m^A^Mftrj^'s^yv '^% i A ••^ •...■@^-^; ^1^ ?^i^ Uli ' ililMM I IItllini'if iW ®I|p p, ^, 1|m pkarg ^orilj ©arnlma ^tate College QH5&T S00540732 L il ^T Date Due ^gApfSrW OCT 9, 6 1990 * ciple those great phenomena which without its aid remain a mass of unintelligible miracles. In a word, it raises the knowledge of organic nature to a science. Even now much of mere professional knowledge is wont to style itself science. But as the doctrine of Descent includes all life, it cannot stop on approaching Man. Were we doubtful as to the origin of language, or even forced to admit total ignorance on this point, we could not, from the existence of language, deduce the inapplicability to man of the doctrine of Descent, without, as it seems to us, arbitrarily breaking the chain of ratiocination. We will now return to the preliminary question already indicated, as to the limits of the investigation of nature. It is the more important, as incompetent judges are wont to assert, that these limits are exceeded. The frivolity of the logic by which such accusations are ren- dered plausible to the multitude surpasses all licence. We open, for instance, Luthardt's " Apologetic Lectures on the Fundamental Truths of Christianity," ("Apolo- THE INVESTIGATION OF NATURE AND MIRACLE. 1 3 getische Vortrage iiber die Grundwahrheiten des Christ- enthums,") and see how he defends the reality of miracles. " Miracles," he says, ** are not even miracles. They do not even repeal the laws of nature ; they merely release single occurrences from the dominion of those laws, and place them under the law of a higher will and a higher power. Of this we have many analogies in lower spheres. If my arm hurls a stone into the air, this is contrary to the nature of the stone, and is not an effect of the law of gravitation, but the interposition of a higher power and a higher will, producing effects which are not the effects of the inferior powers. These powers and these laws are not hereby repealed, but still subsist." Let us pause a m.oment. To say that it is contrary to the nature of the stone that gravity should be apparently overpowered for a few moments by muscular agency, is physically absurd. The stone remains the same weight, 'its nature is wholly the same, even while in the motion of projection ; and it is utterly unjustifiable and so- phistical to prate about muscular force as a higher power opposed to gravity. If the stone weighs two hundred-weight, where is the higher power then? But when the champion of supernaturalism has mis- led and prepared his hearers by his worthless analogy^ he proceeds : " Thus in the miracle, a higher causality interposes, and evokes an efiect which is not the effect of the concatenation of those lower causalities, and yet subsequently submits to these concatenations. But this lugher causality ultimately coincides with the highest moral objects of existence. To serve them is nature's higlicst and most glorious pursuit Therefore if miraclr 14 THE DOCTRINE OF DESCENT. Stands in connection with these objects, if its conditions are moral and not arbitrary, it is not contrary to nature and its purpose, but in the highest sense conformable to it" Thus as soon as belief in miracle comes into conflict with the investigation of nature, it says : " You overstep your limits, and must here suspend your judgment. It is a question of a higher moral object ; the domain of ethics is higher than that of physics, and therefore a higher causality, which physicists have no right to criticise, has suspended the chain of cause and effect with which you naturalists are familiar." This passage', in which one of the most learned and honoured champions of the belief in miracle lays down, like a sophist, the limits of the investigation of nature, is, however, among the most moderate of its kind. But our point of view and our logic differ radically from that of antagonists of this description, in one particular, namely, that to us the opposite to knowledge is igno- rance, whereas they supplement knowledge by a so- called higher knowledge, and by faith. While holding by the maxim of Pico della Mirandola, " Philosophy seeks, Theology finds, Religion possesses the Truth,"* it is forgotten that there are truths and truths. The subjective visions and sensations of sound by which the mentally diseased are excited and alarmed, are to them a reality, yet a reality quite different to that of the sights and sounds received through the healthy organs of the senses. Philosophy and science seek that truth which is deduced from the palpable connection of things. But the other truths, so often negatived by the former, are generally impalpable, and are incom- LIMITS OF INVESTIGATION. 1 5 mensurable with scientific truths. We will therefore abide by the words of Goethe : Whoso has art and science found, Religion, too, has he; Who has nor art nor science found. His should religion be.* And now, having provisionally averted uncalled-for objections and conflicts wath ambiguous ideas, we may quietly consider the limits of natural science. Let us first pause at the address delivered with general approval by the physiologist Dubois-Reymond, at the fiftieth assembly of German Naturalists and Physicians. He made reference to a passage in the classical works of Laplace, in the Introduction to the Theory of Science, which we cannot refrain from quoting in full. The author of the "Mechanism of the Heavens," says: "Pre- sent events are connected with the events of the past by a link resting on the obvious principle that a thing cannot begin to exist without a cause which produces it. This maxim, known by the name of the Principle of Sufficient Cause, extends likewise to events with which it is not supposed to come in contact. Even the freest will can- not evoke them without a determining impulse." "We must, therefore, regard the present condition of the uni- verse as the consequence of its former, and the cause of its future, condition. A mind, for a given moment acquainted with all the forces which animate Nature, and the reciprocal relations of the entities of which it is * Wer Wissenschafft und Kunst besitst. Hat audi Religion ; Wer jene L»eiden nicht besiizt. Dor babe Religion. f6 THE DOCTRINE OF DESCENT. composed — possessed, moreover, of powers of compre- hension sufficient to submit all these facts to analysis, would be able to reduce to a single formula the motions of the largest heavenly body and of the lightest atom. To such a mind nothing would be uncertain, and the future, like the past, would lie open before it. The human mind in all the perfection which it has been able to give to astronomy, offers but a faint image of such a mind as this." "All efforts of the human intellect in the search for truth tend to approach the mind above portrayed, but will always remain infinitely removed from it." The Prussian physiologist then quotes the " Thou art like the Spirit whom thou comprehendest" of Faust ;* and is of opinion that, in the abstract, the formula of the universe is therefore not impenetrable to the human intellect. But we own we are cordially indifferent to an abstract perfection which never comes to light, and regard the unattainableness of this vague formula of the universe as a very endurable limit to human inquiry. But independently of the dubious consolation of the formula of the universe, w^e must agree with Dubois- Reymond, when he considers that the limits, before which the highest conceivable intelligence must pause, are also insurmountable to man. In accordance with the views now prevailing among physicists and biologists, Dubois-Reymond has thus specified the only limit given to the investigation of nature^: "The knowledge of natural science, more closely defined above, is no real knowledge. In the attempt to comprehend the constant, to which the mutations in * Du gfleichst dem Geist, den du begreifst. ATOMS IDEALLY REPRESENTED. 1 7 the material world may be traced back, we stumble on insoluble contradictions. An atom contemplated as a minute, indivisible, inert mass, from which forces ema- nate, is a chimera. In the impossibility of compre- hending the nature of matter and force lies the only limit to the knowledge of natural science." These propositions require some elucidation. Eeyond the subdivision mechanically possible, we must think of substance or matter as consisting of particles ultimately indivisible. Of these atoms, according to the present standpoint of science, we are obliged to admit as many different species as are not chemically reducible to more simple elements. Now there is no doubt that these atoms are, in the actual sense of the word, imaginary, hypothetical quantities ; and theory seems to indicate that all matter, in the most different phenomena in the material world, is based on a single species of atom. Every manual of physics or physiology will show that, in order to understand and calculate the properties of these atoms and their combinations into the ingre- dients of compound bodies, susceptible of chemical analysis, they are ideally represented under various material forms, spherical, cubical, &c. ; furthermore, that in their combinations and co-operations as bodies, they must be contemplated as surrounded by a rarefied atmosphere of an universally diffused ether. But the atom itself, and therefore v the nature of matter, is something incomprehensible, unattainable. In these atoms, forces are inherent, which display themselves in attractions and repulsions, and in motion in general. But the final cause of these motions, and how far these motions are, as it were, identical with the existence of 1 8 THE DOCTRINE OF DESCENT. the atoms, is likewise included in the incomprehensi- bility of matter. " If we pass over this," says Dubois-Reymond again, " the universe is approximately comprehensible. Even the appearance on the earth of life in the abstract does not render it incomprehensible. For life in the abstract, contemplated from the standpoint of the theoretical investigation of nature, is merely the arrangement of molecules in a state of more or less stable equilibrium, and the introduction of an exchange of material, partly by their own elastic force, partly by motion trans- ferred from without. It is a misapprehension to see anything supernatural in this." This is the point which is usually contested with the greatest vehemence. If all the motions and states of quiescence of the inanimate world can be thoroughly explained, the inexplicable must commence with the basis of life. The imputation cast upon the reasoning powers by this assumption may be formularized as follows, in the question put by another sound and thoughtful physiologist, A, Fick :* " Are the charac- teristics of such a particle, as already explained, applicable and effective during the period of its sojourn in an organism ? Thus, for instance, will the motions of a particle of oxygen be affected and altered by a neighbouring particle of hydrogen, in accordance with the same laws, when one or both form part of an organism, as when they are out of it } " To reply in the negative is to avow the vitalistic conception of life, that is, to take refuge in unknown forces quite extraneous to matter, and to admit that the self- same particle can vary its nature, according HEAT, A MODE OF MOTION. I9 to vihcthei it be internal or external to an organism, is, in other words, to affirm a miracle. If this is weighed against the physical view, " which in its perfection reduces every organic process to a problem of pure mechanics," it may be done in the certainly impartial words of the naturalist just quoted : " I am of opinion that the mechanical view of organic life is demonstrated only when all the motions in an organism are shown to be the effects of forces, which at other times also are inherent in the atoms. But similarly I should regard the vitalistic view as proved, if in any case a particular motion actually observed to take place in an organism were shown to be mechanically impossible. At pre- sent, neither is to be thought of. Nevertheless, if a decision must be made without full proof, I provisionally profess myself unequivocally in favour of the mechanical view. Not only does it recommend itself a priori by its superior probability and simplicity, but the progress of scientific development raises it almost to a certainty. When it is seen how certain phenomena — such as the evolution of animal heat, which it was formerly believed could be explained only by vital force — are now ascribed, even by those who in general assume the existence of a special vital force, to the universally active forces of the material particles, we find ourseK^es almost forced to the conviction that by degrees all the phenomena of life will become susceptible of mechanical explana- tion." For the elucidation of the example just given of animal heat, let us observe that modern physics have learnt to know heat as a peculiar mode of motion. The motion of the hammer as it falls upon the anvil is not lost, but 20 THE DOCTRINE OF DESCENT. is transformed into the atomic motion of the places struck, a motion, invisible, it is true, but sensible as heat. But likewise the combination of the particle of oxygen introduced into the animal body by the respi- ration, with the un-oxygenated constituents of the blood, is a motion subject to computation, and manifesting itself as oxydation, combustion, or the evolution of animal heat. This chemical act of combustion keeps the animal steam-engine in motion. In this way, by the application of mechanical prin- ciples, modern physiology has traced to their causes a great number of organic processes, and the phantom of vital force, which formerly reigned paramount over the whole intestinal canal, incited the glandular cells and the muscular fibres to their offices, and glided along the nerves, now scarcely knows where to breed disturbance. Thus the investigation of nature does not shrink from enrolling life and the processes of life in the world of the comprehensible. We are foiled only at the conception of matter and force. But we are much further advanced than Schopenhauer and his adherents, who for the idea of Force substitute that of Will ; for we have analyzed into their several self-conditioned momenta a multitude of processes, which the word " Will," incomprehensible in itself, is supposed to explain in their totality ; and much further also than the fashionable philosopher of the day, von Hartman, who regales us with the agency of the •' unknown " in the domain of the organic world. "And yet," Dubois-Reymond thus formulates another limit, " a new incomprehensible appears in the shape of consciousness even in its lowest form, the sensation of desire and aversion. It is, once for all, incomprchen* CONSCIOUSNESS. 21 sible how, to a mass of molecules of nitrogen, oxygen, hydrogen, carbon, phosphorus, and so on, it can be otherwise than indifferent how they lie or move ; here, therefore, is the other limit to the knowledge of natural science. Even the mind imagined by Laplace cannot go beyond this, to say nothing of our own. Whether the two limits to natural science are not, perchance, identical, it is, moreover, impossible to determine." In these last words the possibility is indicated that consciousness may be an attribute of matter, or may appertain to the nature of the atoms. And we may add, that the attempt has of late been repeatedly made to generalize the sensory process, and to demonstrate it to be the universal characteristic of matter, as by von Zollner, in his work on the Nature of Comets, which has created such a justifiable sensation. He holds that, if by means of delicately-formed organs of sensation it were possible to observe the molecular motions in a crystal mechanically injured in any part, it could not be unconditionally denied that the motions, hereby excited, take place absolutely without any simul- taneous excitement of sensation. We must either re- nounce the possibility of comprehending the pheno- menon of sensation in the organism, or " hypothetically add to the universal attributes of nature, one which would cause the simplest and most elementary opera- tions of nature to be combined, in the same ratio, with a process of sensation." It might be imagined that reflections of this kind would lead to the delusive abysses of speculation ; but if, still speaking only of organisms, we descend from the manifestations elicited by sensations of desire and 22 THE DOCTRINE OF DESCENT. aversion in the higher consciousness of man and of the superior animals, till we see all reaction to external ex- citation dwindle into the scarce perceptible motions of the simplest protoplasmic animalculae, it is evident that there can be no question here of either consciousness or will. We cannot then separate the idea of those sensations of desire and aversion, by which mo- tions are excited, from the elementary attributes of matter, as we are wont to do with regard to the higher animals.* In precisely the same sense, it was said some years ago by one of the most talented investigators of lan- guage — Lazarus Geiger, now unfortunately deceased : ^ '* But how is it, if further down, below the world of nerves, a sensation should exist which we are not capa- ble of understanding ? And it probably must be so. For as a body that we feel could not exist unless it consisted of atoms that we do not feel, and as we could not see a motion were it not accompanied by waves of light which we do not see, neither could a complex living being experience a sensation strong enough for us to feel it also, in consequence of the motion by which it is manifested, if something similar, though far weaker and imperceptible to us, did not occur in the elements, that is to say, in the atoms. If we only con- sider that we are as little capable of knowing that the falling stone feels nothing, as that it does feel ; it is fully open to us to decide, in accordance with the greatest probability, that the vrorld is susceptible of explanation." We have examined the limits which the investigation of nature has prescribed for itself. The organic world. MAN HAS RISEN FROM A LOWER GRADE. 23 t far from rearing itself before us as an incomprehensible entity, invites us to fathom its nature, and promises to reflect fresh light upon the inanimate world. We must now pass in review a great portion of ani- mate nature, and shall then arrive at the same con- clusion as the linguistic inquirer, to whom — we again quote his words — " it became, on historic grounds, incon- trovertibly certain that man has risen from a lower, an animal grade.'* 24 THE DOCTRINE OF DESCENT. II. The Animal World in its Present State. In order to approach the doctrine of Descent, and to prepare for its necessity, we purpose next to pass in review a main part of its object, — the present condition of the animal world in its general outHnes. Organisms, as every one may see, are distinguished from animate bodies by a certain mutability of existence ; a sequence and alternation of phenomena, combined with constant absorption and expulsion of matter. These changes, which are ultimately molecular motions, and are there- fore calculable, definable, and susceptible of investiga- tion, take place in particles in a state of saturation — that is to say, soaked in water and aqueous fluids ; and this peculiar, yet purely mechanical condition, suffices for the explanation and comprehension of many of the neces- sary phenomena of life. Experience shows that this capacity for saturation, and this mobility, essentially characterize the combinations of carbon ; and the sum of these motions and displacements, of which a great part has already been susceptible of mathematically cer- tain investigation, is termed Life. Now it is impossible to resist the impression that there are simple and composite, lower and higher, living beings ; and we likewise feel, more strongly than words will express, a certain antithesis between the plant and LOWER FORMS OF LIFE. 25 the animal. Poetically regarded, the plant is the passive organism as described by Riickert : ** I am the garden flower And meekly bide the hour, The guise, with which you come Within my narrow room." * The antithesis of the passive, quiescent plant and the pugnacious active animal diminishes, however, as we descend in the scale of both kingdoms. The more highly developed animal evinces its animal nature by the vivacity with which it reacts to external influences and excitations. In the lower animals the phenomena of Hfe assume a more vegetal character, and in many- groups of lower beings, which Haeckel has recently comprised under the name Protista, we see the pro- cesses of metamorphosis of tissue, nutrition, and repro- duction taking place, indeed, but in a manner so simple and undifferentiated, that we too must attribute to these beings a neutral position betwixt plants and animals. We gain the conviction that the roots of the vegetal and animal kingdoms are not completely sundered, but, to continue the simile, merge imperceptibly into each other by means of a connective tissue. In this inter- mediate kingdom, I still believe that the " primordial slime " (Urschleim), or Bathybius, has a right to hold a position. Many tiiousand cubic miles of the sea- bottom consist of a slime or mud composed in part of manifestly earthy inorganic portions, in part of * " Ich bin die Blum im Garten Und muss in Uemuth warten, Wann und auf welche Weise Du triitst in meine Kreise." 26 THE DOCTRINE OF DESCENT. peculiarly formed calcareous corpuscles, still perhaps am- biguous in their nature (the Coccoliths and Rhabdoliths), and finally, which is the main point, of an albuminous substance. This slime, the so-called Bathybius, does not, however, exhibit individuality, or the definiteness of a separate existence ; it resembles the shapeless mineral substances, each particle of which bears the characteristics of the whole. The conception of an organism as a being composed of various parts, with various offices or functions, and appearing under a definite form gradually developed, is in our day so inherent and intuitive, that it is only with great exertion that we are able to accommodate our- selves to the idea of a living mass either absolutely formless and undefined, or defined arbitrarily and acci- dentally. Let any one, who either cannot or will not do this, pause for a moment to contemplate another simple being — for instance, Haeckel's " Protamoeba." A small albuminous mass increases by the absorption of nutriment, and by the appropriation of matter, until it reaches a certain circumference, and then propagates itself by spontaneous fission into two equal parts. To our means of observation, these and similar beings are the simplest organisms devoid of organs. While ac- centuating the limits of research as restricted by inade- quate means of observation,'- we maintain the validity of Rollet's retort,^ that our reason cannot properly admit such homogeneous organisms, performing all the functions of life solely by means of their atomic con- stitution ; that we are dealing with the still utterly unknov/n structure of the molecules formed by the PROTISTA. 27 aggregation of atoms ; and that if Bi ucke says, " Apart from the molecular structure, we must also ascribe to living cells another structure of a different order of com- plexity, and this is what we denote by organization," we must likewise ascribe this yet unknown combination to the Monera of Haeckel. But independently of this complexity of the molecular structure, it is of extreme importance to the investiga- tion of animate nature to have become acquainted with bodies which present the simplest structure to the as- sisted eye, and to anatomical research. The substance which characterizes them is found again in plants as well as in animals ; and plants and animals must now be regarded as two classes of organisms, in which the processes of self-preservation and reproduction have, in different ways, assumed the character of a higher com- plexity and development, by the differentiation of the originally homogeneous substance into various morpho- logical structures and organs. As we shall have another opportunity of expressing an opinion in regard to the beginnings of animal life, and its points of contact with protista and plants, we shall transfer ourselves from the dubious boundary line into the midst of the animal kingdom, in order to master our subject by sifting and arranging it. The first impression of infinite variety is succeeded by another, that there are lower and higher animals. On this point com.plete harmony prevails. For if, from teleo- logical considerations, invalid in our eyes, the nature o\ every creature were said to be perfect, that is, in corre- spondence with its purpose or idea, every one takes it for granted and self-evident that a standard of excellence 28 THE DOCTRINE OF DESCENT. exists, without taking account of the scale by which it rises or sinks. This standard will, however, soon be made mani- fest by the comparison of a lower with a higher animal. Let us select the fresh-water polype and the bee. The little animal, several lines in length, which in our waters usually lives adhering to a plant, is a hollow cylinder, of which the body-wall is formed of two layers of cells. The external layer gives rise to fibres which act as muscles, and to a supporting membrane which may be compared to a skeleton. The mouth is sur- rounded by arms of similar construction, and varying in number from four to six. The surface of the body is studded with numerous little stinging vesicles, which by their contact stun any smaller animalculae straying within the reach of the polype, and render them an easy prey. This is, in a few words, the construction of the animal. It possesses no arterial system, no special respiratory apparatus ; the functions of the nerves and the sensory organs are performed by the individual parts of the surface. Reproduction is usually effected by the budding of gemmules, which fall off at maturity, but occasionally also by the produce of very simple sexual organs. On the other hand, hours do not suffice to describe the structure of a bee. Even externally, its body, which. possesses so highly complicated a structure, pro- mises a rich development of the interior. The man- ducatory apparatus can be rendered comprehensible only by comparison with the oral organs of the whole insect world. The various divisions of the alimentary canal are each provided with special glands. The rich psychical life, all the actions which imply intelligence, SYSTEMATIC ARRANGEMENT. 2g calculation, and perception of external situation, are rendered possible by a highly developed nervous system, and the marvellously complex sensory organs combined with it, of which the eyes are especially remarkable. Independently of the generative organs, consisting of manifold parts of greater or less importance, the history of the multiplication and development of the bee de- mands a study of itself. The function, and therewith the rank and value, of the bee's body seem to us higher than that of the polype in proportion as it is more complex. The superior com- plexity and variety of the parts is anatomically evident, and similarly the higher phase of the life. The superior energy of the existence, the functional capacity and per- fection of the bee as contrasted with the feebleness of the polype, is obviousb/ a result, or more correctly an expression, of the greater mechanical and physiological division of labour. In one animal, as in the other, life is spent in the function of self-preservation and the maintenance of the species, or reproduction ; in both, the cycle of phenomena is limited, unbroken ; but the means of execution are very different, and therefore the general effect is different. In the variety and correlation of the organs destined for the different manifestations of life, we have a standard for the rank of the animals. This rank has a twofold character, general and special. In other words, the position of an animal in the system is defined, first, by the general attributes, which it has in common with the forms harmonizing wdth it in the main characters of their organization ; and, secondly, by the more special characteristics, which place the animal in it^ 30 THE DOCTRINE OF DESCENT. own rank and station among its own immediate kindred. Some insight into this classification of the animal kingdom is naturally indispensable to any one, who wishes to test and understand its reasons, and to render an account of it is an essential part of our task. Since Cuvier's reconstruction of Zoology in the early part of this century, our science has been familiarized with the expression "type," or "fundamental form," introduced, long before, by Bufifon. Cuvier, by ex- tensive dissections and comparisons, first proved that animals were not, as people were formerly inclined to suppose, made on a last or shaped upon a block ; but that they fall into several great divisions, in each of which expression is given to a peculiar constitution, arrangement, and distribution of the organs ; in short, to a peculiar style. The sum of these characteristic peculiarities, as well as the whole of the species united in it, was termed a " type." Various views, it is true, even now prevail as to the extent of several of these types or families, as we will already term them ; but if we dis- regard the dubious, and in many ways suspicious, exis- tences, generally comprised under the name of primordial animals, there is a general agreement as to the following number, but less as to the sequence of the animal types, than as to those groups, each of which has its peculiar physiognomy and special characteristic structure. The class Coelenterata includes the Polypes and Medusae, and in the closest connection with it stands the interesting class of the Spongiadae, especially in- structive as affording direct evidence of the doctrine of Descent. The organs of these animals are nearly always TYPES AND FAMILIES. 31 arranged radially round an axis, passing through the dorsal and ventral pole. The cavity, which in most other animals — for instance, in man — is termed the abdominal cavity, the space between the intestinal wall and the abdominal parietes, is deficient in them ; but, on the other hand, from the stomach proceed in general various kinds of tubes and branchia, which to a certain extent replace the abdominal cavity. Fig. 2 represents a Fig. 2. Medusa, Tiaropsis DIadema, after Agasslz. The darkly- shaded organs form the so-called ccelenteric apparatus. Of the Echinoderms, the reader is probably ac- quainted, at least with the star-fish (Asterias) and the sea-urchin (Echinus), of which the general form is like- wise usually radiate. Besides a peculiar chalky deposit, or greater or less calcification of the skin covering, a system of water-canals forms a characteristic of this family. With these are connected the rows of suckers, which, by protrusion and retraction, serve as organs of locomotion. On account of the radiate structure pre- vailing among the Echinoderms, Medusas, and Polypes, 32 THE DOCTRINE OF DESCENT. Cuvier believed them to be more nearly related, and in- cluded them altogether, under the name of Radiata. This similarity, however, is only superficial, for whilst, on the one hand, anatomy discloses the great difference of the Coelenterata and Echinodermata, the history of evolution still more decidedly banishes the Echinodermata from this position, and connects them more closely with the next division. In this, that of the Vermes, the systematlzer of the old school finds his real difficulty ; in so many ways do they deviate from each other, so great is the distance between the lower and the higher forms ; and after deducting the distinctive marks of orders, so little remains as a common character, so variegated is the host of smaller scattered groups, and even of single species, which demand admittance to the system of the Vermes. If we attempt to describe their typical nature in a few words, it must be something like this : The Vermes are more or less elongated, symmetric animals, which possess no actual legs, but effect their locomotion by means of a muscular system, closely combined with the integuments, which frequently become an actual muscular cylinder. To this we will add, that the per- plexities and difficulties in reference to points of classi- fication are transformed into sources of knowledge for the adherent of the doctrine of Descent. The relations of the previous family with the type oi the Articulata is so conspicuous, that the " kinship " of the two was never questioned, even by the older zoologists. The very name of one, the highest division of the Vermes, that is, of the Annelids, or segmented worms, indicate this connection. This distinctive mark GRADATIONS WITHIN THE TYPE. 33 of the Crustacea, Arachnida, Myriopoda, and Insecta, is that their bodies are constructed of sharply-defined rings or segments, the legs, antennae and mandibles likewise sharing in this segmented character. A faithful expres- sion of this segmentation is afforded by the nervous system, which lies, ladder-like, on the ventral side, that is, beneath the intestinal canal, only encircling the gullet with its anterior loop. The display of segmentation is favoured by a deposit of horny substance, which gives a skeleton- like stiffness to the integuments. The direct reverse is shown in the inteGTuments of the MoUusca, our mussels, snails, and cuttle-fish. For although so many are supplied with protecting scales and shells, these are mere excretions from the actual skin, which remains soft, and characteristically moist and slimy, owing to the secretions of numerous glands contained in it, and has an inclination to lay itself in folds, and form a mantle-like investment to the body. The body therefore remains more or less clumsy ; it pos- sesses none of the grace of the Articulata, and especially of the insect ; it is destitute of segmentation, and this deficiency is likewise evinced in the nervous system. This consists only of a ring, encircling the oesophagus, and a few smaller ganglia. We shall most readily come to an understanding as to the Vertebrata, the family with which man is insepa- rably united. The essential part is the vertebral column, that portion of the internal and persistently bon^ or cartilaginous skeleton, in which the main portion of the nervous system is contained. It is thus established that the systematic classification of the animal kingdom is based on certain prominent 34 THE DOCTRINE OF DESCENT. characteristics of form and internal structure ; and it is very easy to select from every type forms in which the distinctive marks, comprised in the systematic diagnosis, may be displayed in full perfection. But this is imme- diately succeeded by a further observation, that of gra- dations within the type. When we previously compared the polype and the bee, and were obliged to assign to each a very different rank, a portion of this difference of grade is certainly due to the difference of the family; but the forms united by family characteristics likewise diverge widely from each other, and the systematist speaks of lower and higher classes within every type, of lower and higher orders within every class. Reason is compelled to this by the same considerations which forced themselves upon us in the comparison of the polype and the bee. Why does the mussel stand low^er than the snail.-* Because it does not possess a head, because its nervous system is not so concentrated and so voluminous, because its sensory organs are more de- fective. In one, as in the other, the structural material is present in quantities sufficient for the completion of the type ; but in the snail it is more developed, and the single circumstance of the integration of various parts to form the head confers a higher dignity upon the snail. It is needless to illustrate this gradation within the families by further examples ; the most superficial com- parison of a fish with a bird or a mammal, of one of the parasitic Crustacea with a crayfish or an insect, shows, as the older zoology represented it, that in the actual forms the ground plan, or "ideal types," find very diversified expression. A further result of this descriptive inquiry is the TREE-LIKE GROUriNG. 35 tree-like grouping of the members of the same family. The reciprocal relations of the various families can- not be represented in a simple line ; though in former days more importance was attributed to the general indications of the relative value of the types. On the other hand, descriptive zoology had long been compelled- to devise tables of affinity for the systematic subdivisions, descending even to species according to the criterion of anatomical perfection ; and these found expression only in diagrams of highly ramified trees. Branches ap- peared which terminated after a brief extension ; others are greatly elongated with numerous side branches ; in every branch characteristic phenomena and series are made manifest. Let us attempt it with the Vertebrata, for example. Even with the fishes we fall into great perplexity ; which to place at the end as being the highest. But take which we will, the sharks or our teleostei, the am- phibians cannot be annexed in a direct line, nor does the elongated branch line of the latter merge, as might be imagined, into the rejDtiles. The birds, on their side, offer a sharp contrast to the mammals, and this separa- tion and divergence extend to all the subdivisions. We must figuratively represent family branches, clusters of genera, and tufts of species, which latter ramify into sub-species and varieties. With this representation of the tree-like distribution of the system, we shall gladly revert to the comparison of the members of different types, with reference to their functional value. The bee in itself is manifestly a far more complex organism than the lowest fish-like animal, the lancelet ; and in these two we compare a low form of a high type, and a high form 4 jS THE DOCTRINE OF DESCENT. of a low type. By varying and combining comparisons of this sort, and taking account of the points of connection between the various types, to which we shall immediately refer, the figure of the systematic trees completes itself into one vast tree, of which the main branches are re- presented by the types. Had the system atizers of the old school been familiar with the construction of plants and animals, they would have first established the diagnoses and distinctive cha- racters, and then called to life the types and their species: for their chief torment has been, that the dia^jnoses are liable to so many exceptions, and that the characters of the fundamental forms are without any absolute value. Roughly and generally speaking, polypes are radiate in form, but not a few are bilateral, or symmetric on two sides. Most snails possess well-marked mantle-folds, but we can scarcely speak of the mantle of many thoroughly worm-like slugs. Head and skull seem an inalienable mark of the vertebrata, yet the lancelet has no such head, but merely an anterior end. Nevertheless, it may be objected, it has a vertebral column ; yet this, the special badge of nobility of the vertebrate animals, like the auditory appa- ratus, and the notochord, is, even if only transiently, a possession of the Ascidians, a class of animals which in their mature condition do not bear the remotest re- semblance to the Vertebrata. When we become aware of these deviations from so-called laws of form and structure, seemingly well established, we are prepared for a manifest failure of the system, in regard to con- necting forms, and forms of uncertain position in the system. INTERMEDIATE FORMS. 37 If the result of the systematic sifting and arrange- ment within the individual types can be comprised in diagrams of trees, forms intermediate to the members of the types, classes, orders, &c., follow as a matter of course. For if the figure be correct, every ramification of the branches must include species diverging very slightly from the species standing in the lowest portions of the bouP"h from which it branches off. And thus all systematizing, in fact, amounted to the insertion of the risrht intermediate forms between each two forms devi- ating from each other in a higher degree ; nay, in some cases, intermediate forms were sought where none exist. The older zoology always regarded the duck-mole (Or- nithorhynchus) as the mammal most nearly allied to the birds, though the cause of the bird-like appearance of the lowest mammal known, is by no means to be sought in a direct relationship, but in a remote cousinhood. But we must draw attention, not to these connecting forms, which natural history assumes as perfectly self- evident, but to those which are, as it were, inconvenient to systematic description, and threaten to render illusory the groundwork so laboriously gained. There are some fish like animals, the Dipnoi, (Lepidosirens and their congeners) with the cliaracters of Amphibians. The Infusoria possess many characteristics of the so-called primordial animals, but in other ways they differ from them, and point to the lowest Turbellaria. A minute animal inhabiting our seas in countless multitudes, i.e. the Sagitta, is neither a true annelid nor a legitimate mollusc. The Peripatus, a creature recently and very thoroughly investigated, combines in a highly remark. ?ble manner the characteristics of the Articulata ; for 38 THE DOCTRINE OF DESCENT. instance, tracheae with those of the Annelida — as the lateral organs. The combination is inexplicable and paradoxical from the point of view of the old system. Example after example might be thus accumulated to show that the rigid partitions of the system are scarcely raised before they are again broken down in every direction ; and this in direct ratio with the increase of special science. As before said, descriptive natural history necessarily gained this experience. It then spoke of exceptions and deviations, without being able to adduce any reason why the classes and types should be able to break through their limits, and indeed most frequently without feeling any need of accounting for the failure of the rigid system. III. The Plienomena of Reproductior. in the Animal World. The faculty of giving existence to new life is part of the evidence of life. A crystal does not reproduce itself, it can only be resolved into its elementary consti- tuents ; and in the natural course of things, or in an artificial manner, these may be induced to form another crystalline combination. But this is not that con- tinuity of reproduction which links Individual to indivi- dual, is not procreation wrapped in a cloud of mystery. Herein, it seems, consists a stubborn opposition. Yet, if the distinction between animate and inanimate nature has been recognized as one not entirely absolute ; especially if the possibility, nay even the necessity, has been perceived of the primordial generation or parent- less origin of the lowest organic beings from inorganic matter (of which more hereafter), and if the nature of nutrition and growth is understood to be entirely dependent on the power of obtaining material, — the mystery of reproduction henceforth disappears. Gene- ration is no longer a mystical event ; and the origin of an organism in or from an organism, the emission or development of innumerable germs, may, like the origin of a new crystal, be analyzed into the motions of elements, as yet accessible only to the eye of Imagi- 40 THE DOCTRINE OF DESCENT. nation. By this we mean to say that in the province of reproduction the Hmits of inquiry are neitlier narrow nor pecuUar. We will therefore now proceed to describe the process of reproduction and development in the animal kinG:;dom. If, as must be generally admitted, the most essential characteristics are common to the highest and the lowest life, — and it is only the complexity of the vital processes, together with the variety of the parts by which they are performed, that give rise to graduated diversities, — it will, of course, be in the simplest organisms that we shall most readily recognize the nature of these vital processes. The simplest beings, discovered by Haeckel, such as the Protamceba, those minute albuminous masses of sar- code, increase to a certain extent. Why these dimensions should v^ary only within definite narrow limits, and why, on attaining a certain extent, the molecules should gravitate into two halves, we do not know ; at any rate it is an affair of relations of cohesion, theoretically susceptible of computation. It is enough that at a certain size the coherence of the parts is loosened in a central zone, the individual becomics faithless to its name, and divides into two halves, of which each from the moment of separation begins an individual life, while from the commencement of the fission prepara- tions were being made for their self-dependence. This is the simplest case of reproduction, a multiplication by division. Frequently, however, it does not stop at bisection ; the motion of the minute constituents, which causes the fission, proceeds in such a manner that the halves are again divided, and the quarters yet again, the whole being thus divided into a greater number of FISSION — GEMMiNTION. 4I portions, and the parent-creature is resolved into a swarm of off-shoots. This multiplication by mere division of the mass pre- supposes that the organism thus reproducing itself pos- sesses no high complexity. The bisection of a beetle or a bird is inconceivable as a means of propagation. Yet Stein's valualDle observations on the reproductive process of the Infusoria, make us acquainted with organisms standing far above these simple so-called Monera, of which the subdivisions undergo a series of profound metamorphoses, before separating as self- dependent individuals. This transformation, combined with fission, leads to reproduction by gemmation. As the fission of these low organisms depends on the attainment of a certain limit of growth conditional on adequate nourishment, the case now more frequently occurs that the individual discharges the superfluity of material obtained at a definite part of the body, and forms a bud or gemmule. We are already acquainted with reproduction by gemmation in the simplest organ- ism, the cell ; for all healing and cicatrization in higher beings, even to the re-integration of the mutilated limbs of amphibians, is effected only by the reproduction by fission and gemmation of the elementary morphological constituents. But it lies in the nature of the process of gemmation, that it should extend far higher than rission in the scale of organisms ; it is the origination of a new being from one already existing, the latter, meanwhile, preserving its individuality wholly or for the greater part, and yet being able to transfer to the progeny its own characteristics in their full integrity. The simplest case of gemmation is where the parent 42 THE DOCTRINE OF DESCENT. animal produces one or more gemmules similar to itself, capable in their turn of producing similar gemmules. Of this, every collection of corals gives numerous examples, and shows how the diversified appearance of the several genera of coral depends merely on minor modifications of this mode of reproduction. Yet single corals exist in which, on careful comparison, not only may accidental deviations be already discerned, but regularly recurring variations between parent and progeny, as Semper has recently shown in Madrepores and Fungiform corals. This brings us to the highly-important phenomenon of Alternate Generation, which we must elucidate by a few examples before entering upon the nature of sexual reproduction. Figure 3 shows in A a, polype-shaped being with cruciform tentacles, on which its discoverer, Dujardin, bestowed the generic name of Cross-polype, or Stauri- dium. This animal, growing like a polype upon a stalk, forms above its lower cross, gemmules which make their appearance as spherical balls, gradually assume a bell-like shape, and detach themselves on attaining the structure and form of a Medusa or sea-nettle. The Medusa (termed Cladonema Radiatum, Fig. 3 B) is thus the offspring of its utterly dissimilar parent, the Stauridium ; it repro- duces itself in the sexual method, and from its eggs proceed Stauridia. The two generations thus alter- nate; the cross-polype is an intermediate generation in the development of the Medusa, so that the sexual genera- tion never originates directly from its egg. In the tape-worm, we have an illustration of the same process, only in a somewhat more complicated form. It is known that from the intestinal canal of individuals ALTERNATE GENERATION. 43 afflicted with tape-worm, issue so-called somites or seg- ments of the tape-worm. These somites are usually filled with such an extraordinary number of ova that they seem like mere packets of eggs. It appears, how- FIG. 3. ever, from the evolutionary history of the tape-worm, and its relations with other annulosa, namely with leeches and Turbellaria, that notwithstanding their in- completeness and deficiency of organs, these somites are equivalent to sexually mature individuals ; or, according to Haeckel's definition, are endowed with personality. If the tape-worm now comported itself like most other animals, somites would be directly developed from its eggs. But to this there is a very circuitous proceeding. 14 THE DOCTRINE OF DESCENT. If the egg of a tape-worm, by chance and good luck, strays into a congenial stomach, — for example, the egg of the human tape-worm, Tcenia solium, into the stomach of a pig, the embryo wanders out of the stomach in which it quitted the egg, and makes its way into the muscles, where it swells out into a sort of cyst. This cyst is the first intermediate generation. It produces a peg-shaped gemmule, which, however, fails of its object as long as the " bladder worm," or " Gargol," remains in the flesh of the pig. It is only when this comes, raw or imperfectly cooked, into the human stomiach, that the time has arrived for the release of the pupa. It emerges from its parent the cyst, and the pupa, in which we now recognize the head and thorax of the t^pe-worm imago, represents a second intermediate generation. Its pro- ductiveness is forthwith displayed; it becomes elon- gated, and as its ribbon-like form increases, shooting out from the posterior portion of the cervix, the more distinctly marked become the transverse stripes and "somites ;" in other words, the individuals of the third or sexual generation. In the evolutionary cycles just discussed, there is an alternation of asexual and sexual reproduction ; and before examining some other cases of asexual multi- plication, we must make ourselves acquainted with the facts of sexual reproduction. The characteristic of this is, that it requires for the generation of the new individual the union of two different products or morphological elements, the ovum and the sperm. The ovum is always, in the first in- stance, a simple cell, of which the nucleus is termed the germinal vesicle, and the nucleole the germinal spot. GERM-CELLS. 45 In manyanlmals it is provided with a sheath or membrane of its own ; in others it remains naked, and in that case frequently displays the remarkable movements of pro- toplasm. The germ-cells of different classes of animals vary considerably in their microscopic dimensions ; nevertheless, in the whole animal kingdom, from the sponges and polypes up to the mammals inclusive of man, they are essentially similar. Nor do non-essential differences appear until the primitive germ-cell is more abundantly provided with yelk and albumen, and has surrounded itself with a specially thick and perforated shell, as in insects and fishes, or with a peculiarly formed sheath, in the shape of a double concave lens, as, for instance,* in some Turbellaria. As a rule, the ova are formed in special organs, the ovaries. The other sexual element, the sperm, contains, as its peculiar active constituents, the spermatozoa (fig. 4 s), which consist of a pointed, elliptic, or occasionally of a hook-shaped, head, and a thread-like body. As long as the sperm is capable of fecundation, the filamentous appendage performs serpentine movements, and the development of the spermatozoa from cells, as well as the comparison of their movements with the vibrating movements of ciliated and flagellate cells, enable us to recognize them also as modified cell structures. The vehement dispute of last century between Evo- lutionists and Epigenists has now a merely historical interest. The former maintained that either in the ovum or in the sperm-corpuscle the whole future organism 4-6 THE DOCTRINE OF DESCENT. was prefigured in all its parts, and that it hence required only the development of the infinitely minute organs already existing. The others, who carried off the victory, saw in the ovum the yet undifferentiated material which subsequent to fecundation had still to be transformed into the various morphological elements and organs. But it is scarcely twenty years ago since the process of fecunda- tion was discovered, and since it was proved that at least one sperm corpuscle, and, as a rule, several or many, must penetrate into the interior of the ovum and unite materially with its substance in order to produce an effectual fecundation. The course of our demonstration obliges us to place sexual in sharp contrast with asexual genesis. But here, again, recent times have produced a series of equalizing and conciliatory observations which must not be neglected by us, bent as we are on tracing the antecedents of the doctrine of evolution, and demon- ftrating the transition taking place throughout organic Nature. In the cases of alternate generation selected above, the generations which do not produce ova and spermatozoa, reproduce themselves by external gemma- tion. Now, there is manifestly no great physiological difference if the deposition of the material from which the progeny is formed takes place, not externally, but in and by special internal organs. One of the most familiar examples occurs in the evolutionary cycle or alternate generation of the genus Distoma of the Entozoa. In the ventral cavity of one larval genera- tion arise cell-spheres, or germs, which develope into the second generation — the Cercaria. Great excitement was likewise aroused by the dis- DEVELOPMENT OF UNFERTILIZED OVA. 47 covery of the germ-formation of the larvae of some di- pterous insects (Cecidomyia, Miastor). In the ventral cavity of the maggots of these flies arises a second gen- eration of maggots, of which the origin was primarily attributed to a simple germ-formation, until it was shown that these germs proceed from the situation of the sexual glands (which in many insects are deve- loped at a very early stage), and must therefore be regarded as unfertilized ova. The second generation of maggots lives at the expense of its parent, consumes its fatty substance, and afterwards destroys the other organs ; while of the pelican-like parent nothing finally remains but the skin, as a protecting cover to the offspring, which very soon emerges. Without mentioning other cases in which it may be questionable whether germs or unfertilized ova attain development, we will point out a few of those in which development, without fecundation, is established with complete certainty. The queen bee, partly from the natural course of its life, partly from various accidenis in which fecundation could not take place, lays regularly a number of unfertilized eggs, from which issue drones, or male individuals ; or if exceptionally eggs are laid by workers, which are imperfectly developed female bees not susceptible of fecundation, these eggs likewise produce drones only. Von Siebold's highly interesting experiments on the reproduction of a wasp (Polistes Gallica), have shown that the hybernating fertilized females, who found a new colony in the spring, deposit eggs whence issue female individuals, and occasionally males. This virgin generation then produces eggs from which males are developed. With various butterflies, tS THE DOCTRINE OF DESCENT. on the contrary, the unfertlHzed eggs produce females only ; and it is the same with several of the lower crustaceans. We will now revert to the consideration of the evolu- tionary processes displayed in sexual reproduction after fecundation has taken place. Development invariably commences with a process of cell-formation, the bifurca- tion or formation of the germinal membrane, after the completion of \\'hich, instead of the one primitive cell, ,1 large number of cells are usually in existence, as the material for the distribution and construction of the embryo. Ova developing parthenogenetically, without fecundation, likewise commence their development by this multiplication of cells ; and even the ova of ani- mals, in which development never takes place without previous fecundation, exhibit an incomplete bifurcation, if not fertilized at a certain stage of maturity. This process, it is true, has been as yet demonstrated only in the ova of the frog and the domestic fowl ; but these cases are sufficient to divest the bifurcation of the character of an independent phenomenon, exclusively restricted to sexual reproduction. Even before the appearance of C. E. von Baer's really classical and fundamental work on the " Evolutionary History of Animals " (Entwickelungsgeschichte der Thiere),^ the view, founded on incomplete observations, had become established, that in the various stages of their development the higher animals passed through the forms of the lower ones. In this, natural philosophy did not confine itself to the limits of the types ; and hence did not pause at the hypothesis that the mam- malian embryo was successively a fish, an amphibian TYPES OF DEVELOPMENT. 49 and in a certain sense, by a particular gradual evolution of the organs, a bird also, but made the embryo like- wise repeat and surpass the lower types. To this false tendency, acting on vague analogies, a stop was put by the great naturalist just named. He showed that a number of coincidences might, indeed, be demonstrated between the embryo of the higher and the permanent form of the lower animals, but that this resemblance rested essentially on the fact that in the embryo of the higher animal the differentiation of the general funda- mental mass had not yet set in, and that in the progress of development it passes through stages which are per- manent in the series of inferior animals. On the other hand, he positively repudiated the asser- tion that the embryos of the higher types actually pass through forms permanent in the lower ones. He says that the type of each animal seems from the first to fix itself in the embryo, and to regulate its whole development As regards the vertebrate animals in particular, the further we go back in the history of their development, the more do we find the embryos alike, both on the whole and in the individual parts. " Only gradually do the characters appear which mark the greater, and later those which mark the smaller divi- sions of the Vertebrata. Thus from the general type the special one is evolved." Von Baer thus held that the analogy consisted only in the embryonic states of the various animal forms; but he was obliged to go beyond the circle of the types, and he thought it probable that among all embryos of verte- brate, as well as invertebrate animals, developed from a true ovum, there is a conformity in the condition of the 50 THE DOCTRINE OF DESCENT. germ at a period when the type has not yet manifested itself. This led him to the question, "Whether, at the beginning of development, all animals are not essen- tially alike, and whether a common primordial form does not exist for all ? " " It might," he finally thinks, " be maintained, not without reason, that the simple cyst-like form is the common fundamental form from which all animals are developed, not merely in idea, but historically." When the barrier which it was formerly thought necessary to erect between asexual multiplication and multiplication caused by fecundation had been recog- nized as non-existent, and it was perceived that all development amounts to the multiplication and meta- morphosis of the primitive germ or egg-cell, the cell was necessarily regarded, in the acceptation of the older investigators, as the common fundamental form. But although the descriptive history of evolution does not go back to this elementary organism, and considers even the bifurcation as merely a preparation for actual development, at any rate the earliest rudimentary larval conditions of different types may be compared with each other. The discoveries of the last ten years with reference to this subject are so numerous, and such striking analogies have been advanced, that we must needs go much further than, at that time, was possible for Von Baer. It is not merely a question of those general analogies in the segregation of tissues from an indiffer- ent rudimentary mass, but of homologies in the distri- bution, form, and composition of the embryos and larvae, of which the after effects are of profound importance EARLIEST CONDITION OF LARVA. 51 to the later and actual typical impress. With this object, let us consider the larva of a calcareous sponge at the stage which Haeckel has designated as the Gastrula phase. The diam-am ^rives the section of a larva of this description, which at this period is nothing more than a stomach provided with an orifice (fig. 5 OCTRTNE OF DESCENT. scriber of Nature, is rendered comprehensible only by the confident style as well as by the neatness of his diagnoses, by which, with a single stroke, he put an end to the indefinite character of Natural History, and appeared to contemporaries and posterity as a lawgiver. The exaltation of species as the basis of all systematic com- prehension had never been so explicitly proclaimed. His opinions culminate in the maxim,'* " Reason teaches that at the beginning of things, a pair of each particular species was created." Lut with Linnaeus this said reason looks rather strange, for it is subservient to the strictest Scriptural belief, and he endeavours to harmonize his geological conceptions with this standpoint. One very effective geological phenomenon was espe- cially striking to him, namely, the upheaval of a great portion of the Scandinavian coast. It proceeds more rapidly than the subsidence of another part ; its phe- nomena are far mightier ; and thus the idea might be formed that the continent had risen from the sea in regular progression. " I believe that I am not straying far from the truth," he says, " if I affirm that in the infancy of the world all the mainland was submerged and covered by an enormous ocean, save one single island in this immeasurable sea, on which all animals dwelt and plants grew luxuriantly."'* It follows that all species of plants likewise existed in this lovely garden, as it is expressly said that Adam named every animal ; consequently all insects must have been assembled in Paradise, but insects cannot be imagined without plants. Linnaeus then makes the first attempt at animal geography by making the animals disperse themselves from this centre. But the summary CUVIER. 85 'of his idea of species is invariably, "We reckon as many species as the Infinite Being created at the beginning."'" And his authority was so powerful that the age of Voltaire and of Diderot devoutly accepted this obvious dogma, and transmitted it to posterity as a maxim impossible to question. Linnaeus was, however, so little of an" anatomist that in this province Zoology required a completely fresh foundation, and, in the capacity of a second Linnaeus, Cuvier stood forth. ^^ His school styles itself the school of facts, yet it was by no means without a tincture of philosophy. On the contrary, the definite and simple nature of his principles and deductions could not fail to be imposing. He epitomized the summary of his obser- vations as "Laws of Organization;" and he applied the Ideological view, the principe des causes finales, with great advantage to the knowledge and restoration of antediluvian animals. The question of the persistency or mutability of species thrust itself forcibly upon him. For this an external cause was given by the Egyptian expedition and the investigation of mummified animals. Etienne Geoffroy St. Hilaire and Lamarck attacked the persistency of species, and held that, especially consider- ing the stability of external conditions, the Egyptian period was far too short for the identity of the mummies with the species now extant, to make it possible to infer the immutability of species; but the question was curtly despatched and silenced by the predominating school of Cuvier. Meanwhile, Cuvier not only increased the accumu- lation of facts, but, as we have already hinted, he grouped them so happily and with such philosophical S6 THE DOCTRINE OF DESCENT. skill that he undoubtedly approached the object at which he aimed — the Natural System. He supplied the first reliable information respecting extinct species. With regard to those v. hich had replaced them in subsequent periods, he was not, as is generally supposed, an un- qualified partizan of new creations, but he refrained from any fixed opinion. " I will not," he says,"* " positively affirm that for the production of tlie present animals a new creation was required. I merely say they did not live in the same locality, and must have come from elsewhere." Geoffroy Saint Hilaire, on the contrary, does not doubt that the animals now living are descended, by an unbroken succession of generations, from the extinct races of the antediluvian age. Cuvier's method involved the danger of introducing dogmatism into natural science, and it is therefore justifiable to refer in this place to one of Cuvier's imme- diate disciples only recently deceased — Louis Agassiz, who in the most rigidly ciidactic manner adheres to the systematic categories, and invests them with fine-sound- ing definitions as "embodied creative ideas." '^ Accord- ing to him, species belong to a particular period in the world's history, and bear definite relations to the physical conditions predominant at the time, as well as to the contemporaneous plants and animals. Species are founded on well-defined relations of individuals to one another and the world in which they live, as well as on the proportions and mutual relations of their parts, and on their ornamentation. Individuals, as representatives of species, bear the closest relations to one another ; they exhibit definite relations also to the surrounding element, and their AGASSIZ. 8/ existence is limited within a definite period. Of genera he says, " Genera are groups of animals most closely connected together, and diverging from one another neither in the form nor in the composition of their structure, but simply in the ultimate structural pecu- liarities of some of their parts." " Individuals, as repre- sentatives of genera, have a definite and specific ultimate structure, identical with that of the representatives of other species." We may pronounce these definitions to be mere phrases, and inquire with Haeckel : "Of what nature are these ' ultimate structural peculiarities of some of their parts' which are supposed alone to define the genus as such, and to be exclusively characteristic of each genus ? We ask every S3^stematizer whether he may not equally well apply this definition to species, varieties, &c., and whether it is not finally the * ulti- mate structural peculiarities of some of their parts ' which produce the characteristic forms of the species, the variety, &c. In vain do we search in the ** Essay on Classification " for a single example of the manner in which, for instance, the genera of oxen or antelopes, the races of hyaenas and dogs, or the two great genera of our fresh-water bivalve shells, the Unio and Ano- donta, are actually distinguished by " the ultimate struc- tural peculiarities of some of their parts.** Several of these definitions given by Agassiz may be interchanged point-blank, so general and merely negative are their statements. He characterizes the classes " by the man- ner in which the plan of the type is executed as far as ways and means are concerned." The orders, " by the degree of complication of the structure of the types." S3 THE DOCTRINE OF DESCENT. These phrases are hiterchangeable, but, hke all dogma- tism, they make a great impression on those who from ignorance of the facts are incapable of criticising for themselves, and they are readily quoted to confute an unbelieving investigation of nature by one made in faith. It might be thought that if the affair were so simple, and systematic ideas so firmly fixed, nothing would be easier than to establish the system. And so Agassiz maintains. He says that if a single species of any of the great animal groups were present, and admitted of investigation, the character of die type, class, family, genus, and species, might be determined. The weak- ness of this and similar statements may best be demon- strated by examining the basis of all dogmatic system, — the " species." If this idea be mutable, if the species be not given once for all, but variable, according to time and circumstances, the implications of the higher and more general ideas of genus, family, &c., must necessarily ensue. The keenest and most logical criti- cism on the deeply-rooted scholastic idea of "species" v/as made by Haeckel,'"^'^ after Darwin, in his classical u^ork on the " Origin of Species," had completely ex- posed the old doctrine and practice of zoology and botany. In what follows we shall adhere to Haeckel. We have seen above that Linnaeus accepted the Crea- tion as an irrevocable scriptural doctrine, and it is really absurd that many naturalists who have long abandoned any other dogm.a, should abide by this one. Therefore as the Bible mentions the creation of species, this legend was made the basis of all science. It is true there are net now many wIto appeal to scriptural testimony, LINN/EAN DEFINITION OF SPECIES. 89 Those who defend the stability of species rather imagine that, with Cuvier, they are entitled to interpret facts in their own favour ; whereas they partly remain uncon- sciously involved in hereditary prejudice, and partly contrive to be deliberately blind to all that evidently contradicts the immutability of species. Since Linnoeus referred to the Creation, he attributed the individuals to a species, of which the pedigree ascended in direct line to the pair which proceeded from the hand of the Creator. Owing to the state of science in general, an examination of this pedigree was totally impossible in his time ; and, indeed, with the strict reliance on sacred tradition, it was scarcely neces- sary. Cuvier, although a very unprejudiced and cool observer, nevertheless radically accepted the Linnaean definition of species. According to him, the species is the aggregate of individuals descending from one another and from common ancestors, and of those who resemble them as strongly as they resemble one another.'' "In this definition," says Ilaeckel, "to which the majority have ever since more or less closely adhered, two things are obviously required of an individual as belonging to a species : in the first place, a certain degree of resemblance or approximate similarity of character ; and secondly, a kindred connection by the bond of a common descent. In the numerous attempts of later authors to complete the definition, the chief stress is laid sometimes on the genealogical consangui- nily of all the individuals, sometimes on morphological uniformity in all essential characters. But it may be generally asserted that in the practical application cf )0 THE DOCTRINE OF DESCENT. the idea of species, in the discrimination and nomencla- ture of the individual species, the latter criterion alone has almost always been employed, while the former has been entirely neglected. Later, it is true, the genea- logical idea of the common descent of all individuals of each separate species was supplemented by the physio- logical definition that all the individuals of every species are capable of producing fertile offspring, by intercross- ing, whereas sexual intercourse between individuals of different species produces only sterile offspring or none at all. In practice, however, it was considered quite enough if, among a number of extremely similar animals under investigation, uniformity in all essential characters could be established, and no inquiry was made whether these individuals ascribed to the same species were actually of common origin, and capable, by crossing, of pro- ducing fertile offspring. The physiological definition was no more applied in the practical discrimination of animal and vegetal species, than was the pre-supposed common descent from a single ancestral pair. On the other hand, two closely allied forms were distinguished v/ithout scruple as two different 'good species,' when- ever in a number of similar individuals examined a con- stant difference could be demonstrated, even though of a merely subordinate character. Here, again, no pains were taken to ascertain whether the two different series were not really descended from common ancestors, and were really capable of generating in conjunction only sterile hybrids, if any." That this radical condemnation of the post-Linnaean manufacture of species is not too severe, is shown by one fact among others ; that within the fraternity such NO ESSENTIAL CHARACTERISTICS OF SPECIES. 9I ultcr discord as to the limitations of species prevailed, and still prevails, that no agreement can be arrived at respecting the basis of the description of species, the •'* essential characteristics." Altliough Agassiz lays down the diagnosis of tlie species, a decision is required in each case as to the mutual relations of the parts, the orna- mentation, &c. As in the absence of birds'-nests, snail- shells, butterflies, &c., it is impossible, when it comes to the erection of species, to pre-determine what may be the " essential characteristics " of the species they are to form, subjective opinions and arbitrary decisions have full play; and within a certain domain, well known by its forms, there are among the systematizers no two autho- rities who are agreed as to the number of species into which the material before them should be divided. The most unbridled license in the manufacture of species prevailed, however, among the Palaeontologists during a period when, in the endeavour to fix the sub- divisions of geological strata as accurately as possible by means of their organic contents, the separation of species was carried incredibly far, into the most minute and often into individual deviations. A certain mutability of species could not fail to obtrude itself on the most purblind eye ; ramifications were made of sub-species, sports of nature, and varieties characterized by "less essential" peculiari- ties acquired by means of climate and inheritance. There was, however, always a reservation that their crosses with one another and with the main species should produce fertile offspring, whereas towards other species their relations were identical with those of the main species. Of course, in this separation of the species into sub-species, subjective opinion was even 53 THE DOCTRINE OF DESCENT. less fettered by tradition and law than in the definition of species. The Hterature of ornithology during the last f(^rty years could furnish thousands of the strangest examples of the Babel-like confusion which was thus introduced. There is no question that a great, perhaps the greater, number of organisms now existing are in a condition in which, according to their internal and external re- lations, they may be characterized by Natural History as so-called species, and for the purpose of recognition and scientific treatment in general, must needs be so characterized. But this stability, as may be shown both directly and by analogy, is under all circumstances only temporary, and we have whole classes of organisms to which it is impossible, even with the widest reservations, to apply the old idea of species, with its immutability of essential characteristics. If we are able to furnish incon- trovertible proofs of the existence of such non-specific groups, the old system and the dogma of species are once for all set aside, and the positive basis of a new doctrine is secured. This evidence is supplied in two directions. Some classes of organisms in their present state vacillate and fluctuate in form, in such a manner that it is utterly impossible; to fix the characteristics of species or genus. They are in an extreme grade of mutability, which, in others, has given way to an apparent state of repose. Other series of facts, exhibiting the most obvious muta- bility of species, are displayed by certain antediluvian groups in the succession of forms called '' species." Even before the appearance of Darwin's work on the " Origin of Species," Carpenter, in the course of his researchea on the Foraminifera, arrived at the con- SERIES OF rCRMS IN SPONGES. Cj elusion, proved in special instances, that in this group of low organisms which secrete the most delicate cal- careous shells, there could be no question of " species," but only of "series of forms." Forms which the sys- tematists had reduced to different genera and families, he beheld developing themselves from one another. These Foraminifera are, however, so simple in structure, the history of their individual evolution or Ontogenesis is, as yet, so little known ; they contribute so little microscopic detail, which might formulate the law of transmutation of species, that the champions of persist- ency of species might still seek refuge in the assertion that Carpenter's series of forms are mere varieties, and only prove that the true " species " have not yet been found. We may now turn with advantage to the class of the Spongiadse, the importance of which in the question of species I was the first to point out.^* With them, as I summed up my researches, it is not as with the Forami- nifera, merely an affair of the general habit of the form, of the variable grouping of the chamber systems; but the variability exists still more specially in the microscopic detail than in the coarser constituents. In the Forami- nifera we may speak of microscopic forms, but not pro- perly of microscopic constituents. But in the sponges we discern the transformation of the finer morphological constituents, the rudimentary organs, and we thereby gain an insight into the mutability of the whole. In this respect the calcareous sponges are somewhat differently circumstanced from the rest, and from the silicious sponges in particular. In the former, the variability of tlie microscopic parts is limited to a smaller circle of forms, 94 THE DOCTRINE OF DESCENT. whereas the habit of the series of individuals is incredi- bly variable. This variability of the whole body is not lacking in the silicious sponges ; in the genus Tedania, for instance, established by Gray from some of my earlier Reniera, we see how their stubbornly coherent needle-like forms recur from Trieste to Florida and Iceland, under the most heterogeneous disguises. In some varieties, how- ever, one of these spicula already manifests a tendency to deviations. This very point, the possibility of tracing- in detail the metamorphoses of organs, which, on the assumption of their stability, appeared to provide the system with the most substantial basis for the erection of genera and species, renders the investigation peculiarly attractive. Even among the Algierian sponges, I have adduced striking examples, and they accumulate in proportion as the horizon is extended. We arrive gradually at the conviction that no reasonable dependence can be placed on any ** characteristic ;" that with a certain constancy in microscopic constituents, the outward bodily form, with its coarser distinctive marks, varies far beyond the limits of the so-called species and genera ; and that, with like external habits, the internal particles, which we looked upon as specific, are transformed into others, as it were, under our hands. "Any one" — thus con- cludes this section of my work on the Fauna of the Atlantic Sponges, — ** who, with regard to sponges, makes his chief business the manufacture of species and genera, is reduced ad absiirdiim, as Haeckel has shown with exquisite irony in his Prodrome to the Monograph on the Calcareous Sponges." In my specific researches I confined myself essentially NO ABSOLUTE SPECIES EXISTS. 95 to the sillcious sponges, and by thousands of microscopic observations, by measurements, by drawings, by facts and inferences, had produced evidence, which acute op- ponents of the immutabihty of species had not brought forward before me, that in these sponges, species and genera, and consequently fixed systematic unities in general, had no existence. The other division of the same class, the calcareous sponges, had been treated with unrivalled mastery by Haeckel in his Monograph.^^ Pie was able not only to confirm my statements, but, owing to the smaller compass and the greater facility of observing the group selected for study, to advance with more sequence and continuity from the observation of details to the whole, to portray its morphology, physiology, and evolutionary history with the utmost completeness. He then challenged the obstructive party with the assertion that, according to subjective opinion, either one or 591 species of calcareous sponges might be accepted, but "that no absolute species exists, and that species and varieties cannot be sharply sepa- rated." Whoever after these demonstrations cleaves to the phantom of species, without either proving that the facts have been falsely observed, or that they may be interpreted otherwise than in favour of the stability of species, — w^ioever, as Agassiz has recently done, ignoring any such researches, publicly asseverates that in no single case has the mutability of any species been exhibited, — scarcely preserves the right to partici- pate in the great controversy by which Natural Science is now perturbed. There is, however, as we have already mentioned, a second direction in which the mobility of " species " must 96 THE DOCTRINE OF DESCENT. be demonstrated, not the direction of breadth, but of height and depth. This mutability of the Spongiadte affords the extremely important evidence that, so to speak, an entire class has, even now, not attained a state of comparative repose. But to confirm the mu- tability of species, evidence of mutability in lapse of time is justly demanded; the transition of the forms succeeding one another historically in the strata of the earth. In former editions I expressed a belief that a highly instructive instance of the modification of a species in time might be seen in the Planorbis from the fresh- water deposits of Steinheim, described by Hilgendorf. But this very example has served to warn us how cautious we must be in accepting proofs, since later observers have sought in vain for the regular succession of strata that was said to exist, and the series of modi- fied forms of Planorbis multiformis contained in them ; indeed, they convinced themselves on the contrary, that the unusually divergent forms of this snail occur mingled throughout. However, other evidences of such modifica- tion abound, and the zeal of some recent Palaeontologists — Waagen, Zittel, Kayser, Neumayr, and Wurtenberger — has proved, by following up the species, so called, of Brachiopoda and Ammonitidse through vast geological periods, that it is impossible to arrange these large groups under true species. I will allow these naturalists to speak for themselves. Kayser concludes from an investigation of the Brachiopoda of the Devonian^ beds of the Eifel : — " No order of animals, perhaps, yields such strong evidence in favour of the Darwinian theory as the Brachiopoda. METAMORPHOSIS OF AMMONITES. 97 Any one who, like myself, has had occasion to trace a great number of these fossils through several strata of considerable extent and thickness, and to handle hun- dreds of every species as heretofore classified, must often have been astonished, to see how wide is the divergence in many of these species. Often he must have lost all hope of ever arriving at a clear delimita- tion of species; again and again must he have felt less inclined to dismiss the idea put forward by Darwin, ' that our species are in fact only an artificial concept, a mere formula/ Kayser consequently finds himself com- pelled to adopt purely artificial limits, and speak of 'Form-series,' as other writers do of Ammonites. Waagen reminds us that, long before Darwin, Queenstedt had suggested the Genetic connection of successive forms in geological strata, and he then says : — " There are but few among the Palseontologists who have recently studied the Ammonitidae under the light of the Theory of Descent, to whom the facts have not brought convic- tion. The existence of Form.-series, such as have been shown again and again by late investigators — series in which each more recent form deviates but slightly from its precursor, till the sum total of these small divergences results in a wide dissimilarity from the original species — points with coercive clearness to the assumption of Genetic connection." Zittel and Neumayr are of the same mind. Neumayr writes : — " There is hardly any fact which speaks so conclusively for the validity of the Theory of Descent as the existence of Form-series, which has been proved in many cases, and of which more instances will certainly be found now that attention has been directed to the matter. A peculiarly beautiful example is to be seen — 98 THE DOCTRINE OF DESCENT. the form here described of Oppelia Darwinii, as a tran- sition form between the normal type of Pennilobata and the abnormal semi-forms." Neumayr confirms many of Wilrtenberger's most important discoveries, which I will proceed to quote. L. Wiirtenberger applied his researches to thousands of samples from the groups of the Planulate Ammonites with ribbed shells, and of the Armate Ammonites with prickly shells. In summing up his results he says, among other things : " How among the Ammonites of the Planulate and Armate groups, the species are to be branched off" from one another, I should be reluctant and unable to give any instructions, for to me this question appears utterly hopeless. For in groups of fossil organ- isms, in which, as in the present case, so many connect- ing links between the most extreme forms are actually before us, that the transition is regularly carried on, the species is far less susceptible of apprehension than in the organic forms of the present world, which at least denote the existing limits of the great pedigree of the organic world. With respect to these fossil forms, it is funda- mentally indifferent whether a very short, or a somewhat longer portion of any branch be honoured by a special name, and looked upon as a species. The prickly Am- monites, classified under the name of Armata, are so intrinsically connected, that it becomes an impossibility to separate the accepted species sharply from one another. The same observation applies also to the group of which the manifold forms are distinguished by their ribbed shells, and termed Planulata." It has further transpired that the Armata, or Custata, originated trom the Planulata. SPECIES AND HYBRIDS. 99 We shall return later to Wurtenberger's preliminary communications. It was our object here to inform our readers how and where modern natural inquiry sets aside the phantom of species, and to enable them to judge for themselves what series of observations are opposed to the asseverations that in no single case has evidence been given of the transition of one species into another. For the old school falls into the dilemma of proclaiming whole orders and classes to be *' species," and the species, formerly so beautifully defined, to be varieties. The untenableness of the physiological part of the definition of species has been conclusively shown first by Darwin and afterwards by Haeckel. It is known that even in a state of freedom good species not infrequently breed together, and that domesticated species, such as the horse and the ass, have been crossed for thousands of years. But hybrids, the produce of this intercourse, were supposed to be only exceptionally fertile, and at any rate not to produce fertile progeny for more than a few generations. On the other hand, it was considered certain that the produce of ci'osses among varieties are fertile in unbroken succession. The dogma of the ste- rility of hybrids was formed w^ithout any experimental or general observation, and by ill-luck was apparently confirmed by the most ancient and best known hybridi- zations of the mule and the hinny. To this familiar example, in which the fertility of hybrids proves abortive, we will oppose only one case of pi'opagation successfully accomplish