tion in another species of plant. For in the sweet pea 
 it is possible for the coupling between these characters 
 to be broken down, so that a plant which exhibits 
 green leaf axils may, under certain circumstances, bear 
 coloured flowers. In such a plant the leaf-axU-colour 
 and the flower-colour must clearly be represented by 
 independent allelomorphs. 
 
 In other cases, again, there may be coupling between 
 characters which have no obvious relation to one 
 another at all. In illustration we may take the case 
 of a cross between two strains of peas, one of which had 
 white flowers and opened its buds several days earlier 
 than the second, the blossoms of which were purple. 
 
 The Fj plants (with purple blossoms) came into flower 
 at a period intermediate between those of the parents. 
 In Fj 506 plants were grown successfully. Some of 
 these flowered as early as the white parent, and others 
 as late as the purple parent ; but the majority of the 
 plants ranged between these two extremes, so that it 
 was impossible to rank the individuals into definite 
 classes in respect of so indefinite a character as time 
 of flowering. On making a perfectly arbitrary division, 
 however, it was found that 175 purple and 104 white 
 plants were in flower on a certain day, and that 208 
 purple and 19 white plants did not open their buds 
 
200 
 
 MENDELISM 
 
 until afterwards. There is, therefore, clearly some 
 coupling between the presence of white blossoms and 
 early flowering on the one hand, and between lateness 
 and purple flowers on the other. Two characters 
 more diverse than colour of the flowers and time of 
 flowering could at first sight scarcely be imagined. 
 
 In this last example the two characters concerned 
 do not appear to be completely, but only partially 
 coupled. This phenomenon of partial gametic coupling 
 was discovered by Bateson and Punnett in the Sweet 
 Pea, and has since been observed in a number of plants 
 and in several animals. [In animals, by far the most 
 completely known case is that of the Fruit-fly {Droso- 
 phila), investigated by T. H. Morgan and his school, 
 see p. 270.] In the Sweet Pea, one of the first cases 
 to be investigated was concerned with the shape of the 
 pollen grains, whether oval or round, and the colour 
 of the flowers, whether blue or red. 
 
 When blue-oval is crossed with red-round, the 
 Fi plants are all blue-oval. But in F2, instead of 
 getting 9 blue-oval, 3 blue-round, 3 red-oval, and 
 I red-round, the numbers of the different kinds of 
 plants obtained closely approached the following 
 proportion — 177 : 15 : 15 : 49. 
 
 Such a series would be produced if the allelomorphs 
 concerned were associated in the gametes in the 
 following proportion: 7 blue-oval, i blue-round, 
 I red-oval, 7 red-round, as may easily be verified by 
 multiplication. Enormous numbers of plants must 
 naturally be examined before it can be asserted that 
 the series actually chosen is really the correct one ; in 
 fact, mere statistics are hardly capable of proving so 
 
COUPLING 
 
 201 
 
 complicated a proportion as this in the absence of 
 independent considerations. 
 
 [In the case of other characters, the coupling between 
 members of distinct pairs was closer, and suggested 
 gametic ratios approaching 15:1:1:15 or 31 : i : i : 31, 
 so that Bateson and Punnett have supposed that the 
 sum of the series is always an exact power of 2 
 (8, 16, 64, etc.), and that the phenomenon is due to a 
 differential division of a germ-cell followed by more 
 divisions of one of the daughter cells than of the other. 
 If, for example, a primitive germ-cell containing all 
 four factors — those for oval and round pollen, blue and 
 red flowers^ — divides twice in such a way that the four 
 cells produced contain respectively the factors blue 
 and oval, blue and round, red and oval, red and round, 
 and then the cells bearing blue-oval and red-round 
 divide, while the others do not, the observed excess of 
 germ-cells bearing these characters will be accounted 
 for. But Professor Morgan in Drosophila does not find 
 these ratios of 7 : i, 15 : i, etc., and has offered an en- 
 tirely different explanation of the facts which will be 
 mentioned in the chapter on Cytology (Chapter IX.). 
 
 It should be noticed that characters which are 
 coupled are always introduced into the double hetero- 
 zygote from the same parent. If, in the Sweet Pea, 
 one parent is blue-oval, the other red-round, the blue 
 flower will be coupled with the oval pollen and the red 
 flow'er with the round pollen in the grandchildren. 
 But if such characters are introduced from different 
 parents, the converse phenomenon of gametic repulsion 
 is seen. If a blue-flowered round-pollened plant is 
 
203 
 
 MENDELISM 
 
 crossed with a red oval-pollened plant, the blue, 
 instead of being coupled with the oval pollen in the 
 gametes of the heterozygote, is ‘ repelled ’ by it, and 
 the great majority of the gametes will bear the blue- 
 round or red-oval characters. This is obviously only 
 another form of coupling, in which a dominant char- 
 acter is coupled with a recessive instead of two 
 dominants being coupled together, and the general 
 law of coupling (or as the Americans call it, linkage) 
 may be expressed thus: In the case of characters 
 which show gametic coupling, those factors which are 
 associated together in the homozygotes used as parents 
 tend to be associated together in the gametes produced 
 by the double (or multiple) heterozygote offspring. 
 The question of coupling of Mendelian characters with 
 a sex-determining factor (‘ sex-hmited inheritance ’) 
 will be mentioned in Chapter IX.] 
 
 The second class of complications that we have to 
 deal with — although the term complication may be 
 to a certain extent justified in connection with it — 
 does not involve any exception to Mendel’s law of 
 segregation. The phenomenon of so-called reversion 
 on crossing has long been familiar to biologists. Its 
 meaning, however, was totally obscure, and even the 
 Mendelian was at first unable to offer any explanation. 
 The phenomenon consists in the appearance, in the 
 offspring of a cross, of a character which was not 
 visibly present in either parent, and in many cases this 
 character can properly be regarded as ancestral — it is 
 a character which has been lost by both parents in 
 the course of their divergent evolution from a common 
 
MASKED CHARACTERS 
 
 203 
 
 primitive form. Now, these cases differ entirely from 
 those of the appearance of a heterozygote form on 
 crossing, such as are due to the combined action of the 
 two parental allelomorphs in the cross-bred offspring, 
 because in true cases of reversion a certain proportion 
 of the reversionary individuals of Fg are found to breed 
 true, which a simple heterozygote will never do. 
 
 It has been found that the essential part of this 
 phenomenon of reversion on crossing consists in the 
 existence in the parents of certain hereditary factors — 
 allelomorphs, in fact — ^which, although by themselves 
 invisible, yet, when combined in cross-breeding with 
 certain other allelomorphs, belonging to independent 
 pairs, lead to the appearance of new visible characters. 
 
 The term reversion cannot properly be applied to 
 these phenomena as a class, because, in the first place, 
 characters may arise in this way which cannot be 
 regarded as ancestral, and, secondly, because reversions 
 may take place in other ways ; for example, the 
 reappearance of a simple recessive character would 
 legitimately be ranked among reversions. The best 
 general name for the class of phenomena we are about 
 to describe is perhaps masking of characters, or cryfto- 
 merism, the latter being the term employed by Tscher- 
 mak, who was the first to describe these phenomena 
 in connection with Mendelian ratios. 
 
 In the simpler cases an invisible or masked factor 
 derived from one parent, on becoming associated with 
 a different factor born by the other parent, and already 
 visibly represented among the external features of this 
 second parent, maJces itself apparent among the visible 
 characteristics of the heterozygote. In such a case 
 
204 
 
 MENDELISM 
 
 the characteristic appearance exhibited by the hetero- 
 zygote may subsequently become permanent, owing 
 to the building up of a type which is a homozygote in 
 respect of both the necessary factors. 
 
 This may be made clearer by a definite illustration. 
 
 A pea-plant characterized by the presence of a greyish 
 or brownish testa to its seeds (grey) was crossed with 
 a plant having nearly colourless testas (white). The 
 testas of the Fj plants were marked with bright purple 
 dots on a grey ground (purplel. These hybrid plants 
 were self-pollinated, and in Eg the three types appeared 
 in the following proportions : 9 pm pie, 3 grey, 4 white. 
 What is the meaning of this ratio ? In order to 
 complete the ordinary expectation for a simple Men- 
 delian case in which two pairs of allelomorphs are 
 concerned (di-hybridism) we must write down the 
 following expression : 
 
 ( purple \ / no purple \ / purple \ . j / no purple/ 
 
 " \ grey / ' ^ I grey / ‘ \ no grey / ‘ \ no grey J 
 
 But it would seem that the purple character cannot 
 appear when the grey colour, or some factor con- 
 stantly associated with this colour, is absent, as is 
 the case in the original white parent from which the 
 factor for purple spots was derived. Consequently, the 
 
 three / 1 plants are indistinguishable from the 
 
 (no greyj 
 
 /no P'lrplel Qj. vyhites, and we thus arrive at 
 
 I no grey J 
 
 the result which was described as being the one actually 
 obtained — namely, 9 purple : 3 grey ; 4 white. 
 
 In other respects this example is precisely like the 
 
MASKED CHARACTERS 
 
 205 
 
 case of two pairs of allelomorphs described on p. 187. 
 We may write A foi presence of grey pigment, a for 
 absence of grey pigment, B for presence of purple, and 
 b for its absence. Then the original cross was of the 
 form Ab xaB, from which AaBb resulted in F,. And 
 the visible characters of the types which appeared 
 in Fj would be represented by gAB ^Ab + {3aB 
 lab). On referring to the account given on p. 187 it 
 will be seen that one in nine of the purple plants is of 
 the constitution ABAB, and may be expected to 
 breed true. 
 
 A precisely similar result may be obtained in Fj in 
 cases where there is no reversion in Fj. In the 
 following example a white pea, which did not contain 
 tire masked purple factor, was crossed with a ' maple- 
 seeded ’ pea. The characteristic feature of maple is a 
 marbling of brown spots on a grey ground colour. In 
 Fj the marbling was dominant, and the seeds resembled 
 the maple parent. 
 
 In Fj there appeared 9 maple : 3 grey : 4 white — 
 i.e., the same ratio as in the previous case, this time 
 without reversion. This ratio is brought about by the 
 simple combinations of two pairs of allelomorphs A-a, 
 and C-c, C being unable to manifest itself unless A is 
 present in the same zygote. As a matter of fact, in 
 this particular case C does sometimes just manage 
 to appear in the absence of A, the result being a 
 white seed with a sort of faint ‘ ghost ’ of a maple 
 marking. 
 
 When a strain bearing both maple marking and 
 purple spots is crossed with a white in which neither 
 of these factors is latent, we cam easily calculate tire 
 
206 
 
 MENDELISM 
 
 ratio to be expected in Fa by using the formula 
 (A+ 2 Aa + a) (B + 2 Bb + b) (C + 2 Cc + c). The result 
 works out as follows (writing m for maple, p for purple, 
 and g for grey):: ^jmpg, gmg, gpg, 3g, (gmp, 3m, 
 3p, iw). Since g is absent from all the members of 
 the series enclosed in the bracket, these appear white, 
 or nearly so, the total number of whites being thus 16. 
 And the numbers obtained in an actual experiment 
 accorded closely with the expected ratio 27 ; 9 : g : 3 : 16. 
 
 Among the sixteen whites, some wiU be bearing the 
 factors for m and p, others that for p only, others that 
 for m only, whilst one in sixteen will contain neither of 
 these factors. Until such invisible differences between 
 the different white plants are actually proved to be 
 present the whole account so far given will remain more 
 or less hypothetical. The proof is obtainable by cross- 
 ing the different whites with a pure grey strain. The 
 grey factor being thus introduced, the whites which 
 contain a /> or an w factor will exhibit the same in 
 their offspring. A number of the whites obtained in 
 Fa and in later generations were actually crossed with 
 the same grey-seeded plant. Some of the offspring 
 showed both the maple and the purple character, 
 others the maple without the purple, others the purple 
 without the maple, and others, again, showed neither ; 
 the seeds of these last being exactly like those of the 
 grey parent owing to simple dominance of the grey 
 allelomorph over white. 
 
 The first example of this kind of phenomenon to be 
 observed in the case of animals was one described by the 
 French zoologist Cu6not [in mice]. Cu6not’s original 
 account has had to be somewhat modified in view 
 
EPISTATIC CHARACTERS 207 
 
 of more recent work by Miss Durham, and her 
 account of the facts runs as follows. For the sake of 
 simplicity we shall deal in the first instance with 
 only four types of colour [in mice]— the ‘agouti’ or 
 wild grey colour, black, * chocolate,’ and albino. The 
 behaviour of these colours in heredity can be described 
 in terms of three pairs of allelomorphs : 
 
 Gg : The presence and absence of the factor which 
 gives the ‘ agouti ’ or grey pattern in the hairs. 
 
 Bb : Presence and absence of the black determiner. 
 
 Cc : Presence and absence of colour. 
 
 Where C is present without G or B, the colour is 
 chocolate, the proper formula for such an animal 
 being CCggbb or Ccggbb. Black mice may be CCggBB, 
 etc., and grey mice CCGGBB, etc. 
 
 All albino mice are to be represented as those from 
 which C — i.e., the chocolate colour — is absent ; but 
 either G or B, or both, may be present (but masked) 
 in an albino individual. 
 
 When B and C are both present, the colour is black, 
 and not chocolate. We cannot, however, speak of 
 black as being dominant to chocolate, since these two 
 factors belong to independent allelomorphic pairs. A 
 new term is therefore required for this relationship, 
 and also for the relationship between grey and black. 
 Bateson’s suggestion for the required terminology may 
 be given in his own words : ‘ We can, perhaps, best 
 express the relation between the grey and the black 
 by the use of the metaphor " higher and lower,” and 
 I therefore suggest the term epistatic as applicable to 
 characters which have to be, as it were, lifted oS in 
 order to allow the lower or hypostatic character to 
 
2o8 
 
 MENDELISM 
 
 appear.’ Thus grey is epistatic to black, and black 
 is epistatic to chocolate. 
 
 A curious phenomenon appears in the case of yellow 
 mice, which must be briefly mentioned here on account 
 of its bearing upon a subject discussed in the next 
 chapter. Yellow appears to be epistatic to grey as 
 well as to black, but yellow mice, so far as the evidence 
 goes, are always heterozygous. Cu6not’s experiment 
 to demonstrate this fact was as follows : 
 
 When YyGGCC is crossed with yyGGCC, equal 
 numbers of yellow and, grey offspring are to be ex- 
 pected, since G is hypostatic to Y. In various crosses 
 of this nature Cuenot actually obtained 177 yellows 
 and 178 greys, from which we may deduce that the 
 heterozygote yellow was giving off the expected pro- 
 portion of gametes bearing the yellow character {i.e., 
 50 per cent.). 
 
 When such heterozygous yellows are bred together, 
 the expected result would be as follows ; 
 
 YyGGCC X YyGGCC - YYGGCC + 2 YyGGCC + yyGGCC 
 
 ' . ' 
 
 3 yellow I grey 
 
 Eighty-one yellow mice were actually obtained in 
 this way. Among them some twenty-seven would 
 naturally be expected to be pure dominant, and to give 
 yellow only when crossed with black or grey indi- 
 viduals. To Cuenot’s astonishment, he found on making 
 the necessary crosses that every one of these eighty-one 
 yellows gave some black or grey among its offspring ; 
 not one of them was a pure homozygous yellow. 
 
 The facts are explicable in one of two ways. On the 
 
YELLOW MICE 
 
 209 
 
 earlier hypothesis there was supposed to be some 
 obstacle in the way of the fertilization of a yellow- 
 bearing egg by a spermatozoon bearing the same 
 character. In this case, since spermatozoa are in 
 great numerical excess as compared with eggs, it is 
 still possible for every Y-bearing egg to be fertilized 
 by a spermatozoon lacking Y, as well as half the non- 
 yellow eggs by Y-bearing sperms, giving a ratio of 
 3 ; I in jF 2 from yellow by yellow. On the other 
 hand, it is possible that pure yellow zygotes are formed 
 in fertilization, but for some unknown cause are incap- 
 able of development. In this case a third of the yellow 
 progeny would be wanting, and the expected ratio would 
 be 2 : I. Cu6not’s figures led to the belief that the first 
 hypothesis was the correct one, but in further experi- 
 ments by Castle and by Miss Durham the ratio was 
 found to approach 2 : i in a majority of cases.* 
 
 We have still to describe a case in which two latent 
 factors, one derived from each parent, give rise, by 
 their simultaneous presence in the zygote produced, 
 to the appearance of an entirely new character. The 
 following example is the first one of the kind to be 
 completely elucidated, and is one of those studied by 
 Messrs. Bateson and Punnett and Miss Saunders. 
 
 The white-flowered variety of sweet-pea known as 
 Emily Henderson was found to exist in two forms, 
 only to be distinguished from one another by the 
 shape of the pollen grains which they produced. In 
 one of the two the shape of the pollen is elliptical 
 
 * [It is now definitely established that homozygous yellow zygotes 
 are formed, but die during embryonic life. Disintegrating embryos 
 have been found in the uterus.] 
 
 14 
 
210 
 
 MENDELISM 
 
 (long pollen), in the other it is approximately spherical 
 (round pollen). Sweet-peas normally undergo self- 
 poUination, so that the two types naturally remain 
 distinct. Let us see what happened when the long- 
 and the round-pollened forms were crossed together. 
 
 The cross-bred plants (Fj) had coloured flowers — ■ 
 flowers of the old - fas'hioned purple type known to 
 florists as Purple Invincible, which is characterized by 
 a purple standard and blue wings. The pollen pro- 
 duced by these plants was of the long type. Thus as 
 regards the shape of the pollen grains there was simple 
 dominance. But the union of two white-flowered 
 types has given rise to a series of plants all possessing 
 a definite colour character — purple with blue wings. 
 This character is very probably the same as that 
 exhibited by the common ancestor of all our cultivated 
 sweet-peas. Here, then, we seem to have a clear case 
 of reversion to the ancestral type on crossing. We 
 shall find that the Mendelian principles will enable us 
 to arrive at a dear conception of the mechanism of 
 this process. 
 
 The cross-bred plants were self-pollinated, and in Fj 
 the following types made their appearance in approxi- 
 mately the proportions given • 
 
 Purple Invincible 
 
 
 
 3 ' 
 
 Picotee ••• ••• 
 
 27J 
 
 1 1 1 ) 
 
 
 Painted Lady 
 
 27] 
 
 
 I , 
 
 Tinged white ... 
 
 White 
 
 9] 
 
 112 
 
 1 u 
 
 
LATENT CHARACTERS 
 
 211 
 
 Painted Lady is a well-known colour type which is 
 characterized by the presence of a red standard and 
 white wings. Picotee and tinged white are also forms 
 well known to the sweet-pea fancy. They appear to 
 be diluted forms of the purple and Painted Lady types 
 respectively, their appearance depending upon the 
 presence of a definite diluting factor in addition to the 
 factor for the colour in question, or perhaps more 
 properly upon the absence of the proper strengthening 
 factor which converts Picotee into purple, and tinged 
 white into Painted Lady. 
 
 The following explanation of the result so far 
 described has now been well established by further 
 experiment. In the first place, we may consider all the 
 coloured forms together as a single group opposed to 
 white. It is now clear that the coloured type of Fj is 
 due to the meeting together of two factors, one of them 
 borne by one white parent and the other by the second 
 and it is necessary for both these factors to be simul- 
 taneously present in order that any colour may make 
 its appearance. We may call these two factors C and 
 R, denoting the absence of either by c and r respectively. 
 By the simple Mendelian behaviour of these two pairs 
 of factors C-c and R-r, the ratio of nine coloured 
 plants to seven white appearing in Fa is readily 
 explicable, and the way in which this happens is shown 
 in the diagram (Fig. i6). 
 
 To explain the presence of the four different types of 
 coloured plants which make their appearance in Fg, 
 two further pairs of allelomorphs are called in. The 
 
 dominant member (B) of one of these, when present 
 
 14 — 3 
 
212 
 
 MENDELISM 
 
 in combination with C and R, produces the purple 
 or Picotee colour (blue), whilst its absence (6) in pre- 
 sence of C and R is accompanied by the appearance 
 of the red colours — Painted Lady and tinged white. 
 
 Purple Invincible and Painted Lady are regarded as 
 intensified forms of Picotee and tinged white respec- 
 tively. The presence of the second factor {T) is attended 
 by the development of the full colours purple and 
 
 CR 
 
 cR 
 
 Cr 
 
 c r 
 
 The shaded squares represent coloured plants, the blank squares 
 white plants. 
 
 Painted Lady ; its absence it) causes the appearance of 
 the diluted forms Picotee and tinged white. 
 
 B and T may be present when either C or i? or both 
 are absent ; the resulting plant has then white flowers. 
 And it is interesting to notice that the ultimate recessive 
 white, containing c r h t, occurs only once among 256 
 individuals of F^. The whole apparently complex system 
 of floral colours is thus explained by the simple Men- 
 delian behaviour of four separate pairs of allelomorphs. 
 Bateson and his collaborators have, therefore, provided 
 
 §miM 
 
 rI 
 
 
 pa 
 
 llil 
 
 0 0 
 
 i 
 
 fm. 
 
 cr 
 
 cR 
 
 
 Cr 
 
 C r 
 
 cr 
 
 Cr 
 
 llpr 
 
 Cr 
 
 cr 
 
 cr 
 
 cr 
 
 Fig 16. 
 
MENDELIAN CHARACTERS 
 
 213 
 
 a complete account of the phenomenon of reversion 
 on crossing, an account which has already been demon- 
 strated to hold good in other instances besides that 
 of the sweet-pea. The facts are expressed in the 
 following manner by their discoverers r ‘ “ Reversion ” 
 is thus seen to be a simple and orderly phenomenon 
 due to the meeting of factors belonging to distinct 
 though complementary allelomorphic pairs, which at 
 some moment in the phylogeny of the varieties have 
 each lost their complement.’* 
 
 Dr. G. H. Shull has classified the different known 
 types of latency in the following manner : 
 
 1. Latency due to separation. In this form of 
 latency an allelomorph, when acting alone, has no 
 external manifestation, and is only rendered visible 
 by combination with another allelomorph belonging 
 to a distinct pair. In this way such ratios as 9 : 3 : 4 
 or 9 : 7 may arise. We have already observed an 
 example of this phenomenon in the case of the two 
 factors required to produce colour in the sweet-pea. 
 
 2. Rare cases have been described of latency due 
 to combination. This, instead of being due to separa- 
 tion, is the result of a union in the same zygote of two 
 dominant allelomorphs, either of which alone will 
 produce a manifest character. When both allelo- 
 morphs are present the character fails to appear. 
 
 3. Latency due to hypostasis, or the masking of 
 one dominant character by another character also 
 dominant and epistatic to the masked character, has 
 already been described in the case of the coat colours 
 
 ♦ Proceedings of the Royal Society, B. vol. 77, p. 238. 
 
214 MENDELISM 
 
 of mice. The simplest typical ratio thus produced is 
 
 12 : 3 : I. 
 
 4 . The last type of latency described by Dr. Shull 
 is of a totally different kind. It is represented by the 
 phenomenon to which the term ‘ latency ' wa's origin- 
 ally applied by de Vries. This latency consists in the 
 disappearance of certain characters under the influence 
 of poor nutrition, or other changes of conditions. 
 Examples of this phenomenon described in the chapter 
 on Mutation were the submerged and floating leaves 
 of the water-ranunculus, and the red and white colour 
 of the flowers of a species of primula. 
 
 We may now proceed to pass in rapid review a 
 selection of the more remarkable instances of Mendel ian 
 inheritance which have been so far demonstrated. 
 
 The ease with which characteristics of colour can be 
 distinguished and defined has naturally led to a good 
 deal of attention being paid to the phenomena of their 
 inheritance. In this way many cases of simple domi- 
 nance have been discovered in plants and in animals, 
 as well as several examples of reversion in Fj, followed 
 in both cases by a Mendelian segregation of characters. 
 
 Thus the colours of many flowers afford perfectly 
 simple phenomena, whilst other cases, like the sweet- 
 peas and the closely similar case of stocks studied by 
 Miss Saunders, have required long and arduous ex- 
 periment for their elucidation. No case of this kind 
 hitherto examined has been definitely proved to be 
 non-Mendelian. 
 
 Colour characters which follow Mendel’s law have 
 been observed in mice, rats, rabbits, guinea-pigs. 
 
MENDELIAN CHARACTERS 
 
 215 
 
 pigeons, fowls, cats, and so on. In butterflies and 
 other insects, and even in snails, similar phenomena 
 have been described. The study of the larger domestic 
 animals awaits for the present the proper endowment 
 of these researches. When this takes place, the 
 inheritance of far more important characters than 
 colour will be adequately studied to the great profit of 
 all who are concerned in the breeding industry. Hurst 
 has already shown from an examination of the stud- 
 book that the bay and brown colours of thoroughbred 
 horses are Mendelian dominants to chestnut.* 
 
 Other characters of the most diverse kinds are also 
 similarly inherited. We have already referred to 
 structural characters in maize and in peas. Stature 
 is a character which is definitely inherited in many 
 plants. Among more subtle characters a similar mode 
 of transmission has been fomid in the case of differences 
 in chemical composition, and in that of immunity 
 from and susceptibility to the attacks of certain 
 diseases. The thrum-eyed condition of the primrose 
 has been shown by Bateson and Gregory to be a 
 Mendelian dominant to the pin-eyed condition, so that 
 we have here a partial solution of a problem which is 
 rendered specially interesting from the fact that it 
 baffled Darwin. 
 
 A study of numerous pedigrees has enabled Bateson 
 and others to show that in the case of the human race 
 certain congenital diseases are simply transmitted from 
 parent to offspring in accordance with Mendel’s law. 
 
 * [Other colour characters have now been shown to have 
 Mendelian inheritance in horses, cattle, sheep, etc.] 
 
2i6 
 
 MENDELISM 
 
 How far the influence of the Mendelian principles 
 may extend we do not yet know. But it is certain that 
 very few, if any, cases have so far been discovered in 
 which differentiating characters do not behave in this 
 way when the types which exhibit them are crossed 
 together. Experiments have now been made upon a 
 great variety of plants and animals, involving a con- 
 siderable diversity of kinds of characters. Nevertheless 
 it is scarcely possible to cite a case in which it is 
 definitely and certainly known that Mendel’s law, 
 subject to the modifications already described, does 
 not hold good. Cases of various kinds are, indeed, 
 recorded, but these records are derived from experi- 
 ments either carried out before the bearings of the 
 Mendelian phenomena were at all fully appreciated, or 
 — ^and this is the most frequent case — without any 
 knowledge at all of Mendci^’s discovery. 
 
 Thus a considerable number of cases were formerly 
 described in which the first cross or heterozygote of Fj 
 bred true instead of segregating in F*. There is some 
 doubt whether any case of this kind will really stand 
 criticism ; Millardet’s case, for example, which was 
 mentioned at the end of the last chapter but one, has 
 never been confirmed. It is quite certain that among 
 all the numerous crosses studied during the last six 
 years no example of the kind has been substantiated. 
 The most recent cases to be described of a first cross 
 breeding true are those of de Vries, and at these we 
 are bound to pause, because de Vries is surpassed by 
 no recent observer in weight of authority. Neverthe- 
 less, de Vries’ cases are of so complex a kind that we 
 
MENDELIAN CHARACTERS 217 
 
 have some hesitation in accepting them without further 
 study. 
 
 It must be admitted that the suggestion that Men- 
 delian segregation may be a universal phenomenon 
 accompanying the formation of the germ-cells of all 
 animals and plants is one which will be disputed by 
 many biologists. There is ample evidence, they will 
 say, of first crosses breeding true in the case of 
 numerous plant and animal hybrids. But there are 
 seyeral points to be considered before we can accept 
 the mere fact of a uniform progeny as sufficient evidence 
 that there was nothing in the nature of segregation 
 amongst the gametes of the hybrid. 
 
 To prove a negative is proverbially difficult, and 
 the attempt to show that hybrid characters exist 
 which do not segregate in the germ cells of the hybrid 
 is no exception to this rule. In favour of the opposite 
 contention the following considerations may be 
 alleged : 
 
 The characters for which non-segregation has been 
 asserted are generally complex and difficult to define. 
 Such characters, for instance, are the general habit 
 of a plant or the general shape of a flower. Characters 
 such as these may reasonably be supposed to owe 
 their appearance to the interaction of a considerable 
 number of independent allelomorphs. 
 
 Where two such factors are concerned, in cases 
 where the heterozygote form is intermediate, we 
 should expect one out of sixteen Fa individuals to 
 resemble each parent form, the remaining fourteen 
 plants being more or less intermediate. 
 
2i8 
 
 MENDELISM 
 
 With three such factors we should expect only one 
 of each parent type to appear among sixty-four 
 individuals. 
 
 There seems to be no reason for doubting that the 
 total number of visible factors, even, which go to 
 make up the total shape of a flower may be consider- 
 ably greater .than this ; and the number of invisible 
 allelomorphs upon which these depend may be pro- 
 portionately more numerous still. In such a case, in 
 an experiment where the total number of offspring 
 grown was limited, types like the original parents 
 might never be seen, simply because the sample taken 
 was not sufficiently large. 
 
 Again, we have to consider the possibility of a 
 differential fertility among the various allelomorphic 
 combinations. This might lead to an intermediate 
 form, actually breeding true in spite of the fact that 
 segregation was going on in its germ - cells. The 
 possibility has to be borne in mind that among the 
 offspring of two widely different species those indi- 
 viduals which more nearly resemble their immediate 
 hybrid parent may have a better chance of survival 
 than the forms which have more characters in common 
 with a single pure grandparent. The only piece of evi- 
 dence we have bearing upon this point is the fact that we 
 know the hybrid form itself to be capable of surviving. 
 
 De Vries has published a very interesting observa- 
 tion, which ought to be mentioned in this connec- 
 tion. It appears that two forms of (Enothera, which 
 originated in a cross, and are distinguished as 
 Lcsia and Velutina respectively, possess female gametes 
 
NORMAL FERTILIZATION 
 
 219 
 
 half of which can be shown to bear the tall and half 
 the dwarf character. All the functional pollen grains 
 of L^ta bear the tall character, and all the functional 
 pollen grains of Velutina bear the dwarf character. 
 De Vries does not offer the obvious suggestion that 
 half of the total number of pollen grains in each case 
 are impotent.* If this suggestion should prove to 
 represent the truth, OBnothera Iceta would afford a 
 case of a hybrid which breeds true in spite of the fact 
 that typical segregation is taking place among its 
 germ-cells. In the light of this discovery it is clear 
 that non-segregation cannot properly be asserted in 
 any given case until the hybrid has been crossed with 
 each of the parent forms on a considerable scale. 
 
 We may now turn for a brief space to some of the 
 cases in which we have as yet no certain knowledge of 
 the manner in which inheritance proceeds. 
 
 The most obvious extension of Mendel’s law to 
 processes where it cannot be directly shown to hold 
 good is to suppose that the same rule applies to cases 
 of normal fertilization as to hybrid fertilizations. We 
 should then picture the former process as taking 
 place in somewhat the following way. Every visible 
 character of the individual which can be separately 
 distinguished, and which on cross-breeding would be 
 inherited on ordinary Mendelian lines, must be repre- 
 sented in the gametes by a definite factor of some kind, 
 
 *[In an analogous case in Drosophila it seems certain that the 
 explanation is of this kind. See H. J. Muller ‘Genetic Variability, 
 Twin Hybrids and Constant Hybrids, in a case of balanced lethal 
 factors.’ Genetics, vol. iii., 1918, p. 422-499.] 
 
220 
 
 MENDELISM 
 
 possibly by a definite substance or combination of sub- 
 stances. The pair of parental factors for a particular 
 character would combine on fertilization, and at the 
 formation of the gametes in the offspring its members 
 would separate as perfectly definite entities, to re- 
 combine when these gametes meet once more with their 
 corresponding mates. Such a definite segregation of 
 characters taking place within a pure strain would be 
 very difficult of absolute demonstration, but it is hard to 
 avoid the conclusion that this is a true deduction from 
 the facts observed when cross-breeding takes place. 
 Such a segregation would formerly have been thought a 
 very small assumption in comparison with that of the 
 segregation of pairs of allelomorphs of which no trace 
 is externally visible, and yet the latter assumption 
 has now been shown to be perfectly well established. 
 
 This idea of unit characters, capable of being inherited 
 independently of one another, is one of the most 
 important conceptions which has ever been introduced 
 into the science of biology, and the introduction of it 
 has followed as the direct result of Mendel’s work. It 
 is a conception which has led to a complete change in 
 our ideas of heredity, since we no longer look upon 
 the individual as a unit, but find ourselves compelled 
 to study separately the independent characters of 
 which the individual is built up. The idea of the 
 individual as a living mosaic — ^an idea put forward long 
 ago by Naudin with only a partial realization of its 
 significance — has thus returned to us. In this con- 
 nection a curious problem presents itself. What 
 would be left if we could imagine all the separable 
 
ESTABLISHMENT OF NEW SPECIES 221 
 
 characters of a living creature as having been taken 
 away ? Would there, or would there not, be any 
 residuum ? Upon this knotty point there is a disagree- 
 ment among authorities, and so we may be content 
 to leave it, since the question is hardly one which is 
 capable of a practical solution. 
 
 A phenomenon to which it is scarcely doubtful 
 that Mendelian principles apply is that of sex. In 
 the male and female sexes of the majority of 
 animals we have a very clear example of a pair of 
 definite differentiating characters. And the fact that 
 in the majority of forms the two sexes make their 
 appearance in nearly equal numbers, may be thought 
 to point clearly to the conclusion that the separation 
 of the sexes depends upon some quite simple gametic 
 process. Light has recently been thrown upon this 
 question from the side of the study of the minute 
 structure of the gametes, and we shall defer the further 
 discussion of the problem to the chapter which deals 
 with microscopic phenomena within the cell. 
 
 A proper understanding of Mendel’s law enables us 
 to escape certain theoretical difficulties which have 
 long been prominent in the minds of students of 
 evolution. Many evolutionists were accustomed to 
 argue that a new form suddenly arising in the midst of 
 an old-established species could not give rise to a new 
 and permanent variety or elementary species, because 
 it would immediately be ‘ swamped ' by intercrossing 
 with the parent species from which it was derived. 
 If, however, the character distinguishing the new 
 type is allelomorphic to the corresponding character, 
 
222 
 
 MENDELISM 
 
 or absence of a character, shown by the parent form 
 this difficulty disappears. For suppose as an extreme 
 case that the new type arises as a single individual 
 only, which is therefore compelled to mate with a 
 member of the original species. If the new character 
 is recessive it will disappear in the immediate offspring 
 of this cross. But half the genn-cells produced by 
 the cross-bred form will bear the new character pure 
 and undiluted. If any of these cross-breds mate to- 
 gether the new type wiU appear in a quarter of their 
 offspring. Even if all of them mate with members of 
 the original t5q)e, half the offspring of such matings 
 will be heterozygous, and sooner or later the hetero- 
 zygotes will be sure to mate with one another, and 
 give rise once more to the novel type of individuals. 
 If the new form has any structural or other advantage 
 over the old species, the former will tend to survive at 
 the expense of the parent type, and it may survive 
 if it is only equally w'ell fitted for the battle of life. In 
 the case of dominance of the mew form the same 
 process will take place, only it will be apparently more 
 rapid in the early stages because the cross-breds will 
 themselves exhibit the new character. In this case, even 
 if the new type has a very marked advantage over the 
 parent form, the process of completely supplanting the 
 latter will be delayed, because the old type of character 
 can survive concealed in heterozygote individuals. 
 
 Let us pause for a moment to sum up the novel ideas 
 which have so far been presented in this and the 
 preceding chapter. 
 
 We found in the first place that irom the point 
 
SUMMARY 
 
 223 
 
 of view of heredity we must look upon an animal or a 
 plant as a composite being, made up of a great number 
 of unit characters, each capable of separate description, 
 and all inherited independently of one another. 
 
 When a pair of nearly-related animals or plants 
 are mated together, when, in fact, like is bred with 
 like, and with still greater certainty in cases of self- 
 fertilization such as are not uncommon among plants, 
 every unit character borne by one gamete finds a 
 corresponding mate among the characters borne by the 
 second gamete. It naturally follows that a series of 
 characters similar to those of the parent or parents 
 make their appearance in the offspring. 
 
 When a pair of individuals belonging to distinct 
 varieties or races are mated together, the result is the 
 same in the case of the majority of characters exhibited 
 by each of them. For separate varieties of the same 
 species differ from one another in a small number of 
 units only, and organisms which differ in more than a 
 few unit characters refuse altogether to unite for the 
 production of offspring. From the study of the precise 
 behaviour of those characters in which a pair of 
 parental organisms differ, a flood of light has been 
 thrown upon the phenomena of inheritance. 
 
 We find, as a rule, that opposed to every differen- 
 tiating unit character of one parent there exists a 
 corresponding but different character in the other 
 parent. One parent may have smooth seeds and the 
 other wrinkled seeds, for example. Very frequently 
 the corresponding feature consists in the absence — or 
 failure to appear — of a particular character, as, for 
 
224 
 
 MENDELISM 
 
 instance, when the non-development of pigment leads 
 to the appearance of white flowers. 
 
 We can now realize how necessary it is, in order to 
 avoid hopeless confusion, to follow the behaviour of 
 each pair of characters in the offspring separately. 
 
 The result of the meeting between the two opposed 
 characters of the same pair we saw to be different in 
 different cases. There may arise in the offspring 
 (i) the appearance of a simple blend of the two parental 
 characters. Or (2) one character may be more or less 
 dominant over the other. Or (3) the combination of 
 the two parental characters in the offspring may give 
 rise to an appearance quite different from that of either 
 of them, very much in the same way as in chemistry 
 oxygen and hydrogen when combined give rise to water. 
 Or (4) we may get further complications in which un- 
 suspected characters, present in an invisible condition 
 in one or both parents, take a part, often giving rise 
 to the appearance of a supposed reversion. 
 
 The most important phenomenon of all, however, 
 is that which is found to occur at the formation of the 
 germ cells of the heterozgyote plant or animal. What- 
 ever the appearance of the hybrid form may have been, 
 at this stage in its history the determining factors for 
 each member of the pair of parental allelomorphs 
 reappear in their entirety in certain cells which by 
 their division give rise to the gametes, and at one 
 of the divisions in question the parental characters (in 
 a potential condition) separate completely from one 
 another, so that half the gametes bear one allelomorph 
 and half of them the other. In cases where more than 
 
SUMMARY 
 
 225 
 
 one pair of allelomorphs has taken part in the cross, the 
 members of each pair are found, as a rule, to undergo 
 this process of segregation quite independently of all 
 the other pairs. 
 
 The result of this phenomenon of segregation is 
 that we find our conception of what constitutes purity 
 in a strain of animals or plants to be completely 
 altered. We now know that purity does not depend 
 upon the length of time during which the race has 
 exhibited a constant character. A strain of absolute 
 purity may arise from the second generation of a cross. 
 Such a pure strain may show an entirely new com- 
 bination of the parental characters. But this is so 
 far the only kind of novelty which we can produce at 
 will. We know almost nothing as to the method by 
 which entirely new characters arise. We can only 
 take advantage of such characters when they happen 
 to make their appearance. 
 
 I would draw special attention to the definiteness of 
 the characters with which we deal. We do not evoke 
 improved features by gradual selection ; the characters 
 are either there or they are not. Let it be further 
 remembered that every process of this kind which has 
 been worked out in the case of a plant can be paralleled 
 by similar phenomena taking place in some one or 
 other of the higher animals. 
 
 On the mind of a biologist familiar with what 
 was known of heredity only about twenty years 
 since, these facts must fall with a sense of complete 
 novelty. The ideas current even so short a time ago 
 are not so much extended, or even altered, as replaced 
 
 15 
 
226 
 
 MENDELISM 
 
 by an entirely new set of ideas. And it may be 
 remarked in passing that the biologist of fifty years 
 ago and more was much nearer to our present line of 
 inquiry. 
 
 We have seen enough to enable us to recognise 
 very clearly the vital importance of an understanding 
 of the constitution of the gametes in all questions of 
 heredity. There must exist in the gametes, in an 
 uncombined condition, those units which by their 
 combination in zygotic organisms lead to the appear- 
 ance of the characters which we can recognise. But 
 we have seen that, owing to the appearance of . domi- 
 nance and other kindred phenomena, the visible external 
 characters of an organism are not a complete guide to 
 the nature of its gametes. It is only by careful 
 breeding that we can distinguish the heterozygote from 
 the pure dominant form — ^to take the simplest possible 
 example of this difficulty. For this reason it has now 
 become the chief business of the student of heredity 
 to determine by experiment what combinations of 
 allelomorphs are present in the gametes of the indi- 
 viduals with which he is working. 
 
 The behaviour of these allelomorphs has now been 
 disentangled in many cases of very considerable com- 
 plexity ; and all such cases as have been so far examined 
 in detail have proved explicable in terms of a larger or 
 smaller number of allelomorphic pairs, all of which 
 obey Mendel’s law — ^with the single exception of those 
 cases in which coupling between the allelomorphs of 
 different pairs introduces a slight further complication. 
 
MENDELISM AND BIOMETRY 
 
 227 
 
 Although it is perhaps scarcely probable that Mendel’s 
 law will ultimately prove universal in its application, 
 nevertheless the few exceptions recorded by competent 
 observers still require further examination before they 
 can be accepted as invalidating the law in any single 
 instance. 
 
 The question naturally arises as to how far the Men- 
 delian rule of inheritance agrees with or contradicts 
 those estimations of hereditary values which have been 
 arrived at by the labours of the biometricians. 
 
 So long ago as 1902 Mr. G. Udny Yule endeavoured 
 with some apparent success to reconcile the Mendelian 
 results with those of biometry. Progress has been 
 rapid during the subsequent years, and what we have 
 now before us is rather the question of reconciling the 
 biometrical conclusions with the firmly established 
 facts of Mendelian inheritance. 
 
 In 1902 Yule considered the case of a pair of simple 
 Mendelian characters, A and a, exhibited in a mixed 
 population breeding together at random, in such a way 
 that the total number of germ cells bearing A and a 
 respectively might be regarded as equal in any genera- 
 tion. In such a case it will always be an even chance 
 whether a recessive parent will produce a dominant 
 or a recessive child, because the chance of its gamete 
 (a) mating with A or « is the same. A knowledge 
 of the ancestry of the recessive parent makes no 
 difference to the result. Consequently the case of 
 the pure recessive does not fall in with any possible 
 theory of ancestral heredity. 
 
 15—2 
 
228 
 
 MENDELISM 
 
 But on turning to the dominant parent, the case is 
 found to be different. For such an one may be either 
 a pure dominant homozygote giving off ^-gametes 
 only, or it may be a heterozygote giving off equal 
 numbers of A- and a-gametes. Yule shows that if both 
 the parents of the A individual exhibited the character 
 A , the proportionate number of its offspring which may 
 on the average be expected to show the A character is 
 greater than would have been the case if one of its 
 parents exhibited the character a. And in a si mil ar 
 way a knowledge of the characters shown by the grand- 
 parents adds something to the certainty of the pre- 
 diction as to the proportionate numbers of offspring of 
 the two kinds which are to be expected, when the 
 average of a number of cases is taken according to the 
 usual statistical method. 
 
 Yule therefore regarded the case of the dominant 
 character as showing conformity with the law of 
 ancestral heredity, according to his own statement of 
 that generalization, which was to the following effect : 
 The law that ‘ the mean character of the offspring can he 
 calculated with the more exactness, the more extensive 
 our knowledge of the corresponding characters of the 
 ancestry, may be termed the law of ancestral heredity.’* 
 
 It may be remarked in passing that Yule’s dis- 
 tinction of the problems of genetics into those of 
 intra-racial heredity and those of hybridization cannot 
 now be regarded as holding good, unless the term 
 hybridization is to be extended to many cases — e.g., 
 tjiat of the inheritance of coat colour in thoroughbred 
 ♦ ‘ New Phytologist,* vol. i., p. 202. 
 
MENDELISM AND BIOMETRY 
 
 229 
 
 horses, which would have been classed unhesitatingly 
 as instances of heredity by all biometricians in 1902. 
 Bateson’s instinct did not fail him when he divided 
 these problems into those of continuity and those of 
 discontinuity respectively, although at the present time 
 the realm of continuous variation and inheritance is 
 being steadily encroached upon owing to the analysis 
 of complex characters into definite constituents. 
 
 In 1904 Karl Pearson struck a blow at the prospect 
 of conformity between biometrical and Mendelian 
 results in his memoir, ‘ On a Generalized Theory of 
 Alternative Inheritance, with special reference to 
 Mendel’s Laws.’ Pearson’s treatment of the subject 
 involved advanced mathematical reasoning, and we 
 can, therefore, only give a brief summary of his main 
 results. Pearson proposes special terms for the A and 
 the a elements respectively of a couplet or pair of 
 allelomorphs. He proposes to call the A element a 
 frotogene, and the a element an allogene, and he thus 
 distinguishes between the two sorts of homozygotes 
 by calling AA a protozygote and aa an allozygote. 
 
 Pearson considered the case of a population breeding 
 together at random, in which a single measurable 
 character, such as stature, is determined by the combined 
 action of an indefinite numiber of pairs of allelomorphs, 
 and he proceeded to work out the value of parental 
 correlation which was to be expected under these 
 circumstances. This value he found to be exactly 
 one-third, a value which happens to be identical with 
 Galton’s original determination of parental correlation 
 from his statistics of human stature. A considerable 
 
230 
 
 MENDELISM 
 
 number of determinations of parental correlation 
 have, however, since been made in the case of all 
 kinds of characters. The values show considerable 
 variation, but the average which they indicate is much 
 nearer to 0’5 than to 0-33. Pearson therefore con- 
 cluded that in none of these cases could anything 
 resembling Mendelian inheritance be taking place, 
 and that the latter is, in fact, the exception rather than 
 the rule.* 
 
 Mendelians, aware of the certainty of their own 
 results, and being convinced that these facts must 
 have a very wide application, were thereupon driven 
 reluctantly to the conclusion that something was 
 seriously wrong with the methods adopted by biome- 
 tricians for determining the coefficients of correlation. 
 It seems, however, that this conclusion may have been 
 arrived at with undue haste. 
 
 In August, 1906, Mr. Yule read before the Inter- 
 national Congress of Hybridization assembled in 
 London a very interesting paper on ‘ The Theory of 
 Inheritance of Quantitative Compound Characters on 
 the basis of Mendel’s Laws.’ Though some difficulty 
 was then experienced in following his argument by 
 an audience unaccustomed to statistical methods, 
 Yule’s conclusion is really very simple. 
 
 Yule points out that the only character dealt with 
 in Pearson’s memoir is the number of protogenicor 
 allogenic couplets present in the individual, and it is the 
 proportionate number of these couplets present in the 
 parent and in the offspring respectively which is taken 
 as determining the value of the correlation coefficient. 
 
MENDELISM AND BIOMETRY 
 
 231 
 
 Consequently Pearson’s treatment of the subject does 
 not justify his statement that the Mendelian theory 
 gives a rigid value for the coefficients of parental cor- 
 relation for all races and characters — a conclusion 
 which he regards as fatal to this theory, because the 
 coefficients for different characters and races, as found 
 statistically, show considerable individual differences, 
 and seem to cluster round a value considerably higher 
 than that indicated by his elaboration of the theory of 
 the pure gamete. Yule thereupon discusses a somewhat 
 more general case, and considers the inheritance of a 
 length made up of a number of distinct segments, each 
 of which is determined by an independent pair of 
 allelomorphs. Supposing each segment to take the 
 length a, b, or c, according as the corresponding proto- 
 zygote, heterozygote, or allozygote is present. Yule 
 arrives at an equation from which the correlation 
 between parent and offspring may be found. From 
 that equation the following results are deducible : 
 
 If there is dominance — i.e., ii a = b, or b-c, the corre- 
 lation coefficient is the same as that found by Pearson 
 — i.e., one-third. 
 
 But if the heterozygote always gives rise to a 
 length exactly intermediate between those due to 
 the respective homozygotes, the correlation is found 
 to be one-half. 
 
 Cases of partial dominance will give an intermediate 
 value. Consequently, according to the degree of 
 imperfection of dominance, and without assuming any 
 other disturbing circumstances, values of parental 
 correlation varying from 0‘33 to 0-5 are to be expected 
 
232 
 
 MENDELISM 
 
 on the Mendelian theory of inheritance when applied 
 to populations. These figures are calculated on the 
 supposition that there is random mating of the parents, 
 but if there were a tendency for like to mate with like 
 the correlation values would become still higher. Yule 
 therefore concludes that ‘ there is therefore no diffi- 
 culty in accounting for a coefficient of 0-5 on the 
 theory of segregation, but such a value probably 
 indicates an absence of the somatic phenomenon of 
 dominance. In the case of characters like stature, 
 span, etc., in man this does not seem very improb- 
 able.’ 
 
 It is impossible to bring the present chapter to a con- 
 clusion without some reference to the practical aspects 
 of the Mendelian discovery. The progress of experi- 
 mental research in this field during the last few 
 years has been so rapid, that there is little ground for 
 astonishment in the fact that only a small proportion 
 of those to whom the discovery of the Mendelian 
 method is of the very highest importance from a com- 
 mercial point of view have yet arrived at any serious 
 appreciation of it. The improvement of the breeds of 
 cultivated plants and domestic animals is a subje«t of 
 vital importance to the whole human race, quite apart 
 from the question of the commercial profit which it 
 represents for those whose business it is to be directly 
 concerned with the process — ^the actual plant- and 
 animal-breeders themselves. 
 
 Hitherto the methods of amelioration which have 
 been adopted have depended largely upon guess-work. 
 
PRACTICAL APPLICATIONS 
 
 233 
 
 or at the best upon the result of practical experience. 
 We are now within sight of the day when a complete 
 system of precise scientific methods will have been 
 elaborated. The time required for the development 
 and application of these methods must chiefly depend 
 upon the apathy or enterprise of those in whose hands 
 rests the means of subsidizing this kind of work, for 
 without proper resources the progress of any such 
 study must of necessity be slower than it would be 
 if properly - equipped establishments were at the 
 disposal of duly trained experimenters receiving an 
 adequate remuneration. 
 
 The practical application of Mendelism cannot be 
 better illustrated than by an account of Prof. R. H. 
 Biffen’s work upon the improvement of cereals, particu- 
 larly of wheat — work which exhibits an extraordinary 
 contrast in point of scientific exactness with everything 
 of the kind which has been previously undertaken. 
 This contrast was remarkably displayed at one of the 
 morning sessions of the recent International Congress 
 on Hybridization and Plant Breeding, held under the 
 auspices of the Royal Horticultural Society. On that 
 occasion a series of communications upon the subject 
 of cereals culminated in an admirable account given by 
 I'rof. Biffen of the way in which the problems of their 
 improvement have been overcome at the experimental 
 farm of the Cambridge University Department of 
 Agriculture. And it was a gratifying sign of better 
 times to observe the enthusiastic interest with which 
 practical men greeted his communication. 
 
 As a preliminary measure Biffen has worked out the 
 
234 
 
 MENDELISM 
 
 inheritance of a number of comparatively simple 
 characters, many of which have little practical impor- 
 tance. But the fact of their strictly Mendelian be- 
 haviour showed the possibility of readily obtaining any 
 desired combination of them, and at the same time 
 rendered it highly probable that characters of a more 
 practical value to the farmer would prove similarly 
 amenable to the breeder’s art. 
 
 Thus Biffen found that the following pairs of 
 characters, among others, exhibited simple Mendelian 
 phenomena, the one placed first being in each case the 
 dominant : 
 
 Beardless ears. 
 
 Keeled glumes. 
 
 Felted glumes. 
 
 Red chaff. 
 
 Red grain. 
 
 Thick and hollow stem, 
 
 Bearded ears; 
 
 Round glumes. 
 Glabrous glumes^ 
 White chaff. 
 
 White grain. 
 
 Thin and solid stem. 
 
 And so on. In other cases, again, the Fj generation 
 showed a character intermediate between those of 
 the parents, and in Fg there appeared a ratio corre- 
 sponding to A : 2 Aa : a. 
 
 Thus when Polish wheat (early) was crossed with 
 Rivet wheat (late), the time of ripening of the Fj 
 generation was intermediate between those of the 
 parents. In Fj, 103 early, 210 intermediate, and 
 100 late plants, were counted. Time of ripening is, 
 moreover, clearly a character which may be of con- 
 siderable practical importance. 
 
 In further illustration of what can be done from a com- 
 mercial point of view, we will consider the case of two 
 other characters only — rust immunity and ‘ strength.’ 
 
PRACTICAL APPLICATIONS 
 
 235 
 
 There is a quality of wheat grains known as strength 
 which is essential for the production of a flour such as 
 can be baked into the kind of loaf which is at present 
 the only one saleable in England. This quality un- 
 fortunately happens to be wanting in all the strains of 
 wheat which it has hitherto been possible to grow at 
 a profit in this country. For this reason imported 
 American and Canadian hard wheats, which possess 
 this quality of strength, are worth in England some 
 shillings a quarter more than home-grown wheats. 
 
 When such strong American varieties are grown in 
 this country the majority of them are rapidly found to 
 lose this quality, and to become after a short time as 
 ‘ weak ’ as ordinary English wheats. Some of them do, 
 however, retain their strength, and after several seasons 
 — in one case fourteen — show no signs of deterioration. 
 An example of a wheat of this latter type is afforded 
 by Red Fife, which is the basis of the mixed wheat 
 known commercially as Manitoba Hard, the latter 
 consisting, as a matter of fact, of a mixture of several 
 different varieties. Unfortunately these permanently 
 hard wheats do not yield so large a crop as the com- 
 monly cultivated English varieties, and so their higher 
 price does not make up for the smaller number of 
 bushels per acre obtained when they are grown. 
 
 Biffen therefore set to work upon the problem of 
 combining hardness or strength with the power of 
 yielding a good crop, and with the other desirable 
 qualities characteristic of the home-grown varieties. 
 With this end in view Manitoba Hard was crossed with 
 a typical English wheat — Rough Chaff. 
 
236 
 
 MENDELISM 
 
 The Fj plants produced grains all of which were 
 fully as hard as those of the Manitoba variety. 
 
 These grains were sown, and it was found that some of 
 the resulting plants produced strong grains and others 
 weak ones, and that the former were to the latter very 
 nearly in the numerical ratio of 3:1. Actually they 
 were as 152 : 48 in a sample of 200 taken at random. 
 
 In order to obtain confirmation of this most impor- 
 tant result, Biffen sent samples of the grains borne 
 by the plants to a well-known authority on milling 
 wheats, requesting his judgment upon them, but with- 
 out telling him their manner of origin. The answer 
 was even more satisfactory than could possibly have 
 been anticipated. Certain of the samples were stated 
 by the expert to belong to the variety Red Fife, 
 which is the name of the particular strain of Manitoba 
 Hard originally made use of in the experiments, 
 whilst others were assigned to a definite strain of 
 ordinary weak English wheat. The segregation of 
 these characters was, therefore, complete, strength 
 being a Mendelian dominant to weakness. 
 
 In the next generation certain of the dominant 
 plants, as was to be expected, bred true, and amongst 
 them were individuals which combined with strength 
 of grain the other desirable qualities of the second 
 parent. The problem has, therefore, been completely 
 solved, and there can be little doubt that whe.n these 
 new types are brought into general cultivation the 
 profit obtainable from the growing of wheat in this 
 country will be increased by several shillings to the 
 acre of crop grown. 
 
PRACTICAL APPLICATIONS 
 
 237 
 
 We may next turn to an even more important 
 achievement. In many countries the annual loss of 
 crop due to the attacks of yellow rust, Puccinia 
 glumarum, amounts on a moderate estimate to a con- 
 siderable number of millions of pounds sterling. 
 Certain strains of wheat exist, indeed, which are more 
 or less completely immune to the ravages of this fungus, 
 but these are usually wanting in other qualities which 
 are indispensable to the farmer. If it should be found 
 that immunity to rust is a simple Mendelian allelo- 
 morph, it would be possible to combine this quality 
 with any other useful character which obeyed the same 
 law of inheritance — as several useful characters have 
 already been shown to do. At one time it must have 
 been thought that a similar method of inheritance of 
 the character rust -immunity was too excellent a boon 
 to be reasonably hoped for. 
 
 Among a great number of strains of wheat grown 
 on the Cambridge experimental farm, several types 
 showed marked differences in the degree of their 
 immunity from, or susceptibility to, the attacks of 
 Puccinia glumarum. Among them Mr. Biffen found 
 one which was apparently quite immune, and, though 
 grown in the midst of numbers of rusted plants, itself 
 never showed a trace of infection. Of another type, 
 known as Michigan bronze, no single individual ever 
 escaped the rust, and so badly were the plants of this 
 strain diseased that very few ripe grains could ever be 
 obtained from them. 
 
 Biffen crossed these two types together. In the 
 first generation every plant without exception was 
 
238 
 
 MENDELISM 
 
 badly rusted, but fortunately a considerable number 
 of ripe grains was obtained, and these were sown 
 to produce the second generation. When the plants 
 of this generation had grown up it was observed 
 that among a majority of badly-rusted plants certain 
 individuals stood out fresh and green, being entirely 
 free from infection. On examination it was found that 
 every plant could be placed in one or other of two 
 categories — either it was badly rusted or it was 
 entirely free from rust ; and the numbers of the two 
 kinds of plants were as follows : 1,609 infected, 523 
 immune. 
 
 It is clear, then, that immunity and susceptibility 
 to the attacks of yellow rust behave as a simple pair of 
 Mendelian characters, immunity being -recessive. And 
 it is, therefore, possible to obtain by crossing, in three 
 generations, a pure rust-free strain containing any other 
 desired quality which is similarly capable of definite 
 inheritance. 
 
 BIBLIOGRAPHY. 
 
 Mendel, G. J. : Briefe an Carl Nageli. 
 
 Bateson, W. : Mendel’s Principles of Heredity, 1909. 
 Bateson, W. ; Punnett, R. C. ; and Saunders, Miss E. R. : 
 Reports to the Evolution Committee of the Royal Society, 
 1902-1909. 
 
 The Journal of Genetics, 1910 — 
 
 Punnett, R. C, ; Mendelism, fifth edition, 1919. 
 
CHAPTER IX 
 
 RECENT CYTOLOGY 
 
 Every living creature may be regarded as being built 
 up of a number of structural units which are known 
 as cells. In the case of some of the simplest animals 
 and plants, indeed, the whole body of the organism is 
 composed of a single cell — a small mass of living proto- 
 plasm, containing, as a rule, only one nucleus. But 
 in all the higher animals and plants the adult body is 
 made up of a great number of such cells living in 
 intimate association with one another. 
 
 The living material of which the cell is composed is 
 known as protoplasm. Protoplasm is a highly com- 
 plicated and unstable combination of substances, 
 amongst the constituents of which the chemical 
 elements, carbon, oxygen, hydrogen, nitrogen, and 
 sulphur, play the chief parts. Its consistency is slimy 
 and semifluid. 
 
 Concerning the nucleus — ^the most essential and 
 characteristic of ceU organs — more will have to be said 
 later on. Other important organs of cells are a wall 
 or membrane which externally surrounds them, one 
 or more vacuoles or cavities containing a watery fluid, 
 or sometimes a gas, and a certain number of more solid 
 
240 
 
 RECENT CYTOLOGY 
 
 bodies or flastids. Certain plastids present in the 
 majority of plants are of particular importance as con- 
 taining the green substance chlorophyll, which plays 
 an essential part in the fixation of carbon from the 
 atmosphere. 
 
 Amongst unicellular organisms — the creatures 
 already mentioned as being made up of a single cell 
 only — those which contain chlorophyll and are pro- 
 vided with a firm cell wall, built up of a material 
 known as cellulose, are usually regarded as simple 
 plants ; whilst those in which chlorophyU and a cell 
 wall are absent aie looked upon as simple types of 
 animals. Similarly slight differences distinguish the 
 cells which build up the fabric of the higher plants 
 from those of which the bodies of the more complicated 
 animals are composed, so that in almost all essential 
 points an account of the behaviour of the cells of the 
 members of one kingdom will apply equally well to 
 those of the other. After a few further preliminary 
 remarks we shall, therefore, for the sake of simplicity, 
 speak of a generalized type of cell, the behaviour of 
 which, except in points of detail, will resemble that of 
 the actual cells of plants or animals indifferently. But 
 in order to convey a more definite idea of an uni- 
 cellular animal to those who are unfamiliar with the 
 rudiments of Elementary Biology, we may refer briefly 
 to the well-known form Amceba, which will serve as 
 an excellent type of an animal consisting of a single 
 free-living cell. 
 
 This little creature consists of a mass of protoplasm 
 enclosing a nucleus which is more or less centrally 
 
THE CELL 
 
 241 
 
 situated and approximately spherical in form. The 
 protoplasm is divided into an outer hyaline and an 
 inner granular portion, the former being limited exter- 
 nally by a very delicate membrane. The shape of the 
 animal is irregular, and, moreover, undergoes gradual 
 alteration owing to the characteristic amoeboid move- 
 ments. These consist in a slow protrusion and with- 
 drawal of processes of the body, enabling the animal 
 to change its position by a kind of flowing movement, 
 and also to engulf its food, which consists of various 
 
 Fig. 17.— Amoeba. 
 
 n, Nucleus ; /, food particle. 
 
 Fig. 18.— Pleurococcus. 
 
 n, Nucleus ; w, cell -wall ; 
 chi, chloroplast. 
 
 minute organic particles, by the simple process of 
 flowing around it. 
 
 In contrast with Amceha the unicellular plant Pleuro- 
 coccus is motionless, and is surrounded by a firm wall 
 of cellulose. In addition to a central nucleus, the 
 plant contains, embedded in its peripheral protoplasm, 
 several plastids which bear the chlorophyll concerned 
 in the assimilation of carbon from the gases of the 
 atmosphere. This chlorophyll lends a green colour to 
 the whole contents of the cell, and in its natural habitat 
 the plant is quite conspicuous. The green powdery 
 substance often to be seen on the bark of trees, 
 
243 
 
 RECENT CYTOLOGY 
 
 especially on the side turned to the north, and in 
 similar shady situations, consists, as a rule, of great 
 numbers of minute Pleurococcus plants, although the 
 size of a single specimen may be represented by a 
 diameter of little more than the two-thousandth part 
 of an inch. 
 
 We are more particularly concerned, however, with 
 the higher animals and plants, the bodies of which are 
 built up of a great number of separate cells. Some 
 of these cells may be modified in various ways, but they 
 all conform, at least in the youthful condition, to types 
 not far removed from those of Aniceba and Pleurococcus 
 respectively. Certain parts of these higher organisms, 
 indeed, such as the bones of vertebrate animals and 
 the wood of trees, do not consist solely of living cells, 
 but are composed to a great extent of dead material 
 excreted or built up by the activity of living cells. 
 These latter have, then, either ceased to live, or they 
 may continue to exist in the interstices of the hard 
 skeletal framework. 
 
 New cells come into existence in only one way — 
 namely, by a process of division which takes place in a 
 pre-existing cell. In comparatively rare cases a cell 
 may give off a small bud which forthwith develops 
 into a new cell like the old one. In such a case we may 
 speak of the cell which gives off the bud as the mother- 
 cell, and of the cell into which the bud develops as 
 , the daughter-cell. But by far the most frequent 
 ; method of cell-reproduction, and the only one which 
 I is characteristic of the higher animals and plants, takes 
 I place by the equal division of an old cell into two 
 
THE CELL 
 
 243 
 
 new ones. In this case, it is only by a stretch of 
 language that we can speak of parent- and daughter- 
 cells, for the individuality of the pre-existing cell is 
 completely lost, and two fresh individualities have now 
 taken its place. 
 
 Since all the cells of the animal or plant body arise 
 by the bipartition of pre-existing cells, it is clear that if 
 we follow these processes far enough back, in the case 
 of any individual organism, we may arrive at a period 
 at which only one cell was present. And under 
 ordinary circumstances this is actually the case. 
 Every individual among the higher animals and plants, 
 iarising by the ordinary sexual method, existed at the 
 earliest stage of its embryonic history in the form of a 
 single cell, the fertilized ovum. And the first obvious 
 process in the development or embryology of the young 
 organism consisted in the division of this primitive 
 cell into two new cells. Each of these new cells then 
 divided again in like manner, and the multiplication of 
 cells continued until all the innumerable cells which 
 build up the organs of the adult body had finally 
 come into existence. When growth is completed ceU- 
 divisions continue more slowly, producing new cells to 
 make good the wear and tear of the bodily tissues. 
 
 As the number of cells increased, their relation to 
 one another in space was constantly changing. Dif- 
 ferent cells, too, became modified in different ways ; 
 for instance, the cells on the outside of the young 
 embryo took on a different form from those within, in 
 accordance with the different conditions to which 
 they were exposed, and a host of other changes took 
 
 16—2 
 
244 
 
 RECENT CYTOLOGY 
 
 place too numerous for us to follow in detail. Thus 
 the complicated structure of the adult organism was 
 gradually arrived at by a process of development in 
 which cell- multiplication played a most prominent and 
 essential part. 
 
 We have next to inquire what is the method of origin 
 of the original embryonic cell — ^the fertilized ovum — 
 from which the new animal or plant develops. 
 
 As is indeed implied by the expression ‘ fertilized 
 ovum,’ this cell arises by the fusion together of two 
 independent cells, such fusion constituting the process 
 of fertilization or impregnation. One of the cells 
 which took part in the fusion was derived from one 
 parent organism, and bore the distinguishing character- 
 istics of the cells which composed that parent — or at 
 least some part of those characteristics — ^whilst the 
 other was in like manner derived from the second 
 parent. 
 
 It is to be observed that this fusion together of a 
 pair of cells, derived (in the case we are considering — 
 namely, that of ordinary biparental reproduction) 
 from two separate individuals, results in the formation 
 of a complete new individuality, which arises definitely 
 at that point of time at which the fusion of the two 
 conjugating cells takes place. In this way the cells 
 of the offspring are seen to be of double origin, and it 
 is found that traits and characters derived from both 
 the father and the mother can co-exist in them side by 
 side. 
 
 The cells which take part in the above-mentioned 
 fusion are known as gametes, or germ-cells — male and 
 
THE CELL 
 
 245 
 
 female respectively, according to the sex of the parent 
 from which each is derived. In animals the female 
 gamete is known as the ovum, and the male as the 
 spermatozoon, and the product of their fusion, as already 
 said, is called the fertilized ovum. Germ-cells of a 
 similar kind- arise in a slightly different way in plants. 
 The germ-cells are produced in special parts of the 
 organism known as the generative organs, which in 
 flowering plants are represented by the pistils and 
 stamens. 
 
 A more convenient expression for the fertilized ovum 
 is that of zygote, a term which we have previously 
 encountered in the shape of the homo- and hetero- 
 zygotes of the Mendelian. By an expansion of meaning 
 the term zygote is -also used to express the whole 
 organism which ultimately arises from the product of 
 fusion of a pair of gametes, and by this use the impor- 
 tance of the gamete, as opposed to the zygotic organism 
 as a whole, is brought into due prominence. 
 
 We find, then, that the succession of generations in 
 the higher animals and plants, according to the common 
 use of this expression, depends upon the succession 
 of a much larger number of cell -generations. By 
 repeated divisions, each giving rise to a new generation 
 of cells, the fertilized ovum gradually develops into 
 the adult organism. By the division of certain 
 members of the later generations of cells which compose 
 this organism the gametes are produced. By the 
 conjugation of a pair of gametes a zygote of the 
 second generation arises, and the same processes are 
 continually repeated. 
 
246 
 
 RECENT CYTOLOGY 
 
 Each of the cells hitherto referred to possesses a single 
 nucleus, which is usually a more or less spherical body 
 occupying a central position within the cell. Nuclei, 
 like the cells which contain them, arise only by the 
 division of pre-existing nuclei. Thus the history of 
 the nuclei is in every way similar to the history of the 
 cells, of which they constitute so important a part. 
 In fertilization the nuclei of the conjugating cells or 
 gametes fuse together to form the single nucleus of 
 the fertilized ovum, and every division of this cell, as 
 well as of its cell-progeny, is preceded by a division 
 of the nucleus into two similar portions. 
 
 We may forthwith concentrate our attention upon 
 the nucleus as being that part of the cell which is of 
 primary importance from the point of view of heredity, 
 for it is now generally recognised that the nucleus 
 is the part of the cell in which hereditary features 
 are in some way carried. And we may next consider 
 a little more closely the structure of the nucleus as 
 seen under high powers of the microscope. 
 
 In what is somewhat improperly called its resting 
 condition — a condition which is characteristic of nuclei 
 at all times when they are not actually undergoing 
 division, or preparing for that process — ^the nucleus may 
 be seen to be bounded by a more or less definite nuclear 
 membrane. The internal structure of such a nucleus is 
 described as reticular — that is to say, at least two 
 different substances are differentiated within the 
 nucleus, one of them forming a reticulated meshwork, 
 the interspaces of which are occupied by the other 
 (Fig. 19, p. 249). 
 
THE NUCLEUS 
 
 247 
 
 In entering into a detailed description of the changes 
 which take place in the finer structure of the nucleus, 
 it must be clearly understood that the more minute 
 features alluded to are only to be seen with any degree 
 of definiteness in dead cells which have been killed 
 practically instantaneously by the action of some 
 powerful chemical poison. Under suitable conditions 
 it is believed that treatment of this kind fixes the con- 
 stituent parts of the nucleus in very nearly the same 
 relative positions as they occupied in life at the 
 moment immediately preceding the death of the cell. 
 The tissues containing the cells to be examined are then 
 usually cut into very thin sections, and other chemicals 
 are applied to them, the result of this treatment being 
 to stain different parts of the nucleus of different 
 colours and with different degrees of intensity. It is to 
 the behaviour of the structures thus made visible that 
 our description applies, since it is impossible to follow 
 these changes in actually living cells except to a very 
 imperfect extent. It may be pointed out, however, 
 that we have every reason for believing that the 
 differential effect produced by the processes of fixing 
 and staining only serves to render more clearly visible 
 real differences which actually existed during the life 
 of the cell, and some indications of many of these 
 differences have even been actually seen in living cells 
 under exceptionally favourable conditions. 
 
 The nucleus, when treated in the manner described, 
 is seen to be built up of a network of branching fibrils, 
 the meshes of which enclose a comparatively clear and 
 hyaline substance. The fibrils of the network are made 
 
248 
 
 RECENT CYTOLOGY 
 
 up of a material of comparatively weak staining 
 capacity ; embedded in this substance are numerous 
 granules of a very intensely staining material which 
 is known as chromatin. There are strong reasons for 
 believing that the chromatin of the nucleus is of 
 special importance from the point of view of the 
 mechanism of heredity. This reticular structure of 
 the nucleus is indicated in a diagrammatic fashion in 
 Fig. 19. 
 
 Further light is thrown upon the detailed structure 
 of the nucleus by the changes which become visible 
 during the process of nuclear division. This process, 
 which is known as mitosis, we must now proceed to 
 describe. 
 
 In the description of mitosis which follows, the 
 account of this process has been somewhat generalized 
 and simplified, and Figs. 19 to 26, which illustrate the 
 phenomena, are purely diagrammatic. It is hoped 
 that the most important features of this complicated 
 process may be in this way rendered comprehensible ; 
 and although in different organisms considerable 
 variations in the details of the process are to be met 
 with, yet in their general features all ordinary mitoses 
 in animals and plants commonly coiSbrm to the 
 essential type of our description. 
 
 The first change in the appearance of the nucleus 
 which indicates that a division is about to take place 
 consists in a rearrangement of the chromatin network, 
 which now takes on the appearance of a tangled 
 thread (Fig. 20). The outwardly-directed loops of 
 this skein often correspond to the separate portions 
 
CHROMOSOMES 
 
 24Q 
 
 into which the thread eventually breaks up. The 
 thread gradually grows shorter and thicker, and 
 presently becomes divided into a number of pieces 
 which are known as chromosomes. In the chromo- 
 somes the shortening and thickening process is con- 
 tinued until these bodies arrive finally at the form of 
 stumpy rods, each of which often becomes bent into 
 
 the form of a horseshoe. Meanwhile the nuclear 
 membrane breaks down, so that the hyaline substance 
 of the nucleus becomes continuous with that of the 
 cell body surrounding it. A fresh phenomenon now 
 becomes visible. A spindle-shaped arrangement makes 
 its appearance consisting of a nmnber of minute fibrils 
 which connect together two points — the poles of the 
 spindle — situated at opposite ends of the cell. The 
 
250 
 
 RECENT CYTOLOGY 
 
 chromosomes now change their position so that they 
 come to lie in the plane of the equator of the spindle, 
 and about this time, but sometimes earlier, each 
 chromosome splits longitudinally into two equal 
 portions (Figs. 22, 23). This splitting in the case of 
 each chromosome takes place in the equatorial plane 
 of the spindle, so that one member of each pair of 
 daughter chromosomes faces towards one pole of the 
 spindle, and the second towards the other pole. The 
 members of each pair of daughter chromosomes now 
 begin to move away from one another towards the' 
 two poles of the spindle, and as they do so the first 
 indication of a dividing wall between the two new 
 cells begins to make its appearance in the equatorial 
 plane. 
 
 Arriving at the poles, the daughter chromosomes 
 begin to elongate, and to put out processes which 
 finally meet and fuse with those of their neighbours 
 to form the chromatin reticulum of the new nuclei 
 (Fig. 25). Surrounding each new nucleus, thus 
 developing at either pole of the now rapidly dis- 
 appearing spindle, a new nuclear membrane makes its 
 appearance ; the dividing wall in the position of the 
 equator of the spindle develops into a complete 
 partition (at least in the case of plants, in which, 
 however, a number of minute passages are left pene- 
 trating the cell wall and preserving the communication 
 between the protoplasmic contents of the separate 
 cells) ; and the division into two new cells is thus 
 completed (Fig. 26). Each new cell is provided with 
 a nucleus into which has entered precisely its fair 
 
CHROMOSOMES 
 
 251 
 
 share of the chromatin which was present in the 
 parent nucleus. 
 
 A great deal of evidence has recently accumulated 
 to show that chromosomes are very definite and 
 important organs. In the first place, the number of 
 chromosomes which make their appearance at each cell 
 division is the same in all the cells of any given 
 creature, and this numerical constancy further extends 
 to the cells of all the members of a particular species, 
 though in members of allied species the number of 
 chromosomes may be different. In widely separated 
 species the number of chromosomes varies consider- 
 ably; thus from 2 to 200 have been counted in the 
 case of various different members of the animal and 
 vegetable kingdoms. One of the commonest numbers 
 found Is twelve, and this number occurs in a con- 
 siderable variety of different animals and plants. 
 
 Next it has been shown that the chromosomes 
 which arise at the beginning of a nuclear division are 
 identical with those daughter chromosomes of the 
 preceding division which originally entered into the 
 nucleus now about to divide. An example of the 
 kind of evidence upon which this conclusion is based 
 may next be given. 
 
 Figs. 27, 28, and 29 show the three possible arrange- 
 ments of the four chromosomes which are found in the 
 cells of the worm-like animal Ascaris, as seen from the 
 direction of the pole of the spindle in the dividing 
 nucleus. Of these arrangements, that shown in 
 Fig. 29 is much the least common. Now in this par- 
 ticular case the chromosomes, when they first mak« 
 
RECENT CYTOLOGY 
 
 253 
 
 their appearance immediately before the process of 
 division, are found with their extremities situated in 
 little pockets or bulgings of the nuclear membrane, 
 so that their exact position is very definitely marked ; 
 and the arrangement of the chromosomes may be any 
 one of those already indicated. Boveri observed that 
 
 Fig. 27. Fig. 28. Fig. 29. 
 
 in the case of two neighbouring cells which had 
 originated by the division of the same mother-cell, the 
 chromosomes made their appearance in both cases in 
 the uncommon position of Fig. 29. Figs. 30 and 31 
 indicate their actual arrangement. The conclusion to 
 be drawn from this observation is that the same 
 
 Fig. 30. 
 
 Fig. 31. 
 
 chromosomes have preserved their individuality right 
 through the resting stage of the nucleus, to reappear 
 in the same position at the outset of a new phase of 
 division. 
 
 If is believed, then, that the same stages which the 
 chromosomes passed through at the close of one 
 
CHROMOSOMES 
 
 253 
 
 nuclear division, giving rise to the nuclear reticulum in 
 the daughter nucleus, are repeated in the reverse order 
 at the outset of the next division ; the same processes 
 are withdrawn into the same chromosomes, and these 
 shorten into structures identical with those which 
 passed into the nucleus at its first formation, except 
 that they have increased in bulk during the interval.* 
 
 Boveri, in fact, concludes that the separate chromo- 
 somes are to be looked upon as distinct individuals — 
 almost as separate simple organisms— which preserve 
 their individuality throughout the history of the cell, 
 and reproduce themselves, just as cells and nuclei do, 
 by a process of bipartition. As far as the chromo- 
 somes themselves are concerned, their typical or 
 resting form is that of the short simple rods seen in 
 mitosis. The branched anastomozing character seen 
 during the stage of the nuclear reticulum is associated 
 with the active co-operation of the chromosomes in the 
 physiological processes going forward within the 
 nucleus. For this reason the term ‘ resting stage ’ 
 applied to this condition of the nucleus is a particularly 
 inappropriate one. 
 
 Boveri illustrates the amount of credence which he 
 would attach to this theory of the individual persis- 
 tence of the chromosomes throughout the resting 
 condition of the nucleus, by means of the following 
 analogy ; ‘ We make water from oxygen and hydrogen, 
 and from this water we can obtain oxygen and hydrogen 
 again in the same proportions. Just in the same way 
 as the chemist on the evidence of these facts regards 
 *[A number of similar examples have been described since this 
 paragraph was written.] 
 
254 
 
 RECENT CYTOLOGY 
 
 water as containing oxygen and hydrogen, although 
 the properties of these substances are completely m 
 abeyance, so 1 believe it to be with equally good 
 reason that our theory regards the individual chromo- 
 somes as being presented in the resting nucleus.’’'^ 
 
 Since Boveri expressed this opinion Rosenberg has 
 produced further evidence of an equally convincing 
 kind. He finds that in the case of certain plants the 
 chromosomes do not pass over mto a continuous 
 reticulum during the resting condition of the nucleus, 
 but remain separate, so that the same number of 
 chromatic bodies can be coimted during this stage as 
 during the actual process of mitosis. 
 
 Boveri has also produced evidence to show that 
 different chromosomes play different parts in the 
 economy of the organism. For example, when dif- 
 ferent chromosomes were artificially removed from 
 the nucleus of an embryonic cell by taking advantage 
 of certain abnormal methods of division, the embryos 
 which arose from these cells • developed to different 
 extents and in different abnormal ways. 
 
 This result is of particular interest, because it gives 
 full corroboration to the suspicion, previously enter- 
 tained, that the chromosomes are specially concerned 
 with hereditary processes — with the building up of 
 particular parts of the developing organism into shapes 
 which resemble those of the corresponding parts dis- 
 played by other members of the same species ; and it 
 seems further to show that particular chromosomes 
 
 * Dr. T. Boveri, ‘ Ergebnisse fiber die Konstitution der 
 Chromatischen Substanz des Zellkems,’ p. 22. 
 
CHROMOSOMES 255 
 
 may be specially concerned in the development of 
 particular parts. 
 
 Sutton and others have shown that the different 
 cl .romosomes contained in the same nucleus of a partic- 
 ular animal may be of different shapes and sizes, so 
 th'at each is individually recognisable. It is thus pos- 
 sible to demonstrate that an identically similar set of 
 chromosomes appear at each of several successive cell 
 divisions. In this way additional evidence is afforded 
 of the individual persistence of the chromosomes and 
 of their separate identity. 
 
 We have already pointed out how, in the process of 
 fertilization, the two conjugating germ-cells, as well 
 as the nuclei which they contain, become completely 
 fused together to form a single cell containing only one 
 nucleus. It might have been expected that the sepa- 
 rate chromosomes contained in the conjugating nuclei 
 would also fuse together in pairs during this process, but 
 this is not the case. The paternal and maternal chromo- - 
 somes remain separate, so that the nucleus of the zygote 
 contains twice as many chromosomes as does that of 
 either of the gametes by the fusion of which it arose. 
 This double number of chromosomes reappears at every 
 cell division during the embryonic history of the 
 zygote, and thus the fact is accounted for that [with 
 certain exceptions to be mentioned later (p. 272)] 
 the number of chromosomes in a somatic nucleus is 
 always even. Thus we see that the chromosomes 
 derived from the two parents are present side by side in 
 the nuclei of the offspring, and reproduce themselves by 
 bipartition at every nuclear division which takes place 
 
RECENT CYTOLOGY 
 
 256 
 
 in the zygote. In this way every somatic nucleus of 
 the latter contains a double set of chromosomes, half 
 of them being descended from the chromosomes intro- 
 duced by one parent, whilst the other half came from 
 the second parent. 
 
 There is reason to believe that the set of chromo- 
 somes derived from one parent is complete in itself, 
 containing everything necessary for the development 
 of a normal individual. Indeed, in some cases of 
 parthenogenesis (development of the unfertilized egg), 
 egg cells have been known to develop which contained 
 only a single set of chromosomes. Boveri proved 
 very prettily that the paternal set of chromosomes is 
 equally adequate for complete development. By dint 
 of violent shaking Boveri contrived to remove the 
 nucleus from the egg-ceUs of a sea-urchin, and he 
 afterwards allowed a sperm-nucleus to enter the 
 enucleated egg, which presently developed into a 
 complete embryo. Thus it was shown that the 
 paternal as well as the maternal set of chromosomes 
 is sufficient by itself to determine the proper production 
 of all the organs of the embryo. But Boveri also 
 showed that if any chromosome of the paternal (or 
 maternal) set were wanting in such a case, normal 
 development of the embryo could no longer take place. 
 Let it once more be emphasized that the somatic cells 
 of an ordinary organism contain a double complement 
 of essential nuclear material. 
 
 Since the gametes contain only half as many chromo- 
 somes as the somatic cells, and since the number of 
 chromosomes present in the latter is constant for each 
 
CHROMOSOMES 
 
 257 
 
 species, it follows that either during the formation of 
 the gametes, or at some one or other of the cell 
 divisions leading up to their formation, there must occur 
 a reduction in the number of chromosomes to one-half 
 of their former number. In the case of the higher 
 animals this reduction takes place during the two cell 
 divisions which directly lead up to the formation of the 
 gametes themselves. In plants, on the other hand, the 
 reduction takes place during the formation of those cells 
 which are known as spores. From these, after a certain 
 number of intervening cell generations, the gametes 
 take their origin. These intervening cell divisions in 
 plants are characterized in every case by the appear- 
 ance of the reduced number of chromosomes. In 
 the higher plants, in fact, a generation is, as it were, 
 interposed between the reducing division and the 
 actual formation of the gametes. For the spores are 
 themselves unicellular reproductive bodies like the 
 gametes, but differ from the latter in the fact that 
 they develop without undergoing conjugation, and 
 give rise to a larger or smaller mass of tissue consisting 
 of cells with the reduced number of chromosomes. 
 From the fact that the cells of this gamete-bearing 
 generation contain half as many chromosomes as 
 those of the spore-bearing generation with which it 
 alternates, the generation produced from the spores 
 has been spoken of as the A;-generation in contrast 
 with the ordinary, or 2X-, generation. In animals the 
 Af-generation is reduced to a single generation of cells 
 only, which is represented by the gametes themselves. 
 
 We must next proceed to examine the actual 
 
 17 
 
RECENT CYTOLOGY 
 
 258 
 
 method by which the reduction in the number of the 
 chromosomes is brought about. 
 
 The simplest type of the process of reduction of the 
 chromosomes takes place at the formation of the male 
 germ-ceUs, or spermatozoa, of animals. For the sake of 
 clearness we shall consider the case of an animal in 
 which the somatic cells contain four chromosomes only, 
 and in which the reduced number characteristic of the 
 gametes is therefore two. 
 
 The reduction in number of the chromosomes takes 
 place during two successive cell divisions which 
 immediately lead up to the formation of the germ- 
 cells. A particular mother-cell divides twice in rapid 
 succession, and the four cells thus arising develop into 
 spermatozoa without further subdivision. During 
 these two nuclear divisions the somatic number of 
 chromosomes becomes halved, giving rise to the 
 number characteristic of the gametes. 
 
 Immediately before the first of these divisions the 
 chromosomes become closely associated together in 
 pairs, and in certain cases it has been shown that one 
 member of each pair is very probably the descendant 
 of a chromosome derived from the male parent, whilst 
 the other member of the pair is the descendant of the 
 corresponding maternal chromosome. 
 
 This association of the chromosomes in pairs may 
 be so close, and may take place so early, that when 
 these bodies are first visibly differentiated only half 
 the usual nmnber of them is to be seen. But in these 
 cases, too, it is reasonable to believe that each of 
 the chromosomes actually visible consists of a maternal 
 
THE REDUCING DIVISION 
 
 259 
 
 and a paternal member fused together. Each /of the 
 visible chromatic bodies next divides into four parts, 
 the set of four deeply staining bodies being known as a 
 tetrad. Thus when there are four somatic chromosomes 
 the number of tetrads appearing will be two (Fig. 32). 
 A mitosis now takes place, during which there is no 
 further division of chromosomes, but half of each 
 
 tetrad passes to either pole of the nuclear spindle, 
 so that each daughter nucleus comes to contain two 
 half-tetrads, each consisting of a pair of deeply-staining 
 bodies (Fig. 34). This division is not followed by the 
 production of a resting nucleus, for before any nuclear 
 reticulum is formed, and while the half-tetrads still 
 retain their definite appearance, the datighter nuclei 
 divide again. At this second division in each nucleus 
 
 17—3 
 
26 o 
 
 RECENT CYTOLOGY 
 
 the separate members of each of the two half-tetrads 
 pass to opposite poles (Figs. 35, 36). In the nucleus 
 of each of the four cells which thus arise there is, there- 
 fore, present one quarter of each of the four chromo- 
 somes which originally appeared — one member, that 
 is to say, of each tetrad (Fig. 37). Each of the cells of 
 which we have thus traced the origin develops directly 
 into a single spermatozoon. 
 
 The method of development, or maturation, of the 
 ova, or egg-cells, of animals is in all essential respects 
 similar to the process by which the spermatozoa arise. 
 It differs, however, in the fact that of the four cells 
 which result from the corresponding divisions, one is 
 very large and constitutes the ovum, whilst the other 
 three are very minute, and are apparently of no further 
 importance. In the accompanying diagrams (Figs. 40 
 to 42), the smaller cells, or polar bodies, have been 
 
THE REDUCING DIVISION 
 
 261 
 
 enormously exaggerated relatively to the size of the 
 ovum itself. 
 
 The original tetrad is believed in all cases, and has 
 been actually observed in a few cases, to arise by a 
 separation of the two fused chromosomes, followed by 
 a division of each of these bodies into two. In cases 
 where the chromosomes retain their rod-like appear- 
 ance throughout these changes there would seem to be 
 some doubt as to whether the first of the divisions 
 giving rise to the ‘ tetrad ’ is transverse or longitudinal 
 in direction, and it is possible that the process may be 
 different in different cases. But it is generally agreed 
 that the first division separates the two original 
 chromosomes, and that at the first of the two nuclear 
 divisions which ensue the members of a pair of parental 
 chromosomes pass into separate nuclei. The second 
 division, on the other hand, like an ordinary mitosis, 
 separates halves of chromosomes. This agreement 
 among authorities is explained by the circumstance 
 that those observers who have seen a longitudinal 
 first division believe that the parental chromosomes 
 conjugated side by side, whilst those who describe a 
 transverse division describe also an end-to-end con- 
 jugation of the chromosomes. 
 
 The first of these two ideas is the one illustrated in 
 the accompanying diagram (Fig. 43), representing the 
 behaviour of a single pair of parental chromosomes 
 during the two nuclear divisions which give rise to 
 four sperm cells. The chromosome derived from one 
 parent is shaded, whilst the other is left blank. 
 
 Thuj> the first of the two gamete-producing divisions 
 
263 
 
 RECENT CYTOLOGY 
 
 differs from all other mitoses in the fact that in it an 
 actual separation of whole chromosomes takes place ; 
 it is a qualitative and not only a quantitative division. 
 It is to this mitosis that the term reducing division is 
 properly applied. 
 
 We have to notice that at one stage of the process 
 now described the chromosomes derived from the two 
 parents are in a close state of fusion. It would seem 
 as if the actual conjugation of chromosomes, which 
 failed to take place when the conjugating gametes and 
 their nuclei fused together in the formation of the 
 zygote, was only delayed, and now occurs hundreds or 
 
 thousands of cell generations after the actual process 
 of fertilization, and immediately before the production 
 of those cells which are to give rise to the new 
 individual. 
 
 It may be pointed out that, although the chromo- 
 somes which emerge from this fusion seem to be 
 identical with those which entered into it, yet it is 
 difficult to believe that they have not undergone some 
 change, or exercised some mutual influence upon one 
 another. If no such influence has been exerted, it is 
 difficult to imagine any possible reason for the process 
 of fusion taking place at all. 
 
 In the higher plants a similar reducing division 
 
THE INDUCING DIVISION 263 
 
 takes place at the formation of the spores, which arise 
 in sets of four, each set corresponding to a group 
 of four spermatozoa, or to the ovum and the three 
 polar bodies of an animal. In the case of flowering 
 plants the nuclei contained in the spores make a few 
 further divisions, at each of which the reduced number 
 of chromosomes is to be observed, and one or more of 
 the cells thus finally produced take on the character of 
 germ-cells. The spores are of two kinds, large and 
 small, the latter being the pollen grains. The larger 
 spores give rise to female gametes and the smaller to male, 
 and fertilization takes place in the ordinary manner by 
 a fusion between the nuclei of these germ-cells. 
 
 We have seen so far that the number of chromosomes 
 contained in the somatic nuclei of a given species is 
 always the same, and is always even.* We have also 
 seen that this number is made up of two separate sets 
 derived respectively from the two parents, and that 
 the members of the two sets preserve their separate 
 individuality right through the long series of nuclear 
 divisions which take place during the development of 
 the individual .zygote. A fusion of chromosomes of 
 paternal and maternal origin respectively takes place 
 only in the direct line of ancestry of the germ-cells 
 which are destined to give rise to new members of the 
 species. This process of fusion takes place in animals 
 immediately before the formation of the actual germ- 
 cells, but in plants a larger or smaller number of ceU 
 generations earlier. After fusion the paternal and 
 maternal chromosomes apparently separate, and the 
 • Except in cases such as are described on pp. 270-276. 
 
264 
 
 RECENT CYTOLOGY 
 
 nuclear division which ensues differs from all other 
 mitoses in the fact that instead of merely dissevering 
 halves of chromosomes, the actual somatic chromo- 
 somes separate and become distributed equally between 
 the resulting nuclei ; so that in these nuclei, and in the 
 germ nuclei which arise by their division, the number 
 of chromosomes is reduced to half the somatic number. 
 When fertilization takes place the somatic number of 
 chromosomes is restored by the union of nuclei, each 
 of which contains half that number. 
 
 Is it possible to throw any further light upon the 
 meaning of these facts regarding the behaviour of the 
 minute constituent parts of organisms ? 
 
 Let us return to Mendel’s experimental discovery, 
 of which an account was given in the last two chapters, 
 and let us consider the case of a cross between parents 
 which differ in respect of two pairs of allelomorphs. 
 Expressing these pairs as A-a and B-h, Mendel 
 showed that the germ-cells of the cross-bred or hetero- 
 zygote bear in equal numbers the combinations ^5, Ab, 
 aB, and ab. Now, it seems clear from this behaviour 
 that the allelomorphs must be represented in the cells 
 of the organism by some kind of definite particles, 
 which remain distinct from one another throughout 
 aU the cell divisions of the body, since we know that 
 at the formation of the germ-ceUs these characters are 
 capable of becoming completely segregated. Let us, 
 then, trace the behaviour of the allelomorphs in a 
 diagrammatic way, regarding each as a distinct par- 
 ticle. These particles we may distinguish by certain 
 
AND MENDEL’S LAW 
 
 265 
 
 letters. A and a are the allelomorphs of one pair, 
 B and b those of the other, and we will suppose that 
 one of the parents exhibits the characters A and b 
 and the other the characters a and B (Fig. 44). Then, 
 in the zygote resulting from fertilization. A, a, B, and b 
 will aU be present. 
 
 Since all the cells, at least in the direct line of 
 ancestry of the gametes, must contain every allelo- 
 morph, it will be necessary for the particle representing 
 each allelomorph always to divide into two before a 
 ceU division takes place, for only in this way can some- 
 thing corresponding to each allelomorph pass into each 
 of the two cells produced by the division. And a 
 similar process will be repeated at each somatic 
 mitosis (Fig. 44). At the formation of the germ-cells, 
 however, or at some preceding cell division, the two 
 members of each pair of allelomorphs must become 
 separated from one another in such a way that the 
 particles originally derived from different parents 
 pass over into different cells. When two pairs of 
 allelomorphs are concerned, this process of separation 
 can take place in either of the two ways shown in 
 Fig, 45. And the experimental evidence shows that 
 the two methods occur with equal frequency in the 
 formation of the germ-cells of the same heterozygote. 
 
 Anyone who has succeeded in following the above 
 account of the behaviour of the supposed particles 
 representing Mendelian allelomorphs in the cells of a 
 hybrid organism, on comparing it with the preceding 
 description of the behaviour of chromosomes in the 
 
266 
 
 RECENT CYTOLOGY 
 
AND MENDEL’S LAW 
 
 267 
 
 somatic and reducing divisions respectively, can 
 scarcely fail to be struck by the extraordinary simi- 
 larity between the two processes. It seems quite 
 clear that there must be some real connection between 
 the behaviour of chromosomes as seen microscopically 
 on the one hand, and the behaviour of allelomorphic 
 characters as deduced from the results of experiment 
 on the other ; and that the evidence derived from these 
 two forms of study is bound to be of considerable 
 mutual benefit. 
 
 At first sight it might be thought that the chromo- 
 somes are the actual bearers of Mendehan characters, 
 in the sense that each chromosome represents a single 
 allelomorph ; and, indeed, there is no fundamental 
 difference between the behaviour of chromosomes and 
 that of our supposed character-bearing particles. But 
 there is, at least in some cases, a fatal objection to 
 this belief in the fact that in certain plants the number 
 of separate allelomorphic pairs which may be bom by 
 a hybrid is greater than the reduced number of chromo- 
 somes which the gefm-eeUs of this hybrid contain. 
 For instance, in the case of the pea the reduced 
 number of chromosomes is seven, and Mendel himself 
 described the behaviour of seven independent pairs of 
 allelomorphs in peas. Recent study has revealed the 
 presence of at least four additional pairs of allelo- 
 morphs in these plants, aU of which are probably equally 
 independent of one another. 
 
 We must, therefore, seek a different explanation, 
 and de Vries has recently suggested one which up to 
 
268 
 
 RECENT CYTOLOGY 
 
 the present time appears the most likely to represent 
 the true account of the phenomena. De Vries’ ex- 
 planation is associated with the finer structure of the 
 chromosomes themselves, a subject upon which we 
 have not hitherto entered. Under high powers of the 
 microscope, and after very careful preparation, it is 
 possible to observe that each chromosome contains a 
 number of separate darkly-staining granules which are 
 known as chromomeres. When the pairs of parental 
 chromosomes fuse together previous to the reducing 
 division, the chromomeres which they contain appear 
 to meet together in corresponding pairs. The members 
 of each pair fuse together completely, afterwards 
 separating as the chromosomes separate; 
 
 Fig. 46. 
 
 De Vries supposes the Mendelian allelomorphs to 
 be contained in the chromomeres, and that when these 
 granules fuse together an exchange of allelomorphs 
 takes place between the chromosomes. This ex- 
 change proceeds in such a way that when the chromo- 
 somes separate after fusion, it is a matter of simple 
 chance whether a particular allelomorph has remained 
 in the chromomere which originally contained it, or 
 has passed over into the other member of the pair. 
 Thus, in a sufficient number of cases we should get 
 aU possible chance distributions of allelomorphs be- 
 
CHROMOMERES 
 
 269 
 
 tween the two chromosomes, except that, of course, the 
 two members of the same pair of allelomorphs would 
 never coexist in the same chromosome. Since the 
 two chromosomes of a pair pass into different germ- 
 cells, precisely that chance distribution of allelomoi’phs 
 which is required on the Mendelian theory would thus 
 be arrived at. 
 
 De Vries’ explanation throws light on one pheno- 
 menon which is not accounted for on the supposition 
 that each chromosome represents a separate allelo- 
 morph. In the diagrams previously given of the 
 behaviour of Mendelian characters within the cells we 
 have given no indication of a conjugation in pairs 
 previous to the reducing division. Such a process of 
 fusion is, however, one of the most marked phenomena 
 in the behaviour of the chromosomes at the parallel 
 stage of their existence. On the chromosome-allelo- 
 morph view, the phenomenon of mitosis as bringing 
 about an equal division of hereditary particles between 
 the cells, and the process of reduction in the number 
 of the chromosomes, are both accounted for, but there 
 is no explanation of the fusion between the pairs of 
 chromosomes. On de Vries’ view, however, this process 
 is necessary in order to bring about the necessary redis- 
 tribution of allelomorphs between the chromosomes, and 
 so between the germ-cells into which the latter pass. 
 
 [More recently Professor T. H. Morgan has brought 
 forward a much more complete theory of the relation 
 between chromomeres and Mendelian characters, based 
 on the behaviour of characters which show gametic 
 
270 
 
 RECENT CYTOLOGY 
 
 coupling. He supposes that the factors for all the 
 characters of a species which show gametic coupling 
 with one another are borne by the same chromosome. 
 In the Fruit-fly {Drosophila) there is rather striking 
 evidence that this is the case, for there are four groups 
 of coupled characters, which may be called A, B, C, D. 
 Any character in one group is more or less closely 
 coupled with any other character in the same group, 
 but quite independent of characters in the other 
 groups. Groups A, B, and C each include a number 
 of characters, while group D is very small, having only 
 two or three characters which are known at present. 
 Now, in the germ-cell of Drosophila there are four 
 chromosomes, three large and one very small, and the 
 inference is therefore made that each chromosome 
 corresponds with a group of coupled characters. 
 Further, one chromosome is an ‘ idiochromosome,’ or 
 sex-chromosome (see the pages immediately following), 
 and all the characters of group A are ‘ sex-limited ’ in 
 transmission — i.e., they are transmitted by the male 
 only to those offspring which also receive the sex- 
 chromosome. The correspondence between the groups 
 of coupled characters and of the chromosomes in 
 Drosophila is thus so close as to make the hypothesis 
 very probable that the chromosomes actually are the 
 bearers of Mendelian characters.* 
 
 * For further evidence of a different kind that these char- 
 acters are borne by chromosomes in Drosophila, the reader is 
 referred to C. B. Bridges' paper, ‘Non-Disjunction as Proof of 
 the Chromosome Theory of Heredity,* in Genetics^ voL i., No. i, 
 January, 1916, or to Morgan's ‘Physical Basis of Heredity,’ 1919* 
 
CHROMOMERES 271 
 
 The problem then arises, how is it that the char- 
 acters are only partially, and not completely, coupled ? 
 Morgan’s answer is that when the chromosomes unite 
 in pairs before the reduction division they do not 
 simply lie side by side, but become twisted round one 
 another, and that instead of untwisting and becoming 
 separated unchanged before the nuclear division, they 
 may break across, so that what was part of one chromo- 
 some becomes attached to what was part of the 
 other. 
 
 If now the determiners for the characters are arranged 
 in series along the chromosome like the chromomeres 
 represented in Fig. 46, when two determiners are placed 
 close together, it is unlikely that the break will occur 
 between them, and such characters will be closely 
 coupled. But if they are placed farther apart the 
 break will more often come between them, and they 
 will more often become separated, so that the coupling 
 is less strong. And finally, if the determiners are 
 very far apart, the break will occur equally often in 
 such a way that they remain associated or become 
 separated, and in such a case there will be no 
 coupling. 
 
 It should be mentioned that the twisting of chromo- 
 somes round each other has not been observed in 
 Drosophila, which is not well suited to cytological work, 
 but has been seen in other animals and plants.] 
 
 The possibility still remains that in certain cases 
 particular characters may be associated with par- 
 ticular chromosomes as a whole, and we shall next 
 
272 
 
 RECENT CYTOLOGY 
 
 proceed to describe what actually seems to be an 
 example of this sort.* 
 
 The case we have to describe is directly concerned 
 with one of the most interesting and elusive of bio- 
 logical problems — namely, the problem of the heredity 
 of sex. Until recently opinion has been largely 
 dominated by the view that sex-production is in 
 general controlled by the influence of external con- 
 ditions. A large number of the earlier researches, 
 and some of the later ones, have, in fact, seemed to 
 show that sex is thus determined. The most recent 
 knowledge appears, however, to point conclusively to 
 the belief that sex is already determined in the fer- 
 tilized ovum. The fact that, so far as the evidence 
 goes, where more than one individual develops from 
 the same fertilized egg all are of the same sex seems 
 to point conclusively in this direction; and further 
 valuable evidence has recently been adduced. Professor 
 E. B. Wilson has recently investigated the behaviour 
 of the chromosomes in the somatic cells and in the 
 germ-cells of a particular species of insect known as 
 Protenor heljragi. The case afforded by this animal 
 is remarkable, inasmuch as the somatic cells in the 
 male, and only in the male, contain an odd number 
 
 * [In the last few years definite evidence of the relation 
 between chromosomes as a whole and hereditary characters 
 has been obtained in several plants; e.g., the lata variety of 
 CEnothera is characterized by the possession of an extra 
 chromosome, and the variety gigas has twice the normal 
 number (Gates). Remarkable observations of a similar kind 
 have been made by R. P. Gregory in Primula sinensis (Proc. 
 Roy. Soc., B. 87, 1914, p. 484).] 
 
HEREDITY OF SEX 
 
 273 
 
 of chromosomes. An irregularity is accordingly intro- 
 duced into the process of fusion of the chromosomes 
 in pairs, which, as already described, always precedes 
 the formation of the germ-cells with their reduced 
 number of chromosomes. In the case of the male 
 Protenor all the chromosomes fuse in pairs except one, 
 which is, of necessity, left over. This odd chromosome 
 is described as the heterotropic chromosome. The 
 female Protenor has one more chromosome in its 
 somatic cells than the male, thus making up an even 
 number; that is to say, in the female the pair to 
 the odd chromosome of the male is present, so that 
 there are two heterotropic chromosomes, or idio- 
 chromosomes. These fuse and separate in the reducing 
 division, which thus proceeds in the normal manner 
 in this sex. In the male, on the other hand, when 
 the reducing division occurs, the heterotropic chromo- 
 some passes complete into one of the resulting cells. 
 In the second gamete -producing division, every chro- 
 mosome present having divided into two, the products 
 of this division pass into different gametes. These 
 latter divisions are of two kinds, since in one of them 
 the heterotropic chromosome takes part, whilst in the 
 other it is wanting; consequently, two out of the 
 four spermatozoa eventually produced contain the 
 heterotropic chromosome and two do not. (Only one 
 spermatozoon of each kind is shown in Fig. 47.) Thus 
 there is a differentiation of the spermatozoa into two 
 different kinds, and one of these kinds contains a 
 chromosome less than the other. On the other hand, 
 
 18 
 
274 
 
 RECENT CYTOLOGY 
 
 every egg (as well as every polar body) contains an 
 idiochromosome . 
 
 In fertilization some of the eggs become impregnated 
 by spermatozoa containing the heterotropic chromo- 
 some. Such eggs invariably develop into females 
 having a pair of idiochromosomes in each somatic 
 
 ov, ovum ; ph, polar body ; sp, spermatozoa {a and b the two kinds). 
 
 cell. Other eggs are fertilized by spermatozoa lacking 
 the heterotropic chromosome, and these become males, 
 their somatic cells containing only the single hetero- 
 tropic chromosome derived from the egg. The ac- 
 companying diagram illustrates the behaviour of the 
 chromosomes during these processes. The hetero- 
 tropic chromosomes are represented as black, whilst 
 
HEREDITY OF SEX 
 
 275 
 
 the remaining chromosomes are left white, and for the 
 sake of simplicity only two pairs of the latter are 
 indicated in the somatic cells. 
 
 [Cases like that of Protenor, in which the male has 
 one ‘ sex-chromosome ’ and the female two, are now 
 known in a large number of animals — for example, in 
 many Insects, in certain Worms, Molluscs, etc., and in 
 several Mammals, including (probably) Man.] The 
 facts render it scarcely possible to doubt that there 
 is a causal relationship between the characteristics of 
 the female sex and the presence of two heterotropic 
 chromosomes, and that a similar connection exists 
 between maleness and the presence of only one. 
 
 [Several explanations of the mechanism of sex-deter- 
 mination by means of these ‘ sex-chromosomes ’ have 
 been suggested. It was originally supposed that the 
 two chromosomes of the female, though alike in appear- 
 ance, were different physiologically, one bearing a 
 factor for femaleness and the other for maleness, and 
 that the female character was dominant. The hypo- 
 thesis now most generally accepted, however, is that 
 the sex-chromosomes are aU alike, and that the differ- 
 ence between the two sexes is produced by the presence 
 of two sex-chromosomes in the female and one in the 
 male. The case would then be comparable with that 
 of a pair of Mendelian allelomorphs in which the 
 dominant character represents the presence of some- 
 thing which is absent in the recessive. In cases of the 
 kind here considered, femaleness is due to the presence 
 of a determiner (the extra chromosome) which is 
 
I 
 
 276 RECENT CYTOLOGY 
 
 absent in the male. As will be seen later, there are 
 other cases in which this relation is reversed, and in 
 which the male has a chromosome which is lacking in 
 the female.] 
 
 In various other insects, some of them closely allied 
 to Protenor, the somatic cells of the male, like those of 
 the female, contain each a pair of idiochromosomes ; 
 but in the male one member of the pair is much larger 
 than the other, whilst in the female they are of equal 
 size. The behaviour of the larger member of the 
 unequal pair of chromosomes, in the various nuclear 
 processes which occur during the life-history, is pre- 
 cisely like that of the single heterotropic chromosome 
 of Protenor. It is still possible to regard this chromo- 
 some as representing a single sex-chromosome, and to 
 suppose that the process of sex determination is pre- 
 cisely similar in the two cases. On this supposition, 
 the smaller idiochromosome is regarded as being with- 
 out function so far as sex is concerned. 
 
 In a third group of insects, some nearly related to 
 Protenor, both male and female sexes bear alike a pair 
 of idiochromosomes of equal size. Here, again, it is 
 possible to apply the same theory of sex determination 
 by simply disregarding one of the idiochromosomes of 
 the male as unimportant.-^ We may suppose, in fact, 
 that one of these chromosomes corresponds to the 
 smaller idiochromosome of the preceding case, and that 
 it takes no essential part in these phenomena. The 
 fact that this chromosome takes no active part in these 
 processes may, indeed, have led to its reduction in the 
 
HEREDITY OF SEX 
 
 277 
 
 second of the three species, and to its final disappear- 
 ance in the first. 
 
 Thus, by dint of a good deal of speculation, Wilson 
 and others have arrived at a possible Mendelian descrip- 
 tion of the phenomenon of sex in a species in which 
 the chromosomes of male and female are alike; and it 
 is a description which has its basis in actual phenomena 
 observed in a number of related animals.- 
 
 [In the cases considered above, the female has two 
 similar idiochromosomes and the male either only one, 
 or two which are unlike one another. The female thus 
 produces eggs which are all alike, while the male pro- 
 duces two kinds of spermatozoa, one bearing an idio- 
 chromosome like those of the female, the other either 
 no such chromosome or a dissimilar one. Now, in 
 animals belonging to groups in which phenomena of 
 this sort have been observed, it has also been observed 
 that there are certain characters which are transmitted 
 by the male only to his daughters, while the female 
 transmits them impartially to her offspring of both 
 sexes. The best -known example of this sort is Droso- 
 phila (see.41. in which all the characters of group 
 A are transmitted by the male only to his daughters 
 (sex-limited transmission by the male). In Man the 
 same sort of thing occurs in several abnormalities, 
 notably in colour-blindness. Colour-blindness is due 
 to the loss of a factor for normal vision, and this factor 
 for normality is transmitted by a man only to his 
 daughters, but by a woman to her children of both 
 sexes. Hence a colour-blind man married to a normal 
 
278 
 
 RECENT CYTOLOGY 
 
 woman has normal children, while a colour-blind woman 
 married to a normal man has normal daughters and 
 colour-blind sons. Colour-blind men are frequent, 
 because the daughters of colour-blind men, though 
 normal themselves, transmit the affection to half their 
 sons; but colour-blind women are very rare, because 
 they can only be produced by the marriage of a colour- 
 blind man to a woman who is herself heterozygous for 
 the affection. 
 
 In Moths and Butterflies, and also in Birds, exactly 
 the converse condition is found. In them, instead of 
 the male transmitting characters only to his daughters, 
 it is the female which transmits certain characters 
 only to her male offspring, while the male transmits to 
 sons and daughters alike. In Fowls, for example, if a 
 barred hen is mated with an unbarred cock, all the 
 cockerels will be barred and the pullets plain, while a 
 cock of a barred breed mated with an unbarred hen 
 produces barred chickens of both sexes. And in the 
 Currant Moth {Abraxas grossulariata) , in which this 
 form of sex-limited inheritance was first investigated, 
 it has been shown by Doncaster that, at least in certain 
 strains, the female has a single idiochromosome and 
 the male two — i.e., exactly the converse condition of 
 that found in Protenor, etc. In this case, therefore, 
 there are two sorts of eggs, male-producing and female- 
 producing, but only one kind of spermatozoa, and it is 
 the male-producing eggs (those which have the extra 
 chromosome) which bear the sex-limited character. 
 
 These facts, therefore, seem to prove that sex is 
 
 1 
 
HEREDITY OF SEX 
 
 279 
 
 inherited as a Mendelian factor, in some cases female- 
 ness, in others maleness, being due to the presence of a 
 factor which is absent in the other sex; and the whole 
 category of characters which have sex-limited trans- 
 mission appears to be only a special case of gametic 
 coupling, due, if the chromosome theory is correct, to 
 the association of the factors for sex-limited char- 
 acters and for sex itself in the same chromosome.] 
 
 By way of further illustrating the far-reaching im- 
 portance of the information which has been rendered 
 available by the combined use of experimental and 
 cytological methods, we may here briefly criticise the 
 celebrated theory of inheritance put forward by Weis- 
 mann in 1892 under the name of the ‘ Germ-Plasm 
 Theory.’ Some notice of this theory, which might 
 otherwise have been permitted to go the way of similar 
 valuable provisional hypotheses, is rendered almost 
 necessary by the circumstance of its having been 
 revived in a prominent manner in the English transla- 
 tion of Weismann’s book, ‘ The Evolution Theory.’ 
 In this book, published in 1904, the bearing of the 
 Mendelian evidence upon the subject of inheritance is 
 practically ignored ; although, in the face of the definite 
 experimental information now rendered available, 
 biologists are beginning to realize that the circum- 
 stantial evidence, formerly so much relied upon, will 
 in future constitute a much less prominent feature in 
 these discussions. 
 
 Weismann’s theory of inheritance, and the Theory 
 of Ancestral Heredity in its original form, are based 
 
28 o 
 
 RECENT CYTOLOGY 
 
 upon a common assumption, which is now shown by 
 Mendel’s discovery to have been unfounded. This is 
 the assumption that all ancestors of the same degree — 
 e.g., grandparents — make a substantially equal con- 
 tribution to the hereditary qualities of the offspring. 
 Mendel has shown that in the case of particular 
 hereditary characteristics this is not the case. 
 
 But if we venture to criticise Weismann’s conception 
 in the light of more recent knowledge, it must not be 
 forgotten that biology, and especially modern cytology, 
 owes a great debt to Weismann. To Weismann is due 
 the conception of the isolation of the germ-cells from 
 somatic influences, a view which is in complete accord- 
 ance with the Mendelian view of the inheritance of 
 definite characters. And it was Weismann who first 
 emphasized the belief that the chromosomes represent 
 those parts of the nucleus which are specially concerned 
 in the process of heredity. These conceptions— 
 which, indeed, constitute an essential part of his own 
 theory of heredity — have stood the test of time in an 
 admirable manner. 
 
 Let us turn our attention, then, for a short space to 
 the Germ-Plasm Theory of inheritance. On Weis- 
 mann’s theory, as in most other theories of heredity 
 from the time of Darwin and Nageli downwards, the 
 separate parts of the living organism are supposed to 
 be represented by separate material particles in the 
 germ-cells. These representative particles are known 
 as determinants. A complete set of determinants in 
 which every part of the organism is thus represented 
 
THE GERM-PLASM THEORY 281 
 
 constitutes an id. So far Weismann’s hypothesis is in 
 close agreement with the idea of representative particles 
 which we are driven to adopt by the facts of Mendelian 
 inheritance, except that, following de Vries, we should 
 speak of separate characters rather than parts as 
 being thus represented ; for there seems to be no doubt 
 that the same character-determinant can affect the 
 development of a number of different parts. But at 
 the next step the Mendelian parts company with 
 Weismann. The latter assumes that the cells of an 
 organism contain a large number of ids, or complete 
 sets of determinants, half of the total number being 
 derived from either parent, and that, although at the 
 reducing division which precedes the formation of the 
 gametes the total number of ids is reduced to half of 
 what it was in the somatic cells, still several ids 
 derived from each parent are present in every germ- 
 cell. 
 
 Thus the reduced number of chromosomes in the 
 germ-cells is regarded as containing all the primary 
 constituents of both parents. And it is an essential 
 point in Weismann’s theory that he regards a given 
 germ-cell as containing a considerable number of ids 
 derived from its ancestors, all near ancestors being 
 thus represented. 
 
 But Mendel’s experiments and others of the same 
 kind show, in the case of a great number of different 
 characters, that although every essential character is 
 represented in every germ-cell, yet each Mendelian 
 character is represented by a paternal or a maternal 
 
282 
 
 RECENT CYTOLOGY 
 
 determinant only, and not by both. Thus, not 
 only are all immediate ancestors not represented 
 in the germ - cells in respect of any particular 
 character, but only one of the parents is so repre- 
 sented — to the complete exclusion, so far as we can 
 tell, of the other parent. In fact, we are led to 
 believe that the germ-cells contain one set of de- 
 terminants only — a single id — whilst the somatic cells 
 contain two ids only. The Mendelian theory is thus 
 seen to be considerably simpler than the germ-plasm 
 theory, which it replaces. At the same time it must 
 not be forgotten that many of the conceptions used in 
 the Mendelian expression of the facts are borrowed 
 from Weismann’s theory, and that but for Weismann’s 
 work it would have been impossible for us to have 
 got so far in the co-ordination of the facts derived 
 from experiment and microscopic observation re- 
 spectively. 
 
 The preceding sketch may serve to show how Mendel’s 
 observations have been found to throw light upon many 
 of the facts of c3d;ology the meaning of which was pre- 
 viously obscure; and how it affords at the same time 
 a criterion by which may be tested the truth of theories 
 based upon the interpretation of minute phenomena 
 only made visible by the highest powers of the micro- 
 scope. The disinterment of Mendel’s discovery took 
 place in 1900 ; and the rapid manner in which the 
 facts of cytology have been found to fall into line with 
 Mendelian conceptions augurs well for the future 
 progress of discovery in these fields. 
 
THE GERM-PLASM THEORY 
 
 283 
 
 BIBLIOGRAPHY. 
 
 Wilson, E. B. : The Cell in Development and Inheritance 
 second edition, 1900. 
 
 ,, „ Recent Researches on the Determination 
 
 and Heredity of Sex. Science, January 8, 
 1909. 
 
 BoverI, T. : Ergebnisse liber die Konstitution der chroma- 
 tischen Substanz des Zellkerns, 1904. 
 
 Weismann, a.: The Germ-Plasm: a Theory of Heredity 
 (English translation by W. N. Parker and H. Ronnfeldt), 
 
 1893- 
 
 Morgan, T. H. : Heredity and Sex, 1914. 
 
 ,, ,, The Mechanism of Mendelian Heredity, 1915. 
 
 „ „ The Physical Basis of Heredity, 1919, 
 
 Doncaster, L. : The Determination of Sex, 1914. 
 
CHAPTER X 
 
 EUGENICS 
 
 Eugenics is defined as ‘ the study of agencies under 
 social control that may improve or impair the racial 
 qualities of future generations, either physically or 
 mentally.’ Sir Francis Galton, the pioneer of this 
 study, may well be regarded as one of the foremost 
 figures in recent science. His claim to this regard is 
 based upon a pre-eminence in many different branches 
 of knowledge — a claim which those to whom his work 
 is as yet unknown will find substantiated with all 
 becoming modesty in his ‘ Memories of My Life. 
 Much of Galton’s work in other fields may be 
 forgotten, overshadowed as it is by his foundation 
 of the science we are about to describe. It will be 
 for future generations to discover that niche in the 
 temple of fame which is fitting for the man who first 
 pointed out a feasible direction in which humanity 
 might set about its own improvement, instead of 
 resting content in the vain expectation of being im- 
 proved vicariously. 
 
 We have neither the mathematical equipment, nor 
 have we here the space, to enter into the statistical 
 inquiries upon which Galton’s conclusions are mainly 
 
 284 
 
Francis Galton. 
 
 [To face p. 284. 
 
THE LIBRARY 
 Of THE 
 
 UNIVERSITY OF ILLINOIS 
 
GALTON’S RESEARCHES 
 
 285 
 
 based. For an account of these the reader must be re- 
 ferred to the admirable popular treatises enumerated at 
 the close of the present chapter. In spite of the rather 
 wide divergence of these studies from those with which 
 this book is mainly concerned, the agreement between 
 the conclusions on race-improvement drawn by the 
 students of Genetics, on the one hand, and by those 
 of Biometry, on the other, is a remarkable one, and 
 may perhaps be taken to indicate that both these 
 methods are right in their several directions. 
 
 Consequently we propose attempting a summary 
 of the line of researches and arguments which led 
 Galton to his conclusion that the human race 
 is capable of vast improvements in physique, in 
 beauty, in character, and in intellect. The importance 
 of this conclusion is “augmented by the corollary that 
 acquisition of these improvements leads to a keener 
 appreciation of their value, and, incidentally, to the 
 greater happiness of mankind. But the student of 
 Eugenics does not rest satisfied with conclusions. He 
 proposes to utilize the great forces of fashion and 
 public opinion as agents of modification and improve- 
 ment by diverting their influence into the right 
 direction and out of their present remarkably wrong 
 direction. 
 
 The first link in the chain of evidence was forged 
 long ago by Darwin, when he showed how far man 
 had already risen from a simpler and lower type of 
 animal. The evolution of man is now a part of the 
 ordinary intellectual creed of most educated men, and 
 yet few politicians or charitable people pause to apply 
 
286 
 
 EUGENICS 
 
 this belief to the present or to the future. Few 
 realize the full significance of the inferences that, if 
 man has risen in the past, he may still rise higher in 
 ages yet to come ; if the type has undergone modifica- 
 tion in the past, it may even now be changing. Those 
 who move the forces of money or of popular opinion 
 should take heed how their actions affect the rate 
 and direction of this possibly momentous change, 
 i _ The evolution of man, like that of other animals, 
 
 I is believed to have been largely due to the effect of 
 \ selection. Whether the variations selected were great 
 or small makes little difference to the argument. In 
 former days this selective action was exerted by fierce 
 competition, which led to the survival of individuals 
 endowed with certain qualities and to the extinction 
 of other individuals differently constituted. 
 
 Modem civilization and humanitarianism have 
 effectually set aside the action of natural selection. 
 The result at the present day indicates that the upward 
 progress of the race has probably almost ceased, and 
 that there is every danger of finding that a downward 
 journey has begun. This is an inevitable conclusion 
 to those who accept the well-established facts (i) that 
 men are not born equal, but differ widely in their 
 birthright in respect of every physical and mental 
 character, and (2) that, although every man is free to 
 become the father of a family, yet the tendency to 
 bring up large families is becoming relatively smaller 
 among those classes of society which we must regard 
 as the best endowed, both physically and mentally. 
 
 Vi Are there any practicable means by which this 
 
PROGRESS OF THE RACE 
 
 287 
 
 tfndency to degenerate can be combated and changed 
 into an upward bias ? In order to discover such a 
 means it is needful to gather all possible information 
 with regard to the primary factors of organic evolution 
 in the human race — that is to say, variation and 
 heredity. Galton was the first to show that the laws 
 of variation and heredity in man are closely similar 
 to the same laws in other organisips. He showed, too, 
 that mental and moral characters are inherited just 
 as strongly as physical characters — a fact which is 
 by no means so generally appreciated as it should 
 be, in spite of the further evidence adduced by Prof. 
 Karl Pearson. 
 
 Many people believe that the progress of the race 
 can be directly and permanently affected by improve- 
 ments in education and the amelioration of social 
 conditions. It is certain that the qualities of any 
 person — health, character, efficiency, and so forth, 
 depend upon his environment and upbringing, as well 
 as upon his hereditary endowments. It is therefore 
 necessary, before we proceed farther, to arrive at some 
 estimate of the relative importance of inherent qualities 
 and of education — of nature and nurture respectively. 
 Practically the only piece of good evidence upon this 
 point is one which we owe to the researches of Galton. 
 
 Galton’s data are derived from the history of twins. 
 Now, human twins are of two kinds. One of these 
 kinds corresponds to the young of those animals which 
 normally bear two or more at a birth, each being 
 derived from a separate ovum, whilst the second kind 
 is the result of the development of two embryos from 
 
288 
 
 EUGENICS 
 
 the same fertilized ovum. In the former case the 
 twins bear no more resemblance to one another than 
 any other ordinary pair of brothers or sisters, and they 
 are often of opposite sex ; in the latter case the twins 
 are known as ‘ identical,’ and are always of the same 
 sex. Our present knowledge of Genetics — not, of 
 course, available to Galton when he first wrote upon 
 this subject — leads us to believe that such twins are 
 indeed identical, and bear precisely the same hereditary 
 endowment.* It is as though a single individual were 
 divided into two parts, and each part grew into a 
 complete person. Galton quotes numerous stories 
 of the frequent confusion between identical twins. 
 ‘ I have one case,’ he writes, ‘ in which a doubt remains 
 whether the children were not changed in their bath, 
 and the presumed A is not really B, and vice versa.’ 
 In the records thus collected there is excellent material 
 for discovering how far identical twins can come to 
 differ from one another when exposed to different 
 conditions, and, on the other hand, for ascertaining 
 how far distinct twins brought up under similar con- 
 ditions can come to resemble one another. Galton 
 obtained information with regard to eighty cases of 
 probably identical twins. In many of these cases the 
 twins remained closely alike in temper and character, 
 as well as in appearance, up to an advanced age. 
 When differences arose in later life, these were generally 
 due to some illness or accident which affected one of 
 the twins. The gradual influence of a number of small 
 causes seemed to have very little effect in this respect. 
 
 * [Cases are, however, known In which twins, apparently 
 ‘ identical ’ in most characters, differ in one or more which are 
 certairffy congenital.] 
 
EFFECT OF ENVIRONMENT 
 
 289 
 
 ‘ In not a single instance,’ Galton writes, ‘ have I met 
 with a word about the growing dissimilarity being due 
 to the action of the firm, free will of one or both the 
 twins which had triumphed over natural tendencies ; 
 and yet a large proportion of my correspondents happen 
 to be clergymen, whose bent of mind is opposed, as I 
 feel assured, to a necessitarian view of life.’ The only 
 conclusion which can be drawn from these observations 
 is that the relative influence of nurture as opposed to 
 nature is very much smaller than has been generally 
 supposed; and this inference is confirmed by the 
 history of dissimilar twins. The descriptions of the 
 latter agreed without exception in showing an entire 
 absence of convergence of character in cases where 
 the whole training and environment were closely 
 similar. From this evidence it seems right to conclude 
 that the hereditary nature of a man is more important 
 than his training and circumstances in determining 
 his adult mental and physical equipment, and the 
 result of common observation may be said to be in 
 agreement with this conclusion. 
 
 You may educate generation after generation, and 
 yet the starting-point from which each individual has 
 to begin his struggle upwards may remain the same, 
 even though each may struggle a little farther than the 
 one who came before him. On the other hand, we 
 have all of us met a few of those happy people to 
 whom it seemed second nature to do the right thing, 
 and for whom the difficulties of life appear to have 
 no menace. These qualities are those of nature, and 
 not of nurture, and their children will inherit them. 
 Some are born great, some achieve greatness, and 
 
 19 
 
290 
 
 EUGENICS 
 
 some have greatness thrust upon them. But those 
 who are bom great — meaning by this not those in 
 high position, but great in themselves — are the men 
 and women to whose descendants we must look for 
 the future greatness of mankind. 
 
 If this view be justified, we shall be obliged to revise 
 very carefully our ideas of what is desirable in social 
 legislation. Important as education, sanitation, and 
 the like may be, their effects are strictly limited. 
 ;The relative birth-rate of good and bad stock, on the 
 other hand, is the fundamental factor. Its influence 
 in a single generation may be so small as to be barely 
 recognizable, but its effect increases from generation 
 to generation, and, moreover, it is an effect which, once 
 produced, is quite irrevocable. We have to consider, 
 therefore, how the relative incidence of the birth-rate 
 falls at the present time, and what are the causes 
 which affect it. We have to consider especially 
 whether existing and proposed legislation of which the 
 intention is to improve the education and condition 
 of upbringing of children has any effect, direct or in- 
 direct, upon the relative birth-rate of different classes 
 of society. For if such legislation be found to favour 
 the rapid reproduction of the less-efficient, it will 
 become a matter for serious consideration whether the 
 advantages of mental and physical improvement in the 
 individual are not being purchased too dearly at the 
 expense of posterity. 
 
 Individuals, as well as Governments, will do well 
 to look closely into the possible results of the best- 
 intentioned proceedings. To quote Galton once more : 
 
SIZE OF FAMILIES 
 
 291 
 
 * It is known that a considerable part of the huge 
 stream of British charity furthers by indirect and 
 unsuspected ways the production of the Unfit ; it is 
 most desirable that money and other attention be- 
 stowed on harmful forms of charity should be diverted 
 to the production and well-being of the Fit. For 
 clearness of explanation we may divide newly married 
 couples into three classes, with respect to the probable 
 civic worth of their offspring. There would be a small 
 class of “ desirables,” a large class of ” passables,” of 
 whom nothing more will be said here, and a small 
 class of " undesirables.” It would clearly be advan- 
 tageous to the country if social and moral support, as 
 well as timely material help, were extended to the 
 " desirables,” and not monopolized, as it is now apt 
 to be, by the “ undesirables.” 
 
 Let us consider the relative birth-rates of different 
 classes of the community. This is in itself a very large 
 question. We have only space here to record a few 
 figures, taken from a paper by Prof. Karl Pearson 
 than whom no better authority could be quoted. 
 
 Here are a few of the figures, in terms of the average 
 size of family, childless marriages being excluded ; 
 
 Group I. 
 
 Criminals . . 
 
 English deaf-mutes . . 
 London, mentally defective 
 
 6-6 
 
 62 
 
 7-0 
 
 Group II. 
 
 English middle class 
 London normal artisan 
 English intellectual class 
 
292 
 
 EUGENICS 
 
 This means, if one takes into consideration the fact 
 that members of the first group probably marry several 
 years earlier, on the average, than members of the 
 second group, that the mentally defective class is 
 reproducing itself about t\yice as rapidly as the intel- 
 lectual class. There can be no doubt as to what the 
 result of such a proportion as this must be. Let us 
 quote Prof. Pearson’s own conclusion : ‘ The progress 
 of the race inevitably demands a dominant fertility 
 in the fitter stocks. If that principle be not recognized 
 as axiomatic by the mentally and bodily fit themselves, 
 if the statesman do not accept it as a guide in social 
 legislation, then the race will degenerate until, sinking 
 into barbarism, it may rise again through the toilsome 
 stages of purification by crude natural selection.’ Or, 
 it may be added, until it be annihilated by a more 
 efficient neighbour. 
 
 With regard to the cause of these differences in the 
 birth-rate of different sections of the community, there 
 can be very little doubt that they are largely due to 
 deliberate restraint on the part of the more prudent 
 and intelligent classes. When a person has attained 
 by his own industry to a certain standard of comfort, 
 and even luxury, he naturally desires to give his 
 children a fair start, and to insure for them an equal 
 degree of ease and security ; and if he finds that this is 
 more easily done when the number of his family is 
 limited, it is difficult to blame him for taking advantage 
 of the fact. If this class of men had more children 
 on the average than are produced by people in in- 
 ferior stations, it would mean that the children of the 
 
CARE OF INFERIOR STOCKS 
 
 293 
 
 comfortably situated would have to be contented with 
 positions somewhat inferior, on the average, to those 
 of their parents. This is precisely the condition of 
 affairs most desirable from the point of view of race- 
 improvement and from that of national efficiency, 
 since any given position would thus be recruited from a 
 better and not from a worse class than the one which 
 previously occupied it. The individual may be par- 
 doned if this is not what he desires to happen in the 
 case of his own children. 
 
 National education and the proposed feeding and 
 care of the children of inferior stocks at the cost of the 
 State are measures which will have certain definite 
 effects upon the relative birth-rates of different classes. 
 
 It is proposed to do all this at the expense of the 
 fitter stock, which is thus rendered still less capable 
 of raising, as well as still less disposed to raise, large 
 families of healthy children. Such measures can only 
 be justified by making at the same time every possible 
 effort to correct these dangerous differences in the 
 incidence of the birth-rate. Legislation in these two 
 directions ought to go hand in hand. Indeed, the 
 improvement in the supply of children ought for every 
 reason to precede the improvement in the care and 
 education of children ; for if the State cares for the 
 children, it has a right to insist that the supply of 
 children shall be the best possible, and this is far from 
 being the case at present. 
 
 Remedies for the existing condition of things have 
 been proposed by would-be philanthropists from Plato 
 downwards. But against aU suggestions for running 
 
294 
 
 EUGENICS 
 
 the human race on the principles of the stud farm the 
 objection holds good which was put forward by Huxley : 
 ‘ Who is competent to do the necessary selecting ? 
 How can the pigeons be their own Sir John Sebright ?’ 
 ‘ The points of a good or of a bad citizen,’ says Huxley, 
 * are really far harder to discern than those of a puppy 
 or a short-horn calf. Many do not show themselves 
 before the practical difficulties of life stimulate man- 
 hood to full exertion. And by that time the mischief 
 is done. The evil stock, if it be one, has had time to 
 multiply, and the selection is nullified.’ And there is 
 another objection. The ruthlessness necessary for the 
 carr3fing out of the method of deliberate selection is 
 in itself so imsocial a quality that, if it were ever to 
 arise, society would probably be far worse off than 
 before. The method is in itself directly opposed to 
 the development of the higher social qualities. 
 
 The student of Eugenics must therefore endeavour 
 to devise other methods, both for encouraging the 
 fertility of the better stock and for discouraging that 
 of the inferior stock. A considerable number of 
 specific suggestions have already been made. ‘ Not 
 a few medical men,’ writes Heron, ‘ are urging that 
 propagation among the obviously unfit — those affected 
 with definite hereditary taints : the imbeciles, the 
 idiotic, the sufferers from S3rphilis and tuberculosis, 
 should be authoritatively restrained.’ Can it be urged 
 that such a proceeding would be unduly tyrannous ? 
 Surely if these people understood the irrevocable laws 
 of heredity — if they only knew — they would be them- 
 selves unwilling to hand on a tainted existence to 
 
OBJECT OF EUGENICS 
 
 295 
 
 future generations ! If there are people so debased 
 that this argument does not appeal to them, surely 
 such a crime against Society as a marriage of this kind is 
 at least as open to coercive treatment as many of the 
 acts which are treated as criminal by existing laws ! We 
 elaborately prevent and punish paltry offences against 
 property, and yet deliberate crimes like marriages 
 between the Unfit are not recognized as criminal. 
 
 Various suggestions for encouraging the multiplica- 
 tion of the Fit have also been made. 
 
 Mr. Sidney Webb, in his ‘ Decline of the Birth-Rate,' 
 has suggested indiscriminate help to all parents, since 
 this should afford encouragement to those who limit 
 their families for prudential reasons and at the same 
 time leave the thriftless where they are. But here 
 it is necessary to point out that the existence of an 
 unlimited population must of necessity bring want 
 and misery to the lowest strata of society. The first 
 object of Eugenics, Galton tells us, ‘ is to check the 
 birth-rate of the unfit, instead of allowing them to 
 come into being, though doomed in large numbers to 
 perish prematurely. The second object is the improve- 
 ment of the race by furthering the productivity of 
 the Fit by early marriages and healthful rearing of 
 their children. Natural selection rests upon excessive 
 production and wholesale destruction ; Eugenics upon 
 bringing into the world no more individuals than can 
 properly be cared for, and those only of the best 
 stock.' 
 
 In the ideal socialistic community, in which, in 
 addition to all the present varieties of civil servants, 
 
3^6 
 
 EUGENICS 
 
 all those engaged in education, all members of the 
 Legislature, all doctors and all lawyers, would no doubt 
 derive their incomes from the State, a recent sugges- 
 tion of Mr. McDougal’s might be put into practice. 
 This is to graduate incomes according to the number 
 of children. Thus the position of married people 
 would be made much fairer on some such scheme as 
 the following : Supposing the salary of a particular 
 post under existing conditions to be £700 a year, a 
 bachelor occupying the post would be paid only £400 
 a year (say). On his marriage an addition of £100 per 
 annum would be made to his income, and a similar 
 increment would again take place at the birth of 
 every child. Under the conditions here postulated 
 this system would apply to the bulk of the more 
 intellectual members of the community, and, incident- 
 ally, it would have a special advantage, to which we 
 may now make allusion.* 
 
 Although a nation’s welfare depends to a very great 
 extent upon the mental and bodily health of the rank 
 and file of its citizens, yet the birth of an occasional 
 genius makes an enormous difference to the progress 
 of the world. Now, Galton has shown quite con- 
 clusively that there is a much greater chance of a 
 genius appearing among the children of eminently 
 intelligent parents than in an average family. There- 
 fore, if the fertility of the more intellectual classes is 
 encouraged, the chance of obtaining a genius now and 
 again is much increased. 
 
 * It is not to be inferred from this paragraph that the 
 writer is an advocate of socialism. 
 
CRIMINAL TAINT HEREDITARY 297 
 
 It must not be supposed that the writer is a special 
 advocate of all or any of the suggestions which have 
 been mentioned above. They are only alluded to in 
 order to indicate the directions in which the problem 
 of race- improvement may be attacked. 
 
 The point of view which has been adopted in the 
 present chapter is very well summed up in the following 
 paragraph from a paper by Prof. Karl Pearson : 
 
 ‘ As we have found conscientiousness is inherited, 
 so I have little doubt that the criminal tendency 
 descends in stocks. To-day we feed our criminals up, 
 and we feed up the insane ; we let both out of the 
 prison or the asylum “ reformed ” or " cured,” as the 
 case may be, only after a few months to return to 
 State supervision, leaving behind them the germs of 
 a new generation of deteriorants. The average number 
 of crimes due to the convicts in His Majesty's prisons 
 to-day is ten apiece. We cannot reform the criminal 
 nor cure the insane from the standpoint of heredity ; 
 the taint varies not with their moral or mental conduct. 
 These are the products of the somatic cells ; the disease 
 lies deeper in their germinal constitution. Education 
 for the criminal, fresh air for the tuberculous, rest and 
 food for the neurotic — these are excellent. They may 
 bring control, sound lungs, and sanity to the individual; 
 but they will not save the offspring from the need of 
 like treatment, nor from the danger of collapse, when 
 the time comes. They cannot make a nation sound in 
 mind and body ; they merely screen degeneracy behind 
 a throng of arrested degenerates. Our highly developed 
 human sympathy will no longer allow us to watch 
 
EUGENICS 
 
 298 
 
 the State purify itself by aid of crude natural selection. 
 We see pain and suffering only to relieve it, without 
 inquiry as to the moral character of the sufferer, or as 
 to his national or racial value. And this is right. No 
 man is responsible for his own being ; and nature and 
 nurture, over which he had no control, have made him 
 the being he is, good or evil. But here science steps 
 in, crying, “ Let the reprieve be accepted, but next 
 remind the social conscience of its duty to the race. 
 No nation can preserve its efficiency unless dominant 
 fertility be associated with the mentally and physically 
 fitter stocks. The reprieve is granted, but let there 
 be no heritage if you would build up and preserve a 
 virile and efficient people.’ 
 
 Signs are not wanting that a few thoughtful people 
 outside the ring of scientific enthusiasts are beginning 
 to take a serious interest in the questions of race- 
 improvement and national efficiency. But before any 
 real progress is likely to be made, a knowledge of the 
 fundamental facts of heredity must have become very 
 much more widely distributed than is at present the 
 case. Valuable work is being done in this direction 
 by the Eugenics Education Society, the object of which 
 is to popularize and carry into practical effect the idea 
 of national Eugenics, as defined at the beginning of 
 the present chapter. The intention of this society to 
 make known generally the fact that biological principles 
 apply to the production of human beings is one which 
 cannot be too highly estimated. Since this is pre- 
 cisely the object with which the present sketch has been 
 added to a work otherwise almost purely theoretical 
 
BIBLIOGRAPHY 
 
 299 
 
 in character, I hare thought it proper to conclude 
 my chapter on Eugenics with a brief reference to the 
 aims and objects of this society. 
 
 Galton, F. : 
 
 $t 
 
 BIBLIOGRAPHY. 
 
 : Hereditary Genius. 1869. 
 
 English Men of Science. 1874. 
 
 Inquiries into Human Faculty and its Develop- 
 ment. 1883. 
 
 9» 
 
 97 
 
 Natural Inheritance. 1889. 
 
 Eugenics : Its Definition, Scope, and Aimfc 
 
 (Sociological Society Papers, 1905). 
 The Eugenics Review, 1909 — 
 
 Schuster, E.: Eugenics, 1912. (Collins.) 
 
CHAPTER XI 
 
 CONCLUDING CHAPTER 
 
 In the preceding chapters a considerable variety ol 
 topics has been dealt with, and in spite of the fact that 
 all are more or less intimately connected with the 
 study of organic evolution, the nearly historical order in 
 which the subject-matter has been in great part pre- 
 sented has inevitably rendered the treatment a little 
 disjointed. 
 
 The method we have so far adopted serves to illus- 
 trate the state of transition in which our studies 
 stand, and which it is our first object to assist in 
 hastening to a close — the transition between the specu- 
 lative philosophy of evolution and the exact science of 
 genetics. 
 
 Future treatises on genetics will make a fair beginning 
 with the law of Mendel, and will then deal with the 
 application of this law in detail ; and in this concluding 
 chapter we may adopt the same method, and proceed 
 to show how Mendel’s discovery affords the connecting- 
 hnk between the various divergent branches which we 
 have already sketched in outline. 
 
 The central generalization, then, around which the 
 subjects considered in the preceding pages are foimd 
 
 300 
 
THE SCIENCE OF GENETICS 
 
 301 
 
 naturally to group themselves is afforded by the law 
 of inheritance discovered by the Abbe Mendel about 
 the year 1865. This discovery has rendered possible 
 that rapid advance of the science of genetics, or the 
 study of the hereditary phenomena of organisms, 
 which has taken place during the first few years of the 
 twentieth century. It is the writer’s avowed opinions 
 that Mendel’s brief paper is the most important con- 
 tribution of its size which has ever been made to bio- 
 logical science. Little apology is therefore needed for 
 formulating once again the law based by Professor 
 Correns upon the conclusions which this paper contains. 
 
 Mendel’s law relates to the inheritance of certain 
 definite characters, which have since been called allelo- 
 morphs. It is a distinctive feature of allelomorphic 
 characters that they are found to group themselves 
 naturally into pairs of more or less antagonistic 
 qualities. In many cases the pair is represented by 
 the presence and absence respectively of a certain 
 definite feature. The two allelomorphs of a pair may 
 be conveniently written as A and a. 
 
 We have seen that the cells of zygotic organisms — 
 organisms, that is to say, which have arisen by the 
 process of sexual reproduction — contain a double 
 complement of hereditary qualities. Such cells may 
 contain A and A, a and a, or A and a. The forms A A 
 and aa are described as homozygotes, the form A a as 
 a heterozygote. In the simpler cases we are enabled 
 to study the behaviour of such a single pair of allelo- 
 morphs by itself, without reference to any other features 
 which the animals or plants under consideration may 
 
302 
 
 CONCLUDING CHAPTER 
 
 display. The demonstration that there exist definite 
 and separable unit characters of this kind is the first 
 great debt that science owes to Mendel. 
 
 Up to the present our certain knowledge of the 
 Mendelian behaviour of unit characters has been con- 
 fined to cases of cross-breeding. In the simplest case 
 which we have to consider, two homozygote forms, A A 
 and aa, are crossed together. 
 
 The external character or visible appearance of the 
 heterozygote A a, produced in this manner, differs in 
 different cases. In the commonest case A represents 
 the dominant allelomorph, and in this case the appear- 
 ance of the heterozygote is practically indistinguish- 
 able from that of the homozygote A A. In other cases 
 the heterozygote Aa is different in appearance from 
 either homozygote AA or aa. Sometimes Aa is inter- 
 mediate between AA and aa, in other cases it is to all 
 appearances totally distinct from either. 
 
 So much for the external appearance of homozygote 
 and heterozygote forms. In the production of the 
 gametes, or germ-cells, we arrive once more at the 
 simplest possible form of hereditary constitution, foi 
 we beheve each feature in the body to be represented 
 in the germ-cells by a single determining factor only. 
 Still confining our attention to the representatives of 
 a single pair of allelomorphs, we find that aU the germ- 
 cells of a homozygote contain only A or only a , as the case 
 may be. But in the case of the germ-ceUs derived from 
 a heterozygote, A and a are represented in an equal 
 number of the gametes produced by the same individual. 
 And the separation between the two allelomorphs is 
 found in almost all cases to be perfectly complete. 
 
MENDEL’S LAW 
 
 303 
 
 This complete segregation of the two allelomorphs in 
 equal numbers of the germ-cells of a heterozygote 
 constitutes the first and most important section of 
 the generalization known as Mendel’s law. 
 
 The second part of the law refers to the fact that, 
 as a general rule, separate pairs of allelomorphs segre- 
 gate quite independently of one another. To this rule 
 a few exceptions have been recorded in cases where 
 apparently distinct pairs of determining factors behave 
 in segregation like a single pair of allelomorphs. In 
 such cases we regard the members of the distinct pairs 
 of allelomorphs as being coupled together, although no 
 serious attempt has yet been made to picture the way 
 in which this coupling comes about.* In other cases the 
 coupling is only partial ; in others, again, there is repul- 
 sion between allelomorphs belonging to distinct pairs. 
 
 The fact that in the great majority of cases separate 
 pairs of allelomorphs segregate independently of one 
 .another leads to the possibility of new combinations of 
 the parental characters being formed in the germ-cells 
 of the cross-bred individuals ; in fact, this must alwa}^ 
 happen when the parent types differ in more than 
 one pair of segregable characters. When two similar 
 germ-cells, each bearing the same new combination of 
 allelomorphs, meet together in fertilization, the result 
 is a new zygotic combination which is a pure type in 
 respect of the characters concerned, and henceforth 
 breeds true. Thus if AB . AB is crossed with ab . ab 
 
 * [Prof. T. H. Morgan and his associates have brought forward 
 remarkable evidence, from their work on the Fruit-fly (Droso- 
 phila), that this coupling is due to the determining factors of the 
 coupled allelomorphs being borne by the same chromosome. 
 See pp. 269-271.] 
 
/ 
 
 304 CONCLUDING CHAPTER 
 
 the heterozygote AB . ab produces in equal numbers 
 the genn-ceUs AB, Ab,aB, and ab. Among the combina- 
 tions of these germ-cells which are represented by the 
 various offspring of the heterozygote there must appear 
 Ab . Ab and aB . aB — novel types which are pure in 
 constitution, and which may form the starting-points 
 for new strains or races. 
 
 Upon this fact depends the enormous importance of 
 Mendel’s law in the breeding of new and useful types 
 of animals and plants. When it is remembered that in 
 wheat, for example, resistance and non-resistance to 
 the attacks of disease, earliness and lateness of 
 ripening, good and bad milling quality, are aU pairs of 
 Mendelian allelomorphs, and that it is now possible to 
 take a different example of these qualities from each 
 of three different strains, and to combine them together 
 in a single new variety with perfect certainty and in 
 four generations, it does not require much imagination 
 to foresee that every department of the animal and 
 plant breeding industries must sooner or later benefit 
 enormously from Mendel’s discovery. 
 
 So far we have only been dealing with the very simplest 
 of Mendelian phenomena, leading to the arithmetical 
 addition and subtraction of definite visible characters. 
 Other kinds of allelomorphs also exist which undergo 
 a similar process of segregation during gamete forma- 
 tion, following Mendel’s law in a perfect manner ; but 
 which may remain entirely invisible and unsuspected 
 so long as certain other allelomorphs, belonging to 
 quite distinct pairs, are excluded from the zygotes in 
 which these invisible factors are concealed. When 
 
MENDEL’S LAW 
 
 305 
 
 this other complementary allelomorph is introduced; 
 however, by crossing with an individual which contains 
 it, the feature previously hidden becomes visible, giving 
 rise to the phenomenon which has long been familiar 
 under the name of reversion on crossing. The demon- 
 stration of these invisible factors, and of the fact that 
 tltey also obey Mendel’s law with perfect regularity, 
 is surely one of the most remarkable discoveries which 
 have ever been made in the whole history of biology. 
 This, again, is a piece of knowledge which may be of 
 the very greatest importance, not only to breeders of 
 bright flowers, some of which are already known to 
 exhibit the phenomenon described, but also in all 
 classes of breeding work where similar facts doubtless 
 await discovery. 
 
 To the man of science, however, the practical aspect 
 of these achievements wiU be of little account in com- 
 parison with the importance of their application to the 
 advance of human knowledge in that most fascinating 
 of scientific studies — biology. Let us, then, turn to 
 consider the way in which Mendel’s discovery affects 
 other branches of biological science. 
 
 We have so recently had occasion to point to the 
 remarkable coalition between Mendelism and cytology 
 that little more need be said here upon the subject. 
 Mendel’s theory has, indeed, thrown a flood of light 
 upon the meaning of the microscopic phenomena 
 exhibited by the minute constituent parts of the cells 
 of living organisms, phenomena the meaning of which 
 could only be vaguely guessed at previously to the 
 introduction of the new method. 
 
 20 
 
30b 
 
 CONCLUDING CHAPTER 
 
 The intimate connection between Mendelism and 
 C5d;ology rests to a large extent upon the close parallel 
 which exists between the behaviour of allelomorphic 
 characters on the one hand and that of chromosomes 
 on the other. 
 
 In the germ-cells of the higher animals the allelo- 
 morphs of the Mendelian become segregated, being 
 reunited in fertilization, and, as a consequence, the 
 cells of the zygote contain twice as many of these 
 factors as do the gametes or germ-cells themselves. 
 
 Similarly, in the cell processes upon which the vital 
 functions of the higher animals are founded, the 
 number of chromosomes characteristic of somatic or 
 zygotic cells becomes halved at the formation of the 
 gametes, the double number being restored by the 
 association of chromosomes derived from two separate 
 gametes in the process of fertilization. We have said 
 that in the higher animals the gametes are sometimes 
 spoken of as constituting an ‘ x- generation, which 
 alternates with the ‘ 2X- generation represented by 
 the zygote. We may justify the use of these expressions 
 by a brief comparative statement of the facts relating 
 to the two so-called generations which recur in the 
 hfe-history of certain families of plants. In doing so 
 we shall begin our account with the most primitive 
 and simplest forms, and then pass on to other types 
 which are regarded as standing on higher planes of 
 evolution. 
 
 What are probably some of the most primitive 
 members of the vegetable kingdom belong to the 
 class of the green algae. This group includes a great 
 
CYTOLOGY OF PLANTS 
 
 307 
 
 number of comparatively lowly organisms, the majority 
 of which dwell submerged beneath the surface of fresh 
 or salt water. In such members of the green algae as 
 have so far been examined from this point of view, it 
 would appear that the 2 ;v-generation is exclusively 
 represented by the single cell which arises as the 
 actual product of conjugation between a pair of 
 gametes. Reduction takes place in the actual zygotic 
 ceU, so that each of the products of this cell’s division 
 shows once more the reduced number of chromosomes. 
 Thus the great bulk — the vegetative mass — of the 
 species is constituted by the j;-generation, and the 
 2A;-generation is composed of a single ceU only — a state 
 of things which is exactly the reverse of what is to be 
 seen in the higher animals. 
 
 In the vegetable kingdom evolution seems to have 
 been accompanied by a gradual increase of the 2X- 
 generation, and a corresponding reduction of the 
 A;-generation in point of importance. Between the 
 two extremes afforded by the algse on the one hand, 
 and the flowering plants on the other, we can trace a 
 series of intermediate stages represented by types in 
 which many other features also must be regarded as 
 standing on intermediate planes of organization. 
 
 As an example of an intermediate condition of this 
 kind, we may take the case of the ferns. 
 
 The fern plant, as commonly understood, represents 
 the 2A;-generation. The method by which the life- 
 history of the fern plant is continued is by the forma- 
 tion of uniceUular reproductive bodies which are 
 
 known as spores. The formation of the spores takes 
 
 20—2 
 
3o8 
 
 CONCLUDING CHAPTER 
 
 place in sets of four, and their production is preceded by 
 a reducing division, so that each spore nucleus contains 
 half as many chromosomes as the nuclei of the fem- 
 plant — ^the spores, in fact, represent the initiation of 
 the X generation. 
 
 Spores take no part in any process of conjugation. 
 They at once germinate and enter on an embryonic 
 development of their own, giving rise to a considerable 
 mass of cells, all of which contain the reduced number 
 of chromosomes. Thus in the case of the fern we have 
 a small but well-developed A;-generation alternating 
 with a much larger 2A?-generation. The mass of 
 cellular tissue making up the Af-generation has been 
 named the prothcdlus. 
 
 Certain cells of the prothallus develop, without 
 change in the number of their chromosomes, into the 
 gametes. These are differentiated in the usual way 
 into male and female — ova and spermatozoids respec- 
 tively. 
 
 Fertihzation of the ovum by the spermatozoid gives 
 rise to a zygote in which the double number of chromo- 
 somes is restored. In this way the 2 :»:-generation or 
 fern plant is initiated, and by the usual processes of 
 cell multiplication and differentiation this body 
 becomes completed, developing its characteristic fronds 
 and so forth. Thus m the ferns the 2A?-generation has 
 arrived at a high degree of development, and represents 
 the chief bulk of the plant. The a:-generation, however, 
 still embodies a considerable mass of cells. 
 
 Turning to the higher plants, among which we may 
 include those which produce t 5 ^ical flowers with 
 
CYTOLOGY OF PLANTS 
 
 309 
 
 stamens or with pistils, or more usually with both, we 
 find that the A;-generation has become still further 
 reduced, so that it no longer occupies an independent 
 phase of the life-history, but has come to be entirely 
 dependent upon the 2 »-generation for its support. 
 
 A plant which bears both stamens and pistils gives 
 rise to spores of two kinds, differing greatly in size. 
 The smaller spores are represented by the pollen-grains, 
 and in these, after one or two cell divisions, unaccom- 
 panied by growth, the one or two male gametes are 
 produced. The small association of cells arising in this 
 way is all that is left of the a;-generation on the male 
 side. 
 
 The nucleus of the larger spore also divides a few 
 times, and one of the final products of division becomes 
 the ovum. Spore and ovum, as well as the few inter- 
 vening cells, bear the reduced number of chromosomes. 
 The a;-generation thus represented is never set free, 
 but remains enclosed in the tissues of the a^f-generation 
 right up to the time of fertilization. In the process 
 of fertilization the double number of chromosomes 
 characteristic of the 2A:-generation is once more 
 arrived at. 
 
 We can look upon the 2A;-generation of the higher 
 plants as being formed by an expansion of the fertilized 
 ovum. The zygote, instead of comprising a single ceU 
 only, by dint of delaying the reducing division, has 
 come to consist of a great mass of cells, all the nuclei 
 of which contain the double number of chromosomes. 
 This fact is also our excuse for applying the same term 
 of zygote to the ceU produced by the conjugation of 
 
310 
 
 CONCLUDING CHAPTER 
 
 gametes, as well as to the mass of cells to which the 
 zygote (in the strictest sense) eventually gives rise. 
 In the simplest forms, such as the green algae, the cell- 
 and nuclear-fusion constituting conjugation are imme- 
 diately followed by fusion of the chromosomes, an 
 event which we have seen to be the first step towards 
 a reduction in the number of these bodies. In the 
 higher plants, by delaying this fusion of chromosomes 
 until many cell generations later than the fusion of the 
 nuclei, the advantages associated with the possession 
 of a double nucleus have been obtained for a large and 
 complicated mass of cells. And this mass has gradu- 
 ally advanced in organization and relative importance, 
 until ultimately the ^-generation has been reduced 
 almost to the vanishing point. 
 
 The sex-phenomena of the higher animals can most 
 readily be brought into line with those of the higher 
 plants if we consider that in animals the spore and the 
 gamete are identical ; the x-generation is here con- 
 densed into the smallest possible limits — namely, those 
 of a single cell. 
 
 A female animal produces ova, and a male produces 
 spermatozoa. Similarly, we may regard as a female 
 plant one which produces only the larger variety of 
 spores from which ova arise ; and we may regard as a 
 male plant one which produces only pollen. It is 
 much more usual to find a flowering plant bearing both 
 pistils and stamens, and producing both large and small 
 spores. Such an organism is described as herma- 
 phrodite^bearing both sexes. Among animals ex- 
 amples of hermaphrodite species are also not infrequent, 
 
CYTOLOGY OF PLANTS 
 
 311 
 
 and here, just as in the case of plants, whole families 
 may display this method of reproduction. 
 
 We see, then, that the course of evolution in the 
 vegetable kingdom would appear to have been accom- 
 panied by a gradual increase in the 2X-generation 
 at the expense of the ^-generation. Starting with 
 lowly aquatic organisms, and passing upwards through 
 the mosses and ferns to the flowering plants, we find 
 a steady diminution in the ^-generation, whilst the 
 vegetative labour of the plant is taken over by the 
 2 %-generation. It is, therefore, proper to suppose that 
 organisms in which the main stage in the hfe-history 
 is of double origin, and bears a double complement of 
 hereditary factors, have some advantage over organisms 
 in which this is not the case. We cannot, of course, be 
 certain as to the exact nature of this advantage, but 
 we may point out that it is only in the former kind 
 of organisms that the operation of Mendel’s law can 
 lead to the production of new combinations of parental 
 characters in the body which represents the main stage 
 of the life-history ; and that this circumstance may 
 possibly lead to a greater power of adaptability to 
 external circumstances. 
 
 Perhaps the most interesting application of the infor- 
 mation afforded by Mendel’s discovery is shown in its 
 bearing upon the question of discontinuity in the origin 
 of species. The fact of the definite and discontinuous 
 inheritance of the differentiating features which dis- 
 tinguish cultivated varieties from one another would 
 point very plainly to a belief that such differences had 
 arisen in a definite and discontinuous manner, even if 
 
312 
 
 CONCLUDING CHAPTER 
 
 we did not actually know from direct evidence that 
 the origin of new races under cultivation is usually 
 sudden and complete. 
 
 It is not necessary to repeat Darwin’s demonstration 
 of the close analogy between the origin of varieties 
 under cultivation and the origin of species in Nature. 
 It is more to the purpose to point out that Mendel’s 
 law has already been shown to hold good in the case 
 of many differences which have certainly not arisen 
 under cultivation, and that we have, moreover, sure 
 knowledge of the definite and spontaneous origin of 
 some natural species. 
 
 Here we arrive at a point at which the evidence is 
 not yet by any means complete. We do not laiow 
 whether aU or even many specific differences obey 
 Mendel’s law on crossing, and a sharp limit is put to 
 our researches in this direction by the fact that so^many 
 natural hybrids are sterile. StiU less do we know 
 from direct evidence whether the majority of natural 
 species have arisen discontinuously, although there is 
 much circumstantial evidence which points to the con- 
 clusion that this must have been the case. 
 
 Clearly this discontinuous method of variation is 
 likely to repay some further discussion. That such 
 mutation, or definite variation, is a phenomenon of 
 the germ-ceUs follows from the fact that every germ- 
 cell normally bears the complete specific character. 
 Bateson has shown that we must regard mutation as 
 consisting in the production of new kinds of gametes, 
 which differ from those normally characteristic of the 
 species. Such a change is most readily pictured by 
 
DISCONTINUOUS VARIATION 
 
 313 
 
 imagining an asymmetrical nuclear division taking 
 place immediately before the formation of the germ- 
 cells, and this would lead us to expect a mutating 
 species to give rise to more than one new kind of 
 offspring at the same time. Such was actually the case 
 with the (Enothera Lamarckiana studied by de Vries ; 
 and this observation stands as the most complete piece 
 of evidence of a mutating species so far known to us. 
 We may be assured, then, that the complete potential 
 nature of new types as well as of old ones is already 
 laid down in the germ-ceUs previous to fertUization. 
 As Bateson puts it : ‘ For the first time in the history of 
 evolutionary thought Mendel’s discovery enables us to 
 form some picture of the process which results in 
 genetic variation. It is simply the segregation of a 
 new kind of gamete, bearing one or more characters 
 distinct from those of the type. We can answer one 
 of the oldest questions in philosophy. In terms of 
 the ancient riddle, we may reply that the owl’s egg 
 existed before the owl ; or, if we hesitate about the 
 owl, we may be sure about the bantam.’* 
 
 Let us consider a little more closely the evidence of 
 mutation afforded by de Vries’ studies of (Enothera 
 Lamarckiana. Semi-wild specimens of this species, 
 when transplanted and carefully observed, were found 
 to yield nearly 3 per cent, of seedlings which differed 
 definitely from their parent, and among these mutants 
 some fifteen distinct new sorts were described. Some 
 of the new species equalled or even surpassed the parent 
 
 * British Association, Cambridge, 1904. Address to the 
 Zoological Section, p. 14. 
 
314 
 
 CONCLUDING CHAPTER 
 
 0. Lamarckiana in vigour and prolific habit, and two 
 of them actually became established side by side with 
 the parent type without man’s assistance. 
 
 It is unfortunate from the point of view of de Vries’ 
 interpretation of this case that the behaviour of 
 0. Lamarckiana should suggest in some respects, as 
 Bateson has pointed out, the phenomena of hybridiza- 
 tion. It must be observed in support of de Vries’ 
 view that the species appears to exhibit the same 
 phenomenon in other locahties, and, further, that it 
 has not been possible to make any suggestion as to the 
 second species with which the pure Lamarckiana might 
 be supposed to have been crossed. 
 
 From one point of view, as de Vries has himself 
 pointed out, mutation in (Enothera is clearly a 
 phenomenon of hybrids, and this circumstance of 
 itself introduces considerable complications into the 
 story. 
 
 We saw just now that there is every reason for the 
 conviction that mutation takes place in the germ- 
 cells, and not in the zygote after fertilization. Since 
 the number of mutants given off under the most 
 favourable circumstances did not exceed 3 per cent, 
 of the total offspring, the enormous majority of mutated 
 germ-cells (on de Vries’ view) must unite with germ- 
 ceUs bearing the ordinary specific character. Conse- 
 quently, the new types which appear will in most cases 
 have originated in the form of a cross between a mutated 
 germ-cell and an ordinary germ-cell. And since this 
 is not the final limit to the possible complications of the 
 case, we can easily recognise that the complete inter- 
 
THE MUTATION THEORY 315 
 
 pretation of the behaviour of (Enothera Lamarckiana is 
 not by any means an easy matter. 
 
 As enunciated by de Vries, the theory of mutation 
 amounts to a very complete and definite hypothesis. 
 A large part of this author’s suggestions are, however, 
 almost purely speculative, and for this reason we have 
 treated the whole at somewhat less length than it per- 
 haps deserves. Some of de Vries’ speculations are, 
 indeed, more picturesque than convincing. 
 
 Thus, de Vries regards the number of unit characters 
 — each of which has arisen by a single mutation — to 
 be quite limited, even in the highest organisms. 
 Three or four thousand such characters, he thinks, 
 may go to build up the hereditary endowment of 
 the most complicated species. He further supposes 
 a period of mutation to recur about once in 4,000 
 years. Four thousand multiplied by 4,000 gives 
 16,000,000 — the number of years required to evolve 
 the lords of creation from a ‘ primordial protoplasmic 
 atomic globule.’ And he points out that this estimate 
 is well within the limits of geological time as allowed 
 by the physicist. In this way de Vries believes that 
 his mutation theory removes a difficulty which besets 
 the selection hypothesis — the difficulty, namely, of 
 insufficient time. The selectionist may reasonably 
 reply that the amount of change necessary to produce 
 in 4,000 years, by the gradual method, a difference 
 equal to that represented by a single unit character, 
 might very well be quite imperceptible in a single 
 generation. 
 
 We may summarize our present conclusions as to 
 
3i6 
 
 CONCLUDING CHAPTER 
 
 the discontinuous nature of species in the following 
 manner : A great number of specific characters are, 
 without doubt, definite ; they are inherited as definite 
 entities, and there can be no question that their first 
 coming into existence was a definite event. Every 
 year tends to increase the range of characters to which 
 the conception of discontinuity has to be applied, 
 i Certain groups of characters do, however, seem to 
 exhibit the phenomena of continuity. Let us endeavour 
 to arrive at some closer idea as to the nature of these 
 characters. 
 
 A study of continuous variations very quickly leads 
 to the conclusion that the variable features are those 
 which are especially liable to modification during the 
 lifetime of the individual, owing to the action of ex- 
 ternal circumstances. Such quantitative features of 
 size and shape and number of parts are particularly 
 plastic in the case of plants. 
 
 The habit, or general form and appearance, of a 
 plant is a feature very characteristic of individual 
 species. The presence of a dwarf or of a tall habit 
 does, indeed, constitute a frequent distinction between 
 different strains of garden plants, and the inheritance 
 of these characters in many cases follows Mendel’s 
 law. But leaving aside this particular example, the 
 inheritance of habit is very little understood ; although 
 habit is a feature which is very liable to considerable 
 fluctuations. Habit seems, in fact, usually to afford 
 an example of continuous variability. 
 
 The habit of some species of plants when grown 
 under alpine conditions on mountain summits is so 
 
CONTINUOUS VARIATIONS 
 
 317 
 
 different from that of the same species when growing 
 in the plains, that inexperienced persons might readily 
 suppose two such forms to belong to as many distinct 
 species. At intermediate levels the habit is more or 
 less intermediate. Bonnier made the experiment of 
 dividing individual plants into two portions, plant- 
 ing one part at a high elevation and the other near the 
 level of the sea. In a few years the plant grown on 
 the mountain had taken on the full alpine habit, 
 whilst that grown on the plain retained the ordinary 
 appearance of the species. In this way very con- 
 siderable differences in habit were shown to be directly 
 dependent on external conditions. 
 
 In some few cases the environment determines the 
 production of perfectly definite and discontinuous 
 features. The water ranunculus, when growing sub- 
 merged beneath the surface of a pond, produces leaves 
 the blades of which are cut up into a great number of 
 fine thread-like segments. As soon as the top of the 
 plant reaches the surface of the water those leaf rudi- 
 ments which are just commencing their existence pro- 
 ceed to develop in a totally different fashion. The 
 leaves to which they give rise possess a wide and undi- 
 vided blade, which floats upon the surface of the water. 
 The two sorts of leaves are as utterly different in 
 appearance as it is possible for leaves to be. Yet the 
 effect of external conditions upon the yoimg leaf- 
 rudiment determines which of the two kinds is to 
 appear. 
 
 In this instance we see a discontinuous change in 
 conditions — the change from water to air as a sur- 
 
CONCLUDING CHAPTER 
 
 318 
 
 rounding medium — giving rise to a discontinuous 
 change in structure. Such cases are, however, com- 
 paratively rare. Much more usually the changes in 
 external conditions are continuous, as changes of 
 altitude, moisture, or chemical composition of the soil, 
 and so on ; and the changes induced by them in the 
 plant are similarly of a continuous kind. 
 
 In most animals changes in external circumstances 
 have a much smaller influence on the form and struc- 
 ture of the individual than is the case with plants. In 
 animals considerable modifications are, however, 
 brought about by exercise and the use of different 
 parts, as Lamarck long ago observed. But these 
 modifying factors usually affect all the members of a 
 single species in nearly the same manner. Neverthe- 
 less, some part of the differences between individuals in 
 respect of strength and of proportion, and possibly also 
 of stature, is undoubtedly associated with differences 
 of training and nutrition, as the example of the human 
 race is sufficient to show. Professor Cope has pointed 
 out how the proper development of such structures as 
 the joints of vertebrates depends to a very large 
 extent upon exercise ; and the effect of disuse may be 
 practically tested by anyone whom accident obliges 
 to keep a knee or other joint immovable for any length 
 of time. The so-called play in which the young of 
 many animals indulge — for example, lambs and kittens 
 — must have a great influence upon the perfection of 
 their locomotory functions. 
 
 We can now see more clearly the reason for that 
 great instability of vegetative type which sessile 
 
ACQUIRED VARIATIONS 
 
 319 
 
 animals, like plants, exhibit. No necessity for definite 
 and co-ordinated movements involving their whole 
 structure forces the development of these animals 
 along certain definite paths. External circumstance 
 is, therefore, free to mould them into a host of slightly 
 different shapes. And thus the great variability of 
 the species of corals, for instance, is doubtless deter- 
 mined to a large extent by the influence of different 
 environmental conditions. 
 
 Strictly speaking, the term variability ought not to 
 be applied to modifications of this description. It wiU, 
 perhaps, be most convenient, however, to distinguish 
 true variations — having their origin in differences 
 among the germ-cells — as genetic variations, contrasting 
 them with the acquired variations which arise during 
 the development of individuals. 
 
 Enough has now been said to show that it is a very 
 difficult matter to distinguish in the case of continuous 
 variations between those which are genetic and those 
 which are acquired. 
 
 It is easy to understand how acquired variations 
 come to be continuous, and to obey the law of normal 
 variabihty. We saw that the normal distribution of 
 characters was induced by the random operation of a 
 multitude of small causes. During the development 
 of the individual a great number of different external 
 influences come into play, leading to slight modifications 
 of every part, now in one direction, now in another. 
 This being so, we may be quite sure that a large pro- 
 portion of the normal variability which any species 
 exhibits is acquired. 
 
320 
 
 CONCLUDING CHAPTER 
 
 Now we saw that there seems to be good evidence 
 that normal or continuous variations are inherited. 
 Logic does not, however, permit us to make the step : 
 Acquired variations are continuous variations ; con- 
 tinuous variations are inherited ; therefore acquired 
 variations are inherited. It seems, indeed, to be this 
 fallacy which has led to the long-continued behef in 
 the inheritance of acquired characters as an important 
 factor in organic evolution, in spite of so many argu- 
 ments to the contrary. 
 
 Formal disproof of this proposition is very difficult, 
 and in the meantime the confusion between continuous 
 acquired variations and continuous genetic variations, 
 which is always present in practice, constitutes a very 
 serious drawback to the biometric method of research. 
 At present Johannsen’s explanation of these phenomena 
 seems to afford so much the simplest solution that 
 we may once more repeat his statement of the case, 
 though with the proviso that his hypothesis is not 
 universally accepted. 
 
 Johannsen looks upon a population which, as a 
 whole, exhibits continuous or normal variability, as 
 being capable of analysis into a number of pure hues. 
 In a single pure line genetic variabiUty is sensibly 
 absent. The members of such a pure hne exhibit, 
 however, very considerable acquired variability, so 
 that in this way each line shows a normal variabihty 
 of its own. And the range of this variabihty may 
 greatly exceed the limits which separate two pure hues 
 from one another. The result is to give a completely 
 blurred picture when aU the hnes are looked at simul- 
 
PURE LINES 
 
 321 
 
 taneously, And thus the normal variability of the 
 population as a whole is brought about by the com- 
 bination of these two separate factors. 
 
 This statement applies to the case of an organism 
 in which self-fertilization is the general rule, so that in 
 this way the separate lines are kept distinct. Where 
 cross-fertihzation takes place between the members of 
 different pure lines the case becomes enormously com- 
 plicated, and this is much the most frequent instance 
 which we have actually to deal with. It has been 
 suggested that the members of different lines when 
 crossed together may display Mendelian phenomena, 
 but the existence of so large a proportion of acquired 
 variability renders the problem of analyzing the result 
 almost insuperable. We have seen, however, that the 
 numerical results obtained by the biometricians do 
 not appear to be inconsistent with the existence of 
 Mendelian inheritance in populations. 
 
 We find, then, that the questions of inheritance of 
 acquired characters and of evolution by the aid of ^ 
 continuous genetic variations are not yet absolutely 
 settled. Recent discoveries by Winkler and Baur 
 regarding the nature of so-called ‘ graft-hybrids ’ go 
 far to prove that acquired characters cannot be in- 
 herited in plants. The classical example of a ‘ graft- 
 hybrid ’ is Cytisus Adami, which was produced in 1825 
 by grafting Cytisus pmfureus on the laburnum. This 
 plant, which is more or less intermediate between the 
 two parent species, has been reproduced by further 
 grafting, but its seeds always give rise to plants which 
 are indistinguishable from the laburnum. Adventitious 
 
 21 
 
322 
 
 CONCLUDING CHAPTER 
 
 shoots exactly resembling those of one or other of the 
 parents often appear upon the ‘ hybrid ’ plants. 
 
 It now appears that in Cytism Adami the cells of the 
 two component species remain perfectly distinct, and 
 that its reproductive cells are always of the laburnum 
 type. In spite of the intimate association of the two 
 groups of cells which build up a common plant body, 
 the cells of Cytisus purpureus are unable to transmit 
 any hereditary influence to the cells of the laburnum, 
 and these give rise to offspring which are pure laburnum. 
 „The epidermis of the ‘ graft-hybrid ’ is said to consist 
 wholly of cells of the Cytisus purpureus type. It seems 
 fair to argue that if one species wrapped in the epi- 
 dermis of another receives no heritable influence what- 
 ever from its living integument, it is in the highest 
 degree unlikely that the germ cells will be able to 
 acquire transmissible modifications from an environ- 
 ment wholly external to the plant. The proof may 
 not be absolutely conclusive, but when it is combined 
 with all the other evidence pointing in the same direc- 
 tion, we think that the inheritance of acquired char- 
 acters may be disregarded as a practical factor in 
 evolution. 
 
 Meanwhile the number of cases in which discon- 
 tinuity of inheritance can be shown to hold good is 
 constantly increasing, and the analysis of some cases 
 of supposed continuous variation into discontinuous 
 Mcndelian factors has already been made. It may 
 be safely concluded that a very large part, if not the 
 whole, of evolution has taken place by the discon- 
 tinuous method. 
 
 New little species — Jordan’s species — arise, then, 
 
CONCLUSION 
 
 323 
 
 from time to time, each at a single step, from pre- 
 existing species. Upon the material thus supplied 
 natural selection operates ; the weaker go to the wall, 
 the stronger survive. This is also, in all probability, 
 the way in which adaptations have arisen. Creatures 
 which came into existence displaying a particular new 
 structure, which happened to be fitted for a particular 
 new function or suited to a particular niche in Nature, 
 survived and flourished exceedingly. Those in which 
 undesirable organs appeared perished and were no 
 more seen. To take Aristotle’s example. If a man 
 were to be born with molars in front and incisors at 
 the back of his jaw he would die — at least, in the days 
 before dentistry. Having his teeth in the positions 
 in which they actually stand (although not for this 
 reason only), he survives and rules the world. 
 
 After all, the difference between the point of view 
 thus briefly indicated, and that of Darwin as expressed 
 in the ‘ Origin of Species,’ is only one of detail — of 
 detail as to the particular sort of variations by which 
 evolution chiefly proceeds. Darwin’s analogy between 
 the origin of species in Nature and the origin of races 
 under cultivation may be repeated with emphasis, 
 although Huxley’s famous criticism, to the effect that 
 races which are sterile together have not arisen in 
 cultivation, is not yet completely answered. But 
 this renders the discontinuous origin of such sterility 
 only the more likely ; and when we recall the Mendelian 
 behaviour of such characters as long and short style in 
 the primrose, or sterility of the anthers in the sweet- 
 pea, the solution of the problem does not seem very 
 far to seek. 
 
324 
 
 CONCLUDING CHAPTER 
 
 Let us see how the principles of which an outline has 
 now been given affect the human race itself. The 
 question of improving the human stock in this country 
 has lately excited a good deal of attention. But 
 without a scientific knowledge of the factors upon 
 which improvement and degeneration depend the dis- 
 cussion is not likely to be of much profit, and in such 
 a case misdirected energy may be even worse than 
 apathy. Without venturing to make any very positive 
 suggestions, it may at least be pointed out that our 
 present practice in these matters is in almost every 
 case the very worst possible. 
 
 Professor Karl Pearson has lately shown how the 
 low birth-rate of the professional and middle classes — 
 the classes amongst which the intelligence of the 
 nation is to a large extent segregated — ^leads to the 
 recruiting of these classes from amongst the lower and 
 less intelligent strata of society. In other words, a 
 steady breeding out of intelligence is taking place. 
 Recognising that intelligence is an important factor in 
 national greatness, we proceed to remedy this defect by 
 endeavouring to reduce the infant mortality among the 
 less desirable classes, and by offering every inducement 
 to the production of large families by the lower strata 
 of society ; indeed, we propose to remove from them 
 all responsibility for the production of children, and to 
 feed and house the latter as we already educate them 
 at the expense of the State. 
 
 The principles of heredity teach us that education 
 and training, however beneficial they may be to indi- 
 viduals, have no material effect upon the stock itself. * 
 If they have any effect at all, this is undoubtedly 
 
CONCLUSION 
 
 325 
 
 unimportant in comparison with the effect which would 
 be produced by the selection of individuals who 
 exhibit desirable qualities. The demand for a higher 
 birth-rate ought to apply strictly to desirables. 
 Instead of this the cry is for education and physical 
 training, processes which can have no permanent 
 beneficial effect upon the race. 
 
 One writer who holds to some extent the attention 
 of the intelligent public has recognised the true state 
 of affairs — I mean Mr. Bernard Shaw. Unfortunately 
 the public does not take Mr. Bernard Shaw seriously, 
 wherein, when I recall Mr. Shaw’s published views on 
 such topics as vivisection and the medical profession, 
 the public has my sympathy. Nevertheless I know of 
 no better expression of the moral to be dra^vn from 
 the science of genetics than that which is embodied in 
 the following passage : 
 
 ‘ I do not know whether you have any illusions left 
 on the subject of education, progress, and so forth. 
 I have none. Any pamphleteer can show the way to 
 better things, but when there is no will there is no way. 
 My nurse was fond of remarking that you cannot make 
 a silk purse out of a sow’s ear, and the more I see of 
 the efforts of our churches and universities and literary 
 sages to raise the mass above its own level, the more 
 convinced I am that my nurse was right. Progress 
 can do nothing but make the most of us all as we are, 
 and that most would clearly not be enough even if 
 those who are already raised out of the lowest abysses 
 would allow the others a chance. The bubble of 
 heredity has been pricked, the certainty that acquire- 
 ments are negligible as elements in practical heredity 
 
326 
 
 CONCLUDING CHAPTER 
 
 has demolished the hopes of the educationists as well 
 as the terrors of the degeneracy-mongers, and we now 
 know that there is no hereditary “ governing class ” any 
 more than a hereditary hooliganism. We must either 
 breed political capacity or be ruined by democracy, 
 which was forced on us by the failure of the older 
 alternatives. Yet if despotism failed only for want of 
 a capable benevolent despot, what chance has demo- 
 cracy, which requires a whole population of capable 
 voters — that is, of political critics who, if they cannot 
 govern in person for lack of spare energy or specific 
 talent for administration, can at least recognise and 
 appreciate capacity and benevolence in others, and 
 so govern through capably benevolent representatives ? 
 Where are such voters to be found to-day ? Nowhere. 
 Promiscuous breeding has produced a weakness of 
 character that is too timid to face the full stringency 
 of a thoroughly competitive struggle for existence, and 
 too lazy and petty to organize the commonwealth 
 co-operatively. Being cowards, we defeat natural 
 selection under cover of philanthropy ; being sluggards, 
 we neglect artificial selection under cover of delicacy 
 and morality.’* 
 
 Mr. Shaw recognises, however, that our knowledge 
 is at present insufficient to prescribe for the breeding 
 of a ‘ Superman,’ even if we were able to come to any 
 agreement as to what qualities are the most desirable. 
 Nevertheless it is along the lines which we have en- 
 deavoured to indicate that such knowledge must be 
 sought in the future. 
 
 • ‘ Man and Superman,’ p. xxiii. 
 
GLOSSARY 
 
 {Many technical terms not included in this glossary are 
 printed in italics on their first appearance in the body of the 
 book, and their meaning is then defined. Such definitions 
 may be discovered on a reference to the inde?f,'\ 
 
 Adaptation. — A teleological explanation of the corre- 
 spondence often shown between the structure and habits of 
 a particular creature and the environment in which the 
 creature lives. 
 
 Albino. — An animal or plant characterized by the absence 
 of colouring matter from its external tissues. 
 
 ALGiE. — A group of plants, mostly aquatic and of re- 
 latively simple organization. 
 
 Anther. — The upper part of a stamen, containing the 
 pollen. 
 
 Atom. — The smallest part of a chemical element which 
 can exist as such. 
 
 Axil. — The angle enclosed between the base or stalk of 
 a leaf and the stem upon which the leaf is borne. 
 
 Binomial Nomenclature. — The application of a double 
 name to an animal or plant, the first name being that of 
 the genus, the second that of the species. 
 
 Biology. — The science of the phenomena of life. 
 
 Biometry. — The application of statistical methods to 
 biological problems. 
 
 Botany. — The scientific study of plants. 
 
 Calyx. — The outermost whorl of floral leaves, which in 
 the bud usually encloses the other organs of the flower. 
 
 Character. — I n heredity, a single definable attribute. 
 
 Class. — One of the larger subdivisions of the animal 
 kingdom — ■e,g,, mammals, birds. 
 
 327 
 
328 
 
 GLOSSARY 
 
 Composit-®.-“A family of plants, including the daisy, 
 chrysanthemum, and many others. 
 
 Conjugation. — The process of fusion of a pair of gametes. 
 
 Corolla. — The second envelope of a flower, consisting of 
 petals — leaf-like organs — usually brightly coloured. 
 
 Corpuscle. — A very minute particle. 
 
 Cytology. — The scientific study of the minute con- 
 stituent parts of organisms by the aid of the microscope. 
 
 Denudation. — The wearing away of the earth's surface by 
 the action of rain, rivers, etc. 
 
 Differentiation.— The separation or discrimination of 
 parts which were previously more or less united and uniform. 
 
 Embryo. — A young plant or animal — usually one which is 
 still contained in the seed or the womb. 
 
 Embryology. — The history of the development of young 
 plants or animals from the egg. 
 
 Environment. — Natural surroundings. 
 
 Evolution. — See p. 22. 
 
 Family. — A group of allied genera, as the family of apes 
 {AnthropoidcB), the buttercup family (Ranunctilacece), 
 
 Fauna. — The sum total of animals inhabiting a particular 
 region. 
 
 Fertilization. — The union of male and female repro- 
 ductive cells or gametes. 
 
 Florets. — The separate flowers of a crowded inflorescence. 
 
 Gametes. — Sexual cells which unite in conjugation or 
 fertilization. 
 
 Genus. — A group of allied species. 
 
 Geology. — The study of the earth's crust. 
 
 Geometric Rate of Increase. — Progress consisting in 
 successive multiplications of the preceding number, instead 
 of simply in additions to it. 
 
 Germ-Cells. — See Gametes. 
 
 Herbalist. — One who collects and studies herbs. 
 
 Heredity. — The transference of similar characters from 
 one generation of organisms to another, a process effected by 
 means of the germ-cells or gametes. 
 
GLOSSARY 
 
 329 
 
 Igneous. — Produced in connection with great heat. 
 
 Inbreeding. — The mating together of near relatives for a 
 number of generations. 
 
 Larva. — The young of an insect after it has emerged from 
 the egg — e.g,, a caterpillar. 
 
 Mantidae. — A group of predatory insects. 
 
 Maxillary. — Connected with the mouth parts. 
 
 Morphology. — The study of form and structure. 
 
 Mutation. — The sudden origin of a new species at a single 
 step. 
 
 Organism. — A living creature. 
 
 Ornithologist. — A student of birds. 
 
 Ovary. — In animals the organ which produces ova. In 
 plants the organ which contains the ovules. 
 
 Ovum. — The female gamete. 
 
 Ovule. — The structure surrounding the spore which gives 
 rise to the female gamete or ovum in the higher plants. 
 
 Petal. — One of the (usually) coloured leaves composing 
 the corolla. 
 
 Petaloid. — Resembling the corolla, usually in the circum- 
 stance of being coloured. 
 
 Physiology. — The study of the functions of organisms. 
 
 Pin-eyed. — Having the stigma on a level with the throat 
 of the corolla, and the anthers lower down, enclosed within 
 the tube. 
 
 Pistil. — The central organ of a flower, which contains the 
 ovules, and ultimately becomes the fruit, or the chief part 
 of it. 
 
 Pollen. — Those spores of the flowering plants which 
 produce the male gametes. 
 
 Pollination. — The transference of pollen to the stigma of 
 a plant. 
 
 Primary, Secondary, and Tertiary Epochs. — The three 
 great divisions of geological time during which the known 
 fossiliferous strata were deposited. 
 
 Radical Leaves. — Leaves arising immediately from the 
 root-stock in the form of a rosette. 
 
330 
 
 GLOSSARY 
 
 Reversion. — The reappearance in the offspring of a 
 character proper to a more or less remote ancestor, and not 
 exhibited by the immediate parents. 
 
 Rotifers. — A kind of minute aquatic animals. 
 
 Segment. — One of a series of more or less similar trans- 
 verse divisions. 
 
 Sessile. — Fixed and stationary, but (in the strict sense) 
 without a stalk. 
 
 Somatic. — Belonging to the body of a zygote. 
 
 Species, Linn^an. — A group of organisms of closely 
 similar appearance. 
 
 Species, Jordan’s. — A group of organisms believed to have 
 arisen by a mutation. (Jordan himself did not, however, 
 suppose so.) 
 
 Sport. — A marked mutation — often one occurring under 
 domestication. 
 
 Stamens. — The organs of a flower which bear the pollen. 
 
 Standard. — The large, upright petal at the back of such 
 a flower as that of the sweet-pea. 
 
 Stigma. — The uppermost part of the pistil, upon which the 
 pollen is received. 
 
 Stratum. — A layer. 
 
 Style. — A stalk connecting the stigma with the ovary — 
 part of the pistil. 
 
 Testa. — The skin or coat covering a seed. 
 
 Thrum-eyed. — Having the anthers situated at the throat 
 of the corolla, and the stigma lower down, enclosed in the 
 tube. 
 
 Tube. — The basal tubular portion of a corolla in which 
 the separate petals are closely fused together, as is the case 
 with that of the primrose. 
 
 Unicellular. — Consisting of a single cell. 
 
 Variation, Continuous. — See Chapter IV. 
 
 Variation, Discontinuous. — See Chapter V, 
 
 Wings. — The lateral petals of a pea-flower. 
 
 Zoology. — The scientific study of animals. 
 
 Zygote. — The organism produced by the fusion of a pair of 
 gametes. 
 
INDEX 
 
 Abraxas grossulariata, 278 
 Acquired characters, inheritance 
 of, 39, 65 et seq., 123, 155 
 variations, 319 
 Adaptation, et seq,, 325 
 ^Albino, 132, 327 
 Algae, 306, 327 
 
 Allelomorphs, 185, 195, 203 et 
 seq,, 302 
 Allogene, 229 
 Allozygote, 229 
 Amoeba, 240 
 
 Ancestral heredity, law of, 34, 
 1 13 et seq., 228 
 Andalusian fowl, 192 
 Andreae, Dr., 58 
 Argus pheasant, 63 
 Aristotle, 124, 323 
 Array, 104 
 Ascaris, 251 
 
 Atom, chemical, 19, 24, 127 
 
 Bachmann, 7 
 Barley, 117, 136 
 Basset hound, 107 
 Bateson, Prof. W., viii, 17, 60, 
 61,66,124, 128 etseq., 133,134, 
 192, 200, 215, 238, 312 et seq. 
 Baur, 321 
 Beans, kidney, 117 
 Beetles, Chrysomelid, 155 
 Bentham and Hooker, 8 
 Between-race, 148 
 Biffen, Prof. R. H., 233 et seq. 
 Biometry, 18, 80 et seq., 160, 
 227 et seq. 
 
 Birth-rate, 290 et seq. 
 Blaringhem, 156 
 
 Blind, tactual discrimination in 
 the, 69 
 Bonnier, 317 
 Boveri, 252 et seq., 283 
 Browne, Sir Thomas, 124 
 Burbank, Luther, 44 
 Busbey, Hamilton, 137 
 Butler, Samuel, 79 
 Butterflies, 215, 57 
 tropical, 56 
 
 Casiiarina indica, 59 
 Cat, 70, 215 
 
 Cell, 72 et seq., 239 et seq., 272, 
 274 
 
 Cereals, 233 et seq. 
 
 Cetacea, 52 
 
 Characters, acquired, 39, 65 ei 
 seq., 123, 155 
 mental, no et seq. 
 variations, 319 
 Chromatin, 248 
 Chromomeres, 268 
 Chromosomes, 248 et seq., 272 
 et seq., 306 
 Classes, 88, 97 
 Clover, five-leaved, 150 
 Coat colour of horses, 109, no, 
 ii 3 » 215 
 
 Coefficient of variability, 99 
 Coins, spinning of, 91 et seq. 
 Colour, inheritance of, 13 1, 213 
 et seq. 
 
 of animals, 56 
 Colour-blindness, 277 
 Comparison of species, 12 
 Compositors, tactual discrim- 
 ination in, 69 
 
 331 
 
332 
 
 INDEX 
 
 Conscientiousness, iii 
 Controversial methods, 3 
 Co-ordinated structures, 69 
 variability, 123 
 Cope, Professor, 318 
 Correlated variability, 106 
 Correlation, loi et seq, 
 coefficients, 106 et seq, 
 Correns, 19, 190, 301 
 Coupling, 197 et seq. 
 
 Cowslip, 8 
 Criminals, 297 
 
 Crustaceans, loss of limb by, 
 62 
 
 Cryptomerism, 203 
 Cuenot, 206 et seq. 
 
 Curve, normal, 96 
 Cytisus Adaini, 321 
 purpureus, 321 ^ 
 
 Cytology, 239 et seq., 305 
 
 Dalton, 19, 175 
 
 Darwin, Charles, 2, 3, 16, 33, 
 38, 40, 41, 42 et seq., 54, 
 123, 124, 139, 160, 190, 
 215, 280, 312, 323 
 Erasmus, 29 
 Sir Francis, xi 
 Sir George, 28 
 Daughter cell, 242 
 Davenport, 99 
 Degressive mutation, 153 
 De Maillet, 30 
 Determinants, 280 
 Deviation, 98, 125^ 
 
 De Vries, 17, 19, 75, 76, 83, 12 1, 
 135. 139. ef ^eq., 158, 190, 216, 
 218, 267 et seq., 313 et seq. 
 Di-hybrids, 185 et seq. 
 Discontinuous variation, 128 et 
 seq., 311 et seq. 
 Domestication, changes under, 
 34 et seq. 
 
 Dominance, 192 
 Dominant characters, 179 
 Doncaster, Dr. L., vfi, 21, 27S, 
 283 
 
 Double flowers, 134 
 Drosophila, 200, 270, 277, 303 
 
 Druery, C. T., 36 
 Dyer, Sir W. Thiselton, 35 
 
 Elephant, 46 
 
 Elk, 69, 71 
 
 Embryo, 177 
 
 Embryology, 31, 243 
 
 Emily Henderson (sweet pea), 209 
 
 Empedocles, 29 
 
 Endosperm, 177 
 
 Error, probable, 99 
 
 Eugenics, 20, 284 et seq. 
 
 Education Society, 298 
 Evening primrose, 83 
 Evolution, 22 et seq., 122, 300, 
 
 307 
 
 parallel, 57, 59 
 
 Eye, colour of human, 82, no 
 Ferns, 36, 307 
 
 Fertilization, 162, 243, 244, 246, 
 3 I 3 » 321 
 normal, 219 
 Flat fishes, 38 
 
 Flies, flovrer-frequenting, 58 
 Fluctuating variations, 75 
 Fly, fruit-, 200, 270, 277, 303 
 Formula, Mendelian, 183 et seq. 
 Fowls, 192, 215 
 Franklin, 45 
 
 Fraternal correlation, 106 et seq, 
 Frequency, 98 
 
 Gaertner, C. F. v., 170 
 Galton, Sir Francis, 80, 89, 107 * 
 113, 125 et seq., 284 et seq. 
 Gametes, 178, 244, 245, 306 
 Gardiner, Professor Stanley, xi 
 Gates, R. R., 146, 272 
 Geikie, 28 
 Genetics, 285, 300 
 Genetic variations, 319 
 Geographical distribution, 33 
 Geological succession, 33 
 time, 27, 315 
 
 Germ-cells, 72 et seq,, 178, 244, 
 245, 282, 306 
 
 Germ-plasm theory, 279 et seq. 
 Giraffe, 37 
 
INDEX 
 
 333 
 
 Goethe, 30, 175 
 Goss, John, 169 
 Gradation of organisms, 31 
 Greeks, 29 
 Greenland whale, 52 
 Gregory, R. P., xi, 215, 272 
 Greyhound, coat colour of, no 
 Guinea-pigs, 214 
 Gulick, 50 
 
 Habit of plants, 316 
 Half-race, 148 
 Harwood, 44 
 Hawaiian Islands, 50 
 Helium, 23 
 
 Herbert, Hon. and Rev. W., 169 
 Hermaphrodite, 310 
 Heredity, i, 4, 5, loi, 112 et 
 seq,, 174 et seq., 272 et seq,, 
 
 324 
 
 Heron, 294 
 
 Heterotropic chromosome, 273 
 et seq. 
 
 Heterozygote, 184 et seq., 301 et 
 seq. 
 
 Heterozygote forms,' 192, 302 
 Hieracium, 190 
 Hippocampus, 55 
 Homoeosis, 133, 134 
 Homozygote, 185, 192, 301 et 
 seq. 
 
 Horns, 63, 69 
 
 Horse, American trotting-, 137 - 
 et seq. 
 
 coat colour in, 109, 215 
 extinct, 33 
 Hume, David, 29 
 Hurst, 215 
 Hutton, 26 
 
 Huxley, 20, 124, 294, 323 
 Hybrid, 159 et seq. 
 false, 172 
 graft, 321 
 Hypostasis, 213 
 
 Idiochromosomes, 270, 273 seq* 
 Ids, 281 
 
 Immunity to rust, 237 
 Indian corm 1^6, 177 seq. 
 
 Inheritance, bisexual, 118 
 
 of acquired characters, 65 
 et seq., 155, 321, 324, 325 
 of colour, 131, 213 et seq. 
 of colour-blindness, 277 
 of mental characters, no et 
 seq. 
 
 of physical characters, 103 
 et seq. 
 
 Irish elk, 69 
 Isolation, 50 
 
 Japan, 58 
 Jennings, 116 
 
 Johannsen, Professor, 116 et 
 seq., 122, 320 
 Jordan, 8, 153 
 Jordan’s species, 9, 153, 322 
 
 Kelvin, Lord, 27 
 Knight, T. A., 166 et seq. 
 Knight-Darwin law, 168 
 Kolreuter, J. G., 161 et seq. 
 
 Laburnum, 321 
 
 Lamarck, 15 et seq., 30, 36 ct 
 seq., 65, 123, 318 
 Laplace, 24 
 
 Latent characters, 153, 203 
 Leaf insects, 55 
 Leicestershire sheep, 137 
 Line, pure, 117 et seq., 320 
 .Linkage, 202 
 Linnaeus, 7 e^ seq. 
 
 Lock, R. H., xiii et seq., 196 
 
 Lotsy, Dr., xi 
 
 Lyell, Sir Charles, 25, 41 
 
 MacDougal, Prof., 146, 156, 296 
 
 Maize, 156, 177 et seq., 197 
 
 Malthus, 45, 46 
 
 Manitoba Hard (wheat), 235 
 
 Mantidae, 55 
 
 Maple pea, 205 
 
 Marriage of unfit, 294 et seq. 
 
 State aid for, 296 
 Masked characters, 203 et seq. 
 Masters, 133 
 Matthews, Patrick, 42 
 
334 
 
 INDEX 
 
 Mean of normal curve, 96 
 Medium of normal cun^e, 96 
 Mendel, Gregor, 19, 86, 113, 
 118, 174 et seq,y 264 et seq., 
 300 et seq. 
 
 Mendel’s law, 189, 300 et seq. 
 Mental characters, resemblance 
 in, no 
 Merism, 129 
 M eristic variations, 130 
 Metamorphy, 133 
 Mice, 207, 214 
 
 Michigan bronze (wheat), 237 
 Mid-race, 148 et seq. 
 
 Millardet, 172, 216 
 Mimicry, 54 et seq. 
 
 Mirabilis jalapa, 165 
 longijiora, 165 
 Mitosis, 248 
 Mivart, 38 
 
 Mode of normal curve, 96 
 Mongrel, 160 
 Mono-hybrid, 183 et seq. 
 Morgan, Prof. T. H., 61 et seq., 
 79, 269, 303 
 Morphology, 32, 329 
 Moth, Currant, 278 
 Mother-cell, 242 
 Mutation, 36, 121 et seq., 
 artificial, 156 
 atavistic, 154 
 causes of, 155 
 degressive, 153 
 progressive, 153 
 retrogressive, 154 
 Mutationists, 17 
 
 Nageli, 190, 280 
 
 Natural selection, 42 et seq., 48, 
 
 121, 135 
 
 Naudin, C., 170, 220 
 Nebular hypothesis, 24 
 Neo-Darwinians, 3, 16 
 Neo-Lamarckian, 3 
 Newton, Sir Isaac, 175 
 Nicotiana paniculata, 163 
 glutinosa, 166 
 rustica, 163 
 tdbacum, 166 
 
 Normal variations, 82. et seq. 
 114, 116 
 
 curve, 95 et seq. 
 
 Nuclear division, 248 et seq. 
 
 membrane, 246 
 Nucleus, 177, 239, 246 et seq. 
 Nutrition, 76, 155 
 
 Oenothera alhida, 143 
 biennis, 147 
 gigas, 143 
 
 Lamar chiana, 142 et seq. 
 3 i 3 » 314 
 
 Iceta, 219 
 lata, 143 
 muricata, 147 
 nanella, 144, 147 
 velutina, 219 
 
 ‘ Origin of Species,’ 124, 323 
 Ovum, 72, 177, 24s, 308 
 Oxlip, 8 
 
 Painted Lady (sweet pea), 210 
 Papaver rhoeas, 140 
 Parental correlation, loi et seq 
 Partial gametic coupling, 200 
 Pea, garden, 84, 166 et seq., 190 
 198, 204 
 
 sweet, 209 et seq. 
 
 Peacock, 63, 132 
 Pearson, Karl, 77, 80 et seq. 
 108, no, 116, 136, 229, 287 
 
 324 
 
 Phasmidae, 55 
 Phyllopteryx, 55 
 Physical science, 4 
 Picotee (sweet pea), 210 
 Pigeon, 34, 45, 215 
 Plastids, 240 
 Pleurococcus, 241 
 Polish wheat, 234 
 Polygon, Gabon’s, 126 
 Population, 117, 118, 320 
 selection in, 119 
 Poulton, Professor, 57 
 Primrose, 8, 215 
 evening, 83 
 pin-eyed, 215 
 thrum-eyed, 215 
 
INDEX 
 
 335 
 
 Primula pyramidalis , 194 
 sinensis, 15 1, 194, 272 
 stellata, 194 
 
 Probability, theory of, 91 
 Progressive mutation, 153 
 Pronuha, 54 
 
 Protective resemblance, 54 et 
 seq. 
 
 Protenor helfragi, 272 
 Prothallus, 308 
 Protogene, 229 
 Protoplasm, 239 
 Protozoa, 116 
 Protozygote, 229 
 Puccinia glumarum, 237 
 Punnett, Prof. R. C., 47, 192, 
 200, 238 
 
 Pure lines, 116 et seq,, 320 et 
 seq. 
 
 .selection in, 119 
 Purple Invincible (sweet pea), 
 210 
 
 Quartile, 97 
 Quetelet, 80, 86, 93 
 
 Rabbit, 214 
 Radium, 23 
 Raimannia, 156 
 Ranunculus, 7 
 
 water, 150, 317 
 Recessive characters, 179 
 Red Fife (wheat), 235 
 Reducing division, 258 et seq. 
 Regeneration, 61 et seq. 
 Regression, 107 
 
 Resemblance, protective, 54 et 
 seq. 
 
 Retrogressive mutation, 154 
 Reversion on crossing, 202 ct 
 seq,, 213 
 
 Rivet wheat, 234 
 Romanes, G. J., 41, 79 
 Rosenberg, 254 
 Rotifers, 47, 330 
 Rough Chaff (wheat), 235 
 Rust immunity, 237 
 
 Saunders, Miss, 209, 214 
 Saxifraga hypnoides, 59 
 
 Schuster, E., 299 
 Sea-horse, 55 
 Sea-urchin, 256 
 Sedgwick, A., 75 
 Selection, artificial, 44, 114 et 
 seq,, 135, 141 
 
 natural, 42 et seq,, 48 ct 
 seq,, 76, 141, 157, 295 
 sexual, 62 et seq. 
 
 Selective fertilization, 209, 274 
 Sex, 221, 272 et seq, 
 
 Shaw, G. B., 325 
 
 Sheep, Leicestershire, 137, 215 
 
 Shirley poppies, 140 
 
 Shull, Dr. G. H., 213 
 
 Snails, 215 
 
 Snakes, 37 
 
 Somatic cells, 74, 272, 274 
 Species, idea of, 6 
 
 Jordan’s, 9, 153, 322 
 Linnaean, 7 et seq. 
 
 Origin of, 5, 124, 147 ci 
 seq,, 323 
 
 Spencer, Herbert, 23, 40, 47, 65 
 et seq,, 71, 122 
 Spermatozoa, 245, 310 
 Spindle, nuclear, 250 
 Spores, 257, 307 
 Standard deviation, 98 
 Statistics, 12, 81 
 collection of, 86 
 Stature, 90, 106 
 Sterility, 323 
 .Stick insects, 55 
 Stocks, 214 
 Strawberries, 172 
 Strength of pull, 89 
 of wheat flour, 235 
 Structural characters, inherit- 
 ance of, 185 
 
 Substantive variation, 13 1 
 Subtypes, 126, 127 
 Sugar-beet industr}^, 135 
 Superman, 326 
 Survival of the fittest, 47 
 of useless structures, 157 
 Swamping effect of crossing, 
 221 
 
 Sweet peas, 209 et seq. 
 
336 
 
 INDEX 
 
 \ Symmetry, 129 
 SyrphidcB, 58 
 
 Tactual discrimination, 66 et 
 seq, 
 
 .Tetrads, 259 
 
 Thomson, Sir J. J., 24, 127 
 Throw-back, 154 
 Tinged-white sweet pea, 21 1 
 Tower, W. L., 155 
 Treviranus, 30 
 Trifoliiim pratense, 150 
 Tschermak, 19, 190, 203 
 Types, primary and subordinate, 
 125 
 
 Twins, 287 et seq. 
 
 Unicellular animals, 240, 330 
 Unit characters, 176, 220, 315 
 Uranium, 23 
 Usher, Bishop, 27 
 
 Variability, acquired, 319 
 correlated, 106 
 genetic, 319 
 coefficient, 99 
 Variate, 97 
 Variation, 14 
 
 continuous, 14, 43, 76 et 
 seq., 82, 155, 316 et seq. 
 
 \ ansition— continued : 
 
 discontinuous, 14, 43, 66, 
 I35 j 316 et seq. 
 meristic, 130 
 negative, 14 1 
 
 similar and simultaneous, 
 132 
 
 substantive, 130, 13 1 
 
 Wading birds, 38 
 
 Wallace, A. R., i6, 35, 43, 65, 
 
 88, 157 
 
 Webb, Sidney, 295 
 Weismann, 16, 51, 52, 65, 72 et 
 seq,, 157, 279 et seq. 
 
 Wells, Dr. W. C., 42 
 Werner, 25 
 Whale, 52 
 
 Wheat, 136, 233 et seq, 
 Whetham, W. C. D., 23, 41 
 Wilks, Rev. W., 140 
 Willey, A., 79 
 
 Wilson, Prof. E. B., 74, 272 et 
 seq, 
 
 Winkler, 321 
 
 X-generation, 306 
 
 Yucca plant, 54 
 Yule, G. U., 227 et seq» 
 
 Zygote, 185, 245 
 
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