BIOLOGY LIBRARY G INHERITANCE OF ACQUIRED CHARACTERS Eugenic Rignano UPON THE Inheritance of Acquired Characters A HYPOTHESIS OF HEREDITY, DEVELOPMENT, AND ASSIMILATION AUTHORIZED ENGLISH TRANSLATION BY BASIL C. H. HARVEY ASSISTANT PROFESSOR OF ANATOMY, UNIVERSITY OF CHICAGO WITH AN APPENDIX UPON THE MNEMONIC ORIGIN AND NATURE OF THE AFFECTIVE OR NATURAL TENDENCIES CHICAGO THE OPEN COURT PUBLISHING CO. 191 1 § Hf 3 Is BIOLOG? LIBRARY G Copyright, 1911, by The Open Court Pub. Company, Chicago. CONTENTS INTRODUCTION 5 CHAPTER I. Ontogeny, as a Recapitulation of Phylogeny suggests the Idea of a continuous Action exerted by the Germ Substance upon the Soma throughout the whole of Development II CHAPTER II. 1. Phenomena which indicate a continuous formative Action which is exerted by Parts of the Soma upon the other Parts throughout the Whole of Development 19 2. Hypothesis of the Nature of the formative Stimulus 29 CHAPTER III. j. Phenomena which point to the Existence of a central Zone of Development 53 2. Hypothesis of the Structure of the germinal Substance 76 CHAPTER IV. 1. Phenomena which refute simple Epigenesis 104 2. Phenomena which refute Preformation 121 3. Inadmissibility of a homogeneous germ Substance 144 4. Inadmissibility of preformistic Germs 150 CHAPTER V. The Question of the Inheritance of acquired Characters 159 CHAPTER VI. The most important of the existing biogenetic Theories in Rela- tion to the Inheritance of acquired Characters 224 CHAPTER VII. The centroepigenetic Hypothesis and the Explanation of In- heritance afforded by it 289 CHAPTER VIII. The Phenomenon of Memory and the vital Phenomenon 316 Conclusion 356 Appendix 359 Index 401 242218 "Some deny flatly the possibility df ever arriving at an understanding of the nature of life. But if we ask ourselves in what this understanding of the nature of life could consist, from the point of view of positive philos- ophy, ive have no difficulty in recognizing that such an understanding must be reduced to comparing vital phe- nomena with some physico-chemical model already known, suitably modified by the particular special conditions im- posed upon it so that just these special conditions shall determine the differences which exist between this vital phenomenon and that phenomenon of the inorganic world most closely related to it. If this be so, it is then the duty of science emphatically to refuse to give up the attempt to understand the nature of living matter, for that would be to belie the Spirit of all scientific endeavor. For whether it be clearly recognized or not, it is just this search for the nature of the vital principle ivhich properly constitutes the principal object and the final goal of all biologic study in general" — E. RIGNANO, in "Acquired Characters," p. 334- TRANSLATOR'S PREFACE Rignano is a student of Biology who has also the training of an engineer and physicist. His attack on bio- logical problems is from that side. In this book he offers an explanation on a physical basis of assimilation, cell di- vision, and the biogenetic law of recapitulation in ontog- eny, and he suggests a mechanism whereby the inheritance of acquired characters may be effected. Such a study of the most fundamental and difficult of biological problems can not fail to be of the greatest in- terest to all students of science. It points out a way to the understanding of the essential nature of living matter. Therefore the translator has gladly consented to prepare for publication this translation first made for his own sat- isfaction. It has been revised by the author. University of Chicago, 1911. BASIL HARVEY, PREFACE TO THE ENGLISH EDITION This work which appeared first in French in 1906 and later in German and Italian now appears after some years in English thanks to the interest shown by several Eng- lish and American biologists and philosophers, who have expressed a desire to have its circulation facilitated among the savants of their countries. This new indica- tion of the growing favor acquired in a short time by the theories here advanced, notwithstanding the author's fear that their novelty would stand somewhat in the way of their reception, is the best reward he could desire for the great difficulty he has encountered in the study of these — the most fundamental problems of biology. And he wishes to pay here his sad and affectionate respect to the revered memory of Professor C. O. Whitman, one of the first to interest himself in this English edition, and at the same time to express his gratitude to his friend Dr. Basil Harvey for the great care which he has given to the translation, as well as to Dr. Paul Carus and The Open Court Publishing Co., who have been so good as to be- come the publishers. Milan, March, 1911. E. R. INTRODUCTION The question of the inheritance of acquired characters is one which, by its generality, by its importance for the theory of the origin of species, and by its close connec- tion with still more difficult questions concerning the essential nature of life lying in the border land between physical chemistry and biology, passes beyond the con- fines of pure biology, and enters the wider field of posi- tive philosophy in the sense of August Comte, that is of scientific philosophy, which concerns itself with the most general results of the various sciences and with their fundamental interrelations. Is it any wonder then that this much discussed but unsolved question excites the keenest interest in philosophers and even induces some of them though they are not specialists, to attempt to study it thoroughly utilizing the abundant and valuable mate- rial which biologists and naturalists can now supply? It is so with the author of the present study. Formerly when he had not yet formed any fixed and definite opinion upon this subject, he had been inclined in a few philosophical and sociological studies to prefer Weismann's theory of the non-inheritance of acquired characters to the contrary theory of Lamarck. The rea- son for this inclination even though no logically tenable opinion had been formed, lay in the demonstrated inabil- ity of any of the biological theories which had then been 5 6 Introduction devised to give any explanation whatever, even an un- satisfactory one, of the mechanism of that inheritance. Yet the author never lost sight of the fact that natural selection in no way sufficed to explain phylogenetic evolu- tion completely, and he was always convinced that non-inheritance was irreconcilable with the fundamental biogenetic law that ontogeny is only a recapitulation of phylogeny. This law, whose remote and immediate consequences constantly stimulated the reflection of the author, has finally led him in a purely inductive way to the new biogenetic hypothesis about to be presented. It seemed to the author that he ought to devote a special effort to the elaboration and exposition of this hypothesis, for he saw from the outset that it promised an explanation not only for the inheritance of acquired characters but also, and quite independently, for a whole series of fundamental biological phenomena, and how it afforded an outlet from the blind alley into which onto- genetic biology seems to have run: for while some facts lead us to reject epigenesis as it is commonly understood, others force us to reject pre formation, and similarly while a whole series of reasons force us to hold as inadmissable a homogeneous germinal substance or a sub- stance only chemically heterogeneous, another whole series of reasons obliges us to hold no less inadmissable a germinal substance constituted by the germs of the preformists. The author knows well that he must not entertain any oversanguine expectations. In the position of biological science today we can deal only with preliminary hypoth- eses, of which each gives way to its successor and each, taking in a greater number of phenomena than its pred- ecessor prepares the way for a later hypothesis which Introduction 7 is able in its turn to include a number greater still. So soon as a hypothesis pushes a bit nearer to the beleaguered fortress, so to speak, and indicates new lines for study, observation and research, one must admit that it has ful- filled its purpose. And this applies to our view of the new biogenetic hypothesis which we here submit to the judg- ment of biologists and of positive philosophers in general. We have believed it expedient to follow in the exposi- tion of this theory the order in which it was conceived and built up, and so the first chapter describes briefly the inductive way in which the author, starting out from the fundamental biogenetic law, was led to the conception of his hypothesis. In the three following chapters are col- lected and arranged as concisely as possible the principal, different, biogenetic facts which, quite independently of the ever controverted question of the inheritance of acquired characters, serve best to set forth and define the new hypothesis and which, since they find in it their most complete explanation, confirm it again directly or in- directly in a deductive way. After having then undertaken in the fifth chapter a brief examination of the question of the inheritance or the non-inheritance of acquired characters which until then we had laid entirely aside, we pass in the sixth chapter to the critical exposition of the principal biogene- tic theories which are current at present. And we do this not only with the object of showing their inadequacy to explain the mechanism of inheritance, but rather in order that the perception of the reason of this inadequacy may aid us in discovering the necessary and sufficient con- ditions required in any theory which seeks to explain this inheritance. After that we go on again in the seventh chapter with the examination of our hypothesis whose 8 Introduction conception and elaboration up till then was considered quite independently of the question of the possibility or impossibility of the inheritance of acquired characters, but which supported by an elementary hypothetical phenomenon, which in certain respects finds its counter- part in the inorganic world, becomes recognizable at once as a most complete explanation of this inheritance. Finally in the last chapter we endeavor to show how this elementary hypothetical phenomenon on which the new biogenetic theory rests, explains also a fundamental, psychic phenomenon, to wit, memory and indeed the most characteristic properties of the vital phenomenon in gen- eral. And so this elementary hypothetical phenomenon seems to us capable of bringing together within it and referring to one basis not only the whole group of genetic phenomena, but all vital phenomena whatever in the very widest sense of the word. Since, for the reasons above stated, the inheritance of acquired characters is a question affecting positive philos- ophy in the Comtian sense or scientific philosophy, the author ventures to hope that biologists and naturalists may not regard him as an unbidden intruder into their domain, but rather since he is the first to recognize the many gaps and shortcomings of his work, he ventures to hope that he may count upon especial consideration, be- cause of the great difficulties with which he who is no specialist has had to contend in studies of so difficult a nature. Milan, May, 1905. E. R. ONTOGENY CHAPTER ONE ONTOGENY, AS A RECAPITULATION OF PHYLOGENY, SUG- GESTS THE IDEA OF A CONTINUOUS ACTION EXERTED BY THE GERM SUBSTANCE UPON THE SOMA THROUGH- OUT THE WHOLE OF DEVELOPMENT. Everyone knows the fundamental biogenetic law of Haeckel : ontogeny is a recapitulation of phylogeny, that is, the development of the individual is a rapid resume of the development of the species, a short reproduction of the endless chain of its ancestors. The most important facts establishing this law, now perhaps irrefutably, are so well known that we hardly need to mention them here; for example: solipedation develops gradually in the horse and only in the last stages of its development; many whales which later instead of teeth have the so called whalebone have teeth in their jaws while they are still in a fetal condition and cannot take any nourishment ; the serpent while it is in the embryonic state possesses its two pair of limbs, and so on. "The development of the organism," writes Roux, "is not merely a production of the complex from the simple by the most direct route. The ways are devious; and many a forward step must be retraced. We mention only the well known examples of the gill clefts and gill arteries and their ultimate concrescence, the notochord also, and the pituitary and pineal glands, structures quite super- ii 12 Bio genetic Law of Recapitulation fluous and functionless from the first." 1 Development of this sort, proceeding toward its goal not by the direct line but by byways and often backwards, would be incom- prehensible were it not for the fundamental biogenctic law. Also Delage draws attention to the fact that all those structures which disappear during the progress of development must nevertheless have their significance.2 Similarly Oscar Hertwig notes expressly that there exist many embryonal organs "which never come into a position to perform the function which they have once performed during the course of phylogeny." 3 We must then regard this fundamental biogenetic law as true. We can even suppose it to be a close approxima- tion, that ontogeny represents phylogeny exactly. It is true that during the first ontogenetic stages phylogeny is only epitomized, but this becomes steadily less true the farther development proceeds, and during the later stages ontogeny can be regarded as an almost exact repetition of all the corresponding phylogenetic stages. The human embryo, on account of the more numerous and careful researches of which it has been the object, serves better than any other to illustrate this almost exact phylogenetic repetition in the later stages. Its develop- ment demonstrates even to the smallest details how the embryo passes through the whole series of forms of the pithecanthropoids, its immediate ancestors. Thus for example the articulations of the leg in man show during 1Wilhelm Roux : Der Kampf cler Teile im Organismus. Leipzig, Engelmann, 1881. P. 59. 2Delage: L'heredite et les grands problemes de la biologic generate. Paris, Schleicher, 1903. P. 176. 'Oscar Hertwig: Die Zelle und die Gewebe. Zweites Btich. Jena, Fischer, 1898. P. 232. Significance of This Law 13 fetal life a much closer resemblance to those of anthro- poids than during adult life. At a certain stage of development the great toe instead of being parallel to the others forms an angle with their direction as in the apes. In the same way many of the bones of the foot of the newborn infant, in their form, in their respective angles of inclination, etc., resemble very closely those of the climbing foot of the anthropoid apes, particularly of the gorilla. In the attempt to see what significance the funda- mental biogenetic law can have for the biologist we can come somewhat nearer to the question by supposing ontogeny to be an exact repetition of phylogeny instead of a rapid resume of it. It is true that it will be necessary later to make some important corrections in this first ap- proximation and to study the significance or cause of the abbreviation and suppression in ontogeny of many phylo- genetic stages ; and this more intimate study will allow us to penetrate further into the innermost nature of the phenomenon. But for the present we desire by this tenta- tive supposition that ontogeny is an exact repetition of phylogeny, to have the great advantage of defining the phenomenon to be studied more simply and precisely, and of making our comprehension of it correspondingly easier. It is by this means, that is by successive degrees of gradual approximation that mechanical, physical, and chemical researches have usually proceeded. This first degree of approximation of the fundamental biogenetic law will permit us then to make the twro fol- lowing statements : Each stage of the ontogenetic devel- opment of any organism represents exactly one species among the ancestors of that organism. Two species hav- ing a common ancestor have an identical ontogenetic 14 Biogenetic Law of Recapitulation development up to the stage corresponding to that com- mon ancestor; they do not commence to diverge until they have passed that stage. But since all the various theories of heredity admit that two distinct species descending from a common re- mote ancestor possess germinal substances different from each other, the question at once presents itself: If these germinal substances are different, how then is it possible that throughout a long series of stages up to the stage corresponding to the common ancestor they present like ontogenetic forms, the very same as those through which the ancestor passed? If the germinal substance of one species is different from that of the other should they not from the very beginning show a totally unlike series of forms ? A germinal substance in process of development con- stitutes to a certain extent a dynamic system of forces in continual transformation. But two systems commencing to give rise to two series of successive transformations which throughout a long time are quite alike must neces- sarily be themselves alike. And if at a given moment one series diverges from the other it is necessary to attribute this divergence to one of two causes; either to some ex- ternal circumstance acting at that moment, or to some internal impulse becoming active just at that moment. "The parallelism in the phenomena of ontogeny and phylogeny," says Delage, "shows that first something develops which is similar to what was developed in the ancestors, and that then something which remained till then inactive is added and development proceeds further." 4 4Delage: L'heredite etc., P. 457. Succession of Ontogenetic Stages 15 In other words the biogenetic. law implies that up to each stage of development the productive cause of develop- ment remains the same as that which produced the ancestral species corresponding to that stage. We should next ascertain whether the new circum- stance now added or the new force becoming active only at this stage and causing the subsequent development is to be sought for within or without the various parts of the organism which are actually in process of formation. If at the start we limit ourselves for the sake of ' simplicity to the consideration of morphological trans- formations only, each stage of development whether ontogenetic or phylogenetic will appear to us only as a special mode of distribution of the organic substance con- stituting the organism. \ But this distribution is modified during the life of the adult individual only by new func- tional stimuli, that is to say, only by agents which are external to the structure in progress of modification. In other words the impulse by which the corresponding por- tion of living organic substance is constrained to distribute itself differently does not reside within this portion but comes to it from without. Until the contrary is proven we may accept the state- ment that the properties of living organic substance dur- ing development are not different essentially from those which it presents when development is completed. Con- sequently when any particular mode of distribution of the organic substance becomes altered during the prog- ress from one given ontogenetic stage to the succeeding stage, we can admit as a provisional hypothesis that this different distribution is effected by some provocation external to the parts which change. This provocation cannot be constituted merely by the 1 6 Biogenetic Law of Recapitulation morphological and physiological state of the other part of the organism at that moment; for in the corresponding phylogenetic state the two portions were in perfect equilibrium with each other. It is necessary then to sup- pose that somewhere in the remaining parts of the or- ganism, there enters into play just at that moment and only at that moment, some factor which was not present in the ancestral species. Further, since the alteration in the organism during ontogeny is not confined to a single part of the organism but affects several parts at the same time, and since the impulse which comes into play at the end of each stage of development compelling the passage to the successive stage must lie external to each of the parts undergoing transformation, it cannot lie in any of these parts. This will be possible, however, only on condition that among all the different parts of the organism there is at least one part which is not itself subject to any substantial change, but in which there comes into activity a series of specific energies one after another of which each provokes the passage of all the other parts of the organism to the next ontogenetic stage. This special part may be called the central zone of development. And one can give the name of centro- epigenesis to this hypothesis by which ontogenetic development is made to depend on an infinite number of different influences which this zone gradually exerts upon all the remainder of the organism by activating succes- sively a regular series of specific energies, each remain- ing in a potential state up to the time of its activation. Now the part which actually does remain unaltered from the first segmentation of the egg up to the giving off of the reproductive cells by the new organism is the Centroepigenesis vs. Pre formation and Epigenesis 17 germinal substance, and one suspects at once that it may be just this substance which constitutes the central zone. If so it would follow that the central zone must be at the same time the germinative zone, that is to say, the place whence the sexual cells get the germinal substance which makes them capable of reproduction. Let us hasten to add now that the central zone must coincide with the effective germinal zone, but may possibly be quite separate and distinct from the apparent germinal zone. The latter would be then only the place of formation of the sexual cells, inasmuch as these constitute in a certain sense the mere envelope in which later the germinal sub- stance is assembled, which alone is able to give them reproductive capacity. The hypothesis of centroepigenesis includes then that of a continuous action exercised by the germinal sub- stance upon the soma throughout the whole duration of its development. We shall endeavor in the second chapter to learn what is the nature of this action and we shall reserve for consideration in the third chapter the central zone itself, as well as other facts and arguments which make its existence seem probable and which serve to make the hypothesis clearer. We shall limit ourselves here to putting in special light the fundamental characteristics which differentiate this hypothesis as well from the preformistic as from the epigenetic theories. •/ While Weismann and the preformists in general con- sider that the germ plasm separates itself before the com- mencement of development from the portion set apart to form the new organism, and remains passively aside in a detached part of the soma until it later steps in to form the future sexual cells; consequently it would not control 1 8 Bio genetic Law of Recapitulation development nor indeed have any part whatever in it; that would fall entirely to the other portion alone, and it would be just this passivity which would secure the in- alterability of the germ plasm: While on the other hand the epigenesists consider that the idioplasm would participate in an important and con- tinuous way in development, because it would be present and active at every instant and in all cells; it would remain, however, in spite of this participation permanently unaltered, so that the cells of the soma would never be- come differentiated by nuclear somatization from the germ cells, but on the contrary retain the capacity of reproduction to the same extent : The centroepigenetic hypothesis postulates on the con- trary that the germinal substance, although limited to a single zone and separated and differentiated from the rest of the soma, nevertheless exercises its epigenetic, forma- tive action upon all the rest of the organism and during the whole of development, without undergoing any altera- tion whatever through this participation in development. But this hypothesis thus sketched must now be made more precise and clear by the consideration of other series of phenomena, while at the same time the proof of the facts is undertaken. And to this we propose to proceed in the chapters which follow. CHAPTER TWO PHENOMENA WHICH INDICATE A CONTINUOUS FORMA- TIVE ACTION WHICH IS EXERTED BY PARTS OF THE SOMA UPON THE OTHER PARTS THROUGHOUT THE WHOLE OF DEVELOPMENT HYPOTHESIS OF THE NATURE OF THE FORMATIVE ACTION. i. Phenomena Which Indicate a Continued Formative Action Among the phenomena which seem to indicate indis- putably a continuous formative action exercised by a more or less great part of the soma upon the other parts throughout the whole of development, those of the re- generation of amputated organs take a first place. It is known that when the antennae of a snail, the chelae of a crab, the feet of a salamander or the head of a worm are amputated, these organs are reproduced even when the amputation is performed during adult life. Spallanzani has cut the feet and tail off the same sala- mander six successive times, and Bonnett seven times, and each time feet were reproduced of exactly the same size as the former ones without any increase or decrease in any part. These facts show that the formative agent whatever it may be is always external to the part formed, and that it exercises upon the whole development of that part and throughout its entire duration a continuous ac- tion, and further that it remains itself unaltered even 19 2O Indications of Continued Formative Influence after the completion of its work and consequently is ca- pable of renewing it at every favorable opportunity. If on account of unusual conditions the regeneration of the amputated part proceeds in an abnormal fashion, the remaining part continues always in spite of that to be capable of normal regeneration. For example : an axolotl had a foot bitten off. The foot was reproduced but badly formed. This foot was amputated and a third was developed which was quite normal.5 We shall later at a proper place treat of the ill-starred attempts of the preformists to bring their theory into accord with similar phenomena, and of the arguments and the special regenerative processes which the epigene- sists have brought forward in support of their theory. Here it may merely be noted that while epigenetic theories furnish an immediate explanation for all phenomena of regeneration, the preformation theory on the contrary must have recourse to the addition of complicated sub- sidiary hypotheses which are entirely opposed to the principal one. If the morphological capacity does not reside in the somatic cells of the cut surface, which by their multiplica- tion produce the regenerated organ, but is outside these, it follows that the continuous action exercised upon all cells at the end of the regenerated part as also upon all cells which do not lie at the cut surface, by the remaining part of the organism must be a mediate action exercised from a distance, and therefore must traverse intermediate cells. A yet more striking demonstration of a continued, 6Darwin: The variation of animals and plants under domestica- tion. Eighth impression of the second edition. London, Murray. 1899- P. 357, 358. Mediate Formative Influence in Post-generation 21 formative action exercised mediately and at a distance by one part of the organism upon another during the entire development of this latter is furnished by the famous ex- periments of Roux in the post-generation of his half embryos. The words in which he describes the process of this post-generation which he has observed in half embryos obtained from frogs' eggs after he had killed one of the first two blastomeres with a hot needle, deserve to be reproduced here in full. But we must remember at the outset that while the uninjured blastomere develops into only a half embryo, the injured blastomere lying beside its neighbor produces often a late fragmentation of its protoplasmic mass con- sisting only of undifferentiated cells. And this is effected in one of the following two ways, either through only partial killing of the nucleus concerned some individual fragments of it continuing to live and multiply, or through an emigration of naked nuclei from the part of the egg remaining intact into the protoplasm of the in- jured blastomere. Now we quote Roux. "Postgeneration of germ layers of half organisms proceeds always from the already differentiated germ layers of the normally developed half of the egg. It extends thence first to the yolk mass subsequently cel- lulized, especially where such a germinal layer is in con- tact with such a mass by a broken surface and conse- quently by the lateral surfaces of its cells." 'The formation commencing at this point proceeds steadily and continuously through the yolk mass of the undeveloped half of the egg. About the free margin of the advancing germinal differentiation there are to be found gradual transition stages between the undiffer- 22 Indications of Continued Formative Influence entiated yolk cells and the cells of the already completely differentiated germinal layer. With the elimination of other possibilities this leads us to the conclusion that the progressive differentiation is accomplished in material already in position before differentiation commenced, and remaining there throughout it, and therefore in passive yolk material by direct transformation of the yolk cells (accompanied in the case of the ectoderm and mesoderm also by the division of these cells)." "As to the location of the causes of these processes," continues Roux, "we can draw a few further conclusions." "Since the yolk cell material later differentiated to form the germ layers in the manner described above has been quite disordered in its substance forming the bodies of the cells, by the operation, and since also the nuclear material of the cells which are later formed from it has never yet taken its place by virtue of a typical division, but, being derived partly from the nucleus of the half operated upon and partly from the emigrated naked nuclei of the half remaining intact, owes its disposition to the chance of the moment, therefore the conception, possible in the case of normal development that at typical places there is always deposited typical material capable of quite definite independent development, cannot be admitted in this case." "We must conclude rather, that the cause of this typ- ical formation of the germ layers of the first developed half of the egg, extending into the half operated upon, lies in forces which proceed from the germ layers of the first half." "I conclude, then, that in our postgeneration the progressive differentiation extends in space as the result of a direct assimilating and differentiating action exercised Continuous Formative Influence 23 by differentiated cells upon other less differentiated cells which are immediately adjacent to them." "In the latter process very different degrees of action are possible. There can, for example, emanate from the differentiated cells an influence which simply sets free the process of differentiation allowing the entire series of necessary changes after this preliminary impulse to proceed of itself. Or, each of these changes may not merely receive from the differentiated cell a simple initial impulse, but may on the contrary be determined ,by that throughout. Between these two extremes one can imagine a \vhole series of intermediate stages. On account of the at first atypical disposition of the material which at last becomes typically differentiated I am inclined to think that the action of the differentiated cells upon the nondifferentiated cells is not a mere liberating one or a mere stimulating one." ° These facts of postgeneration indicate then above all that the action of the half embryo already formed upon the other half in process of formation is exercised in a continuous manner throughout the whole development of this latter. One would be led also to this conclusion, that there is a continuous action exercised throughout the whole of development, by the fact that the postgeneration of the undeveloped half goes on with greater rapidity, so that it soon overtakes the other half and proceeds with it to the same stage of development. "Wilhelm Roux : Uber die kiinstliche Hervorbringung ,,halber" Embryonen durch Zerstorung einer der beiden ersten Furchungszellen, sowie uber die Nachentwicklung, Postgeneration der fehlenden Korperhalfte. Virchows Archiv, Bd. 114. October 1888, P. 279 — 282. Gesamm. Abhandl., Zw. Band. P. 507 — 509. 24 Indications of Continued Formative Influence This continued action leads us to conclude also that as a further consequence a remote action is exercised by the half embryo already formed upon the parts of the other half which are not in direct contact with the first. This remote action can be transmitted only through all the intermediate cell layers which lie between the surface of contact of the two half embryos and the remote parts in process of postgeneration. It would seem necessary then to conclude that the first half embryo exercises upon the far removed parts of the second a remote, mediate and continuous action. We shall now provisionally assume that postgenera- tion does not differ in its essential nature from direct gen- eration, and note some consequences which follow for the hypothesis of the central zone of development which during normal ontogenesis would exert an action similar to that which the half of the embryo already formed exerts upon the other half during postgeneration. The nuclei obtained as the result of the first divisions of the egg and destined to become somatic, even though derived from those which according to this hypothesis would later form the central zone, would then have to be considered, in relation to the somatizing stimulus exerted by that zone, as quite like the indifferent nuclei of the embryonal half capable of post generation, which receive now this now that stimulus without any preference from the half already developed, and become somatized in consequence now in one way now in another way according as they may chance to be disposed. The indifference in respect to somatizing stimuli of these nuclei, which we should at least at the outset suppose like those nuclei from which they are derived, and which later according to this hypothesis give rise Elastomer es Which Acquire Formative Control 25 to the effective germinal zone, leads us to a second hypothesis, namely: that the especially germinative energies of those nuclei destined to become somatic may be once for all silenced, that is once for all put in a potential state incapable of activation, on account of the preponderance which the nuclei that go to form the effective germinal zone or central zone acquire. In fact we can suppose that the first blastomeric nuclei, though exactly alike qualitatively, are different quanti- tatively, that is to say are furnished with amounts of energy which do not chance to be quite the same in all, perhaps on account of special conditions of nutrition or, perhaps, on account of special conditions of the proto- plasm in which they are placed. Then, as soon as the moment comes when because of the nature of the com- mencing transformation, such as perhaps imagination or some such thing, embryonal development can no longer proceed after the same fashion in all cells, certain ones will necessarily gain the upper hand, namely those which possess more potential energy. The other blastomeres whose nuclei would no longer be able to activate their germinal energies will from now on conduct themselves, in relation to the stimuli of the nuclei of those blastomeres which constitute the central zone of development, just like cells with indifferent nuclei. And with the progressive sornatization of these latter the mass of their respective specific elements to which is due the persistence, potentially at least, of their germinal energies, will gradually diminish and finally disappear. There is only a single conceivable exception, namely the case in which at the beginning of development or in inferior forms, these blastomeres or cells just beginning 26 Indications of Continued Formative Influence to be somatized accidentally remain isolated, in which case the activation of their potential germinal energies would be permitted, which might have been impossible had they remained united with the other blastomeres or cells. The possiblity of such receptive indifference toward somatizing stimuli in nuclei which if they were isolated or in other conditions would on the contrary possess and give practical demonstration of very definite specific qualities, is indicated by those cases in which all the nuclear material of the post generated half embryo is furnished by the half already developed. Indeed it often happens that on the side operated upon the nutritive yolk alone is utilized; and into this latter emigrate nuclei formed by normal division of the nuclei of already somatized cells in the developed half of the egg. And these emigrated nuclei then bring about the division of the yolk mass of the part operated upon into small indifferent cells only: "With the formation of a right or left half embryo," Roux observes in a later study, "the formative capacity of the uninjured half of the egg is not yet exhausted. On the contrary certain experimental results permit the conclusion that in many cases there is an emigration from it of nuclei, and perhaps indeed, of a little protoplasm also, going out from those points which by the accident of position have the most intimate contact with the side operated upon, toward the contiguous half of the egg deprived of its own capacity of development. These nuclei become distributed throughout the whole of the large mass of yolk and thereupon follows later a breaking up of the half operated upon into cells, and this is not as in normal division a division of the whole mass first into two nearly Cells Partially Differentiated Can Be Altered 27 equal and consequently large cells, which divide again later in their turn each into two correspondingly smaller cells and so on; but on the contrary the breaking up is from the first into small cells." It is upon these indifferent cells that the half of the egg already developed exercises its formative action. These nuclei which arise from cells of the half of the embryo already developed must nevertheless possess very definite specific properties; some come from ectodermic cells, others from mesodermic and others from entodermic cells. And if the medullary plate, the notochord, etc., have already been formed, the vagrant nuclei come also from cells which are in an advanced stage of development. And yet when they have once emigrated and have become scattered through the yolk of the injured side, they remain no less indifferent in relation to the formative stimuli which come off later from the part already formed, than in the cases where they arise entirely from the injured half of the egg. From whatever cells of the embryonal half already developed they may have been produced, they are capable of any somatization whatever, for this depends only on the place at which they happen to stop or become arrested during their migration into the yolk plasma of the injured half of the egg. The same thing can take place in the blastomeric nuclei also as soon as they once find themselves outside the group, which, according to the hypothesis above stated, would form the central zone of development: in relation to the ontogenetic stimuli which from now on 7Wilhelm Roux : Uber das entwicklungsmechanische Vermogen jeder der beiden ersten Furchungszellen des Eies. Verhandlungen der anat. Ges. Wien. June 1892. P. 34, 35. Gesamm. AbhandL Zw. Bd. P. 782, 783. 28 Indications of Continued Formative Influence are sent out from this zone, they conduct themselves in a quite indifferent way, though for a greater or less time they preserve their germinative capacity potentially. While the experiments of Roux carried on carefully and with astonishing exactness of observation, dem- onstrate directly the continued remote and mediate action which the formative part exercises upon the part being formed throughout the whole of development, the exist- ence of this action is confirmed by other investigations, in a way which though indirect is not any less certain on that account. They comprise all the cases in which the part removed is regenerated by cells histologically different from those of normal generation, for instance in which organs or tissues of ectodermic, mesodermic, or entodermic origin are reproduced in the regeneration by tissues having a different blastodermic origin. It will suffice for our object to recall the most typical example which has stirred up the two hostile camps of the epigenesists and the preformists; we refer to that of the regeneration of the lens in the eye of the tritons, which, after its extirpation, is reproduced from the cells of the iris, that is to say, from a material quite different in character from that of which it is formed in normal generation.8 The double epithelial layer of the iris, from the marginal proliferation of which the new lens springs, must exercise upon the lens in process of formation a continuous action persisting throughout the development 8See e. g. Erik Miiller: Uber die Regeneration der Augenlinse nach Exstirpation derselben beim Triton. Archiv f. mikrosk. Anat. und Entwicklungsgesch. Band, XLVII. erstes Heft. Bonn. Cohen, 1896. P. 23 ff., especially 29 and 30. The Formative Stimulus the Determining Factor 29 of the lens. For the cells of the iris cannot preserve within them potentially any trace of a formative capacity, or of a germinal "anlage," or of any "determinant" which provokes the formation of the lens, seeing that in normal development the latter takes its origin from another tissue. In these examples, both in the post generation of Roux's half embryos and in the regeneration of the lens in the triton, the cells which serve as constructive material appear then to be absolutely incapable of any auto-transformation and ready on the contrary to differ- entiate themselves and to dispose themselves indifferently in any manner whatever, according to the formative stimulus to which they happen to be exposed. At this point the fundamental biological question pre- sents itself: What is the nature of these formative stimuli, of this continued action which the formative part exercises upon the part being formed? The attempt to build up a hypothesis relating to so important a question is the object of the studies presented in the second part of this chapter. 2. Hypothesis of the Nature of the Formative Stimulus If, in our study of the nature of the formative stimulus in the development of organisms, we start with the primitive pluricellular form, consisting simply of aggre- gations of cells that are all alike, we observe that during some stages of their ontogeny the essential nature and the behaviour of these cells is clearly determined by phenomena of nervous nature in the widest sense of the wrord. For example, phenomena of this nature exist undoubtedly in the little mononuclear amoebae into which the spores of the myxomycetes become changed, also in 30 Nature of the Formative Stimulus the zoospores which move about by means of their vibratile flagella and into which these mononuclear amoebae become transformed. And it is certain that phenomena likewise nervous in character must come into play when the cells of Magosphera planula become separated from one another and each one moves off independently by means of its cilia. One is justified then in suspecting that when these cells are united in a colony they may then also be the seat of phenomena of a nervous nature, and that the other ontogenetic stages of these minute organisms ought also to be attributed to such phenomena. But then we should have to refer the development of higher organisms also to phenomena of the same nature. As a support of this hypothesis we recall the well known fact that all cells of organisms, from these most primitive pluricellular forms up, are united to one another by a network of intercellular protoplasmic bridges. We recall the example which the different species of Volvox afford, a genus of lower algae consisting of a vesicle formed of a single layer of cells, very much like the blastula stage of the development of animals. All the species of Volvox show a perfectly typical and regular form of union between the different cells of the body. In Volvox aureus for example, the superficial cells of the trophic hemisphere of the vesicle lie in a thick soft gelatinous mucus and each is not only provided with two long flagellae, but also is connected with each of the five or six adjoining cells by a long, thin protoplasmic filament. In the germinal hemisphere the protoplasmic filaments are more numerous so that each of the great spores which arise there is connected with each one of the large neighboring cells by bundles of from three to Stimulus to Formation of Cell Membranes 31 six filaments. These protoplasmic connections persist for some time even when the spores have been divided into two, four, or more constituent parts.9 An experiment which has already become famous appears to indicate that the intercellular bridges, as if they were successors of the vibratile protoplasmic fila- ments, substituted for them, are traversed incessantly by nervous currents or discharges emanating from the nucleus. For this experiment we are indebted to Pfeffer. After having detached by plasmplysis the cell mem- brane of the nucleated protoplasmic body of a plant cell, and dividing the cell into halves, one containing a nucleus and one without any, he observed that only the nucleated half had surrounded itself with a new cell membrane. If however, the part deprived of the nucleus remained united to the nucleated fragment even by only a very fine protoplasmic filament, it also \vas capable of secreting its little cellulose membrane. Pfeffer varied his experiment also in the following manner. He prepared cells of a moss protonema in such a way that an entirely isolated, anucleate mass of proto- plasm remained united to the neighboring cell which con- tained a nucleus by means of thin filaments piercing the cell wall. In this case a membrane was formed round the anucleate fragment. But the membrane was not formed if the neighboring cell had been itself deprived of its nucleus. In the formation of this cellular membrane in anucleated parts of the cytoplasm united by protoplasmic filaments with other nucleated portions, the maximum intervening distance observed by Pfeffer was 3.7 mm. 'Oscar Hertwig-: Die Zelle und die Gewebe. Zw. Buch, P. 34, 35- Fig. 16, 17. 32 Nature of the Formative Stimulus "But the nucleus can certainly exercise the membrane forming stimulus at an even greater distance." If the nucleus remains united with a whole chain of anucleated bits of cytoplasm "the production of the membrane appears to advance centrifugally and so to commence a little later in the more remote portions of cytoplasm, than in the bits nearer the nucleus." 10 From these experiments one is inclined to think, this author concludes, "that the production of a cellular membrane required the continuous transmission and cooperation of certain states of motion and vibration which radiate out from cell nuclei or rather owe their origin to the reciprocal action of nucleus and cytoplasm." Oscar Hertwig makes in this connection the following remark: "This experiment proves that the stimulus necessary for membrane formation can be transmitted by thin connecting filaments which traverse the septum interposed between two cells. Nothing hinders us then from assuming that some similar transmission goes on in other functional conditions." n In these observations of Pfeffer the formation of the membrane goes on independently of the situation and remoteness of the nucleus and of the geometric form of the line of communication which may be straight or curved in any way, and consequently, (and we must keep this especially in mind,) just as though this formation were effected by a specifically stimulating current, passing 10Pfeffer: Uber den EinflujS des Zellkerns auf die Bilduhg der Zellhaut. Berichte iiber die Verhandl. der konigl. sachs. Gesellsch. d. Wissensch. zu Leipzig. 1897. P. 507. "Oscar Hertwig: Die Zelle und die Gewebe. Zw. Buch, P. 40, 41. Currents of Nervous Energy in Skin Cells 33 out from the nucleus and quite independent of the form and extent of the conductor which carries it. But it is very probable that this nervous current or discharge which is conducted from the nucleated cell along the protoplasmic filaments to the anucleated frag- ment of the contiguous cell, also passes across into the fragment even when it contains a nucleus and so also when it is replaced by an entire cell. This leads us to the conception that wherever intercellular protoplasmic connections are present, the various nuclear currents or -discharges stream through these connections and so permit a general nervous flux throughout the whole net- work of these protoplasmic bridges, in the meshes of which the nuclei themselves would constitute the nodal points. In this way one would have a continuous circulation or distribution of nervous energy throughout the entire organism. This supposition is supported by the experiments of //"Siegfried Garten. On his own arm he cut out a little disc of skin one centimeter in diameter so as to lay bare the muscle fibers. Without suturing the wound he covered it with an aseptic dressing and left it to the process of granulation. After the wound was completely covered with epithelium except only a small circle of 1.75 mm. radius he cut out the whole piece again and enough around it to reach to the area in which the skin was in quite normal condition. Microscopic observation gave the following result: Studying it from the center of the wound out, one met first of all a greater or less number of wedge shaped epithelial cells with the long axes radially disposed. Surrounding this came next an annular zone, 0.45 mm. broad, of fusiform epithelial cells whose long axes were 34 Nature of the Formative Stimulus almost without exception tangentially disposed, that is perpendicularly to the radius of the wound, and the protoplasmic filaments within the cells ran parallel with their axes. In correspondence with this interior cellular arrangement, the intercellular bridges into which the protoplasmic filaments were prolonged ran for the most part from tip to tip of the fusiform cells and parallel to their long axes. Outside this annular zone, (from 2 to 2.5 mm. outside the inner epithelial border,) were found large cells in which the filaments and protoplasmic bridges were remarkably well developed and there also were found mitoses. It is therefore in this area that new cells are formed. In this zone the intercellular spaces are larger than elsewhere, from 3 to 6 /* wide, whereas 1.8 to 3 ^ represents the mean normal figure for the epidermis. This considerable enlargement of the intercellular spaces makes it possible for these cells to store up the larger quantity of nutritive fluid which is necessary for their more intense activity.12 If one admits for the moment that the intercellular bridges are traversed by a continuous nervous flux this result will find in this hypothesis its immediate explanation. In order to make our idea clearer let us consider the concrete case of a stream of flowing water, which at a certain point divides up into several branches. Sooner or later a dynamic equilibrium is established and the quantity of water flowing during each unit of time into each of the branches respectively will be constant. If 12Siegfried Garten: Die Interzellularbriicken der Epithelien und ihre Funktion. Archiv fur Anatomic und Physiologic. Leipzig. 1895- P. 407—409. Action of Nervous Currents in Skin Wounds 35 we now effect artificially the obstruction of some of these branches the entire volume of water must flow through the others which remain open and their volume of water will consequently be increased in proportion. In the same way let us suppose that through the intercellular bridges of the epidermis for example, there go similar nervous currents, in which when once the organism has attained its adult age dynamic equilibrium is established. And let us remove a little disc of skin as Siegfried Garten did, then the nervous flux which heretofore had taken its way through the filaments and protoplasmic bridges of the cells situated in the little disc removed will now find these ways closed. Consequently the entire flux must take a roundabout way through the neighboring parts surrounding the little disc. The augmentation of the nervous current in these ways will have as its result an augmentation of the trophic stimulus which it exercises; so that the cells situated along these ways will grow and proliferate more rapidly, thus producing a zone of reproduction characterized by numerous mitoses. The augmentation of the vital processes of these cells will in consequence of increased osmotic attraction, attract a greater quantity of nutritive fluid, exactly as the wick of a lamp which is stimulated by a current of air draws up by capillarity a larger quantity of combustible fluid. And this greater quantity of nutritive fluid pressing in between one cell and another, will distend the intercellular spaces so that in this zone they undergo an enlargement. The further the new formation of the skin proceeds, the shorter will be the way through which the abnormally increased nervous flux tends to pass, that is to say that the zone of the 36 Nature of the Formative Stimulus most intense flux and the zone of reproduction thereby determined will approach the center of the wound. The tangential disposition of the cells of the circular zone between the reproductive zone and the central granular zone, would be due just to this nervous flux which commences to flow through the new formed cells, but is still always forced to flow around the wound in some such way as the water of a river flows around the circular pier of a bridge. The direction of the cell axis would thus be determined by the direction of the current. According to Siegfried Garten's views, on the con- trary, there would exist all around the wound, in spite of the aseptic dressing, an augmentation of the blood circulation, which would have as its consequence an increase of pressure and of the amount of nutritive fluid in the tissue. And consequently there would arise an augmentation of volume of the intercellular spaces and an increased formation of new cells in the reproduc- tive zone. As for the tangential disposition of the cells of the surrounding zone, he believes that one can explain that by the theory of sphincter action, that is through the contraction of the intercellular bridges of these cells. In consequence of this contraction the long axis of the cells would turn into the direction of the tractile force which is exercised upon the cells. At the same time the consequent shortening of the respective circular zones of these cells would cause the epithelium to press in toward the center of the granulating surface, and thereby effect the gradual contraction of the opening of the wound.13 But the increased blood supply and the shortening "Siegfried Garten: Ibid. P. 409 — 411. Effects of Nervous Currents in Skin Wounds 37 of the intercellular bridges which this explanation pre- supposes would require to be explained themselves. It is to be remembered further that Roux in his studies upon the struggle of the parts of the organism has shown that the great affluence of necessary nutritive fluids is always the consequence rather than the cause of the grqwth of the organic substance. This experiment of Siegfried Garten argues strongly in favor then of the hypothesis that a continuous nervous flux traverses the intercellular bridges. The nuclei, as foci of nervous energy, would be precisely the sources which feed it, and which in normal conditions preserve it unaltered. At the same time, the nervous flux dis- charged by the other nuclei in passing through any one nucleus acts like a functional trophic stimulus, in so far as it is favorable to the specific vital process of this nucleus. Each increase or decrease of this current passing through certain nuclei caused by conditions lying without these nuclei, would have as a result an aug- mentation or diminution first of their mass and later of their number. This augmentation of the nervous flux in given zones following the ablation of neighboring parts would thus be the general cause of the active proliferation of cells by which all the phenomena of reproduction commence. Later when there comes into play a disturbance of the equilibrium, one can conceive that its reestablish- ment can proceed and spread from any one whatever of the numerous parts which surround the part cut off. In other and more general terms one understands that the reestablishment of the normal distribution of nervous energy, necessary for the reformation of an organ, in case it be prevented from following the ordinary way, 38 Nature of the Formative Stimulus can reach the organ by any other way and indeed by a way quite the opposite of the normal as happens for example in any distribution of electric or hydrodynamic energy, and that consequently it amounts to the same thing whether the regeneration of the tissue or of the organ ablated proceeds in the same way as in ontogeny or by any other way. And finally we must suppose that regeneration is nothing else than a particular case of generation or reproduction and that the nature of one is substantially identical with that of the other: for, to use the words of Delage "generation is only the regeneration of a complete organism by a portion of greater or less size attached to it or detached from it;" 14 so the causes of the regeneration, for instance of a little disc of skin which has been removed, must be essentially the same as those which effect a complete reproduction. Between the two phenomena the following difference will however exist : The regeneration of a little disc of skin will be due, according to what we have just stated, to the fact that the continuous nervous flux which flows through the whole organism and particularly through those parts which were contiguous with the part removed, would tend to reestablish its dynamic equilibrium, dis- turbed by the operation. If we accept the fundamental biogenetic law in its first degree of approximation, complete generation, on the contrary, would be a whole series of transitions of the nervous energy circulating or distributed in the developing organism, from one dynamic system to the other next in order, both meanwhile being in a state of equilibrium since they were already formerly 14Delage : L'heredite et les grands problemes de la biologic generate. P. 98. Protoplasmic Connections of Adjoining Cells 39 in equilibrium in both the corresponding phylogenetic forms. To effect the transition it is necessary then that in each ontogenetic stage there suddenly supervene at some point of the system a change, which disturbs the established equilibrium and provokes the passage of the continuous nervous flux to a new dynamic equilibrium. If the intercellular bridges have really the significance which we have attributed to them, one can see how they must be present in all organisms and in all stages of development. It is superfluous to go more thoroughly here into the fact that this is exactly what is fully confirmed by histologic investigations which are being ever more carefully prosecuted. We recall for example the protoplasmic connection observed by Hammar between the segmentation spheres of the sea urchin egg: The cells of the blastula are covered all over the outside of it by a protoplasmic layer which adheres in each cell only to the part of its surface which is directed toward the outside. This layer in a few preparations not sufficiently protected against drying separated itself a little from the blastula. There appeared then thin filaments, variable in number and more or less regular in disposition, which extended from the granular protoplasm of the different blastomeres to the interior of this layer and produced in this way a manifold connection of the different cells with one another.15 Sedgewick has observed the same thing in the develop- ment of the eggs of Peripatus. The two cells which come from the cleavage of the eggs are not completely 15Hammar: Ubereinen primaren Zusammenhang zwischen den Furchungszellen des Seeigeleies. Archiv fur mikrosk. Anat. und Entwicklungsgesch. Bd. XLVII. Erstes Heft. Bonn, Cohen. 1896 P. I4ff. 4O Nature of the Formative Stimulus separated but remain connected by protoplasmic fila- ments; the cells which arise from each of the first two cells are associated in the same manner, and this con- tinues indefinitely. So that during the whole of develop- ment from the first segmentation of the egg up to the adult stage all the cells of the organism remain in inter- communication by means of these protoplasmic bridges. "The connection of cell with cell is not a secondary feature acquired late in development, but is primary dating from the very beginning of development." 16 It is quite unnecessary to recall the universality of intercellular bridges not only between the cells of each tissue but also between the cells of different tissues: between the epithelial cells of gland ducts and contiguous smooth muscle cells, between epithelial cells and con- nective tissue cells, between connective tissue cells and endothelial cells, between smooth muscle cells and con- nective tissue cells, between connective tissue cells and striated muscle fibers, between striated muscle fibers and epithelial cells, and so on.17 It is necessary to be remarked further that in animals this circulation of nervous currents can and at least in certain stages of development must certainly utilize not only the protoplasmic network uniting the cells to "Adam Sedgwick: The Development of the Cape Species of Peripatus. Quart. Journ. of microscopical Science. XXVI, 1886. P. 198 — 200, 206. "See for instance: Heidenhain: Uber das Vorkommen von Interzellularbriicken zwischen glatten Muskelzellen und Epithelzellen des ausseren Keimblattes und deren theoretische Bedeutung. Anat. Anzeiger, VIII, No. 12 and 13; May 13, 1893; P. 4°4 — 4*0 J and Schuberg: Uber den Zusammenhang verschiedener Gewebezellen ina tierischen Organismus. From the Sitzungsberichten der Phys.-Med. Gesellschaft zu Wurzburg. Sitzung vorn Febr. 25, 1893; P. i— 8. The Whole Organism a Great Protoplasmic Plexus 41 one another, but also the nervous system itself with all its fibers and fibrils in so far as it is developed. This leads to the conclusion that in the adult organism the ordinary nervous currents passing along the various nerves as a result of ordinary nervous discharges con- stitute only momentary intensifications of permanent nervous currents which pass continually through these nerves. The great frequency with which conductors of nuclear stimuli in general and intercellular bridges in particular, are found in both animal and vegetable kingdoms, is as we have said, even by itself a very strong support for the hypothesis proposed by us of a nervous circulation or distribution throughout the whole organism. In this hypothesis we approach, even though only in certain respects, the most recent theories of some botan- ists and physiologists who in consideration of this striking general protoplasmic connection between the different cells, regard the multicellular organism not as a mere assembly or colony of cells, but rather as a single voluminous protoplasmic body in which the nuclei are inserted at different intervals as centers or foci of energy, (synergids of Sachs), and in which the mem- branes and other intermediate structures have produced only incomplete divisions and serve merely as supports of the organism. For example, according to Sedgewick the body of the adult animal would be only an immense syncytium whose nuclei or centers of force are dispersed throughout a single protoplasmic network binding together the whole organism.18 We approach especially the conception which Oscar 18Adam Sedgwick: The Development of the Cape Species of Peripatus. P. 205 — 206. 42 Nature of the Formative Stimulus Hertwig seems to have of protoplasmic connections, for while it is true that he never discusses such a continuous nervous flux and speaks only of the transmission of nuclear stimuli, nevertheless this latter conception in our mind implies the former. To sum up our conception of these protoplasmic connections we could almost adopt the very words of this investigator : "It is probable that this transmission of nuclear stimuli by protoplasmic fila- ments will be much less rapid and less intensive than nerve conduction, but perhaps for this very reason will be more continuous and by reason of its duration more efficacious." 19 It is quite unnecessary to draw especial attention to the fact that the vegetable kingdom is not in any way an obstacle to this conception of a continuous nervous flux throughout the whole organism, for if nervous phe- nomena are less apparent in it than in the animal kingdom, they constitute nevertheless just as in the latter the essence of the vital phenomenon. From amoeboid movements, from the vibrations of cilia and flagella up to the most complex psychic phe- nomena, everywhere where life is, one finds also processes of nervous nature. The reticular or fibrillar structure which protoplasm in general exhibits, protoplasmic currents, especially those in very long, fine filaments, such as for example those in the pseudopodia of the rhizipod Gromia oviformis, which by their peculiar character make one suspect that they may be only the consequence and sensible effect of currents provoked by an energy of another kind, the striations consisting of bundles of curved parallel lines without sharp angles in "Oscar Hertwig: Die Zelle und die Gewebe. Zw. Buch. P. 40. Protoplasmic Nervous Currents Universal 43 each little layer of the cell membranes of plants, the formation of a membrane in an anuclear fragment of a vegetable cell in consequence of a nuclear stimulus trans- mitted from the nucleus of another cell along any proto- plasmic conductor, all these facts speak together in favor of the hypothesis that all living substance is traversed by vital nervous energy in the form of currents. Claude Bernard has already remarked that anaesthe- tics act not only upon the nervous system but also upon the cells of every animal or vegetable tissue, in that they destroy or suspend the vital activity of every cell in the same way, and from that he concludes that all vital processes in general are substantially identical.20 This substantial identity is demonstrated further by the fact that phenomena of irrito-contractility present themselves in the same way in both animal and vegetable kingdoms and that there are all possible transition stages between plants considered "especially sensitive" and those considered not sensitive at all. The whole group of "especially sensitive" plants present especially well developed intercellular protoplasmic connections.21 We recall further the microscopic movements of the protoplasm within the membranes of plant cells, which led Huxley to make the well known definition that a plant is only "an animal shut up in a wooden box." Everybody knows that in addition to these micro- scopic movements there are now known in plants also various movements visible to the naked eye, which cor- 20Claude Bernard : Legons sur les phenomenes de la vie communs aux animaux et aux vegetaux. Paris, Bailliere, 1878. P. 289 — 290. 21Macfarlane : Irrito-contractility in plants. Biol. Lect. at the Mar. Biol. Lab. of Wood's Holl. Summer Session 1893. Boston, U. S. A., Ginn. 1894. P. 189, 204. 44 Nature of the Formative Stimulus respond fully with the reflex movements of animals ; that is, in which one can distinguish as in animals a region of perception and another of motility, as well as the transmission of a stimulus from the perceptive region to the motor region. Leaving aside the very well known example of Mimosa it suffices to recall for example, that in certain plants one can demonstrate that the sensibility of the root to gravitation resides in its extreme tip while the bending movements of that same root in order to resume its vertical position after it has been disturbed takes place in another part. In the same way the vertical position of the stem is maintained. But a yet more typical example is furnished by the grass, Setaria. "It has a remarkable manner of germination; as soon as the seed germinates it does not produce a simple cylindrical stem but one terminating in a wedge shaped tip like a lance head. When a group of Setaria is lighted from one side it inclines strongly toward that side and all the lance tips point toward the light. But these tips are not curved at all, on the contrary the whole bending is produced in the stem, although it is clearly these tips which are sensitive to light and not the stems. It is easy to prove this by covering the tops of some stems with an opaque cap: the grass stalks so protected remain vertical while others incline their stems toward the light. The part that bends has not then any sensitiveness to light and the part sensitive to light does not bend. The little lance is the organ of perception, the stem the motile region, and it is clear that a stimulus is transmitted from the tip to the stem." 22 22Francis Darwin: Le mouvement chez les plantes. Revue scientifique. March I, 1902. P. 265. Specific Modes of Being of Nervous Currents 45 Consequently the view is not only justified but almost required that in plant bodies also there is a nervous cir- culation or distribution, which, though it manifests itself only in its variations dependent on some change in the external influences, is certainly none the less present during the repose of the organism when it is in a state of dynamic equilibrium. It is this constant circulation or continuous distribution of nervous energy, which con- stitutes in plants quite as well as in animals the "small yet mighty link" which unites the parts of the organism into a "sympathetic whole," a function which Lewes ascribes especially to the nervous system.23 As to the properties of each of the respective excita- tations or currents which constitute this general nervous flux, it is sufficient for our purpose to suppose that these latter appear in specifically differing modes of existence which are capable now of combining now of disjoining; we mean by this that two specific modes of being are able for example to combine with each other and so to give rise to a third specific mode of being, or indeed that this latter can break up giving rise to the two preceding specific modes of being or even to others different from them. While for the present we are not in a position to discover what these different specific modes of being really are, yet we can and indeed must necessarily regard them as existing since in different tissues different specific nuclear stimuli must certainly be present in the cells. These different specific modes of being might consist in something analogous with the intensity of the continuous electric current, or perhaps in a rhythmic form correspond- 28Lewes : The physical basis of mind. New edition. London. Kegan Paul, Trench, Triibner & Co. 1893. P- 61. 46 Nature of the Formative Stimulus ing somewhat for example with the alternating electric current, or indeed might arise in another way of which perhaps we can not in advance form any conception at all. It is sufficient for our purpose, we repeat, to suppose that these specific modes of being can combine and break up according to laws which are definite even though so far quite unknown. For the very fact of the existence of these laws would imply also the existence of correspond- ing laws on which the circulation or distribution of nervous energy in definite networks would depend. If one accepts such a hypothesis of the circulation or the distribution of nervous energy in the organism, one could find the immediate explanation of certain phenomena of development whose cause has so far remained a secret. These phenomena consist in the recip- rocal influences which certain parts of the embryo, even though widely separated, exert upon one another in spite of the lack of any functional adaptation and by which the development of these parts is wholly or partially determined. One can, indeed, attempt to give the beginning of an explanation of these phenomena of correlation, by supposing the different parts which exercise this reciprocal influence to be situated, maybe upon the same partial network of the general circulatory system, maybe upon different partial networks which nevertheless come off at one common given point from the same principal branch, or maybe finally, in the case of contiguous parts, upon different partial networks which are however provided with direct communications between some of their respective nuclei. The absence of any analogous reciprocal action between other parts also contiguous may be explained by the lack of any such direct con- Correlative Differentiation and Growth tf nection between the nuclei of the two partial networks belonging to these parts, a fact which renders these net- works in certain respects quite independent of one another if at the same time they do not come from a common principal branch. It would be "a very important matter," writes Delage "to know if secondary protoplasmic connections are formed between neighboring cells which are not sisters but which have been brought only secondarily into contact with one another, for example after an invagination in animals or through grafting in plants." 24 In the cases in which this did not occur one could then have the simultaneous and to a certain extent independent exist- ence of partial circulatory systems even though they lie close together or perhaps even enclose each other. Roux designates by the term "correlative or dependent differentiation" the complete or partial determination of development by reciprocal influences of epigenetic nature which become established, in a certain measure at least as he admits himself, between the cells. We can then designate these partial networks of the general cir- culatory system by the name "networks of correlation." And we shall see that, conformably with our theoretical conjectures very many processes seem actually to prove that each of these networks is capable of existence by itself, independent within certain limits of other partial networks. Among the phenomena of correlative differentiation in development belong also those which are called com- pensatory growths. The investigations of Ribbert ( 1889) and his pupils upon the mammals have shown especially "Delage: L'heredite et les grands problemes de la biologic gen- erale. P. 33. 48 Nature of the Formative Stimulus well, that after the cutting off of organs not yet in function, for example of the testicle or of an infantile ovary or several infantile milk glands, the other similar organs underwent in their respective parts a proportionate growth as though they would thus compensate for absent parts. It would appear from this that the networks of correlation belonging to each of these parts of like organs must come off from a common principal branch in such a way that the whole current of this branch prevented from the usual division by the absence of one part of the network would now discharge itself entirely into the remaining organ. The hypothesis of the continuous circulation or the continuous general distribution of nervous energy which thus finds its support in certain special phenomena of development, affords better than any other an explana- tion for the fundamental process of every ontogeny, which consists as Roux has very aptly said only in a series of unequal localizations of growth. "A given region grows," writes Delage, "while the neighboring parts by which it is surrounded, grow much less or not at all. This part must necessarily then project outward or become invaginated and form a cavity. But at a given moment growth ceases in this place and goes over to another place, and the same phenomenon is now repeated at this new place." 25 The morphological means which ontogeny employs is then always the same, always of the same identical nature even when the tissues already partially differentiated commence to differ from one another in their most essential properties. 25Delage: L'heredite et les grands problemes de la biologic generate. P. 174. Explanation of Ontogenetic Involutions 49 The hypothesis of a continuous trophic nervous flux being admitted, these serial unequal localizations of growth can be explained by changes in the distribution of this flux at each new ontogenetic stage, from causes which we shall examine in the next chapter. These changes of distribution bring about now here, now there, a great affluence of nervous energy and thereby induce at the corresponding points proliferation of the cells from which must necessarily arise later the invagination or evagination in question. But the ontogenetic phenomena which most clearly call for the conception of such a distribution of nervous energy which continually changes and shifts, streaming now through one region now another of the developing organism, are the phenomena of involution; that is to say phenomena of reduction presented by the tissues of an organ which after being formed in the course of ontogeny tends at a later stage to disappear ; for example the involution of the tail of a tadpole during its meta- morphosis into a frog. The atrophy and degeneration of the skin, of the notocord, of nerve and muscle fibers, by which this involution is produced, have been described particularly by Osborn. He as well as Metschnikoff has established in this connection the great phagocytic activity of certain cells and the formation of true and false giant cells. Nevertheless he does not attribute to the phagocytes the most important role in the elimination of material. The whole of the process, in fact, results in the gathering together of the cellular material in process of dis- integration and conducting it into the lymph and blood vessels for utilization later in the construction of other organs and tissues peculiar to the adult animal. 50 Nature of the Formative Stimulus This would indicate then that the greater phagocytic activity would be not so much the cause as rather the effect of the diminution of the vital resistance of the organ. And this latter would seem necessarily to be due exclusively to the fact that the organ itself would be at this ontogenetic stage abandoned by the particular energy which had formed it, and which up till now had maintained it in full vital activity, and which now has not indeed ceased to exist, but has turned to other regions. This transference of cellular material in process of disintegration to other organs and tissues in process of formation would seem in fact to demonstrate that simultaneously with the diminution or the cessation of the trophic stimulus in one given region there appears an increase of that stimulus in another region. This utilization, as nutritive material, of the substance of cells which are disintegrating is rendered necessary by the fact that animals during their metamorphosis take almost no nourishment. It follows that, if the nutritive material which the abandoned parts give up to the parts now more abundantly infused with trophic energy, should be insufficient at the normal rapidity of disintegration, the disappearance of the tissues must be accelerated. This is indicated by Osborn's researches upon the influence of fasting upon metamorphosis, from which it appears that it is appreciably accelerated by inanition, just because of the more rapid reduction and absorption of the organs about to disappear.26 In the disappearance of the tail of the tadpole one has not a senile but rather a premature involution of tissues, 26Osborn: Alte und neue Probleme der Phylogenese. Ergebn. d. Anat. u. Entwicklungsgesch., herausg. v. Merkel u. Bonnet. Bd. III. 1893. Wiesbaden, Bergmann. 1894. P. 198. Distribution of Energy Explains Ontogeny 51 "in which nature destroys in a manner which may seem to us cruel, cells which have just been produced." The hypothesis of the distribution of trophic nervous energy seems to us the only one which can give a satisfactory ex- planation of this phenomenon. The struggle of the parts of the organism cannot in fact be the exclusive cause of this involution of young tissues. This struggle is not sufficient by itself to explain the exactness of the epoch and of the stage of develop- ment at which this physiological involution takes place. Even if we were willing to admit that this struggle has some effective participation in the production of this phenomenon, we must nevertheless admit that in addition an inciting ontogenetic stimulus, as Roux would say, must at the appointed time exert its trophic action upon the parts destined to victory, while it abandons others previously favored, but which now are devoted to destruc- tion. The distribution of trophic nervous energy with its changes would thus always remain the only cause of the phenomenon. But if ontogenetic physiological involutions are due to the fact that the distribution of trophic nervous energy abandons one region to turn to another, similar changes and shifting of this distribution must then be likewise the cause of all invaginations and evaginations, of all morphological phenomena in general and, with much probability, consequently, of those ontogenetic phenomena also which are not exclusively morphological in nature. To produce each one of these serial ontogenetic mod- ifications in the distribution of trophic nervous energy it would suffice theoretically that at the required moment there become active, were it only upon one certain point of the circulatory system, a single new definite specific 52 Nature of the Formative Stimulus current, different from the current previously present at the same point. And this would be exactly the role, which as we have already said the central zone of development plays. This zone therefore will be the object of our study in the next chapter. CHAPTER THREE PHENOMENA WHICH POINT TO THE EXISTENCE OF A CENTRAL ZONE OF DEVELOPMENT HYPOTHESIS OF THE STRUCTURE OF THE GERMINAL SUBSTANCE. /. Phenomena Which Point to the Existence of a Central Zone of Development The only group of organisms in which one can say that the existence of a central zone of development is di- rectly demonstrated is that of the unicellular organisms, in which this zone is constituted by the nucleus. In pluricellular organisms it is only indirectly that we are able to arrive at the conclusion that the central zone exists. Experiment has shown that the necessary and suffi- cient condition for the ontogenetic development of the Infusoria is the presence of a nucleus. This latter con- stitutes therefore for them an effective central zone of development and consequently ontogenesis consists in them in a true and proper centroepigenesis. If one divides an amoeba or a rhizopod or an infuso- rian already completely developed, into many pieces, that one of these fragments which remains provided with its nucleus though it be the smallest of all, is yet capable of reproducing by new formation all the missing organs and of developing again into a normal individual ; whereas the 53 54 Indications of a Central Zone of Development other fragments, without nuclei, are incapable of it even though they may be much larger. Especially we recall the researches upon artificial di- vision of the Infusoria made by Nussbaum and Gruber. If, for example, one cuts a stentor into three pieces, of which each contains one portion of its moniliform nucleus, in the space of twenty- four hours each piece regenerates the missing part. The anterior extremity regenerates the posterior and vice versa ; the middle piece reforms the two extremities, that is to say, both the rather complex peri- stomal region with its mouth, its pharynx, its long cilia, etc., and also the simpler posterior part. If however the fragment retains no part either of the paranucleus or of the nucleus proper, even though it may be of much greater size than those fragments retaining the nuclei, no trace of regeneration is observed, a fact which does not prevent the piece concerned from continuing to live for a while, even for two or three days, nor from retaining com- pletely the capacity of locomotion, of vibration of cilia, of pulsation of the contractile vesicle, of defecation, of capturing, engulfing and digesting its food.27 Gruber reports however the following experiment which has caused a good deal of surprise, for according to the view of some biologists it seems to be opposed to the results of earlier researches. He selected a Stentor coerelus which showed already the first stages of spontaneous division, that is there had already commenced in it the formation of a lateral, per- 27See e. g. Balbiani : Recherches experimentales sur la merotomie des infusoires cilies. Recueil Zool. Suisse, t. V, no. I, 1888. P. 48— 49, 54; und Verworn: Die physiologische Bedeutung des Zellkerns. Archiv fur die gesamte Physiologic, Band. 41. Bonn, Strau/3, 1892. P. 13-14- Objection of Anuclear Regeneration in Stentor 55 istomal, ciliated area, and he divided it in two halves. Since in this stage the chief nucleus, ordinarily monili- form, contracts into a round or bean shaped mass, Gruber was able to remove it completely from both halves. The division of the animal was effected in such a way as to produce about the same two halves as would later have been produced by spontaneous division. In the fragment which contained the original peristome the simple cicatrization of the wound was enough to repro- duce a complete individual. In the other fragment which contained the posterior extremity the wound closed in the same way and the anterior extremity continued its development until the peristomal area and the buccal spiral were completely formed.28 This result seemed to contradict the view that the formative action of a nucleus, as a developmental center for the unicellular organism, was exerted continuously throughout the whole duration of development. But the following considerations show that this premature con- clusion is quite fallacious. We should remember in this connection another ob- servation of Gruber. He cut off from the anterior end of a Stentor a fragment absolutely without a nucleus, but containing a small portion of the peristomal band. The cicatrization of the wound was followed by the ordinary contraction of the fragment and thereby the small portion of the peristomal band was given the appearance of a complete Infusorian such as would be formed by regen- 28Gruber: Uber kiinstliche Teilung der Infusorien. Zweite Mitteilung. Biol. Centralbl. Band, V, No. 5; May I, 1885. P. 139— 140; und: Beitrage zur Kenntnis der Physiologic und Biologic der Protozoen; Berichte der Naturforschenden Gesellsch. zu Freiburg i. B., Freiburg i. B., Mohr, 1886, P. 13—14. 56 Indications of a Central Zone of Development eration. But that this was not actually the case was demonstrated by more careful observation by which it was recognized that the completeness was only apparent, for no part altogether lost was reproduced and no new mouth was formed in the place of the old one which had been removed.29 From this one could almost infer that some analogous phenomenon is the effective cause whereby the organs of the peristomal field, as soon as they are all formed in their essential parts, become arranged in the posterior anuclear half in about the same way as they would be arranged after the completion of spontaneous division. Even if we admit a true and proper continuation of development, we must yet bear in mind first that it is not at all certain that this posterior half was completely de- prived of macro- or micro-nuclear substance. For the micro-nuclei sometimes attain the number of fifty- four or sixty-six in Stentor coereleus, and it is always difficult to see them, especially in individuals in process of sponta- neous division.30 Secondly we must above all things get a clear under- standing of what the remaining alive for a while of anuclear fragments of adult individuals can signify, keep- ing in view at the same time the absolute generative inca- pacity of these fragments. They signify nothing else than a posthumous persistence for a while of the special action or series of actions, partly simultaneous, partly succes- 29Gruber: Uber Kiinstliche Teilung der Infusorien. Zweite Mitteilung. Biol. Centralbl., Bd., V. No. 5; May i. P. 139—140. 80 H. P. Johnson : A Contribution to the Morphology and Biology of the Stentors. Journal of Morphology, vol. VIII, no. 3. Boston, U. S. A., Ginn, August 1893. P. 499- Posthumous Action of the Nucleus 57 sive, which the nucleus was exerting at the moment of its excision or shortly before. We can suppose, as we shall see better later, that this posthumous action of the nucleus ("Nachwirkung") may be explained in the following way : Each of the different nervous currents which the nucleus can discharge simul- taneously or successively into the protoplasm deposits in it, of all the nuclear substances just that one which had given origin to it, perhaps by reproducing it partially while on its way. This substance once deposited in the protoplasm, would act as a reserve which, while incapable of growth by itself because it lies outside the nucleus, would nevertheless preserve for a time, until its gradual exhaustion, the capacity of producing the same current again. It would produce in relation to the excision of the nucleus, the same effect as would be produced by a very slow propagation of trie respective current through the protoplasm.31 Therefore one can easily understand how in Gruber's experiment, in which the adoral ciliated zone of the an- imal was already in an advanced stage of formation, the whole series of simultaneous or successive formative stimuli had been already discharged shortly before the excision of the nucleus, and that therefore it remained only to await the slow unfolding of their effects, which would bring to completion the development already far advanced. "Compare the partly similar, partly different hypothesis of Ver- worn on the posthumous action ("Nachwirkung") of the nucleus, which may be attributed to a reserve material built up gradually by the nucleus and given off to the protoplasm, and persisting till the protoplasm is used up, in the above mentioned article: Die phy- siologische Bedeutung des Zellkerns, P. 90; also: Die Bewegung der lebendigen Substanz. Jena, Fischer. 1892. 58 Indications of a Central Zone of Development To one or other of these conclusions, either to the presence of an undetected micronucleus or to the pos- thumous action of the nucleus, one is necessarily driven, as we said, by the fact that only the nucleated fragments of an infusorian already completely developed, are capable of regeneration. For this capacity of reorganization, as one may call it, of the protoplasmic substance, which gives to it again the form of the complete individual but of correspondingly smaller size, cannot possibly arise either from the properties of this sub- stance itself or from its specific "physiological units" for which the adult form of the individual would constitute the only state of equilibrium. This is quite impossible because a mass of protoplasm as large as the nucleated part, but which contains no nucleus, does not manifest the slightest tendency to regenerate, although it is capable of surviving its ablation for several days. The impulse tending to produce the specific form of the ordinary equilibrium is present only when the nucleus is not lacking. Nevertheless the material which disposes itself in this definite specific form of equilibrium is not the nuclear ma- terial but the protoplasmic. The nuclear substance with- out participating itself in the process of reorganization, merely provokes it in the protoplasmic substance, which in this respect remains totally indifferent. This is dem- onstrated among other things by the observation of Gruber that the four nucleated fragments into which one individual was cut by a transverse and a longitudinal in- cision required in all cases the same time to regain their complete specific form including the adoral ciliated zone. The anterior end which already contained a portion of it and which one would suppose to be better adapted in its Unicellular Organisms Like Pluricellular 59 protoplasm to this new formation requires just as long as the posterior end remote from the adoral zone.32 This impulse to reorganization which enables proto- plasmic substance already organized to take on any other organization whatever speaks strongly in favor of the hypothesis that it is due to a special formative energy ("formgestaltenden Energie" as Nussbaum would call it) emanating from the nucleus, which using the proto- plasmic substance merely as a support or vehicle or as an indifferent constructive material, would be in reality the only quiddity which tends to dispose itself in that partic- ular form, which constitutes for it the only possible system in dynamic equilibrium. For the reasons above stated we must think that this formative energy is prob- ably nervous in nature. This ontogenetic function of the nucleus which we see in unicellular organisms permits of very important deduc- tions in relation to all organisms whatever. For the com- plicated unicellular organisms whose manifold organs have different and mutually independent functions, such as for example a Stentor coereleus or a Paramoecium caudatum, are not essentially different from pluricellular organisms, but on the contrary are comparable with them in all essential respects. "Between the internal differentiations of a complex cell," says Delage, "and so between the body of certain Infusoria, and the organs of the pluricellular being there exists I think only a casual difference, which depends not so much on the requirements of the differentiation as on the size of the organism." 33 82Gruber: Uber kunstliche Teilung bei Infusorien. Zweite Mitteilung. Biol. Centralbl., Bd. V. No. 5; May I. P. 138. "Delage: L'heredite et les grands problemes de la biologic gen- erale. P. 97. 60 Indications of a Central Zone of Development "However great," says Whitman quoting Gruber "the difference between an infusorian and a highly organized animal it cannot be a qualitative one. We can assume that the same vital elements serve in both as the founda- tion, only in ever new combinations. This kinship declares itself very clearly in the correspondence of many organs of the Infusoria with those of the higher or- ganisms. We mention only the membranellae of the Infusoria which are quite similar to the corner cells of the mollusk Cyclas cornea." 34 But we have already seen that when one cuts the infusorian into several nucleated fragments the membra- nellae can be formed from any given part of the proto- plasm of the original individual, and can be arranged in definite relation to one another under the influence of the nucleus as a center from which the formative activity of the entire organism radiates. It is then probable that the formation and manner of disposition of the corner cells of the mollusk Cyclas cornea may be due also to a similar process of centroepigenetic nature. But this justifies us in suspecting that in all pluricellular organisms whatso- ever, every formative process commencing with normal ontogeny is of centroepigenetic nature. This hypothesis is supported for example by the ex- periments of King upon regeneration in Asteria vulgaris. These have given among others the following results : each of the arms cut off close to the body can remain liv- ing by itself for two weeks but is incapable of regenerat- 34Whitman : The Inadequacy of the Cell-Theory of Development. Biol. Lect. at the Mar. Biol. Lab. of Wood's Holl, Summer Session 1893; Boston U. S. A., Ginn, 1894. P. 118; and Journal of Morphol- ogy, Boston U. S. A., August 1893. Vol. VIII, No. 3, P. 651— 652; Fig. 2 and 3. Regenerations Indicate Central Zone 61 ing the entire animal. If a fifth part of the body disc remains regeneration can occur in exceptional cases. If half of the disc is present the absent parts are always reformed. If one amputates all five arms of the same animal by five transverse cuts, the first one very near the body, the others at four different distances from it, then after a certain time, — the same for all five arms, — the regen- erated portion is largest for the first, and proportionately smaller for the other four according as the site of amputa- tion was farther from the central disc. This regenerated part which has developed from the amputated arm is much smaller in diameter than the original arm which was cut off. That is an indication that the regeneration is not produced by the cooperation of all the parts immediately adjacent to the surface of amputation.35 These experiments thus seem to indicate a distinct zone from which the process of regeneration proceeds and to the activity of which it is due. From another side the existence of this formative central zone is almost required by the results of the similar experiments of Roux, which we have mentioned above, on the formation of half embryos of frogs. These experiments show, in brief, that each half, right or left, anterior or posterior, can develop independently. If one admits also that this development is always en- tirely epigenetic in nature, that is that it is due entirely to correlative differentiations which the cells produce in one 35Helen Dean King: Regeneration in Asteria vulgaris. Arch. f. Entwicklungsmech, d, Org., Band, 7. Heft. 2. and 3. Leipzig, Engel- mann. October 18, 1898. P. 351—361. Table VIII, especially Fig. ii. 62 'Indications of a Central Zone of Development another, it follows that at least each quarter of the embryo must have its own system of correlation networks independent of the other quarter systems. But the four quadrants have one zone which is common to all. Con- sequently at least these four independent systems of cor- relation networks for the four quadrants must come off from this common zone. This suffices to warrant the statement that this latter must belong to a central zone of development, in the sense which we have set forth above; and that from this zone must branch off and diverge independently of one another the different great correlation networks or principal branches for the general distribution of nervous energy. These latter divide further into progressively smaller ramifications, one could almost say just as the great arterial trunks coming off from the heart continually sub- divide down to the terminal capillary vessels. It is nevertheless advisable to study these phenomena more closely. We must postulate that each of the two blastomeric nuclei obtained in the frog's egg after the first segmentation, when it once becomes completely isolated, is capable of giving rise to a complete embryo. But in the experiments of Roux, the disposition of the nutritive yoke or deutoplasm in the uninjured blastomere remains, thanks to the retention of its place by the injured blasto- mere, the same as the disposition which would have existed in this same blastomere if development had pro- ceeded normally. Therefore, of all the specific potential energies which the uninjured blastomeric nucleus would be capable of activating successively, there would com- mence and continue to be activated only those which in normal development would have flowed directly into the Interpretation of Partial Formations 63 half of the egg affording such a definite deutoplasmic disposition. So it becomes clear that when once the development of the uninjured half of the egg has commenced, the central zone concerned, even though it may be exactly the same as in the complete embryo, would activate only the specific potential energies proper to the corresponding half embryo and would produce only a half formation. This half formation no matter how independent the great correlation networks might be, could nevertheless at each instant of its development, render the system of nuclear actions and reactions an incomplete one, without equilibrium, because at the plane of separation there would not be opposed to its own nervous tensions any equivalent system of tensions of the absent half. This incomplete system of nuclear tensions, not of itself in equilibrium, could nevertheless be prevented, even though only transiently, from equilibrating itself normally, by the special distribution of the nutritive yolk and by the presence of the injured half of the egg, still placed op- posite that which is developing. But as soon as the con- tinual increase of energy in the nuclear system overcomes these artificial barriers, equilibrium is once for all upset and postgeneration will appear. It is necessary nevertheless to note that some derm- monsters which proceed almost to the completion of their development would seem on the contrary to indicate for this incomplete system of nuclear actions and reactions a practical equilibrium existing from the commencement and persisting through all the stages of development. The typical example of these demimonsters is the famous Hemitherium anterius which Roux so thoroughly describes. It is constituted by the almost fully developed 64 Indications of a Central Zone of Development foetus of a half-calf, in which all the posterior part of the trunk was missing as if it had been removed by a transverse cut.36 However that may be, one fact which speaks in favor of the hypothesis that the nutritive yolk acts as a tem- porary dam to a system of nuclear actions and reactions not of itself of equilibrium, is that these half embryos can be obtained only from those eggs in which, as in the Amphibia and Ctenophera, the nutritive yolk is abundant. Whereas in the case of animals whose eggs are poor in nutritive yolk, and present cleavage cells that are almost identical, as in the Echinodermata and Ascidiae, either a complete organism develops at once from the isolated blastomere or postgeneration appears very early.37 Another fact which speaks in favor of this hypothesis is that toward the end of a spawning period when the vitality of the egg, and consequently also of the blasto- meric nuclei, is lower, the formation of pure half embryos is much easier to effect.38 While thus, after exclusion of pre formation theories one can say that the formation of half embryos is a direct proof of the hypothesis of a centroepigenesis with its ramifying, independent correlation networks, this hypoth- 36Wilhelm Roux : Uber die kunstliche Hervorbringung ,,halber" Embryonen etc. Virchows Archiv, Bd. 114. Oct. 1888. P. 132. Gesamm. Abhandl. Zw. Bd. P. 442. 37Wilhelm Roux: Uber das entwicklungsmechanische Vermogen der beiden ersten Furchungszellen des Eies. Verhandlungen der Anat. Gesellsch. Wien, 1892. P. 55—56. Gesamm. Abhandl. Zw. Bd. P. 809-^810. 38Wilhelm Roux: Die Methoden zur Hervorbringung halber Froschembryonen und zum Nachweis der Beziehung der ersten Furchungsebenen des Froscheies zur Medianebene des Embryo. Anatomischer Anzeiger, Bd. IX. February 1894. P. 257. Gesamm. Abhandl. Zw. Bd. P. 954. Development of Parts Not Due to Local Relations 65 esJs becomes confirmed indirectly by a whole series of other facts which Roux also has described and commented upon with his customary carefulness. Natural or artificial headless monsters for example, and in general all monsters lacking entire parts but other- wise normal, prove that no formative action or reaction is exerted by the head or by these other parts upon the rest of the organism. They speak therefore in favor of a centroepigenesis with independent networks of correlation in which it is necessary to suppose that the formative action must stream out entirely from a center toward the periphery. In all these monsters of which some part is lacking, there need be present only one part of the body namely the seat of the central zone of development. Roux in his researches upon the formation of half- embryos once observed, as an example of the disturbance of correlations of mass by the absence of one embryonal half, a lateral dislocation of the notochord and a corre- sponding retardation of development of the dorsal part of the endoblast lying near the median line as marked by the semi-medulla and by the ventral parts. "It is an interest- ing fact/' he remarks, "that the axial parts can be laid down and developed with so considerable a shifting in relation to one another. For this indicates further that the development of many parts even of the main parts is not included in the form as such; the embryo does not live a formal life." 39 These facts would also confirm the hypothesis of independent networks of correlation, the displacement of whose material would not alter their "Wilhelm Roux: Uber die kunstliche Hervorbringung halber Embryonen etc Virchows Archiv. Bd. 114. October 1888. P. 132. Gesamm. Abhandl. Zw. Bd. P. 442. 66 Indications of a Central Zone of Development respective formative capacity, because it would leave un- altered the reciprocal relations of the different parts in each network. Further, the deformations which the entire organism, and consequently also each of its different networks, undergo, would not seem to alter markedly the internal, reciprocal relations of the different parts of each net- work; thus from frog's eggs which had been compressed continuously during their development, the blastula and gastrula having been forcibly flattened, folded, and bent, there developed embryos whose internal and external aspect was just the same as though they had been allowed from the first to develop in a normal fashion and had undergone the deformation only later.40 "In the development of frog's eggs it happens very often," Roux writes further, "that the primitive mouth of the gastrula is not yet closed when the medullary folds appear, and this condition can persist in part up till the time of the closure of the medullary tube and the forma- tion of the branchial elevations and adhesion cups. The formation of these latter can proceed in a manner which appears quite normal in the anterior half of the body even though the posterior half of the body may have quite an abnormal form, the primitive mouth remaining persist- ently wide open. Another instance, yet more surprising, is that in which notwithstanding the entire absence of the medullary ridges, the gastrula gradually exchanges its round form for a pear shaped one, a thing which or- dinarily occurs only after the formation and development 40Wilhelm Roux : Uber die ersten Teilungen des Froscheies und ihre Beziehungen zu der Organbildung des Embryo. Anatomischer Anzeiger, Band VIII. 1893. N. 18. P. 608—609. Gesamm. Abhandl. Zw. Bd. P. 926. Anachronisms Show Local Relations Inessential 67 of the medullary tube. These modes of behavior and others like them indicate that the parts which continue to develop normally require for their development neither the absent parts, nor that the remaining parts should be at the stage of development normally corresponding, and thus that they can develop alone, independent to a cor- responding extent of those absent or backward."41 "Anachronisms of development," continues Roux in a later research, "appear also in the relative retardation or acceleration of development of one of the germ layers in relation to the others. For example several embryos otherwise normal, in which the medullary fold is still quite undifferentiated, show already in the mesoderm, in the entoderm, and in the chorda dorsalis, formations which appear normally only about the time of closure of the medullary tube. In this case there is an evident re- tardation of development of the ectoderm in relation to the development of the other two layers. There occur also inequalities of lesser degree in the rapidity of development of the two lateral halves of the body, and thus it is possible to observe two different stages of development in the same object." 42 "If such large parts of the organism," concludes our author in a still later study, "can remain behind in their development or indeed remain absent, without thereby producing any disturbance in the development of the others, it follows surely that the development of these "Wilhelm Roux: Zur Orientierung iiber einige Probleme der embryonalen Entwicklung. Zeitschrift fur Biologic. Bd. XXI. Miinchen, Jul 1885. P. 478—479. Ges-amm. Abhandl. Zw. Bd. P. 203—204. *2Wilhelm Roux: Uber die kiinstliche Hervorbringung halber Embryonen, usw. Virchows Archiv, P. 128 — 129. Gesamm. Abhandl. Zw. Bd. P. 438. 68 Indications of a Central Zone of Development latter is not at all bound up by reciprocal actions with the absent parts, and also that it is not accomplished by the reciprocal action of parts of the whole organism." 43 These conditions are just those existing in a centro- epigenesis with ramifying and independent networks of correlation. The formation of double monsters with double symmetry in the disposition of their organs is particularly in accord with centroepigenesis. Concerning this Roux expresses himself in this way: "In these double forma- tions the fragment which is lacking in a symmetrically similar manner from each of the two individuals, can be any selected piece limited by a plane surface ; and in them the organs are nearly all present and normal in form up to the plane of union, as if from two embryos, fully developed and ready to be born, one had cut off two symmetrical pieces so as to leave plane surfaces, and the twins had then been reunited by the cut surfaces." "The simultaneous development of two formations so exten- sively united, into two distinct bodies, of which each is centered in itself, indicates directly that there are not any general reciprocal actions operating to combine them into a single whole." 44 According to the centroepigenetic hypothesis, the formation of these monsters would be due to the fact that the two blastomeres concerned, which are quite identical with each other since they arise from the segmentation of one and the same egg, have become, on account of ab- 43Wilhelm Roux: Uber Mosaikarbeit und neuere Entwicklungs- hypothesen. Anat. Hefte, Edited by Mcrkel and Bonnet, Febru- arheft 1893. P. 320. Gesamm. Abhandl. Zw. Bd. P. 859. 44Wilhelm Roux: Uber Mosaikarbeit etc. Anat. Hefte, P. 320. Gesamm. Abhandl. Zw. Bd. P. 859—860. Centroepigenesis Accords with General Phenomena 69 normal circumstances which have made them independent of each other, two distinct nodal points, — two central zones of development. The necessary consequences of the independent action of these two centers of develop- ment would be the centration of each individual by itself. On the other hand, the similarity of the two first blasto- meric nuclei, which through multiplication give rise to both of the two central zones concerned, would bring about a similarity of the formative actions given off from these zones. Therefore those formative stimuli which act upon all points of any one of the infinite number of planes symmetrical in relation to these centers, would offset and annul one another, because they would be equal and opposite but only in so far as they do act along these planes. In this way could be explained why the parts lacking in both individuals must always be alike. While thus all these different facts which have at- tracted the careful attention of Roux and been made the special object of his studies confirm, some directly others indirectly, the hypothesis of centroepigenesis with ram- ifying and independent networks of correlation, a further support, indirect indeed, but nevertheless very real, will be brought forward in the following chapter. For we shall endeavor there to show that if a whole series of facts compels us to throw over preformation, a series of other facts forces us to throw over simple epigenesis also, and this would give a high degree of probability to any other hypothesis with which both series of facts should be in accord. Finally, the symmetrical disposition which the greater number of organisms present in relation to a point, to a straight line, or to a plane, and also the advance of growth along diverging ramifications, which indeed is a 70 Indications of a Central Zone of Development general law of organic development, are phenomena which indicate in their turn that this centroepigenesis with ramifying, independent networks for the distribu- tion of nervous energy, is also in harmony with the most general biologic phenomena. One could always bring forward the objection that it is difficult to conceive how this series of energies acting one after the other, all upon the same point of the de- veloping organism, could give rise to a series of dynamic systems of distributions as complex as we must neces- sarily suppose those to be which constitute the successive stages of ontogeny. To dissipate all doubts in this mat- ter one may draw attention to the following simple hydro-dynamic experiment. Let us imagine a very large cylindrical glass container almost filled with water, and having a hole at a selected point in the bottom of it. By means of a suitable force pump let us make more water enter the vessel through this hole with a velocity varying from one moment to another. To make the idea clear and the phenomenon more intelligible let us suppose that the velocity varies sharply each second, sometimes increasing, sometimes diminishing very considerably. On account of the great amount of water already contained in the vessel, the series of successive systems of very complex currents which will be produced each moment by the incoming water, whose newly added mass interpenetrates and dis- places the other in a manner which we can make appar- ent in part by previously coloring the water to be injected, will depend on the entire series, and exclusively on the series, of different velocities with which the water is forced into the container. A given series of dynamic actions, qualitatively alike Centroepigenesis in Plants 71 but quantitatively different, starting successively always from the same point could thus give rise to a succession of dynamic systems of as complex a configuration as one could imagine. Different series, that is to say those in which quantitative variations of the same dynamic ac- tion succeed one another in different ways, would natur- ally give rise also to dynamic systems of different configuration. We can compare, though only roughly, this water which enters the container always by the same opening and with a velocity changing every instant, to the series of nervous currents of different specificity which, accord- ing to our hypothesis, would be discharged into the soma in the course of development, or into the great mass of yolk, by the activity of the germinal substance, always from one and the same point of the organism, which would thus constitute the central zone of development. It would be proper at this point to touch upon the probable location of this central zone. But important as it is we do not need to stop long over it. It is necessary at the outset to notice in a very general way that in plants, and especially in the higher plants, one must regard the leaf as the true individual and one must attribute to it a centroepigenesis of its own. The flower would then be merely the product of numerous centroepigeneses not entirely independent of one another: The corresponding simultaneous or rapidly successive activations of multiple centers, and the reciprocal action of these centers upon one another, would be indeed the agents by which modifications of each of the centro- epigeneses is effected, so as to produce for example here a petal and there a pistil instead of an ordinary leaf. It would also be possible for a center or a definite 72 Indications of a Central Zone of Development group of these centers, perhaps on account of some special situation, to become, in the course or at the end of the particular centroepigenesis producing the different parts of the flower, and in relation to all the other centers and so to the whole flower, the director to a further de- velopment; so that there would be present for the whole development of the flower, or for a part of it at least, a centroepigenesis of the second degree. By analogy one can conceive also of the possibility of other centroepigeneses of still higher degrees (composite flowers, etc.). "It may appear," writes Le Dantec, "that the sexual individual belongs to a higher order than the asexual in- dividual and may originate from the individualization of an association of parts which are like asexual individuals* It occurs perhaps in the Medusae ; it is certain in Phane- rogams. A flower corresponds morphologically to an assemblage in a fixed form, of parts which are like the asexual individuals of the plant. Goethe had already noted this peculiarity. The asexual individual of a plant is the internode with its leaf and auxiliary bud : flowers are much more complex." 45 Centroepigeneses of a second or higher grade could thus serve to explain the transformation of simple colonies of individuals, (e. g., of the ancestors of the present echinoderms), into complicated organisms, which tend steadily toward an individualization of their own. One could explain easily the transformation, for example, of the original individuals of the colony, becoming more and more differentiated from one another, into the organs of this organism (siphonophores). With the growth of "Le Dantec: Traite de Biologic. Paris, Alcan, 1903, P. 413. Location and Structure of the Central Zone 73 centralization (annelids, arthropods), centroepigeneses of the second or higher degrees would approximate grad- ually more and more the simple centroepigeneses of the first degree. "The gastrula," adds Le Dantec further, "itself a morphologic unit of higher order than the cell, can itself bud off other gastrulas, just as the cell can produce other cells by budding. This budding may take place always in the same direction and so give rise to linear associations of gastrulas, as in the worms, arthropods, etc. ; or it may take place in every direction and thus produce plant- like associations, for example the fibroid polyps or coral polyps; it may proceed radially and so give rise to the echinoderms. Even the vertebrates themselves would be, according to this, the result of an individualized assem- blage of a linear series originally comparable with an annelid worm. That is the theory of human polyzoism of Durand de Gros and Edmond Perrier." 46 After what has been said thus far the probable loca- tion of the central zone of development of the various types of organisms need not be especially treated of here. The place in which we must suppose it to be, which naturally lies in the plane of symmetry of the organism, appears almost self evident from what we have said above, and will become steadily clearer from what will be said in continuation. It may here be remarked merely that this zone cannot be imagined as any special tissue marked off distinctly from the surrounding somatic tis- sues; but must rather be a simple, indistinguishable part of some tissue whose special, somatic functions in the adult individual are such as predispose it best, in the 40Le Dantec : Traite de Biologic. P. 412. 74 Indications of a Central Zone of Development descendant organism, to the definite function of develop-' ment, and which differentiates itself gradually from the other parts of this tissue by quite inappreciable and gradual transitions. We have already said that the centroepigenetic hypothesis makes it necessary to distinguish the effective germinal zone, or true place of origin of the germinal substance, from the apparent germinal zone, which would be nothing else than the receiving station for the sub- stance separated out or secreted by the effective germinal zone, or the place where the sexual cells concerned are built up out of this material. While we must regard only the effective germinal zone as the central zone of develop- ment, it is clear that the apparent germinal zone can be located at any part whatever of the organism. In the higher plants the apparent germinal zone of the asexual budding cells, and that of the female, sexual cells, would seem generally to coincide approximately with the actual zone, that is, with the corresponding central zone of the leaf and of the flowrer. In this way can be explained the heretofore puzzling phenomenon of the Xenia, in which as is known the flower after a hybrid fecundation, often takes on the form, size, color and tissue structure, characteristic of the variety from which the pollen comes ; that is, as Darwin has already observed "in which the male element not only influences the germ as is its proper function, but at the same time influences various parts of the mother plant, in the same manner as it influences the same parts in the seminal offspring from the same two parents." 47 47Darwin : The Var. of Animals and Plants under Domestication. Eighth impression of the second edition. Vol. I. London, Murray, 1882. Chap. XI, P. 433. Determination of Location of Central Zone 75 In the pluricellular animals we can start out with the simple supposition that if a central zone of development is present, it is probably that in which the blastomeres multiply with unlike rapidity ; that is, it will be constituted by these blastomeres which multiply most rapidly. For this greater rapidity would in the majority of cases indicate a greater vitality or energy, which may be produced by a richer protoplasm or by any other special condition favoring the nutrition. And this greater energy would bring it about that the cells possessing it would win the upper hand over the others. "The zone," writes Oscar Hertwig, "where the smallest embryonal cells lie, which are also those which divide most rapidly, becomes the place of gastrular invagination ; it becomes something like a fixed center of crystallization for the animal development." 48 But these blastomeres are those which, in the vertebrates for example, later constitute the medullary tube and afterward the spinal cord. As we shall see, everything seems to lead to the con- clusion that in animals with specialized nervous systems the central zone is constituted by the least differentiated part of the nervous system itself; in the vertebrates probably by the innermost periependymal part of the spinal cord. This part after completion of its activity in determining development would, on account of the so- matic function pertaining to it, likewise constitute the place where the infinitely manifold nervous activities of all the rest of the nervous system or rather of the whole organism are faintly re-echoed. As we have already said, this location of the central 48Oscar Hertwig: Zeit- und Streitfr?gen der Biologic. Heft. 2. Mechanik und Biologic. Jena, Fischer, 1897. P. 180. 76 Hypothesis of Structure of Germ Substance zone is almost self-evident from what has preceded, and will be still more so from what is to follow. Here it may only incidentally be remarked, (since we have treated of the subject more thoroughly in another place), that this hypothesis concerning the location of the central zone in vertebrates, and in general in those animals which have a specialized nervous system, finds a very strong support also in the numerous researches and obseryations upon the influence which the nervous system exercises upon de- velopment and regeneration.49 From this we can now pass on to the discussion, even though very briefly, of the question of the probable com- position and structure of the substance which constitutes this zone, and which is consequently none other than the germinal substance. This affords an opportunity of speaking of the not essential but subordinate difference by which the germinal nuclei are probably distinguished from the somatic. 2. Hypothesis Upon the Structure of the Germinal Substance We have seen that according to the centroepigenetic hypothesis, ontogeny can be attributed to a series of mod- *°See e. g. Wolff: Die physiologischen Grundlagen der Lehre von den Degenerationszeichen. Virchows Archiv, Bd. 164, 1902;— Richard Rubin : Versuche iiber die Beziehung des Nervensystems zur Regeneration bei Amphibien. Archiv f. Entw.-Mech. d. Org., Bd. XVI, Heft i ; March 13, 1903 ; — Kurt Goldstein : Kritische und ex- perimentelle Beitrage zur Frage nach dem Einflu/3 des Zentralner- vensystems auf die embryonale Entwicklung und die Regeneration. Arch, fur Entw.-Mech. d. Org., Bd. XVIII, Heft i; February 26, 1904; — Eugenic Rignano: Die zentroepigenetische Hypothese und der *, EinflujS des Zentralnervensystems auf die embryonale Entwicklung und die Regeneration, Arch. f. Entw.-Mech. d. Org., Bd. XXI, Heft. 4; September n, 1906 Composed of Specific Potential Elements 77 ifications in the general distribution of nervous energy of the organism. If we consider the law of Haeckel in its first degree of approximation, we must suppose as we have already said, that this distribution of nervous energy constitutes by itself at each ontogenetic stage a system in dynamic equilibrium, because the same distribution of this energy was in equilibrium in the corresponding an- cestors. To provoke the transition from one dynamic sys- tem to the other, it is necessary that at each ontogenetic stage there become active in the central zone a new spe- cific energy, which disturbs the dynamic equilibrium which has just been formed and effects the transition to a new dynamic equilibrium. This leads to the supposition that the germinal sub- stance may be constituted by a number of material par- ticles of which each would be able to activate only the corresponding specific nervous energy. We can desig- nate each of these particles by the term specific potential element. We must here provisionally postulate the possibility that there may be substances capable of containing in the respective potential condition, not only definite forms of energy but also different specific modes of the same form of energy. We shall take up the question again later in order to make it clearer and to handle it more thoroughly. It may be remarked here, however, that a chemical analy- sis of the material particles of the nucleus, which actually contain the hereditary mass, could hardly throw much light upon the eventual differentiation of the different ma- terials which compose the entire germinal substance. For, at least for the moment, it can give only the com- position of the possibly homogeneous residue into which 78 Hypothesis of Structure of Germ Substance all these different substances break up or decompose as soon as life has gone out of them. In conformity with the views of most biologists, we can assume that the hereditary mass, and therefore all the specific potential elements, are preserved and distrib- uted during what is called the resting stage, in the gran- ules of chromatin which are disposed like the beads of a rosary upon the nuclear reticulum ; but during mitosis, in the chromosomes, and particularly in the little discs of chromatin which the chromatic filaments into which the nuclear reticulum and its granules contract, often present, superimposed upon one another and separated by inter- vening layers of linin. We must nevertheless note that this mode of dispo- sition of the different specific potential elements in the nucleus is of importance to us only in relation to nuclear division. For we must hold with the epigenesists, as we shall see soon, that this division always proceeds in a manner qualitatively the same. But this disposition is of absolutely no importance to us in relation to the effects which it will have upon the serial activation of these elements. From this latter point of view we can indeed suppose these elements to be scattered through the germinal nucleus and mixed with one another in any way whatever. For, as we shall see better later, the activation of every specific potential element in the proper ontogenetic stage, depends in no way upon its position in relation to the others, but rather on the condition that at this stage only its activation requires the doing of only a moderate amount of work — an amount which does not require more energy than the total quantity inherent in each element. Its activation in any other stage would require, on ac- Nuclear Somatization and Equal Cell Division 79 count of the much greater modification which it would then induce in the distribution of nervous energy already existing, the doing of a large amount of work, — an amount which would require more energy than the quan- tity present in each element. The centroepigenetic hypothesis of a single limited zone containing the germinal substance, and the other conception following upon it, that the germinal sub- stance may consist of a number of different, material particles, each representing one particular, specific, poten- tial element brings up the question of nuclear somatiza- tion. We postulate the existence of a central germinal zone distinct from the soma. We must not forget neverthe- less that all nuclei arise by division from the first, that of the egg. If we also admit with the epigenesists a qualitatively equal nuclear division, then the nuclei des- tined to become somatic must at first be equivalent with those destined to become the central zone of development. In what way then is the nuclear somatization brought about in the cells which later must constitute all the dif- ferent tissues of the body? There presents itself at once the preliminary ques- tion: Must we really admit this nuclear division to be always qualitatively equal ? Or shall we rather hold with the preformists that in addition to equal nuclear divi- sions there may be also unequal divisions ? On this point we believe we ought to agree unconditionally with the epigenesists. There does not exist any observation which gives even the slightest ground for the conclusion that there is a qualitatively unequal division. "By the most thorough study of the longitudinal division of the chromosomes," 8o Hypothesis of Structure of Germ Substance writes Strassburger, "it is absolutely impossible to hunt out anything but equal division. Unequal division is not presented at all. There is not a single fact to support the notion that it exists." 50 We shall here limit ourselves to mentioning only two orders of facts which speak directly against unequal divi- sion : First, it does not occur in any nucleus in the vast realm of unicellular and of primitive pluricellular forms, con- sisting of colonies of like cells; for in them the facts of heredity show directly that nuclear division is always equal. But especially the oft repeated and keenly discussed experiments upon the relative shifting and isolation of blastomeres afford direct proof that nuclear division is equal in the first segmentations of the egg. We recall for example the experiments of Chabry upon the Ascidians, those of Wilson upon Amphioxus, those of Herbst on the separation of the blastomeres of the sea urchin merely by adding chloride of potassium to ordinary sea water, of Driesch on the Echinus microtuberculatus, of Oscar Hert- wig upon frogs' eggs, of Raffaello Zoja on the Medusae and so on. These experiments in which one of the first blastomeres, or one of the four, or eight, or sixteen, or thirty-two first blastomeres, produce when isolated an entire embryo, perfectly formed but proportionally smaller, or in which the blastomeres, though shuffled about in any way whatever, nevertheless developed in a perfectly normal way, lead with the greatest certainty that any one could desire to the conviction that in the 50Stra£burger : Uber periodische Reduktion der Chromosomen- zahl im Entwicklungsgang der Organismen. Biol. Centralbl., XIV. No. 23-24. Leipzig, Dec. i, and 15, 1894. P. 835. Cell Division Hereditarily Equal 81 successive blastomeric divisions the nuclei always remain like the first, that of the egg from which they sprung. The contrary cases of the formation of half embryos, or of incomplete embryos, after the separation or killing of one of the two first, or of several of the first blast o- meres, are found always and only in embryos rich in deutoplasm, so that, as we have already seen, they cannot afford any ground for the conclusion that the nucleus or nuclei from which these incomplete embryos develop must be different from the first nucleus, that of the egg. We shall examine in the following chapter the very complex and untenable subsidiary hypotheses, to which the partisans of unequal division have been driven, in order to bring their principal hypothesis into accord with experiments upon the isolation and displacement of blastomeres, and also with other equally irreconcilable processes, such as post-generation and regeneration. Here we shall only quote and adopt the conclusion which Oscar Hertwig has drawn from these experiments, namely : "It is self-evident that such an interchange of blastomeres without injury to the product of development, is possible only if one nucleus has the same characters as the others, that is, only if all the nuclei are produced from the seg- mentation nucleus by hereditarily equal division." 51 In order that this hereditarily equal division may be materially possible in a germinal nucleus constituted by innumerable, different, infinitely small particles, — that is in order to permit the division of each of these particles or substances between the two daughter nuclei, — it would be sufficient that they become disposed during mitosis in little transverse layers one over another in the various "Oscar Hertwig: Die Zelle und die Gewebe. Zw. Buch. P. 69. 82 Hypothesis of Structure of Germ Substance little discs of chromatin of the chromatic filament, in exactly the same disposition as that which these little discs actually do present in the chromatic filament. Admitting then that nuclear division is always quali- tatively equal we must now ask: does this really mean that the nuclei of all cells must remain alike throughout the whole of development? But before taking up this question we must first answer a preliminary one : must we exclude nuclear som- atization with the epigenesists or admit it with the pre- formists ? If it seemed to us impossible to disagree with the epigenesists upon the first question of a qualitatively equal nuclear division, it seems on the contrary impossible to disagree with the preformists on this second of a nuclear somatization. We shall certainly not repeat here all the arguments by which these latter support their thesis. They can be summed up in their essential parts in the following words of Weismann. "The chromatin is able to imprint upon the cell in the nucleus of which it lies a specific character. Just as the thousands of cells which make up the organism possess very different characters and very different functions, so the chromatin which controls them cannot be every where alike, but must rather be different in different kinds of cells/' 52 The epigenesists, on the contrary, are well known to be inclined to the view that all the somatic cells of the organism have, without distinction, like nuclei con- stituted by the same idioplasm. Oscar Hertwig indeed ventures the assertion that each somatic cell if it were "Weismann: Das Keimplasma, eine Theorie der Vererbung. Jena, Fischer, 1892. P. 43 and 268. Cells Become Differentiated and Somatized 83 possible to put it in conditions which would render it capable of nourishing itself and preserving its life independently separated from the rest of the organism, could function as a germ cell.53 And certain processes which appear in all the lower organisms, with tissues that are not very highly specialized, appear to justify this view. If one places a piece of a Begonia phyllomaniaca in some earth in moist air, after cutting through the leaf- ribs in different places, one finds after some time, in the neighborhood of each wound, one or more little new plants. Any fragment whatever of a hydra or medusa possesses the power of reforming an entire animal without increasing its mass, but rather by a process of differentiation and rearrangement of cells already existing. A theory which admits equal nuclear division and also a slow and gradual nuclear somatization, resulting from the action of a determinate zone constituted only by the germinal substance, would reconcile the different and contradictory phenomena brought forward by the epi- genesists on the one side, and by the preformists on the other. Oscar Hertwig who as we have just seen is a zealous partisan of the idioplasmic equality of all nuclei, is com- mitted in another place to the possibility of a certain nuclear somatization. "The hypothesis of a hereditarily equal nuclear division does not imply the view that the 'anlage' substance must therefore be an immutable thing. . . . The idioplasms of certain groups of cells of an organism which find themselves permanently in unlike conditions in consequence of their different spatial and functional disposition in the body as a whole, dif- "Oscar Hertwig: Die Zelle und die Gewebe. Zw. Buch. P, 304-305. 84 Hypothesis of Structure of Germ Substance ferentiatcd as it is through the division of labor, can receive to a certain extent the stamp of local character." 54 If we admit that each new specific current while passing through a nucleus deposits there the substance which was capable of producing it, and which would be capable on occasion of reproducing this same specific current, — a hypothesis of which we reserve a better exposition till later, — we can conceive of nuclear soma- tization as a gradual and constant acquisition of new, specific, potential, somatic elements. The fact that each cell, as long as its differentiation has not progressed too far, can upon occasion, provided it be isolated from its neighbors, arise to the rank of a germinal cell, appears to indicate that these new somatic elements, thus gradually acquired, would from the start be simply added to the germinal elements already existing, without altering them at all, but merely relegating them to the potential state, from which, under normal conditions, they would not again emerge. In other terms we must suppose that all the germinal elements remain unaltered in the nuclei undergoing somatization as long as the number or the mass of acquired somatic elements does not progress beyond a given limit. But when this limit is once passed, then the require- ments of nutrition or of space would cause the different germinal elements to disappear gradually, and the nucleus concerned would thus lose all generative capacity. Further even those somatic elements, which each nucleus acquired one after another at each successive stage of development, will gradually disappear after "Oscar Hertwig : Zeit- und Streitfragen der Biologic. Praforma- tion oder Epigenese? Jena, Fischer, 1894. P. 142 — 143. How Somatization Reduces Germinal Capacity 85 ontogeny is completed, for then the nuclei are always exposed to one specific current or to a limited group of specific currents peculiar to the adult state. There would remain over only a small or very inconsiderable number of those somatic elements which were acquired last and which would thenceforth continually increase in mass. The cell would thus lose by degrees its undifferentiated embryonic aspect, and its exclusively somatic characters would steadily increase. While according to Weismann there would be a fundamental distinction between the germinal nuclei set apart for the preservation of the entire hereditary mass, and the somatic nuclei which from the first would receive only such particles of that hereditary mass as are indis- pensable for their function, and the ontogenetic passage from one to the other would take place suddenly and directly at the very commencement of development; according to the centroepigenetic hypothesis, on the contrary, there would not exist any essential difference between them, because they differ from each other only in the number and the specificity of their respective potential elements, and the passage from one to the other would be effected gradually and slowly. And this transition would be due we repeat only to the constant acquisition by the nuclei destined to become somatic, of ne\v specific potential elements, which at first are simply added to the germinal elements already present but finish by causing the latter gradually to disappear on account of the requirements of nutrition and space and by taking their place themselves. Without needing to have recourse to a reserve idioplasm, or to any other equally involved subsidiary hypothesis, one can explain in this way the phenomena, 86 Hypothesis of Structure of Germ Substance common in plants, of the retention by some cells of the germinal capacity even though they belong to somatic tissues which have already advanced to a certain degree of differentiation. In the same way is easily explained how a given piece of a hydra or medusa reorganizes itself so as to reproduce the entire individual without any correspond- ing increase of its mass. For since histologic differentiation in the hydra is not very pronounced one can surmise a priori that in all or nearly all their cells, the whole of the specific potential elements must coexist with the somatic elements peculiar for each cell and acquired by it during develop- ment. The separation of the fragment from all the rest of the organism, which arrests the general circulation of nervous energy, will therefore cause the somatic elements which were active in the intact individual to return to the exclusively potential state and thus enable the germinal elements to become active again. That cell or group of cells which surpasses the others in vigor will have its germinal elements activated first and will then form a central zone directing development of the others; and the distribution of nervous energy, which again passes through the wonted series of ontogenetic stages, will now proceed in the fragment in the same way as formerly in the entire individual. There are often external circumstances which deter- mine what cells of the fragment shall constitute the central zone. Thus if one cuts off from the trunk of the hydra both the tail end and the head end at the same time, and then places the fragment with the lower cut surface down, the head is reproduced at the same end as formerly, whereas if one turns it over so that the former head end More Vigorous Cell Groups Direct Development 87 of the polyp sticks into the sand of the aquarium, the head is reproduced at the aboral pole. Sometimes two distinct groups of cells seem to struggle with each other to constitute the central zone of development, and both may attain their object thus producing double monsters. If from the trunk of a hydra one cuts off at the same time both the tail and the head end, and suspends the fragment horizontally in the water it forms a head at both ends. In an analogous way, Morgan in his experiment on the regeneration of Planaria maculata once obtained from a fragment which had been cut off by two trans- verse sections, two heads one at the front end, the other at the hind end.55 This simultaneous activation of two centers of develop- ment can go on also at the commencement of ontogeny in the cells of the blastula itself. "From causes which are yet beyond our knowledge, there are often produced (in fish eggs) two gastrular invaginations instead of one, at separate points of the blastula. And it depends on the position of these two invaginations, which could also be designated crystallization centres for the further development of the embryos, how the embryonic cells of the germinal disc are then drawn into the process of development, given very definite positions in relation to one another, and utilized for the formation of organs." 56 The analogous phenomena of heteromorphosis in general can likewise be explained, through the similar, "Morgan : Experimental Studies on the Regeneration of Plan- aria maculata. Arch. f. Entwicklungsmech. d. Org. Bd. VII. 2 and Heft, 3. Leipzig, Engelman. Oct. 18, 1898. P. 381, 395. 56Oscar Hertwig: Zeit- und Streitfragen der Biologic. Prafor- mation oder Epigenese? P. 60 88 Hypothesis of Structure of Germ Substance abnormal activation of new centres of development after amputations, incisions, or in any other abnormal condi- tions whatever in the most different regions of the organ- ism which otherwise would have continued to constitute definite somatic parts of it. In Planaria maculata for example these new develop- mental centers of heteromorphous formations, of which this animal affords perhaps the most typical cases, appear, according to the results of recent researches, to be formed always from one of the two ends of the piece of the lateral nerve tract which is separated by the operation from the other parts of this tract.57 Indeed it appears to be indicated by these latest and most careful researches that those pieces of the planar ian which contain no part of this nerve tract are not able to regenerate themselves, any more than are those frag- ments of infusoria which contain no part of the nucleus.58 Therefore this animal forms perhaps the transition from those pluricellular organisms in which all the somatic cells preserve throughout their capacity of regeneration undiminished, to those in which this capacity exists in the adult in only a very definite and special zone. The phenomenon which more than any other speaks in favor of a nuclear somatization arising in the higher multicellular organisms during development, is the cir- cumstance that the capacity of regeneration diminishes with age; for it is very much greater in embryos than in fully developed animals. For example, if the feet of an adult frog are cut off, they do not grow again, whereas "Charles Russell Bardeen : Factors in Heteromorphosis in Plan- ariae. Arch. f. Entwicklungsmech. d. Org. Bd., XVII. i. Heft. 13 March 1903. P. 1—20, esp. P. 6—8; Fig. 5, 6, 7. "Charles Russell Bardeen: Ibid. P. 2, 3. Transformations Show Epigenetic Somatization 89 Barfurth has demonstrated that during the first stages of development this regeneration is complete. And Roux has found that if one cuts quite young frog embryos into longitudinal or antero-posterior halves the missing parts are completely regenerated in a few hours.59 On the other hand, if the regeneration of the optic lens in the triton from a tissue other than that from which it is developed in ontogeny, is of itself enough to exclude preformation decisively, it is nevertheless not in any way incompatible with the most complete nuclear somatization. If one admits this latter, the histological transformation of certain cells would indicate only the possibility that in certain ways somatized nuclei may become differently somatized, if unusual influences are exerted upon them by neighboring nuclei, that is if nervous energies other than the usual ones act upon them; and they would undergo this new somatization through the gradual acquisition of new specific, potential somatic elements, different from the former ones. By itself this transformation certainly does not prove that all nuclei of the different cells consist of like idioplasm. Further there is no firm support for this supposed idioplasmic identity of the nuclei in the researches deal- ing with vegetable and animal grafts. In order to support such a hypothesis effectively, these investigations would have to show a closer relation- ship or "harmonicity" (as Vochting would say) between different tissues of the same individual or of individuals belonging to the same species, than between like tissues "Roux: Uber die verschiedene Entwicklung isolierter erster Blastomeren. Arch. f. Entwicklungsmech. der Organismen, 1895, Band, I. Heft 4. P. 614. 90 Hypothesis of Structure of Germ Substance of different species. But this demonstration has not been afforded in a single instance of animal grafting. There are indeed examples of transfusion of blood which do not succeed when they are made from one animal to another of different species, whereas they do succeed perfectly between animals of the same species. There are the experiments of Bert upon the successful transplantation of a fragment of the tail of a mouse into the subcutaneous tissue at another part of the body of the same individual or of another individual of the same or a related species, as Mus decumanus and Mus rattus, whereas such a transplantation is not successful between species farther removed, such as Mus rattus and Mus silvaticus. There are the experiments of Oilier and Schmitt on the transplantation of fragments of bony tissue, which were successful in transplantations from one part to another of the same individual or to another individual of the same species, but failed be- tween individuals of different species.60 But all these experiments indicate only that there is more affinity between the parts of the same tissue or of similar tissues when they belong to individuals of the same species or of related species than when they are taken from individuals of different species. They do not prove at all that there is more affinity between parts of different tissues, which come from the same individual or from individuals of the same species, than between parts of the same tissue taken irom different species. On the other hand Joest's experiments have demon- strated the possibility of true heteroplastic grafts in the annelids, in which it is easy to obtain transplantations 60Oscar Hertwig: Die Zclle und die Gewebe. II, P. 248. Grafts Do Not Show Idioplasmic Identity 91 between different species. In contrast with the results obtained by Oilier and Schmitt, the transplantation to man of portions of bony tissue and of horny tissue which were taken from carnivorous or rodent mammals, has been entirely successful. It is known indeed that the transplantation of a cock's comb to a cow's ear has been successfully effected. Born, in his famous experiments, has succeeded in transplanting definite parts of young embryos of Rana esculenta to corresponding parts of other embryos, not only of the same species but also of different species (Rana fusca, arvalis and esculenta), and indeed of different genera (Rana esculenta and Bombinator igneus). 61 All these experiments show that the plasticity or capacity of transformation of living organic substance reaches much farther than between individuals of the same species. Therefore it cannot be explained by the idioplasmic identity of the nuclei, which in any case could exist only between tissues of the same individual and between individuals of the same species. There are certain grafts in plants that appear to justify the conclusion that there is a single nuclear idioplasm, identical throughout the whole plant. For in grafts between plants of the same species there has even been obtained the union of parts which have nothing in common with each other at all, as for example a root with a leaf. Whereas if one attempts to transplant even in quite normal relations, parts of plants belonging to "Compare e. g. Oscar Hertwig: Die Zelle und die Gewebe. II. P 23; Delage: L'Heredite etc. P. 114; G. Born: Uber Verwach- sungsversuche mit Amphibienlarven. Leipzig, Engelmann. 1897. P. I46ff. 92 Hypothesis of Structure of Germ Substance different species the result of the graft is not certain and often unfavorable. This can be explained by the fact that in numerous species of plants, as we have seen, nearly all the cells preserve the reproductive capacity. What one calls "vegetative affinity" is then perhaps nothing else than a direct effect of the retention of the whole of the specific germinal potential elements in addition to the somatic elements peculiar to each of the different tissues, in all or nearly all the different nuclei which do not pass beyond a certain degree of differentiation. In drawing a conclusion from all that has been said so far, we are confronted with this apparent paradox : on the one side, it seems that in conformity with the epigenesists we must reject a nuclear division which during one and the same development must be sometimes qualitatively equal, sometimes unequal, as inadmissable and refuted by the facts, and instead of this admit only a nuclear division always qualitatively equal. On the other hand it appears that in conformity with the preformists, one must likewise exclude a nuclear substance, identical in all the cells of the same organism, and must accept on the contrary, the hypothesis of an actual nuclear somatization. It follows that this nuclear somatization can be effected only gradually and only by a process of epigenetic nature. But when one has once admitted equal nuclear divis- ion and gradual nuclear somatization by a process of epigenetic nature, there follows therefrom necessarily the hypothesis of centroepigenesis. For if the nuclei of the cells of the different tissues of the body finally become completely somatized it is certain that some certain ones of the nuclei constituting the organism do not become Elasticity of Developing Organisms 93 somatized at all, namely those whose function it is to supply the reproductive cells with germinal substance. And if the first nuclei become somatized by a process of epigenetic nature, this process even though it involve the entire organism, must leave the other nuclei unaltered. But this would be possible only when this process is dependent on influences proceeding from the zone of germinal nuclei, and being exerted by it in such a manner that the germinal substance concerned does not become altered at all. The continuity of the germinal substance, the spec- ificity of the nuclei, and the epigenetic nature of the formative processes of organisms, — these three concep- tions which individually are favored by a great number of biologists — imply together the conception of centro- epigenesis. Another fact which has been considered perhaps less than it deserves, supports the hypothesis that the process of development is not only of epigenetic nature, but also depends upon influences coming off incessantly and suc- cessively from a point which is external to all the trans- forming parts, but which remains itself unchangeable; namely the elasticity by virtue of which developing organ- isms, much more than those completely developed, are able not only to undergo without injury enormous changes of form but also to resume their original form as soon as the disturbing influence ceases. And just to this greater elasticity of the young organism is to be attributed the fact that it is much less plastic than the adult organism. In fact the centroepigenetic hypothesis would permit one to deduce this a priori. For according to it the young organism is so much more elastic, because in it all the cells, being less specialized, are thus much more 94 Hypothesis of Structure of Germ Substance easily able to assume any new somatic character what- ever which may be imposed upon them. It makes no difference in the case of the still unspecialized cell, or even in that of the cell which is in the first stages of specialization, whether the somatizing stimulus is onto- genetic, proceeding by indirect ways from the central zone, or functional, induced by the environment. For the embryonic cell is in itself thoroughly plastic. Con- sequently the young soma would also be plastic if it were not continually influenced by the formative stimuli proceeding from the central zone of development. This influence though it is more feeble than the functional stim- ulus proceeding from the environment and consequently unable to resist it, has nevertheless the advantage of be- ing continuously in action, and so of gaining the ground lost, as soon as the action of the environment ceases. The cells of the adult soma are on the contrary less plastic, because they are already considerably specialized. But every modification which their limited plasticity per- mits in them remains, since the opposition of the central zone of development has already ceased. The adult organism is much less elastic. But in respect to the permanence of results it is more plastic than the young one. And, as already said, this is entirely confirmed not only by the most commonplace phenomena, but also by the most careful embryologic researches. In fact con- siderable changes of form, which would be destructive to an adult organism are, on the contrary, very well borne by the young. But attentive observation of these processes shows us also, as stated, that the younger the organism the greater is its elasticity, which tends, when the disturbing action has ceased, to restore it to its Roux's Self Regulating Mechanism 95 primitive state. Thus it is that a wound or a fracture is never so detrimental to the child as to the adult; but it is also true that with the same intensity and duration of the educative influence directed toward the modifica- tion of inborn tendencies the results are more permanent the older the child is. This elasticity of development is proved by Roux with his customary care in the following way. In one of his experiments on the effects of passive deformations in the first stages of development he suc- ceeded in bending a few frogs' embryos within their gelatin envelopes by squeezing them between needles. "If the needles were removed immediately after the deforma- tion, the embryo at once took on again its previous form ; if they remained however a few hours the deformation tended to be a persistent one and disappeared again only in the course of several hours; a proof that an inter- nal adaptation to the new form had already commenced, but which was in its turn caused to disappear in the course of further development, perhaps by the action of growth forces inhibited during the deformation but re- sumed upon the restoration of the normal form." 62 Roux gives this dynamic elasticity of development the name "mechanism of self regulation." Let us note again that the absence of this elasticity in adult organisms, which remain plastic in relation to the somewhat persist- ent, deforming influences of the environment, would de- note that this mechanism is active only during embryonic life. Now the continued action exercised by the central zone of development constitutes precisely such a mechan- "Wilhelm Roux: Zur Orientierung iiber einige Probleme der embryonalen Entwicklung. Zeitschr. f. Biol. Bd. XXI. Miinchen, July 1885. P. 515—516. Gesamm. Abhandl. Zw. Bd. P. 245. 96 Hypothesis of Structure of Germ Substance ism of self -regulation, active during the whole of onto- geny but ceasing upon the completion of development. Another example of the dynamic elasticity of devel- opment, no less characteristic in certain respects than the preceding, has been repeatedly observed by Roux in the postgeneration of his half embryos. "In the postgenera- tion of the mesoblast it can be observed that very young yolk cells with nuclei not yet stainable, and also the re- mains of substances not yet cellulized, hinder the differen- tiation, and so divert the mesoblastic formation toward the interior or divide the formation into two layers; but after the circumvention of this obstacle the further dif- ferentiation soon resumes its normal course ; a procedure in its essence extremely puzzling." 63 It may be merely noted here that this elasticity of de- velopment helps to explain the interpolation of certain newer ontogenetic formations or stages (placenta and similar things) in the series of older ontogenetic stages, without markedly altering the earlier or later members or even the last member of this ancient series. "We have reason to believe/' says Orr, "that the man- ner of growth for some particular period of the develop- ment may be secondarily changed without radically affect- ing either the preceding or succeeding growth. As an ex- ample of this may be mentioned the embryonic organs and embryonic modifications which adapt the embryo to undergo partial development in the body of the parent, and allow it to receive nutriment from the parent, e. g. the placenta." 64 esWilhelm Roux: Uber die kiinstliche Hervorbringung halber Embryonen usw. Virchows Archiv. Bd. 114, October 1888. P. 276. Gesamm. Abhandl. Zw. Bd. P. 504. "Orr: A Theory of Development and Heredity. New York, Macmillan, 1893. P. 210, Temporarily Disturbing Action of Yolk Mass 97 These embryonal organs and modifications which in- terpolate themselves in the series of ontogenetic stages, leaving these latter unchanged, can then also serve as proof that developing organisms are elastic but not plas- tic, while contrariwise grown organisms remain plastic but not elastic. To these facts one can add that the large accumulation of yolk in the egg cells exerts a great influence on the first stages of development, but subsequently exerts abso- lutely no influence on the other stages. "The organiza- tion of the egg," says Hertwig, "which depends on the disposition of the deutoplasm, has fundamentally only a subordinate influence, and that of a secondary and tran- sient nature in the developmental process." "Eggs of ani- mals which belong to different races can present a very similar type of cleavage and similar early embryonic forms, while eggs from closely related divisions of one and the same race divide in very different ways, and dif- fer very extraordinarily in the nature of the blastula and gastrula. The deposition of yolk material in the egg im- prints a quite characteristic stamp upon the first embry- onic stages, — the cleavage process, the blastula, gastrula and so on, — but it has no influence on the essence of the animal species itself, nor on the formation of any special species of animal." 65 One has here then developments, which, altered in the first stages by the influence exerted by the yolk mate- rial, later resume their normal course, exactly as though they had undergone no alteration. In other words the yolk substance alters the normal development only tem- porarily, only for so long as its action continues to make "Oscar Hertwig : Die Zelle und die Gewebe. II. P. 265—266. 98 Hypothesis of Structure of Germ Substance itself felt. This process corresponds essentially to that described in the above mentioned researches on the trans- position of the blastomeres, in which the latter were com- pressed for example between two plates and so all com- pelled to lie in the same plane. But when the pressure ceased they resumed at once their normal disposition. Each of these two processes constitutes another proof of the self-regulating capacity or elasticity of devel- opment which finds in centroepigenesis its most simple explanation. Centroepigenesis implies further, as we have seen, that the distribution of nervous energy in each stage of de- velopment forms in itself a system in complete dynamic equilibrium, which becomes disturbed and replaced by an- other system in equilibrium, only through the activation by the central zone of a new specific potential element. This is the conception from which as a starting point we have built up our hypothesis. It follows that if the activation of the specific potential elements successive to any given stage is prevented through certain abnormal circumstances, development will stop without thereby causing the organism thus remain- ing behind in an earlier ontogenetic stage, to cease to form a dynamic system in complete equilibrium. Such transitory or permanent arrests of development are extremely numerous, much more numerous than com- monly believed. All the phenomena called atavistic rever- sion belong in this category. Metamorphoses also, with the exception of certain characteristic and remarkable phenomena which have been added later, are only similar arrests of development, which proceeds at once on its course as soon as external conditions, and with them also Atavism and Variation in Crosses 99 the conditions within the organism, become again favor- able to further development. As a typical example of these arrests of development may be mentioned the well known case of the aquatic salamanders, (newts). These tailed amphibians at a cer- tain stage of their ontogeny take to the land, lose their gills, and become accustomed to respiration by lungs. If however they are prevented from doing that by impris- onment in a closed aquarium, they retain their gills, and the triton is halted for life at a low stage of development, which its near relatives, the perennibranchs never pass. The hypothesis of centroepigenesis, which has thus been derived in its entirety from the fundamental bio- genetic law taken in its first degree of approximation, that is in the sense of an exact repetition of phylogeny by ontogeny, implies also that in two species arising from a common ancestor, the series of specific potential elements remain the same up to the ontogenetic stage corre- sponding to this common ancestor, and only after this stage do the series of elements concerned in the two spe- cies diverge from one another. It follows that in crosses development can go on very well so long as the two series of germinal elements are identical, but it becomes hindered as soon as the elements concerned, which strive to become active at the same time, thwart one another by their difference. And through this hindrance to development the organism will take on a form similar to that of the common ancestor. Further a few germinal elements too feeble heretofore in relation to the others, and therefore unable to become active during the development of organisms of the pure strain, are able, if common to both races, to acquire by their union a pre- ponderance over the others different in the two species, ioo Hypothesis of Structure of Germ Substance and so to secure the energy necessary to their activation, and thus to bring out in the cross certain characters of the ancestor which otherwise would not be found in the existing species in any of its ontogenetic stages. In this way then, by the arrest of development at the ontogenetic stage at which the respective germinal ele- ments of the two species begin to diverge from one an- other, can be explained in the most direct possible manner the above mentioned phenomena of atavistic reversion which all hybrids present. "The offspring of a cross of two such species," writes Orr, "might therefore continue its development so long as the two inherited impulses were alike, but when the im- pulses begin to impel growth in opposite directions, de- velopment must cease. This explains why the imper- fectly developed offspring of a crossed species resembles an ancestral form." 66 For example the distinct, colored, transverse stripes on the foreleg and shoulder of the mule, which in the horse and the ass are quite rare and usually very faint, arise in this way and must be referred to the common an- cestor of both species. From the crossing of certain races of pigeons arise birds which have the slate colored plu- mage of the wild dove, even though the races concerned in the crossing possess quite a different color. But it has been proven that these races branched off directly from the wild races. In the same way the mixed breeds of do- mestic ducks recall the wild ducks. And the hybrid of a German and Japanese pig is quite similar to a wild boar. The hybrids of Datura ferox and Datura laevis regularly have blue flowers instead of the white of their parents; "Orr: A Theory of Development and Heredity. P. 230 — 231. 'Activation of Last Element Ends Development 101 and Darwin proves that this is a reversion to a blue flow- ering ancestor. The tendency to incubate, which domes- tic hens so often lose, always appears again in their hybrids. The hybrids of ducks show a tendency to mi- grate. The mule is harder to break thoroughly than the horse or the ass.67 These examples afford, in our estimation, the most certain proof that the ontogenetic stimuli of two species arising from a common ancestor must remain alike dur- ing a long series of earlier developmental stages, and only in later stages begin to diverge from one another. And this is just what the centroepigenetic hypothesis implies, but what no other hypothesis has yet been able to explain. Further the hypothesis of centroepigenesis teaches us that the series of like germinal elements must be shorter the farther removed the species are from the common ancestor. Now Morgan as is well known has obtained hybrids in which, for example, eggs from Asteria were fecundated by sperm from Arbacia, which belongs to the genus Echinus. The two parent forms belong here not only to two different genera but also to two different classes. But these hybrids have never got beyond the larval form, the pluteus which represents only one of the first stages of ontogeny.68 The hypothesis of centroepigenesis, finally, regards development as completed at the moment when all the germinal elements have achieved activation. We note that the central zone is then no longer required to employ its acquisitions of nutritive material for the growth of its 67Darwin: Animals and Plants under Domestication. II. P. 13— 21 : Crossing as a direct Cause of Reversion ; P. 254. "Morgan: Experimental Studies on Echinoderm Eggs. Anat Anzeiger, Bd. IX. No. 5 and 6; Dec. 23, 1893. P. 151—152. IO2 Hypothesis of Structure of Germ Substance entire mass or for the restoration of the masses of any of its specific elements, as at the time when these latter were being used up in proportion as they became activated. And perhaps this explains also why the sex cells, which according to our hypothesis form only the container for the germinal substance given off by the central zone, usually become "ripe" only at the end of development. When the continuous activation of new specific po- tential elements ceases, the disturbing influences exercised by the central zone upon the dynamic equilibrium of each ontogenetic stage will cease also. And thus the organism arrives at the final equilibrium of the adult condition. But now the functional stimulus in the widest sense of the term can come into play, with the innumerable variations possible for it, as new causes of perturbation. So just as formerly the perturbing influence of the central zone upset the just formed equilibrium, and there- by provoked a transition to the next ontogenetic stage, so now each persisting alteration of the functional stimu- lus disturbs the dynamic equilibrium of the adult condi- tion and thereby causes also a different distribution of the general nervous energy. Through each cell of the entire organism, or of definite portions of the organism, there will consequently flow a nervous current specifically dif- ferent from that present before, and also specifically dif- ferent from one cell to another. There is formed and deposited therefore in the nu- cleus of each of these cells a particular specific potential element, which will add itself to the element or elements already present. But all the elements, the new as well as the old, which are deposited in the somatic nuclei will be lost with the death of the individual ; and only those will be preserved from annihilation, which have been depos- How New Elements Are Deposited 103 ited in the nuclear substance of the central zone. The permanent change of the functional stimulus will thus have as its result, in so far as the species is concerned, only the simple addition of a new specific potential ele- ment to the germ substance. We must therefore now study in what way this new element behaves during the ontogenesis of the succeeding organism. But this investigation will form the object of one of the next chapters. After we have thus brought to a conclusion this short review of the most important processes, which according to our view, if they do not exactly prove the centroepi- genetic hypothesis, yet make it most probable, we now pass to the following chapter, which will afford as stated still another proof, even though a quite indirect one, for this hypothesis. For in showing that while a whole series of facts compels us to reject simple epigenesis, and a whole series of other phenomena compels us to reject preforma- tion, it will make it seem very probable that a hypothesis which is able to bring both series of phenomena into accord must come close to the truth. CHAPTER FOUR PHENOMENA WHICH REFUTE SIMPLE EPIGENESIS ; AND PHENOMENA WHICH REFUTE PREFORMATION. INAD- MISSABILITY OF A HOMOGENEOUS GERM SUBSTANCE; AND INADMISSABILITY OF PREFORMISTIC GERMS. I. Phenomena Which Refute Simple Epigenesis Roux designates, with the expression "self-differenti- ation'' of a certain part of the organism, the process in which, according to a certain hypothesis, "the cause of whatever is specific in the differentiation of that part lies within this latter." And he calls "dependent or correla- tive differentiation" the opposite process, in which, ac- cording to other hypotheses, whatever is specific in the alteration which goes on in a certain part of the organism during development is determined by causes lying outside this part.69 If an ontogeny consisted only of self-differentiations, we should designate the development as evolutionary. If on the contrary, an ontogeny were produced only through dependent differentiations, we should call that a process of epigenetic nature. "Wilhelm Roux: Die Methoden zur Hervorbringung balbcr Froschembryonen und zum Nachweis der Beziehung der ersten Furchungsebene des Froscheies zur Medianebene des Embryo. Anat. Anzeiger, Bd. IX. February 1894, P. 277 — 278. Gesamm. Abhandl., II, P. 978. 104 Evolution and Epigenesis 105 We note that theoretically a mixed or intermediate hypothesis would be conceivable, according to which a given part of the organism would be differentiated through the cooperation of causes lying within and with- out it. In case however the causes lying at any moment of ontogeny within the part concerned, do not arise through any antecedent process of epigenetic nature, the develop- ment at least up to this time must be considered as essen- tially purely evolutionary. But if, on the contrary, the internal causes do arise through an antecedent process of epigenetic nature the whole development would then be essentially of that nature. Whitman states that the conception of modern evo- lutionists differs essentially from that of the earlier ovists and spermatists ; for they excluded the formation of new structural parts during development, a thing which is nat- urally admitted by the evolutionists of to-day. Accord- ing to Mivart's definition which Whitman accepts com- pletely, "the term evolution may be employed, as it has been, to denote that the successive formation of parts pre- viously not existent is due not to their imposition from without but to their generation from within." 70 According to this definition which is essentially iden- tical with that of Roux above cited, evolution, it may here be repeated, limits to a minimum the influence which the various other parts of the organism exert upon the development of each part, or considers it as absolutely non-existent, since each part contains within itself, or in any event in its immediate neighborhood, the causes of its progressive development. According to the epigenetic "Whitman: Evolution and Epigenesis. Biol. Lect. at the Mar. Biol. Lab. of Woods Holl, Summer Session 1894. Boston, U. S. A., Ginu, 1896. P. 224. io6 Phenomena Refuting Simple Epigenesis hypothesis, this influence is, on the contrary, of the very greatest importance and is considered to be the only cause of each development. We also can accept Mivart's definition in this sense. We note that it does not include in any way the con- ception of preformistic germs; for it is possible that the internal causes involved arise gradually in the course of development and need not be already present in the germ substance. In the first case, one has evolution without preformistic germs; in the latter, evolution with pre- formistic germs, which we would call pre formation proper. This pre formation proper, for example Weis- mann's type, is also included in the definition of evolution just given; it forms however only a special case of it, which is limited and approximates more the conception of pre formation which the ovists and spermatists enter- tained. The processes of epigenetic nature can be regarded likewise as belonging to two kinds, corresponding to the above mentioned categories of evolution. For one can conceive of processes of epigenetic nature both with pre- formistic germs and without preformistic germs, and both cases are actually met with. In the first case the causes which bring about each specificity of development would be already present in the germinal substance. Only their liberation or acti- vation in opportune time and place depends upon the reciprocal action of the different parts of the organism upon one another (for example DeVries, Oscar Hertwig, etc.). In the second, on the contrary, the causes pro- ducing the different specificities of development arise only gradually in the course of ontogeny, and always in con- Theoretically Conceivable Kinds of Epigenesis 107 sequence of the reciprocal action of the different parts of the organism upon one another. We will call the first of these processes epigenesis with preformistic germs, the other, epigenesis without preformistic germs, or epigenesis proper. Further, each of these two processes can, theoretically, be divided again into the two following categories. One can conceive of the reciprocal action of the dif- ferent parts of the organism upon one another, as such that no part whatever should ever be considered different in any way, in so far as its formative action on the other parts is concerned, from these other parts, but rather all are to be regarded as equally necessary and of equal value in this respect. Or, on the other hand one can suppose that among all the parts there is one whose action upon the other parts differs through some peculiarity from the corresponding action of all other parts, so that it acquires in comparison with the latter much greater importance. We shall designate the first process with the name "simple epigenesis," or briefly "epigenesis/' which would be equally possible either with or without preformistic germs, and we shall call the second in which the form- ative action would on the contrary become specially local- ized in a definite zone of the organism, by the name of "localized or centralized epigenesis," or briefly "centro- epigenesis." Practically it would be conceivable only without preformistic germs. Finally, in all the different theories without pre- formistic germs, one could conceive of the germinal sub- stance as formed of a single homogeneous substance (or a homogeneous mixture of different chemical substances), or of a material which though not consisting of pre- io8 Phenomena Refuting Simple Epigenesis formistic germs, would nevertheless be formed of a greater or less number of specific parts different from one another. Of all these hypotheses which one can form concern- ing the nature of the developmental process and the structure of the germinal substance, we need discuss here only the following chief ones, and consider these only very briefly, mentioning the others only casually in passing. We arrange them in the following way: Concerning the nature of the developmental process : 1. Simple epigenesis with preformistic germs or without such. 2. Evolution with preformistic germs, i. e., pre- formation proper. Concerning the structure of the germinal substance : 1. Germinal substance consisting of homogeneous material. 2. Germinal substance consisting of heterogeneous material. Here belongs the special case in which this substance consists of preformistic germs. We can now pass on without further comment to a rapid review of the most important phenomena, on account of which simple epigenesis with preformistic germs or without such, cannot be admitted. This will oblige us sometimes to return to the phenomena and arguments with which we were occupied in the preceding chapter. With the chief facts which are opposed to simple epigenesis we must now range the production, already mentioned and discussed, of right and left, anterior and posterior, half embryos of frogs, which resulted from the killing by a hot needle of one of the two first Partial Developments 109 blastomeres. As is well known it was these very half embryos that caused Roux to construct his evolutionistic theory, in which he compares development, at least in so far as the four quarters of the embryo come into consideration, with a mosaic work. So long as the half formations arising from isolated blastomeres are limited to the very first divisions, so long for instance as one of the first two blastomeres, when isolated, limits itself to giving half of the total number of micromeres, or so long as the first cleavage spheres arising from the isolated blastomeres succeed each other and arrange themselves as if the two blastomeres had remained united, so long there is still nothing to be seen in these phenomena which would afford any proof against simple epigenesis. For in general we can suppose that the deutoplasm alone is the immediate cause of the num- ber, and of the different relative sizes and disposition of the first blastomeres. If then the relations to the yolk of the blastomere and of the whole of the blastomeric group, could not by themselves constitute any proof change through the isolation of the blastomere, it is clear that the first cleavages must proceed exactly as though no isolation whatever had taken place. So for example the isolated blastomeres of the two or four cell stage of the egg of the gastropod, Ilyanassa obsoleta, which divide in essentially the same manner as they would if they were part of the complete blastomeric group, could not by themselves constitute any proof whatever for or against any given developmental theory, so long as the separated blastomeric group does not take on any really specific form. For in this Ilyanassa obsoleta the yolk is distinguished by its great mass, thickness and no Phenomena Refuting Simple Epigenesis density.71 And these peculiarities make it certain that the relation of the isolated blastomere to the yolk plasm is not different from that which would have existed had it remained united to the other blastomere. But the preponderance of the determinative action of the yolk is limited usually to the early stages preceding the gastrula. It has no influence whatever on the final form of the embryo, no more than has for example, the temporary compression of the blastomeres between two plates or the shuffling of them. Therefore the early cleavage stages have no specific morphological signi- ficance, as is evident also from the above mentioned fact, that different related species and even quite widely separated species can present almost identical cleavage systems.72 It follows from this that as soon as development commences to take on its really specific form, that indi- cates that the action of the germ substance is preponder- ating over the action of the yolkplasm no matter in what way the latter may formerly have acted. Consequently simple epigenesis certainly cannot have recourse, in order to explain the half embryos of Roux, to the fact that the deutoplasm remained unchanged in the unsegmented blastomere. For these half embryos arise at very advanced stages of development and represent quite specific formations. Nevertheless Driesch seems to want to explain the half- formations in this way: "Each particle of the sur- 71H. E. Crampton, Jr. : Experimental Studies on Gasteropod De- velopment. Arch. f. Entwicklungsmech. d. Organismen. Bd. 3; Heft. i. Leipzig, Engelmann, March 24, 1896. 72Cf. E. B. Wilson: The Cell-lineage of Nereis. Journ. of Morph., Vol. VI, No. 3. Boston, U. S. A., Ginn, July 1892. P. 455. Certain Regenerations ill viving half preserves as is shown by Roux's figures, the position which it would have had in normal development. Then after segmentation has taken place, the same form- ative factors act on each particle and on each blastomere respectively, which would have acted upon them in nor- mal development, consequently also the same forms result ; Ergo : half-embryo." 73 From the epigenetic standpoint this explanation is inadmissible. For when the half embryo begins to take on the characteristic form of its species, and thereby indicates as we have seen that the specific action of the germ substance has from that time become preponderant over that of the deutoplasm, one could not assert that the same organ forming factors continue their action, for that would be to deny that there is any formative action at all exerted by the idioplasmic nuclear substance of one entire half, left or right, anterior or posterior, upon the other, developing half. This would be exactly the op- posite of what simple epigenesis postulates, for it attributes the tendency of development to take on its specific form of equilibrium to the reciprocal action of all the innumerable little masses of one and the same idioplasm, which are active at the same time in all the nuclei of the entire organism. Roux then can rightfully assert that the half embryos constitute by themselves the most direct and decisive refutation of the theory of epigenesis. If we pass on now from half embryos to the regen- eration of amputated organs, we know that this con- stitutes one of the most important arguments that the epigenesists ordinarily advance against preformation. "Driesch: Analytische Theorie der organischen Entwicklung. Leipiz, Engelmann, 1894. P. 15 — 16. H2 Phenomena Refuting Simple Epigenesis But the preformationists on their side cite certain partic- ular cases of regeneration as unfavorable to epigenesis: "Regeneration" remarks Roux, "takes place in tritons when all four extremities are removed at one time, from which it follows that for the formation of new extremities in one antimere, the presence of the other extremities is not in the least necessary, so that for this formation it is not necessary that there be any formative correlating influence extended from them." 74 The anachronisms of development in which, for instance, certain parts remain behind other parts in their formation, or in which the germ layers may even develop with uneven speed, or one entire half of the body may take a jump ahead of the other half so that one can sometimes observe two different degrees of development in the two halves of the same embryo, belong likewise to the number of phenomena which simple epigenesis is incapable of explaining: "How the (epigenetic) con- ceptions of O. Hertwig," Roux writes further, — and his words, already quoted above, deserve to be repeated here, — "can be reconciled with these anachronisms in the development of the germ layers which I have observed, or indeed with the absence of the lower layer — the endo- blast (Anentoblastia), while both of the other two layers remain essentially normal in the disposition of their parts, or finally with the formation of half embryos, may well be left to the reader's own judgment. For if such large parts can remain behind in their development, or indeed be lacking altogether, and the other parts be in no wise disturbed thereby in their development, it surely follows that the development of these latter is not con- 74Wilhelm Roux : Uber Mosaikarbeit etc. Anat. Hefte, Febr. 1893, P. 299. Gesamm. Abhand. II, P. 839. Headless but Otherwise Normal Monsters 113 nected by reciprocal actions with the absent parts, and therefore is not carried on by the reciprocal action of all the parts of the whole, one upon another." 75 It is the same with headless monsters as with all monsters which lack entire parts of the organism but are nevertheless normal in the other parts. Because they show likewise that there does not exist any formative action exercised by the head, or by other parts, upon the rest of the organism. While thus the head can be absent in development, the presence of certain other parts seems on the con- trary to be indispensable in headless omphalosite mon- sters: "When one studies," writes Dareste, "headless, omphalosite monsters comparatively, one notes that the trunk is aknost complete in some cases but in others incomplete. And upon this fact is based Isadore Geoffrey Saint Hilaire's division of headless monsters into three different types: the true acephali, in which the thoracic region is as well developed as the abdominal region; the paracephali, which have only the abdominal region; and the mylacephali in which only the sacral region is pres- ent. These three types arise through inequalities in the development of the cerebro-spinal axis. But how is it that the posterior part of this axis is always present, while the anterior part is lacking to a greater or less extent ? Why does not the reverse appear in other cases ? This depends evidently on some as yet unknown fact of embryogeny. For the present we must be content with the mere question." 76 "Wilhelm Roux: Uber Mosaikarbeit etc. Anat. Hefte, P. 320. Gesamm. Abhandl. II. P. 859. "Dareste: Recherches sur la production artificielle des mon- struosites. Paris, Reinwald, 1891. P. 495. 114 Phenomena Refuting Simple Epigenesis Nevertheless some other varieties of omphalosite monsters seem to show that the presence of any part whatever of the vertebral axis is sufficient to permit at least partial development; for instance in the cephalic monsters the embryo consists generally of the head alone.77 We note however that these monsters com- monly contain the anterior extremity of the spinal cord which can have been only slightly differentiated in the embryonic stage at which the incomplete development is arrested. In some of these cephalic omphalosite mon- sters, a large part of this anterior extremity of the spinal cord may even have undergone a process of reabsorption after the previous arrest of the partial development. As we shall see soon Born succeeded in producing artificially a thing like these cephalic omaphalosite mon- sters by grafting upon a complete tadpole a piece removed from another tadpole, and consisting only of the head and a small part of the elongated medulla. Concerning the double monsters with double sym- metry, it will be worth while to repeat once more in extenso the following statement of Roux, even though we have reported it already, for the most part, in the preceding chapter: "This additional fact speaks directly against the achievement of development of the individual through a general, reciprocal, formative cooperation of all parts to form a whole; namely that in the chief class of double monsters, and so in those double formations which cor- respond to the law which I formulated of the double symmetry of the anlagen of organs, the piece absent in a symmetrically similar way from each of the two "Dareste : Recherches sur la prod, artif. des monstr. P. 498. Connected Partial Embryos of Roux 115 individuals may actually be any given piece whatever that is limited by a plane surface; and that in them the organs are nearly all present in their normal form up to the plane of reunion, just as if two symmetrical pieces had been cut away so as to leave two plane surfaces, from two twins, after they were fully developed and ready for birth, and the foetuses had then been united by the cut surfaces. This normal formation of defective organs up to any given plane of separation as, for example, the 8-shaped double cornea or double lens of the third eye common to both organisms, speaks likewise strongly in favor of a capacity of self-differentiation possessed even by parts of these organs, as the simul- taneous development of two structures united so exten- sively, to form bodies of which each is self centered, indicates directly the absence of the action of general reciprocal influences, connecting them into one whole." 78 We remark nevertheless in our turn that the evolu- tionistic theory does not in any way explain as Roux asserts, how the two organisms are limited by a plane surface which is perfectly symmetrical, rather than by any kind of irregular surface whatever. This theory merely shows that it is possible that the development of organs which differentiate themselves automatically, may be arrested at any given surface, without thereby dis- turbing the normal form of any of the remaining portions, not even in the neighborhood of the surface where development is arrested. But it does not explain why the surface of division must be a plane surface, and perfectly symmetrical in the two individuals. One should rather expect here a manifold reciprocal inter- "Wilhelm Roux: Uber Mosaikarbeit etc. Anat. Hefte, P. 320. Gesamm. Abhandl. II. P. 859—860. n6 Phenomena Refuting Simple Epigenesis locking of the two organisms which would give a most asymmetrical and irregular dividing surface. The continuation of development in the tail frag- ment of the tadpole seems to speak likewise against both epigenesis and preformation. For in his experiments upon tadpoles, Born has proven the accuracy of Vulpian's statement that the amputated tails not only continue to live for sometime (some even thirteen days), but con- tinue to grow and to differentiate into their various tissues. He has further observed the following processes of new formation. "A few days after the amputation, the margins of the fin of the amputated tail commence to grow over the cut surface of the axis ; they unite in front of it to form a high semicircular tail fin. The axis is not entirely without participation in this process, for prolongations of the notochord as well as of the spinal cord grow into the newly formed fin, but the metameric muscu- lature shows nothing like this and terminates sharply at the original surface of amputation. But this prolonga- tion of the notochord, like that of the spinal cord, even in the most favorable instances, scarcely extends half as far forward in front of the cut end of the original axis as does the newly formed marginal fin. This latter is formed of typical embryonic mucous connective tissue with a few pigment cells scattered through it. I have not been able to discover in it any rudiments of vessels." "This observation teaches then," continues Born, "that the provision of yolk in the tail end cut off from a tadpole does not serve merely, as Vulpian has already shown, for further growth, and further differentiation of the tissues, and for the formation of a new structure growing out from the cut surface, but it shows that Growth of Amputated Parts and Grafts 117 besides the fin margin, the notochord and the spinal cord take part in this new formation. It is interesting that the tail of a tadpole is capable of such a regenerative new formation not only in the caudal direction but also in the opposite direction." 79 We say then that these phenomena, which the ampu- tated tail of the tadpole presents, can be cited by the pre- formationists against the epigenesists, as well as by the latter against the former. For the former can object that the progress of the histological differentiation in the fragment of tail cut off from all the rest of the organism would denote the absence of any action of the organism upon the development of this part of the body, and the epigenesists on their side could show that the power of the tail to regenerate even in the direction from the tail toward the head, could not be explained by pre formation, even with the aid of reserve idioplasm, for that could effect regeneration only in the direction from the head toward the tail. It is the same with Bern's celebrated experiments on the grafting of certain fragments of tadpoles upon one another or upon complete tadpoles, which are opposed to simple epigenesis and at the same time indicate a process of epigenetic nature. In the first place they are opposed to epigenesis. For in all grafts of parts of tadpoles upon complete tadpoles, the grafted parts have continued their development regu- larly as if they had remained united to their own organisms. Therefore the rest of this organism has not under normal conditions any influence" upon the development of these portions. 7BBorn: Uber Verwachsungsversuche mit Amphibienlarven. Leipzig, Engelmann, 1897. P. 32 — 33. n8 Phenomena Refuting Simple Epigenesis Thus for example a larva of Rana esculenta from which there had been cut off the most anterior part of the head including the eye anlagen, the anterior part of the brain, the nasal groove, and the primitive mouth, was so grafted upon the caudal half of the abdomen of a complete larva, as to form an acute angle between the back of the former and the abdomen of the latter, the abdomen of the former being turned toward the head of the latter. After allowing the double larva twelve days of development, it was killed and it showed that "all the organs of the partial larva up to the surface of amputation and union had developed quite as com- pletely as though there was no part of them lacking and as though their normal environment and their or- dinary relations were quite undisturbed." 80 The anterior portion of a larva so short that it scarcely extended beyond the commencement of the elon- gated spinal cord, was grafted upon the abdomen of a complete larva, and continued to develop normally. "All parts developed completely up to the surface of amputation: the cartilaginous trabeculae, the quadrates with the chewing muscles covering them, behind the mouth cavity the cartilages of Meckel, the cartilages of the lower jaw and behind these again the hyoids." 81 Upon these and other similar examples, Born bases the following conclusions : "Although up to the moment of grafting there had been no trace (of primordial cra- nium) present, and the mesoderm from which it develops remained in a quite indifferent and almost primitive con- 80Born : Ubcr Verwachsungsversuche mit Amphibienlarven. P. 97. 81Born: Uber Verwachsungsversuche mit Amphibienlarven. P. 108—109. Nuclear Somatization 119 dition, nevertheless the complicated and characteristic parts of the head were developed up to the surface of amputation completely and in their perfect form, and not only entire structure but also parts of these structures." "Whatever development there is going on beyond the stage at which amputation was performed depends essentially on self-differentiation of the individual parts; no correlative influence of the neighboring parts or o'f the entire organism can ever be recognized, either nega- tively or positively. Thus this development beyond the stage at which amputation was performed, corresponds entirely with Roux's mosaic theory." 82 Nevertheless, we shall see soon that another whole series of Bern's experiments, as also those just recorded if one considers them from another point of view, are no less opposed to evolutionary hypotheses in general and to hypotheses of pre formation properly so called in particular. In short, the observations and experiments which we have thus far cited, from the half-embryos of Roux to the tadpole fragments of Born, all show the possibility that individual parts of the organism, provided they con- tain any part whatever of the vertebral axis, can develop independently of the remaining parts, and are sufficient by themselves alone to prove the inadmissibility of simple epigenesis. But the preformists had yet another fundamental objection to make to the epigenesists, who have sought so far in vain for a reply to it : namely that epigenesis requires the renunciation of nuclear somatization. For these two hypotheses are absolutely irreconcilable. It 82Born : Uber Verwaclisungsversuche mit Amphibienlarven. P. 204—205. I2O Phenomena Refuting Simple Epigenesis follows therefore that each fact or each argument which speaks in favor of nuclear somatization, is at the same time a proof against epigenesis. And we saw precisely in the preceding chapter, that there is a whole series of facts and arguments, which it would be useless to repeat here, but which compel us to admit this nuclear somatization as an incontrovertible fact. The preformationists can finally object to epigenesis and not unreasonably, that by its "attainment of equilib- rium," it does not explain the termination of ontogeny as well as preformation does. For why should the reciprocal actions of all parts, on which development depended up to that time, suddenly cease to effect any further change when once the adult stage is reached? Because it is only then, reply the epigenesists, that the dynamic equilibrium is attained. But if the successive ontogenetic forms repeat the phylogenetic, how comes it that the dynamic equilibrium which once existed in each of these latter does not remain existent in any of the former? And if the absence of equilibrium at all these stages is due to some alteration of the formative living substance, how then could this new substance pass again during a long series of stages through the same phylogenetic ancestral forms? The preformists, on the contrary, have no trouble in explaining the arrest of development since according to their theory it would follow only at the moment when there would be present in each cell only a single kind of preformistic or determinant germs. Having thus made a rapid review of the principal objections which compel us to reject epigenesis, we can pass on to an equally brief consideration of a number Complexity of Preformistic Germ Plasm 121 of objections which stand in the way of admitting pre formation. 2. Facts Which Compel the Rejection of Preformation If, limiting ourselves to the most typical theory of pre formation to which all the others can be finally reduced, we consider that of Weismann, we encounter at the outset a very simple argument which is yet so formidable that it is in itself enough to discourage the most firmly convinced partisan of that doctrine. For the preformation theory of Weismann forces him to suppose for the infinitely numerous particles constituting the different determinants or groups of determinants, an excessively complicated architecture or excessively complicated arbitrary mode of disposition. Now the elementary fact of reproduction demonstrates that the constitution of the germ plasm, whatever it may be, does not become at all altered when this latter divides and distributes itself among the incalculable number of germ cells which can be produced by each organism and by all its succeeding generations. Weis- mann must first then explain to us how the subdivision of a given germ plasm into new parts in each of which this very complicated structure is preserved uninjured or is accurately reproduced again, is possible. That is fundamentally the same difficulty which the old ovists and spermatists encountered, and which they endeavored to overcome by their idea of the encasement of the germs.83 Weismann has since endeavored to weaken the force 83Among others compare e. g. Oscar Hertwig: Praformation oder Epigenese? P. n. 122 Facts Compelling Us to Reject Pre formation of this objection by taking into consideration, besides the architecture of the germ plasm itself, also the uneven rapidity of multiplication of the different determinants and the reciprocal forces of attraction exercised by these latter upon one another, as factors which determine the orderly division of each plasm or each id.84 But, as this author himself admits, the architecture of the germ plasm remains necessarily the principal factor, and conse- quently the objection of the incompatibility of this arbi- trary architecture with constant complete division of the plasm without alteration, remains in all its force. Another argument which presents itself likewise against preformation is, that with the exception of the partial developments cited in the preceding section of parts which each contained one very definite zone of the organism, it has never been possible to obtain the devel- opment or the continuation of development of somatic parts, even though they are capable of living for some time after they have been detached from the rest of the organism. One would certainly not regard as a continuation of normal development the mere increase in mass which takes place in parts cut off from the fetal organism, when they are transplanted upon tissues which on account of their great richness in blood vessels are especially capable of affording abundant nourishment to their new guests. This simple increase in mass depends for the most part on an actual multiplication of the respective cells, which proceeds in directions determined either by nutri- tion alone or by the path of least resistance in the environ- 84Weismann: Das Keimplasma. Eine Theorie der Vererbung. Jena, Fischer, 1892. P. 86. Transplanted Parts Soon Cease to Develop 123 ment, accompanied either by no morphologic alterations or by quite aspecific ones, depending upon whether the portion cut away consisted of a formless fragment of tissue, or of an organ whose proper form was already indicated. We may mention, for example, Zahn's trans- plantations of portions of cartilaginous or bony fetal tissues to the lungs and kidneys of other individuals of the same or different species,85 or Fischer's transplanta- tions of anterior and posterior extremities of chicken embryos (especially of one incubated only eleven days) to the comb or ruff of the cock.86 It is true that both, and especially Fischer, have observed that in these extremities of chicken embryos, ossification, which at the time of amputation had not commenced at all or had scarcely commenced, was initiated or continued in the transplanted extremities.87 But this process of ossification can be considered only as the mere accumulation, and consequent intensification, of the effects of the specific vital activity which was already at work before the amputation, and which persists unaltered after the transplantation. Consequently we think that Roux is quite wrong when, apropos of these experiments of Zahn, Fischer, and others, he expresses himself as follows: "These experiments have demonstrated that many isolated embryonic parts can not only grow but even become "Zahn : Uber das Schicksal der in den Organismus implantierten Gewebe. Virchows Archiv, Bd. 95. Drittes Heft, 5. March 1884; especially e. g. P. 374—375, 380, 381. "Fischer: Uber Transplantationen von organischem Material. Deutsche Zeitschrift der Chirurgie, Bd. 17. Erstes, Zw., Dr. u. Viertes Heft, 1882; especially e. g. P. 362—363, 370—371. "E. g. Zahn: Uber das Schicksal etc. P. 382ff.— Fischer : Uber Transplantationen etc. P. 370, 374. 124 Facts Compelling Us to Reject Pre formation histologically differentiated in an approximately normal way. It follows that the differentiation of these parts is not a function of reciprocal actions between these parts and other parts. Therefore there is thus already proven a certain histologic and morphologic self -differ- entiation of many parts of the developing egg." 88 This is not correct. For these experiments show, we repeat, only a mere increase in mass of these tissues, which is morphologically without any specific character; and the continuation of the histologic differentiation which had already commenced or was on the point of commencing at the moment of amputation, is explicable by the simple accumulation of the effects of the same vital process which merely persists exactly as it was before the amputation. Another argument against pre formation is the great capacity of modification of the organism while it is undergoing development, as well as when fully developed, to which it owes its remarkable power of adapting itself to quite abnormal conditions. For the preformation theory with its determinants, which are bound up with one another into a solid structure, and of which each determines the formation even of the smallest particles and their most minute variations, implies undeniably a great morphological rigidity, which is not reconcilable with the great mutability of the organism. "Galls," says Oscar Hertwig, for example, "are valuable witnesses against the germ theory of Weismann. They teach us that cells of plant bodies can serve quite other purposes than could have been foreseen during 8ftWilhelm Roux : Zur Orientierung iiber einige Probleme der embryonalen Entwicklung. Zeitschrift fur Biologic; Bd. XXI. July 1885. P. 480—482. Gcsammelte Abhandlungen. II, P. 206—207. Adaptability and Alterability of Structures 125 development, that they can adapt their form to new conditions, and that their specific form is not determined by special determinants in the nucleus but by external stimuli." 89 The stomach of the tern, which ordinarily feeds on fish, is lined by a soft mucous membrane. If one feeds it with wheat for a few weeks its stomach develops a superficial horny coat, its musculature is strengthened and it takes on the character of a gizzard.90 If these stomachs belonged to two varieties of the same species, Weismann would have no hesitation in attribut- ing the diversity to special, and thus different, determi- nants which, as the facts show, do not really exist. Loeb has demonstrated that the colored design of the yolk sac of a fish embryo (Fundulus) is not in itself predetermined, but depends upon the distribution of blood vessels. The pigment cells are at first distributed uniformly but when the circulation of the yolk sac is established, they migrate toward the vessels, attracted probably, as Loeb supposes, by a chemical substance in the blood, and give rise thus to a definite design. Graf has likewise recently demonstrated that the color designs of the leech are not themselves inherited, but that they depend upon the disposition of muscle fibers in which the amoeboid pigment cells lie. It would be absurd, concludes Wilson, to imagine in all of these cases a special series of determinants for each individual color design.91 89Oscar Hertwig: Zeit- und Streitfragen der Biologic. I. Pra- formation oder Epigenese? P. 48 — 49. 90Delage: L'heredite etc. P. 604. 91E. B. Wilson : The embryological Criterion of Homology. Biol. Lect. at the Mar. Biol. Lab. of Wood's Holl ; Summer Session 1894. Boston, U. S. A., Ginn. 1896. P. 116. 126 Facts Compelling Us to Reject Pre formation Everybody knows the peculiar static structure of bone. Substance becomes accumulated in bone at the points of greatest pressure, and attains thus its best possible utiliza- tion. Now it is known, as J. Wolff discovered and as Kastor, Martiny and J. Rabe have confirmed, that similar structures are formed also in quite new and abnormal circumstances in connection with new static conditions, for example, in bones broken and reset at an angle. "From this it follows," says Roux, "that these forma- tions do not need to be fixed and inherited, but arise of themselves whenever the conditions exist. As the static structure of bones is developed in a clearly recognizable form only after the first years of life, one can not say anything of the necessarily hereditary transmission of it, without special researches upon this point." 92 Teratogenesis in general, both natural and artificial, is quite opposed to preformation. It denotes that the organism, at any rate while it is still in process of devel- opment, can adapt itself to exceptional conditions which are quite different from the normal. And it accom- plishes this by producing abnormal formations whose development must consequently be due only to a process of epigenetic nature, and cannot be of preformistic nature. Let us consider one of the simplest examples. In the hemiteratic spina bifida, the spinal opening is ordi- narily covered over by a layer consisting of fibrous tissue like that of scars, which in some cases takes on all the characters of the skin. Then the spinal opening is not visible from the outside. But when the spinal opening is in the lumbar region it is not rare for a 82Roux : Der Kainpf der Teile im Organismus. P. 28. Adaptability Irreconcilable with Pre formation 127 considerably developed tuft of hair to appear on the outside.93 Would Weismann also presuppose its own determinants for this tuft? And if this tuft arises without being represented in the germ plasm by its own determinants, why cannot the same process occur in normal development for other parts of the organism? Normal and abnormal development do not differ essen- tially from each other; and the causes which produce them are of the same nature in both. Weismann certainly recognized the great importance of the capacity of alteration by functional adaptation, possessed by both embryonic and adult organisms. "If this principle did not exist," he writes, "the organism would be like a building, of which each stone is already prepared before the situation and use of the building is determined. Such a predetermined ontogenesis could not produce any organism capable of living. The influ- ences under which organisms exist during their develop- ment are never exactly the same and to be able to adapt themselves to them they must possess a certain freedom." 94 But we cannot repeat often enough, that this great capacity of adaptation is absolutely irreconcilable with his theory of determinants, or with any preformistic composition of the germ substance. If functional adapta- tion effects "the adjustment of the primary hereditary anlagen, that is of the determinants, to new circum- stances," 95 this signifies that these new circumstances "Dareste: Recherches sur la production artificielle des mon- struosites. P. 327, 538. "Weismann: The Effect of External Influences upon Develop- ment. The Romanes Lectures, 1894. P. 16 — 17. "Weismann: Ibid, P. 16. 128 Facts Compelling Us to Reject Pre formation external or internal to the individual exert upon these determinants a certain formative action. But if one assumes that a certain formative action becomes thus exerted upon each of the determinants by abnormal cir- cumstances in their development, one must then assume that a similar formative action is exerted upon each of these same -determinants by the other parts of the organ- ism when the development of these parts proceeds quite normally. But what remains then of the preformistic action of these determinants which should fashion the organism like a piece of mosaic-work? The experiments of Born also, which, as we have seen above, are absolutely opposed to simple epigenesis are just as little reconcilable with preformation, because they denote in general the epigenetic nature of the process of growth. We need recall here only the union of portions of different tadpoles ; for example, of the anterior portion of one tadpole with the posterior portion of another. In this latter case, if the anterior portion were limited by a section passing through the medulla oblongata, while the posterior portion had been obtained by a section passing through the medulla spinalis, it would follow that the two surfaces of amputation of the medulla which ought to match exactly would present on the con- trary unlike forms and surfaces. In spite of this a short time after the two ends of the medulla were united, the two half tadpoles having meanwhile continued their development, they showed a union in which no angle or sharp fault persisted but gentle curves of transition were present instead. The two medullary canals went over into one another also gradually and without inter- ruption, so that one could no longer recognize the exact Secondary C captation in Grafts 129 point of their union. "This is an example of the phe- nomenon observed also in all the other organs, that after the growing together of two sections of unequal size and form, the uneven character of the union, present at first, disappears and gradually a smooth connection of the surfaces is established." 96 The union of the corresponding organs of the two fragments is quite intimate: for example in the case which we have just cited, longitudinal sections of the two united medullas showed that the fibers of the white substance of the medulla spinalis went over continuously into those of the white substance of the elongated medulla oblongata.97 Among the experiments of Born, those upon the artificial production of double monsters are especially remarkable. They were obtained by joining two tadpoles together in the most diverse ways. In the case which we now report he cut away from each of two tadpoles the upper part of the ventricles of the brain and joined the two cut surfaces together so that both the tails and the bellies of the two tadpoles constituting the double monster were directed in opposite ways. Here also, there took place, in so far as the two tadpoles developed, a complete union so that after sometime there could not be perceived any transition stage between the surfaces of the corresponding organs which were united together. "It is impossible to believe," remarks Born in this connection, "that in placing these two tadpoles one upon the other, the small ventricular clefts and the external walls of the two brains could be applied exactly one upon "G. Born : Uber Verwachsungsversuche mit Amphibienlarven. P. 53—54- 97G. Born : Uber Verwachsungsversuche usw. P. 56. 130 Facts Compelling Us to Reject Pre formation another." In such cases, which are repeated over and over, there is no escape from the admission that with the progress of growth there has taken place in the blended organs a sort of smoothing out, "Ausglattung," of the external and internal walls, and perhaps even a transitory modification of the normal form, caused simply by the influence of the similar organs growing together.98 In other cases there is more than a simple smoothing out of the two surfaces which would not fit together at first. Thus in the union of two parts of the intestine of double monsters, thoracopagi, gastropagi, and ventro- pagi, obtained by cutting off from each of two tadpoles a thin layer of the abdomen, and superimposing the two cut surfaces as usual, there results an exact conjunction of the two thin-walled intestinal tubes in such a man- ner as to constitute a single tube without any trace of the junction which was made. So exact a coaptation can certainly not be effected by the simple superposition of the two tadpoles." Sometimes the corresponding organs of the two larvae seem as though seeking each other and reaching out to each other. They both deviate from their ordinary direction in order to be able to unite and extend one into the other. This phenomenon appears character- istically in the fusion of the two vascular systems as is demonstrated in the clearest manner by certain experi- ments in grafting definite portions of tadpoles upon complete tadpoles: as for example, the grafting of the posterior heartless portion of a tadpole upon the abdomen of another complete tadpole : the fusion of the two vas- cular systems is so complete that a single blood circulation MG. Born: Ibid. P. 141. "G. Born : Ibid. P. 69—86. Form Not Dependent on Number of Divisions 131 is produced, the heart of the complete tadpole putting in circulation the blood of the grafted portion also.100 This phenomenon is presented also very strikingly in the mutual prolongation one into another of the pronephric and other secretory ducts. Thus in a double monster obtained by uniting the anterior portions of two tadpoles, the left pronephric ducts met and grew together although at first their extremely fine cut ends certainly lay some distance apart and their directions crossed almost at right angles.101 All these phenomena are hard to reconcile with the rigidity implied in Weismann's determinants. They speak on the contrary entirely in favor of a general process of growth that is epigenetic in nature; for only to a process of growth of this nature can be ascribed all the phenomena of adaptation and of deviation from the normal form which result in the complete and exact conjunction of the various corresponding portions of different individuals. Against the rigid preformation of Weismann, which attributes development exclusively to qualitative nuclear divisions, Roux himself furnishes a most appropriate argument which even the most pronounced anti-preform- ists rarely cite. "In the larger animals of the same species the cells are not correspondingly larger than in individuals which in consequence of lack of nourishment have re- mained smaller. Thus the unequal size of the individuals must be associated with an unequal number of cell divisions, which by the method of qualitative differentia- tion assumed by Weismann must lead to a very essential 100G. Born : Ibid. P. 87-88. 101G. Born: Ibid. P. 144. 132 Facts Compelling Us to Reject Pre formation disturbance. It follows that qualitative differentiation can not have any close association with the number of cell divisions, nor indeed with the process of cell division itself, so that it is not possible for any definite qualitative alteration to be so associated with each individual cell division as to produce a definite character in each soma- tic cell of the tenth, eleventh, twelfth, twentieth, and fiftieth generation from the egg cell in consequence of this number of generations." 102 To circumvent this objection to the preformation theory, Roux has recourse to the hypothesis of a self regulating mechanism of frankly epigenetic nature which really amounts to reducing the part played by pre formative processes in ontogeny to a wholly subordinate role.103 But the strongest objections against preformation are on the one hand the above mentioned experiments on isolation of blastomeres, and the production of double monsters from a single egg and other similar ones, and on the other hand the experiments upon regeneration. The experiments upon the isolation of blastomeres which showed that each one could produce an entire individual are, as we have already explained in the pre- ceding chapter, a convincing proof that at least the first nuclear divisions are not qualitatively unequal. Having recourse to a reserve idioplasm implies the renuncia- tion of preformist theories. For in the first place, it really admits that the whole of the idioplasm (active 02Wilhelm Roux : Uber die Bestimmung der Hauptrichtungen des Froschembryo im Ei und iiber die erste Teilung des Froscheies. Sep.-Abdruck aus der Breslauer arztlichen Zeitschrift, 1885, P. 35. Gesamm. Abhandl. II. P. 316—317. 103Wilhelm Roux : Ibid. P. 35, 317. Double Formations 133 plus reserve idioplasm) remains after division just as it was before; in the second place, the conception by which one makes the activation or non-activation of the reserve idioplasm depend upon the abnormal or nor- mal relations of the different nuclei with one another is very similar to that of epigenesis with preformistic germs, such as would correspond somewhat with the hypothesis of DeVries or that of Oscar Hertwig. The formation of double monsters from a single egg constitutes essentially an analogous case to that of the formation of entire individuals from isolated blasto- meres. We mention for example the experiments of Wilson in which following the simple displacement in relation to each other of the first two blastomeres of the egg of Amphioxus, each of those blastomeres pro- duced a gastrula united along a more or less extensive surface with the gastrula produced by the other blasto- mere in such a manner as to give rise to numerous and very varied forms of double gastrulas in which the axes and respective blastopores of the twin gastrulas wrere oriented in the most diverse ways in relation to one another.104 The same thing occurred in the similar double monsters obtained by Oscar Schultze from frogs' eggs, which were produced by compressing the egg be- tween two horizontal plates and turning them over immediately after the first segmentation.105 If these double monsters, — and Roux has remarked 104E. B. Wilson: Amphioxus and the Mosaic-Theory of Devel- opment. Journ. of Morphology, vol. VIII, No. 3. Boston, U. S. A., Ginn. August 1893; P. 591—595. Table XXXIV. 106O. Schultze: Die kiinstliche Erzeugung von Doppelbildungen bei Froschlarven mit Hilfe abnormer Gravitationswirkung. Arch. f. Entwicklungsmech. d. Organismen Bd. I. Heft 2. Leipzig, Engel- mann, 1894. P. 276—284. Tables XI and XII. 134 Facts Compelling Us to Reject Preformation this himself, as we have seen in the case of those with double symmetry, — are opposed on one side to simple epigenesis because they show that between the two organisms, even though they have so great a part of the body in common there do not exist any general reciprocal actions tending to make a single whole of the two bodies, they are on the other side, also opposed to preformation, in that they demonstrate in the same man- ner as do the experiments upon isolation of blastomeres, the equipotency or qualitative identity of the two first segmentation nuclei. And this equipotency is not limited only to the two first but exists also in all the first blastomeric nuclei as is demonstrated by the inverse phenomenon obtained by Morgan of the formation of a single embryo from two blastulas of Sphaerechinus which had grown together of themselves.106 Finally preformation as we have said, is quite irrecon- cilable with all the manifold processes of regeneration without exception. Above all, Weismann interprets in fundamentally the same sense as wre, the experiments and observations of Roux upon the peculiar regeneration constituted by the postgeneration or completion of the half embryos which we have so often mentioned. For they signify as he himself admits, "that this completion took place by a kind of cell infection, of such a nature that mere contiguity, for example with ectoderm cells, caused the as yet undifferen- tiated cells of the side operated upon to become developed into ectodermal cells, while similar contiguity with me- 108E. H. Morgan: The Formation of one Embryo from two Blastttlae. Arch. f. Entwicklungsmech. d. Org., 1895. Bd. II. Heft I. P. 65—71. Remodeling of Old Tissues in Regeneration 135 soblastic cells made them become mesoblastic cells." And Weismann finally calls into question the incontestability of these facts, just because such a cell determination de- pendent upon contiguity would upset at once his whole theory of preformation.107 But there are also many cases of regeneration proper in which one has a remodeling of old tissues into new tissues that are quite different, and they constitute phenomena which are analogous in this respect with post- generation. As an example may be mentioned the regeneration of Planaria maculata. Fragments of this worm obtained by two transverse sections regenerate the head and the tail by producing new cells. But after their formation, this head and this tail do not grow any further, but the entire subsequent growth in length of the body takes place in the older more pigmented parts, so that the normal relative proportions of the planaria are restored simply by a remodeling of the older tissues. "The fragment of the worm reacquires its normal form but not through the addition of new tissue at the anterior and posterior extremities, except to a very small extent. The transformation is produced chiefly in the old tissue after the head and tail are developed. Thus we find here not only the capacity of regeneration but also a subsequent self-regulation by means of which the normal relations of the parts characteristic for the species become re-established." But that is not all. For in an- imals regenerated from lateral fragments, the longitudinal axis of the new worm is found often in the older tissue, so that one portion of the old material which was in the right side of the old animal becomes now part of the left 10TWeismann : Das Keimplasma. P. 192. 136 Facts Compelling Us to Reject Pre formation side of the new animal or vice versa ; and the development of the new pharynx which is found in the exact lon- gitudinal axis, indicates that it can be produced indiffer- ently from any part whatever of the old tissue.108 This remodeling of old tissues into new tissues differ- ing from them indicates that the supposed determinants of Weismann have not by themselves any value, for as soon as the tissue finds itself in conditions different from the normal ones it takes on forms and acquires properties which would require determinants of quite another nature. "The organism," writes Whitman, "dominates cell forma- tion using for the same purpose one, several, or many cells, massing its material and directing its movements and shaping its organs as if cells did not exist or as if they existed only in complete subordination, if I may so speak, to its will." 109 And one would not know how to give any better proof of the correctness of this statement than that which is constituted by these particular regener- ations, which utilize the material already existing to remodel it into the new. And not only are these phenomena of peculiar regen- eration irreconcilable with pre formation but the very fact of regeneration in general is irreconcilable with it. "The germ tissue of the new organ," whites Hertwig, "does not contain any remnant of the amputated organ itself from which it could be reproduced by simple growth. The buds destined to reconstitute the eye-bear- L08E. M. Morgan : Experimental Studies on the Regeneration of Planaria maculata. Arch. f. Entwicklungsmech. d. Org. Bd. VII. Heft 2. and 3. Leipzig, Engelmann. Oct. 18, 1898. P. 385, 389, 395 -396. "'Whitman: The Inadequacy of the Cell-Theory of Develop- ment. Biol. Lect. at the Mar. Biol. Lab. of Wood's Holl, Summer Session 1893. Boston, U. S. A., Ginn 1894. P. 119. Weismann's Accessory Idioplasm 137 ing tentacles of the snail contain no trace whatever of retinal cells nor of pigment cells, nor of any other sensory cells whatever. Similarly the buds for the extremities do not contain any trace of the material of the carpus and phalanges nor of the muscles and tendons belonging to them. It is a complete new formation." no The explanation which Weismann endeavors to give of these complete new formations produced in every re- generation is well known: "If each cell of the completely developed bone con- tains within it only that kind of idioplasm which con- trols it and which is consequently the molecular expres- sion of its own particular nature, it would be impossible to understand how the regeneration could be effected of a bone which had been, for instance, cut through lon- gitudinally. Supposing that because of the wound there would become exercised upon the cells of the stump a stimulus which caused them to proliferate, a mass of bony tissue would indeed be produced but never a bone of def- inite size and shape. This can take place only in case the cells undergoing proliferation possess, besides their ac- tive determinants, an additional supply of determinants which control the missing part about to be reformed. It is then evident that, if we wish to transport the Nisus formativus of Blumenbach into the cell and indeed into its idioplasm, we must assume that each cell capable of regeneration contains besides its principal idioplasm, also an accessory idioplasm ('Neben-Idioplasni), consisting of the determinants of the portion of the amputated organ which can be regenerated by it. Thus, for instance, the cells of the humerus must contain besides their own con- 110Oscar Hertwig: Die Zelle und die Gewebe. II. P. 180. 138 Facts Compelling Us to Reject Pre formation trolling determinants also all the determinants of the forearm and of the hand as accessory idioplasm, for they can cause the entire chain of these bones to be formed anew; and the cells of the radius must contain as acces- sory idioplasm all the determinants of the radial portion of the wrist, hand and fingers. "We can regard this theoretical requirement as quite realizable also, since when the whole organ commences to be formed, the necessary accessory idioplasm can very well separate from the disintegrating embryonic idio- plasm. We need only assume that this accessory idio- plasm remains henceforth inactive in the nuclear sub- stance of the cell until some cause for regeneration arises."111 We note at once that, according to this hypothesis, there is no reason at all why there should be held in re- serve in each part of the bone only the accessory idio- plasm capable of regenerating the bony parts distal to that point, but never any other capable of regenerating a larger or smaller part. Each particular reserve idioplasm, when once it has separated itself in a given cell from the principal idioplasm, and been segregated in the nucleus of the cell itself in the latent state, will be able to preserve itself unaltered through many generations of cells. Con- sequently there must be present at any point at which a bone may be broken several accessory idioplasms, each capable of regenerating a more or less long portion of the bone which was broken, and perhaps also of some other bone. In the illustrative case cited by Weismann the sec- ond phalanx should contain besides the reserve idioplasm capable of regenerating the second and third phalanx, 111 Weismann : Das Keimplasma. P. 136—138. Regeneration and Generation 139 also that which is capable of regenerating all three pha- langes; or the distal part of the second phalanx should contain, besides the accessory idioplasm capable of regen- erating the distal portions of the second phalanx and the whole of the third phalanx, also that which is capable of regenerating both the third phalanx and the entire second phalanx itself. Why, then, should only that accessory idioplasm become activated which is capable of regenerat- ing just the particular part cut off ? Further, Weismann himself recognizes that when dif- ferent tissues and organs are cut through, "it is only the harmonious equipment of the cells of a definite cross sec- tion with groups of determinants, different but mutually adaptable, in accord among themselves and compliment- ary, that could make regeneration of the higher type pos- sible." 112 But really it is not easy to conceive how this harmonious equipment of reserve idioplasm could be guaranteed in the great number of different cells of a complex section. Roux has seen so clearly this impossibility of explain- ing the phenomena of regeneration by preformation theories, that he asserts that in "direct" or "typical" gen- eration self-differentiation may have the preponderance over differentiation due to reciprocal actions among the parts without nevertheless entirely excluding it; but in regeneration, which he calls "indirect" or "atypical" generation, he admits that differentiation of epigenetic nature must necessarily prevail over preformation.113 Weismann has rightly been unwilling to fall into the contradiction of imagining two different natures for two 112Weisann: Ibid. P. 297—298. 113Roux: "Uber Mosaikarbeit usw. Anat. Hefte. P. 279 — 331. Gesamm. Abhandl. II. P. 819—870. 140 Facts Compelling Us to Reject Pre formation processes which are essentially identical with each other, but has been thereby driven to an attempt at explanation, which is wholly artificial and indefensible. In order to make the artificiality of his interpretation of the most difficult cases clearer, let us consider further the following examples. It is known that regeneration is not usually an exact repetition of the ontogenetic process. "Until the last few years," writes Delage, "it has been regarded as a dogma that regeneration is a repetition of ontogeny. That is that the regenerating organ or limb goes through the successive stages of development through which it went in its first formation. Yet the question has not been thoroughly enough investigated to permit the statement that it always does this, and in many cases it is certain that it does not proceed in this way. Thus a round tailed salamander regenerated a round tail from the first and not the flattened finlike tail of the larva, the crab regenerates an adult foot and not a foot like that of its larva, Zoaea. The limb or organ regenerated after a wound arrives at once at the stage which corresponds to the age at which regeneration takes place." m Further, regenerations of ectodermic tissues at the expense of entodermal or mesodermal tissues are not rare. We have already seen how the crystalline lens, embryologically of ectodermic origin, regenerates in the triton from the mesodermic iris. The anterior intestine of Tubifex rivulorum, whose ontogenetic origin is ectodermal, regenerates, with the exception of a small portion at the end, from entodermal tissues.115 114] 1161 'Delage : L'heredite etc. P. 104 — 105. 'H. Haase : t)ber Regenerationsvorgange bei Tubifex rivulorum Lam. mit besonderer Berucksichtigung des Darmkanals und Ner- Regeneration Not Exact Repetition 141 Finally, cases are not rare in which a regenerating organ alters its form, as in the lizard in which the new tail has a skeleton not formed of individual vertebrae at all, but of a little continuous, cartilaginous cylinder. Now the epigenetic theories explain very easily how it comes that the part amputated can follow in its regen- eration a shorter road than in its ontogeny (caenogenetic regeneration), and how in many cases after completion of the process, it may have a form quite different from that of the original part. For the remaining part of the body, on which the morphologic determination of the ampu- tated part depends, is now in the adult state while for- merly it was in the embryonic state. The altered condition in which it now exerts its formative action upon the part in process of regenera- tion explains the diversity, not only of the earlier results obtained, in which development and regeneration proceed in different ways, but also of the final results, in which the regenerated part is of abnormal conformation. For the differences of conformation which are produced at the commencement of the process of regeneration cannot always be smoothed out and effaced when, at the end of the regenerative process, the condition of the rest of the organism from which the formative action is exerted upon the part in process of regeneration becomes again the same in relation to that latter as at the end of normal ontogeny. Weismann on the contrary, whose above quoted ex- planation is clearly no more adapted to these cases, is forced to take refuge in the following subsidiary hypothesis : vensystems. Zeitscher. f. wissensch. Zoologie. Bd., 65. Zw. Heft, 1898. P. 229—235. 142 Facts Compelling Us To Refect Pre formation "In caenogenetic regeneration, (and a fortiori when the regenerated part remains of abnormal conformation), one cannot but admit that certain double or multiplied determinants must be present beside one another in the germ plasm, some of them being destined to embryonic development, others to regeneration. These latter must have their interior forces and particularly their growing force so arranged in advance as to split off, either alone or together with neighboring determinants of regenera- tion, as reserve idioplasm, at the proper moment of development." 116 Epigenetic theories contain in themselves an imme- diate explanation of the well known fact that when a worm is cut in two, the anterior part regenerates the posterior while the posterior regenerates the anterior. Weismann on the contrary is forced to have recourse to the following artificial hypothesis : "As the two halves become always complete again, no matter at what place the worm is cut, it therefore follows that the cells situated in any particular transverse planes of the body are not merely provided with reserve determinants for generating in some planes the head, in others the tail, but every cell must be able to act in either way, according to whether it happens to lie anteriorly or posteriorly to this plane. In order therefore to explain the twofold reaction of these cells, and stick to our fundamental view, — which regards the cells concerned in regeneration, as arranged and con- trolled by forces lying within themselves, and not by any external directing power, — it seems to me that we must assume that each of them contains two different reserve determinants, one for reconstruction of the head, the 116Weismann : Das Keimplasma. P. 145 — 146, 147. Weismann's Explanation of Regeneration 143 other for that of the tail end, and that one or the other becomes active according as the stimulus due to lying un- covered, is applied to the anterior or posterior surface of the cell concerned." m Finally according to epigenetic theories the regenera- tion of the hydra is a process which does not differ es- sentially from any other process of regeneration, but ac- cording to Weismann's theory the following complicated additional explanation becomes necessary. "If one divides a hydra in a longitudinal plane the two halves grow again into entire individuals, irrespective of the plane of section. As a transverse section of the animal at any point which may be selected is followed likewise by the complete reconstruction of each of the two halves it follows that every part of the body of the hydra must be capable of regeneration in a threefold direction, namely in the three directions of space. As the body is differently constructed in these three directions we are forced to the conclusion that each of its cells must contain groups of determinants of three different kinds. * * * And it cannot be the quality, but the direction from whicfi the stimulus of the wound comes to each cell, which will decide for it which of the three groups of determinants will become active/' 118 We believe that we do not pronounce too severe a judgment, if we affirm that so artificial a hypothesis demonstrates the absolute incompetency of preformation theories to explain the phenomena of regeneration. What is the conclusion which can be drawn from all that we have said thus far in the present chapter? 11TWeismann: Das Keimplasma. P. 169. ll8Weismann : Das Keimplasma. P. 170. 144 ^ Homogeneous Germ Substance Inadmissable The first part has shown us that simple epigenesis is directly and decidedly controverted by a whole series of indubitable facts and results which no one now opposes. The second part, which refutes preformation, shows us on the contrary that the nature of every process of development is really epigenetic. And thus a correspondingly greater probability is es- tablished for those hypotheses which, concentrating the power of sending forth the controlling influences of development into a single well defined zone of the orga- nism, thereby explain quite as well as epigenesis the facts that are irreconcilable with preformation, and are at the same time in accord also with all the facts which simple epigenesis is incapable of explaining. j. Inadmissibility of a Homogeneous Germinal Substance It will not be necessary to give this question more than a very brief consideration, for it is sufficient to men- tion the chief argument which all the partisans of pre- formistic germs, the epigenesists as well as the preform- ists proper, have repeated incessantly and still repeat. The fact that in doing this each uses almost the same ex- pressions as the others shows how conclusive this argument is. "The considerations," remarks Wilson, "which have led to the rehabilitation of the theory of pangenesis are based upon the facts of what Galton has called particu- late inheritance. The phenomena of atavism, the char- acters of hybrids, the facts of spontaneous variation, all show that even the most minute characteristics may ap- pear or disappear independently, may be modified inde- Paniculate Inheritance 145 pendently, may be inherited independently from either parent, without in any way disturbing the equilibrium of the organism, or showing any correlation with other variations. These facts, it is argued (by the partisans of pre formation), compel us to believe that hereditary char- acters are represented in the idioplasm by distinct and definite germs (pangens, idioblasts, biophores, etc.), which may vary, appear or disappear, become active or latent, without affecting the general architecture of the substance of which they form a part. Under any other theory we must suppose variations to be caused by changes in the molecular composition of the idioplasm as a whole, and no writer has shown even in the most ap- proximate manner how particulate inheritance can thus be conceived." 119 It is well known that this is really the principal argu- ment, one might say the only one, which Galton brings up in defense of his germs, substituted by him for the gemmules of Darwin: "The independent origin of the several parts of the body can be argued from the separate inheritance of their peculiarities. If a child has its father's eyes and its mother's mouth these two features must have had a separate origin. Now, it is observed that peculiarities even of a microscopic kind are transmissible by inheritance, therefore it may be concluded that the most minute parts of the body have separate origins." 12° The argument which DeVries brings up in favor of his pangens, or material particles representative of the 118E. B. Wilson: The Mosaic Theory of Development. Biol. Lect. at the Mar. Biol. Lab. of Wood's Holl: Summer Session 1893. Boston, U. S. A., Ginn, 1894. P- 3— 4- 120Francis Galton : A Theory of Heredity. Journ. of the Anthro- pological Institute. January 1876. P. 331. 146 A Homogeneous Germ Substance Inadmissible different characters of the organism, is quite similar to that of Galton. It is summed up in the following passages from his book: "Many species of plants," writes he, "have the power of producing definite chemical com- pounds: among the most important of these are the red and blue coloring substances of flowers : also the various tannic acids, the alkaloids, the etherial oils, and numer- ous other products. A small number only of these com- pounds are limited to a single species of plants: a large number are present in two or more species, systematically far removed from one another. There is no reason to believe that there is a different mode of production of the same compound in each particular case: on the contrary one would naturally expect the same compound, in what- ever place one meets it, to be produced always by the same chemical mechanism. " "Similarly we must admit the possibility of a break- ing down of the morphologic signs of species. Morphol- ogy is clearly not yet far enough advanced to permit of such an analysis in each particular case. But the same coarse or fine notching at the leaf margins are repeated in numerous species and the customary terminology in- forms us in advance that all forms of leaf patterns are composed of a relatively small number of more simple characters." "This shows that the character of each individual species is made up of numerous hereditary peculiarities of which the most part are present also in an almost infinite number of other species. * * * According to this view, we would regard each species as an extremely com- plicated figure, and the organic world in its entirety as the result of innumerable different permutations and com- binations of a relatively small number of factors." Independence of Hereditary Peculiarities 147 "Experiments upon the production of varieties teaches us further that nearly every peculiarity can vary inde- pendently of the others. Many varieties in fact diverge from their stock form by only a single character; for in- stance the white blooming variety of a species with red flowers. In the same way the villosity, the armanent of spines or thorns, the green color of leaves, each of these characters can vary by itself and can even disappear en- tirely, and all the other hereditary properties remain perfectly unaltered." It follows that: "The hereditary anlagen, of which the hereditary peculiarities are the visible signs, are inde- pendents units which may have had their origin at differ- ent epochs, and which may also be lost independently. They are miscible with one another in almost all propor- tions, since each peculiarity can pass through all inter- mediate degrees from its complete absence to its greatest development." "Independence and miscibility, these are the essential properties of the hereditary anlagen of all organisms." 121 Quite similar to this argument of DeVries is that of Weismann in favor of his preformation or of preformistic germs in general : "It is impossible for one part of the body to vary independently of the others, and for these variations to be hereditary, if it is not represented in the germ plasm by a special particle, a variation of which in- duces a corresponding variation of the part in question. If this were represented, together with other parts of the body, by a single particle of the germ plasm, then a change of this latter would have as its consequence the variation of all the parts which are determined by it. In 121De Vries : Intracellulare Pangenesis. Jena, Fischer, 1889. P. MI. 17, 32, 33- English Translation by C. Stuart Gager. Open Court Publishing Co. 1910. 148 A Homogeneous Germ Substance Inadmissible those parts of the body which are independently and hereditarily variable, we have thus an exact measure for determining the number of the little vital particles which must compose the germ plasm: they cannot be fewer." "We are then logically forced to assume that a special element exists in the germ plasm for each of these pecul- iarities, not because the inheritance of even the smallest details is possible, but because each of these parts of the body can have its variations inherited individually, each by itself. If all men possessed a certain depression in front of the ear, one could not conclude that because it was hereditary, it must be represented in the germ plasm by a special element * * * the fact which forces us to accept this hypothesis is that all men do not possess this depression, that we can imagine two people who resemble each other in all other respects but of whom one pos- sesses this depression and the other does not." 122 This is then the great and only argument of all the theories of preformistic germs. One cannot fail to see that it really possesses a very great value against such theories as that of Spencer, who supposes the germ plasm to be constituted by a homogene- ous substance. In the almost complete darkness in which we still find ourselves in respect to the nature and causes of ontogenetic phenomena, there are very few things which we can venture to call impossible. Nevertheless the supposition which is implied in the epigenetic theories of the Spencer type, namely, that a homogeneous germ substance a little different chemically from an other, is able to give rise to an individual quite identical with that which the other substance produces, except for one little 122Weismann : Das Keimplasma- P. 72 — 74. Theories of Chemical Development 149 peculiarity in a definite part of the organism, if it does not seem quite impossible, certainly seems difficult to con- ceive. It is true that these theories of the Spencer type can always bring up the objection that to a visible varia- tion of a certain group of cells there might possibly cor- respond similar variations in all the other cells of the organism, but always so small that they are not appre- ciable. But such an explanation of the especially inherit- able variations would be formal rather than actual. This applies equally well, it may here be said paren- thetically, to evolutionary theories without preformed germs, such for example as the theories which are called those of the chemical development of the egg. They start out usually with a heterogeneous germ substance, con- stituted by multiple and diverse chemical substances, from chemical interactions of which new chemical compounds are formed later, which give place in their turn, — in each cell as in a separate crucible independent of the others, — to new chemical reactions and consequently to new com- pounds different in the different cells, and so on up to the end of development. But one cannot conceive how each one of these components of the germ substance, which commences to exercise its chemical action upon the other constituents from the very first moment of development, even though it be the only point in which one germ differs from another, can bring about an alteration of the or- ganism limited to a single point rather than an alteration extended over the entire organism. The argument brought up by the partisans of pre- formistic germs, both epigenesists and preformationists properly so called, is then really weighty enough to force us to hold as inadmissible every biogenetic hypothesis which starts out from or is based upon a homogeneous 150 Inadmissibility of Preformistic Germs germinal substance, even though it were as complex as one could imagine ; and a germ substance which is hetero- geneous indeed but each of whose components would nevertheless commence to be active from the very first moment of development must be no less certainly excluded. On the other hand we ask : Is it in general possible to conceive, much less accept, these preformistic germs, of which each is set apart for some infinitesimal part of the body, — any part provided that it can vary independently of the others? Would the supposition of germs of this nature constitute any explanation whatever of this par- ticulate inheritance, or would this not rather be a pure and simple repetition in other words of the phenomenon which one pretends to explain? That is what we propose to consider very briefly in the following last section of' this chapter. 4. Inadmissibility of Preformistic Germs We note in advance that the independently variable and inheritable peculiarities of the organism are not limited merely to the form and structure of entire groups of cells, but can include even the chemical characters of each cell. One would arrive thus at the absurdity that not only each cell, as Darwin's pangenesis already admits, but almost each molecule of the organism must have its representative in the germ plasm. Besides this material impossibility the idea of pre- formistic germs encounters insurmountable difficulties from the point of view of their conceivability. Is it a conceivable thing that there is for instance a preformistic germ of a certain nervous tic, or of a par- Instincts and P articulate Inheritance 151 ticular instinct, that there is a pangen or a group of pangens for the instinct of the hunting dog, that there is a determinant or group of determinants of the instinct of the new-born chick, which knows already how to peck at the wheat and swallow it ? How can we conceive of these instincts which are the consequences of very complicated combinations and interconnections of almost innumerable centers arid nerve tracts, as due to one separate germ, which having come up at the opportune moment of on- togeny, and at the exact point of the organism, produces them by itself, automatically, and we may say independ- ently of all the rest of the organism already formed? And yet these instincts actually constitute variable and inheritable peculiarities of the organism, susceptible of being present or absent independently of all the other peculiarities of the organism. But if, in order to explain this "particulate inheritance" one has recourse to germs especially preformed just for this, would this constitute anything else than a purely verbal explanation without any real inherent significance? "A man, for example," Le Dantec very rightly says, "is composed of about sixty trillions of cells, and he is nevertheless reproduced by sexual elements of very small size : this is the phenomenon to be explained. It has been thought that the difficulty would be less, or at least would not appear so distinctly, if one were to divide the problem into sixty trillion parts, if one could replace the reproduc- tion of the man by sixty trillions partial reproductions; and there have been consequently imagined infinitely small particles which are to the cells as the whole germinal substance is to the man." 123 128Le Dantec : Traite de Biologic. P. 22^—22^. 152 Inadmissibility of Preformistic Germs The consequence of this has been that the problem has become enormously complicated because it has given birth to this other very great problem: How comes it that these sixty trillions of autonomous and therefore inde- pendent individual parts can constitute a complete and harmonious whole? It results from this that preformistic germs, which by themselves are quite inadmissible, become yet more so when they are separated from preformistic doctrines properly so called. And Weismann endeavored to show that they are inseparable. "DeVries," he says, "once mentions the zebra stripes. How can such .a character be transmissible if in the germ the different pangens are free one beside another, without being bound up into firm groups inheritable as such? Zebra-pangens cannot give it, for the striping of the zebra is no cell character. Perhaps there are pangens which for brevity we can call "whites" or "blacks," whose presence would produce white or black color in the cell. But the striping of the zebra does not consist in the development of the black or of the white in the interior of the cell, but rather in regular alternations of thousands of black or white cells arranged so as to form stripes." "DeVries mentions also the long stemmed variety of the alpine Primula acaulis occasionally produced by atavistic return to a remote stem form. Here again the character of the long stem cannot be due to 'long stem pangens/ because the long stem is not an intracellular property ; neither is the specific form of the leaves, etc. ; the dentate border of a leaf cannot be due to the presence of 'dentate pangens/ but is due to a special arrangement of the marginal cells. The same is true of nearly all the characters which we designate as visible properties of the Incapable of Explaining Participate Inheritance 153 species, genus, or family etc., and so of the size, struc- ture, and shape of a leaf, of the characteristic and often constant spots upon the leaflets of flowers (orchids, and so on). All these properties manifest themselves only by the orderly cooperation of many cells. Or think of the prop- erties of the human individual, of the form of the skull, of the nose, and so on. All these very characteristic properties cannot be due only to the presence in the germ of pangens which must form the hundreds and thousands of different cells which compose the property in question ; they must be due rather to a fixed grouping of the pan- gens or of some other corresponding primary element of the protoplasm, transmissible in its fixity from generation to generation." 124 But when under the pressure of logical necessity we pass from simple preformistic germs, either free or in- termingled in any way, to germs built up together into a fixed structure, we fall at once into all the difficulties and contradictions of pure Weismannian pre formation, which we have already discussed, beginning with the one which we have seen to present itself first, namely that it is quite inexplicable how this "fixed grouping of the pangens" can divide in successive germ plasms and nevertheless remain unaltered in its structure. Preformed germs, materially impossible and theoreti- cally inconceivable, are nothing else than empty, wordy names, and appear besides to be quite incapable of ex- plaining even the most important phenomena of particu- late inheritance on account of which they were especially devised, and which constitute the only excuse for their existence, when once they are separated from the stronger "*Weismann : Das Keimplasma. P. 22 — 23. 154 Inadmissibility of Preformistic Germs preformistic theories, the absolute inadmissibility of which we have seen above. * * * What conclusion can be drawn from the last two sec- tions of the present chapter ? The penultimate section has shown us that the actual independence in variation and inheritance of the various and particular characters of all the rest of the organism can be explained neither by a homogeneous germ sub- stance, nor by a heterogeneous germ substance of which all the various constituents would become active from the first moment of development. The last section has dem- onstrated to us the inadmissibility of preformistic germs although at first they appear to constitute the most im- mediate explanation of the mutual independence of the various particular characters. It remains then for us to see if a heterogeneous germ substance without preformistic germs, but whose con- stituent parts instead of entering all into action from the first moment of development, become active successively from the commencement to the end of development, can give the adequate explanation of particulate inheritance which we are seeking. Let us consider first the phenomena of particulate inheritance which are shown by the presence in the child of certain paternal characters simultaneously with other maternal characters, intermingled but yet clearly distin- guishable from one another. For the sake of clearness we shall overlook for the moment all sexual peculiarities and limit ourselves to considering only the clearly asexual paternal and maternal characters. "The form of the skull," remarks Weismann for instance, "can be paternal and the face maternal; the form of the entire head and Successively Activated Specific Elements 155 the face can be maternal and the eyes, in spite of that, be entirely paternal in character ; the dimple which the father had on his chin may be found again in the child, although in the form of its face and nose it may resemble the mother rather than the father." 125 Let us note at the same time that the germ substance of the fertilized egg must contain the anlagen of both the paternal and maternal germ substance, and that the former as well as the latter, since they correspond one to another in pairs, tend to become active in pairs simulta- neously or almost simultaneously, except in the cases \vhere they are of such nature as to be reciprocally exclusive. Now if we suppose that the process may be of epi- genetic nature, and if we suppose also that the different anlagen of the germ substance becoming successively ac- tive, are all located in one definite zone of the organism from which they send forth their formative action, then it is clear that the different points of the soma must experience the determinative influence of the paternal and maternal germinal anlagen at the same time. Consequently when the corresponding anlagen com- posing each couple are quite identical, as will be the case especially during the first stages of development and per- haps also at subsequent stages more or less advanced, then the two respective, determinative actions will be- come fused into one, and there would result the exact reproduction of the entirely similar characters which the two parents possess in common. When on the contrary the corresponding anlagen com- posing each pair are different, provided that they are not, 125 We^mann : Das Keimplasma. P. 377. 156 Explanation of Particulate Inheritance we repeat, different to such a degree that the activation of one prevents that of the other, the two formative actions will be likewise different for all the points of the soma upon which their action would be perferably or exclusively directed, and they would be able thus either to combine and thus unite into a single resultant formative action, or, by developing their respective characters separately, to bring about an intimate interlacing of them, in such a way as to cause the appearance of a com- mingled intermediate character, or finally the paternal character developed by one of the formative actions can at a given point prevail over the maternal to such an ex- tent as to appear in all its purity, while perhaps the reverse appears in a neighboring point of the soma, and the maternal character comes alone to development. A characteristic example of this interlacement of pa- ternal and maternal characters remaining in part distinct but in part fused, is shown us by the hybrid arising from the spontaneous crossing of Vitis aestivalis and Vitis labrusca, the epidermis of whose leaves is formed like a mosaic, the cells of which belong either to the purely paternal type or to the purely maternal type, or to an intermediate form.126 If now after considering the phenomena of particulate inheritance due to sexual reproduction, we consider the phenomena of this particulate inheritance in its broadest extent and most general significance in order to be able to answer the question ; how can the simple fact be explained that two individuals can be altogether alike except for a single definite peculiarity at a single given point of the 136Strasburger : t)ber periodische Reduktion der Chromosomen- zahl im Entwicklungsgang der Organismen. Biol. Centralbl., Bd., XIV. No. 23 and 24, Dec. i. and 15, 1894. P. 850. Explanation of P articulate Inheritance 157 organism ? It will not be difficult to convince us that the possibility of this phenomenon will be fully provided for by the same hypothesis of the structure of the germ sub- stance which has served to explain for us the same phenomenon in so far as it is due to sexual reproduction. Let us imagine, for example, two germinal substances constituted by two series of specific anlagen, which are qualitatively alike, but in one of which a certain entire group of these anlagen is furnished with a little less potential energy than in the other. We do not need to suppose, even though we could, that this certain group of specific anlagen is of such a nature that its activation in the above mentioned common zone from which formative stimuli are given out should determine preferably or exclu- sively just that part of the organism which shows itself capable of independent variation, such as for example the dimple in front of the ear of which Weismann speaks. Instead we could very well suppose that this group, either by itself or in combination with others, brings about definite ontogenetic modifications not only in this one part but also in many other parts, perhaps even in all the cells of the organism without exception. But the epigenetic nature which we attribute to the process of development implies the idea that the activation at a given stage of ontogeny of a definite specific anlage must exert very different influences not only qualitatively but quantitatively upon the individual parts of the soma that are already formed. It is thus conceivable that a very small amount of potential energy in a given group of specific germinal anlagen might exert inappre- ciable effects or indeed no effect at all on a definite or even great part of the organism, but yet exert quite an appreciable or even considerable effect upon another very 158 Explanation of P articulate Inheritance small part of it, and so much the more since the effects produced by this particular group of germinal anlagen must combine with those produced by all the others both antecedent and subsequent. It would amount to the same thing if this given group of specific anlagen differed from the corresponding group of the other germinal substance not only quantitatively but also qualitatively to a certain extent. We believe the final result to be that we can affirm that the hypothesis of a heterogeneous germinal sub- stance whose anlagen do not all enter into action from the first moment of development, but rather become active successively one by one, throughout the entire course of development, explains the phenomena for which pre- formistic germs were especially devised quite as satis- factorily as they do, and at the same time is not open to any of the formidable objections, which demonstate with certainty the untenability of the hypothesis of pre- formistic germs. CHAPTER FIVE THE QUESTION OF THE INHERITANCE OF ACQUIRED CHARACTERS The great service of Weismann, which is not yet appreciated highly enough, is that he brought forward this matter of the inheritance of acquired characters, and questioned its existence, which previously had been not only tacitly admitted by most biologists, but regarded as not needing proof. And we must recognize the fact that the great and justifiable desire to find for this inheritance some proof which should be irrefutable and not open to any objections has remained so far unfulfilled. It is not proposed here to make a long list of all the facts which have been brought forward as proofs of the Lamarckian principle, but it will be worth while to examine a few in order to show clearly that Weismann and his school are not really far wrong in denying to most of these facts any right to be considered conclusive proof. We shall leave aside the question as to whether calves have really been born without horns, as alleged, in con- sequence of the breaking off before their conception of the horns of one or other parent; or whether tailless calves were produced by a bull whose tail had been squeezed off at the root by the violent closing of the stable door. It is clear that all these cases and many others like them, which have been reported in dogs, cats, rats, and 160 Inheritance of Acquired Characters so on, can not constitute any satisfactory proof in the absence of reliable observation and confirmation of the facts. Darwin draws especial attention to the inheritance of characters acquired by domestic animals. "The domes- ticated duck," he remarks, "flies less and walks more than the wild duck and the bones of its anterior and posterior limbs have become respectively diminished and increased in comparison with those of the wild duck. A horse is trained to certain paces and the colt inherits similar con- sensual movements. The domesticated rabbit becomes tame from close confinement ; the dog intelligent from as- sociating with man; the retriever is taught to fetch and carry; and these mental endowments and bodily powers are all inherited." 127 These examples, one must admit, deserve all considera- tion, especially the first. But one encounters here the ob- jection which can always be raised against such examples : As functional adaptation has a great modifying influence upon the organism, how can we be certain that the greater size of the bones of the legs in the domestic duck really springs from inheritance of acquired characters rather than from the daily exercise of the individual itself? Would not a wild duck if it were obliged to walk during all its life from its coming out of the egg acquire a similar hypertrophy of these bones? Unfortunately we have not exact measurements on this point which alone could de- cide the question whether hypertrophy acquired during the life of an individual could attain the same degree as that which has been observed in the domestic duck. Several travellers have remarked that when men have 127Darwin : The Variation of Animals and Plants under Vol. II. P. 367. Apparent Instances and Objections 161 disembarked for the first time upon uninhabited islands the animals have not often any fear of them, but after a very few generations the fear of man has become an inborn instinct. Weismann and his followers could object here also that this fear of man is not even now an inborn character, but rather is simply acquired after birth and due to the educa- tion, in the largest sense, which the little animals con- tinually receive from their parents and from all the other adults merely by observation and imitation of their con- duct on definite occasions. "The co-ordination, arrangement, and connections of the ganglion cells which innervate the muscles of speech," says Roux, "are already inborn in us to such an extent that we learn to speak our mother language easiest, while for example Europeans even when brought among the Namas while still children never learn their language as perfectly as the Namas themselves, or do so only with the greatest difficulty." 128 This does not prevent any one fundamentally opposed to the inheritance of acquired characters from objecting that the European language spoken by the parents and ancestors of the child may not be the cause of these dis- positions and inborn connections of the ganglion cells but rather the effect ; in other words, it is not the use of this or that speech which develops such and such inheritable connections; but rather the presence of certain connec- tions due to natural selection has produced certain peculiarities in the character of the language of a given human race. "When young hunting dogs," writes Exner, "which 128Roux: Der Kampf der Teile im Organismus. P. 38. 1 62 Inheritance of Acquired Characters have never been out hunting, nor had occasion to become otherwise acquainted with guns and their effects, hear one in the fields for the first time they start up eagerly, just like old hunting dogs, to retrieve the prey even when they do not see any fall. This demonstrates that since the invention of gunpowder the mnemonic image of a gun- shot and its effects has passed hereditarily into the brain of the dog, and so has been gathered up in the so called instinct." 129 And here, we do not really know what objection the Neo-Darwinians could bring forward ; for it seems to us that they would encounter difficulties in trying to attribute the formation of this instinct in a brain which was abso- lutely tabula rasa in so far as this instinct is concerned, to the artificial selection of the breeders. We must never- theless recognize that even this example does not fulfill, and cannot from the nature of it fulfill all the requisite conditions of exact observation, of measurement, of con- trol, and particularly of comparison which alone could give a single case the value of a decisive proof. A very remarkable example is reported by LeDantec. The shells of Hyatt's oldest cephalopods have the form of a cows horn nearly circular in transverse section. And following the series of these fossils in the more recent strata, one notes that these shells, at first almost straight, are little by little rolled up upon themselves like an Arch- imedes' spiral. The presence of certain characters shows clearly that the rolled up forms are descended from those with the straight shell. In a few types the rolling up is so marked that the successive turns of the spiral press one 129Exner : Physiologic der Gro/3hirnrinde, in Hermann : Hand- buch der Physiologic. Zw. Bd., Zw. Teil. Leipzig, Vogel, 1879. P. 282—283. Apparent Instances and Objections 163 into another, giving rise to a dorsal groove the mechanical production of which is evident since it undoubtedly re- sults from the pressure of the preceding spiral upon the succeeding. Now in a still more recent geological period paleontological discoveries show that the descendants of these cephalopods with a tightly rolled up shell have begun to unroll, and have then the form of an Archimedes, spiral with broader turns which no longer touch one another. But the dorsal groove persists even in these half rolled up shells, a proof that the younger cephalopods have repeated hereditarily this character which was acquired by their ancestors.130 This is certainly a most interesting example, but it has not quite the force of complete proof. For besides the objection, which we shall examine later, that the groove is formed in the non-living substance of the shell, it does not exclude the interpretation, though it be only verbal and without any real foundation, that it was not the rolling up of the spiral upon itself that produced the inherited groove, but rather that both the tight rolling up and the groove were selected and fixed independently of one another by natural selection. The influence of dry, hot climates upon the develop- ment of the horns of cattle and sheep is well known. If certain individuals of a certain breed of cattle are trans- ported from a wet, cold climate to a hot dry climate, the horns increase in length and circumference and the skin thickens. The following fact seems to prove that this acquired elongation of the horns is inheritable. A cowr was transported from Algau in Bavaria where the climate is moist and cold, into the dryer and hotter steppes of 180Le Dantec : Traite de Biologic. P. 296—297. 164 Inheritance of Acquired Characters Hungary. This cow whose horns were 19 cm. long gave birth to a calf with horns 22 cm. long. The calf of this calf born also in Hungary, (presumably from a father of the same race as that of the mother?) had horns 23 cm. long and thicker than those of its mother and grand- mother.131 So of the three centimeters of elongation, due to the action of the environment, which we can regard as functional adaptation in the widest sense, one centimeter would have become hereditary in a single generation. It is however evident that experiments of this nature can- not have any real significance unless they are made on a large scale, so that an average can be established from many instances. And this experiment of Wilckens has been mentioned here just because the way in which it was conducted comes close to possessing the requisite and indispensable conditions of a fundamental proof. Another instance which the partisans of the inherita- bility of acquired characters adduce is brought forward by Spencer: it is that of the Punjabi of India, who have certain muscle imprints on the bones of the leg, and certain facets in the articulations of the hip, knee and foot, which are produced by their habit of squatting upon the ground; and these peculiarities are hereditary, as is demonstrated by the fact that they commence to show themselves even in the foetus. Weismann seeks to demonstrate that they are only the continuation in man of certain peculiarities in the articulations of anthropoid apes which natural selection had already fixed in very ancient times because they LS1Wilckens : Die Theorie erworbener Eigenschaften vom Stand- punkte der landwirtschaftlichen Tierzucht in Bezug auf Weismanns Theorie der Vererbung. Biol. Zentralbl., July 15, 1893. P. 426. Apparent Instances and Objections 165 were useful then. But still, in our opinion, he has not been able to explain correctly why these peculiarities are retained only in the Punjabi, who are also the only ones of all the tribes of the same family who are accustomed to squat in this way.132 One could bring up also as examples the callosities at the knees and sternum which are hereditary in the domestic camels but are lacking in the wild camels. Thus for example the camels of the tame stock of San Rosso re near Pisa (Italy) are covered with hair both over the breast bone and on the knee at birth, but after a few days they lose the hair in the breast bone region, which is then permanently replaced by a horny plate. Every camel up to three months old had these more or less broad, hairless plates, though they still retained the hair on the knee, but the thickened and hardened skin could be felt under it. Of course these camels which were only a few months old were not required to do any work. On wild camels, on the contrary, no such swellings are to be found either in the very young or the adult.133 Still more remarkable is the following fact reported in 1888 by Prof. Fogliata. "A she ass from the Tuscan Appenines, which long had borne the pack saddle, showed on the back and on both sides over the ribs, a very evident pad of soft fat, which in extent and shape was like those which the pressure of an ordinary mountain pack saddle produces. This she ass was put to an ordi- L82Weismann : Neue Gedanken zur Vererbungsfrage. Eine Ant- wort an Herbert Spencer. Jena, Fischer, 1895. P. 54ff. 183Cattaneo : Le gobbe e le callosita dei cammelli in rapporto colla questione della ereditarieta dei caratteri acquisiti. Estratto dai Ren- diconti del R. Istituto Lombardo di sc. e lettere. Serie II, Bd. XXIX, 1896. P. 10 — ii ; and: I fatten della Evoluzione Biologica. Genua, Martini, 1897. P. 40 — 41. 1 66 Inheritance of Acquired Characters nary male ass and bore a female colt which shows the same peculiarity as the mother. Its fat pad which covers the back and reaches almost half way down the ribs is not less than 5 cm. thick, clearly defined, and has an abrupt and perpendicular margin. It is to some extent a distinct and separate fat mass, — a true lipoma, — cer- tainly similar to those which according to Lombroso's description are produced by burden bearing. It has the same character as the hump of the camel, is more or less developed according to the condition of nutrition of the animal, and appears exactly as though it had arisen by the pressure exerted by a saddle on the back. Also the hair is longer and thicker over the whole of the fat layer, which agrees likewise with the observations of Lom- broso on pack animals possessing such lipomas, and is like the hump of camels also, which is covered with thicker, longer wool. It is worthy of remark that this young she ass has never borne a saddle and inherits its peculiarity entirely from the mother, which proves be- yond doubt, that this peculiarity, acquired by pressure on the back, has been inherited.134 Even though this single fact cannot decide the ques- tion finally we do not really see what objection Weismann and his school could urge against it. Finally we have the celebrated experiments of Brown Sequard on guinea pigs, proving the transmissibility to the young of effects produced in the parents by certain accidental lesions. Thus he has demonstrated that epilepsy, produced in one of the parents by section of the sciatic nerve or of a part of the spinal cord, is transmissible to the young. 134Cattaneo : Le gobbe e le callosita dei cammelli etc. P. 9 — 10. Instances Reported by Brown Sequard 167 After section of the sympathetic trunk in the neck there was produced a particular change in the form of the ear or a partial closing of the eyelids, and the same modification of the ear, and the same closure of the eyelid were reported in their respective descendants. A lesion of the medulla oblongata produced exophthal- mos in certain guinea pigs and the same exophthalmos showed itself in the young. Ecchymoses accompanied by dry gangrene and other alterations of the nutrition of the ear have been reported in the descendants of individuals in which this series of , phenomena had been produced by a lesion of the restiform body. Certain other guinea pigs in which section of the sciatic nerve had rendered the hind foot insensible to pain gradually destroyed their toes by gnawing them; it is reported that in their descendants parts of the toes or even whole toes were missing from one of the hind feet. Other individuals in which the sciatic nerve had been cut had descendants which exhibited at first a diseased condition of the same sciatic nerve, and later the phe- nomena characteristic of the onset and decline of epilepsy, particularly the development of an epileptogenous capac- ity in a zone of skin of the head and neck, and a loss of hair toward the decline of the affection. Guinea pigs which had had an eye altered in conse- quence of the transverse section of the restiform body had descendants which all showed more or less imperfec- tion in one or both of the two eyes. In more than a score of guinea pigs representing* the total posterity of individuals in which muscular atrophy had developed after section of the sciatic nerve 1 68 Inheritance of Acquired Characters there appeared just such a muscular atrophy of the thigh and of the leg.135 The desperate endeavor which Weismann has made to refute the results of these experiments, at least in relation to the transmissibility of epilepsy, is well known, objecting that this affection was due only to an infection innoculated in the parents after operation and so trans- mitted to the germ. Brown Sequard has signally over- come this objection by showing that epilepsy is not produced by all nerve sections but only by some, and that further it can be provoked also by the simple crush- ing of the sciatic nerve without any breaking of the skin, and this would exclude the possibility of any infection whatever. Nevertheless it is necessary to recognize the fact that these experiments, while they undoubtedly demonstrate the inheritance of the effects of certain lesions, are not enough to produce a firm and general conviction of the inheritance of acquired characters among the numerous naturalists and biologists who are in no wise blind fol- lowers of Weismann's theories; perhaps because what is inherited in these cases is always somewhat morbid and abnormal. In short the determination of the ques- tion requires certain proof of the inheritance of definite normal peculiarities acquired by functional adaptation. We see then, that whoever proposes systematically to hunt out a weak point in every fact adduced in support of the Lemarckian principle, by which its value as proof can be shaken, can usually if not always find it. But 185Brown Sequard: Faits nouveaux etablissant 1'extreme fre- quence de la transmission, par heredite, d'etats organiques morbides, produits accidentellement chez des ascendants. Comptes Rendus de 1'Acad. der Sciences. T. XCIV, No. 11, March 13, 1882. P. 697—700. The Non-inheritance of Amputations 169 this would justify the assertion that the inheritance of acquired characters has not yet been directly proven, only in the case that there were but a few facts or only a single fact that could be brought forward in proof of it. But when on the contrary there are a very large number of facts favorable to a given principle, even though each one of them by itself would not be an" absolutely incontestable proof, they would in spite of that have, when taken as a whole, a very great value as proof, and this value would be so much the greater if the opponents of the principle, in seeking to deny the incontestability of the individual facts, are forced to • resort to as many specially devised subtleties. On the other hand the non-inheritance of certain gross instantaneous modifications, such as amputations and other similar things, of which Weismann and his followers make so great a case, proves nothing against the inheritance of functional adaptations which are of quite different nature. For let us consider the dynamic equilibrium existing in the adult state in a given small portion of the soma, and let us suppose also that this equilibrium was estab- lished by a process of epigenetic nature dependent upon all the rest of the organism. If this local equilibrium , is suddenly very much disturbed as is the case in amputations, instead of gradually and slowly as in functional adaptations, one can understand that it can and must be promptly restored in the neighborhood of the wound or in any case in the limited area of the stump, before the disturbance has time to extend much farther. Therefore if there is a definite place in the organism to which non-transitory derangements and the variations of equilibrium caused thereby must 170 Inheritance of Acquired Characters penetrate if the corresponding morphological modification is to be inheritable, it follows that amputations, unlike functional adaptations, could not as a rule leave any trace of themselves in the descendants. But in still another very essential point amputations are different from functional adaptation. The ampu- tation of a limb, or of a piece of a tail, does not con- stitute in any way the mode of reaction of the organism to a definite external influence, but rather it is this external influence itself. How then will its reproduction in the new organism be possible? This would be the same thing as expecting that an individual who had been accustomed throughout his life to bear a burden upon his shoulders as exercise, should transmit to his son not only stronger bones and muscles but also the • burden itself which was the cause of this strengthening. So that the most that could be transmitted to the descendants of an animal which had undergone some amputation, would be the mode of reaction of the organ- * ism to this gross external influence, that is all the phe- nomena constituting the cicatrization, properly so called, of the wound, as well as the establishment of a new local equilibrium. We must however bear in mind in this connection that the reproduction in the child of con- siderably thicker and stronger bones and muscles will * not be hindered by the fact that it is not exposed to the same external influence which acted upon the parent, i. e. by the fact that it does not bear the same burden as its father, but that if one does not also repeat the amputation, the repetition of all the phenomena con- stituting the cicatrization of the wound and the reestab- lishment of a new equilibrium could not but be very much hindered and usually quite prevented by the pres- The Decisive Experiment 171 ence of the limb or other part of the body which was destroyed in the parent. In order to make this more apparent let us con- sider some one of the numerous rats from which Weismann cut off the tails, and which that author has rightly brought forward as proving that the surgical operation of amputation is not inherited. Let us sup- pose that in the young rat when once his development was completed and he had arrived at about the age at which the old rat had undergone the amputation, really showed at the spot at which the amputation took place a tendency to reproduce the same phenomena of cicatri- * zation and reestablishment of the new local equilibrium which had supervened in the parent. It is evident that the absence of the tail is a necessary condition in order to make the reproduction of these phenomena materially possible. This tendency must then be hindered and per- haps absolutely suppressed so long as the tail remains a part of the organism. It is interesting in this connection to note that Kohl- wey has obtained in one and the same individual a com- pletely negative result in respect to the inheritance of mutilations, but a positive result in respect to the trans- mission of habit : He cut off the posterior digit from the feet of some pigeons which thereupon turned back another digit in order to retain their perch; and in one instance this habit was reproduced.136 The decisive experiment upon the inheritance of ac- quired characters must leave amputations and similar sudden variations out of consideration, since either their effect is to bring about the reestablishment of an exclu- 16H. Kohlwey: Arten und Rassenbildung. Eine Einfuhrung in das Gebiet der Tierzucht. Leipzig, Engelmann, 1897, P. 6—7. 172 Inheritance of Acquired Characters sively local equilibrium or the repetition in the descendants of the phenomena by which the parent organism reacted is hindered. This experiment must rather be directed toward modifications of the functional adaptation, wihch have a very extensive action and whose repetition in the descendants is not hindered by anything. In order that these experiments on the changes dependent upon functional adaptation may constitute an incontestable proof for or against the inheritance of acquired characters, — which latter are to be understood in Weismann's sense as only somatic and not general peculiarities of the entire organism, — they must be planned in such a way as to make it certain that the change effected by the transforming influence has affected only the soma directly, and for still greater certainty it ought to act upon only a definite part of the soma and not upon the entire soma to the same extent. They must also be undertaken on pluricellular organisms in which the somatic germ cells are clearly differentiated, and there ought to be employed as 'trans forming influ- ences only such as certainly exert no direct influence upon the reproductive cells.137 All plants in which the difference between somatic and germ cells is not a thorough going and definite one will therefore be less suitable for these researches than animals, and particularly higher animals, and all such investigations both in animals and in plants which em- ploy physical or chemical transforming agents exerting a general action on the entire organism, on the somatic cells as well as on the germ cells, as for example tem- 18TCompare J. De Meyer: L'heredite des caracteres acquis est- elle experimentalement verifiable? Archives de Biologic. Tome XXI, No. Ill and IV. Paris, Masson 1905, P. 625, 634 — 639. The Decisive Experiment 173 perature, light or darkness, particular substances that are nutritive, stimulating or poisonous, infections, immuniza- tions, etc., could never afford such incontestable evidence against Weismann's theory, as those investigations which employ agents having a very definitely localized action. Thus for example Heschenhagen's researches upon the adaptability of the lower fungi to sodium chloride have, for the reasons stated, little or no value for the refutation of Weismann's theory, even though they have proved that the spores of the mycelium which had adapted itself to a strongly concentrated saline solution were capable of germinating in concentrations in which the spores of a mycelium arising in normal conditions were incapable of germinating. The same is true for the similar researches carried on by Hunger-Errera, DeMeyer, Pulst and Ray upon the inheritance of changes, mostly physiological rather than morphological in nature, which were brought about in the lower fungi by means of concentrated salt solutions, for example by sodium chloride or copper sulphate or concentrated sugar solu- tions, although these results as well as Heschenhagen's are certainly very interesting from the point of view of the adaptability of organisms to their environment. Also Hoffman's researches upon the inheritance of variations produced by insufficient nourishment in Papaver, Migella, and Argemone, — (a relatively large number of atypical flowers), — and Schubeler's researches upon the inheritance of the more rapid development of barley grains which had been transplanted from the south part of Norway to the north part, prove incontrovertibly the inheritance of the changes induced in organisms through general conditions in their environment, but the general influences very probably exerted in those cases also 174 Inheritance of Acquired Characters by the selected transforming agent upon the whole organ- ism, and our entire ignorance of the real nature of its peculiar action, deprive these experiments of any value as arguments against the theory of Weismann which denies the inheritance of any peculiarly somatic char- acters that have been acquired by means of local func- tional adaptation to external influences that are very definitely and clearly limited. Just as little valuable as proof against Weismann' s theories were the . researches of Standfuss, Fischer, and Bachmetjeff on the inheritance of changes in the color design of butterflies' wings, when the pupae concerned were placed in an unusually high or low temperature, so that Weismann, as we shall see further in the next chapter, could acknowledge the otherwise unimpeachable results of these researches without thereby being com- pelled to retrench his own theory. It would be best therefore to employ mechanical means, and to produce changes whose mode and place of working can be easily observed and definitely limited. But amputations are to be excluded for the reasons given, as are also sudden transformations, and so there remains as the experiments best adapted for the final decision of this disputed question, prolongation or fre- quent repetition of the activity of certain organs or definite parts of organs. We might suggest for example the artificial and therefore extraordinarily frequent extension or contrac- tion of the muscles of the fore or hind legs of a certain animal, such as could be effected in little amphibia or little mammals with the help of an especially devised clock work. Prolonged traction on the tail of the rat leading to its elongation and growth should be substituted The Decisive Experiment 175 for Weismann's amputation which can prove nothing. Similarly light hammering, continually repeated, which a proper mechanism might automatically perform upon certain parts of the skull of hornless animals, would be better than cutting off or breaking the horns in horned animals. All these artificial stimuli would certainly produce in each individual the hypertrophy of the organ upon which they act. It could then be seen whether the repetition of these stimuli throughout a series of generations would be followed by the production of individuals in which these organs would possess at birth even in a small proportion the greater development that had been acquired in several successive generations of its ancestors. The performance of such experiments upon guinea pigs or rats would not seem to present very great practical difficulties ; never- theless, so far as we know, it has never yet occurred to any one to make them or to propose them. But in all these experiments one must never forget that it is just the littleness of the inheritable fraction of an acquired quantitative variation, that constitutes the great difficulty of verification of the Lamarckian principle. Galton proposes as is known to select for experi- ment rather the inheritance or non-inheritance of certain acquired instincts. He advises for example to adopt the method of the following experiment of Mobius upon the pike; Mobius divided a large glass receptacle into two compartments by means of a perfectly transparent glass septum and placed the pike in one compartment and in the other little gudgeons upon which the pike usually feeds. It followed that whenever the pike precipitated himself toward any of the little fishes he was stopped by the glass against which he hit. After several weeks of 176 Inheritance of Acquired Characters useless attempts the pike finally gave up any attempt to catch this unseizable prey and he persisted in this atti- tude even after the glass had been removed. Now Galton advises repeating this same experiment on several genera- tions of pikes, taking care that each generation should always be brought up apart from the preceding to prevent any possibility of the educative influence of imitation, and seeing if one would finally obtain any descendant in which the instinct to throw himself upon the gudgeons would be replaced by the contrary instinct of indifference toward them.188 We should remark in this connection that because one is here concerned with establishing the transmission of an acquired instinct that is opposed to the inborn instinct, experiments of this nature are less advisable than those which seek rather to verify the inheritance of a simple quantitative increase acquired by already existing organs or tendencies. In Galton's experiment the tendency of the descendants to produce the new in- stinct even if it were present through a long series of generations, might not possess sufficient potential energy to enable it to manifest itself through activation because it would have to overcome a pre-existing tendency which in the beginning at any rate is certainly furnished with a greater quantity of potential energy. Therefore it is probable that it would be necessary to submit a very long series of generations to this experiment of the glass par- tition before the new tendency would be able to attain a superiority over the former and to replace it. The first pike upon which Mobius made his experiment 138Galton: Feasible Experiments on the Possibility of transmit- ting acquired Habits by Means of Inheritance. Paper read at the British Association. Nature, October 17, 1889. P. 610. Weismann's Arguments Against Inheritance 1 77 furnishes us itself a proof of this. For from the first impact against the glass partition the inclination con- trary to its instinct must have commenced to arise; nevertheless it effected the replacement of the latter only after a large number of unavailing attempts. From all that we have said thus far it follows that it is much to be desired that new and absolutely incon- testable experiments should once for all finally place the inheritance of acquired characters beyond a doubt. But it also follows, as we have said, that if no one of the proofs which we possess already demonstrates this in- heritance in an absolutely certain way, nevertheless all together they supply a great weight of evidence for it. As we shall see later this is true also of indirect proofs; one cannot say of any one of them that it decides the question in one way or the other, but all together they constitute a strong presumption in favor of the Lamarckian theory. It will be convenient to examine next the chief argu- ments which Weismann has adduced against this theory. They can be reduced in substance to the following: i. "In many animals," he writes, "for instance in many insects, instincts appear which are exercised only once during life. It is sufficient to cite the laying of eggs by ephemerids and many butterflies, the conjugation of bees, the search for proper hiding places in which caterpillars may change into chrysalids, — one species suspends itself, another lying on the ground builds de- fences, a third goes deep into the earth, a fourth spins itself a case in a rolled up leaf, and so on, and so on. Further there belong here the several species of cocoons which some butterflies, especially the bombycids, spin in a fashion so astonishingly complicated and so well 178 Inheritance of Acquired Characters adapted to its purpose, a thing which each individual does only once and from the most ancient times has done only a single time in all its life." 139 2. The second group of facts controverting the inheritance of acquired characters is furnished, accord- ing to Weismann, by the parts which have only a passive function, "in so far as they show that they also become rudimentary and finally disappear if they cease to be used and are not necessary for the preservation of the species. They show that the process of disappearance which the Lamarckians attribute to the inheritance of the direct effects of non-usage cannot be due to this cause, since here the organ in question does not exert any physiological function and so there are of course no effects of such function in the individual life. To this category belong for example the colors of animals, which become unstable when they are no longer needed for protection or as a means of recognition; here also belongs the deterioration of the chitinous cuirass of various crustaceans and insects which thrust one part of their body into protective envelopes." 14° 3. The third argument against the inheritance of acquired characters is that constituted by the neutral individuals among bees, ants, and termites which, accord- ing to Weismann, show that all the adaptations whether positive or negative, isolated or co-ordinated, that are to be observed in propagating individuals, appear also in individuals which do not propagate at all and which therefore do not transmit anything.141 l9BWeismann : Neue Gedanken zur Vererbungsfrage. Fine Ant- wort an Herbert Spencer. P. 61 — 62. 140Weismann : Ibid. P. 62—63. 141Weismann: Ibid. P. 66. Consideration of Weismann's Arguments 179 4. Finally the last argument of Weismann is that it is incomprehensible how the inheritance of acquired characters could be effected.142 In the endeavor to examine these four arguments with the most scrupulous objectivity, we must first divide them into two categories: The fourth is the only one which attacks the principle of inheritance directly; the first, the second, and the third, on the contrary, con- trovert this theory only indirectly, in that they seek to show that many formations are of such a nature or arose under such circumstances that they can be explained only by the theory of natural selection. The conclusion which it is desired to have drawn from this is clear, and is indeed admitted : If natural selection is capable of explaining some formations it will be capable also of explaining all the others; if all formations can be ex- plained by natural selection alone, the inheritance of acquired characters becomes useless for the purpose of explaining the transformation of species; consequently if it is useless it is very probable that it does not exist at all. The impartial reader will admit that this manner of reasoning is deceptive. Even if it be proved that natural selection must necessarily have been capable of producing certain formations with the help of fortuitous individual variations, it does not follow that it must also have been • capable of producing all other phylogenetic formations, especially if they are different in nature from the former. And even if the proof were forthcoming that it is capable of explaining by itself all phylogenetic formations what- ever, it is evident that even this would not constitute 142Weismann: Ibid. P. 6r. 180 Inheritance of Acquired Characters any argument against the inheritance of acquired char- acters. The continuous electric current for example can be produced by a battery as well as by a dynamo; and the fact that one can always explain it as having been pro- duced by a battery does not prevent it from being actually produced by a dynamo in many cases. This being so, let us now examine as succinctly and objectively as we can each of the four arguments : 1. No value can be attributed to the fact of the exercise of a function only a single time during life. In the first place, it is possible that it may formerly have been exercised repeatedly by the ancestors of individuals now living. In the second place this singleness does not exclude in any way its inheritance as a habit acquired by exercise. For the fact of having performed a given function even though only a single time, would certainly leave in the parent organism a potential disposition to perform it again and with greater facility in similar physiological and external circumstances ; therefore the conception that this disposition and this greater facility would be represented in descendent organisms represents only an ordinary case of inheritance. 2. As for the second argument one cannot but recognize that for certain formations the statement of Weismann that they can be due only to natural selection seems very probably true. But it must be remarked that to support his assertion Weismann attributes a merely passive function tosmany parts in which it is very questionable. Why for instance should we not regard the carapace of the turtle as a true and functional adaptation due to the stimulus of the environment to which the skin of the animal had reacted by a secretion constantly richer Consideration of Weismanris Arguments 181 in hard substance, exactly as the skin of the sheep reacts by the secretion of wool? There is nothing to prevent such secretions, produced through functional adaptations, serving later as protective shields and thus becoming useful to the species in still another way. On the other hand the envelopes into which the crustaceans and insects cited by Weismann insert one part of their body might preserve the external surface of that part from the hardening action of external agents, just as houses and clothes may have contributed to the disappearance of the hair in man. For one should not consider the passive function of the hair or of the chiti- nous substance so much as the active function of the * tissues which secrete these substances; and this function is essentially active for it is a specific reaction to external influences. In this respect the hermit crab constitutes one of the most conclusive proofs of the Lamarckian theory. For this crab which is accustomed to insert the hinder part of his body into empty snail shells has completely adapted itself to the conformation of its new habitation, and this bodily adaptation acquired by it has become hereditary so that it is present in advance before the animal inserts itself into its house. According to Weismann's view, the animal must have first adopted the habit and natural selection must have been able to exert its influence only subsequently. Clearly both processes must go on at the same time, the residence in the new habitation and the adaptation to it; and the fact that these exist together can be explained only through functional adaptation and - inheritance of its effects.143 "8G. Cattaneo: I fattori della evoluzione biologica. P. 43 — 45; 1 82 Inheritance of Acquired Characters Similarly, how can one escape attributing the colors of butterflies' scales to functional adaptation when one sees the golden red butterfly Polyommatus phlaeas change its color and take on a black tint merely from transporting it to warmer climates? Further, indubitable instances are known in which the color of the environment stimulates the outer surface of the animal directly or indirectly to take on the same color. Thus some arctic animals and birds become per- fectly white in winter, putting themselves thus in con- formity with the general color of the environment. Certain butterflies present phenomena of protective poly- chromatism in the sense that they always take on the color of their environment, and this should not appear strange for there is nothing inadmissible in the sup- position that a very sensitive skin can suffer much greater discomfort when the light rays which strike it are of a color different from its owrn than when they are of a like color with it. This discomfort would correspond to the disagreeable sense of heat or cold which makes itself felt over the surface of the body when its tem- perature differs from that of its environment. Consequently if every functional adaptation of the living substance to external agents consists in such a modification of its own vital processes that these find in the external agents no longer obstacles but rather co- operative stimuli, one can understand the tendency of every especially sensitive organ to make the color of its surface conform with that of its environment. This would not prevent the identity of colors from being of and Cesare Lombroso: Ancora dei caratteri acquisiti ; Paguri, Cam- melli e Zebu. Rivista di Scienze Biologiche. Vol. II, No. 3. 1900. P. 2-3. Consideration of Weismann's Arguments 183 service to the animals in a protective way also, but never- theless the productive cause would remain always a functional adaptation. From the preceding instances in which the action of the color of the environment upon the external surface of the animal appears to be direct we pass on to those in which this action is indirect. Thus many fishes, amphibians, reptiles and cephalopods are capable of changing their color in a very short time and thus of putting themselves always in accord with the very vari- able color of the environment. The color of the environ- ment which determines that of the animal does not act nevertheless, in this case, directly upon the elements of the skin, the chromoblasts, which produce the color; but by a complicated nervous apparatus connecting these elements with the part which is first stimulated by the color. This part is sometimes constituted merely by the nerve ends of the skin, at other times by the retinal nerve ends of the eye. In the latter case if the optic lobes of the brain are artificially destroyed the capacity of changing color disappears.144 Further, according to LeDantec, many colorations of the skin that are now fixed and correspond to the hence- forth unchanging color of the environment are derived from former colors that changed voluntarily writh the different colors of the environment, of which one certain color has remained, persisting to the exclusion of all the others.145 One could perhaps also adopt the opposite view. Just 144Weismann : The Effect of external Influences upon Develop- ment. P. 26 — 27. I45Le Dantec: Lamarckiens et Darwiniens. Paris, Alcan, 1904, Chap. XIV : Le mimetisme lamarckien. P. 129 — 149, 184 Inheritance of Acquired Characters as the secretion of gastric juice was originally a func- tional adaptation of the wall of the stomach to certain foods but finally is poured out before the foods them- selves are ingested but only tasted, in consequence of psychic associations; so the assimilation to the color of the environment, which originally was a functional adaptation of the elements of the skin producing the color, the chromatophores, may have gradually come to be produced in anticipation and finally exclusively by the perception through the eyes of the color of the environment. However that may be, all these facts show that far from seeing in the protective colors of animals merely the result of fortuitous variations which have become fixed by natural selection just for their passive protective function, we have legitimate reason for holding on the contrary that usually they are the direct result of true functional adaptation. In this way would be explained the possibility that as soon as the color of the environment alters, the pro- tective color of the animal might also become unstable or disappear entirely, and one would not be compelled to resort for the explanation of the phenomenon, as Weismann is, to panmyxia or to any other complicated process of natural selection. For this tendency to give up the color can in this case also be attributed to the simple circumstance that when the color of the environ- ment was altered the functional stimulus ceased, which was the sole cause of the color of the animal. It is true that Weismann points out certain cases of more typical mimicry which seem to prove the cor- rectness of his views very especially because they seem to show that natural selection had undoubtedly been Consideration of Weismann's Arguments 185 alone sufficient to produce in some instances extraor- dinarily complicated formations down to the most minute peculiarities. The example of Kallima, a well known leaf-like butterfly, will at once occur to every one. Never- theless it is known that certain Lamarckians have had the hardihood to wish to attribute these very perfect resemblances to former, voluntary, chromoblastic, mime- tic changes which do not now exist in the animal but which can be demonstrated even now in certain other animals, for instance in some fishes. The absence of change in the object taken as a model which was imitated only voluntarily at first, has resulted in the gradual with- drawal of the imitative color mechanism from the con- trol of the animal's will.146 Here it is sufficient to remark that the last word has certainly not yet been said upon this voluntary mimicry which rightly excites the greatest interest. In any case such imitative formations as that of Kallima cannot be disposed of by simply refer- ring them to natural selection alone, seeing that their protective utility can commence to be manifested only after they have attained an advanced degree of perfection. "A muscle," insists Weismann, "can become greater by use, but a claw, a bristle border, a dentition, a pro- tuberance at an articulation, cannot become thicker, longer or stronger by usage, it can only be used up." 147 But does not the very use of these inactiv.e parts dr, better, the transmission through the inert substance to the living substance of the mechanical action constituted by this repeated use, provoke the living tissue to secrete in larger quantity the chitinous substance of the bristles and of the claws? l*'Le Dantec : Lamarckiens et Darwinians. P. 142 — 145. 147Weismann : Neue Gedanken zur Vererbungsfrage. P. 65. 1 86 Inheritance of Acquired Characters "I need not recall," continues our author, "the host of positive changes undergone by plants which cannot be explained by the Lamarckian theory, — the appropri- ately placed protective spines, bristles and hairs, the poisons, the tannins, the etherial oils of all kinds, and all the purposeful forms of leaves, of flowers and all parts of plants in general. In the case of all these the supposed inheritance of the effects of use and disuse in general does not come into question; in them every- thing proceeds without it, — an incontestable proof that nature does not require this supposed factor for its trans- formations." 148 It is probable on the contrary that many of these changes undergone in the past or in the characters now existing are rather simply the result of the reaction of the plant organs to a certain external or internal stimulus which has not yet been remarked nor indeed suspected. To this category belong very probably, for instance, all the various secretions of chemical substances. Further the very fact that secre- tions that are entirely alike occur in plant species that are quite unlike one another in everything else, speaks, as we shall see at once, in favor of the hypothesis that these same secretions are acquired and inherited char- acters. Other characters which likewise were formerly in all probability functional adaptations or are such now can incidentally serve other purposes and can therefore be useful to the species in other ways also, as we stated. It is evident that Weismann, in order to support his assertion that natural selection is quite capable of ex- plaining by itself the transformation of species, has allowed himself to be misled into denying arbitrarily 148Weismann : Neue Gedanken zur Vererbungsfrage. P. 66. Consideration of Weismann' s Arguments 187 to a large number of modifications, which do not differ in their essence from the others, the character of func- tional adaptations. We would not deny that there are certain forms or structures, which, just because their functional character has not been perceived, have not heretofore received any explanation except through nat- ural selection, even though it does not always furnish an altogether satisfactory explanation. But the deeper one goes into the essence of functional adaptation and the wider its field of action is seen to be, the number of these formations become less and less and with it dwindles away also this seeming almightiness to explain all physiologic transformations whatever, which Weis- mann would like to attribute to natural selection without producing proof for it. 3. The third argument based upon the neutral indi- viduals of ants, bees and termites is well known as the chief question about which turned the polemic between Weismann and Spencer. The latter brought up as one of the strongest arguments in favor of inheritance, the co-adaptation, that is the co-ordinated modification of different parts co-operating to produce a definite physio- logical result. Weismann on the contrary sees in the existence of neutral individuals among the ants, termites and bees a refutation of Spencer's theory, since these individuals in the course of their phylogenetic develop- ment have undergone harmonious modifications of di- ' verse parts, without ever having been capable of repro- duction. To this Spencer replied that all the harmonious modifications of different parts, including the numerous ' instincts which the neutrals present to-day, are only the heritage of those which the ancestors of these now social insects acquired in a state of isolation or in a society in 1 88 Inheritance of Acquired Characters which there was equality and in which there were no castes, the neutrals being nothing else than females incompletely developed because of defective nutrition. It would not be correct to state that the question has been finally decided through this debate. While Weismann has not been able to prove conclusively that these harmonious modifications of the neutrals have been at least partially acquired after the development of castes and when the sterility of the neutrals had already appeared, neither has Spencer been able to demonstrate that all these harmonious modifications had been already acquired by the presocial ancestors. Nevertheless the conception of Spencer that the neutrals are produced by an arrest of development of the females has in our opinion won a decisive victory over that of his opponent, and has in reality taken from the last rampart of the Weismannists all the strength which it had derived from the conception that the neutrals were special formations, which had acquired special characters by fortuitous vari- ations and natural selection only. 4. The fourth and last argument, that of the incon- ceivability of the transmission of acquired characters, has already been considered by Darwin in connection with examples which he had himself communicated of the inheritance of certain peculiarities, particularly of instincts acquired by domestic animals. "Nothing in the whole circuit of physiology," he stated in this connection, "is more wonderful. How can the use or disuse of a partic- ular limb or of the brain affect a small aggregate of reproductive cells seated in a distant part of the body, in such a manner that the being developed from these cells inherits the characters of either one or both parents? Consideration of Weismann' s Arguments 189 Even an imperfect answer to this question would be satisfactory." 149 This argument which Weismann considered as the strongest, without indeed saying so definitely but allow- ing it to be seen, is in reality the feeblest of all. Even admitting that the mechanism of transmission may be at present quite inconceivable, that is no reason for believ- ing that it does not exist, since the number of phenomena and even of natural laws which we must regard as cer- tainly established, even though we cannot so far explain them in any way is, one can well say, infinite. It recalls the former objection to Newton's theory that it is incon- ceivable how the heavenly bodies could mutually attract one another at such a distance, and like this it is of no logical value. Apart from this it can have only one very important practical consequence, (and it has had this effect and as a matter of fact is still producing it), i. e. of bringing the reality of this inheritance into ques- tion with very many investigators and stimulating them therefore to a zealous search for a conclusive experiment which should once for all establish or exclude it. In any case it is interesting to note that Nussbaum whose theory of the continuity of the germ cells sug- gested to Weismann his fundamental conception of the continuity of the germ plasm, is opposed to him in that he does not exclude the possibility of the transmission of acquired characters. For immediately after the expo- sition of his theory he states "since seeds and eggs are stored up in the parent organism, they are therefore subjected to the action of conditions which bring about 149Warwin : The Variation of Animals and Plants under Domes- tication. H. P. 367. 190 Inheritance of Acquired Characters modifications of it, and so the transmission of acquired characters is not excluded." 15° After thus having brought forward and refuted the four principal arguments adduced by Weismann against the Lamarckian theory, we must now examine the value of the corollaries and subsidiary theories which this investigator devised to defend his doctrine from the mani- fold objections which were brought forward from all sides to show its inadmissability. Panmixia presents itself as the first subsidiary . theory. It has entirely succumbed. It was devised by Weismann to explain the progressive phyletic atrophy of the organs which have become useless, and rests on the supposition that as soon as the fortuitous variation of a certain organ has become useless for the species, and is therefore withdrawn from natural selection, the minus variations which this organ would chance to present in certain individuals would no longer cause the disappear- ance of these latter in the struggle for existence. The survival of organisms with such minus variations and their sexual union with individuals which still preserve the organ in its original state would lead gradually to the degeneration, progressive atrophy and final disappear- ance of the organ. Spencer, nevertheless, rightly draws attention to the fact that the appearance of plus variations is just as probable as that of minus variations, and therefore pan- mixia is not at all capable of explaining by itself this , progressive and continuous degeneration of useless organs. 1BONu/9baum : Zur Bifferenzierung des Geschlechts im Tierreich. Arch. f. mikr. Anat, Bd. 18. Erstes Heft. Bonn. Cohen. 1880. P. 113. Panmixia and the Principle of Economy 191 The addition suggested by Romanes that the tendency to atavistic reversion favors minus variations at the ' expense of plus variations, does not suffice.151 For in the first place an atavistic reversion could appear at best only in the phyletic characters acquired last, and in the second place, after the first stages of atrophy had ap- peared it would in any case tend from that time on to insure the preponderance of plus over minus variations. One could nevertheless assert that panmixia is not necessary for Weismann's theory. The principle of the economy of the organism by which every useless and unused organ is harmful because it withdraws nourish- ment from other organs is by itself enough, if one rejects the inheritance of acquired characters, to explain the gradual phyletic disappearance of useless parts. But this hypothesis is easily refuted by some calcula- tions of Spencer, showing that it is impossible that the advantage to the organism of a small inborn and fortu- • itous minus variation in the useless organ, particularly when this is already very much degenerated as is for instance the hind leg of the whale, can procure for the individual an advantage over others and so provoke the phylogenetic passage to a yet greater atrophy. And no great value can be attributed to the counter observation, which Weismann several times repeats, that we are still quite unable to measure the selective efficacy of the • struggle for existence. One need think only of the parasites and particularly of the endoparasites, which have always an excess of nutrition and in which there- fore the advantage of the degeneration of useless organs • would become reduced absolutely to zero. And it is lB1Romanes : A Note on Panmixia. Contemporary Review. Octo- ber 1893. P. 612. 192 Inheritance of Acquired Characters nevertheless in these that this reduction reaches its maximum. However that may be, we might nevertheless admit either that, as panmixia supposes, fortuitous minus variations preponderate over plus variations, or that the principle of the economy of the organism is alone enough to secure the victory to those individuals whose useless organs are most atrophied. But, even in that case how could panmixia and the principle of economy in the organism explain the fact that the atrophic state of organs which have become useless, such as appears in adult organisms, results in the course of ontogeny from an involutive process of these organs which are better developed in the early stages than in the later stages? Although in so doing we anticipate a question which we shall examine again later in all its generality, we may note here that the most that panmixia and the principle of economy could do would be to explain the fact that the more recent a species with a given atrophied organ is, the earlier should be the stage of development at which the organ in question is arrested in ontogeny, whereas in the ancestral species it attained a greater development. But how can they explain how certain tissues and organs develop during ontogeny up to a certain and fairly advanced point and thereafter at a certain moment undergo a physiologic involution result- ing in their degeneration and often in their complete disappearance ? As we pass on now from continuous, gradual atrophy of useless organs to the slowly progressive formation of useful organs and so to phyletic evolution in general, we must declare at the outset that of all the objections that have been urged and which can yet be urged against the All Sufficiency of Natural Selection 193 view that natural selection by itself is sufficient to account for the transformation of species, we shall bring forward only a very small number. For we believe it worth while to limit ourselves to the most characteristic and certain ones, which serve better than the others as an indirect support for the Lamarckian theory. And so much the more since in the case of many objections dis- cussion is idle. For — and this may be said once for all — if our adversary adopts the complete sufficiency of natural selection both as his thesis and as the ground for the defense of this thesis, naturally it will be very difficult, indeed, often quite impossible for us to carry on the contest from a purely logical standpoint. Candidly one could wish that Weismann would prove this omnipotence of natural selection by some facts. But he has still to furnish this proof. For, as we have seen, he has limited himself to showing that among the various hypotheses which have been devised to give account for certain special formations, natural selection is the one which fulfills this purpose relatively best. But when once our adversary sets up this almighti- ness of natural selection as an axiom, to be employed at need as thesis or as the support of the thesis, it will then be very difficult, we repeat, in most cases to point out any contradiction in his tenets, which is the only means by which a logical refutation can proceed and reach any result. In other words, if in order to demon- strate the complete sufficiency of selection Weismann starts off with the supposition that natural selection is omnipotent, how can one by pure reasoning convict him of error? And in fact: One investigator offers the objection that fortuitous variations even though they are useful 194 Inheritance of Acquired Characters must nevertheless in many cases be so inconsiderable in amount, that they could not possibly constitute such an advantage as to give natural selection anything to act on. But Weismann in order to extricate himself from this embarassment needs only to repeat here again his habitual, axiomatic, already mentioned reply that we are unable to measure the degree of selective power of the struggle for existence. Others object that certain characters due to functional adaptation are altogether useless to the species. One can conceive how they might be inborn in individuals if one admits the inheritance of acquired characters, whereas they would be quite inexplicable if one sought to ascribe them to natural selection alone. But Weis- mann has always the answer at hand that one cannot judge of their present or past usefulness. A typical example of these discussions which logical processes are powerless to decide is the question debated in the Spencer- Weismann polemic upon the especially acute taste sense of the tongue papillae. While Spencer attributes it to the continual rubbing of the tongue against the teeth and states that it is without utility for the organism; Weismann on the contrary asserts that it may have been of some use, at least in the past. We do not forget in this connection that the question might also be raised whether this fine sense of taste is really inborn or is not rather acquired anew in each individual after birth. Others regard natural selection as powerless to bring about any transformation because the fortuitous vari- ations or individual deviations, upon which it is able to act, are constantly destroyed by amphimixis. Weismann can always reply that the fortuitous variations or devi- ations preserved in an individual by natural selection are Amphimixis and Nutritive Irregularities in Germs 195 not prevented from passing to descendants by the con- jugation of this individual with others which do not possess any such variations, but are only diluted, so to speak. Therefore all that is necessary is to suppose the struggle for existence to be more severe or to have a higher degree of selective capacity than that which would have been sufficient if there had not been sexual repro- duction. It is well known further that Weismann, in order to afford natural selection abundant and never failing material upon which to act, has made for himself a weapon of sexual reproduction, attributing to it a great fruitfulness in the constant production of new variations. But he seems to have been partly converted finally to the opposite view of the Lamarckians already mentioned, that sexual reproduction only contributes to securing the unity and constancy of the species. For, in order to ex- plain the production of a lot of fortuitous variations, he finally sought refuge in the unavoidable irregularities of nutrition in the germ plasm,152 a thing which makes his hypothesis upon the biologic function of amphimixis quite superfluous. It may be merely noted here that when once one sees in amphimixis a cause tending toward the levelling of individual characters and consequently to- ward the fixity of the species, and thereby reducing by so much the probability that the selective capacity of the struggle for existence is alone sufficient, one must then feel so much the more strongly the necessity of discover- ing some cause of variation capable of acting simul- taneously and in the same way upon at least quite a large part of the individuals of the species, and of t52Weismann : Das Keimplasma. P. 541 — 570. 196 Inheritance of Acquired Characters acting also in this same direction throughout a decidedly large number of successive generations. "One must admit," says Hartmann, very rightly, "that minute and purely accidental variations even if they are useful, are unable to preserve themselves from disappearing again through crossing. Whatever is to be preserved must, as Darwin also admits, appear in a certain quantity, either all at once or successively, because the number of similar variations must be sufficient to overcome the suppression through crossing. But it is not to be expected that similar variations will appear in such frequence by chance, but only as a result of definite external or internal causes which set a definitely directed modification in the place of accidental ones." 153 Of the causes of variation which possess this capac- ity of simultaneous similar and constant action, we know at present only functional adaptation aided by the inher- itance of acquired characters. The very fixity of many species has been rightly urged against Weismann. Natural selection in fact, because of the smallness of fortuitous individual vari- ations, is forced, on the one side in order to explain by itself the development of species, to fall back upon an excessively great degree of selective capacity; but on the other side if this great degree of selective capacity is accorded, it encounters still greater difficulty in account- ing for the contrary phenomenon presented by a host of other species which have remained unaltered even dur- ing a whole series of long geologic periods. The Lamarckian theory does not find any special 1B3Eduard von Hartmann: Die Abstammungslehre seit Darwin. Annalen der Naturphilosophie, herausg. v. W. Ostwald. Zw. Bd., Heft. III. Leipzig, May 26, 1903. P. 289. Transformations Due to Environmental Changes 197 difficulty in this. If the true provocation to alteration comes from the environment and not from natural selection, the evolution of -certain species and the con- stancy of certain others may be explained simply by the respective alteration or stability of their environment. The alteration of the environment could be brought about in the case of a given species not only through natural telluric changes but also for instance by the migration of this species toward other regions, or by the immigration of other species into its territory, often also by the overcrowding of its territory by the species itself. Emigration as a cause of variation of environment does not need to be illustrated by examples. The immigration of other species can immediately induce a very considerable modification of the environ- ment. The immigration of a bird of prey with rapid flight will have as a result that the birds for instance of a certain native species are compelled to fly more rapidly in order to escape it. This repeated greater effort will develop an increase of their swiftness, an increase which would not have been attained, we must believe, by normal daily exercise. For the normal exercise of a given function after the respective organ has once been formed, does not develop it any further but merely causes it to preserve the degree of development already attained. In this way one can readily see also that if the region ravaged by the bird of prey is only one part of the whole territory inhabited by this aboriginal species, one portion only of that species will be forced to become transformed into a swifter variety while the remaining portion can and must remain unaltered. The overcrowding of a given territory by a given 198 Inheritance of Acquired Characters species can not only force this species to emigrate, or at least to widen its range of habitation, thus placing itself in contact with different telluric conditions and with different fauna and flora, but also it can itself induce directly very considerable modifications of the environment. Thus, to take an already famous example, it is pos- sible that the long neck and forelegs of the giraffe are to be ascribed to the overcrowding of the territory inhabited by its ancestors. For if we suppose that these ancestors at a definite time and in a definite region had become altogether too numerous in proportion to the trees present whose leaves served them for nourishment, then all the leaves up to a certain height would naturally have been eaten first, and there would finally remain only those leaves situated very high, so that in order to reach them the animal was forced to stretch out its neck with a greater effort than formerly and to stand upon its hind legs, falling later on the fore legs after plucking off the leaf. And these efforts so very different from the ordinary would have produced quite new morphological adaptations. But it is not at all necessary to suppose that all the individuals of this former species of giraffe were forced indiscriminately to this transformation. For many, perhaps becoming accustomed to another diet, could remain unaltered or undergo transformations of little importance. In other cases, on the contrary, that part of the species which was driven through overcrowding of the territory to change its diet would be compelled to undergo the most considerable transformations. This would be the case, for example, when the overcrowding of a given tract of meadow land by a species feeding exclusively Consideration of Weismanris Arguments 199 on grass and herbs would force some of these animals to feed on tree leaves, others to become transformed into rodents and insectivors or to undergo some transforma- tion of still another kind. In this connection it is to be especially noted that just because a large number of the individuals of the old species will thus have come to seek their nourish- ment elsewhere, no change will appear in those remaining behind in the former conditions of nourishment. In other words the elimination caused by the change in their habits of the overcrowding individuals from among the company of the old species will leave the other individuals of this species, whose number will now be no longer too great, in the same conditions of environ- ment as formerly, without any overcrowding and con- sequently there will not be any further causes provoking in these individuals also a transformation into another species. The change of nutrition will induce then a whole series of changes in the functions of seeking food, hunt- ing, fighting, seizing, chewing etc., but only for that position of the species which has changed its mode of living. Thus it is clear how, during a series of entire geological periods, a certain number of the ancestors of individual species that are now quite different from them were able to preserve themselves unaltered and so to reproduce their descendants unaltered to this day. Man is distinguished from other animals perhaps in this, that whereas the latter do not modify their environ- ment or modify it only indirectly and intermittently by emigration or by overcrowding and consequently make no progress in their development so long as their environ- ment undergoes no alteration from one of the causes 200 Inheritance of Acquired Characters mentioned or from some other accidental cause, man on the contrary modifies his environment directly and continuously by the products of civilization. And this unceasing modification of the environment results in the unceasing evolution of the man. It is thus for example with cerebral development. Civilization itself and the continual progress of science and arts make steadily increasing demands upon the brain. And this mental exercise, steadily increasing from generation to generation, contributes always to the development of the brain. What wonder then, if the cranial capacity of man has become markedly increased even during the three last centuries, as is stated by the anthropologists ? Another cause whereby one portion of a given species can remain unaltered while the remaining portion becomes transformed, is found, when once the inherit- ability of acquired characters is admitted, in the sudden apparition of certain instincts. "Also in the domain of biology," writes Emery, "and very especially in that domain, many characters of organisms seem to me to permit of explanation only by sudden formation. This is especially true of habits and instincts. How could the first Velleius dilatatus arrive gradually at its para- sitic life in the nest of the hornet? The first cuckoo certainly commenced suddenly to deposit its eggs in the nest of a strange bird." 154 The first sudden appearing of a new instinct can be compared to a happy thought. It is a definite association of ideas which is formed for the first time. But when it has once been formed, it is easily possible and indeed 1B4Emery: Gedanken zur Deszendenz- und Vererbungstheorie. Biol., Centralbl., July 15, 1893. P. 416. Appearance and Inheritance of Instincts 201 probable that it will be formed often again in the same individual, and this repetition will produce a constantly increasing, corresponding modification of the nervous tissue in the individual concerned, which will be repro- duced in his descendants. A new association of ideas can arise and actually does arise independently, within certain limits, of the nervous structure of the individual, and therefore independently of the germ substance also which has produced this latter, in so far as the fortu- itous external circumstances which produce this new association exert a strong and overmastering influence: Among a thousand individuals, quite identical in regard to the structure of their nervous mechanism, this new association of ideas will be developed in only one, on ' account of the special external circumstances in which it happens to be placed. But without the inheritance of acquired characters this fortunate new association of ideas, and the repeated employment of it by the individual later, would be com- pletely lost for the species. To assure its transmission from one generation to another there would remain only ' imitation or education in the widest sense of the word. But the fact is that nearly all the instincts are, on the contrary, truly inborn, that is to say they are produced without any psychic educative influence whatever. It is clear also that not all the members of the older species will be able to make use of a new, fortuitously developed instinct through educative imitation and later through heredity, but only the immediate descendants or associates of the individual in whom it was developed. All other members of the species would be excluded. And so those will be the only ones, who, in consequence of this newly adopted habit, will make a thorough 2O2 Inheritance of Acquired Characters change in their former manner of life, and in whom consequently the whole organism will be modified through functional adaptation. In this way may be explained the transformation of only one group of the individuals constituting a species into a new species, while the others remain unchanged. These few examples, even though so briefly outlined, are nevertheless quite enough to show us that the Lamarckian theory is capable of explaining at the same time both the evolution and the fixity of a species. But how can Weismann account for the inalterability and constancy of a given species? It goes without saying that he has no hesitation in attributing it, like variation itself, to natural selection again. But even if one were willing to suppose the environment immutable, is it possible that any species could ever come to such a degree of perfection in relation to its environment that every new variation in any direction whatever must make the conditions of this species worse and make its mem- bers less likely to be victorious in the struggle for existence? Is it not much more probable that however high a degree of adaptation to its environment a species may have attained, it can always become even better equipped for the struggle for existence through further transformations in certain directions, and consequently offer still greater opportunities for natural selection which is everywhere and always upon the qui vvue? We must nevertheless be careful, in relation to this question of the fixity of species, not to attribute to the arguments which we have just set forth any greater value than they actually possess, especially because we know nothing, or only a very little, concerning the immediate circumstances that have actually existed in the develop- Adaptations in Tissue Structure 203 ment of even a single species, and still less, if that is possible, concerning the true mode of procedure of this natural selection which is so difficult to control. So that we must content ourselves with putting forward the views which we have just outlined merely as further con- jectures speaking in favor of the Lamarckian theory, without seeking to attribute to them the value of logical proof. Structural relations in general and the most remark- able ones in particular, such as the static structure of bone, of certain tendons, of certain membranes, the dynamic structure of the smooth and striated muscle tissues and other similar formations, which represent the most perfect functional adaptation and the best utilization of the material down to the minutest and most delicate details, testify likewise in favor of the transmissibility of acquired characters. "All these formations of con- nective tissue, muscle and bone," writes Roux, "could never have been developed in such regularity and com- pleteness by Darwinian selection from individual varia- tions of form, since here there must necessarily have been thousands of fibers and cellules already accidentally arranged in this purposeful fashion in order to produce even the smallest advantage appreciable in the economy and capable of being acted upon by natural selection, and so much the more since in the extremity of hunger these would be exactly the parts, the heart excepted, which, thanks to the small amount of metabolism in them, • would suffer last of all, — much later than other more vitally important organs with more active metabolism. These formations could not therefore arise from the selection of individual variations in form, but are rather derived only from those qualities of the respective tissues 2O4 Inheritance of Acquired Characters to which is directly due the fashioning of the adaptation even to the smallest details." 155 But Weismann could very lightly deny to these very perfect structural formations any value as even indirect proof of the principle of inheritance. From his point of view he needs only to object that since they are useful to the species they can then be very easily explained by natural selection alone, and no refutation would be possible. Inborn characters have the tendency to become like those which the ancestors acquired by functional adapta- tion, and this coincidence speaks also in favor of the hypothesis of the inheritance of acquired characters. But against this also Weismann would have no lack of words and apparent arguments. Functional adaptation, he could reply, renders the species more capable of resistance. The greater the individual disposition to this adaptation the more rapidly the adaptation will proceed. Consequently those individuals upon which this disposition is especially impressed will survive, and above all those individuals in which this adaptation is already existent potentially in their germ plasm. Thus the coincidence in question could be explained without requiring the adoption of the inheritance of acquired characters. In this way one would arrive at the conclusion that all characters susceptible of being produced by the innu- merable functional adaptations must for that very reason be always useful to the species, so that they can be fixed even when they happen to be actually produced by fortu- itous inborn variation. We shall not consider this further 155Roux : Der Kampf der Teile im Organismus. P. 30- Similarity of Adaptations in Different Species 205 for it would lead us back to the question which Spencer has raised with his example of the more acute sense of taste in the papillae of the tongue, and of which we have spoken above. But apart from that, is it possible for this argument, however unassailable it may be from a purely logical point of view, really to weaken the strong presumption in favor of transmissibility which is derived from the fact that inborn structural relations always follow — though slowly and tardily — those acquired in life through functional adaptation, as the shadow follows the body? Another fact among those which speak most con- vincingly in favor of the inheritance of acquired char- acters is the similarity of certain structures in different species which are subjected to the action of the same mechanical conditions. Without needing to bring up the most typical and most familiar case, viz., the trans- formation of the extremities of the whale into fins, it would be enough to recall as examples the like char- acter of the leg joints in the two-hoofed animals (Diplartha, Cope) and the rodents, which is attributed to their rapid locomotion; the like structure of the extremity of the radius in the edentates and the quadru- mana which possess the power of supinating the hand; the like reduction in number of the digits in many orders of digitigrade mammals which run about on the dry hard ground; the like modifications in the form and development of crests on the skull following like employ- ment of the canine teeth as fighting teeth in all orders in which these teeth are strongly developed.156 158Cope : The mechanical Causes of the Development of the hard Parts of the Mammalia. Journal of Morphology, vol. VIII, No. 2. Boston, U. S. A., Ginn, Sept. 1899. P. 159—163, 164—165, 175—176, 182—183, 201—203, 273. 206 Inheritance of Acquired Characters For if these structures had all arisen through nat- ural selection only, selecting the most fit from among all the chance variations, one could not explain how, in different species, even though they were subjected in respect to that particular organ to like mechanical con- ditions, it could lead to one and the same result. In fact how could one affirm that the structure of any given organ must be of one certain character only and no other, in order to render the species most fit for the struggle for existence? So mere chance must be invoked to account for the fact that of the numerous structures among which natural selection could choose, it has selected in the most different species subjected to the action of the same mechanical conditions in relation to only one of their organs, just one single structure for this organ, absolutely alike for all these different species. A similar phenomenon, which leads one toward the same view, is mentioned by DeVries, as already noted, in support of his theory of pangens or preformistic germs, representative of definite characters ; namely, that the most diverse species of plants have often the power of producing a greater or less number of identical chem- ical compounds. "Insectivorous plants, for example, belong to the most different natural families, neverthe- less they all possess the faculty of producing from their leaves the necessary mixture of an enzyme and an acid requisite for dissolving albuminous bodies." 157 Darwin himself has already remarked that this mixture is quite similar to the gastric juice of the higher animals. Now without wishing to touch anew upon the ques- tion of preformistic germs which we have already dis- "7De Vries : Intracellulare Pangenesis. P. 8— 10. Similarity of Adaptations in Different Species 207 cussed, it may merely be remarked here that the existence of these like properties, acquired in the most different species, is easier to explain by the Lamarckian theory than by natural selection. For just as substances exposed, for example, to identical calorific influences finally all take on the same degree of temperature, and yet all remain quite different from one another in other characters, so when quite different species are exposed, on account of the environment, or nourishment, or peculiar condi- tions of light, or of any other cause whatever, to func- tional stimuli which incite them, for example, to secrete tannic acids or alkaloids, and so on, these secretions, acquired by means of functional adaptation and trans- mitted later by heredity, must come to be present in several species, even though all other characters can remain different. Here it seems to us, there remains for Weismann nothing else than to affirm that these like characters may have been fixed by natural selection in the most different species because, from the likeness of the func- tional stimuli to which, according to the hypothesis, these species were exposed, the same characters must have been the most useful for every one without exception. But this must first be proved. And so much the more since in this case it is more difficult than in other cases to see the absolute necessity that all characters or pecu- liarities whatever, which are due to the reaction of the organism to the most different external influences and often to insignificant ones, must always be useful to the individual, and that therefore they must also possess this utility even when they are produced rather by means of inborn fortuitous variations. If the soundness of this conception, required by the 208 Inheritance of Acquired Characters theory of Weismann, of the selective utility of every inborn character which happens to be a repetition of a functional adaptation induced by reaction to any external influence whatever, is very doubtful, and in any case far from having yet been proved ; yet the utility of one part of these acquired characters is proven and indubitably established. And indeed it is just from this utility that one of the strongest arguments against the presumed non-inheritance of acquired characters is derived. Since in fact the usefulness of some functional adaptations to the individual is great and sometimes extremely great, it must immediately follow from that, according to Weismann' s view, that the inheritance of acquired characters is itself the result of natural selection. For the species in which this inheritance began to mani- fest itself even though to a slight extent would certainly have had an advantage over the others, just because the adaptation to the environment in their descendants could go on with ever increasing rapidity. "As modifications acquired by use during life," writes Cope, "are necessarily useful, it follows that if one accepts the post-Darwinian or Weismannian theory the only mode of acquisition of useful variations which we know is excluded from the process of organic development." "Each generation should commence, in the matter of useful characters acquired by use, at the same point at which its ancestors had commenced, so that an accumu- lation or development of these characters would hardly be possible. The influence of the environment as well as the energies of the living being would be incapable of developing in a given generation more than only that which this generation could acquire during its single life. How could evolution, then, account for the law, which Inheritance in Unicellular Forms 209 paleontology has demonstrated in so splendid a manner, of the gradual modification of certain parts through long geologic ages toward ideals of mechanical perfection, for example, of the gradual perfecting of the skeletal articulations? Not only does the post-Darwinian school afford no explanation of this but with the acceptance of its theories this progress is indeed impossible." 158 And Osborn's view is similar. "Living matter," writes he, "is characterized by a capacity of adaptive or purposeful reaction. If this capacity is inherited in the Protozoa, thanks to the simplicity of their process of propagation, it must be the same in the Metazoa. For each newly developed metazoon which retains the advan- tage of the inheritance of the adaptive reaction would be preserved, while each individual which loses this would degenerate. The mechanism of the inheritance of onto- genic adaptation must then have been developed in passing from the unicellular to the pluricellular forms by means of natural selection." 159 Weismann it seems to us could reply only by saying that natural selection may not have established the inher- itance of acquired characters in the pluricellular forms also, because the production of the mechanism neces- sary for that purpose had become materially impossible by reason of the structure of the metazoic organism. But this assertion, which would limit the capacities of living organic substance, must appear a little too hazard- 158Cope : The mechanical Causes of the Development of the hard Parts of the Mammalia. Journ. of Marph., vol. VIII, No. 2, Boston, U. S. A. Girni, Sept 1889. P. 140—141. "8Osborn : Alte und neue Probleme der Phylogenese. Ergebnisse der Anatomic und Entwicklungsgeschichte, herausgegeben von Mer- kel und Bonnet. Band. III. 1893. Wiesbaden, Bergmann, 1894, P. 607. 2 io Inheritance of Acquired Characters ous and quite unfounded, especially when one thinks that apart from that there is no process or phenomenon in the organic world which Weismann does not ascribe to the almightiness of natural selection. Thus the power of regeneration, sexual reproduction, the physiological necessity of death in the pluricellular forms, all are based upon natural selection. And is the almightiness of nat- ural selection insufficient only for the inheritance of acquired characters ? After bringing to a close this rapid review of the objections which Weismann can always oppose to any argument, thus enabling him to save himself from com- plete defeat at least with the help of mere words and with a semblance of logic, it remains for us, before passing to the following chapter, to examine two other objections to his views which seem to us particularly im- portant, in that the arguments which he opposes to them appear to involve his own theory in the most striking contradictions. These are the inexplicability of co- ordinated variations, and the repetition of phylogeny by ontogeny, and with these we shall close the present chapter. The objection to the conception of the complete sufficiency of natural selection which arises from co- ordinated variations is well known. When the utility of certain modifications of the organism depends upon the correlative development of many quite different parts natural selection cannot account for the inter- dependent, phylogenetic modifications, since, for the production of these latter, it can act at best only upon special fortuitous variations which are independent of others and from that very fact totally useless. Roux for example describes in a masterly way the Coordinated Variations 211 contemporaneous formation of thousands and millions of new characters each adapted to the others and all combined to perform some function such as must be necessary in the phylogenetic passage from an aquatic to a terrestrial life. And he concludes in these words: "One must necessarily conclude that functional adaptation, such as is produced in alteration of the conditions of life, can bring about purposeful co-ordinations simulta- neously in all organs of the body concerned. And the characteristic feature of this simultaneity of action in millions of parts must be the fact that it is opposed to the action of natural selection which can never develop simultaneously more than a very limited number of purposeful characters." 16° \Yeismann on whom the force of this objection aris- ing from correlative development is not lost, has sought to get around the difficulty by setting over against it, as we have noted above, the neuter forms in bees, ants, and termites. He does not deny the extraordinary diffi- culty of explaining co-ordinated variations by natural selection, but expects to show that in spite of it there exist undoubted examples in which this difficulty was overcome by natural selection. As to the polemic which raged between Weismann and Spencer on the subject of these neuters, we have already seen how in our view Spencer has succeeded in driving his opponent from any tenable ground by demonstrating convincingly that the neuters are really nothing else than incompletely developed females. We shall not return here to what has already been said. But it is worth while to observe that Weismann thus 160Roux: Der Kampf der Teile im Organismus. P. 39 — 44. 212 Inheritance of Acquired Characters deprived of his last stronghold saw himself forced to give an explanation for these co-ordinated variations also, which should prove, at least from a theoretical point of view, that they also might possibly be produced through natural selection. But it is just in this attempted explanation that he has fallen into the most evident contradiction, a thing which was inevitable anyway seeing that his thesis is untenable. It is worth while to spend a little more time examining this contradiction. He utilizes for this purpose a theory which, though introduced only at the last to supplement or replace the earlier, already discussed theories of panmyxia and the economy of the organism, was originally intended to give if possible some better explanation than the earlier theories offered of the continuous regression of useless organs even after any further regression is of no more value for natural selection. According to this theory when once the involutive process has begun in a given organ from any external provocation whatever, it would acquire in this very way an intrinsic tendency to bring about more and more retrogression. And the tendency acquired by this organ and now inherent in it toward constant phylogenetic regression would be accounted for by the following consideration. Weismann affirms that when the tendency to degen- erate once appears in an organ especially well developed, let us suppose by natural selection, that proves that it is represented from that time on in the germ plasm by determinants "of smaller growing power." "But since," he continues, "growth and assimilation are physiologic functions, just as are contraction and secretion, so the fundamental principle of intraselection is applicable to them: the functional stimulus strengthens the function- "Determinants of Smaller Growing Power" 213 ing organ, and the part which performs its function more energetically attracts to itself more nutrition and repairs with interest its loss of matter more rapidly than does the part performing its function less energetically. So in the struggle of the parts for nutrition the more feeble determinants will be at a disadvantage, they become slowly but constantly feebler in the course of generations until finally they degenerate completely." 161 So that according to the view of this investigator: 1 'It is not the functional change that is inherited; but variations in the biologic value of a part, (for example, of an organ which has become useless), give the im- pulse to the regressive or progressive variations of the germ plasm and these only would establish the heredi- tary functional change of the somatic part." 162 Now it is just this conception of determinants "of smaller growing power" which is quite inadmissible, and which constitutes a contradiction in terms. For what signifies in general "weaker" or "stronger" determinants? The determinants of a small organ are by their very definition not feebler than those of a larger organ; they are only qualitatively different from them. Also when the variation of an organ consists in a diminution of its mass, this is not a diminution of grow- ing power of the respective determinants but an alteration of these latter, which is nothing else than their replace- ment by other qualitatively different determinants. Would Weismann himself say that the respective determi- nants of the fore legs of the kangaroo are provided with a smaller power of growth than those of the hind legs? Or that the fingers of the human hand have determinants 161 Weismann : Neue Gedanke nzur Vererbungsfrage. P. 14 — 15. "2Weismann : Ibid. P. 59. 214 Inheritance of Acquired Characters whose power of growth stands in exact proportion with their length? Would the shorter fingers therefore have a phylogenetic tendency to become constantly still shorter, and the longer fingers a tendency to become steadily longer yet? If that were so it would lead directly into this absurdity, that the formation in phylogeny of new organs or of new structures in general could never have any commencement, since originally their determinants, just because of the very smallness of these formations, must have been provided with only a very small power of growth and therefore could never progress side by side with determinants which must in any case be stronger since they belong to organs or structures already developed. If on the contrary this is not the case and cannot be the case because it contains an unavoidable contradiction, then the determinants of any degenerated organ what- ever, such for example as the hind leg of the immediate ancestor of the whale, cannot be regarded as feebler, but must rather be regarded as qualitatively different from those of the complete organ. Consequently there cannot exist for the degenerated organ any phylogenetic tendency to become still more rudimentary. Weismann would give, as we have said, a quite similar explanation of co-ordinated variations : If there were, for example an increase of the weight o£ the head, as a direct result let us suppose of natural selection, certain muscles of the body after having received an initial impulse from natural selection itself, would acquire a phylogenetic tendency to grow part passu with the weight of the head. For the first operation of natural selection would be to eliminate individuals whose muscles were too feeble. Then even if we suppose that Weismann's Explanation of Coordinated Variations 215 every fortuitous variation that is possible, both plus and minus, actually develops, the net result after the elimi- nation of the minus variations must be that the muscles would be stronger. But this initial increase, this first impulse toward strengthening, would be in turn the cause of a phylogenetic tendency to a further strengthen- ing, because it would indicate that these muscles were represented in the germ plasm by determinants which are endowed with a greater power of growth, and con- sequently with greater power of assimilation. "The affluence of nutritive fluids would become proportionally augmented and would contribute likewise to giving the plus variations a preponderance over minus variations. There would thus be a phylogenetic tendency toward the continual increase of these muscles and it would endure just as long as the increase in the weight of the head, and would stop when the latter stopped. For in this case the plus variations of the determinants would be eliminated by individual selection, as soon as they attained selectable value." 163 But this artificially constructed hypothesis, which did not hold good at all in the case of rudimentary organs, is still less adapted to the case of co-ordinated varia- tions. For in these phenomena it appears still more clearly that in phylogenetic changes there are concerned not simply exclusively plus or minus variations, but transformations which might be constituted by a com- bination of increases in one direction and decreases in another, or might not be susceptible of being decomposed into merely quantitative variations. It should be noted further that for certain correlative, histological varia- LG3Weismann : Neue Gedanken zur Vererbungsfrage. P. 22. 2i6 Inheritance of Acquired Characters tions of physico-chemical nature, which are concerned in any way in the fundamental specific characters of vital processes, the expressions increase and decrease have no significance at all. Nevertheless it would not have been advisable not to mention here these later explanations of the atrophy of organs which have become useless and of co-ordinated variations, because the fact that Weismann substituted them for his earlier ones, shows that he himself regarded the earlier explanations as insufficient, and because the artificiality of these new explanations shows very clearly the almost insurmountable difficulty encountered in the attempt to explain these phylogenetic phenomena if the inheritance of acquired characters is rejected. But the phenomenon which more than any other remains an enigma when the inheritance of acquired characters is rejected, and which when this inheritance is accepted becomes not only self explanatory, but sets the whole mechanism of inheritance in the clearest light, is that of the repetition of phylogeny by ontogeny, and just because of this we reserved it for the last. "Whenever a new species is formed," writes Delage, "it is accomplished by the addition of one or more new characters, at the end of ontogeny, after all the old specific characters have already appeared. And since this goes on from the very commencement it is evident that the characters must appear in ontogeny, in the same sequence as in their phylogenetic formation." 164 But if there is no inheritance of acquired characters why should the new character be invariably just added to those already present, and only after the development of the latter is completed ? Why should it not be possible "4Delage : L'heredite etc. P. 366. New Characters in Phlogeny 217 for each variation of the germ substance to appear or to become active, either from the beginning, or at any time at all during the ontogeny? "The phenomena of latency" says Osborn, "speak absolutely against Weismann's conception, according to which phylogenetic development would take place in the germ plasm by selection of advantageous elements, and elimination of disadvantageous elements. These phe- nomena of latency indicate that the phylogenetic process does not consist in an elimination but in a shoving of certain characters into the background (Zurickdrangung) during the later stages of ontogeny." Osborn cites as example the well known experi- ments of Cunningham on the color of the asymmetrical flat fishes, pleuronectids, on whose lower colorless side artificial illumination is followed by a reappearance of the pigment disposed in the same designs and in the same colors as on the upper side, and also Agassiz's experi- ments according to which the young of these same fishes retain their original symmetry when they are kept at the surface of the water for a longer time than under normal conditions. "According to these experiments," Osborn says very rightly, "progressive inheritance (and so phylo- geny) appears to represent rather a process of substitu- tion or of addition than one of true elimination in Weis- mann's sense." 165 Thus these facts also speak in favor of the conception that phylogeny rests upon an addition of new characters and their superimposition upon the old. We can see that to explain by the inheritance of ac- quired characters this addition of a new character to the 1*5Osborn: Alte und neue Probleme der Phylogenese. Ergebn. d. Anat. u Entwicklungsgesch., herausg. v. Merkel u. Bonnet. Bd. III. 1893. Weisbaden, Bergmann, 1894. P. 610, 619. 218 Inheritance of Acquired Characters old only after the completion of the development of these latter, it is sufficient to suppose that the agent of trans- mission of an acquired character becomes active in onto- geny, only when the young organism finds itself in the same conditions in which the parent organism was when it acquired this character. As soon as one admits this condition for the mech- anism of inheritance the law of the repetition of phylo- geny by ontogeny appears to be merely the immediate consequence of the inheritance of acquired characters. For so long as the embryo is developing in the egg or in the maternal body and so long as it is nourished, sup- ported and protected by its parents, it is withdrawn from the changing influences of the environment. It is only when the individual is left to himself that he finds him- self driven perhaps to new functional adaptations. In other words it is only in the adult state, after it has com- pleted or almost completed its specific development, that the organism in general can find itself in conditions neces- sary for the acquisition of new characters. But another fact also can explain why new phylo- genetic characters are acquired only when all the old ones are already quite developed. We have indeed already seen that the organism undergoing development is much more elastic but much less plastic than the adult, so that the modifications which arise in it from the action of an external force, even when it acts for a long time, have the tendency to disappear without leaving any trace be- hind so long as the organism has not yet completed its development, whereas this tendency is no longer inherent in the adult organism. We have already mentioned the experiment of Roux, in which he distorted a few frog embryos within their Inheritance Explains Bio genetic Lcnv 219 gelatinous envelope by compressing them between needles : "If the needles were withdrawn again immediately after the deformation, the embryo at once resumed its earlier form. If on the contrary they were held in place for several hours the deformation became from the first a persistent one, and only after several hours would the embryos resume their original form — a proof that an in- ternal adaptation to the new form had already com- menced, but that this adaptation is nevertheless caused to disappear again in the course of further development, per- haps by the action of those very forces of growth which bring about the restoration of the normal form, and which were inhibited during the time of the deformation." 166 We have thought it worth while to mention again this very characteristic example of the elasticity of develop- ment, because it, better than others which we have already mentioned in the course of our investigation of the cause of this elasticity, helps us to explain the rule inviolably followed in the evolution of species, of the addition of new phylogenetic characters to those already present. For from this it is very evident that those phylogenetic characters whose appearance is caused dur- , ing ontogeny to some extent by the action of external influences, have the tendency to disappear again promptly as soon as the cause which produced them has ceased to • act. So that, unless we have an extraordinary influence, whose intensity and insistent action during ontogeny through the course of successive generations give it an L88Roux : Zur Orientierung iiber einige Probleme der embryonalen Entwicklung. Zeitschr. f. Biol. ; Bd. XXI. Minchen. July 1885. P- 515, 5i6. Gesamm. Abhandl. II. P. 245. 220 Inheritance of Acquired Characters overmastering power, it will not leave behind any trace in the individual and certainly none in the species. Thus the inheritance of acquired characters, thanks to these two facts that the embryo is usually withdrawn from the influence of the environment, and that or- ganisms undergoing development are elastic and not plastic, shows itself to be completely capable of account- ing for the fundamental biogenetic law. Whatever in it could seem marvelous and enigmatic finds its natural so- ultion and the law itself becomes an immediate and neces- sary consequence of this inheritance. What on the contrary is the explanation which Weis- mann is able to give for this law ? He thinks to explain it merely by the following laconic words : "The biogenetic law rests upon this, that phylogenetic development is accomplished partly by the addition of new ontogenetic stages at the end of ontogeny. In order that this latter may be attained, the preceding terminal stages must each time be run through again." 167 But in this Weismann leaves just the most important part of the question out of consideration. Why can phylogenetic development take place only by the addition of new ontogenetic stages at the end of ontogeny ? According to Weismann's theory, there is no reason whatever why one should believe the determinants cor- responding to the last ontogenetic stage to be the only ones to undergo modifications, for one cannot forget that according to this theory each cell of each ontogenetic stage must have its own determinants.168 The same causes of differences in the nutrition or any other thing, which are capable of modifying the determinants cor- 187Weismann : Das Keimplasma. P. no. 168Weismann : Ibid., e. g. P. 97, 100, 232—233, 596. Weismann's Theory Cannot Explain Bio genetic Law 221 responding to the last ontogenetic stage, must be also capable of transforming in the most different ways the determinants of the other stages. According to that, each phylogenetic stage would have its own ontogeny, which would differ completely even in the first stages of development from the ontogeneses of the preceding phyletic stages. And there is no more reason for the supposition that the only way in which the determinants corresponding to the last ontogenetic stage could undergo modification must be by "obtaining a greater power of growth, aug- menting consequently in number, differentiating each in a new fashion, and adding thus at the end of the old ontogeny one or more generations of cells." 169 For these determinants could perhaps undergo any merely qualitative variation whatever without first augmenting in number, that is to say could become differentiated at once in a new way so that the part determined by them should at once take on a form different from the old one without needing first to pass through its preceding phylogenetic state. We need just to recall again the example which we have already cited above, furnished by one of the most characteristic manifestations of the fundamental bio- genetic law, namely ontogenetic involution, in order to demonstrate in the clearest way the absolute inability of Weismann's theory, to account for that law. For accord- ing to that theory one would understand for instance that the tail of the ancestor of the tadpole, or that the limbs of the ancestor of the existing serpent may have become con- stantly shorter in the course of phylogeny by virtue of "'Weismann : Ibid. P. no. 222 Inheritance of Acquired Characters natural selection, panmixia or something else, and that consequently they have become arrested in successive ontogeneses at successively earlier stages of development. But the question cannot be repeated often enough; how can this theory explain the growth of these organs up to a certain stage of development and their retrogression and disappearance in the later stages? In short, it seems to us that one cannot imagine a more complete overthrow even from a purely logical point of view, where it is only a matter of avoiding con- tradiction on one's own premises, than thrt suffered by Weismann in the attempt to find an explanation of the repetition of phylogeny by ontogeny, and one can hardly bring forward a more thorough failure of a theory built up laboriously with the object of explaining all the differ- ent phenomena of heredity, even the most peculiar and secondary ones, than appears in the fact that this theory is not even capable of giving the least explanation of the most general biogenetic phenomenon — the one which underlies all the others. And this contradiction and this failure do not appear so much in the minute and partic- ular parts of Weismann's theory, in which it deals with this or that peculiar detail, but much rather in the theory itself in all its generality, which disputes the inheritance of acquired characters. Weismann and his supporters can, if the most evident facts are not enough for them, deny this law of recapitulation. But that they admit it and nevertheless dispute inheritance, this is a contradic- tion from which the opponents of the Lamarckian principle cannot escape now or ever — a destructive rock upon which all their theories are wrecked. ******* If now we sum up succinctly the discussion in this Summary of This Chapter 223 chapter, we are able to affirm that although no fact or argument is capable by itself alone of affording an ir- refutable and unconditional proof either direct or indirect, of the inheritance of acquired characters, nevertheless the sum total of the facts and the arguments which are fa- vorable to it is so weighty that one is not only justified in believing but is even compelled to believe that the • Lamarckian principle is in all probability correct. But the difficulties of explaining the mechanism of . inheritance are so great, that many investigators may have thought them to be insurmountable. It is conceiv- able that many others, like Roux, have been led to dis- pute its existence, just in order to free themselves in that way from a veritable nightmare. But this position is no longer possible. The objective examination of the question leads to the conviction that the inheritance of acquired characters is to be considered as in all probability a reality, there- fore we are in duty bound to seek an explanation of this phenomenon by some hypothesis, even if it be only a * provisional one. So in the following chapter we propose to examine comparatively a few of the most recent and most im- portant hypotheses which have been devised for the explanation of inheritance. After that in the penultimate chapter we shall set forth more thoroughly the explana- tion of the Lamarckian principle which the centro- epigenetic hypothesis can give. Further evidence that somatic changes induced in animals by environmental influences may be repeated in their descendants as a result of germinal influences is furnished by Sumner and by Kammerer. See Archiv fur Entwicklungs — Mechanik der Orgari- ismen, Leipzig, June and September, 1910. (Translator.) CHAPTER SIX THE MOST IMPORTANT OF THE EXISTING BIOGENETIC THEORIES IN RELATION TO THE INHERITANCE OF ACQUIRED CHARACTERS. We believe it is unnecessary to discuss here in an exhaustive way the fact that the question of the admis- sibility or inadmissibility of the Lamarckian principle re- mains always distinct from, and entirely independent of the question of the evolutionary or epigenetic nature of development. Darwin in his evolutionary theory with preformistic germs, — a true theory of preformation, — accepts inheritance ; Galton limits it to a few cases ; Weis- mann excludes it unconditionally. Hertwig accepts it in his epigenetic theory, although he does not exclude some sort of preformistic germs; DeVries excludes it. Roux who was inclined at first to believe that this inheritance might exist in combination with the chemical develop- ment of the egg, a theory frankly evolutionary without preformistic germs, has finally regarded the two theories as irreconcilable. The only theories which appear es- pecially inclined toward the complete acceptance of the Lamarckian conception, are the epigenetic theories with- out preformistic germs, for example that of Spencer. We can now pass on to the rapid review of the principal biogenetic theories current today, with especial reference to their direct or indirect relation to the ques- 224 Spencer 225 tion of inheritance and to the conceptions of inheritance which have been formed. Spencer This author's idea of "physiological units/' inter- mediate between the morphological units or cells and the chemical units or molecules, and representing the last irreducible vital elements, is well known.170 If one supposes that in each organism there exists only a single variety of these units, Spencer believes the explanation of the inheritance of acquired characters would follow immediately from that. "Just as the physiological units because of their special polarities build themselves into an organism of a special structure, so on the other hand, if the structure of this organism is modified by modified function, it will impress some corresponding modification upon the struc- ture and polarities of its units. The units and the aggregate must act and react on each other. If nothing prevents, the units will mould the aggregate into a form which will be in equilibrium with their pre-existing polar- ities. If contrariwise the aggregate is made by incident actions to take a new form, its forces must tend to mould the units into harmony with this new form. And to say that the physiological units are in any degree so moulded as to bring their polar forces towards equilibrium with the forces of the modified aggregate, is to say that when separated in the shape of reproductive centers, these units will tend to build themselves up into an aggregate mod- ified in the same direction." 171 170Spencer: Principles of Biology, Sixth edition; London, Will- iams and Norgate. 1898. Vol. I. Chap. IV, $66. P. 224—226. 171Spencer : Ibid. Vol. I. Chap. VIII : Heredity, $ 84. P. 319- 226 Theories Treating of Inheritance It seems to us quite superfluous to expose any further here the pure verbality of such an explanation without any real content. Neither shall we go more closely into the objection, which is apparent on the very surface, that physiological units identical throughout the whole or- ganism cannot form muscles here, bones there, nerves elsewhere, all of which represent special tissues with totally different physical, chemical and vital properties. We limit ourselves rather to noting that, according to this, the inheritability of even quantitative and partial modifications, for example the transmission of the merely greater development of a tissue or an organ already ex- isting, must be attributed to a uniform, qualitative change of all the physiologic units of the organism. And not- withstanding that, the properties of each group of these units, not excepting the group constituting the tissue which has undergone a simple increase in mass, must re- main identically the same as they were before. Let us consider the case which Spencer himself quotes and regards as one of the examples of the inheritance of acquired characters, namely, the increase in size or greater development of the great toe as well as the diminution or regression of the little toe, as a result of the fact that our ape-like ancestors gave up life in the trees for life on the surface of the ground.172 Is it possible that so very local a morphologic change has transformed qualitatively the physiological units of the entire organism? And apart from the fact that the change is limited to a certain very small part of the body, it must yet be borne in mind that one has to do here with no new quality nor with any new material introduced 172Spencer : A Rejoinder to Prof. Weismann. London, Williams and Norgate. 1893. P. 3^- Spencer 227 into the organism by the new function, nor consequently \vith any new physical or chemical or biological character which the organism has now for the first time acquired ; but on the contrary there is involved only a different dis- tribution of already existing qualities of matter. But how can a change of quality in imaginary physiological units, which would have proceeded uniformly in the whole organism, accord with the fact that all the qualities and properties of this organism remain unaltered, and there is merely another distribution of these materials? Let us consider as a further example the instinct of new born chickens. "In the first minutes of life," writes Jastrow, "chickens follow with their eyes the movements of crawling insects, turning their heads with the precision of an old fowl. In from two to fifteen minutes they pecked at some speck or insect, showing not merely an instinctive perception of distance but an original ability to judge, to measure distance with something like in- fallible accuracy. A chicken hooded as it emerged from the shell was unhooded when three days old; six minutes later it followed with its head and eyes the movements of a fly twelve inches distant, and about ten minutes later made a vigorous dart at the fly, seized and swallowed it at the first stroke." 173 Spencer would rightly attribute this instinct to the long practice acquired by the ancestors of the chicken. But if he wished to explain this inheritance through the alteration of specific physiological units of the entire organism, such an explanation would not be taken ser- iously. How could the new physiologic units, capable of effecting this local modification constituted by the 173Jastrow: The Problems of Comparative Psychology. The Popular Science Monthly. New- York. Nov. 1892. P. 36—37. 228 Theories Treating of Inheritance formation of a few new nerve paths, at the same time reproduce those parts of the organism which remained entirely unaffected by this local change ? Finally, how can the hypothesis of Spencer account for the law of repetition of phylogeny by ontogeny? If the explanation of the inheritance of acquired characters by means of physiological units were accepted, this law would be futile. For the new physiologic units with changed polarity must take on at once in the daughter organism that form to which the parent organism had last attained, without needing to pass first through the preceding forms. The physiologic units were devised in order to permit the comparison of the formation of the organism with that of a crystal. But a substance which because of a slight qualitative alteration of its molecules changes its form of crystallization, goes over from the very first commencement of crystallization into a form different from the preceding, and takes on at once the form which it will have after the completion of crystallization. A comparison between organisms and crystals is therefore inadmissible; and this inability is especially evident when it is attempted in this way to explain the laws and phenomena of development, in which organisms and crystals are totally different and are even antagonistic. Haacke The conception of Haacke is much like that of Spencer. "According to my view/' says he, "we have to do not only with the genetic continuity of the germ cells of one generation with those of the generation immediately pre- ceding and following, but also with a material continuity Haacke 229 of the germ cells with the other cells of the body. The body represents a system in equilibrium; if this changes the germ cells developing in it change also. But the equilibrium of the system constituted by the body becomes directly altered by the acquisition of new characters; con- sequently the changes which it undergoes must be trans- mitted also to the germ cells. But no matter whether the germ cells become changed as a result of the acquisition of new characters by the body which surrounds them, or whether they remain unchanged, they always inherit the same thing, namely, the capacity to form that body with which they were in equilibrium." 174 Like Spencer he supposes that this equilibrium is due to the tendency possessed by an infinite number of par- ticles, identical throughout the whole organism, to dis- pose themselves in this way only. His rhomboidal gemmes, grouped into composite units or gemmaria, are fundamentally nothing else than the physiological units of Spencer. The geometric form attributed to them, which emphasizes the static character of this explanation, does not make it in any way more acceptable. Nevertheless there are to be noted and carefully con- sidered, here perhaps, even more than in Spencer, the close interaction and the reciprocal equilibrating influence, which would always exist between the soma and the germ substance, — that is to say, between the organism and that small portion of its units contained in the reproductive cells, — not only throughout the whole development of the individual but also after the completion of development when the organism becomes subject to the modifications which external agents induce in it. 174Haacke: Kritische Beitrage zur Theorie der Vererbung und Formbildung. Biol. Centralbl., Bd. XV. 1895. P. 568. 230 Theories Treating of Inheritance Sedgwick This investigator deduces the possibility of the in- heritance of acquired characters from his conception that the pluricellular organism is simply a great syncytium. "If the protoplasm of the body is essentially a syncy- tium and the ovum until maturity a part of that syncytium, the separation of the generative products does not differ essentially from the internal gemmation of a protozoan, and the inheritance by the offspring of pecul- iarities first appearing in the parent, though not ex- plained, is rendered less mysterious; for the protoplasm of the whole body being continuous, we must naturally be inclined to think that every change in the molecular con- stitution of any parts of it would naturally be expected to spread in time, through the whole mass." 175 This conception which recalls somewhat Naegeli's idea of an idioplasmic network, extending its meshes throughout the whole body, though it gives a hint of the possible mechanism of inheritance by means of this proto- plasmic continuity, nevertheless does not give even the most vague and remote notion of the nature of this mechanism. Bard According to this author the cells participate in onto- genetic development in two ways. The first way is by their specific division or qualitative nuclear division, as in Weismann's theory of preformistic germs. The second rests upon a special action of the germ cells upon the somatic cells, acting indeed at a distance but nevertheless 175Adam Sedgwick: The Development of the Cape Species of Peripatus. Quart. Journ. of Microscopical Sc. Vol. XXVI. 1886- P. 206. Bard 231 not by any mediate path, like the influence exerted by electric induction-currents in the production of induced currents, and Bard has therefore given this process the name of vital induction. But this induction would be exercised not merely by the germ cells upon the somatic, but also by the latter upon the former. And the modified soma is capable of bringing about the inheritance of the modifications it has undergone, by means of an influence of exactly this na- ture exerted upon the germ cells contained in it.176 But the inheritance of these new characters by the next succeeding organism by the means of the germ cells which give rise to this latter can be accepted only upon the supposition that it is effected by means of the same vital induction which had already transmitted the char- acters of the paternal soma into the germ, now acting in the reverse way. And Bard himself seems to admit this. But if this is true for the characters acquired last, it must also be true for all the characters acquired phylogeneti- cally. Consequently the role which specific cell division or qualitative nuclear division in the Weismannian sense would play in the histologic differentiation and in the whole development would become reduced to nothing. We shall limit ourselves then to drawing attention to the inconceivability, made more evident by the considera- tions just mentioned, of the idea that the germ cells could participate in the development in two such extremely different modes of action simultaneously, and the lack of any experimental proof for this supposed vital induction. L76Bard: Influence specifique a distance des elements cellulaires les tins sur les autres. Archives de Medecine experimentale ; i. serie, t. II. Paris, Masson. May i, 1890; and La specificite cellulaire et ses principales consequences. La Sernaine Medicale. Paris, March 10, 1894- 232 Theories Treating of Inheritance Tornier Tornier believes that the nervous system acts as in- termediary, transmitting the acquired characters from the soma to the germ cells and then fixing them in the latter. "In the more highly organized individuals each adapt- ation of the active end organs is accompanied by a corresponding and equivalent adaptation in the central nervous system. The central nervous system in its turn transmits the acquired character to the sexual organ forming with it a single functional and nutri- tive unit, and especially to the sexual cells causing them to undergo an equivalent transformation. When the sexual cells become later generative cells, the property acquired by the parent is by this means inherited by the descendents." m One does not see nevertheless how the modification undergone by the sexual cells could be reversible ; that is to say how these cells could produce in the descendants the new character which was acquired by the parent or- ganism and to which their own modification was due. To state it more exactly, one does not see at all how it will have satisfied the condition to which we shall often have occasion to return, and which appears indispensable to this reversibility, namely that during ontogeny there is produced at the right time and the right place an action, which is of exactly the same nature as that by which this part of the paternal soma had reacted to the modifying action of external influences. It is necessary nevertheless to note the important role which is thus attributed to the nervous system as the in- "Tornier: t)ber Hyperdaktylie, Regeneration und Vererbung. Arch. f. Entwicklungsmech. d. Org. Bd. III. Heft 4. and Bd. IV. Heft i. Leipzig, Engelmann. 1896. P. 192. Her twig 233 termediary through which must pass all the characters newly acquired by the soma and thanks to which they become transformed so that they can then be inherited by following generations. Oscar Hertwig Oscar Hertwig gives in the following words the es- sentials of his theory of biogenesis. "The cells, necessarily equal specifically on account of their origin in the segmentation of the egg, which are combined to constitute an organic system of a higher order, have their character determined by the relations in which they become placed during the course of develop- ment. Their state becomes modified when these relations are modified. For the cell organism is an extremely ir- ritable substance so that the slightest influences are suffi- cient to bring about modifications in it." "Contrary to the mosaic theory of Roux and the germ plasm theory of Weismann, the theory of biogenesis is based upon the principle, that from the commencement of development, the cells arising from the segmentation of the egg are constantly in the most intimate relation with one another, and the character of the developmental pro- cess is determined essentially by this fact. The cells do not take their especial future character of their own initia- tive, but their character becomes determined according to laws which result from the co-operative action of all the cells during the successive stages of development of the entire organism." "The relations of the rapidly multiplying cells of the substance capable of development are constantly chang- ing in accordance with general laws, and the relations between these internal factors and those which are with- 234 Theories Treating of Inheritance out the organism are likewise undergoing continual change, and because of these changes in relations new conformations become produced at each stage of the developmental process in a variety becoming ever more complex." 178 Nevertheless this does not hinder one, according to that investigator, from considering the organism in its entirety as a single physiological unit because of the idio- plasmic identity of the nuclei of all its cells; — a thing which he thinks, makes the inheritance of acquired characters conceivable.179 For to explain the latter, Hertwig brings up the cases of infection, immunization, and other similar examples, in relation to which the organism can really be regarded as a single entity. He quotes for instance the experi- ments of Ehrlich who has succeeded by the administration of extremely small doses of ricin in making rats immune to this poison which is very powerful for them, and in establishing the fact that this immunity was acquired not only by the walls of the digestive canal with which the poison comes into immediate contact, but also by all the other tissues of the body, such as for example the sub- cutaneous tissues and the ocular conjunctiva, and even by the germ cells as was proved by the fact of the trans- mission of this immunity to the young born of immunized parents.180 Just as all the cells of the body are accessible to the action of ricin and thanks to that fact all undergo a material modification, which some of them, namely the germ cells, transmit later to the descendants as an im- 178Oscar Hertwig: Die Zelle und die Gewebc. TI. P. 75, 144, 156. 17BOscar Hertwig: Ibid. II. P. 241. 180Oscar Hertwig: Ibid. II. P. 240 ff. Hertwig 235 munity against ricin, just so according to Hertwig's view do all the cells behave toward acquired characters in gen- eral. "In the same way as the cell is sensitive to the action of ricin, which brings about an enduring material modification of it, and this becomes inherited as im- munity to ricin, so I think every cell is sensitive also to the influence of the general condition of the body, which brings about material modifications of its substance, that is of its idioplasm or hereditary material, which is es- pecially susceptible of such material modifications, and these correspond to the cause as its effect both in the cells of the soma and also in the sexual products." 181 We shall not consider here the fact demonstrated by Ehrlich, that in the instance in which only one of the parents was immunized, the immunity was transmitted very well to the young of an immunized mother but on the contrary was not transmitted to the young of an im- munized father; a fact which seems to confirm the hypothesis of Ehrlich that the immunity against ricin was due to the formation of an anti-ricin, with which the protoplasm of all the cells became impregnated, but with which the spermatazoon could not become impregnated because it is almost entirely devoid of protoplasm, show- ing consequently that one has to do here not at all with a permanent modification of the nuclear idioplasm. But even apart from that and even admitting the hypothesis that the immunity against ricin was due from the be- ginning or at the time to the acquisition by the idioplasm of a new and persistent character,182 it is still evident that this is not a just comparison. For in the case of infections, immunizations, and so 181Oscar Hertwig: Ibid. II. P. 242. 18£Oscar Hertwig: Ibid. P. 2^. 236 Theories Treating of Inheritance on, there would be a single identical influence exerted upon the nuclei of all the cells without exception, which makes it conceivable that thereby the special nuclei of the different cells, and consequently of the germ cells also, could all inherit the new reactive property, which would be added to the other special characters already present in each nucleus and different in the different cells. But in the case, for instance, of a certain muscle, which develops to a greater size because of a definite modification in its local, trophic, functional stimulus, is it possible to make the analogous statement that there is thus obtained a new state of the body which brings about a modification in the idioplasmic substance of all the cells of the organism without exception — the same modifica- tion in each of them ? Certainly this modification induced in the trophic functional stimulus of a muscle and the greater development thereby provoked in it will exert an influence on all or nearly all parts of the organism; but the most probable supposition and that best corresponding with the facts would be that the reaction is different in each part. This case at least is quite different from that in which one has to do with the transmission of a definite infection or immunity, and cannot be compared with it without further consideration. In short Hertwig supposes that every local material modification which appears at a given point of the idio- plasm as a reaction to "a new functional stimulus extends at once throughout the whole idioplasm, so that the latter becomes modified uniformly everywhere, like a true physiologic unit : "In the organism considered as a physiologic unit of life the actions of all individual organs, tissues and cells must be combined into a complex common action, the Hertwig 237 nature of which will be conditional upon the general state of the organism ; this action will be felt by each individual part and in so far as it amounts to a lasting modification of the idioplasm it becomes a newly acquired character." At every fresh modification of the general state of the organism, "the total heritage of the organism becomes enriched by a new member, by a new anlage which mani- fests itself again in the development of the succeeding organism, in that now the newly developing individual reproduces more or less 'from the germ out* or from internal causes the character which its parents had ac- quired during their lives from intercourse with the outer world." 183 Does Hertwig in this say that the reproductive sub- stance is constituted by a heaping up of a whole series of material modifications, which correspond to the successive phylogenetic general states of the body, and constitute as many potential tendencies ? That is hard to decide, because all that relates to his conception of the idioplasm structure is obscure and often contradictory. Thus in some places he seems to admit that his idioplasm may be constituted by preformistic germs, so that his theory would belong with that of De Vries to the group of theories of epigenesis with pre- formistic germs. In other places on the contrary where he speaks of general states of the idioplasm, and other similar things, every idea of preformistic germs seems to be excluded, so that his theory appears to be very similar to those of epigenesis without preformistic germs, like that of Spencer. The same is true also of this heaping up of different material modifications representing the suc- 188Oscar Hertwig: Ibid. II. P. 242, 243. 238 Theories Treating of Inheritance cessive phylogenetic states; he appears sometimes to exclude, sometimes to accept it. If he accepts this heaping up, the explanation of the inheritance of acquired characters which the hypothesis of biogenesis could give would be reduced to this : The uniform modification into which are summed up during their extension throughout the whole body the different transformations in the idioplasmic nuclear sub- stance that are brought about in consequence of the acquisition of new local characters is added to the preceding phylogenetic modifications without altering them, but merely reducing them to the potential state. Then in the next following ontogeny, when the required stage of development is attained, and this recently ac- quired idioplasmic modification becomes active in its turn, it induces the same general state of the body as was induced in the parent as a result of the acquisition of new local characters, and this general state, because of the reversibility of the relation between action and reaction, tends to bring about the formation of this character once again. But one must not be deceived even by this. Even supposing this to be the explanation that the biogenetic hypothesis could afford for the inheritance of acquired characters, it would consist rather in mere words than in ideas. For, as we have said above, this supposed summing up of all these different, simultaneous, local variations into a single idioplasmic modification, including them all and uniform for the entire organism, lacks not only any basis in fact but also any possibility of conception. And the following questions remain unanswered : In what do these different general states of the idioplasm consist ? In what way do some come to be added to the others Hertwig 239 during ontogeny one after another in the same order as in phylogeny? How does each of the successive general states of the idioplasm, identical for the entire organism, exert upon each individual cell so many special actions, which are not only quite different from one another but also are the exact reverse of those which by their union had produced this general state during phylogeny? But however that may be, it is not at all certain that Hertwig accepts this heaping up. For, as we have said above, if he seems in certain places inclined to accept it, he appears in others to reject it absolutely. He seems to accept it for example when he approves and adopts the following passage from Nageli : "The unfolding of the anlagen of the idioplasm fol- lows faithfully the phylogenetic order. While the or- ganism, developing in ontogeny, runs successively through the stages through which its phylogenetic stem has run, the idioplasmatic anlagen become developed in just that order in which they came into existence." And this conception appears confirmed in the follow- ing words of Hertwig: "On account of the progressive multiplication of the cells, their combined action is con- stantly producing new embryonic states in the same serial order as that in which they appeared during phylogeny. The individual cells are brought into new relations to one another and to the external world, and through these successive reciprocal relations the anlagen latent in the cells become awakened." 184 In other places on the contrary he seems to reject completely this heaping up of a wrhole series of anlagen, of which each should correspond to the phylogenetic state 184Oscar Hertwig: Die Zelle und die Gewebe. II. P. 251, 253. 240 Theories Treating of Inheritance in which it had arisen, and to suppose instead that when once the idioplasm has been modified there remains in it nothing more of the preceding states, not even in a latent or potential condition. At least this would seem to be indicated in the following passages. "The theory of biogenesis makes it necessary for us to introduce into Haeckel's statement of the fundamental biogenetic law, a few modifications and explanatory addi- tions through which the contradiction (between this law and this theory) may be avoided. We should drop the expression: repetition of the forms of extinct ancestors, and substitute for it: repetition of forms which obey the laws of organic development and which progress from the simple to the complex. We should lay the emphasis upon this, that in embryonic forms, just as in the adult forms of animals, are expressed general laws of develop- ment of organized living substance." "The periodically repeated development of pluricellu- lar individuals from unicellular representatives of the species, or individual ontogeny, is brought about in general accordance with the same rules as the preceding onto- genies, but becomes each time a little modified correspond- ing to the extent to which the characteristic cell of the species was modified in phylogeny." "That certain conditions of form recur in the develop- ment of animals with such great constancy, and in the main in similar ways, is due chiefly to the fact that in all circumstances they furnish the prerequisite conditions under which the next later stages of ontogeny can be produced." "The unicellular organism, on account of its very nature, can be transformed into a pluricellular organism Her twig 241 only by division. It follows that in all living beings onto- geny must commence by the cleavage process." "An organism constituted by layers and groups of cells disposed in a definite order can be formed from a heap of cells only on condition that the cells while they are multiplying, begin to be arranged in separate assem- blages and so progress in accordance with certain rules from the very simple initial forms to more complex ones. Thus the gastrula implies as prerequisite the simpler stage of the germinal vesicle. Thus the embryonic cells must first be disposed in germinal layers, which constitute the basis for further processes of differentiation in their territory. The anlage of an eye in a vertebrate can be formed only after a nerve tube has been separated from the outer germinal layer, since in it is included also the material for the formation of the optic vesicles." "Certain forms then become firmly fixed in the developmental process, despite all the constantly acting, modifying factors, because it is only by means of them that the complicated final state can be reached in the most simple way and in the most suitable manner." 185 Thus as we said, Hertwig seems in this really to sup- pose, contrary to what he asserted above, that the idio- plasm is not at all a heaping up of numerous anlagen representing respectively the successive steps of the evolu- tion of the species, but so transforms itself with each new phyletic acquisition that it preserves no trace of preceding phyletic states. In this he is in complete accord with the hypothesis of Spencer, from which in fact he quotes long passages and makes them his own. And accordingly he supposes, 185Oscar Hertwig: Die Zelle und die Gewebe. II. P. 273, 274, 276. 242 Theories Treating of Inheritance as appears from his own words here quoted, that if the organism appears to pass again during ontogeny through the preceding phyletic stages this depends merely upon the fact that there is no other materially possible way for the idioplasm to attain the phylogenetic equilibrium just acquired. But to accept this is to deny the law of repetition. One notes that Hertwig was lead to reject this law, as he himself admits, only because he wished to avoid the objection which Weismann had already urged against Nageli ; namely that if one supposes that different phylo- genetic forms may be due to respective idioplasms qual- itatively different from one another it is not possible to understand how the same forms when they succeed one another in the ontogeny of a single species can then depend only upon "different conditions of tension and movement," of one and the same idioplasm. But it seems to us that Hertwig has attempted in vain to circumvent this obstacle. "A very general and very astonishing fact," writes Delage, "is that ontogeny almost never follows a direct and simple course. The cells almost never dispose them- selves from the beginning in the order which would bring the embryo soonest to its final conformation. Ontogeny approaches its goal gradually, but as though compelled to tack against a contrary wind, and its long tacks carry it sometimes far to the side. It shapes a number of rudi- ments, permits purposeless members to arise, opens gill clefts in a lung breathing animal only to close them again, and so on." 186 These tacks, these deviations hither and thither, cer- 180Delage: L'heredite etc. P. 175—176. Driesch 243 tainly do not denote any endeavor of the idioplasmic substance to proceed by the shortest route to the condi- tion of its equilibrium. They indicate that it is quite impossible on the one hand to accept the passage of ontogenetic through phylogenetic forms and on the other hand to refuse to this process the significance of an actual repetition of phylogeny by ontogeny. In other words one must seek the cause of this repetition not merely in biologic laws of maintenance of the equilibrium in an existing homogeneous idioplasm, but chiefly in the entire past of the species and just in the historic fact that it passed during its development through such and such phylogenetic forms. And so the objection urged by Weismann against Nageli can be urged in its full force and even more justly against Hertwig. For though Nageli gave no explana- tion of their causes and ways of action, he nevertheless accepted the activation of a whole series of different anlagen of the idioplasm in exactly the same serial order as in their phylogenetic appearance. Hertwig on the con- trary after he had first accepted this activation of succes- sive anlagen of the idioplasm finally rejected it. Driesch This author's conception of organic development cannot in its very nature afford any explanation whatever of the inheritance of acquired characters and conse- quently, admitting that this inheritance exists, ought for this very reason to be considered inadmissible. It can be summed up in the following words of its author. ''Each cell concerned in the ontogenesis in so far as it possesses a nucleus really carries within it the sum total of all anlagen ; in so far however as it possesses a specific 244 Theories Treating of Inheritance protoplasmic body it is, just on account of this latter, susceptible of being acted upon by only certain causes (causes which effect the liberation of different individual nuclear energies)." "We believe that the capacity of reacting to stimuli resides in the nucleus, but that the capacity of receiving them resides in the protoplasm, which is chemically specific in each elementary organ. The protoplasm is thus the medium, (the zone of perception), between the liberating causes and the nucleus, (the zone of action)." "The appearance of elementary processes comes about in each ontogeny through a liberation of potentialities. * * * I break the whole of ontogeny up into a series of liberated effects." "Each liberating cause produces not only a chemical specificity and thereby the new elementary process as such, but it produces through this specificity at the same time the limitation by which the new cell is capable of receiv- ing later only certain further liberating causes." 187 The especially striking thing in this conception is the absence of any real, continuous, reciprocal dependence of the different parts one upon another throughout the whole course of development. Each cell will preserve in its nucleus, it is true, the germinal substance uninjured; but the successive liberation of special nuclear energies which will impart to it its own especial character, depends fundamentally only upon the specific properties which its protoplasm had already acquired earlier, and not upon the reciprocal actions which exist between all parts of the body throughout the whole duration of ontogeny, as it would according to Hertwig's theory, for instance. 187Dnesch: Analytische Theorie der organischen Entwicklung. P. 81, 49, 60, 82. Driesch 245 For the specific protoplasm which a certain cell has already acquired at a given moment of ontogeny by re- ceiving only one certain stimulus corresponding to its immediately preceding specificity, would really become the special cause by which among all possible nuclear energies only that one becomes liberated which should become active at that instant. The activation of this new nuclear energy would in its turn modify the specificity of the protoplasm of this cell and of its immediate descend- ants; and this protoplasm so altered would then become the cause of the reception of a new specific stimulus and of the consequent liberation of the next required nuclear energy ; and so on up to the completion of development. Each cell of any given ontogenetic stage would thus come to carry in itself all that is necessary to determine its own future character and that of its most remote descendants, with the exception of the various stimuli which it is the duty of the protoplasm to select and to receive. One thing is not quite clear in this. Do these liberat- ing causes of the different nuclear energies, that is these stimuli, among which each protoplasm should select and receive only those belonging to it, come only from the world outside the organism, or also from the reciprocal actions of the individual parts in the interior of the organism ? In the first case it would be necessary to place Driesch among the out and out evolutionists; in the second, his theory would be a mixed one, that is it would rest upon phenomena of evolutionistic nature combined with others of epigenetic nature. We shall not set forth any further here what enor- mous difficulties one would encounter in either case if one sought to build up in accordance with this theory any 246 Theories Treating of Inheritance conception of a mechanism for the inheritance of acquired characters. Even if the hereditary substance should be preserved in its entirety, and unaltered, in the nuclei of all cells without exception throughout development, this would be ascribable only to qualitatively equal nuclear division. But one could not but ask why the modifica- tions which supervene in the hereditary nuclear substance of such and such somatic cells in consequence of a new local functional adaptation in the adult stage should not remain limited to these cells alone. Very noteworthy, however, in Driesch's theory is the conception that ontogeny takes place by means of a series of successive liberations of different energies remaining up till then in the potential state, and also that one result of the liberation of each of these energies and of the effects which it produces is that the necessary and sufficient conditions for the liberation of the next follow- ing potential energy are brought about. Herbst The epigenetic conception of Herbst is still less capable if possible, than that of Driesch of rendering con- ceivable any mechanism whatever for the inheritance of acquired characters. He mentions at first several experiments upon the way in which unicellular organisms and cells react to cer- tain stimuli, and also a great number of facts serving to show the great dependence of plant ontogenies especially upon external influences. While it is evident that external influences constitute most often only liberating or releasing stimuli, they seem on the contrary to become in certain formations real formative stimuli. In these formations there is involved not only a true ontogenesis Herbst 247 but an actual phylogenesis which would repeat itself in each generation ex novo, because of the repetition always in the same way of the same external formative influence. After having thus stored up a rich harvest of facts upon the physiological actions exerted by the most differ- ent stimuli upon organisms or upon given parts of organisms, — actions which are properly nothing else than functional adaptations in a broad sense of the word, — • Herbst believed he was able to build upon them his epigenetic conception, by which he makes development fundamentaly dependent upon a whole series of directive stimuli. "Just as freely moving organisms are influenced in the direction of their movements by external agents, so do independent tissue cells react to definite directive stimuli, and thereby make possible the production of a large number of ontogenetic formative processes/' "Just as the germinal vesicles of Cuscuta, for ex- ample, develop their stinging barbs at the points of contact with the plant upon which they lodge, just as in the leaf stalks of Helleborus niger the traction of a weight causes the formation of new mechanical elements which otherwise do not appear, or just as roots may be made to grow on a grass stalk because of the secretion of a gall fly, so, in the interior of an animal embryo in process of development a given organ can cause new formative pro- cesses to come into existence in another organ by means of contact, pressure, traction, by a specific product of metabolism or in some other way." And so, "just as in plants and sessile animals morphological formations of the most different kinds are produced through formative stimuli which either arise from the external world or are exerted by one organ of 248 Theories Treating of Inheritance the organism upon another; so also the morphological changes in animal ontogeny arise in the same way through manifold inductive stimuli "Inductionsreize" which are almost always of internal origin." 188 This conception, especially if it is extended to the whole process of development in general, would really amount to a reduction of ontogeny, exactly as in the plant formations above mentioned in which external agents act as true formative stimuli, almost to a phylogeny, con- stantly repeating itself anew in each generation. And the fact that successively arising generations would in spite of that remain always alike is to be attributed to the repetition, proceeding always in the same way, of succes- sive functional stimuli both without and within the organism in process of development, which are produced gradually one out of the other through the principle of fructifying causality and give rise each time to these new phylogeneses. In this respect the conception of Herbst recalls the purely mechanical explanation of development given by His, who refers the appearance of the same ontogenetic phenomena every time to the repetition of definite me- chanical influences, proceeding always in the same way. Since each influence, itself induced by the influences preceding it, induces in its turn the influences following it, then if only the first link of the chain is constantly repeated in the same way in each generation, that would be enough to cause the same thing to happen in the case of all the others. 188Herbst: t)ber die Bedeutung der Reizphysiologie fur die kausale Auffassung von Vorgangen in der tierischen Ontogenese. Biol. Centralbl. 1894. Bd. XIV. No. 18—22. P. 771 ; and Bd. XV, N. 20—24. P- 852- Herbst 249 One notes that these theories of Herbst and His and other similar ones imply an extraordinary independence and autonomy even during ontogeny not only in each part of the organism but even in each cell. The development of each particle, even the most minute, would thus depend fundamentally upon the independence of its response to the action of its immediate or close environment, even though this response is given in a very definite manner and is in no wise arbitrary. But this is hard to reconcile with the mutual adaptedness which there must necessarily be between the different parts constituting a single co- ordinated whole. And it is still more irreconcilable with the constancy and precision with which even the most minute peculiarities of the organism are reproduced in each development, even when the conditions of the en- vironment in which it takes place do not always remain alike in respect to nutrition, temperature or other factors. And so much the more, since the principle of fructify- ing causality, employed to explain this constancy and rigorous precision with which the same series of onto- genetic phenomena is always repeated, is a sword with two edges. For, admitting that only a single one of the numberless intermediate links of the chain should find itself in a somewhat different condition in relation to its environment, or should differ in any way even though inconsiderably from the corresponding link of the preced- ing generation, a thing which one might well assert occurs in every ontogeny, then the remaining portions of the chain would find themselves, because of the modifications accumulating and multiplying like an avalanche, altered throughout, and therefore in the last links also. The mutual adaptedness of numberless different parts and their co-ordination into a single harmonious whole, and 250 Theories Treating of Inheritance the constant precision in the repetition even of the most minute peculiarities, stipulate fundamentally an individual ontogenetic factor, which is directive and at the same time co-ordinative, acting at each moment of develop- ment throughout the entire organism even to the smallest single parts of it. But ontogenetic theories like those of Herbst or His take a stand diametrically opposed to this. It is also almost superfluous to remark that they cannot give the least account of the repetition of phylogeny by ontogeny, and still less of the inheritance of acquired characters. For the latter, as we can well assert without needing to fear that we get too far from the truth, requires ab- solutely the condition, certainly not sufficient, yet at least necessary, that just this ontogenetic factor of individual nature should act everywhere and incessantly and also that it should not give up the control of development even for a moment, so that it may thus be in a position to experience in itself each variation even the smallest appearing in the organism in consequence of any new functional adaptation. But the theories of Herbst and His, and all others like them, which have recourse only to the principle of fructifying causality, rest upon the conception that the successive influences would always be left to themselves by their respective special causes, as soon as they had once been produced and launched, so to speak, into development, to produce in their turn new influences. Therefore these theories necessarily exclude any inheritance of acquired characters. But if this latter actually exists they become, as we have said, quite untenable from this point of view also. Orr 251 Orr Orr takes as his starting point the conception that he has formed concerning the way in which the pluricellular organisms arise from the unicellular. It is easy to understand that after the unicellular organism in the course of generations had attained a certain size, its external surface might have become transformed in consequence of contact stimuli into a denser protective layer, and thereby have lost its repro- ductive capacity, which would have been preserved only in the inner part of the organism. "When such an organism as this would be divided into a number of pieces by the natural process of repro- duction those parts of the protoplasm which had not undergone a grosser material differentiation would be like the protoplasmic germs of all its ancestors, capable of responding to the same stimuli, and therefore of devel- oping in the same manner. The only difference between these and the ancestral germs would be the increased complexity of their nervous co-ordinations. But, on the other hand, part of the organism which has been dif- ferentiated into the denser outer layer would be in structure so different from the germs of the species that it would be incapable of responding to any of their accus- tomed stimuli and therefore incapable of repeating the development." "But at every step in the evolution a part of the protoplasm retains its original qualities, only changing its nervous condition to a condition of greater complexity of co-ordinations. In this way the original protoplasm -gradually adds to itself the co-ordinations for developing 252 Theories Treating of Inheritance in each generation, first cell walls, and then the differ- entiated organs." 189 From the whole work of Orr it seems to result with- out any possible doubt, that according to his view this non-differentiated part of the protoplasm is present in all the cells of the organism, is everywhere quite similar, and is continuous to the extent that stimulating influ- ences can be transmitted from any given part whatever to all parts of the animal, so that it constitutes a complete physiological unit.190 But on the other hand it is never to be seen quite clearly what this investigator understands by this greater complexity of co-ordination. The fact that the nervous system presides over all physiologic activities of the organism makes him think rightly that development also may be dependent on similar nervous phenomena. Only in the nervous system of the organism is clearly to be seen what is to be understood by a greater complexity of nervous co-ordinations, because it is constituted by numberless points, distinct from one another and con- nected with one another in more or less complicated ways by direct or indirect nerve tracts. In undifferentiated protoplasm on the contrary, which remains always en- tirely similar in the most different parts of the body, and a fortiori in that infinitely small part contained in the germ cells, one could not conceive in what these sup- posed nervous co-ordinations and this ever greater com- plexity of co-ordinations could consist, and what their significance would be. The following passages do not clear up any of the 186Orr: A Theory of Development and Heredity. New York, Macmillan, 1893. P. 127 — 128. 180E. g. Ibid. P. 124. Orr 253 obscurity : "When we consider the protoplasm's respon- siveness to stimuli and to the effects of repetition or prac- tice, with the intricate co-ordinations that may thereby be effected, also the impressions made by stimuli which remain long fixed as "memory," we are led directly to suppose that the property which is the basis of bodily development in organisms is the same property which we recognize as the basis of psychic activity and psychic development." "On the same principle that a thought in the mind calls up an associated thought, or one tone of music calls up another, or one action in an oft repeated series of actions calls up the next subsequent action or actions, so the initial stimulus being given to an incipient organ- ism, its responsive activity each time tends to produce by association the next activity in the oft repeated series and so on through the successive steps of growth and development." m The points of contact between the mnemonic phenome- non of the association or succession of ideas and the phe- nomenon of ontogenetic development have already been very rightly brought forward by others and we shall return to them for closer examination in the last chapter. It may be remarked here merely that the first can in no way serve to explain the second. For in the first place it belongs to a class of phenomena still more specialized and more complex than the phenomenon to be explained, in the second place the conditions of origin and of repeti- tion are quite different in the two phenomena. When a melody strikes the ear for the first time and is later often repeated it leaves behind impressions on 181Orr: Ibid. P. 238—239, 142. 254 Theories Treating of Inheritance several nerve centers and unites them by new nerve tracts becoming always smoother. These new impressions and tracts remain then unaltered in the same places in which they were produced and it is just in their continuance in the place of their origin that there must be sought the reason of the always greater ease with which these melodies are reawakened in our memory. When the muscles of the hand become accustomed to producing a musical exercise, the greater development of the muscles and the greater complexity of the nervous co-ordinations which connect them with the brain constitute well defined material alterations which remain unaltered in the places where they arise and make the exercise, at first difficult, always easier. In the development of the organism on the contrary the causes of the repetition each time of always the same ontogenetic stages must reside in a single cell, the germ cell. But this cell is not in any way the place in which are produced the material alterations which were acquired by the parent organism and handed over to the descend- ants, like the stronger development of certain muscles, the greater complexity of certain nervous co-ordinations and other similar variations. Of the stronger develop- ment of muscles, of the greater complexity of the nervous co-ordinations which are produced in the parent organ- ism, there remains absolutely nothing, in so far as they represent alterations of muscles and nerves, in the little particle of matter which is destined to produce the descendants. Consequently the comparison of the two phenomena, although certainly very suggestive, is not sufficient by itself to afford in any way an explanation of ontogenetic phenomena. Orr 255 Orr continues in these words: "The co-ordination of forces which determines development is not to be considered a definite, localized mechanism, wound up, and ready to go when touched. If such were the case we ought to find one such mechanism allotted to a cer- tain definite number of cells; but instead we find that each piece (of a hydra), regardless of the number of cells, or whether it be the half or the twentieth of the hydra, is capable of producing only one new individual." "The quality upon which development depends seems to reside in a small piece just as well as in a large piece and moreover equally in all parts." "I think we can best compare the inheritance of the plan and potentiality of development in the clump of protoplasm to inherence of ideas and potentialities of volition in the brain substance, not as though each idea and potentiality were located there in its own minute definite limited space, and attached to a definite mechan- ism of matter; but rather we should think of development and mental potentialities as dependent upon certain states of living matter, which states are the result of the entire past history of that living matter and which thus deter- mine the method of response to external stimuli, and the direction which shall be taken by the new energy con- stantly entering from the outside." 192 This recalls again the above mentioned conception of Nageli and Hertwig of idioplasm which is both general and mnemonic, with all its short-comings which consist in complete indefiniteness or worse yet in lack of content masked by empty words. Nevertheless it is worthy of notice that however 1B2Orr: Ibid. P. 172—173. 256 Theories Treating of Inheritance indefinite the theory of Orr may be, it contains a clearly expressed and very remarkable idea, namely: the con- ception that nervous activity is the only general phe- nomenon and basis of life. Orr attributes to it therefore the great function of forming by itself the whole mechan- ism of development as well as of the inheritance of acquired characters, and seeks to explain through it the striking analogy between this mechanism and the mnemonic phenomenon. Cope In order to explain the inheritance of acquired char- acters Cope starts out with the following investigations upon butterflies. By exposing larvae which were near the stage of pupation to different colors, the correspond- ing colors were produced in the chrysalids developed. In another experiment larvae, which were in the act of weaving cocoons, on exposure to certain colors were induced to weave cocoons of corresponding colors. "In the first experiment/' explains Cope, "the dynamic effect produced by the exposure was stored for the period which elapsed between the exposure of the larva and the full development of the pupa. The second experi- ment demonstrates that a stimulus may be transmitted to a gland so as to modify the character of its secretion in a new direction. From both experiments we learn the transmissibility of energy from the point of stimulus to a remote region of the body, and its conversion into growth energy (in this case by physiogenesis). This prepares us to look upon heredity as an allied phe- nomenon, i. e. the transmission of a special energy from a point of stimulus to the germ cells, and its compo- Cope 257 sition there with the emphytogenic (inherited) energy into bathmism (or evolutionary energy)."193 From this he at once draws the conclusion that as soon as a new character is acquired by the soma in con- sequence of a definite stimulus, it appears at the same time in the germ plasm also. This simultaneous double acquisition of the same character by the soma and by the germ represents his theory of "diplogenesis :" "The effects of use and disuse are twofold; viz.: the effect on the soma and the effect on the germ plasm. Those who sustain the view that acquired characters are inherited must I believe understand it as thus stated. The char- acter must be potentially acquired by the germ plasm as well as actually by the soma. Those who insist that acquired characters are not inherited forget that the character acquired by the soma is identical with that acquired by the germ plasm, so that the character acquired by the former is inherited but not directly. It is acquired contemporaneously by the germ plasm and in- herited from it. There is then truth in the two appar- ently opposed positions, and they appear to me to be harmonized by this theory of diplogenesis." 194 It is almost unnecessary in this connection to remark that, if one sticks to the letter, this supposed double acquisition of the same character by the soma and by the germ, lacks any foundation in fact and indeed appears inconceivable. For in the first place the two experi- ments quoted above concern phenomena too special, too complex, and as yet too little analysed to permit of their utilization as foundations for any theory. In the 193Cope: The primary Factors of organic Evolution. Chicago. The Open Court publishing Company. 1896- P. 440. 194Cope : Ibid. P. 442, 443. 258 Theories Treating of Inheritance second place if this character instead of consisting in a general change existing in all the cells concerned, is rather any given local morphological or physiological modification whatever of definite organs or tissues, it will evidently be impossible to represent this modification as acquired at the same time also by the germ plasm, since in the latter these organs and tissues do not exist. It seems nevertheless that Cope's meaning is that each even local morphological or physiological modifi- cation of the soma must always correspond at the same time to a certain specific, dynamic state of the proto- plasm in general, and that it is this new dynamic state which would be acquired at the same time by the proto- plasm of the soma and by that of the germ. For to explain "the way in which the influences which acted upon the general structure reach the germ cells," he builds up his "dynamic theory," taken from the do- main of molecular physics, and has recourse to that spe- cial form of energy mentioned above, which he calls "bathmism." And this bathmism would consist acord- ing to this author "in a mode of motion of the molecules of living protoplasm by which the latter build tissue at particular points, and do not do so at other points." "This action is most easily observed in the begin- nings of growth, as in the segmentation of the oosperm, the formation of the blastodermic layers, of the gastrula, of the primitive groove, etc. In the meroblastic embryo the energy is evidently in excess at one point of the oosperm and in defect at another. This is a simple example of the location of growth force or bathmism. In all folding or invagination there is excess of growth at the region which becomes the convex space of the fold; i. e. a location or especial activity of bathmism at Cope 259 that point. All modifications of form can thus be traced to activity of this energy at particular points/' "The building energy being thus understood to be a mode of molecular motion, we are not at liberty to suppose that its existence is dependent on the dimensions of the organic body which exhibits it. It is as characteristic of the organic unit or plastidule as the mode of motion which builds the crystal is of the simplest molecular aggregate from which the crystal arises. Bathmism has, however, no other resemblance to crystalloid cohesion. The latter is a simple energy which acts within geo- metrically related spaces, without regard to anything else than the present compulsion of superior weight- energy. In bathmism we see the resultant of innumerable antecedent influences, which builds an organism con- structed for adaptation to the varied and irregularly occuring contingencies \vhich characterize the life of liv- ing beings. As this resultant is distinctive for every species, bathmism must be regarded as a generic term, and the characteristic growth energy of each species as distinct species of energy, which present also diversities expressive of the peculiarities of individuals." 195 It would be superfluous to bring forward the extraor- dinary indefiniteness and, we can almost say, pure verbalism without any foundation in fact of this theory of Cope's which approximates closely Haeckel's theory of perigenesis with its undulatory plastidular movement. We shall confine ourselves to remarking merely that each given dynamic state of the protoplasm peculiar to a given species, when it thus represents in itself the result- ant of all dynamic states, which were peculiar to the 19ECope : Ibid. P. 447—449. 260 Theories Treating of Inheritance protoplasm during the whole course of phytogeny, would nevertheless not cease on that account to constitute still a special dynamic state which is quite different from the preceding ones, and which cannot possibly preserve materially even the smallest trace of them. Therefore this theory of Cope leaves the repetition of phylogeny by ontogeny quite as incomprehensible as did that of Haeckel. And on the other hand one cannot see how the protoplasm could be in the same identical dynamic state in all the most different parts of the soma and yet give rise to specific biologic phenomena correspondingly different in each of these parts. It would have been on the contrary a much more suggestive idea, had Cope sought to reduce all the dif- ferent, contemporaneous, physiological and morphological variations of the organism, not so much to a single and everywhere uniform change of this given growth energy, as rather to numerous specific variations of a single generic form of energy, so that the latter would thus represent to a certain extent the common denom- inator of all these unlike morphological and physiological variations. For this is in any case one of the means which every theory must employ which essays to explain the inheritance of acquired characters. For when once all variations of the most manifold forms of energy, acting simultaneously upon the most different points of the organism, are attributed to as many specific varia- tions of a single form of energy as the basis common to all of them, then it would be easy to combine with it the conception that for each complex state of the organism there might appear in the germ a single, well defined specific mode of being of this common form of energy, as the resultant of all these specific different modes Cope 261 which are active simultaneously, each in its own way in the most different points of the soma. And just as the resultant of several forces acting upon one point at the same moment can be decomposed again into its former component parts, all of which would still act simulta- neously, so it is conceivable how this particular mode of being of the common form of energy which arose and was stored up in this way in the germ can become decomposed again at the proper time at all the various points of the new organism into the same modes of being as formerly, which had already been its components in the parent organism. To mitigate the fault of indefiniteness in his theory this investigator, just like Haeckel, Orr and many others, also compares the ontogeny thus produced by bathmism with the mnemonic phenomenon. And although he has thereby certainly neither removed or even diminished the general vagueness which characterizes his whole theory he succeeds nevertheless in expressing here a remarkable and suggestive idea. "We may compare the building of the embryo to the unfolding of a record or memory which is stored in the central nervous system of the parent and impressed in greater or less part on the germ plasm during its construction, in the order in which it was stored. This record may be supposed to be woven into the texture of every organic cell and to be destroyed by specializa- tion in modified cells in proportion as they are incapable of reproducing anything but themselves." "In the case of the germ plasma no other specialization exists so that the entire record may be repeated stage after stage, thus producing the succession of type- structures which embryology has made familiar to us. 262 Theories Treating of Inheritance In the process of embryonic growth, one mode of motion would generate its sucessor in obedience to the molecular structural record first laid down in the ovum and sperma- tozooid, and then combined and recomposed on the union of the two in the oospore, or fertilized ovum." "Were all cells identical in characters, every one would retain the structural record or memory of its past physical history as do the unicellular organisms. Evolu- tion has however so modified most of the structural units of the organic body that none but the nervous and reproductive cells retain this record in greater or less perfection. The nervous cells have been specialized as the recipients of new impressions, and the excitors of definite corresponding movements in the cells of the re- mainder of the organism. The somatic cells retain only the record or memory of their special function. On the other hand the reproductive cells which most nearly resemble the independent unicellular organisms, retain first the impression received during their primitive unicellular ancestral condition; and second, those which they have acquired through the organism of which they have been and are only a part." 196 As we shall devote ourselves in the last chapter to the comparison of the ontogenetic phenomenon with the mnemonic, it will suffice here to bring forward, as a contradiction to the same author's assertion reported above in respect to a single dynamic mode in the whole organism, the complete mnemonic somatization of the specialized somatic cells, or nuclear somatization, which this investigator recognized, and also his suggestive sub- stantial equalization of the nerve cells with the repro- 186Cope: Ibid. P. 451—453- Delage 263 ductive cells as the only cells endowed with unsomatized memory, and consequently as the only ones which would be likewise capable of preserving more or less com- pletely the memory of past generations. Nevertheless he should in our opinion have limited this equalization with the reproductive cells to those nerve cells which are least differentiated. Delage According to Delage, "The egg is like a star thrown out by an initial force into the midst of a system of stars in movement. Its trajectory will be influenced and determined by all the stars whose sphere of action it traverses, but nevertheless, if anything had been altered in its mass or in its initial movement, it \vould not have been what it is. It is not dependent on the system alone nor is it at any point independent of it. Every other similar mass thrown out at the same point, with the same force in the same direction will reproduce a tra- jectory identical with its own; but every difference even the most minute, in any one of these three factors will be able to induce considerable differences in the form of this curve." 197 This comparison leaves the repetition of phylogeny by ontogeny and the inheritance of acquired characters out of consideration. The inheritance of acquired characters is nevertheless accepted in part at least by Delage, who explains it thus : "When a new chemical compound introduced into the organism produces different effects at different points, that is undoubtedly due to this, that it finds at each i97Delage: L'heredite etc. P. 802—803. 264 Theories Treating of Inheritance individual point a different cell substance as the pre- dominant element. Then if the egg contains the sub- stance characteristic of certain cells of the organism, it must be affected at the same time as these cells and by the same influences. According as these influences exert an exciting or depressing influence and so provoke the corresponding organ to further development or to atrophy, there will be produced a similar action in the egg, the corresponding substances will undergo a certain growth or a certain regression and when the egg develops, the cells whose task it is to localize these substances within them, will experience the effects of this regression or of this growth." 198 In the first place it is to be noted here that the organs whose modifications produce new phyletic stages do not usually either develop or atrophy uniformly in all direc- tions. Indeed, specific morphological alteration consists rather in a growth or diminution always proportionally unlike in different directions. The particular substance which has increased in the egg can serve at most for the explanation of a quantitative increase in mass of the organ, but not for a morphological increase, different in each different direction, like that which the parent organ- ism has experienced. In the second place this explanation cannot be satis- factory since there may be growth in one organ, while another organ consisting of the same tissue, such as nervous, muscular, bony tissue, etc., may remain un- changed or even regress. These organs consisting of the same tissue ought all to grow or diminish alike with ie8Delage : Ibid. P. 837. Delage 265 each change which their particular substance experiences in the egg. In the third place, this would at all events suffice only for the explanation of inheritance of those char- acters which develop in the parent organism under the influence of definite chemical actions, distributed through- out the whole body, and acting only upon those particles or cells of the body which have a certain chemical com- position. But what explanation could that give of true morphological inheritance and so of the inheritability of the growth or of the atrophy of an organ resulting from too much or too little use? — of the inheritability of the spongy structure of bone, of the conformation of the eye, and of all functional adaptations in general? Yet Delage gives the following explanation of the inheritance of the atrophy of unused organs. "That only is determined in the egg, which is not determined by functional excitation, but the amount determined by the latter is enormous." The absolute uselessness of slight reductions of the atrophied femur of the whale and the consequent inefficacy, in this respect, of natural selection, and on the other hand, the impos- sibility of understanding how the slight reduction in volume which the femur undergoes during the life of the individual can extend its influence to the egg, and determine in that the modification necessary for the reproduction, in the following generations, of this new reduction in volume, "forces us to admit," continues the author, "that neither in consequence of a fortuitous variation fixed by natural selection, nor in consequence of an acquired and inherited modification does the egg of the existing whale differ in so far as the femur is concerned, from the eggs which produced the whales 266 Theories Treating of Inheritance of centuries ago, whose femur was only a very little greater than that of the whales of today." It still remains then to explain, without devising some improbable hypothesis, how the same kind of egg can produce two different forms. And that is not very difficult when one keeps functional excitation in mind." "When an animal has a femur 20 cm. long, that does not indicate that in its egg all the conditions were present for the formation of a bone of this length. That indicates only that the elements necessary for it are there, which with the co-operation of the functional stimulus can form a femur of such length. We cannot know just what part this latter takes in the result, but it must be considerable." "While the whale had still a femur, which though not normal was yet only half atrophied, the femur pro- ducing factors inherent in the egg were perhaps suffi- cient to produce a bone of only the size of that present in the whales of today, and the functional stimulus, which as Roux has shown, begins to operate even in embryonal life, did the rest. It is therefore not astonish- ing that upon the cessation of the functional stimulus, the femur became reduced to a very little rudiment." 19° But the embryonal functional stimulus in the whales of many centuries ago whose femurs were only a very little bigger than those of the whales of today, cannot have been different from the embryonal functional stimulus of the whales of today, no matter how much one may limit direct morphological action of the egg, if one starts out from the hypothesis that the eggs concerned are quite identical. Why should the embryonal functional stimu- 199Delage : Ibid. P. 854—85?. Delage 267 lus in respect to the useless femur, already so atrophied, have been greater in the former than in the latter? The progressive diminution of the femur remains thus entirely unexplained. Finally Delage accounts for the parallelism between ontogeny and phylogeny in the following way: "The functional stimulus appears, we agree with Roux, even in embryonal life, but is at this time certainly weaker than after birth. There results from that a note- worthy consequence which escaped Roux. That is that at least the relative atrophy of the organ becomes more marked the older the animal becomes, and that, in respect to the atrophied organ, the young, and above all the embryo, must differ much less from the ancestral forms. Thus there is explained at once the parallelism between ontogeny and phylogeny in everything which is depend- ent upon atrophy or hypertrophy induced by use or dis- use, that is in very many cases." 20° Is it inactivity that really causes in serpents the retro- gression during embryonal life of the already partially developed limbs? Or is it use that in the salamanders causes to any extent the development, during embryonal life, of the same limbs? Whence come these very unlike embryonal processes of activity or inactivity? Why did not this same inactivity, consequently this same atrophy, show itself in the embryos of the remote ancestors of the serpents of today? Why does the inactivity and consequently the atrophy of these extremities depend, in the egg of the present day serpents, upon conditions within the embryonic organism itself, and manifest itself at exactly that ontogenetic moment, which corresponds 200Delage : Ibid. P. 856—857. 268 Theories Treating of Inheritance to the phylogenetic moment at which it was produced in their ancestors, whereas in the latter this reaction commenced only when the animal was exposed, after leaving the egg, to the influences of the external world, and was rendered necessary only in consequence of very definite circumstances external to the organism? We see thus that the question of the repetition of phylogeny by ontogeny finds no answer at all. It seems to us further that there remains only the erroneous view that entire phylogenetic epochs could have gone by, without leaving behind any trace in the egg, so that the progress of each ontogeny would be nothing else than a phylogeny which is almost entirely repeated each time. In the second edition of his work Delage recognized himself that the significance attributed by him to the functional stimulus in ontogeny is exaggerated.201 And he admits that he was embarrassed in explaining by it both the special fact of the formation of an organ so complicated and so well adapted to its purpose as the eye, for which during embryonal life there was never- theless lacking any functional stimulus, and also the phenomena of regeneration, or the general fact that nearly all organs without exception show, from the first stages of their development on, an adaptation to the func- tions which they \vill perform only later.202 LeDantec According to LeDantec's theory, each individual, living, elementary mass or plastid "contains a mixture of different plastic substances, which are distributed in such a way that assimilation represented by the equation : 201Delage: Ibid. P. 862. Remark. 202Delage : Ibid. P. 870, Le Dantec 269 a + Q = X a + R (where a = mass of each single specific assimilating substance; Q = mass of the nutritive substances absorbed ; x = coefficient > i ; and R = mass of the substances of refuse), multiplies all these sub- stances and preserves to them their original proportions. Destruction however, or at least certain destructions, act separately on each of these substances and alter the pro- portions of the mixture and consequently the characters of the plastid." 203 From this it appears that LeDantec attributes all variations which the plastids or organisms in general can undergo, to total or partial destructions of the dif- ferent, particular plastic substances (a) already present, but never to the production of new plastic substances. The numbers and characters of plastic substances, which participate in the formation of the complex substance of the plastid or rather of the organism in general, may be different in different species. The difference between different species and between different individuals of the same species, would consist only in the proportions in which the special plastic substances (a) peculiar to this species, are united. "We are inclined to regard the living substances of the plastids as mixtures of different plastic substances, the substances (a). The species of the plastids would be determined by the nature or quality of these plastic substances; their individual peculiarities, their person- ality, would depend upon the proportions of the mixture of these specific plastic substances. In the same way we must regard the individual substances in the higher organisms as characterized by a mixture in definite proportions of lfiLe Dantec: Traite de Biologic. P. 93. 270 Theories Treating of Inheritance the living substances characteristic of their species. We are able in this way to present even a mathematical defini- tion of the personality of a given individual of a species, to a certain extent an arithmetical personal description of this individual, namely the list of co-efficients of the mixture of his specific substances." 204 The proportions of this mixture persist unaltered in all cells of the same organism. Upon this mixture depends the quality of the chemical reactions, i. e. of the molecular movements; upon these latter again depend the molar movements or osmotic currents of nutritive and excretive material; upon the molar movement finally depends the form of each plastid as well as that of the most complicated organism: "It is absolutely useless to suppose, in the egg which produces man, other characters present than for example those of a simple hepatic or epithelial assimilative ele- ment, determining by this assimilation the molar move- ments around it. These molar movements associated with the movements which result from assimilation in neighboring elements, and also with the existence of the skeleton such as is constituted in a thenceforth unchange- able form from the moment of its first anlage, determine the conditions of local equilibrium from which the local form of the body results. Analogously as soon as a human element (the fecundated egg) is capable of liv- ing by itself alone, the molar movements, which assimila- tion provokes first in this element alone and later in all which are derived from it, determine the successive forms of the growing mass arising from assimilation. The phenomenon appears from the outside then to be 204 Le Dantec : Ibid. P. 267. Le Dantec 271 quite otherwise than as though this element had assimi- lated without being isolated, as though for example it had belonged to a man in process of growth. It would then have been the destiny of this element, thanks to the combined "dynamisms" of the neighboring elements, to build up a part of the man, but not a whole man." 205 Finally the transformations to which the living sub- stance would be forced by the constraint of external influences may be hereditary, i. e. can take place anew in the descendant organism without any further need of the action of the same constraint, because they would alter the living substance itself in a corresponding way, so that it adapts itself to the new conditions of equilibrium : "If assimilation were the only possible phenomenon of living matter there would not take place any alteration of the living substance through external influences; but upon the truly vital phenomena of assimilation are super- imposed as we have seen phenomena of destruction, and the co-operation of these two phenomena can result in changes in the nature of the substance, in the definite proportions of the mixture forming it; thus education can modify heredity." "Since the form is the result of the molar move- ments of the metabolism of all cells of the body, a variation imposed on the form reacts upon these molar movements by which again the molecular movements in the interior of the cell are determined. Then in con- sequence of this form imposed on the body mass, there will occur within the cells phenomena of destruction, i. e., of variation. The variations may take any direction 808 Le Dantec: Ibid. P. 257—258. 272 Theories Treating of Inheritance whatever; but natural selection (which acts in each cell of the organism among the different plastidular varia- tions) intervenes and fixes only those which are adapted to the new conditions of equilibrium." 206 We would just remark here, that the alteration undergone by the molar movements within each cell will be different in the different cells. For it would be incom- prehensible how in the very complex structure of the organism, a local change of form, imposed by external agents, could induce quite identical alterations in the molar movements of all the other cells of the body indiscriminately. Consequently the alterations of the liv- ing substance which internal natural selection preserves as fittest will likewise be different in different cells. How then can there be any question of the survival, in con- sequence of this internal natural selection, of one single plastidular variation identical at all points of the organism ? LeDantec, like Hertwig, has recourse to the example of immunization. But as we have already seen, this case is quite different from the more or less local changes of form, which individuals experience in consequence of particular functional adaptations. In the case of immuni- zation the transforming cause, i. e. antibacterion, is the same for all cells. In the case of a morphological alteration on the contrary the transforming cause, that is, as we would concede it, the variation experienced by the molar movement concerned, is different in each cell. Even if one were willing to assume an identical variation of the living substance at all points of the organism indiscriminately, that would not explain the 20flLe Dantec : Ibid. P. 270, 298. Le Dantec 273 law of the repetition of phylogeny by ontogeny, as we have already had occasion to remark in connection with the similar hypotheses of Spencer, Hertwig and several others. This requires, as we have seen, the conception of the addition of a new substance to all those formerly present. All variations of the organism are ascribed by LeDantec, as we have already seen, to total or partial destruction of some of the different plastic substances (a), which make up the living substance, whereby their quantitative proportions become changed; but never to the formation of new plastic substances. Likewise dif- ferent species would differ from one another in the number and quality of the plastic substances (a). From this it follows: i, that no further development can be effected by any given living matter, if the number of its substances has become very small, and thus an abso- lute inalterability must be established as soon as this number is reduced to one; 2, that the development of the species can have been produced only by successive total destructions of an always greater number of these plastic substances; 3, that the further a species is devel- oped, the smaller therefore must be the number of the plastic substances which form its respective living matter. One would thus arrive at the absurdity, that the simpler the living substance is the more complex must be the organisms formed from it. Finally LeDantec, like Spencer, Hertwig and the others is unable to explain histological differentiation by this supposed similarity of living substance in all parts of the organism : "A muscular element differs entirely from a nervous epithelial element, and these differences are manifested 274 Theories Treating of Inheritance not only in the form of cells but also in their mode of activity. Now what is the nature of these differences? We do not know. Are they physical in character? That would be hard to believe, because of the difference of the chemical excreta of these elements. If the dif- ferences are of chemical nature they must leave uninjured the hereditary patrimony (the living substance similar at all points of the organism). Now it is entirely impos- sible that quantitative variations can be produced in the elements, and leave untouched a quantitative character already present. Perhaps there is properly speaking no quantitative variation, but only a modification in the nature of the non-living accessory substances which fill out the aggregate at different points of the organism according to the special conditions obtaining at these points. To all these questions we have as yet no answer." 207 Before we leave this investigator we must bring up one last point, namely : the logical necessity which forces him to regard the living substance as similar at all points of the organism. According to him, this conception is a logical consequence of the inheritance of acquired characters which he holds as a fact already proved beyond a doubt. For, says he, let us consider any given morphological variation acquired by the organism and transmissible to its descendant. And let us assume that the hereditary patrimony, i. e. the living substance ( a ), originally common to all elements of the individual by descent from the egg, can, under the influence of the morphological variation experienced by the latter, have been replaced, here by a different substance ( /3 ) , there 207Le Dantec : Ibid. P. 461—462. Le Dantec 275 by another substance ( i ) and so on, in such a way that the whole of the "dynamism" existing in this hetero- geneous mass, finds its expression in a form of equili- brium F, which preserves accurately, without any need of further constraint, those forms of equilibrium which the individual had acquired in consequence of the com- pulsion of external influences. "If that were so," continues LeDantec, "this form could not be hereditary. For the substance 0 produces the form F only with the assistance of cells of the sub- stances y and 8 , which are simultaneously present in other elements of the altered individual, and no one of these substances which does not belong to the sum total of the elements is by itself a consequence of the total form F. If then one detaches from this form a few pieces capable of reproducing themselves, these pieces endowed with different substances or heritages will give rise to different individuals, namely to individuals or groups of cells like those whose total constituted the form F, but of which none had this form. There is thus absolutely no reason existing why any one of these individuals should take the form F. If then observation teaches us that acquired characters can be inherited we are thus obliged to suppose that in the case in which they are hereditary they were acquired by the parent organism in a homogeneous manner." 208 Thus if it were possible to explain this inheritance and at the same time to accept, nevertheless, the most complete diversity of the substances constituting individ- ual parts of the organism, LeDantec would be perhaps the first to renounce with joy his single individual sub- 208Le Dantec : Ibid. P. 294—295. 276 Theories Treating of Inheritance stance, similar throughout the whole organism, which as he himself states, makes histological differentiation at least inexplicable. THEORIES OF CHEMICAL DEVELOPMENT In his fundamental work "The Struggle of the Parts of the Organism," and therefore at a time, before Roux had yet reached the preformistic view of idioplasm or germ plasm which he later very clearly adopted, and which in many respects is like the conception of Weis- mann; when also he still considered development to be rather the complex result of a long series of purely chemical phenomena, and nevertheless had not yet wel- comed Weismann's theory of the non-inheritance of acquired characters as a deliverance from a nightmare, at that time he sought to explain this inheritance in the following way: First he notes that the germ plasm although it be- comes separated at the very commencement of develop- ment from the organism in process of formation, "remains nevertheless dependent upon and in relation with this organism; for it must be fed and grow and multiply and to that end it receives its nourishment from its parent by chemical metabolism, and might still be influenced in its own nature in this way." 209 He supposes further that on the one hand each structural formation may be conditioned by certain spe- cial, chemical relations, and vice versa that each variation of form which the adult organism undergoes through functional adaptation produces in its turn a certain *09Roux : Der Kampf der Teile im Organismus. P. 60. Theories of Chemical Development 277 special chemical change. This chemical change would later become transmitted to the germ plasm by means of the metabolism.210 One can not rightly comprehend here, how a special chemical modification, produced in the germ plasm by a change of form in the adult organism, can later give rise to such a development by that germ plasm as to reproduce the same change of form at the proper time in the new organism. If the chemical variation cor- responding to a definite change of form were provoked by the germ plasm in the new organism only at the time when this latter reached the same age and conse- quently a state of being which would be the same in its entirety as that of the parent organism when this given variation of form supervened in it, and were confined to the same limited zone in which this chemical variation \vas produced in that parent organism, then the concep- tion of an actual reversibility of the phenomena would not be in itself at all impossible, that is it would not be impossible that the same chemical phenomenon might provoke in the new organism the same variation of form by which it had itself been produced in the parent organ- ism. But in our case on the contrary this chemical varia- tion, no matter whether it transforms the whole chemical composition of the germ plasm or only a part, will com- mence to act upon the new organism immediately, at the very commencement of its development, and will modify therefore not merely a limited part of the cells of the organism, but all the cells without exception. How then could this same chemical change, which operates immedi- ately at the commencement of development and conse- 210Roux: Ibid. P. 61. 278 Theories Treating of Inheritance quently upon all stages of development and upon all cells of the organism, call forth the same result as if it had come to act upon only a very definite point and at a very definite time of the development of this organ- ism? It seems to us that we ought much rather to conclude that these results must be very different and that with them there can be no question of any similarity whatever. This impossibility of explaining the inheritance of acquired characters by Roux's earlier theory is not limited to it alone, but pertains to all theories of chemical develop- ment in general. And the fault lies not only in the above mentioned impossibility of the reversibility of the phenomena of inheritance which we have just considered but also in a still more generally characteristic circum- stance, which is likewise common to all these theories of chemical development, and which we have elsewhere already stated for other theories. And it is mostly from it that this impossibility of reversibility comes. It con- sists in this, that according to all of these theories as soon as the germinal substance has once given the initial impulse to development it is unable to exercise even the slightest influence upon the further course of this develop- ment. If thus the reins by which development is directed are let fall, and each bond severed which connects the changes of the soma with those of the germ and vice versa, then it is impossible to conceive how this union could later be re-established, as soon as the need was felt of transmitting to the germ and fixing in it the requisite variation, corresponding to that which appeared in the soma as the result of a new functional adaptation. Hofmeister's theory can be considered as an especially typical example of this complete abandonment of develop- Theories of Chemical Development 279 ment to itself, which constitutes the great defect of all theories of chemical development. This investigator believes that the chemical activity of the cell is due in general to colloidal ferments which are contained within them and of which each is destined for a special chemical process. He admits thereby the existence of numerous colloidal ferments in cells with multiple chemical processes, and he sees in ontogeny the result of a series of chemical reactions which follow one another according to the principle of fructifying causality : "During the development of the embryo there takes place a chemical differentiation parallel with the morpho- logical differentiation. The formation of new chemical anlagen indicates the appearance of different ferments at definite stages of embryonal development." — "One could hardly form a better idea of the chemical transformations going on during the early development of the embryo than by supposing that at first only a very small number of ferments become active, and that these transform existing material into new substance, among which pro- ferments or ferments of another kind appear, through which the first then become annihilated, and which become supplanted in their turn by a new generation of ferments which they have themselves produced and so on until the cycle of new chemical formations requisite for the history of the race is run through. The epigenesis of form would be then only the expression of the epigenesis of chemical forces/' 211 We shall pass over the fact that all these theories of chemical development have yet to explain the connection :11Hofmeister: La chimie de la cellule. Revue generale des sciences; Aug. 15, 1902. P. 730—731. 280 Theories Treating of Inheritance between each chemical and the corresponding morpho- logical stage of "development ; for this morphological character of different chemical reactions has not so far been observed in any phenomena of the inorganic world, since it has absolutely no analogue in the process of crystallization which is a property of the molecular struc- ture of already formed, stable substances, that is of substances in perfect statico-chemical equilibrium. But we may mention the fact — and after all which has been said above no further proof of it is required — that the fundamental phenomena, such as the regeneration of amputated organs, the occasional reappearance especially in crosses of atavistic characters long since disappeared, and especially the ontogenetic repetition of phylogeny and the inheritance of acquired characters, not only find no explanation in all these hypotheses of chemical development but are on the contrary absolutely irrecon- cilable with them. Darwin, Gallon, DeVries, Weismann It would be useless for our purpose to tarry especially over any one of these four theories, the underlying idea of all being the same identical conception of preformistic germs. The progressive elaboration of this idea which has proceeded gradually from the first to the last of these theories presents however the following noteworthy phenomenon. Preformistic germs, which were devised by Darwin, one could well say, chiefly for the purpose of accounting for the inheritance of acquired characters, were then deprived by Galton in great part but not com- pletely of this property, and finally with DeVries, and still more with Weismann became themselves the greatest difficulty for accepting that inheritance. Darwin 28 1 Of Darwin's pangenesis it is necessary here to men- tion only the conception that the sexual or reproductive organs in general were not so much the place of refuge to which the germ plasm withdrew immediately after its separation from the soma at the very commencement of development, as rather the containers of the germinal substance continually produced and secreted by other parts of the organism lying without these organs, so that they build up as it were the sexual or reproductive cells out of this valuable material thus received and accumulated.212 In Darwin's hypothesis this conception of the repro- ductive organs as mere glands for the reception and giving up again of the germinal substance was intimately associated, although in its essence quite separate and independent, writh his further conception of the free circulation of the gemmules throughout the organism; and he supposes, as is known, that these gemmules were produced and secreted continuously during the adult state by all somatic cells indiscriminately, — by those already present as well as by those just appearing in consequence of a new functional adaptation. Now if Galton by his experiments on the transfusion of blood from a rabbit of one species to the blood vessels of another belonging to a related species, has provoked a thoroughly justifiabile doubt of this supposed circulation of gemmules, especially in so far as it was carried on in the blood vessels, the original idea remained nevertheless unshaken, that is that the germinal substance is assembled in the sexual glands after it has been formed in some real place of origin external to them. 212Darwin : The Variation of Animals and Plants under Domesti- cation. II. P. 370, 379. 282 Theories Treating of Inheritance Because of the fact that the theory of Darwin derives the germinal substance from all parts of the soma rather than from one well defined region of it, its partisans could certainly not object to these experiments that they leave the conception still possible that the germinal sub- stance might perhaps be transmitted from such a special well defined region to the sexual organs only along certain very definite special ways, which might be quite different from the blood vessels. And on account of the nature and properties attributed to the gemmules they would be still less able to advance the conjecture that a substance might possibly be reproduced at a distance, quite like another substance, by the direct influence of the latter, by means of some other means of connection of such nature that it would not require any real and proper material trans- mission. From this the conclusion may be drawn, that all theories which do not exclude or perhaps even include one or the other of these two hypotheses upon the manner of transmission or upon the means of reproduction at a distance of the germinal substance, are completely justi- fied in accepting Darwin's conception of the sexual glands, acording to which the latter have only the func- tion of receiving and accumulating a substance the real origin of which is outside these organs. In the case of Galton we shall recall only that he was the first who introduced the theory that stirp, — i. e., the germ plasm consisting of numerous germs or of gem- mules which remain behind after the extrusion of the particles concerned directly in the formation of the new organism — separated itself entirely from the soma immediately, at the commencement of development. Through this separation of the stirp from the soma he opened the way which later led necessarily to the uncon- Gait on; DeVries; Weismann 283 ditional rejection of the inheritance of acquired char- acters. Nevertheless he did not immediately venture to go so far but continued to admit as a sort of concession that in the adult organism a gemmule might occasionally escape from the somatic cell, which had produced it and was also its customary abode, even though this cell had been only shortly before acquired in consequence of a new functional adaptation ; then this gemmule might be taken up by the reproductive organs and become likewise a part of the stirp and the acquired character which had ap- peared in the somatic cells might thus be inherited.213 In the case of DeVries we should remark that he assumes that the germinal substance, that is the sum total of the pangens, is present equally in all nuclei only be- cause he, as we have also seen in the case of Driesch, took it for granted that nuclear divisions are qualitatively equal. If then a nucleus of a somatic cell acquired new pangens, as a consequence of a new local functional adaptation, then they would have to remain in the place where they arose and could not enter the reproductive cells also. And so much the more since he also asserts that the substance which will later actually form the sexual cells separate itself from the soma immediately, at the commencement of development, and passes along cer- tain "Keimbahnen," which may be recognized, chiefly because upon them the greater part of the pangens remain inactive.214 In respect to Weismann we remark once again that in consequence of a more rigorous logical elaboration of the doctrine of preformistic germs, which has convinced him 213Galton : A Theory of Heredity. Journ. of the Anthropo- logical Institute. January 1876. P. 342 — 343. 21*De Vries: Intracellulare Pangenesis. P. 188—189. 284 Theories Treating of Inheritance of the necessity of regarding them as bound up with one another into a rigid structure, he has been led, through the conception of these preformistic germs which was forced upon him by particulate inheritance, to deny most energetically every possibility of the inheritance by the germ of characters which the soma had acquired by functional adaptation. Weismann admits, it is true, that sometimes external influences acting uniformly upon the whole organism, like temperature and other such things, can alter the deter- minants of the soma and the corresponding determinants of the germ at the same time and in the same direction; as occurs for example in the determinants of the wing scales of the butterfly Polyommatus phlaeas, whose color changes as we have seen when it is transported to a warmer climate. But the cases which permit of this ex- planation, which resembles in many respects the above discussed diplogenesis of Cope, — the only cases which Weismann admits, — are limited by this investigator to so small a number, and are also of so peculiar a kind that it would be wrong to assert that he held less determinedly to his earlier stand as an opponent of the Lamarckian theory. We may point out however, the following contradic- tions. He admits inheritance in unicellular organisms while he denies it in the pluricellular and thinks he can justify this by saying simply that as the unicellular divide always into two equal halves they need only preserve what they have acquired, in order to transmit it unaltered to the new individuals. But this is not right. For new functional adaptations acquired by the anterior end of the infusorian Stentor, for instance the acquisition of "Mem- branelles" by the peristome in consequence of the fusion Weismann 285 of several cilia, would then become transmitted only to that one of the two new individuals to which the anterior part falls in the division, and could in no wise be trans- mitted to the other individual in which this anterior part is formed anew. If one assumes on the contrary, that transmission goes on by means of the nuclei, and can therefore proceed equally into both of the two newly forming individuals, one could not then understand, wherein the transmission of somatic modifications in the unicellular animals, which is accomplished by means of a part of the organism containing in itself no membranelles and quite distinct from them, would differ from the trans- mission of any modification experienced by any organ of a pluricellular organism, which likewise goes on by means of a fragment containing no part of the modified organ and quite distinct from it. So much the more since the substantial identity of the complex unicellular with the pluricellular organisms, which we have already discussed above, corresponds also with a substantial identity in their development, as is shown by the fact that the funda- mental biogenetic law of the repetition of phylogeny by ontogeny is followed in the development of unicellular animals also, as for example, in the formation of the new frontal field in the division of Stentor coereleus.215 And in relation to all these theories with preformistic germs from Darwin to Weismann we might mention once more the insurmountable difficulties that would be encountered if one were required to explain by them this very fundamental law, either in unicellular or pluricellu- lar organisms. This impossibility and the fact that in the 213Johnson: A contribution to the Morphology and Biology of the Stentors. Journ. of Morphol. ; vol. VIII, No. 3, Boston, U. S. A., Ginn, August 1893. P. 519. 286 Theories Treating of Inheritance end the acceptance of these germs has led necessarily to systems which reject the inheritance of acquired char- acters concur to prove, although more proof is certainly no longer necessary after all the other considerations which we have developed in an earlier chapter, that the very idea of these preformistic germs is untenable, as is thus every theory founded upon them. However limited the number of theories or hypoth- eses selected by us, and however rapid and brief the critical exposition which we have made of them, it seems to us nevertheless that it is unnecessary to proceed further with our examination. For it has shown us that among the principal theories, which up to the present have been devised to explain the inheritance of acquired characters, none has accomplished this difficult task, and it has already served another purpose for which chiefly we undertook it. This purpose consisted on the one hand in bringing to light in other theories the most suggestive and fruitful ideas put forward; on the other hand in deter- mining the conditions which are necessary and sufficient to render possible the inheritance of acquired characters, and a critical examination of concrete theories already developed has certainly helped to put these conditions in evidence better than simple reflection upon them could have done. If we take a look over the road which we have thus far traveled we see that among these conditions those which have appeared to us as the essential and fundamental ones are the three following: I. All the manifold physical, chemical, morphologi- cal, and physiologic variations, which can appear in the most different parts of the organism, are to be ascribed Conditions Necessary for Inheritance 287 to specific alterations of a single form of energy, so that the latter appears, as it were, the common denominator for these variations that are quite unlike in nature and whose combination or separation is thus permitted as often as required. 2. The determinative influence which the germ sub- stance in its totality exerts upon the soma cannot be limited to the first moment of the first cleavage of the egg but must persist throughout the whole of ontogeny up to the adult condition, so that the germinal substance never as it were loses touch with the soma, but rather remains in a continual state of reciprocal action and reaction with it. 3. The influence exerted by the soma in this way must be reversible, that is the germ substance must be influenced in such a way that it can call forth again at the proper moment, at the numberless different points of the soma of the new organism, all the same, respective, special, somatic conditions by whose complex modes of being the germ substance itself was already influenced in the parent organism in so special a way. This last condition which alone implies in itself the whole question of inheritance falls again into two parts. First the germ substance must be influenced always in such a way that it is capable of giving back at the required moment the same influence, qualitatively identical but in the reverse direction, which it had already experienced as the single resultant of all the elementary somatic in- fluences to which this germinal substance was simulta- neously subjected in the preceding organism. Second: the germ substance which thus gives back again the influence, by which it was influenced, qualitatively identical but in reverse direction, must be localized at a 288 Theories Treating of Inheritance single definite point of the organism, which is always the same, both when the parent soma exerts its influence upon the germ substance which it contains, and also when the latter exerts upon the new organism its own determina- tive ontogenetic influence. It is therefore our task now to investigate in the following chapter whether the centroepigenetic hypothesis already set forth above really satisfies all these conditions and is consequently capable of really affording the ade- quate explanation for the inheritance of acquired characters, which we seek. CHAPTER SEVEN THE CENTROEPIGENETIC HYPOTHESIS AND THE EXPLANA- TION OF INHERITANCE AFFORDED BY IT. As we said at the end of the third chapter, when the end of development is reached, and there comes with it a cessation of the steady activation by the central zone of new specific potential elements, there is also a cessation of the perturbing action which that zone had up till then exercised upon the dynamic equilibrium of each onto- genetic stage; so that the organism attains at that moment the final equilibrium of the adult state. But we should note that a new perturbing influence can now come into play, namely, the functional stimulus in the widest sense with all its innumerable variations. In the same manner, we said, as the perturbing action of the central zone had formerly upset the equili- brium which was but just formed, and so provoked the passage of the organism to a new ontogenetic state, so now each lasting change of the functional stimulus by disturbing the dynamic equilibrium of the adult state, will induce another general distribution of nervous energy. Consequently each cell of the entire organism or of cer- tain portions of the organism will now be traversed by a nervous flux which is specifically different from that be- fore existing, and specifically different in different cells. In each nucleus of these cells, we continued, a par- 289 290 Explanation of Inheritance ticular specific potential element will consequently be formed and deposited, which will be added to the element or elements already present. All these elements, new as well as old, deposited in the somatic nuclei, will, however, be lost with the death of the individual; and those alone will escape this destruction which are deposited in the germinal substance of the central zone. The lasting variation of the functional stimulus will thus have had for its total effect, in so far as the species is concerned, the simple addition of a new specific potential element to the germinal substance. Arrived at this point, we reserved for one of the following chapters the examination of the manner in which this new element would act in the ontogeny of the next following organism. It is then with this examina- tion that we must now occupy ourselves in the present chapter. We should first dwell a little more in detail upon this hypothesis, which we mentioned only in passing in con- nection with the posthumous action, or "Nachwirkung," of the nucleus in enucleated fragments of unicellular forms. We mean the hypothesis that the substance which constitutes each specific element, and which is cap- able of giving as discharge a single well-determined specific nerve-current, is also the same and only substance w-hich this specific nerve-current can in its turn form and deposit. This should not appear so very strange to us, since the inorganic world itself presents a phenomenon similar in certain respects. The substance which actually con- stitutes the charge of ordinary electric accumulators is capable of giving back inversely, during its discharge, the same kind of energy which it had previously received, Accumulators of Specific Nervous Energy 291 and by which it had itself been deposited, namely the continuous electric current. The most important differ- ence consists in this, that an electric accumulator is capable of restoring always only one and the same kind of energy, but not solely such or such intensity of current. It constitutes, for that reason, only a generic potential element; but such accumulators would attain the com- pleteness of specific potential elements — receiving and restoring instruments of the greatest delicacy — if one could make it possible that each one of them should re- ceive and restore only a single definite intensity of current. The similarities and differences which nerve currents present, in comparison with electric currents, quite war- rant us in assuming in nerve currents some of the properties of electric currents, and in attributing at the same time to the first other properties which the electric do not possess, provided these qualities are not incom- patible with the others. It is known that, if we designate by E the electro- motor force of an accumulator or of any electro-chemical generator, it can furnish currents of any intensity i whatever, according to the resistance R of the circuit, according to the equation i=E/R. Thus, — even though the terms of motor force, of resistance, of intensity, or more generally of specificity, transferred from electric to nervous currents are very indefinite — we may very well venture, nevertheless, as a preliminary trial hypothesis, to attribute to nervous currents as among the properties which they might have analogous to electric currents precisely those contained in this equation. As it involves nothing incompatible with the prop- erties expressed by this equation, we may imagine a nervous accumulator, constituted by a given substance 292 Explanation of Inheritance capable of being produced and deposited solely by a current of a definite intensity or specificity, and at the same time capable of producing, by its decomposition, this current only, of the same intensity or specificity i is that of the charge. This accumulator, then, will discharge itself and produce this current as often as its nervo-motive force, which we may still call E, is sufficiently great to overcome the respective resistance, according to the equation : E=iR. Finally, we can assume that the magnitude of this nervo-motive force, is proportional to the quantity or mass of the substance which has been gradually deposited and accumulated, as if the successive infinitesimal deposits of this substance were innumerable little Ley den jars ar- ranged in serial order. Then the greater the mass of the specific substance of this nervous accumulator the greater in proportion will be the resistance which its dis- charge will be able to overcome. At the same time, this accumulator, capable of surmounting by its current of a fixed intensity i a given resistance R, will be capable also of surmounting every other resistance less great than R ; for to effect that, it will suffice that it be not the total quantity of material at disposal that enters into action, but only a portion more or less large, so as to furnish for each resistance R'