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diff --git a/old/65049-0.txt b/old/65049-0.txt deleted file mode 100644 index a01baaa..0000000 --- a/old/65049-0.txt +++ /dev/null @@ -1,18893 +0,0 @@ -The Project Gutenberg eBook of The Evolution Theory, Vol. 2 of 2, by August -Weismann - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: The Evolution Theory, Vol. 2 of 2 - -Author: August Weismann - -Translator: J. Arthur Thomson - Margaret R. Thomson - -Release Date: April 10, 2021 [eBook #65049] - -Language: English - -Character set encoding: UTF-8 - -Produced by: Constanze Hofmann, Alan, Marilynda Fraser-Cunliffe and the - Online Distributed Proofreading Team at https://www.pgdp.net - (This book was produced from images made available by the - HathiTrust Digital Library.) - -*** START OF THE PROJECT GUTENBERG EBOOK THE EVOLUTION THEORY, VOL. 2 OF -2 *** - - - - - THE - EVOLUTION THEORY - - VOLUME II - - - - - THE - - EVOLUTION THEORY - - BY - - DR. AUGUST WEISMANN - - PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF FREIBURG IN BREISGAU - - - TRANSLATED WITH THE AUTHOR'S CO-OPERATION - - BY - - J. ARTHUR THOMSON - - REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN - - AND - - MARGARET R. THOMSON - - - ILLUSTRATED - - IN TWO VOLUMES - - VOL. II - - - LONDON - EDWARD ARNOLD - 41 & 43 MADDOX STREET, BOND STREET, W. - - 1904 - - _All rights reserved_ - - - - -CONTENTS - - - LECTURE PAGE - - XX. REGENERATION 1 - - XXI. REGENERATION (_continued_) 23 - - XXII. SHARE OF THE PARENTS IN THE BUILDING UP OF THE - OFFSPRING 37 - - XXIII. EXAMINATION OF THE HYPOTHESIS OF THE TRANSMISSIBILITY - OF FUNCTIONAL MODIFICATIONS 62 - - XXIV. OBJECTIONS TO THE THESIS THAT FUNCTIONAL MODIFICATIONS - ARE NOT TRANSMITTED 80 - - XXV. GERMINAL SELECTION 113 - - XXVI. GERMINAL SELECTION (_continued_) 136 - - XXVII. THE BIOGENETIC LAW 159 - - XXVIII. THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 192 - - XXIX. THE GENERAL SIGNIFICANCE OF AMPHIMIXIS (_continued_) 210 - - XXX. IN-BREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION, - AND THEIR CONSEQUENCES 238 - - XXXI. THE INFLUENCES OF ENVIRONMENT 265 - - XXXII. INFLUENCE OF ISOLATION ON THE FORMATION OF - SPECIES 280 - - XXXIII. ORIGIN OF THE SPECIFIC TYPE 299 - - XXXIV. ORIGIN OF THE SPECIFIC TYPE (_continued_) 330 - - XXXV. THE ORIGIN AND THE EXTINCTION OF SPECIES 346 - - XXXVI. SPONTANEOUS GENERATION AND EVOLUTION: CONCLUSION 364 - - INDEX 397 - - - - -LIST OF ILLUSTRATIONS - - - FIGURE PAGE - - 35 _B_ (repeated). _Hydra viridis_, the Green Freshwater Polyp. 4 - - 96. A Planarian cut transversely into nine pieces 6 - - 97. A Planarian which has been divided into two by a - longitudinal cut 14 - - 98. The leg of a Crab, adapted for self-mutilation or - autotomy 17 - - 99. Regeneration of the lens in a Newt's eye 21 - - 100. Regeneration of Planarians 25 - - 101. A Starfish arm 27 - - 76 (repeated). Diagram of the maturation divisions of the ovum 39 - - 82 (repeated). Fertilization in the Lily 59 - - 91 (repeated). Hind-leg of a Grasshopper 83 - - 102. Brush and comb on the leg of a Bee 84 - - 103. Claw on the leg of a 'Beach-fly' 85 - - 104. Digging leg of the Mole-cricket 86 - - 105. Ovary of a fertile Queen-Ant and ovaries of a Worker 91 - - 106. Three Workers of the same species of Indian Ant 97 - - 107 _A_, _B_. Larva of a Caddis-fly 105 - - 107 _C_. Leptocephalus stage of an American Eel 133 - - 108. Nauplius larva of one of the lower Crustaceans 161 - - 109 _A_, _B_. Metamorphosis of one of the higher Crustacea, - a Shrimp 162 - - 109 _C_. Second Zoæa stage 163 - - 109 _D_, _E_. Mysis-stage and fully-formed Shrimp 164 - - 70 (repeated). Daphnella 166 - - 110. The largest of the Daphnids (_Leptodora hyalina_), with - summer ova beneath the shell 166 - - 111. Nauplius larva from the winter egg of _Leptodora hyalina_ 167 - - 112. Development of the parasitic Crustacean _Sacculina - carcini_ 168, 242 - - 113. The two sexes of the parasitic Crustacean _Chondracanthus - gibbosus_ 170 - - 114. Zoæa-larva of a Crab 171 - - 115. Caterpillar of the Humming-bird Hawk-moth _Macroglossa - stellatarum_ 178 - - 3 (repeated). Full-grown caterpillar of the Eyed Hawk-moth 178 - - 4 (repeated). Full-grown caterpillar of the Eyed Hawk-moth 179 - - 8 (repeated). Caterpillars of the Buckthorn Hawk-moth 179 - - 116. Development of the eye-spots in the caterpillar of the - Elephant Hawk-moth _Chærocampa elpenor_ 180 - - 117. Caterpillar of the Bed-straw Hawk-moth _Deilephila galii_ 181 - - 118. Two stages in the life-history of the Spurge Hawk-moth - _Deilephila euphorbiæ_ 182 - - 119. Caterpillar of the Poplar Hawk-moth _Smerinthus populi_ 184 - - 120. _A_, Symmetrical, and _B_, asymmetrical curve of frequency 207 - - 121. Life-cycle of _Coccidium lithobii_ 214 - - 122. Conjugation of a Coccidium (_Adelea ovata_) 216 - - 123. Conjugation of _Coccidium proprium_ 218 - - 79 (repeated). The two maturation divisions of the 'drone eggs' 236 - - 124. Alternation of generations in a Gall-wasp 245 - - 125. The two kinds of galls formed by the species 246 - - 126. Ovipositor and ovum of the two generations of the same - species of Gall-wasp 247 - - 127. Life-cycle of the Vine-pest (_Phylloxera vastatrix_) 249 - - 128. Heterostylism 254 - - 38 (repeated). A fragment of a Lichen 261 - - 129. Aberration of _Arctia caja_, produced by low temperature 276 - - 130. Skeleton of a Greenland Whale, with the contour of the body 313 - - 131. Peridineæ: species of _Ceratium_ 325 - - - - -LECTURE XX - -REGENERATION - - Budding and division--Every theory of regeneration in - the meantime only provisional, a mere 'portmanteau - theory'--Regeneration not a primary character--Volvox--Hydra--Vital - affinities--Planarians--Heteromorphoses--Enemies of - Hydroid-colonies--Regeneration in Plants--In Amphibians--In - Earthworms--Different degrees of regenerative capacity according - to the liability of the part to injury--Different results of - longitudinal halving in Earthworms and in Planarians--Regeneration - in Birds--The disappearance of the power of regeneration is very - slow--Morgan's experiments on Hermit-crabs--Autotomy in Crustaceans - and Insects--Regeneration of the lens in Triton. - - -We have endeavoured to explain the handing on of the complement -of heritable qualities from one generation to another as due to a -continuity of the germ-plasm, and we assumed that the germ-cells never -arise except from cells in the 'germ-track'; that is, from cells which -are equipped, from the fertilized egg-cell onwards, with a complete -sample of slumbering germ-plasm, and are thereby enabled to become -germ-cells, and, subsequently, new individuals, in which the aggregate -of inherited primary constituents implied in the germ-plasm can again -attain to development. - -We have now to consider other cases of inheritance in relation to the -same problem--the origin of their hereditary equipment. - -We know, of course, that new individuals may arise apart from -germ-cells, that, in many of the lower animals and in plants, they may -arise by budding and fission. - -For both these cases the germ-plasm theory will suffice, with a -somewhat modified form of the same assumption which we made in regard -to the formation of germ-cells. The origin of a new individual by -budding seems often, indeed, to proceed from any set of somatic cells -in the mother animal; but somatic cells, if they contain solely the -determinants controlling themselves, cannot possibly give rise to a -complete new individual, since this presupposes the presence of _all_ -the determinants of the species. But as these determinants cannot be -formed _de novo_, the budding cells must contain in addition to the -usual controlling somatic determinants, idioplasm in a latent, inactive -state, which only becomes active under certain internal or external -influences, and then gives rise to the formation of a bud. The source -of this accessory idioplasm must, however, be looked for only in the -egg-cell. - -In plants this bud-idioplasm must be complete germ-plasm, because -the budding starts only from one kind of cell, the cambium-cells; -but in animals in which--as it seems--it always proceeds from at -least two different kinds of cells--those of the ectoderm and those -of the endoderm--the matter is more complex. In this case these two -kinds of cells will contain as bud-idioplasm two different groups of -determinants, which mutually complete each other and form perfect -germ-plasm, and only the co-operation of these two sets will give rise -to the formation of a bud. I will not, however, go further into detail -in regard to these relations, for the theory can do nothing more here -than formulate what has been observed; it is hardly in a position to -help us to a better understanding of the facts. - -The case is not much clearer in regard to the processes which lead to -the replacing of lost parts. The manifold phenomena of regeneration -can also be brought into harmony with the theory, if we attribute to -those cells from which the replacing or entire reconstruction of the -lost part arises an 'accessory-idioplasm,' which, at least, contains -the determinants indispensable to the building up of the part. It is -possible that the assumed accessory idioplasm frequently contains -a much larger complex of determinants, and that it depends on the -liberating stimuli which, and how many of these, will become active. - -If we take a survey of regenerative phenomena in the animal kingdom, -it strikes us at once that the capacity is very different in different -species, extraordinarily great in some and very slight in others. -In general it is greater in lower animals than in higher, but, -nevertheless, the degree of differentiation cannot be the only factor -that determines the capacity for regeneration. That unicellular -organisms can completely replace lost parts, that even a piece of an -infusorian can reconstruct the whole animal if only the piece contain -a part of the nucleus, we have already seen when discussing the -significance of the nuclear substance. In this case the nucleus must -contain the complete germ-plasm, that is, the collective determinants -of the species, and these induce the reconstruction of the lost part, -though they do so in a way that is still entirely obscure to us. In the -meantime, our interpretation will not carry us further, either here or -in regard to any other order of vital phenomena. To go further would be -little short of propounding a causal theory of life itself; it would -mean having a complete and real 'explanation' of what 'life' is. As yet -no one has been able to claim this position. We can see the different -stages through which every organism passes, and that they arise one -out of the other; we can even penetrate down to the succession of those -delicate and marvellously complex processes which effect nuclear and -cell-division; but we are still far from being able to deduce, except -quite empirically, from the present state of a cell what the succeeding -one will be, that is, from being able to understand the succession of -events as a necessary nexus which could be predicted. How a biophor -comes to develop from itself the phenomena of life is quite unknown to -us; we know neither the interaction of the ultimate material particles -nor the forces which bring it about; we cannot tell what moves the -hordes of different kinds of biophors to range themselves together in -a particular order, what molecular displacements and variations arise -from this, or what influence the external world has, and so forth. -We see only the visible outcome of an endless number of invisible -movements--growth, division, multiplication, reconstruction, and -differentiation. - -As long as we are so far from an understanding of life no theory of -regeneration can be anything more than a 'portmanteau theory,' as -Delage once expressed himself in relation to the whole theory of -inheritance, a theory which is like a portmanteau in that one can -only take out of it what has previously been put in. If we wish to -explain the renewal of the aboral band of cilia in a Stentor, we -first pack our trunk, in this case the nucleus of the Infusorian, -with the determinants of the ciliated region, and then think of these -as being liberated by the stimulus of wounding, and being brought to -and arranged in the proper place by unknown forces to reconstruct the -ciliary region in some unknown way. No one could be more clearly aware -than I am that this is not an exhaustive causal explanation of the -process itself. Nevertheless, it is not quite without value, inasmuch -as it allows us at least to bring the facts together in rational -order--in this case the dependence of the faculty of regeneration on -the presence of nuclear substance--under a formula which we can use -provisionally, that is, with which we can raise new questions. As soon -as we ascend higher in the series of organisms the theory gains a -greater value, for, while we leave altogether out of account any answer -to the ultimate question, and thus renounce for the present the attempt -to find out how the determinants set to work to call to life the parts -which they control, we are brought face to face with other, in a sense, -preliminary questions which we _can_ solve, and the solution of which -seems to me at least not entirely without value. - -The first of these questions runs thus: Is the power of regeneration a -fundamental, primary character of every living being in the sense that -it is present everywhere in equal strength, independently of external -conditions, and thus is an inevitable outcome of the primary characters -of the living substance? Or is it, though primaeval in its beginnings, -a phenomenon of adaptation, which depends on a special mechanism, and -does not occur everywhere in equal extent and potency? - -We have already become acquainted with some facts which must incline us -to the latter view. The globular Alga-colonies of _Volvox_ (Fig. 63) -consist of two kinds of cells, of which only one kind, the reproductive -cells, possess the power of reproducing the whole, the others, the -flagellate, or, as we called them, somatic cells, being only able to -produce their like, but never the whole. - -New investigations which have been carried out by Dr. Otto Hübner in my -Institute have placed these facts beyond doubt. We may conclude that, -in this case, a disintegration of the germ-plasm has taken place during -ontogeny, by means of differential cell-division, so that only the -reproductive cells receive the complete germ-plasm, while the somatic -cells receive only the determinants necessary to their own specific -differentiation, the somatic determinants. - -In this case regeneration and reproduction coincide; there is no -regeneration except the origin of a new individual from a reproductive -cell. - -[Illustration: FIG. 35 _B_ (repeated). _Hydra viridis_, the Green -Freshwater Polyp. Section through the body-wall, somewhere in the -direction of _ov_ in Fig. 35 _A_. _Eiz_, the ovum lying in the ectoderm -(_ect_), and including zoochlorellæ (_schl_) which have immigrated -from the endoderm (_ent_) through the supporting lamella (_st_). After -Hamann.] - -Let us now ascend to the lowest of the Metazoa, for instance, the -freshwater polyp, Hydra (Fig. 35 _A_), and we find a high degree of -regenerative capacity in the restricted sense, for, in addition to -the power of producing germ-cells, that is, cells which, when two -combine in amphimixis, give rise again to a new animal, almost any -part of the polyp can regrow a whole animal. Not only has Hydra been -cut in from two to twenty different pieces, but it has even been -chopped up into innumerable fragments, and yet each of these, under -favourable circumstances, was able to grow again into a complete -animal. Nevertheless, we are not justified in concluding that every -cell possesses the power of reproducing the whole. If, with the help -of a bristle, we turn one of these polyps outside in like the finger -of a glove, and then prevent it turning right again by sticking the -bristle transversely through it, it does not live, but soon dies, -obviously because the cells of the two layers of the body, ectoderm -and endoderm, cannot mutually replace each other, and cannot mutually -produce each other. The inner layer, now turned outwards, cannot resist -the influence of the water, and the outer layer, now turned inwards, -cannot effect digestion; in short, one cannot be transformed into the -other, and we must therefore conclude that both are specialized, that -they no longer contain the complete germ-plasm, but only the specific -determinants of ectoderm and endoderm respectively. - -The animal's high regenerative capacity must therefore depend on the -fact that certain cells of the ectoderm are equipped with the complete -determinant-complex of the ectoderm, in the form of an inactive -accessory idioplasm, which is excited to regenerative activity by -the stimulus of wounding, and that, in the same way, the cells of -the endoderm are equipped with the whole determinant-complex of the -endoderm. It need not be decided whether all or only many of the -cells, perhaps the younger ones, are thus adapted for regeneration; -in any case a great many of them must be distributed throughout the -whole body, with perhaps the exception of the tentacles, which are by -themselves unable to reproduce the whole animal. When the animal is -mutilated, the cells of both layers, equipped with their respective -determinant-aggregates, co-operate in reproducing the whole from a part. - -It is true that even with these assumptions we only reach the threshold -of a real explanation. For, given that all the determinants of the -species must be present in a fragment, we are not in a position to show -how these set about reconstructing the animal in its integrity, and the -most that we can say is, that it must depend on the specific kind of -stimulus to which each of the cells is exposed through its direct and -more remote environment, which determinants are to be first liberated, -and therefore which parts are to be reconstructed. - -That there are at work regulative forces, such as we were already -compelled to assume in regard to the division and regeneration of -unicellular organisms, as to the nature of which we cannot yet make -any definite statement, but which we may call 'polarities,' or, as I -prefer to say, 'affinities,' is shown by countless experiments which -have been made, particularly with the freshwater polyp. Thus Rand cut -off the anterior end of the polyp with its circle of tentacles, and the -excised disk of living substance lengthened in a transverse direction, -so that half the tentacles came to lie to the right, the other half to -the left, while the body developed between these two groups, so that -they became further and further separated from each other, till finally -the original transverse axis of the animal became the longitudinal -axis. One group of tentacles survived and surrounded the new mouth, -while the other at the opposite aboral pole, the new foot, died off. -This total change of structure in the polyp, as to the arrangement of -its main parts, points to unknown forces, which cannot depend on the -determinants as such, but on the vital characters of the living parts, -and on the interactions of these with one another. - -[Illustration: FIG. 96. A Planarian cut transversely into nine pieces. -The regeneration of seven of these into entire animals is shown. After -Morgan.] - -The same holds true of all the lower Metazoa that have highly developed -regenerative capacity, not only of polyps, but of worms such as the -Planarians. Through the experiments of Loeb, Morgan, Voigt, Bickford, -and others, we know that these animals respond to almost every -mutilation by complete reconstruction, that they may, for instance, as -is indicated in Fig. 96, be cut transversely into nine or ten pieces -with the result that each of these pieces grows again to a whole -animal, unless external influences are unfavourable and prevent it. - -Something similar happens if the head be cut off a Tubularia-polyp, it -forms a new head with proboscis and tentacles. It does so, at least, -if the stalk of the polyp be left in the normal position; but if it -be stuck into the sand in the reverse position a head arises at the -end which is uppermost, where the roots arose previously, and the -previous head-end now sends out roots. By suspending a beheaded stalk -horizontally in the water a head can be caused to develop at each end -of the stalk, so that we must assume that every part of the polyp is, -under some circumstances, capable of developing a head, and that it -must be 'circumstances'--in this case gravity, contact with earth or -with water, and the mutual influence of the parts of the animal upon -each other--which decide what is to be produced. Loeb, who was the -first to observe this form of regeneration, called it heteromorphosis, -to express the fact that particular parts of the animal might be -produced at quite different places from those originally intended for -them. - -It would certainly be erroneous to range these cases of heteromorphosis -against the determinant theory, but they certainly do not afford any -special evidence of its validity as an interpretation, for all that we -can say here again is that all, or at least many, cells of the animal -must contain the full determinant-complex of the ectoderm, and others -those of the endoderm, and that particular groups of determinants -become active when they are affected by certain external or internal -liberating stimuli. In regard to such animals the theory is hardly more -convincing than the rival theory, that the faculty of regeneration is a -general property of living substance, which does not attain to equally -full expression everywhere, because it is met by ever-increasing -difficulties involved in the increasing complexity of structure. The -validity of the theory only begins to be seen when we deal with cases -where it is demonstrable that every part cannot bring forth every -other, where the power of regeneration is limited, and occurs only in -definite parts in a definite degree, and can only start from particular -parts. Here the assumption of a general and primary regenerative -capacity fails. Any one who insists, as O. Hertwig does, that the -idioplasm in all cells of the body is the same, can always plead that, -in the cases in which regeneration does not occur, the fault lies, -not in the regenerative capacity, but in the absence of the adequate -liberating stimuli, and at first sight it does seem as if this position -were unassailable. We shall find, however, that there are facts which -make Hertwig's interpretation quite untenable. - -My own view is that the regenerative capacity is not something -primary, but rather an adaptation to the organism's susceptibility to -injury, that is, a power which occurs in organisms in varying degrees, -proportionate to the degree and frequency of their liability to injury. -Regeneration prevents the injured animal from perishing, or from -living on in a mutilated state, and in this lies an advantage for the -maintenance of the species, which is the greater the more frequently -injuries occur in the species, and the more they menace its life -directly or indirectly. A certain degree of regenerative capacity is -thus indispensable to all multicellular animals, even to the highest -among them. We ourselves, for instance, could not escape the numerous -dangers of infection by bacilli and other micro-organisms if our -protective outer skin did not possess the faculty of regeneration, -at least so far that it can close up a wound and fill up with -cicatrice-tissue a place where a piece of skin has been excised. -Obviously, then, the mechanism which evokes regeneration must have -been preserved in some degree and in some parts at every stage of the -phyletic development, and must have been strengthened or weakened -according to the needs of the relevant organism, being concentrated -in certain parts which were much exposed to injury and withdrawn from -other rarely threatened parts. Thus the great diversity which we can -now observe in the strength and localization of the regenerative -capacity has been brought about. But all this can only be regarded as -adaptation. - -I should like to submit a few examples to show that the regenerative -capacity is by no means uniformly distributed, and that, as far as we -can see, it is greater or less in correspondence with the needs of the -animal, both in regard to the whole and to particular parts. - -It must first be pointed out that those lower Metazoa, like the -Hydroid polyps in particular, which are endowed with such a high and -general power of regeneration, do actually require this for their -safety; they are not only soft, easily injured and torn, but they -are most severely decimated by many enemies. In the beginning of May -I found on the walls of the harbour at Marseilles whole forests of -polyp-stocks of the genera _Campanularia_, _Gonothyræa_, and _Obelia_, -all large and splendidly developed, with thousands of individual polyps -and medusoids, but in a very short time the great majority of the -polyps were eaten up by little spectre-shrimps (Caprellids) and other -crustaceans, worms, and numerous other enemies, and towards the end of -May it was no longer possible to find a fine well-grown colony. It must -therefore be of decisive importance for these species if the stems and -branches, which are spared because protected by horny tubes, possess -the faculty of transforming their simple soft parts into polyp-heads, -or of giving off buds which become polyps, or even of growing a new -stock from the twigs which have been half-eaten and bitten loose from -the stock and have fallen to the ground. If, finally, a torn-off -polyp-stalk (of _Tubularia_) falls to the ground with the wrong side -up, the end which is now the lower will send out roots, and the end -now uppermost will give off a new head. This also appears to us -adaptive, and does not surprise us, since we have been long accustomed -to recognize that what is adapted to an end will realize this if it be -possible at all. Think again of the innumerable adaptations in colour -and form which we discussed in the earlier lectures. I hope later to -be able to show in more detail how it comes to pass that necessity -gives rise to adaptation. In regard to the case of the polyps, we can -understand that, as far as a high degree of regeneration and budding -was possible in these animals at all, it could not but be developed. -Regeneration and budding complete each other in this case, for the -former brings about in the individual 'person' what the latter does -in the colony, namely, a _Restitutio in integrum_. It is readily -intelligible that the former was not difficult to establish where the -latter--the capacity of budding--was already in existence. - -It seems at first sight very striking that the higher plants, which all -depend upon budding, and which form plant-colonies (corms) in the same -sense as the polyps form animal-colonies, only possess the faculty of -true regeneration in a very low degree, although they are extremely -liable to injury. - -We see from this that the two capacities are not co-extensive, that -germ-plasm may be contained in numerous cells of the body in a latent -state, and yet that regeneration of each and every detailed defect -may not be possible. This is the case in the higher plants in regard -to most of their parts. A leaf in which a hole has been cut does not -close the hole with new cell-material; a fern frond from which some -of the pinnules have been cut off does not grow new ones, but remains -mutilated. Even leaves which, if laid on damp earth, readily give off -buds which grow to new plants, as the Begonias do, do not replace a -piece cut out of the leaf; they are not at all adapted to regeneration. - -From the standpoint of utility this is readily intelligible. It was, -so to speak, not worth Nature's while to make such adaptations in the -case of leaves or blossoms, partly because these are very transient -structures, and partly because they are rapidly and easily replaceable -by the development of others of the same kind. Moreover, the leaf in -which we have cut a hole continues to function, but the polyp whose -mouth and tentacles we have cut off could no longer take nourishment -unless it were adapted for regeneration. But that this adaptation -_could_ have been made in the case of plants is proved by the -root-tips which are formed anew when they are injured, and the closing -of wounds on the stem by a 'callus.' - -I shall return to plants when we are dealing with the mechanism -of regeneration, but I must now direct more attention to animals, -inquiring further into the question as to whether the faculty of -regeneration is correlated with the degree of liability to injury to -which the animal is exposed, and with the biological importance of -the injured part, for this must be the case if regeneration be really -regulated by adaptation. - -Hardly any other vertebrate has attained such celebrity on account of -its high regenerative capacity as the water-newt, species of the genus -_Triton_. It can regrow not only its tail, but the legs and their parts -if they are cut off. Spallanzani saw the legs grow six times, after -he had cut them off six times. In the blind newt (_Proteus_) of the -Krainer caves, a near relative of the common newt, the leg regenerated -only after a year and a half, although the animal stands on a lower -stage of organization than the newt, and thus should rather replace -lost parts more easily. But Proteus lives sheltered from danger in -dark, still caves, while Triton is exposed to numerous enemies which -bite off pieces from its tail or legs; and the legs are its chief means -of locomotion, without which it would have difficulty in procuring -food. It is different with the elongated eel-like newt of the marshes -of South Carolina, _Siren lacertina_. This animal moves by wriggling -its very muscular trunk, after the manner of an eel, and in consequence -of the disuse of its hind legs it has almost completely lost them. Even -the fore-legs have become small and weak, and possess only two toes, -and these do not regrow if they are bitten off, or only do so very -slowly. - -Earthworms are exposed to much persecution; not only birds, such as -blackbirds and some woodpeckers, but, above all, the moles prey upon -them, and Dahl has shown that moles often lay up stores of worms in -winter which they have half crippled by a bite, while even Réaumur -knew that moles frequently only half devoured earthworms. It was -thus an obvious advantage to earthworms that a part of the animal -should be able to regrow a whole, and accordingly we find a fairly -well-developed regenerative capacity among them. But it varies greatly -in the different species, and it would be interesting if we knew the -conditions of life well enough to be able to decide whether the faculty -of regeneration rises and falls in proportion to the dangers to which -the species is exposed. Unfortunately we are far from this as yet; we -only know that, in the common earthworms of the genera _Lumbricus_ and -_Allolobophora_, the faculty of regeneration is still very limited, -for at most two worms, and sometimes only one, can develop from an -animal cut into two pieces. Cutting into a greater number of pieces -does not yield a larger number of worms, but usually only one, and -often none at all. - -This corresponds to the behaviour of their enemies, which may often -bite off a piece or tear it away when the worm attempts to escape, but -never cut it up into pieces. The regenerative capacity is more highly -developed in the genus _Allurus_, more highly still in the worms of the -genus _Criodrilus_ which lives in the mud at the bottom of lakes, and -most highly of all in the genus _Lumbriculus_ which lives at the bottom -of small ponds. Long ago Bonnet cut up a specimen of _Lumbriculus_ into -twenty-six pieces, of about two millimetres in length, and he observed -most of these grow to complete worms again. His experiments have often -been repeated in recent times, and have been extended and made more -precise in many ways. Von Bülow was able to get whole animals from -pieces consisting of from four to five somatic segments, and with eight -or nine segments he almost invariably succeeded. A _Lumbriculus_ which -he had cut into fourteen pieces, one of which only measured 3.5 mm. in -length, gave rise to thirteen complete worms with head and tail; only -one piece perished. - -These worms have little enemies with sharp jaws which may gnaw at them -behind or before but cannot swallow them whole. Lyonet, famous for his -analytic dissection of the wood-caterpillar (_Cossus ligniperda_), -observed when he was feeding the larvæ of dragon-flies with these -Lumbriculid worms that 'the anterior end of some whose posterior end -had been gnawed away by the larvæ continued to live on the ground.' We -can thus understand why a high power of regeneration is of use to these -worms, and at the same time why it is advantageous to them to contract -so that they break in pieces on very slight irritation, but to this we -shall refer again. - -The very diverse potency of the faculty of regeneration in animals -belonging to the same small group, and nearly, if not quite, at the -same level of organization, seems to show clearly that we have here to -do with adaptation to different conditions of life, although we cannot -demonstrate this in detail. It would certainly be erroneous to regard -the conditions of life as uniform, since the worms in question not -only live in different places--in the earth, in mud, or in water--and -are thus exposed to different enemies, and since they may also be -quite different in regard to size and speed, in means of defence, and -possibly also of defiance, as is indeed in some measure demonstrable. - -We meet with the same thing in a group of still smaller worms, Rösel's -'water-snakelets,' species of the genus _Nais_. These, too, behave in a -variety of ways in the matter of regeneration, for while many species, -such as _Nais proboscidea_ and _Nais serpentina_ will, if cut into -two or three pieces, become two or three worms respectively, Bonnet -expressly mentions an unnamed species of _Nais_ which does not bear -cutting up at all, and even dies if its head be cut off. - -Thus neither the degree of organization nor the relationship alone -determines the strength of the regenerative capacity. And as nearly -related species may behave quite differently in this respect, so also -do the different parts of one and the same animal; and here, too, the -strength of the capacity seems to depend on the more frequent or rarer -injury of the relevant part and on its importance in the maintenance of -life. Let us take a few examples. - -Parts which, in the natural life of the animal, are never injured, show -in many cases no power of regeneration. This is so in regard to the -internal parts of the newt, whose regenerative capacity is otherwise -so high. I cut half or nearly the whole of a lung away from newts -anæsthetized with ether; the wound closed, _but no renewal of the organ -took place_. The same thing happened when a piece of the spermatic duct -or of the oviduct was cut away. It is true that the kidney enlarges in -higher animals when a piece has been cut out, by the proliferation of -the remaining tissues, but that is a mere physiological substitution, -evoked by the increased functional stimulus, due to the accumulation in -the blood of the constituents of the urine. Such substitution depends -on the growth of parts already existing, and it occurs in man when one -kidney is removed, for the other, as is well known, may then grow to -double its normal size. This is mere hypertrophy of the part that is -left, it is not regeneration in the morphological sense, and it is not -comparable to the re-formation of a cut-off leg in the salamander, or -of a head in the worm, where the growth is not a mere increase of the -remaining stump, but a new formation. It would be regeneration if a -new kidney developed from the remnants of the kidney-tissue, or, in -the liver, if new lobes grew in place of those which were cut off. But -neither of these things happens, and, as far as I am aware, nothing of -the kind has ever been observed, nothing more than new formation of -liver-cells through increase of existing ones; that, however, is not -regeneration in the morphological sense. - - -I have referred to the slight power of regeneration possessed by the -blind Proteus in regard to its legs or tail, and I connected this with -the absence of enemies in its thinly peopled cave-habitat. But the same -animal can regenerate its gills when these are bitten off, and this is -probably associated with the habit that Proteus has, in common with -other newts with external gills, of nibbling at its neighbour's gills. -Thus, the power of regenerating the gills was retained even when the -animals migrated to the quiet caves of Krain, and were thus secured -from the attacks of other enemies. - -In lizards, a leg which has been cut off does not grow again, but an -amputated tail does, and this has quite a definite biological reason, -since the active little animal will seldom be caught by the foot by any -pursuer, but may easily be caught by the tail, which is far behind. -Thus the tail is adapted not only for regeneration, but also for -'autotomy' that is, for breaking off easily when it is caught hold of. - -We have already seen that some segmented worms have a very high -regenerative capacity; yet every part cannot produce every other, and -while, in _Lumbriculus_, any piece of from five to nine segments is -able to grow a new head or tail, neither ten nor twenty nor all the -segments together, if they are _halved longitudinally_, can reproduce -the other half, and the cause of this inability does not lie in the -fact that the animal is thereby hindered from taking food, for even the -transversely cut pieces do not feed until they have grown a new head -and tail. The reason must lie in the fact that the primary constituents -for this kind of regeneration are wanting, and they are so because a -longitudinal splitting of this cylindrical and relatively thin animal -never occurs under natural conditions, and thus could not be provided -against by Nature[1]. - -[1] Morgan maintains that this statement is incorrect, and that -_Lumbriculus_ is capable of lateral regeneration. But if we look into -the matter more closely we find that all he says is, that small gaps -made by cutting a piece out of one side are filled up again, while the -cut pieces perish. If the whole animal be halved, according to Morgan, -both halves die, or if a 'very long piece' be cut out of one side, not -only this piece dies, but also 'the remaining piece.' There is thus, as -I have said, an essential difference between the regenerative capacity -of _Lumbriculus_ and that of _Planaria_. - -That regeneration of this kind could have been arranged for if it had -been useful we learn from the Planarians among the flat worms, in which -every piece cut out of the body, large or very small, from the middle, -from the left side, or from the right side of the animal, grows into -a complete Planarian. The animal can be halved longitudinally, as -in Fig. 97, and each half will grow to a whole. This again is quite -intelligible from the biological point of view, for these flat, soft, -and easily torn animals are exposed to all sorts of injuries, and are, -in point of fact, frequently mutilated by enemies which are unable to -swallow them whole. Von Graaf not infrequently found examples of marine -Planarians (_Macrostomum_) which lacked 'a part of the posterior end -or the whole tail region as far as the food-canal,' and of species of -_Monotus_ he found 'very often' in May specimens with the posterior -end split or broken off. Probably the persecutors of these flat-worms -are some species of Crustacean, but, at any rate, so much is proved, -that the Planarians have abundant opportunities of making use of their -faculty of regeneration, and that the species gains an advantage from -it in respect to its preservation. - -[Illustration: FIG. 97. _A_, a Planarian, which has been divided into -two by a longitudinal cut. Each half can grow into an entire animal. -_B_, the left half at the beginning of the regenerative process. _C_, -the same completed. After Morgan.] - -In contrast to this, worms which live within other animals, and -are thus secure from mutilation, such as the familiar round-worms -(_Nematoda_), have no power of regeneration at all, and do not survive -either longitudinal or transverse division. - -Until recently birds were regarded as possessing a very low degree of -regenerative capacity, and, as a matter of fact, they cannot replace -a leg or a wing wholly or in part; but, what is otherwise unheard of -among higher vertebrates, they can renew the whole anterior portion of -the skeleton of the face, the bill, and can indeed almost reconstruct -it with new bones and horny parts. Von Kennel communicated a case of -this kind in regard to a stork, and for a long time this remained an -isolated case, but a few years ago Bordage showed that, in the cocks -which are used in the Island of Bourbon for the favourite sport of -cock-fighting, the bill is regularly renewed when it has been broken -off or shattered. Quite recently Barfurth gave an account of a case -of complete renewal of a broken bill in a parrot. Yet it should not -astonish us that the bill in birds has such a high regenerative -power, for of all parts in a bird it is the one that is most readily -injured; with it the bird defends itself against its enemies and its -rivals, masters its prey, and tears it to pieces, pecks holes in trees -(woodpecker), or climbs (parrot), or digs and burrows in the ground, -or builds its nest, and so on. That the faculty of regeneration could -be developed to so high a degree in relation to this particular part -of the body, while the rest of the very important but rarely injured -parts do not possess it at all, again points to the conclusion that the -faculty of regeneration has an adaptive character. - -It does not affect matters to discover cases in which we cannot -recognize this relation between the regenerative capacity of a part -and its importance or its liability to injury. Such instances do not -lessen the convincingness of the positive cases, since we do not know -the exact conditions which may lead to the increase of regenerative -capacity in a part, and, above all, since we do not know the rate at -which such an increase may take place. If adaptation in general depends -upon processes of selection, these processes must also be able to give -rise to an increase in the power of regeneration. On the other hand, it -by no means follows that the disappearance of a faculty of regeneration -which was once present in a part, but which has become superfluous -in the course of time, must take place immediately through natural -selection. For it is the very essence of natural selection that it only -furthers what is useful, and only removes what is injurious; over what -is indifferent it has no power at all. Thus it follows that the faculty -of regeneration, when it has once been present in a part, cannot be -set aside by natural selection (personal selection), for it is in no -way injurious to its possessor. If it gradually decreases and becomes -extinct notwithstanding this, when it is of no further use, as seems to -be to some extent the case in regard to the legs and tail of the blind -Proteus, that must depend on other processes, on those which generally -bring about the gradual disappearance of disused parts or capacities. -We shall attempt to probe to the roots of these processes later on; -for the present let it suffice us to know that, according to our -experience, they go on with exceeding slowness, and that it has taken -whole geological periods to eliminate the legs of the snake-ancestors -so completely as has been done from the structure of most of our modern -snakes, while the Proteus which migrated into the caves of Krain as far -back as the Cretaceous period is indeed blind, but still retains its -eyes under the skin, though in a degenerate condition. - -Since the degeneration of disused parts and capacities goes on so -slowly it need not surprise us that we meet many parts which still -possess regenerative capacity, although they are protected from -injury. Thus Morgan found that, in the hermit-crab, the limbs which -are protected within the mollusc shell were quite as ready to regrow -as those which are actually used for walking, and thus are exposed to -possibility of attack, but this proves nothing against the conclusion -we drew from the facts cited above, according to which the faculty of -regeneration comes under the law of adaptation. For the disappearance -of this faculty must take place very much more _slowly than its -growth_. For instance, the development of the tail-fin of the whale has -long been an accomplished fact, while the hind-legs of this colossal -mammal, which were rendered useless by the development of the tail-fin, -still lie concealed in a rudimentary state within the muscles of the -trunk. Yet these limbs must have lost their significance for the animal -exactly at the time that the tail-fin became more powerful. Thus the -retrogression must have taken place more slowly than the progressive -transformation. - -It is clear, then, that the faculty of regeneration is not a primary -character of living beings occurring uniformly in all species of -equally high organization and in all parts of an animal in the same -degree; it is a power which occurs in animals of equal complexity in as -varying degrees as in their parts, and which is manifestly regulated by -adaptation. Between parts with the faculty of regeneration and parts -without it there must be an essential difference; there must be present -in the former something that is wanting in the latter, and, according -to our theory, this is the equipment with regeneration-determinants, -that is, with the determinants of the parts which are to be -reconstructed. - -If this be really so it should be capable of proof, at least in so -far that we should be able to establish that the power of completing -or re-forming a damaged or lost part is a limited one, localized in -certain parts and cell-layers. This can be actually proved, as may -be seen from numerous cases in which the faculty of regeneration is -associated with autotomy, that is, with the power of breaking off or -dropping off a part of the body. Even in worms we find this power, -as we mentioned before in speaking of the high regenerative capacity -of _Lumbriculus_. This worm reproduces in summer by what is called -'schizogony,' that is, by breaking into two, three, or more pieces, and -it does not seem to require a very strong stimulus, such as pressure -of the end of the worm by the jaws of an insect larva, to start this -rupture; it often follows from quite insignificant friction on the -ground. Certainly the power of regeneration is so great in this animal -that it is out of the question to talk of localizing the primary -constituents of regeneration; almost every broken surface is capable of -regeneration. - -But this localization is well illustrated in Insects and Crustaceans, -which possess the power of self-amputation in their appendages, -especially in their legs. As far back as 1826 MacCullock observed -this remarkable power in crabs, and described the mechanism on which -it depends. When the leg is irritated, for instance when it is pinched -at the tip and held fast, it breaks off at a particular place. This -line of breakage lies in the middle of the short second joint (Fig. -98, _A_ and _B_, _s_), just between the insertions of the muscles -(_me_, _mf_, _m_) which extend from this line towards the extremity of -the limb and in the opposite direction towards the body-wall. Between -these muscle-attachments the external skeleton is thin and brittle, and -forms a suture, _s_, which breaks through when the animal contracts -the muscles of the leg convulsively, and thus presses the lower -protuberance (_a_) against a projection (_b_) of the first upper joint. -Crabs require to make a very considerable muscular exertion before they -can throw off the limb, and therefore they can only do it when they are -in full vigour. - -[Illustration: FIG. 98. The leg of a Crab, adapted for self-mutilation -or autotomy. _A_, the first three joints of the limb, _I_, _II_, _III_. -_s_, the suture, that is, a thin area on the second joint which is -predisposed to breakage. _mf_, flexor muscle, _me_, extensor muscle, -both inserted at the suture. _B_, the entire leg with its six joints -and with the suture (_s_). Slightly enlarged. After MacCullock.] - -We have here a quite definite structural adaptation of the parts to a -danger which often recurs--that of falling entirely into the power of -an enemy which has seized the leg. By a sudden violent throwing-off of -the leg the crab escapes from this danger. Quite similar adaptations -are found among certain insects, such as the walking-stick insects -or Phasmids, in which the mechanism is much the same, and lies at an -almost exactly corresponding place, namely, at the line where the -second and third joints of the leg, the 'trochanter' and the 'femur' -meet. In this case the advantage of the arrangement is not merely that -the animals are thus enabled to escape from enemies; it is useful in -another connexion, for a knowledge of which we have to thank Bordage. -This naturalist observed that the Phasmids not infrequently perished at -one of their numerous moultings, by remaining partially fixed in the -discarded husk. Of 100 Phasmids nine died in this way, twenty-two got -free with the loss of one or more legs, and only sixty-nine survived -the moult without any loss at all. - -That the moulting or ecdysis of insects is often hazardous may be -observed in our own country, and it is familiar to every one who has -reared caterpillars. These, too, often fail to get clear of their -'cast' cuticle, and they perish unless artificial aid is given to them. -I have never observed any autotomy in them, but in the Phasmids it -seems to be a much-used 'device' and is therefore of great importance -in the persistence of the species. - -Limbs which are thus thrown off by autotomy regenerate again from -the place at which they broke off, that is from the 'suture.' It -had been noticed even by the earlier observers (e.g. Goodsir) that -there was a jelly-like mass of cells within the joint, and that the -development of the new limb started from this. It might be supposed -that the regeneration-primordium is present in the rest of the leg -also, but that is not the case, for the animal responds to the tearing -off of one joint or of a smaller number than to the suture, not by -regenerating the torn part directly, but by amputating the whole of -the leg up to the suture, and then from this the regeneration of the -whole leg takes place. In the Phasmids the case is similar, but with -the difference that regeneration is possible from three places, from -the tarsal joints, from the lower third of the tibia, and finally, -from the suture between the femur and the trochanter. There is thus -a regeneration-primordium (_Anlage_) at the beginning of the tarsal -joints, another in the tibia, and a third in the 'suture' and the first -must be equipped, as we should express it, with the determinants of -the five tarsal joints, the second with those for the lower end of the -tibia as well, and the third with all the determinants of the whole -leg, from the 'suture' downwards. - -In any case, regeneration is here associated with definite localized -pieces of tissue, and is not a general character of all the cells of -the leg, and, as it obviously runs parallel at the same time with -another adaptation--that of autotomy--there can be no doubt that it too -is dominated by the principle of selection, and that it can not only -be increased, but that it can be concentrated at particular places and -removed from others. But this is only possible if it be bound up with -material particles which may be present in or absent from a tissue, and -which are therefore a supplement to the ordinary essential constituents -of the living cells, although they do not themselves belong to the -essential organization. - -I might cite many more examples of localization of regenerative -capacity, but will confine myself to one other, which seems to me -particularly instructive, because it was first interpreted as an -indication of the existence of an adaptive principle in the organism, -a principle which always creates what is useful. I refer to the -regeneration of the lens in the newt's larva. - -G. Wolff, an obstinate opponent of the theory of selection, attempted -to solve the same problem as I had before me in my experiments on the -regeneration of the internal organs of newts, that is, he tried to -answer the question whether organs which are never exposed to injury or -to complete removal in the conditions of natural life, and which could -not therefore have been influenced in this direction by the processes -of selection, are nevertheless capable of regeneration. He extirpated -the lens from the eye of Triton larvæ, and saw that in a short time it -was formed anew, and from this he concluded that there was here 'a new -adaptiveness appearing for the first time,' and that therefore adaptive -forces must be dominant within the organism. The current theory of -the 'mechanical' origin of vital adjustments seemed to some to be -shaken by this, and the proclamation of the old 'vital force' seemed -imminent. And in truth, if the body were really able to replace, after -artificial injury, parts which are never liable to injury in natural -conditions, and to do so in a most beautiful and appropriate manner, -then there would be nothing for it but at least to regard the faculty -of regeneration as a primary power of living creatures, and to think of -the organism as like a crystal, which invariably completes itself if it -be damaged in any part. But we have to ask whether this is really the -case. - -What makes the regeneration of the lens seem particularly surprising -is the fact that in the fully formed animal it must arise in a manner -different from that in which it develops in the embryo, that is, it -must be formed from different cell-material. In the embryo it arises -by the proliferation and invagination of the epidermic layer of -cells to meet the so-called 'primary' optic vesicle growing out from -the brain--a mode of development which cannot of course be repeated -under the altered conditions in the fully developed animal. The -reconstruction of the organ must therefore take place in a different -way, and if the organism were really able, the very first time the lens -was removed, to react in a manner so perfectly adapted to the end, -and so to inspire certain cells, which had till then had a different -function, that they could put together a lens of flawless beauty and -transparency, we should have reason to suspect that nearly all our -previous conceptions were erroneous, and to fall back upon a belief in -a _spiritus rector_ in the organism. - -But the excision of the lens in these experiments was not by any means -an unprecedented occurrence! It is true enough that newts in their -pools are not liable to an operation for cataract, but it does not -follow that the lens is never liable to injury, and could not therefore -be adapted for regeneration. It can be bitten out along with the rest -of the eye by water-beetles or other enemies, and as far back as the -time of Bonnet and Blumenbach (1781) it was known that the eye of the -newt would renew itself if it were cut out, given that a small portion -of the bulb was left. But if this were removed the possibility of -regeneration was at an end. Thus, before the first artificial excision -of the lens, a regeneration-mechanism must have existed, by means of -which the eye with its lens was reconstructed, and this depends on the -characters of the cells of the eye itself--it is localized in the eye, -and without the presence of a piece of eye-tissue no regeneration can -take place. Is it then so especially remarkable that the lens should -be renewed when it is artificially removed without the rest of the -eye? The mechanism for its renewal is there, and is roused to activity -whether the lens alone or other parts of the eye also be removed. -We do not need, therefore, to assume the existence of a purposeful -or adaptive force; it is more to the point to inquire where the -regeneration-mechanism which suggests this inference is to be found. - -A definite answer to this is given in a detailed experimental work -recently published by Fischel. It corroborates what Wolff had already -found, that the substance of the new lens develops from cells which -cover the posterior surface of the iris, that is, from cells of the -retinal layer of the eye. First, the margin of the pupil begins to -react to the stimulus of the injury (extraction of the lens); its cells -enlarge, become clear, while previously they were filled with dark -pigment, and finally they proliferate. They thus form a cell-vesicle -similar to the ectoderm-vesicle from which the lens arises in the -embryo, and into this the already mentioned retina-cells from the -posterior wall of the iris grow, elongate, and arrange themselves to -form the so-called 'lens-fibres,' on whose form, arrangement, and -transparency the function of the lens depends. This is marvellous -enough, but not more marvellous than that a whole foot should grow -on the cut stump of a newt's leg, or that a whole eye should arise -from a residual fragment. Here, again, we do not know the processes -which cause the arrangement of the cells and their often manifold -locally-conditioned differentiations, in short, we do not know the -_essential nature of regeneration_. But, in the meantime, we can -endeavour to find out which cell-groups regeneration is bound up with -in particular cases, so as to know where the vital particles, the -'determinants,' which condition regeneration, are placed by nature. - -[Illustration: FIG. 99. Regeneration of the lens in the Newt's eye. -_A_, section through the iris (_J_); from its margin and posterior -(retinal) surface the primordium of a new lens (_L_) has developed -after the artificial removal of the old one. _B_, section through the -eye after duplicated regeneration of the lens (_L_) from two areas of -the iris. _Gl_, vitreous humour. _J_, iris. _C_, cornea. _R_, retina. -After Fischel.] - -In this case there can be no doubt on that point: they are the cells -on the posterior wall and the margin of the iris. And it is certainly -not the _absence_ of the lens which gives rise to its renewal, as would -necessarily be the case if it were due to the dominance of an adaptive -force. If the lens, instead of being excised, be simply pressed back -into the vitreous humour occupying the cavity of the eye, a new lens is -developed all the same from the irritated margin of the pupil. And if -by chance this margin has been irritated in two places while extraction -of the lens was being performed, then two small lenses will develop -(Fig. 99, _B_). Indeed, several may begin to develop at the posterior -wall of the iris, although they do not attain to full development; -mechanical irritation of any part of this cell-layer is responded to by -the formation of lenses. This surely disposes of the 'mystical nimbus' -which would dazzle us with a new force of life, always creating what -is appropriate. We have before us an adaptation to the liability of -newts' eyes to injury, which, like all adaptations, is only relatively -perfect, since under the usual conditions of eye injury it gives -rise to a usable lens, but under unusual conditions to unsuitable -structures. It is exactly the same as in the case of animal instincts, -which are all 'calculated' for the _ordinary_ conditions of life, but, -under unusual conditions, may operate in a manner quite unsuited to the -necessary end. The ant-lion has the instinct to bore backwards into the -sand, and he makes the same backward-pressing movements when placed -on a glass plate into which he cannot force the tip of the abdomen. -The same is true of the mole-cricket, which makes its usual digging -movements with the forelegs even on a plate of glass. The wall-bee -roofs over her cell when she has laid an egg in it, but she does so -even if the egg be taken out beforehand, or if a hole be made in the -bottom of the cell, so that the honey which is to serve the larva for -food when it emerges from the egg runs out (Fabre). Her instinct is -calculated for filling the cell _once_ with honey, and _once_ laying -an egg in it, because such disturbances as we may cause artificially -do not occur or occur very rarely in natural conditions. There are -countless facts of this kind, for every instinct and every adaptation -can, in certain circumstances, go astray and become inappropriate. This -should be considered by those who still persist in opposing the theory -of selection, for herein lies one of the most convincing proofs of its -correctness. Adaptations can only arise in reference to the majority of -occurrences, and variations which are only useful in an individual case -must, according to the principle, disappear again. Adaptation always -means the establishment of what is appropriate in an average number of -cases. - -Therefore the inappropriate reaction of the margin of the iris to an -artificial double stimulus affords additional reason for regarding -regeneration as an adaptive phenomenon. If it were the outcome of -an adaptive force it could never be inappropriate; and if it were -the operation of a general and primary power of the organism it -would be exhibited by the nearly-related frog as well as by the -newt. But, in the frog, extraction of the lens gives rise only to a -sac-like proliferation of the cells of the iris margin, which form no -transparent lens, but an opaque cluster of cells, which destroys vision -altogether. It appears, therefore, that the frog no longer requires the -power its ancestors possessed of regenerating a lost lens. - - - - -LECTURE XXI - -REGENERATION (_continued_) - - Phyletic origin of the regenerative capacity--The liberating stimuli - of regeneration--Production of extra heads and tails in Planarians - (Voigt)--Regeneration in the Starfish--Atavistic regeneration in - Insects and Crustaceans--Progressive regeneration--Regeneration has - its roots in the differentiation of organisms--The nuclear substance - of unicellular organisms is the first organ for regeneration--The - ultimate roots of regeneration. - - -In the previous lecture we have considered many different forms of -regeneration, and have recognized them as adaptive phenomena; we have -now to inquire how such regeneration-adaptations have arisen, and this -is a very difficult question even in general, while in particular -cases it is often quite unanswerable at present. In regard to the -case last discussed, the regeneration of the lens in the eye of -Triton, our hypotheses would require to reach back to the time of the -primitive vertebrates with an unpaired eye, for the lens of the paired -vertebrate eye, from Mammals down to the lowest Fishes, does not arise -in embryonic development from the retinal cells, but always from the -corneal epithelium, as the elaborate researches of Rabl have recently -shown. It is true that the unpaired parietal eye of some reptiles forms -its lens from the cells of the retinal layer, but it would be difficult -to demonstrate the possibility of a genetic connexion between it and -paired eyes, and in the meantime we must refrain from elaborating a -hypothesis as to the origin of the marvellous faculty the retinal cells -possess of transforming themselves into lens-fibres. - -But it is easier to form some sort of picture of the origin and -adaptation of the faculty of regeneration in general. - -We saw that the power of regenerating a part can be localized, and -that it does not belong to all the cells of the body, but only -to some of them, and we have to ask how and by what steps it has -been imparted to these. The faculty depends on the possession of a -regeneration-primordium (_Anlage_), and this again, in our mode of -expression, consists of a definite complex of determinants, and as -determinants are the products of an evolution, and thus are vital units -which have arisen historically, they can nowhere suddenly originate -anew in a species, but must be derived directly or indirectly from the -sole basis which, in each species, forms the starting-point of the -individual--that is to say, in the Metazoa, from the germ-plasm of the -ovum. From it the determinant-complex of every regeneration-rudiment -mast in the ultimate instance be derived. - -We may think of the matter thus: all the determinants of the germ-plasm -vary, grow slowly or quickly, and in certain circumstances may be -doubled. In this way there arise what we may call 'supernumerary' -determinants, which are not required in the primary building up of -the body from the ovum, and which may remain in an inactive state in -the nuclei of certain cells, ready to become active under certain -circumstances and to produce anew the part which they control. Such -regeneration-idioplasm will at first come to lie in the younger -cells of the determinate organ, but it is conceivable that under the -influence of selection it may be gradually shifted to other cells of -a later developmental origin, or, conversely, to others in a less -external position, so that, for instance, the regeneration-rudiment for -the finger of a newt may be contained not merely in the cells of the -hand, but in those of the fore-arm or even of the upper arm. - -But all such segregation of determinant-groups cannot have taken place, -as we might perhaps be inclined to think, at the periphery in the organ -itself during its development; it must take place in the germ-plasm of -the ovum, for otherwise it could not be transmissible, and could not be -directed and modified by the processes of selection, as is actually the -case, as I shall show in more detail later on. - -I have already pointed out the importance of the rôle played by -liberating stimuli in regeneration, and not only of extra-organismal -stimuli, such as gravity, but above all of intra-organismal stimuli -that is, the influences exerted in a mysterious manner by other parts -of the animal on the parts which are in process of regeneration. It -is a great merit of the modern tendency in evolution theory that it -has demonstrated the importance of such internal influences. Although -we are still far from being able to define the manner in which these -influences operate, we may say so much, that it depends essentially on -the nature and extent of the loss which parts are reproduced by the -regenerating cells, and, also, on the position and direction of the -injured surface from which the regeneration starts. The influences, -still quite beyond our comprehension, which are exerted on the -regenerating part by the uninjured parts constitute the liberating -stimuli, which evoke the activity of one or other of the determinants -contained in the regeneration-idioplasm. - -[Illustration: FIG. 100. Regeneration of Planarians. _A_, an animal -divided into three parts by two oblique cuts. _B_, the fragments(_a_, -_b_, _c_) in process of regeneration. _C_, an animal with various -oblique incisions in the margin of the body, which have induced the new -formation of heads (_k_), of tails (_s_), and pharynx (_ph_). _A_ and -_B_ after Morgan; _C_ after Walter Voigt.] - -Walter Voigt has shown, by a series of most interesting experiments, -that it is possible not only to cause the development of a new head -in Planarians by cutting them, in which case a tail may grow from the -anterior portion and a head from the posterior portion, but it is -also possible in an intact animal, that is, one with both head and -tail, to cause the production of a second head, or a second tail, or -both at once, at any part of the body margin at will, according to -the direction of the cut. If the margin of the body be cut obliquely -forwards (Fig. 100, _A_) a supernumerary tail arises (_C_, _s_), if -it be cut obliquely backwards a supernumerary head arises (_C_, _k_), -and in this way several heads and several tails may be produced in -the same animal. It is obvious, then, that the interaction, in the -first place, of the cells of the cut surface, but probably also of the -deeper-lying cells, decides which determinants are to come into action, -those of the head or those of the tail, but both must be present at -every part of the cut. How far below the cut surface the cells take -part in this determination we cannot make out, but that it cannot be -due to the co-operation of all parts is clear in this case at least, -since the animal still possesses its original head and tail. The extra -heads and tails thus produced prove, at any rate, that there can be no -question here of the expression of an adaptive principle, a _spiritus -rector_, or a vital force, which always creates what is good, but that -it is rather a purely mechanical process, which takes its course quite -independently of what is useful or disadvantageous, and that it must -take this course according to the given regeneration-mechanism and the -stimulus supplied in the special case. It cannot be supposed that these -supernumerary heads and tails are purposeful, but who would expect an -adaptive reaction from the animal in a case like this, since cuts of -the kind which we make artificially, and _must keep open artificially_ -if the deformities are to develop, hardly occur in nature, and, if they -did occur, would very quickly close up again? Adaptations can only -develop in response to conditions which occur and recur in a majority -of cases, and when they have a useful, that is, species-preserving -result. The adaptiveness of the organism is blind, it does not see the -individual case, it only takes into account the cases in the mass, -and acts as it must after the mechanism has once been evolved. The -case is the same as that of 'aberrant' or mistaken instincts, whose -origin by means of selection is the more clearly proved, since we must -recognize such an instinct as a pure mechanism and not as the outcome -of purposeful forces. - -In the regeneration of Planarians we must think of the -regeneration-idioplasm as containing the full complex of the -collective determinants of the three germinal layers, and possibly -we must add to this cells with the complete germ-plasm for giving -rise to the reproductive cells. But when the amputated tail of the -newt is regenerated, or its leg, or the arm of a starfish, or the -bill of a bird, we have no ground for assuming that the cells, from -which regeneration starts, contain the whole germ-plasm, since the -determinants of the replaceable parts suffice to explain the facts. -We must even dispute the possibility of the presence of the whole -germ-plasm in this case, because the faculty of regeneration of the -relevant cells is really no longer a general one, but is limited to -the reproduction of a particular part. This is seen in the fact that, -in the starfish, whose high regenerative capacity is well known, the -central disk of the body may indeed give rise to new arms[2]; but -an excised arm, to which no part of the disk adheres, is in most -starfishes unable to give rise to the body. Thus the arm does not -contain in its cells the determinants of the disk, but the latter -contains those of the arm. We are not surprised that the amputated -tail of the salamander does not reproduce the whole animal, but this -can only be because the impelling forces to the regeneration of the -whole animal are wanting, that is, that the cut surface only contains -the determinants of the tail and not the complete germ-plasm. It might -be objected here that the tail-piece is too small to give rise to the -whole body, but in _Planaria_ it is only very diminutive heads and -tails which grow from the artificial incisions, and the same is true -of starfishes when only a single arm and a small piece of the disk -have been left. Notwithstanding the small amount of living substance -at their disposal, and although they are at first unable to take -nourishment, they send out very small new arms (Fig. 101), close up the -wounded surface, and, after reconstruction of the mouth and stomach, -begin to feed anew. The new arms may then grow to the normal size. - -[2] I see now that there are contradictory statements in regard to this -case. Possibly these depend on the different behaviour of different -species, and this on the varying frequency of mutilation. Starfishes -which live on the shore between the rocks, for instance on the movable -stones of a breakwater, are very frequently mutilated; in some places -it is rare to find a specimen without traces of former wounds. H. D. -King counted among 1,914 specimens of _Asterias vulgaris_ 206 in the -act of regenerating a part, that is, 10.76 per cent. In the case of -the starfishes from deep water this cause of injury does not of course -exist. - -[Illustration: FIG. 101. A starfish arm, growing four new arms; the -so-called 'comet-form.' After Haeckel.] - -We must therefore assume that, in many cases, the -regeneration-primordium consists of cells which only contain a definite -complex of determinants in the form of latent regeneration-idioplasm, -as, for instance, certain cells of the tail of Triton contain the -determinants of the tail, certain cells of its leg the determinants of -the leg, and so on. In many cases we can speak even more precisely, -and determine from which cells the nerve-centres, from which the -muscles, and from which the missing section of the food-canal will be -formed, as was recently shown by Franz von Wagner in regard to the worm -_Lumbriculus_, whose regenerative capacity is so extraordinarily high. -We must then attribute to each of the relevant cells an equipment of -regeneration-idioplasm, which includes only the relevant complex of -determinants. - -I need not here go further into detail, but I should still like to -show that, in reality, as I assumed in regard to the regenerative -capacity of a part, the root of the regeneration-idioplasm lies -in the germ-plasm, that it is present there as an independent -determinant-group, and, like every other bodily rudiment (_Anlage_), -must be handed on from generation to generation. This assumption is -necessary, as has been already indicated, on the ground that the -faculty of regeneration is hereditary, and hereditarily variable, on -the same ground, therefore, as that on which the whole determinant -theory is based. The regeneration-determinants must be contained _as -such_ in the germ-plasm, otherwise a twofold phyletic development could -not have occurred, as it actually has, in many parts. The tail of the -lizard is adapted for autotomy; it breaks off when it is held by the -tip, and this depends on a special adaptation of the vertebræ, which -are very brittle in a definite plane from the seventh onwards. This is -thus a very effective adaptation to persecution by enemies. The tail -which has been seized remains with the pursuer, but the lizard itself -escapes, and the tail grows again. But this regeneration does not take -place in the same way as in the embryo; no new vertebræ are formed, -but only a 'cartilaginous-tube,' a new structure, a substitute for the -vertebral column; the spinal cord with its nerves is not regenerated -either, and the arrangement of the scales is somewhat different. - -This last point, in particular, indicates that the determinants of -the regeneration-rudiment may pursue an independent phylogenetic path -of their own, for this scale arrangement of the regenerated tail -is an atavistic one, that is, it corresponds to a more primitive -mode of scale arrangement in these Saurians. We know quite a number -of cases similar to this. It not infrequently happens that cut-off -parts regenerate, but that they do so not in the modern form, but in -one that is in all probability phyletically older. Thus the legs of -various Orthoptera, as of the cockroaches and grasshoppers, regenerate -readily, but with a tarsus composed of four joints instead of five[3], -and the long-fingered claws of a shrimp (_Atyoida potimirim_) is -replaced by the older short-fingered type of claw, while in the -Axolotl an atavistic five-fingered hand grows instead of the amputated -four-fingered one. - -[3] New investigations, specially directed to this point, by -R. Godelmann, have shown that 'in the great majority of cases' -the regenerated legs of a Phasmid (_Bacillus rossii_) exhibit a -four-jointed tarsus; but the regeneration of five joints also occurs, -though only after autotomy, and only in seven out of fifty cases -(_Archiv für Entwicklungsmechanik_, Bd. xii, Heft 2, July 1901). -The regeneration-rudiment in this species seems to be in process of -advancing slowly to the five-jointed type. - -This last case shows that it is not merely a lesser power of growth -that accounts for the difference between the regenerated part and -the original, for here more is regenerated than was previously -present. There remains nothing for it but the assumption that the -regeneration-determinants have remained at a lower phyletic level, -while the determinants which direct embryogenesis have varied, and -either developed further or retrogressed. It is easy to understand that -the regeneration-rudiment must vary phyletically much more slowly -than the parts which evolved in the ordinary way and much more slowly -than the determinants of these parts, for natural selection means a -selection of the fittest, and the speed with which the establishment -of a variation is attained depends, _ceteris paribus_, on the number -of individuals that are exposed to selection with respect to the -varying part. If in a species of a million living at the same time -nine-tenths perish by accident, there will remain only 100,000 from -which to select the 1,000 which we will assume constitute the normal -number of the species. The more of these 100,000 which possess the -useful variation the higher will be the percentage of the normally -surviving 1,000 possessing it, and the more rapidly will the useful -variation increase. But when it is a question of the variation of -the regeneration-primordium, the selection will take place not among -all the 100,000 individuals which chance has spared, but only among -those of them which have lost a limb by accident, and thus are in -a position to regenerate it more or less completely. If we assume -that this takes place in 10 per cent. of cases, then selection for -the improvement of the regeneration-apparatus will only take place -among 1,000 individuals, and thus the process of modification of the -regeneration-primordium must go on very much more slowly than that of -the limb itself. - -I do not see how the opponents of the germ-plasm theory can explain -these facts at all, for the appeal to external influences is here -entirely futile, and that to internal liberating stimuli does not -suffice, since these must be different after a part has been cut -off from what they were when the limb developed normally, and also -different from those which prevailed at the normal origin of the -limb in ancestral forms. The four-jointed tarsus of the ancestors -of our cockroaches did not arise as a result of amputation. We -cannot therefore avoid referring the processes of regeneration to -particular 'regeneration-determinants,' which are contained in the -germ-plasm and are handed on in ontogeny with the other determinants -from cell-division to cell-division, till ultimately they reach the -cells which are to respond, or may have to respond, to the stimulus -of injury by some expression of their regenerative capacity. As these -determinants, as has been shown, can often only be very slightly -subject to the influence of selection processes, they will, in many -respects, lag behind in the phyletic development, and will tend to -belong to an ancestral type of the relevant part. They will often -remain for a long time at this ancestral level, and they will always -adapt themselves to new requirements more slowly than the parts which -arise in the normal way, and the determinants representing these in -the germ. But the regeneration-determinants _are_ variable, and, -indeed, are so hereditarily, and independently of the structure of the -normal parts. They thus follow their own path of phyletic development, -and this one fact is enough to secure a preference for the germ-plasm -theory above others that have hitherto been suggested. None of these -has even attempted an explanation of this fact; the tendency has rather -been to call it in question. This, however, can be done at most only in -regard to the explanation of the regenerations as atavistic, certainly -not in regard to the progressive variations of the regenerated part, -such as have been established by Leydig and Fraisse in regard to the -lizard's tail. It may be doubted whether the most primitive insects had -only four tarsal joints, but there is no disputing the kainogenetic -deviation of the lizard's-tail. - -I have interpreted the regenerative capacity as secondary and acquired, -not as a primary power of all living substance, and I should like to -substantiate this in another way. - -Let us go back to the simplest organism conceivable, which must have -represented the beginning of life on our earth, and we see that this -need not have possessed any special power of regeneration, because, for -an organism without differentiation of parts, growth is equivalent to -regeneration. But growth is the direct outcome of one of the primary -characters of the living substance, the capacity of assimilation. -This cannot be an adaptive phenomenon, nor can it have arisen through -selection, because selection presupposes reproduction, and reproduction -is only a periodic form of growth; but growth follows directly from -assimilation. The fundamental characters of the living substance, above -all the dissimilation and assimilation which condition metabolism, must -have been in existence from the first when living substance arose, -and must depend on its unique chemico-physical composition. But the -faculty of regeneration could only be acquired when organisms became -qualitatively differentiated, so that each part was no longer like -every other part or like the whole. As soon as this stage was reached -the faculty of regeneration would necessarily be developed, if further -multiplication was to take place. For when each fragment could no -longer become a whole by simply growing, some arrangement had to be -made by which each fragment should receive, in the form of primary -constituents, what it lacked to make up the whole. We do not know the -first beginning of this adaptation, but, in its further development, -it appears in the form of 'nuclear substance,' enclosed in the nucleus -of the cell, and, as is well known, it is now to be found in all -unicellular organisms. That the nucleus there precedes regeneration -in the sense that without a piece of it the cell-soma is not able to -complete itself alone, we have already seen, and the explanation of -this fact has always seemed to me to be that invisibly minute vital -units relating to the regeneration of the injured part leave the -nucleus and evoke the development of the missing parts by laws and -forces still unknown to us. Loeb has recently claimed that the nucleus -is the cell's organ of oxidation; but if that be true it would still -not exclude the possibility that the nucleus is also and primarily -a storehouse of the material bearers of the primary constituents -of a species. It must be regarded as such when we call to mind the -phenomena of amphimixis in its twofold aspects as conjugation and as -fertilization, and its obvious outcome among higher organisms where it -implies the mingling of the parental qualities. - -Thus the 'nuclear substance' of unicellular organisms is for us the -first demonstrable organ of regeneration, and first of all for normal -regeneration, which takes place at every reproduction, for instance, -of an Infusorian. For we have already seen that, in the transverse -division of a trumpet animalcule (_Stentor_), the anterior part must -develop the posterior half anew, while the posterior half must develop -the much more complex anterior half, with mouth region and spiral -bands of cilia. But as soon as the arrangement for normal reproduction -was elaborated, as soon as the nucleus was present, as a depôt of -'primary constituents,' this implied the possibility of regeneration -in exceptional cases, that is, after injury. The mechanism was already -there, and it came into operation as soon as a part of the animal was -missing. - -It is in the first nucleus, therefore, that we have to look for -the source of all regenerative capacity, both in unicellular and -multicellular organisms. But with the origin of the latter a limitation -took place, either quite at the beginning or a little later, for each -nucleus of the cell-colony no longer contained the whole complex of -'primary constituents' or determinants of the species, but, in many -cases, only the reproductive cell possessed them. As soon as this began -to develop into a whole by cell-division the determinant-complex was -segregated. Thus the first cell-colonies with two kinds of cells arose, -as we have seen in the case of _Volvox_--the reproductive cells with a -complete equipment for regeneration in their nucleus, and the somatic -cells with a limited equipment for regeneration in their nuclei. The -somatic cell could no longer give rise anew to the whole organism, but -could only reproduce itself or its like. - -But as many of the lower Metazoa and Metaphyta possess _the power of -budding_, that is, are able not only to produce a new individual -from definite cells--the reproductive cells--with or without sexual -differentiation, but from other cell-groups also, these must contain -the whole complex of determinants appertaining to the reconstruction -of the organism, and we have to ask how this is reconcilable with the -differentiation of a multicellular organism, whose different kinds of -cells depend, according to our interpretation, on the fact that they -are controlled by different determinants. - -Obviously, there is only one way out of this difficulty, and it is the -one we have already indicated, that although the diffuse regenerative -capacity which we have just alluded to occurs in species which exhibit -gemmation, this does not exclude the control of a cell by a specific -determinant; other determinants may be contained in the cell, in a -state, however, in which they do not affect it, that is, in an inactive -or latent state. - -Thus we arrive in this way also at our earlier assumption that an -inactive accessory-idioplasm is given to all, or at least to many -cell-generations. Only among plants must this necessarily be complete -germ-plasm, and among the lower plant-forms, as in _Caulerpa_ among -the Algæ, in _Marchantia_ among Liverworts, it must be assumed to -be present in nearly all the cells, according to the experiments -in regeneration made by Reinke and Vöchting. But in multicellular -animals which develop from two different germinal layers equipped -with a different complex of determinants budding arises from a -combination of at least two different kinds of cells, and we must -only ascribe to each of these its own peculiar determinant-complex as -regeneration-idioplasm. Higher plants show us that well-marked power -of budding is not necessarily associated with a high regenerative -capacity, the histologically specialized cells among them will contain -no inactive germ-plasm, because they do not need it. But in animals the -power of budding is probably always combined with high regenerative -capacity, as is shown by the Polyps and Medusoids above all, and in a -different way by the Ctenophores, which exhibit no budding and at the -same time a very slight regenerative capacity, although they possess -an organization scarcely higher than that of the Hydromedusæ. In the -Ctenophores each of the first segmentation-cells, when artificially -separated, yields only a half-embryo, and we may conclude from this -that it contains no complete germ-plasm in an inactive state, or at -least very little, and certainly not a sufficient quantity to make it -readily regenerative. - -Undoubtedly, however, the regenerative capacity occurs apart from the -capacity for budding, yet this in no way contradicts the theory. As -we have seen, a high regenerative capacity is to be found among many -animals which occur only as 'persons' and not as colonies or stocks, -but only in those which are readily liable to injury, and only in -the manner conditioned by their injury. In the higher Metazoa the -regenerative power becomes more and more limited, and in the Mammals it -sinks to a mere closing up of wounds. - -If we take a survey of the assumptions we have been compelled to -make from the standpoint of the theory to explain the development of -germ-cells, budding, and regeneration, it would seem as if it were -contradictory to assume that, on the one hand, complete germ-plasm -should be given to certain cell-series as inactive accessory idioplasm, -and, on the other, that very numerous cells, at least in the lower -Metazoa, should have received the idioplasm of budding, and still -more numerous cells that of regeneration. But it is obvious that -among the lower Metazoa the idioplasm of budding and the idioplasm of -regeneration are equivalent; the same idioplasm, which, when liberated -by stimuli unknown to us, co-operates from two or three germinal layers -in the formation of a bud, effects, in response to the known stimulus -of injury, the regeneration of the mutilated part. But germ-cells -can never arise in the Metazoa from the partial budding-idioplasm or -regeneration-idioplasm, because this is not complete germ-plasm, and -because it can only give rise to budding or regeneration through the -co-operation of two or more kinds of cells, while germ-cells always -originate from _one_ cell and never arise from the fusion of cells. -Germ-cells can thus only arise from the cells of the germ-track, and -in no other way, no matter whether the germ-track lie in the ectoderm, -as in the Hydromedusæ, or in the endoderm, as in true jellyfishes -(Acalephæ) and the Ctenophores, or in the mesoderm, as in many higher -groups of animals. It is only apparently that these cells belong to -one particular layer, for in reality they are unique in kind, and they -are simply assisted in their development by one or other cell-layer, -from which they not infrequently emancipate themselves, as happens -so notably in the Hydromedusæ. As we have already said, it is only -among plants that we must think of budding as arising from cells which -contain complete germ-plasm, for here there are no 'germinal layers' -corresponding to those of animal development, and the cells of 'the -growing point' must be equipped with the complete germ-plasm. The -plant, like the Hydroid stock and the Siphonophore colony, is saved -from death, in spite of the frequent loss of its members, mainly by -the fact that it is capable of producing, at almost any part above the -ground, buds which develop into new shoots, with leaves and the like. -This makes a power of regeneration on the part of the individual leaves -and flower-parts superfluous, but at the same time it implies that an -enormous number of cells must be distributed over the whole surface -of the plant, each of which can in certain circumstances become the -starting-point of a bud. That is to say, each must contain, in a latent -state, the complete germ-plasm which is necessary for the production of -an entire plant. - -We must therefore assume that, in the higher colony-forming plants, -germ-plasm is contained in a great many cells, perhaps in all which -are not histologically differentiated, and sometimes even in those -which are so, as, for instance, in the leaves of Begonias. I suppose, -therefore, that in the higher plants the process of development implies -a segregation of the determinant-complexes of the germ-plasm, but that -this takes place at a late stage, and that in a much higher degree than -among animals the individual or the 'person' carries with it germ-plasm -in a latent state. To this must be attributed the fact that the plant -is not only able to make good its losses in twigs and branches by -sending out new shoots, but that cuttings, that is, detached shoots, -are also able to take root, and in general to give rise to what is -necessary to complete themselves according to the position of the part -in question. In the ontogeny of animals, too, we must assume that it -requires a liberating stimulus to rouse the determinants to activity, -that this stimulus is to be sought for in the influence exercised by -the constitution of the cell on the idioplasm contained within it, -and that this constitution in its turn is subject to influences from -external conditions, including the cell-soma itself. We may therefore -suppose that, among plants also, the germ-plasm latent in numerous -cells only becomes active in whole or in part according to the -influences exerted on it by the state of the cell at the moment; but -this varies with external circumstances, according to whether the cell -is exposed to light or lies under ground, according as it is influenced -by gravity, by moisture, chemical stimuli, and so on. - -It might be objected to this that it would be simpler not to assume -a segregation of the germ-plasm into determinant-complexes at all in -order to explain the process of development, but rather to credit each -cell with a complete equipment of germ-plasm from the beginning to the -end of the ontogeny, and to attribute the differences in the cells, -which condition the structure of the plant and its differentiation, -solely to the different influences, external and internal, to which -the cell is exposed, and which rouse some determinants to activity at -one part and others at another. Perhaps the botanists would be more -readily reconciled to this idea, but it seems to me that there are two -points which tell against the possibility of its being correct. In the -first place, it is far from being established that every cell in the -higher plants is capable of giving rise, under favourable conditions, -to a whole new plant; every tree and every higher plant has a multitude -of cells in its leaves, its flowers, and so on, which cannot do this, -which are in fact differentiated in one particular direction, that is, -they contain only one kind of determinants, like the histologically -differentiated cells of the tissues of the human body. Secondly, -there are other organisms besides plants, and a theory of development -cannot be based on the phenomena to be observed among plants alone, -any more than a theory of heredity can. There are obvious differences -in the processes of life among plants as contrasted with those among -animals, but it is improbable that there is any thoroughly fundamental -difference. It is, however, indubitable that the cells forming the -tissues of higher animals, the nerve, muscle, and glandular cells, -are really differentiated in one direction, and are quite incapable, -under any circumstances whatever, of growing into an entire organism, -and even from this alone we might conclude that they contain only one -primordium or determinant. Are we then to assume that the vascular -cells, epidermis-cells, wood-cells, and so on, of the higher plants, -which are also differentiated in one direction, do nevertheless contain -the complete germ-plasm? I do not see any ground for such an assumption. - -To conclude what can be said on the subject of regeneration we must -return to the question of an ultimate explanation of this marvellous -phenomenon. I have declined to attempt any explanation at all, because -I do not consider it possible to give a sufficient one as yet, but I -should like at least to give an indication as to the direction in which -we must look for it. - -We assumed that there is a regeneration-idioplasm, and therefore that -there are 'primary constituents' at certain positions in the body, -but how does it happen that these are able to build up the lost parts -in the proper situation and detail? A theoretical formula might well -be thought out, according to which the determinants of successive -parts would become active successively, and would thus liberate one -another in an appropriate order of sequence, but there would not be -much gained by this, especially as what we already know in regard to -the regrowth of the legs and toes in Triton does not harmonize with -such an assumption. It appears to me more important--though even here -we must still be very vague as to details--to recognize that, in all -vital units, there are forces at work which we do not yet know clearly, -which bind the parts of each unit to one another in a particular order -and relation. We were obliged to assume such forces even in regard -to the lowest units, the biophors, since otherwise they could not be -capable of multiplication by division, on which all organic growth -depends, unless we are to assume, as Nägeli did, a continual _generatio -æquivoca_ of the specific kinds of biophors (his 'micellæ'). But we -shall see later, when we come to speak of spontaneous generation, that -we cannot acquiesce in such an assumption. If, then, we cannot conceive -of a power of division arising from within and depending solely on -growth by means of assimilation, without such attractive and repellent -forces or 'vital affinities' the internal parts would necessarily fall -into disorder at every division. It seems to me therefore that such -'affinities' must be operative at _all stages_ in the life of the vital -units, not only in biophors, but also in the cell, and in the 'person' -as well as in determinant and id. It is true that 'persons' no longer -generally possess the power of multiplying by division, but in plants -and lower animals many do possess it; and the power of giving rise -anew to certain parts is obviously a part of that power of doubling -the whole by division. The ultimate roots of regeneration, then, must -lie in these 'affinities' between the parts, which preside over their -arrangement and are able to maintain it and to give rise to it anew. -In this respect the organism appears to us like a crystal whose broken -points always complete themselves again from the mother-lye after -the same system of crystallization, obviously in this case too as a -result of certain internal directive forces, polarities, which here -again we are unable precisely to define. But the difference between -the organism and the crystal does not--as people have been hitherto -inclined to believe--lie only in the fact that the crystal requires the -mother-lye to complete itself, while the vital unit itself procures the -material for its further growth; it lies also in the fact that such -regeneration is not possible in every organism and at every place, but -that _special_ 'primary constituents' are necessary, without which the -relevant part cannot arise. The indispensableness of these primary -constituents, the determinants, seems to me to depend on the fact -that the new structure cannot be built up simply by procuring organic -material, but _that specially hewn stones, different in every case, -are necessary_, which can only be supplied in virtue of an historical -transmission, or, to abandon the metaphor, because the vital units of -which the organ is to be reconstructed possess a specific character -and have a long history behind them; thus they can only arise from -such vital units as have been handed on through generations, that is, -from the determinants. But these primary constituents are given to the -different forms of life in very varying degrees and in very unequal -distribution, and as far as we can see according to their suitability -to an end. - - - - -LECTURE XXII - -SHARE OF THE PARENTS IN THE BUILDING UP OF THE OFFSPRING - - The ids are 'ancestral plasms'--The reducing division brings - about a diversity of germ-plasm in the germ-cells--Bolles - Lee's 'Neotaxis' even in the primordial germ-cells--Häcker's - observations on the persistent distinctness of the maternal and - paternal chromosomes--Identical twins--The individuality is - determined at fertilization--Unequal share of the ids in the - determination of the offspring--Preponderance of one parent in the - composition of the offspring--Certain ids of the ancestors remain - unchanged in the germ-plasm of the descendants--Struggle of the - Biophors--Alternation of the hereditary sequences in the parts - of the child--Reversion--Datura-hybrids--Zebra-striping in the - horse--Three-toed horses--New experiments in hybridization among - plants by Correns and De Vries--Xenia. - - -As far as the phenomena of regeneration and budding are concerned, we -have not been able to do much more than bring them under a formula, -which harmonizes with the germ-plasm theory. But the case is different -with the actual phenomena of inheritance in the restricted sense, for -instance, with regard to the transmission of individual peculiarities -_from parent to child_. Here the theory really increases our insight -and lets us penetrate deeper into the causes of the phenomena; it is -here no longer a mere 'portmanteau-theory.' - -We are well aware, especially from observation on ourselves, that is, -on Man, that the children of a pair often resemble one another but -are never alike, and that one child frequently resembles one parent, -another the other, while a third may exhibit a mingling of both -parents. How does this come about? Since the germinal substance of both -parents is derived from that of the ovum, from which they themselves -have arisen--and must therefore be the same in all the germ-cells -to which they give rise--new determinants cannot be added, and old -ones cannot be dropped out, and variation of the determinants, the -possibility of which is granted, would still not directly bring about -the familiar mingling of resemblances to the two parents, but would at -most give rise to something new and strange. - -Here the theory helps to elucidate matters. We found ourselves obliged -to assume that the germ-plasm is composed of ids, that is, of -equivalent portions of germ-plasm, each of which contains all the kinds -of determinants appertaining to the building up of an individual, but -each of these kinds in a particular individual form. I have already -called these ids 'ancestral plasms,' and the term is appropriate, in -so far that in every fertilization an equal number of ids from the -father and from the mother are united in the ovum, so that the child -is built up of the ids of his two nearest 'ancestors.' But as the ids -of the parents are derived from those of the grandparents, and these -again from those of the great-grandparents, the ids are in truth the -idioplasm of the ancestors. - -The expression, however, has been very frequently misunderstood, as -if it were intended to mean that the ids retained _unchanged_ for all -time the character of their respective ancestors, and I have even -been credited with supposing that our own ids still consist of the -determinant-complexes of our fish-like or even Amœba-like ancestors. -But in reality no id exactly or completely corresponds to the type, -that is, to the whole being of any one of the ancestors in whose -germ-plasm it was formerly contained, for each of the ancestors had -many ids in his germ-plasm, and his entire constitution was not -determined by any one of these alone, but by the co-operation of them -all. The individual arising from a germ-cell must necessarily be the -result of all the ids which make up his germ-plasm, but undoubtedly the -share taken by some of them may be much stronger than that taken by -others. It is also clear that, if we leave out of account any possible -variation on the part of the ids, each of them belongs, not to one -ancestor only, but to a whole series of ancestors, and must have taken -part in their development, so that it is not the idioplasm of any -particular ancestor, but only ancestral plasm in the general sense. In -this sense we may quite well retain the designation, 'ancestral plasm,' -for the id. - -Thus, according to our view, the germ-plasm consists of ids, each of -which contains all the determinants of the whole ontogeny, but usually -in individually different quality. - -Returning for a moment to the processes by which the reduction of the -chromosomes, that is, of the nuclear rods of germ-plasm in the ovum -and sperm-cell is brought about, we recall the fact that this happens -at the last two divisions of the germ-cell, the so-called 'maturing -divisions.' In these the nuclear substance, as we have seen, is -divided between the two daughter-nuclei in a manner quite different -from the usual one, for a longitudinal splitting of the rods, bands, -or spheres in the equatorial plane of the nucleus does not take -place, but half the number of rods move into the right and half into -the left daughter-nucleus without previous division, so that in each -daughter-nucleus the number of rods is reduced to half (Fig. 76). - -[Illustration: FIG. 76. Diagram of the maturation divisions of the -ovum. _A_, primitive germ-cell. _B_, mother-egg-cell, which has grown -and has doubled the number of its chromosomes. _C_, first maturation -division. _D_, immediately thereafter; _Rk_ 1, the first directive cell -or polar body. _E_, the second maturation spindle has been formed; the -first polar body has divided into two (2 and 3); the four chromosomes -remaining in the ovum lie in the second directive spindle. _F_, -immediately after the second maturation division; 1, the mature ovum; -2, 3, and 4, the three polar cells, each of these four cells containing -two chromosomes.] - -Although the distribution of the rods in this manner takes place twice -in succession, the normal number is not, as we have already seen, -reduced to a quarter, because, long before the occurrence of the first -maturing division, a duplication of the rods by means of longitudinal -division had taken place, and thus the first division differs from an -ordinary division in that the splitting of the rods does not take place -during the process of dividing but long beforehand. Only the second -maturing division differs from all other nuclear divisions known to -us, since it is not associated with any splitting of the rods at all, -but conveys half of the existing rods into each daughter-nucleus. It -is the time reducing division, through which the number of the rods is -reduced to one half[4]. - -[4] Recent investigations have shown that the reduction of the -chromosomes does not always take place exactly in accordance with the -scheme here indicated, but that it differs from it in many cases. But -as investigations on this point are as yet by no means complete, I need -not go into the question further; the ultimate result is the same in -any case. - -This numerical reduction must, however, have other consequences; it -must make the germ-cells of the same individual qualitatively unlike, -that is, in relation to their value in inheritance. Let us assume only -four chromosomes of the rod-form ('idants') as the nuclear elements -of a species, two of which, _A_ and _B_, come from the mother, and -other two, _C_ and _D_, from the father, the last maturing division -may, as far as we can see, result either in removing the combination -_A_ and _B_ from _C_ and _D_, or _A_ and _C_ from _B_ and _D_, or _A_ -and _D_ from _B_ and _C_; there is thus a possibility of one of six -different combinations of rods in any one germ-cell. What is the same -thing, six different kinds of germ-cells differing in their hereditary -primary constituents may _be developed in the same_ individual. As this -new combination, or, as we may call it, neotaxis of the germ-plasm -elements, takes place in female as well as in male individuals, there -is a possibility that, in fertilization, 6 × 6 = 36 individuals with -different primary constituents may arise from the germ-cells of the -same two parents. Of course the number of possible combinations -increases very considerably in proportion to the normal number of -rods, for with eight of these it comes up to 70, and with sixteen to -12,870; the number of individuals differing in their inherited primary -constituents would thus be enormous, for each of the 70 or of the -12,870 different hereditary minglings of the ovum could combine in -amphimixis with 70 or 12,870 different sperm-cells, so that 70 × 70 -and 12,870 × 12,870 offspring individually different in their primary -constituents might arise from the same two parents. In Man there are -said to be sixteen nuclear rods; so that in his case the last-mentioned -number of parental hereditary minglings might occur. This may seem a -disproportionately high number as compared with the small number of -children of a human pair, but we must not judge from the case of Man -alone, and in plants and animals, which we have already discussed, the -number of descendants is very much larger, and is often enormous. We -saw what significance this apparent extravagance on the part of nature -has, for without it adaptation to changed conditions of life would -not be possible, since, if only so many were born as could attain to -reproduction, no selection of the fittest could take place. The same -would be the case if all the young of a species were alike, and even if -all the descendants of a single pair were alike, effective selection -would be excluded, since only as many individualities could be selected -as there were pairs of parents. It is easy to understand that selection -works more effectively the larger the number of descendants of a -species and the more they differ from each other. The chance that the -best possible combination of characters will occur is thereby increased. - -Although we cannot calculate how many individuals of different -combinations of characters natural selection requires to work upon in -order to direct the evolution of the species[5], we can understand -that only as large a choice as possible can secure that the best -possible adaptations of all parts and organs are brought about and -maintained. Precisely in the fact that in every generation such an -enormous superfluity of individuals is produced lies the possibility -of such intensive processes of selection as must continually take -place, if the adaptation of all parts is to be explained. For if among -the thousands of descendants of a fertile species each hundred were -alike among themselves, these hundreds would have, as far as natural -selection was concerned, only the value of a single variant. But such -an all-round adaptation as actually exists in the structure of species -requires as many variants as possible; it requires that each individual -should be a _peculiar complex of hereditary characters_; that is, that -all the fertilized germ-cells of a pair should possess an individually -well-marked character. - -[5] For this reason I have left the number of id-combinations given -above unaltered, though, according to the most recent researches into -the processes of maturation, they are probably too high, since every -conceivable combination does not actually occur. We are here concerned -less with the exact number than with the principle. - -The justification of this postulate becomes all the clearer if we take -into consideration the male germ-cells as well as the female. Let us -think of the enormous number of sperm-cells which are produced by many -animals, and indeed by the highest of them--an almost incalculably -large number which certainly goes far beyond millions. Let us assume -that in Man there may be 12,870 million spermatozoa, then, with -sixteen ids, and with an equally frequent occurrence of all possible -combinations of germ-plasm--there would be 12,870--there would be a -million of each type containing identical germ-plasm. The danger that -several ova would be fertilized by identical sperm-cells would be by no -means small. - -It cannot, therefore, surprise us that other means have been employed -by Nature to secure re-groupings of the ids. The simplest means would -be, if before each division of the primitive germ-cells the nuclear -rods were to divide, and if the split halves were irregularly -intermingled, then at the formation of the next nuclear spindle an -entirely new arrangement of the halves would result. But in animals, at -least, this is certainly not the case; the processes of reduction are -restricted to the maturing divisions. - -Years ago Ischikawa observed that, in the conjugation of _Noctiluca_, -the nuclei of the two animals become closely apposed, but that they do -not fuse, although they behave like a single nucleus in the division -which follows. In this case _paternal and maternal nuclear substance -remain separate_ (Fig. 83, vol. i. p. 317). The same phenomenon has -since been repeatedly observed in many-celled animals, first by Häcker, -then by Rückert in the Copepods, and afterwards by Conklin in the eggs -of a Gastropod (_Crepidula_). But all these observations referred only -to the earlier stages of ovum-segmentation up to twenty-nine cells, -and it could not be affirmed that the distinctiveness of the paternal -and maternal chromosomes lasted till farther on into the ontogeny. -Professor Häcker now informs me, however, that he has been able to -trace this separateness in a Copepod (_Canthocamptus_) not only from -the beginning of segmentation on to the primitive genital cell, but -also through the divisions of this up to the mother-egg-cell[6]. -Thus we may now assume that the paternal and maternal hereditary -bodies remain distinct, not only for a time, but throughout the whole -development, a fact which confirms our assumption of the independence -of the nuclear rods, notwithstanding their apparent breaking-up in -the nuclear reticulum of the 'resting' nucleus. This new knowledge -throws fresh light in another direction; it proves to us that the -remarkable and complicated processes which go on in the nucleus -during the maturing divisions have really the significance which I -long ago ascribed to them[7], that of effecting the maximum diversity -of intermingling of the paternal and maternal hereditary elements. -For Häcker has shown that during the second maturation-division -the paternal and maternal chromosomes are no longer united each -in a special group, but occur scattered about in the nucleus, and -subsequently come together again to form two differently combined -groups. - -[6] Since this was written Häcker has published his results. See -_Anatom. Anzeiger_ xv (1902), p. 440. - -[7] See my essay, _Amphimixis_, Jena, 1891. - -If this were not so, if the maternal and paternal chromosomes remained -separate, then the reducing division would cause only one of these -groups to reach each of the germ-cells, and thus each mature ovum -or sperm-cell would contain either only paternal or only maternal -hereditary bodies. But this would make a reversion to more than three -generations back impossible, and as such reversions undoubtedly occur, -we must conclude that manifold new combinations of the paternal and -maternal chromosomes take place. This obviously happens during the -maturation-divisions, at least in the Metazoa. - -The more numerous the rods or the free individual ids in a species -are, the more numerous are the possible combinations. Whether all the -mathematically possible combinations actually occur is a different -question, which I should not like to answer in the affirmative just -yet; but in any case the _actual_ number of combinations in a species -with many nuclear elements will be greater than in one with few, and -in this respect those species in which the ids occur as independent -granules will have an advantage over those in which they are combined -into rods or bands (idants). These latter, however, afford us a better -possibility of deducing the new combinations of the ids, although the -idants themselves are not outwardly distinguishable from each other. - -I must refrain from going into these highly interesting processes in -more detail just now. So much is certain, that Nature makes use of -_various means_ to bring about the re-combination, and at the same time -the reduction of the ids during the two 'reducing divisions.' This is -proved by the fact recently established by Montgomery, that in many -animal groups reduction results from the _first_ maturing division. -Whether it operates at this stage with rings, bands, double rods, -X-shaped structures, groups of four (tetrads), and so on, all this -serves the same end, the more or less thoroughgoing re-arrangement of -the hereditary vital units. I am convinced that new investigations -into these processes, if they were undertaken from this point of view, -would lead to very important results[8]. It would be important to find -out how great the variations are which thus arise, for it is very -probable that they differ in degree in the different animal-groups. -Even the combination of the ids into rods (idants) indicates that some -species may be more conservative than others in maintaining their -id-combinations, and that there will be among them a greater tenacity -in the hereditary combinations of characters (i.e. of the 'type' of -the parents). If we should succeed in penetrating more deeply into -these processes we should probably also understand why in certain human -families the hereditary characters are transmitted more purely and more -tenaciously than those of other families with which they have mingled, -and so on. It may well be that the persistence of character is due -to the fact that ids which have once combined into rods hold firmly -together, for it seems to me in no way impossible that individual -differences should occur even in these most delicate processes. - -[8] Since this was written for the first edition observations of this -process have been considerably increased, and discussions as to the -exact interpretation of these are in full tide; we are surrounded by -a wealth of new observations, facts, and explanations, without having -attained to a consistent and unified theory. Several naturalists, such -as Boveri, Häcker, Wilson, and others, have attempted interpretations, -but these are in many points contradictory to one another. It is -therefore impossible to enter into the question in detail here; -further light from new observations must be awaited. So much we may -say, however, that it is not chance alone which presides over the -re-arrangement of the chromosomes during the reducing divisions; -_affinities_ play a part also; there are stronger or weaker attractions -between the chromosomes, which aid in determining their relative -position to one another. - -But let us leave these more intimate questions out of account -altogether, and turn our attention to the more obvious and less -delicate phenomena, and we find that the re-arrangement of ids -(Neotaxis) which we have just discussed affords a simple explanation -of the generally observed phenomenon of the differences between -individuals! Each individual is different from every other, not in -the case of Man alone, but in all species in which we can judge of -differences, and this is true not only of descendants of different -parents, but even of those of the same parents. - -Of course the differences between two brothers or two sisters do not -depend entirely on the hereditary basis, but in part also on external -conditions which have affected them from embryonic development onwards. -Let us suppose that of two brothers who have sprung from identical -germ-cells one becomes a sailor, the other a tailor; it would not -surprise us to find them very different in their fiftieth year, one -weather-beaten and tanned, the other pale; one muscular, straight, -and vigorous, the other weakly and of bent carriage. The same primary -constituents develop differently according to the conditions to which -they are exposed. But the two brothers will still resemble each other -in the features of the face, colour of hair, form of eyes, stature and -proportion of limbs, perhaps even in a birthmark, more than any other -human beings of their own or any other family, and this resemblance -will depend upon the identity of the hereditary primary constituents, -on the similar id-combination of the germ-plasm. - -Man himself affords a particularly good example in favour of this -interpretation in the case of so-called 'identical twins.' It is well -known that there are two kinds of twins, those that are not strikingly -alike, and often very different, and those that are alike to the extent -of being mistakable for one another. Among the latter the resemblance -may go so far that the parents find it necessary to mark the children -by some outward sign, so that they may not be continually confused. -We have now every reason to believe that twins of the former kind -are derived from two different ova, and that those of the latter kind -arise from a single ovum, which, after fertilization, has divided into -two ova. This not infrequently occurs in fishes and other animals, -and we can bring it about artificially in a number of species by -experimentally separating the two first blastomeres. - -We have here, then, a case of absolute identity of the germ-plasm in -two individuals, for the id-combination of the two ova derived from the -same process of fertilization must be exactly the same. That in such a -case, notwithstanding the inevitable differences of external influences -to which the twins are exposed from intra-uterine life onwards, such a -high degree of resemblance should arise is a fact of great theoretical -importance. From the basis of the germ-plasm theory we can very well -understand it, for, according to the theory, only precisely similar -combinations of ids can give rise to identical individuals. - -But we learn more than this from the occurrence of identical twins. -They prove above all that the whole future individual is determined at -fertilization, or, to express it theoretically, that the id-composition -of the germ-plasm is decisive for the whole ontogeny. It might have -been supposed that the combination of ids could change again during -development, and that a greater multiplication of some than of others -might take place at certain stages of development, or through certain -chance external influences. It might have been thought that there was a -struggle among the ids in the sense that some of them were suppressed -and set aside. All such suppositions break down in face of the fact of -identical twins, which teaches us that identical germ-plasm evokes an -ontogeny which runs its course as regularly as two chronometers, which -are constructed and regulated alike. - -But when I say that a struggle of the ids, in the sense of a material -setting aside of some of them, cannot take place, I by no means intend -to maintain that the influence which each individual id exerts on the -course of development may not be disproportionate to that exerted by -others, and, under some circumstances, very disproportionate indeed. -I must refrain from entering into this subject in detail now, but I -should like to give at least an indication of what I mean. - -If the germ-plasm consists of ids, these ids collectively must -determine the structure, the whole individuality--let us say, briefly, -the 'type' of the offspring; it is the resultant of all the different -impelling forces which are contained in the different ids. If these -were all equally strong, and all operating in the same direction, -they would necessarily all have the same share in the resultant of -development, the 'type' of the child. But this is not the case. - -Numerous experiments on the hybridization of two species of plant have -taught us that the descendants of such hybridization usually maintain -a medium between the ancestral species; but it is not always the case, -for in many hybrids the character of one species, whether paternal or -maternal, preponderates in the young plant. - -We recognize the same thing still more clearly in Man, whose children -by no means always maintain a medium between the characters of the two -parents, but frequently resemble one--the father or the mother--much -more strongly than the other. - -How can this fact be theoretically explained? Must we ascribe to the -ids of the father or of the mother a greater determining power? Without -excluding such an assumption as on _a priori_ grounds inadmissible, -I am inclined to believe that we do not require it to explain _this_ -phenomenon. For, if we take our stand simply on the fact of the -preponderance of one parent, it follows directly from this that not -all the ids control the type of the child, let the cause of the -non-co-operation of some of them be what it may. But if in this case -only a portion of the ids contained in the germ-plasm controls the -type, this combination of ids suffices to make the child resemble -one parent, the father, for instance, and consequently half the -number of ids is sufficient in some circumstances to determine the -child--taking for granted that the one-sidedness of the inheritance is -complete, which never actually happens. But the half number of ids can -only suffice if it includes the same combinations of ids which have -determined the type in the case of the father; as soon as one or more -ids of this particular combination are replaced by others the paternal -germ-plasm alone is not enough to call forth complete resemblance in -the child. - -But, at the reduction, a change of arrangement of ids takes place, -and a new combination arises, and thus each germ-cell receives its -particular group of ids. It may thus happen that, in one particular -sperm-cell, exactly the same group of ids is contained as that which -determined the type of the father, and that the same is true of a -particular egg-cell in regard to the type of the mother. Let us now -assume that a sperm-cell and an egg-cell meet, which contain both those -groups of ids which had determined the type of the father and of the -mother; if the determining power of the maternal and paternal ids were -equal a child would result which would maintain the medium between -father and mother. - -As is well known this does happen not infrequently, although it is -difficult or impossible to demonstrate it precisely. In plant-hybrids -proof is easier, and it has been established that by far the greater -number of hybrids maintain a medium between the characters of the two -ancestral species. This proves that our assumption of equal strength -of the ids of both species must be correct in general, for we know -definitely in this case, as I shall show later, that the paternal -and the maternal ids are equivalent as regards the characters of the -species. This is the case, for instance, with the hybrids between the -two species of tobacco-plant, _Nicotiana rustica_ and _N. paniculata_, -which were reared by Kölreuter as far back as the eighteenth century, -and which then, as now, maintained a fairly exact medium between the -two ancestral species, and did so in all the individuals. Both species -thus strive to stamp their own character on the young plant, and in -both the hereditary power is equally great; in both it is contained -in the same number of ids, that is, in the half, for both kinds -of sex-cell have undergone reducing division. We have here, then, -strict proof that the half number of ids suffices to reproduce in the -offspring the type of the species, or, more generally, of the parents. - -If we apply these results to the inheritance of individual differences -in Man, we may say, that those germ-cells, to which at the reducing -division the same combination of ids has been handed on as that which -already determined the type of the parent, will endeavour to impress -this image again on the child. If a female cell of this kind combine -with a male which likewise contains the facies-combination of the -parent, in this case the father, the same thing will happen which we -described in the case of the plant-hybrids, that is, a medium form -between the type of the two parents will arise. - -Not infrequently, however, there is a marked preponderance of the one -parent in the type of the child, and we have to inquire whether the -theory gives us any help with regard to such a case. - -One might be inclined to assume a difference in the determining power -of the paternal and maternal ids, but if we cannot show to what extent -and for what reason this power may be different such an assumption -remains rather an evasion than an explanation. Moreover, it would not -always apply to the conditions in Man, for if, for instance, the ids -of a particular mother were in general stronger than those of the -father, all the children of the pair in question would necessarily -take after the mother; but it happens not infrequently that one child -resembles the father preponderantly, and another the mother. Moreover, -the ids pass continually from the male to the female individual, and -conversely, by virtue of the continuity of the germ-plasm, so that the -idea that sex can have anything to do with the relative strength of the -ids is altogether erroneous. - -But, as I have already said, unilateral inheritance occurs even in -the mingling of species-characters, and most clearly in the case of -plant-hybrids. Thus, for instance, hybrids between the two species of -pink, _Dianthus barbatus_ and _Dianthus deltoides_, resemble the latter -species much more closely than the former, and the hybrid between the -two species of foxglove which are wild in Germany, _Digitalis purpurea_ -and _Digitalis lutea_, is much more like the latter than like the -former. - -It might be reasonably asked whether, in these crossings, the normal -number of ids in one species is not greater than in the other. We -know that, among animals at least, differences in the normal number -of chromosomes occur even in very nearly related species. It is not -impossible that this, in many cases, is really the cause of the -diversity of transmitting power in different species. Nevertheless, we -cannot rest satisfied with this, for, in the first place, this cause -could not apply to the apparent unilateral inheritance from one parent -in Man, since the normal number of ids, as far as we know, is strictly -maintained in the same species, and second, this would not explain -certain phenomena of inheritance in plant-hybrids. - -It happens not only frequently, but usually, that the different parts -of the hybrid take after one or other parent in different degree, and -this is the case also with children. In the hybrid between the two -species of tobacco-plant, _Nicotiana rustica_ and _N. paniculata_, -which I have already given as an example of a medium form between -the two parents, such diversities occur regularly in all the hybrid -individuals. Thus the corolla-tube of the hybrid is nearer _N. -paniculata_ in regard to its length, but nearer _N. rustica_ in regard -to its breadth. Many hybrids suggest one parent-form in the leaves, the -other in the blossoms. In the same way in the child the form of eye -may be that of the father, the colour of the iris that of the mother, -the nose maternal, the mouth paternal--in short, the preponderance in -heredity swings hither and thither from part to part, from organ to -organ, from character to character, and this is even the rule though -the oscillations may not always be apparent and are often invisible. - -If we think of the proposition we arrived at earlier, and which was -proved chiefly by the case of identical twins, that the facies or -'type' of the descendant is determined at fertilization, we may be -inclined to regard such an oscillation of the hereditary tendencies -as almost impossible, for it means that, with the given mingling of -parental germ-plasms, the potency of inheritance from the two parents -in every part of the offspring is determined once for all in advance. -But the case of identical twins corroborates these oscillations, for -in them, too, the father predominates in one part, the mother in -another, and it proves, at the same time, that these oscillations do -not depend on any chances whatever in development, but that they are -exactly predetermined in the mingling of the hereditary substances in -the germ-plasm of the fertilized ovum, and are strictly adhered to -throughout development. - -This fact can only be explained thus: the primary constituents of -the different parts and characters of the body are contained in the -parental germ-plasm in varying degrees of hereditary or transmissive -strength, and this can be understood very well from our point of view -without putting anything new into the 'portmanteau' of our theory -(Delage). - -But I must digress a little in order to make this plain. - -When, in speaking of plant-hybrids, I said that the collective ids -of the germ-plasm of a species must be equivalent in regard to the -characters of the species, I did not speak quite precisely; in the -majority of ids, in many cases in an overwhelming majority, this must -be the case, but not actually in all, at least not on the assumption -we make that the transformation of species is accomplished under the -control of natural selection. - -Let us recall what we have already established in regard to the -evolutionary power of natural selection, namely, that the changes which -it controls can never transcend the range of their utility, and it will -be clear to us that, of the many ids which make up the germ-plasm of -the species, only so many will be modified as are necessary to evoke -the character which has varied. Just as the protective resemblance -of an insect to a leaf may be raised to a very high pitch, but can -never become perfect, because an imperfect resemblance is already -sufficient to deceive the persecutor, and the selective process comes -to a standstill because individuals which possessed a still greater -resemblance to a leaf would be no better protected from destruction -than the others, so in the modification of a species the whole of the -ids need not at once be modified, if a majority is sufficient to stamp -the great majority of individuals with the desired variation. But it -may happen that, at the reduction of ids during the development of the -germ-cells, an id-combination with wholly or almost wholly unchanged -ids may come together in one germ-cell, and if another sperm-cell of -this kind meets with an egg-cell similarly constituted, an individual -of the old species must arise. But this must--on our assumption--be -at a disadvantage as compared with the transformed individuals in the -struggle for existence, and will perish in it, and therefore the number -of unmodified ids in the germ-plasm of the species will gradually -diminish. It is obvious, however, that this will take place very -slowly, as we may conclude from the phenomena of reversion, of which I -shall have to speak later on. - -But what is true of the ids is true also of their constituent parts, -the determinants, and that--if I mistake not--is fundamental in the -interpretation of the alternation of hereditary succession in the parts -of the child. - -According to our theory, the ids do not collectively exert a -controlling influence on the cells, not even on the germ-cells, whose -histological differentiation into spermatic or egg-cells can only -depend on control through specific sex-cell determinants. It is the -different determinants of the ids that control; transformations of -the species will, it is true, depend on transformations of the ids, -but this need not necessarily consist in a variation of _all_ the -determinants of the id. If, for instance, two species of butterfly, -_Lycæna agestis_ in Germany and _Lycæna artaxerxes_ in Scotland, only -differ from each other in that the black spot in the middle of the wing -in _L. agestis_ is milk-white in _L. artaxerxes_, no other determinants -in the id of the germ-plasm can be different except those which control -this particular spot. In a majority of the ids in _L. artaxerxes_ the -determinants of this spot must have been modified, let us say, to the -production of 'milk-white.' This majority will increase very slowly if -the white colour has no pronounced advantage for the persistence of -the species, but it will increase gradually, as we have already seen, -though extremely slowly, through the elimination of those individuals -whose germ-plasm at the reducing division has chanced to receive a -majority of ids with the old, unmodified determinants, and which have -therefore reverted to the ancestral form. This will happen whenever the -new character has any use, however small, in maintaining the species. - -But in most modifications of species quite a number of parts and -characters have undergone variation either simultaneously or in -rapid succession; in many cases nearly all the details of structure, -and therefore almost all the determinants of the germ-plasm, must -have varied. We must not, however, assume that all the equivalent -determinants, for instance, all the determinants _K_ in all the ids, -have varied[9], and above all we must not take for granted that -the determinants of different characters or parts of the body, for -instance, the determinants _L_, _M_, or _N_, must all undergo variation -in an equal number of ids. It will depend on two factors whether a new -character is implicit, in the form of varied determinants, in a small -or in a very large majority of ids: first, on the relative age of the -character, and, secondly, on its value in relation to the persistence -of the species. The more important a character is for the species the -more frequently is it decisive for the life or death of the individual, -and the more sharply will individuals not possessing it be eliminated, -and the more rapidly, therefore, will those whose germ-plasm still -contains a majority of the unvaried determinants of this character -tend to disappear. In this way these determinants will tend to sink -down from generation to generation to an ever smaller minority in the -germ-plasm of those that survive. - -[9] By equivalent or homologous determinants I mean the determinants of -different ids which determine _similar parts_, e.g. the scales of that -wing-spot in _Lycæna agestis_ which is alluded to above, and to which -we must refer again in more detail. - -Thus in the ids of any species which has been in some way -transformed--and that is as much as to say, in every species--the -equivalent or homologous determinants are modified in a very varied -percentage. A very modern and at the same time not very important -character _K´_ will only be contained in a small majority of ids, -while in the remainder the original homologous ancestral determinant -_K_ is contained; an older but not very much more important character -_M´_ must have its determinants in a larger majority in the ids, while -a character _V´_ of decisive importance for the preservation of the -species, if it has been in existence long enough, must be represented -in almost all the ids, so that the homologous unvaried determinants of -the ancestral species _V_ can only have persisted in an id here and -there. - -If this argument be correct, many phenomena of inheritance become -intelligible, especially the variability in the expression of the -inheritance in the parts of the offspring, which is more or less -rigidly predetermined at fertilization. For the germ-plasm thus -contains in advance every kind of determinant in diverse _nuances_, -and in definite numerical proportions. In a plant _N´_, for instance, -_Ba´_ may be the determinant of the modern leaf-form, and may occur -in twenty-two out of twenty-four ids of the germ-plasm, while the two -remaining ids still contain unmodified the old leaf-form determinants -_Ba_, which the ancestral form _N_ possessed. But the flower of _N´_ -may be of still more recent origin, and contain the modern flower -determinants _Bl´_ only in sixteen out of twenty-four ids, while in the -other eight the old flower-determinant _Bl_ of the ancestral form has -persisted. Let us now suppose that another nearly related species _P´_ -has, conversely, a recently changed leaf-form but a very ancient form -of flower, so that the former is represented only in sixteen ids by the -determinants of the leaf _ba´_ and the latter in twenty-two ids by the -flower-determinants _bl´_: it is obvious that when the two species are -crossed, notwithstanding the equal number of ids in the germ-plasm, the -leaves of the hybrid will resemble more closely those of the ancestral -form _N_, and the flowers those of the form _P_; it is even conceivable -that in such a case the numerically preponderating leaf-determinants -_N_, and the equally preponderating flower-determinants of _P_ may form -a close phalanx, so to speak, against the much less numerous homologous -determinants of the other species, and that against this power working -in a definite direction the others can make no headway and are simply -condemned to inaction. - -How we may or can picture this as occurring is a question which of -course admits only of being answered very hypothetically, and it leads -us, moreover, into the region of the fundamental phenomena of life, -with the interpretation of which we are not here concerned. For the -present we have assumed that life is a chemico-physical phenomenon, and -we have postponed the deeper explanation of it to the remote future, -that we may confine ourselves in the meantime to the solution of the -problem of inheritance on the basis of the forces resident in the vital -elements. But we may, nevertheless, make the supposition that a kind of -struggle between the different kinds of biophors may take place within -the cell, if the homologous determinants of all the ids for the control -of the cell have entered into it. - -In many cases this struggle will be decided by the numerical -preponderance of one kind of determinant over the other, but it is -certainly conceivable that dynamic differences may also have something -to do with it. - -Let us, however, abstain from trying to penetrate further into the -obscurity of these processes, and let us content ourselves with -establishing that the preponderance of one parent in some or many -parts of the child may be almost if not quite complete, and that this -compels us to assume that the hereditary substance of the other parent -is in such cases rendered inoperative--for we know it is present--since -the ids of both parents all go through the whole ontogeny, and are -contained in every somatic cell. - -Upon this struggle between homologous determinants depends the -possibility of the entire suppression or inhibition of the influence -of one parent, the whole diversity in the mingling of paternal and -maternal character in the body of the offspring. It is in this that we -must seek the explanation of the fact that not only whole bodily parts -of the child, such as arms, legs, the nature of the skin, the form of -the skull, may take after sometimes the father, sometimes the mother -wholly or predominantly, but that the small separate subdivisions of -a complex organ may sometimes turn out more maternal, sometimes more -paternal. Thus intelligence from the mother and will from the father, -musical talent from the father and a talent for drawing from the -mother, may be inherited by the same child. I do not doubt that genius -depends in great part on a happy combination of such mental endowments -of the ancestors in one child. Of course something more is necessary, -namely, the strengthening of certain of these hereditary endowments, -but of this we shall speak later. - -It is, however, not only the immediate ancestors, that is to say, -the parents, that have to be taken account of in this mingling of -hereditary contributions, but also those more remote. Not a few -characters in the child do not occur in either parent, but were present -in the grandparents, and their reappearance is called 'atavism' or -'reversion.' Let us consider this phenomenon in more detail, and try -to find out whether and how far it can be interpreted by means of our -theory. - -The simplest and clearest cases are again found among plant-hybrids. -It may happen, for instance, that a hybrid between two species, when -dusted with its own pollen, gives rise to descendants, some of which -resemble only one of the ancestral forms: thus we have reversion to -one of the grandparents. The explanation of this lies in the different -modes in which the reducing divisions are effected; if they take place -in such a manner that all the paternal ids of the hybrid are separated -from the maternal ids, then the result is germ-cells which are like -those of the grandparents, that is, those of the parent species, and -these, if they happen to combine in amphimixis, must give rise to a -pure seedling of one or other of the two ancestral species. This case -occurs less rarely than was formerly supposed, and than it could do if -absolutely free combination of the idants took place at reduction. If -combination were quite unrestricted, all other possible combinations -would be likely to occur as frequently as these. But recent experiments -have shown that, in many plant-hybrids, the germ-cells of the hybrids -which are fertilized by their own pollen are either purely paternal or -purely maternal. There cannot, therefore, be free combination of the -idants at the reducing divisions; the idants of the two parent-forms -separate from one another and do not combine. It is doubtful, however, -whether the same thing occurs within a race, for instance, in the case -of reproduction within a human race. - -In Man reversion to a grandparent occurs not infrequently, and we -may explain it thus: the id-group which controlled the type of the -grandfather was also contained in the germ-cell which gave rise to -the existence of the father, but it did not dominate the type in -that case because a more powerful id-group was opposed to it in the -germ-cell of the grandmother. When, later on, at the reducing divisions -of the germ-cells of the father, this id-group again arrived in the -sperm-cells of the father, it would predominately control the type of -the child, that is, of the third generation, provided that the egg-cell -with which it combined contained a weaker id-group. - -In the case of ordinary plant-hybrids what are designated reversions -can only be called so in a wide sense, for the ancestral characters -are contained _visibly_ in the parent, although mingled with those -of the other parent. In human families, however, there are undoubted -cases in which one or more characters of the grandparent reappear in -the child which were not in any visible way expressed in the parent, -and must therefore have been contained in the parent's germ-plasm in -a latent state. And there are both in animals and plants reversions -to ancestors lying much further back, to characters and groups of -characters which have not been visible for many generations, and the -occurrence of these can only be explained on the assumption that -certain groups of ancestral determinants have been carried on in the -germ-plasm in too small a number to be able to give rise ordinarily -to the relevant character. Such isolated determinants may, however, -in certain circumstances be strengthened by the amphimixis of two -germ-cells both containing small groups of them, and thus augmented -they may gain a controlling influence. In this case the chances of the -reducing divisions have a part to play, since they bring together the -old unvaried ancestral determinants which, as we have seen, may persist -in the germ-plasm of any species through a long series of generations. -This would, of course, only suffice to bring about a reversion if the -determinants of the ancestral species were still contained in the -germ-plasm in comparative abundance. If this is no longer the case, -something more is necessary, and that is the relative weakness of the -more modern determinants. - -If two white-flowered species of thorn-apple, _Datura ferox_ and -_Datura lævis_, be crossed, there arises a hybrid with bluish violet -flowers and brown stalks instead of green. This was interpreted by -Darwin as a reversion to violet-flowering ancestral species on both -sides, for there are even now a great number of species of _Datura_ -with violet flowers and brown stalks. When the two white forms are -crossed the reversion takes place every time, not merely in some cases, -and we may conclude from this that in both these species there is still -such a strong admixture of the same unvaried ancestral ids that they -always excel the ids of the two modern species crossed, in strength -though certainly not in number. And this superiority must again depend -on the fact that similar determinants of the same part are cumulative -in their effect, while dissimilars are not. - -For this reason reversions to remote ancestors occur readily when -species and breeds are crossed, while they are rare in the normal -inbreeding of a species. The reversions of the breeds of pigeon to -their wild ancestral form, the slaty-blue rock-pigeon, never result, as -Darwin showed, and as we have already noticed, from pure breeding of -one race, but only when two or more breeds are repeatedly crossed with -one another. Even then it occurs by no means always, but only now and -again. The germ-plasm of the breeds must therefore still contain ids -of the rock-pigeon, but in a small number, varying from individual to -individual. If by fortunate reducing divisions and the meeting together -of a sperm-cell rich in ancestral ids with a similarly endowed egg-cell -the number of ancestral ids be raised to such a point that it exceeds -the number of modern-breed ids contained in each one of the conjugating -germ-cells, the ancestral ids control the development and reversion -occurs, for the ancestral ids together have a cumulative effect while -the ids of the two parent breeds are different and therefore, as far -as they are so, cannot be co-operative in their influence. But it must -be understood that they need not be different as far as _all_ their -determinants are concerned, but usually only as regards some groups, -and thus it happens that reversion does not occur in regard to all, but -only in regard to particular characters--thus in the _Datura_ hybrids, -chiefly in regard to the colour of the flowers and the stem, and in -the hybrids between different breeds of pigeon mainly in regard to the -colour and marking of the plumage. - -The reversions of the horse and ass to striped ancestors, which Darwin -has made famous, go much further back into the ancestral history of the -species, for while we know the ancestral form of the domestic pigeon -in the still living rock-dove (_Columba livia_), the ancestral form -common to the horse and the ass is extinct, and we can only suppose -that it was striped like a zebra, because such striping occasionally -occurs in pure horses and pure asses, at least in their youth, although -now only on the legs, and because this striping is often more marked -in the hybrid between the horse and the ass, the mule. In Italy, where -one sees hundreds of mules, the striping is not exactly frequent, -but it may occur in about two per cent., while in America it is said -to be much more frequent. The germ-plasm of the horse and the ass -must therefore contain, in varying numbers, ids whose skin-colour -determinants represent in part still unmodified ancestral characters. -When two germ-cells chance to meet in fertilization, both of which have -received, through a favourable reducing division, a relatively large -number of such ids, a relative majority of these in the fertilized -ovum is opposed to the dissimilar and therefore mutually neutralizing -homologous determinants of horse and ass, and reversion to the -ancestral form occurs. - -These cases of reversion are enough to show us that the old unmodified -ancestral determinants may persist in the germ-plasm through long -series of generations. But an even deeper glimpse into the dim ancient -history of our modern species of horses is afforded by the occurrence -of three-toed horses, references to a small number of which the -palæontologist Marsh was able to discover in literature, and one of -which he was able to observe in life. Julius Caesar possessed a horse -whose three-toed feet represented a reversion to the horses of Tertiary -times, _Mesohippus_, _Miohippus_, and _Protohippus_ or _Hipparion_; for -all these genera possessed, in addition to the strong middle toe, two -weaker and shorter lateral toes. - -In the germ-plasm of our modern horses there must still persist in -certain ids the determinants of the ancestral foot, which, after a long -succession of favourable reducing divisions combined with favourable -chances of fertilization, may come to be in a majority, and may thus be -able to induce a reappearance of a character which has long been hidden -under the surface of the present-day type of the species. - -I do not propose to enter further on the discussion of the phenomena -of inheritance. A more detailed investigation of the phenomena of -reversion is to be found in '_The Germ-plasm_' published ten years -ago; and the discussion could not be resumed here without a critical -consideration of a relatively large series of newly acquired facts -not always harmonizing, and, as yet, not even fully available. The -year 1900 has given us the investigations of three botanists, De -Vries, Correns, and Tschermak, who have sought by experiments in -hybridization between different sorts of peas, beans, maize, and other -plants, to throw light on the phenomena of inheritance, and thus on -the actual processes which occur in the germ-plasm at the reducing -division. This led to the discovery that similar experiments had been -published as far back as 1866 by the Abbot of Brünn, Gregor Mendel, and -that these had been formulated as a law which is now called Mendel's -law. Correns showed, however, that this law, though correct in certain -cases, did not by any means hold good in all, and we must thus postpone -the working of this new material into our theory until a very much -wider basis of facts has been supplied by the botanists. There is less -to be hoped for from the zoologists in regard to this problem owing -to the almost insuperable difficulties in the way of a long series -of experiments in hybridization in animals. I myself have repeatedly -attempted experiments in this direction, and have always had to abandon -them, either because the crossing succeeded too rarely, or because the -hybrids did not reproduce among themselves, or did so defectively, or -because the distinguishing characters of the crossed breeds proved -insufficiently tenacious or diagnostic. But it would be a fine task for -zoological gardens to undertake such experiments from the point of view -of the germ-plasm theory, and their success would afford material for -the criticism of the theory, the more valuable because it is apparent -from the experiments on plants that the processes of heredity are -manifold, and are far from being uniform in different domains[10]. - -[10] Castle and Allen have recently published the results of -experiments in crossing white mice with grey, and these confirm -Mendel's Law. - -I have assumed for my theory that the reducing division took place -according to the laws of chance, and that thus every combination of ids -occurred with equal frequency. This assumption seems to be confirmed, -by the experiments of the botanists I have mentioned, only in so far -that in the crossing of hybrids with one another every combination -of distinctive characters occurred with equal frequency. But, on the -other hand, the splitting of the germ-plasm at the reducing division -seems, as I said before, in many cases to take place in such a way that -the id-groups of the two parents are discretely separated from one -another; this was so in the stocks, peas, beans, and other hybrids. -But even if this were always the case in these, we could hardly infer -that it must be the same everywhere; we should rather expect that the -relationship of the two parents and their ids would bring into play the -finer attractions and repulsions between the ids of the germ-plasm, -and would thus determine their arrangement and grouping. Further -investigations may clear up this point; in the meantime we can only -say that already--even among hybrids--many deviations from Mendel's Law -have been established, for instance, by Bateson and Saunders (1902). - -Before I conclude this lecture I should like to refer briefly to a -phenomenon which Darwin was acquainted with and sought to explain -through his theory of Pangenesis, but which at a later date was -regarded as not sufficiently authenticated to justify any attempt -at a theoretical explanation, since it seemed to contradict all our -conceptions of hereditary substance and its operations. I refer to the -phenomenon to which the botanists have given the pretty name of Xenia -(guest-gifts), and which consists in the fact that in crosses of two -different plants the characters of the male may appear not only in the -young plant but even in the seed, so that a transference of paternal -characters seems to take place from the pollen-tube to the mother, to -the 'tissue of the maternal ovary.' In heads of yellow-grained maize -(_Zea_) it is said that, after dusting with pollen from a blue-seeded -variety, blue seeds appear among the yellow, and similar observations -on other cultivated plants have been on record for more than half a -century. Thus dusting the stigma of green varieties of grape with the -pollen of a dark blue kind is said frequently to give rise to dark blue -fruits. - -Darwin accepted these observations as correct, and endeavoured -to explain them as due to a migration of his 'gemmules' from the -fertilized ovum into the surrounding tissue of the mother-plant. His -explanation was not correct, we can say with confidence now, but he -was right so far, for the phenomena of Xenia do occur; they are not -illusory as most modern botanists seem inclined to believe. I myself -was at first inclined to wait for further facts in proof that the -phenomena of Xenia really occurred before attempting to bring them -into harmony with my theory, and this will not be found fault with -when it is remembered that these cases of Xenia seem to stand in -direct contradiction to the fundamental postulates of the germ-plasm -theory. For this depends essentially on a definite stable structure -of the germ-substance, which lies within the nucleus in the form of -chromosomes, and which cannot pass from one cell to another in any -other way than by cell-division and division of the nucleus; how then -could it pass from the fertilized ovum to the cells of the endosperm -which do not derive their origin from it at all, but from other cells -of the embryo-sac? In point of fact some of my opponents have cited -Xenia as an actual refutation of my theory. - -[Illustration: FIG. 82. Fertilization in the Lily (_Lilium martagon_), -after Guignard. _A_, the embryo-sac before fertilization; _sy_, -synergidæ; _eiz_, ovum; _op_ and _up_, upper and lower 'polar nuclei'; -_ap_, antipodal cells. _B_, the upper part of the embryo-sac, -into which the pollen-tube (_pschl_) has penetrated with the male -sex-nucleus (♂_k_) and its centrosphere; below that is the ovum-nucleus -(♀_k_) with its (also doubled) centrosphere (_csph_). _C_, remains of -the pollen-tube (_pschl_); the two sex-nuclei are closely apposed. -Highly magnified.] - -That cases of Xenia really do occur is now established by the -comprehensive and at the same time exceedingly careful experiments -recently made by C. Correns with _Zea Mais_; it is only necessary to -look through the beautiful figures with which his work is adorned to be -convinced that heads of maize whose blossoms have been dusted with the -pollen of a different kind produce more or less numerous seeds of the -paternal kind, usually mingled with those of the maternal. Thus heads -of the variety _Zea alba_ resulting from fertilization with _Z. cyanea_ -exhibited a majority of white grains, but among them a smaller number -of blue; and the converse experiment, of dusting _Z. cyanea_ with -the pollen of _Z. alba_, yielded heads in which a minority of white -grains appears among a majority of blue. But it is always only the -nutritive layer surrounding the embryo--the endosperm--which exhibits -the character of the paternal species, and even the capsule surrounding -the seed shows nothing of it, but is purely maternal. Thus the heads -of different species with pale-yellow capsule, when dusted with the -pollen of _Z. rubra_, never have red seeds like those of _Z. rubra_, -but always seeds with a pale-yellow skin, while, in the converse -experiment, dusting of the red-skinned species _Z. rubra_ with pollen -from _Z. vulgata_, all the seeds are red, like those of the maternal -species, and the influence of the paternal species only shows when the -strong red skin has been removed, so that the intense yellow colour of -the endosperm, which in the pure maternal species is white, is exposed -to view. Thus the mysterious influence of the pollen never goes beyond -the endosperm, and the riddle of this influence is solved in the most -unexpected manner, indeed was solved even before Correns had securely -established the genuine occurrence of Xenia. The explanation is due -to recent disclosures in regard to the processes of fertilization in -flowering plants. - -It had long been known that the pollen-tube contains not merely one -generative nucleus but two, which arise from one by division. But what -had till recently remained unknown was that not only one of these -penetrated into the embryo-sac to enter into amphimixis with the -egg-cell, but that the other also makes its way in, and there fuses -with the two nuclei which had long been designated the upper and lower -polar-nuclei (Fig. 82, op. cit.). Nawaschin and Guignard demonstrated -that these two nuclei fuse _with the second male_ nucleus; thus two -acts of fertilization are accomplished in the embryo-sac, and one of -these gives rise to the embryo, while the second becomes nothing less -than the endosperm, the nutritive layer which surrounds the embryo, -whose origin from the polar nuclei had been previously recognized. - -Thus the riddle of Xenia is essentially solved. We understand how -paternal primary constituents may find their way into the endosperm, -and indeed must do so regularly; we understand also how the paternal -influence never goes beyond the endosperm. The riddle is thus not only -solved, but at the same time the view which assumes a fixed germ-plasm, -and believes it to lie in the nuclear substance of the germ-cells, -receives further confirmation, if it should need any, for if facts -which are apparently contradictory to a theory can be naturally -brought into harmony with it, this affords a stronger argument for the -correctness of the theory than the power of explaining the facts which -were used in building it up. - -There is much more to be said in regard to Xenia, and I am sure -that much that is of interest will be brought to light by deeper -investigation; theoretical difficulties will still have to be overcome, -and I have already pointed out one of these in my '_Germ-plasm_,' but I -must here rest satisfied with what has been already said. - -We have now passed in review and attempted to fit into the theory a -sufficiently large number of the phenomena of heredity for the purpose -of these lectures. Although, as is natural, much of this must remain -hypothetical, we may accept the following series of propositions as -well founded: there is a hereditary substance, the germ-plasm; it is -contained in very minimal quantity in the germ-cells, and there in -the chromosomes of the nucleus; it consists of primary constituents -or determinants, which in their diverse arrangement beside or upon -one another form an extremely complex structure, the id. Ids and -determinants are living vital unities. Each nucleus contains several, -often many, ids, and the number of ids varies with the species and is -constant for each. The ids of the germ-plasm of each species have had -a historical development, and are derived from the germ-plasm of the -preceding lineage of species; therefore ids can never arise anew but -only through multiplication of already existing ids. - -And now, equipped with this knowledge, let us return to the point -from which we started, and inquire whether the Lamarckian principle -of evolution, the inheritance of functional modifications, must be -accepted or rejected. - - - - -LECTURE XXIII - -EXAMINATION OF THE HYPOTHESIS OF THE TRANSMISSIBILITY OF FUNCTIONAL -MODIFICATIONS - - Darwin's Pangenesis--Alleged proofs of functional - inheritance--Mutilations not transmissible--Brown-Séquard's - experiments on Epilepsy in guinea-pigs--Confusion of infection of the - germ with inheritance, Pebrine, Syphilis, and Alcoholism--Does the - interpretation of the facts require the assumption of the transmission - of functional modifications?--Origin of instincts--The untaught - pointer--Vom Rath's and Morgan's views--Attachment of the dog to his - master--Fearlessness of sea-birds and seals on lonely islands--Flies - and butterflies--Instincts exercised only once in the course of a - lifetime. - - -As I have already said in an earlier lecture, Darwin adhered to -Lamarck's assumption of the transmission of functional adaptations, -and perhaps the easiest way to make clear the theoretical difficulties -which stand in the way of such an assumption is to show how Darwin -sought to present this principle as theoretically conceivable and -possible. - -Darwin was the first to think out a theory of heredity which was worthy -of the name of theory, for it was not merely an idea hastily suggested, -but an attempt, though only in outline, at elaborating a definite -hypothesis. His theory of 'Pangenesis' assumes that cells give rise to -special gemmules which are infinitesimally minute, and of which each -cell brings forth countless hosts in the course of its existence. Each -of these gemmules can give rise to a cell similar to the one in which -it was itself produced, but it cannot do this at all times, but only -under definite circumstances, namely, when it reaches 'those cells -which precede in order of development' those that it has to give rise -to. Darwin calls this the 'elective affinity' of each gemmule for -this particular kind of cell. Thus, from the beginning of development -there arises in every cell a host of gemmules, each of which virtually -represents a specific cell. These gemmules, however, do not remain -where they originated, but migrate from their place of origin into the -blood-stream, and are carried by it in myriads to all parts of the -body. Thus they reach also the ovaries and testes and the germ-cells -lying within these, penetrate into them, and there accumulate, so that -the germ-cells, in the course of life, come to contain gemmules from -all the kinds of cells which have appeared in the organism, and, at -the same time, all the variations which any part may have undergone, -whether due to external or internal influences, or through use and -disuse. - -In this manner Darwin sought to attribute to the germ-cells the power -of giving rise, in the course of their development, to the same -variations as the individual had acquired during its lifetime in -consequence of external conditions or functional influences. - -I abstain from analysing the assumptions here made; their improbability -and their contradictions to established facts are so great that it -is not necessary to emphasize them; the theory shows plainly that it -is necessary to have recourse to very improbable assumptions, if an -attempt is to be made to find a theoretical basis for the transmission -of acquired (somatogenic) characters. Even when Darwin formulated -his theory of Pangenesis his assumptions were hardly reconcileable -with what was known of cell-multiplication; now they are above all -irreconcileable with the fact that the germ-substance never arises -anew, but is always derived from the preceding generation--that is, -with the continuity of the germ-plasm. - -If we were now to try to think out a theoretical justification we -should require to assume that the conditions of all the parts of -the body at every moment, or at least at every period of life, were -reflected in the corresponding primary constituents of the germ-plasm -and thus in the germ-cells. But, as these primary constituents are -quite different from the parts themselves, they would require to vary -in quite a different way from that in which the finished parts had -varied; which is very like supposing that an English telegram to China -is there received in the Chinese language. - -In spite of this almost insuperable theoretical obstacle, various -authors have worked out the idea that the nervous system, which -connects all parts of the body with the brain and thus also with each -other, communicates these conditions to the reproductive organs, and -that thus variations may arise in the germ-cells corresponding to those -which have taken place in remote parts of the body. - -Even supposing it were proved that every germ-cell in ovary or testis -was associated with a nerve-fibre, what could be transmitted to it by -the nerves, except a stronger or weaker nerve-current? There is no such -thing as _qualitative_ differences in the current; how then could the -primary constituents of the germ be influenced by the nerve-current, -either individually or in groups, in harmony with the organs and parts -of the body corresponding to them, much less be caused to vary in a -similar manner? Or are we to imagine that a particular nerve-path leads -to every one of the countless primary constituents? Or does it make -matters more intelligible if we assume that the germ-plasm is without -primary constituents, and suppose that, after each functional variation -of a part, telegraphic notice is sent to the germ-plasm by way of the -brain as to how it has to alter its 'physico-chemical constitution,' -so that the descendants may receive some benefit from the acquired -improvement? - -I am not of the number of those who believe that we already know all, -or at least nearly all, that is essential, but am rather convinced that -whole regions of phenomena are still sealed to us, and I consider it -probable that the nervous system in particular is not yet exhaustively -known to us, either in regard to its functioning or in regard to its -finest structural architecture, although I gratefully recognize the -advances in this domain that the last decades have brought about. In -any case, such assumptions as I have just indicated, or similar ones, -seem to me quite too improbable to furnish any foothold for progress. -Yet we must always remain conscious that we cannot decide as to the -possibility or impossibility of any biological process whatever from -a purely theoretical standpoint, because we can only guess at, not -discern, the fundamental nature of biological processes. At the close -of this lecture I shall return to the question of the theoretical -conceivability of an inheritance of functional adaptations; but first -of all we must consider the facts and be guided by them alone. If they -prove, or even make it seem probable, that such inheritance exists, -then it must be possible, and our task is no longer to deny it, but to -find out how it can come about. - -Let us therefore investigate the question whether an inheritance -of acquired characters, that is, in the first place, of functional -adaptations, is demonstrable from experience. We shall speak later on -of the effect of climatic and similar influences in causing variation; -the case in regard to them is quite different, because they undoubtedly -affect not only the parts of the body but the germ-cells as well. - -When we inquire into the facts which have been brought forward by -the modern adherents of the Lamarckian principle as proofs of the -inheritance of acquired characters in this restricted sense, we shall -find that none of them can withstand criticism. - -First, there are the numerous reputed cases of the inheritance of -mutilations and losses of whole parts of the body. - -It is not without interest to note here how opinion in regard to this -point has altered in the course of the debate. - -At the beginning of the discussion they were all brought forward as -evidence of undoubted value for the Lamarckian principle. - -At the Naturalists' Congress in Wiesbaden in 1887, kittens with only -stumps of tails were exhibited, and they were said to have inherited -this peculiarity from their mother, whose tail, it was asserted, had -been accidentally amputated. The newspapers reported that the case -excited great interest, and biologists of the standing of Rudolf -Virchow declared it to be noteworthy, and regarded it as a proof, if -all the details of it were correct. From many sides similar cases were -brought forward, intended to prove that the amputation of the tail -in cats and dogs could give rise to hereditary degeneration of this -part; even students' fencing-scars were said to have been occasionally -transmitted to their sons (happily not to the daughters); a mutilated -or torn ear-lobe in the mother was said to have given rise to deformity -of the ear in a son; an injury to a father's eye was said to have -caused complete degeneration of the eyes in his children; and deformity -of a father's thumb, due to frostbite, was said to have produced -misshapen thumbs in the children and grandchildren. A multitude of -cases of this kind are to be found in the older textbooks of physiology -by Burdach, and above all by Blumenbach, and the majority have no -more than an anecdotal value, for they are not only related without -any adequate guarantee, but even without the details indispensable to -criticism. - -As far back as the eighteenth century the great philosopher Kant, and -in our own day the anatomist Wilhelm His, gave their verdict decidedly -against such allegations, and absolutely denied any inheritance of -mutilations; and now, after a decade or more of lively debate over -the pros and cons, combined with detailed anatomical investigations, -careful testing of individual cases, and experiment, we are in a -position to give a decided negative and say _there is no inheritance of -mutilations_. - -Let me briefly explain how this result has been reached. - -In the first place, the assertion that congenital stump-tails in dogs -and cats depended on inherited mutilation proved to be unfounded. In -none of the cases of stump-tails brought forward could it even be -proved that the tail of the relevant parent had been torn or cut off, -much less that the occurrence, in parents or grandparents, of short -tails from internal causes was excluded. At the same time anatomical -investigation of such stump-tails as occur in cats in the Isle of Man, -and in many Japanese cats, and are frequently found in the most diverse -breeds of dogs, showed that these had, in their structure, nothing -in common with the remains of a tail that had been cut off, but were -spontaneous degenerations of the whole tail, and are thus deformed -tails, not shortened ones (Bonnet). - -Experiments on mice also showed that the cutting off of the tail, -even when performed on both parents, does not bring about the -slightest diminution in the length of tail in the descendants. I -have myself instituted experiments of this kind, and carried them -out through twenty-two successive generations, without any positive -result. Corroborative results of these experiments on mice have -been communicated by Ritzema Bos and, independently, by Rosenthal, -and a corresponding series of experiments on rats, which these two -investigators carried out, yielded the same negative results. - -When we remember that all the cases which have been brought forward in -support of an inheritance of mutilations refer to a _single_ injury -to one parent, while, in the experiments, the same mutilation was -inflicted on both parents through numerous generations, we must regard -these experiments as a proof that all earlier statements were based -either on a fallacy or on fortuitous coincidence. This conclusion is -confirmed by all that we know otherwise of the effects of oft-repeated -mutilations, as for instance the well-known mutilations and distortions -which many peoples have practised for long, sometimes inconceivably -long, ages on their children, especially circumcision, the breaking -of the incisors, the boring of holes in lip, ear, or nose, and so -forth. No child of any of these races has ever been brought into the -world with one of these marks: they have to be re-impressed on every -generation. - -The experience of breeders agrees with this, and they therefore, as -Wilckens remarks, have long regarded the non-inheritance of mutilations -as an established fact. Thus there are breeds of sheep in which, for -purely practical reasons, the tails have been curtailed quite regularly -for about a century (Kühn); but no sheep with a stump-tail has ever -been born in this breed. This is all the more important because there -are other breeds of sheep (fat-rumped sheep) in which the lack of -the tail is a breed character; it is thus not the case that there is -anything in the intrinsic nature of the tail of the sheep to prevent -it becoming rudimentary. The artificially rounded ear of fox-terriers, -too, though cut for generations, never occurs hereditarily. Mr. Postans -of Eastbourne informs me that the cocks which are to be used for -cock-fighting are docked of their combs and wattles beforehand, and -that this had been done for at least a century, but that no fighting -cock without comb and wattles has been reared. In the same way various -breeds of dog, such as the spaniels, have had their tails cut to half -their length regularly and in both sexes for more than a century, yet -in this case there is no hereditary diminution of the length of tail. -Deformed stump-tails do indeed occur in most breeds of dog, but, as I -said before, their anatomical character is quite different from that of -artificially shortened tails, moreover they may occur in breeds whose -tails have not suffered from the fashion of docking, as, for instance, -in the Dachshund. - -We may therefore affirm that an inheritance of artificially produced -defects and mutilations is quite unproved, and in no way bears out the -supposed inheritance of functional changes. - -This is now admitted by the great majority of the adherents of the -Lamarckian principle, and we may now regard this kind of 'proof' as -disposed of. - -In addition to the above, various sets of facts have been brought -forward as proofs, and in particular the much discussed experiments of -Brown-Séquard on guinea-pigs, from which it was inferred that epilepsy -artificially induced could be transmitted. But these experiments do -not really prove anything in regard to the question at issue, because -epileptic-like convulsions may have very various causes, and these -are, for the most part, quite unknown. Since artificial epilepsy -can be induced in guinea-pigs by the most diverse injuries to the -central or peripheral parts of the nervous system, this of itself -points to the fact that it is not a question of the mere lesion of -anatomical structure, I mean, of the breaking of the continuity of -a definite part, and of its transmission. The result would, in any -case, differ according to whether certain centres of the brain, or -half the spinal cord, or the main nerve-trunks were cut through. There -must, therefore, be something more needed to produce the appearance of -epilepsy--some morbid process which may arise at different parts of -the nervous system, and be continued from them to the brain-centres. -This is corroborated by the fact that it takes at least fourteen days, -and often from six to eight weeks, for epilepsy to develop after the -operation, and that in many cases it does not develop at all. I have -made the suggestion that, during or after the operation, some kind of -pathogenic micro-organism might easily reach the wounded parts, and -there excite inflammation, which may extend centripetally to the brain. -Similar processes have been observed in connexion with lymph-vessels, -and why should they not occur in connexion with nerves? - -It has been objected to this that the guinea-pig's epilepsy may be -produced by blows on the skull, and also by a destructive compression -of the _nervus ischiadicus_ through the skin, and that in both cases -the epilepsy may reappear in the following generation; and this, it -is supposed, shows that the intrusion of microbes is excluded. If -this were so beyond a doubt, and if we could exclude the possibility -that there were previously various microbes within the body, which -could only penetrate into the nervous substance after the cutting -or destruction of the neurilemma, nothing would be gained that -would in any way support the Lamarckian principle. One could only -say: Certain injuries to the nervous system give rise secondarily -in guinea-pigs to morbid phenomena like epilepsy, and all sorts of -functional disturbances of the nervous system often appear in the next -generation, including in rare cases even the phenomena of epileptic -convulsions. That this is a case of the transmission of an acquired -anatomical modification brought about by the injury is not only -unproved, but is decidedly negatived, for the injuries themselves are -never transmitted. Thus what is transmitted must be quite different -from what was acquired, for no one has ever detected in the offspring -the lesion of the nerve-trunk which was cut through in the parent, or -any other result except the disease to which the original injury gives -rise. Moreover, the inheritance of these morbid phenomena has been -again brought into dispute quite recently owing to the investigations -of such experts in nervous diseases as Sommer and Binswanger, and the -correctness of Brown-Séquard's results, which have dragged through the -literature of the subject for so long, has been emphatically denied[11]. - -[11] See H. E. Ziegler's report in _Zool. Centralblatt_, 1900, Nos. 12 -and 13. - -Clearly formulated problems, like that of the inheritance of acquired -characters, should not be confused by bringing into them phenomena -whose causes are quite unknown. What do we know of the real causes -of those central brain-irritations which give rise to the phenomena -of epilepsy? It is certain enough that there are diseases which are -acquired and are yet 'inherited,' but that has nothing to do with -the Lamarckian principle, because it is a question of _infection_ -of the germ, not of a definite variation in the constitution of the -germ. We know this with certainty in regard to the so-called Pebrine, -the silkworm disease which wrought such devastation in its time; the -germs of the pebrine organism have been demonstrated _in the egg_ of -the silk-moth; they multiply, not at once but later, in the young -caterpillar, and it is the half-grown caterpillar, or even the moth, -that succumbs to the disease. - -Whether in this case also the disease germs are transmitted through -the male sex-cells is not proved, as far as I am aware, but that this -can happen is shown by the transmission of syphilis from father to -child. That in this case, also, the exciting cause of the disease is a -micro-organism cannot be doubted, although it has not yet been proved. -Thus even the minute spermatozoon of Man can contain microbes, and -transmit them to the germ of a new individual. - -This discussion of scientific questions ought not to be brought down -to the level of a play upon words, by bringing forward cases like the -above as evidence for the inheritance of 'acquired characters,' as was -done, for instance, by M. Nussbaum, who cited as a proof of this the -migration of the alga-cells which live in the endoderm of the green -freshwater Hydra into the ovum, which is originally colourless, and -originates in the ectoderm of the animal (Fig. 35_B_, p. 169, vol. -i). It seems to me better to make a precise distinction between the -transmission of extraneous micro-organisms through the germ-cells and -the handing on of the germ-plasm with the characters inherent in its -structure. Only the latter is inheritance in the strict scientific -sense, the former is infection of the germ. - -Still less than the cases of inherited traumatic epilepsy can the -morbid constitution of the children of drunkards be regarded as a -proof of the inheritance of somatogenic characters, though this has -often been maintained. I will not lay any stress on the fact that the -allegation itself is, according to the most competent observers, such -as Dr. Thomas Morton[12], far from being established. But even if it -were quite certain that the numerous diseases of the nervous system, -amounting sometimes to mania, which are frequently observed in the -children of drunkards, were really _caused_ by the drinking of the -parents, it ought not to be overlooked that we have here to do not with -the hereditary transmission of somatic variations, but of variations -_directly_ induced in the germ-plasm of the reproductive cells, for -these are exposed to the influence of the alcohol circulating in the -blood, just as any other part of the body is. That by this means -variations in the germ-plasm can be brought about, and that these -may lead to morbid conditions in the children cannot be denied, and -ought not on _a priori_ grounds to be called in question. For we are -acquainted with many other influences--climatic, for instance--which -directly affect and cause variation in the germ-plasm. Whether this -is so in the case of drunkenness, and in what manner it comes about, -whether through direct action of the alcohol, or through infection of -the germ with some microbe, we must leave to the future to decide; the -whole question is out of place here; it can in no way help us to clear -up the problem with which we are now occupied. - -[12] Morton, 'The Problem of Heredity in Reference to Inebriety,' -Proceed. Soc. for the Study of Inebriety, No. 42, Nov. 1894. - -But even if there were not a trace of proof of the transmissibility of -functional modifications, that alone would not justify us in concluding -that the transmission is impossible, for many things may happen that we -are not in a position to prove at present. If it could be shown that -there was a whole group of phenomena that could not be explained in any -other way than on the hypothesis of such inheritance, then we should -be obliged to assume that it really occurred, although it was not -demonstrable, and, indeed, not even theoretically conceivable. This is -the standpoint of the adherents of the Lamarckian principle at present. - -They say there are a great number of transformations which are simply -and easily explained, if we regard them as the effects of inherited -use or disuse, but which admit only of a strained explanation, and -sometimes of none at all, on the basis of natural selection, and these -are not a few isolated cases, but whole categories of them. - -I will submit a few of these, and show at the same time why I cannot -regard them as convincing, even if it be the case that we are not at -present in a position to explain them without the aid of the Lamarckian -principle. But let me hasten to add that it is my belief that we can do -this, although certainly not without first giving a somewhat extended -application to the principle of selection. - -It has often been maintained that the existence of animal instincts is -in itself enough to prove that the Lamarckian principle is operative. -In one of the earlier lectures I showed that at least the greater -number of instincts must have originated in purely reflex actions, -and therefore, like these actions themselves, can only be explained -through natural selection. A reflex action, such as coughing, sneezing, -shutting of the eyelids, and so on, differs from an instinctive -action in the lesser complexity and shorter duration of the series -of movements liberated by a sense-impression, and also in that it -does not require to enter into consciousness at all; but no very -precise boundary can be drawn between the two, and, in any case, both -depend, as we have already seen, on a quite analogous anatomical -basis. It is only a difference in degree whether, at the sight of a -rapidly approaching object, the muscles of the eyelids contract, and -by shutting the lids, protect the eye, or whether the fly, which we -intend to seize with our hand, is impelled by the sight of the rapidly -approaching shadow of the hand to fly quickly up. The action of the fly -may be regarded as reflex, or equally well as instinctive. But there -is also only a difference in degree, not in kind, between this simple -action and the complex and protracted behaviour of a mason-bee, the -sight of whose colony impels her to fly out and fetch clay, with it -gradually to build a neat cell, to fill this with honey, to lay an egg -in it, and finally to furnish the cell with a roof of clay. Since all -reflex mechanisms, and all the natural instincts of animals, contribute -to the maintenance of the species, and are therefore useful, their -origins must be referable to natural selection, and we have only to ask -whether they must be referred to it always, and to it alone. - -It cannot be doubted that, in Man, and in the higher animals voluntary -actions which are often repeated gradually acquire the character -of instinctive actions. The individual movements pertaining to the -particular action are no longer each guided by the will, but a single -exercise of will is enough to liberate the whole complex action, such -as writing, speaking, walking, or the playing of a whole piece of -music; frequently the will-impulse may be absent altogether, and the -action be set going simply by an adequate external stimulus, as in -the case of sleep-walking, which is observed in fatigued children and -soldiers, and in somnambulists. The external stimulus is transmitted -to the proper group of muscles as unfailingly as in the case of true -instincts, and this happens not only in regard to actions which, -like walking, are essential to the life of the species, but also in -regard to those which have arisen from chance habits or exercises. -Often a short practice is sufficient to make an action in this sense -instinctive, and the complexity of the instinct-mechanism gained by -such practice is often astounding. Under some circumstances a person -may play a piece on the piano from the score, and yet be thinking -intently of other things, and be quite unconscious of what is played. -In the same way it may happen that a person dominated by violent -emotion, when trying to free himself from it by reading, may read a -whole page, line by line, without understanding in the least what has -been read. In the last case it is not directly demonstrable that the -reader has made all the complex delicate eye-movements which would be -liberated by the sight of the words, but in the case of playing, the -listeners can perceive that the piece is correctly played, and thus -that the stimulus exercised by each note on the retina of the eye -is translated into the complex muscular movement of arm and finger, -corresponding both to the pitch and the duration of the note, and to -the simultaneousness of several notes. - -In all these cases it is probably not always quite new paths which -are established in the brain, but use is made of particular tracks -in the innumerable nerve-paths already existing in the nerve-cells -(neurons) which are 'more thoroughly trodden' by practice, so that -the distribution of the nerve-current takes place more easily along -them than along others[13]. This much-used metaphor does not indicate -the actual structural changes which have taken place, but it serves -at least to indicate that we have to do with material changes in the -ultimate living elements of the nerve-substance (nerve-biophors) -whether these changes be in position or in quality. Now, if such -brain-structures and mechanisms acquired through exercise in the -individual life could be transmitted, new instincts would certainly -arise in this way, and many naturalists hold this view still. - -[13] This, however, is by no means intended to cast doubt on the -possibility that quite new paths may arise during the individual life, -as is made probable by the recent investigations of Apáthy, Bethe, and -others. - -If the inheritance of acquired characters had already been proved in -other ways, we could not refuse to admit that it might play a part in -the higher animals in the modification and new formation of instincts. -We should then have to admit that habits can be inherited, and that -instincts actually are or may be, as they have often been said to be, -inherited habits. But to make the converse conclusion, and to infer -from the result of the brain-exercise in the individual life and its -similarity to inborn instincts that the latter also depend on inherited -exercise, and that there must therefore be inheritance of acquired -characters, is hardly admissible. - -It might be all very well if there were no other explanation! But as -instincts depend on material brain-mechanisms which are variable, like -every other part of the body, and as, furthermore, they are essential -to the existence of the species, and, down to the minutest detail, are -adapted to the circumstances of life, there is no obstacle in the way -of referring their origin and transformation to processes of selection. - -It has been asserted that the results of training, for instance in -dogs, can be inherited, since the untaught young pointer points at -the game, and the young sheep-dog runs round and barks at the flock -of sheep without biting them. It is, however, often forgotten that, -not only have these breeds arisen under the influence of artificial -selection by Man, but that they are even now strictly selected. My -colleague and friend, Dr. Otto vom Rath, who unhappily died all too -soon for Science, and who was not only a capable investigator, but -an experienced sportsman, told me that huntsmen distinguish very -carefully between the better and the inferior young in a litter, and -that by no means every whelp of a pair of pointers can be used for -hunting game-birds. Lloyd Morgan points out the same thing, and he is -undoubtedly a competent judge in the domain of instinct; he confirms -the statement that the pointer 'often points at the quarry, it may be a -lark's nest, without instruction,' but he says at the same time, that -the power is inborn in very varying degrees, and that, in his opinion, -selection undoubtedly plays a part. - -It must not, therefore, be believed that the habit of the pointer -depends on training; it is only strengthened in each individual by -training, but it depends on an innate predisposition to creep up -to the game, and is thus a form of the hunting instinct. Man has -taken advantage of this, and has increased it, but has certainly not -ingrafted it into the breed by whipping. And something similar will be -found to be true in all cases of so-called inheritance of the effects -of training. It must not be forgotten what astounding results can -be achieved in the individual by training. The elephant is the best -example of this, for it only exceptionally breeds in captivity, and -all the thousands of 'domesticated' elephants in India are tamed wild -elephants. Yet they are as gentle and docile as the horse, which has -been domesticated for thousands of years; they perform all kinds of -tasks with the greatest patience and carefulness, in many cases without -being under constant superintendence. They are indeed animals of great -intelligence; they understand what is required of them, and they -accommodate themselves readily to new conditions of life. - -The attachment of the dog to its master and to Man generally has -often been cited as a proof of the origin of a new instinct by the -inheritance of acquired habitude; but the dog is a sociable animal -even in a wild state, and by living in co-operative association with -Man it has transferred its sociable affections to him. We find exactly -the same thing in the elephant which has been caught wild and tamed. -It is particularly emphasized by those who have accompanied animal -transports in Africa that the young elephants are wild and malicious -towards the blacks who teased and maltreated them, but complaisant and -harmless towards the whites who treated them kindly. The attachment of -elephants to their keepers and to every one who shows them kindness is -familiar enough; it does not depend on a newly acquired impulse, but -on the sociable impulse inherent in the species, which, in the wild -state, causes them to live in fairly large companies, and on their -inoffensive, timid, and, we may almost say, affectionate disposition. - -Of course it is easy enough to give an imaginative theoretical -interpretation of the origin of a new instinct from a newly acquired -habit. We have often heard that sailors have found the birds in distant -uninhabited islands quite free from fear; they let themselves be struck -down with cudgels without attempting to escape. The extermination of -the Dodo three centuries ago is a well-known example of this. Chun, in -his magnificent work on the German Deep Sea Expedition of 1898, has -recently communicated numerous interesting examples of the indifference -of birds towards Man when they have not learned what his presence -means: thus the sea-birds of Kerguelen, penguins, cormorants, gulls, -'kelp-pigeons' (_Chionis_), and others, behaved towards Man very much -like the tame geese of our poultry yards. Even enormous mammals like -the 'sea-elephant,' a seal with a proboscis-like prolongation of the -nose, neither attempted to escape nor showed any hostility to man, but -quietly let itself be caught. Similar tales were told by Steller in -1799, after he had been obliged to pass a winter with his sailors on an -island in the Behring Straits. The numerous gigantic sea-cows (_Rhytina -stelleri_) which lived there were so confiding that they allowed the -boat to come quite up to them, and the sailors were able to kill many -of them from time to time, using their flesh for food. But towards the -end of the winter the animals began to be shy, and, in the following -winter, when other sailors to the polar regions endeavoured to hunt -them too, it was very difficult to secure them; they had recognized -man as an enemy, and fled from him when they saw him from afar. Thus -the same individuals which had earlier carelessly allowed man to come -up to them now avoided him as an enemy. _This was not instinct, it was -a behaviour controlled by the will and founded on experience._ But it -would soon become 'instinctive' if the meeting with the enemy were -often repeated, just like the winding-up of a watch, which is often -done at a wrong time, for instance, on changing clothes during the day, -and thus without reflection. It is quite easy to conceive that if the -material brain-adaptation which causes flight without reflection at -the sight of man were transmissible, the flight-instinct might become -a congenital instinct in the species in question. But this assumption -is unfounded; for, as is shown by the case of the sea-cow, we do not -require it where the animal is of sufficient intelligence to perform by -its own discernment the action necessary to its existence. The action -may thus become 'instinctive' through exercise and imitation in the -_individual_ life, without however attaining to transmissibility. - -But in many cases this is not enough, namely, in all cases in which the -degree of intelligence is not sufficiently high, or where the flight -movement must follow so rapidly that it would be too late if it had to -be regulated by the will, as, for instance, the shutting of the lids -when the eye is threatened, or the flight of the fly or the butterfly -when an enemy approaches. Both fly and butterfly would be lost in every -case if they had voluntarily to set the flight-movement going after -they became conscious of danger. If they had first of all to find -out from whom danger threatened no individual would escape an early -death, and the species would die out. But they possess an instinct -which impels them to fly up with lightning speed, and in an opposite -direction, whenever they have a visual perception of the rapid approach -of any object of whatever nature. For this reason they are difficult -to catch. I once watched the play of a cat, ordinarily very clever at -catching, as she attempted to seize a peacock-butterfly (_Vanessa Io_), -which settled several times on the ground in front of her. Quietly and -slowly she crept within springing distance, but even during the spring -the butterfly flew up just before her nose and escaped every time, and -the cat gave it up after three attempts. - -In this case the beginning of the action cannot lie in a voluntary -action, for the insect cannot know what it means to be caught and -killed, and the same is true of innumerable still lower animal forms, -the hermit-crabs and the Serpulids, which withdraw with lightning -speed into their houses, and so forth. It seems to me important -theoretically, that the same action can be liberated at one time by -the will, at another by the inborn instinct-mechanism. In both cases -quite similar association-changes in the nerve-centres must lie at -the root of the animal's action, but in the first case these are -developed only in the course of the individual life by exercise, while -in the second they are inborn. In the former, they are confined to the -individual, and must be acquired in each generation by imitation of -older individuals (tradition) and by inference from experience, in the -latter they are inherited as a stable character of the species. - -It has been maintained by many that the origin of instincts -through processes of selection is not conceivable, because it is -improbable that the appropriate variations in the nervous system, -which are necessary for the selective establishment of the relevant -brain-mechanism, should occur fortuitously. But this is an objection -directed against the principle of selection itself, and one which -points, I think, to an incompleteness in it, as it was understood by -Darwin and Wallace. The same objection can be made to every adaptation -of an organ through natural selection; it is always doubtful whether -the useful variations will present themselves, as long as they are -due solely to chance, as the discoverers of the selective principle -assumed. We shall attempt later to fill up this gap in the theory, -but, in the meantime, I should like to point out that the process -of selection offers the only possible explanation of the origin of -instincts, since their origin through modifications of voluntary -actions into instinctive actions, with subsequent transmission of the -instinct-mechanism due to exercise in the individual life, has been -shown to be untenable. - -If any one is still unconvinced of this, I can only refer to the cases -we have already discussed of instincts which are only exercised once -in a lifetime, since, in these, the only factor that can transform -a voluntary action into an instinctive one is absent, namely, the -frequent repetition of the action. In this case, if any explanation -is to be attempted at all, it can only be through natural selection, -and as we have assumed once for all that our world does admit of -explanation, we may say, _these instincts have arisen through natural -selection_. - -Even though it may be difficult to think out in detail the process of -the gradual origin of such an instinctive activity, exercised once in -a lifetime, such as that, for instance, which impels the caterpillar -to spin its intricate cocoon, which it makes only once, without ever -having seen one, and thus without being able to imitate the actions -which produce it, we must not push aside the only conceivable solution -of the problem on that account, for then we should have to renounce -all hope of a scientific interpretation of the phenomenon. We may -ask, however, whether there is not something lacking in our present -conception of natural selection, and how it comes about that useful -variations always crop up and are able to increase. - -But if we must explain, through natural selection, such complex series -of actions as are necessary to the making of the cocoon of the silkworm -or of the Saturnia moth (_Saturnia carpini_), what reason have we -for not referring other instincts also to selection, even if they be -repeated several times, or often, in the course of a lifetime? It is -illogical to drag in any other factor, if this one, which has been -proved to operate, is sufficient for an explanation. - -Thus, as far as instincts are concerned, there is no necessity to -make the assumption of an inheritance of functional changes, any more -than there is in regard to any purely morphological modifications. As -the instincts only exercised once show us that even very complicated -impulses may arise without any inheritance of habit, that is, without -inheritance of functional modification, so there are among purely -morphological characters not a few which, though effective, are purely -passive, which are of use to the organism only through their existence, -and not through any real activity, so that they cannot be referred -to exercise, and therefore cannot be due to the transmission of the -results of exercise. And, if this be the case, then transformations -of the most diverse parts may take place without the inheritance of -acquired characters, that is, of functional modifications, and there is -no reason for dragging in an unproved mode of inheritance to explain -a process which can quite well be explained without it. For if any -part whatever can be transformed solely through natural selection, -why, since there is general variability of all parts, should this be -confined to the passive organs alone, when the active ones are equally -variable, and equally important in the struggle for existence? - -There are, indeed, many of these passive parts among animals; I need -only recall the coloration of animals, the whole set of skeletal parts, -so diversely formed, of the Arthropods, the legs, wings, antennæ, -spines, hairs, claws, and so on, none of which can be changed by the -inherited results of exercise, because they are no longer capable of -modification by exercise; they are ready before they are used; they -come into use only after they have been hardened by exposure to the -air, and are no longer plastic; they are at most capable of being -used up or mutilated. Finally, even so convinced an advocate of the -Lamarckian principle as Herbert Spencer has stated that among plants -the great majority of characters and distinctive features cannot be -explained by it, but only through the principle of selection; all the -diverse protective arrangements of individual parts, like thorns, -bristles, hairs, the felt-hairs of certain leaves, the shells of nuts, -the fat and oil in seeds, the varied arrangements for the dispersal -of seeds, and so on, all operate by their presence alone, not through -any real activity which causes them to vary, and the results of which -might be transmitted. An acacia covered all over with thorns seldom -requires to use its weapons even once, and if a hungry ruminant does -prick itself on the thorns it is only a few of these which are thus -'exercised,' the rest remain untouched. - -But since all these parts have originated notwithstanding their -passivity, there must be a principle which evokes them in relation to -the necessities of the conditions of life, and this can only be natural -selection, that is, the self-regulation of variations in reference to -utility. And if there is this principle, we require no other to explain -what is already explained. - -I can quite well understand, however, that many naturalists, and -especially palæontologists, find it difficult to accept this -conclusion. If we think only of those parts that actively function, -and thus change by reason of their function, being strengthened by use -and weakened and diminished in size by disuse, and if, further, we -follow these parts through the history of whole geological epochs, we -may certainly get the impression that the exercise of the parts has -directly caused their phyletic evolution. The direction prescribed by -utility in the course of the individual life and in the phylogeny is -the same, and the intra-selection, that is, the selection of tissues -within the individual animal, leads towards the same improvements -as the selection of 'persons.' Thus it appears as if the phyletic -variations followed those of the individual life, while in reality the -reverse is true; the changes arising from variations in the _germ are -primary, and they determine the course of phylogenesis_, while the -tissue-selection in the individual life only elaborates and improves, -according to the demands made upon it, the material afforded by the -primordial equipment of the germ. - -The American palæontologist, Osborn, cites the case of the horse's -feet as an example in support of his view that modification brought -about by use in the individual life must be transmitted in order that -the phyletic transformations may be brought about, but this example -is perhaps the best that could be chosen to prove the contrary. He -supposes that, in every young horse, the means of locomotion are -improved at every step, so to speak, through the contact with the -ground, and I am quite willing to admit that this is so. But that only -proves that, even now, an elaboration and improvement of the equipment -which the germ affords is indispensable, as it has been at all times -and in all animals, and thus that, notwithstanding the enormous number -of generations which our modern horse has behind it, the functional -acquirements of the individual have not yet been impressed upon the -germ. Why not? Because the horse becomes perfect without this, and -there was no reason why personal selection should perfect the primary -constituents of the germ still further, since the finishing touch of -perfection through use is readily afforded by the conditions of each -individual life. - -Moreover, when Osborn, Cope, and other palæontologists emphasize that, -in phyletic evolutionary series, _definite paths of evolutionary_ -change are adhered to, and are not deviated from either to right or to -left, they are undoubtedly right, but the conclusion which they draw is -not justifiable, whether they assume with Nägeli that there is a power -of development, a principle of perfecting, or whether, as Osborn does, -they assume the transmission of the modifications brought about through -use in the individual life. There remains a third possibility, that -the quiet and constant evolution in a definite direction is guided by -selection, and as, in passively useful parts, that principle alone is -admissible, I see no justification for assuming it to be inoperative -in regard to those which are actively functional. All these variations -which have led up, for instance, to the modern form of the horse's foot -are useful; if they were not, they could not have been produced either -by use or by disuse in the individual life. - -At the same time, here again, we are justified in inquiring whether the -assumption of 'chance' germinal variations, which we have hitherto made -with Darwin and Wallace, affords a sufficient basis for selection. -Osborn says very neatly in this connexion, 'We see with Weismann and -Galton the element of chance; but the dice appear to be loaded, and in -the long run turn "sixes" up. Here arises the question: What loads the -dice?' - -Until recently we might have answered, 'external conditions'; it is -they that load the dice one-sidedly, and condition that the same -straight path of phylogenesis is adhered to, and exactly the same -direction of variations is preferred and maintained. It has to be -asked, however, whether this answer, which is certainly not absolutely -incorrect, is sufficient by itself, whether the dice are not falsified -and one-sidedly loaded in another sense, so that they always throw a -preponderating number of the useful variations. We shall attempt very -soon to solve this problem, but in the meantime I must refer to another -argument in favour of assuming the Lamarckian principle, perhaps -the most important and it may be thought the most difficult of all -to refute, the so-called co-adaptation of the parts of an organism, -that is, the fitting together of many individual organs for a common -purposeful functioning. - - - - -LECTURE XXIV - -OBJECTIONS TO THE THESIS THAT FUNCTIONAL MODIFICATIONS ARE NOT -TRANSMITTED - - Giant stag as an example of co-adaptation or 'harmonious - adaptation'--This occurs even in passively functioning parts--Skeleton - of Arthropods--Stridulating organ of ants and crickets--Limbs - of the mole-cricket--Wing-venation--Colorations which form - mimetic pictures--Harmonious adaptations in worker-bees and - ants--Degeneration of their wings and ovaries--The quality of food - acts as a liberating stimulus--Vom Rath's case of drones fed with - royal food--Transition-forms between females and workers--Wasmann's - explanation of these--The Amazon ants--Two kinds of workers--Appendix: - Zehnder on the case of ants--On the skeleton of Arthropods--Hering's - interpretation of Ehrlich's Ricin experiments--Hering's position in - regard to the transmission of functional modifications. - - -It was Herbert Spencer, the English philosopher, who first brought -the argument of co-adaptation into the field against my view of the -non-inheritance of functionally acquired modifications. He pointed -out that many, if not, indeed, most modifications of bodily parts, to -be effective, implied further changes, often very numerous, in other -parts, and these latter must therefore have changed _simultaneously_ -with the part which was being changed under the control of natural -selection; this, however, is only conceivable as due to an inheritance -of the changes caused by use, since a simultaneous alteration of so -many parts through natural selection would be impossible. If, for -instance, the antlers of our modern stag were to grow to the size of -those of the Giant Stag of the Irish peat-bogs, which measured over -ten feet across from tip to tip, this would mean--as has already been -shown--a simultaneous thickening of the skull, and to bear the heavy -burden, a strengthening of the _ligamentum nuchæ_, of the muscles of -the neck and back, of the bones of the legs and their muscles, and, -finally, of all the nerves supplying the muscles; and how could all -this happen simultaneously with, and in exact proportion to the growth -of the antlers, if it depended--as natural selection assumes--on chance -variations of all these parts? What if the appropriately favourable -variation in one of these organs did not occur? A harmonious variation -of all the parts--bones, muscles, nerves, ligaments--which unite in a -common activity, is an inadmissible assumption, because, in many cases, -such co-operating groups of organs have in the course of evolution -developed in opposite directions. In the giraffe, for instance, the -fore-legs are longer than the hind-legs, which is the reverse of what -obtains in the majority of ruminants; in the kangaroo the hind-legs, -on the contrary, have developed to a disproportionate size, while -the fore-legs have degenerated into relatively small grasping arms. -Co-operating parts, like the fore and hind limbs, may thus follow -opposite paths of evolution; their variations need not always be -directed to the same end. - -The difficulty presented by these so-called co-adaptations or -harmonious correlations cannot be denied, and we must also admit -that, if the results of exercise were inherited, the explanation of -the phenomenon would, in many cases--but not, indeed, in all--be -easy, because the adaptation of the secondarily varying parts in each -individual life would correspond exactly to the altered function of -the part, and would be transmitted to the descendants, and in them -would again be subject to such a degree of variation, according to the -principle of histonal selection, as might be conditioned by the further -progress of the primary variation. The simplicity of the explanation -is striking, if only it were at the same time correct! But there are -whole series of facts, or rather of groups of facts, which prove that -the causes of co-adaptation do not lie in the inheritance of functional -modifications, and this must be recognized, even though we may not -yet be in a position to state the causes of co-adaptation, and to say -whether natural selection suffices to explain it or not. - -I must first point out that co-adaptations occur not only in _actively, -but also in passively functioning parts_. Very numerous instructive -examples are to be found among the Arthropods, whose whole skeleton -belongs to this category. It has been objected that this is not wholly -passive, but that, like the bones of vertebrates, it is stimulated by -the contraction of the muscles and incited to functional reaction, -and that it thickens at places where strong muscles are inserted, and -becomes or remains thin where it is not exposed to any strain from -the muscles. But this is not the case, for the chitinous skeleton -can only offer resistance to the muscular contractions when it is no -longer soft, as it is immediately after it is secreted. As soon as it -has become hard, it can no longer be altered, and can at most be worn -away externally by long use. The proof of this lies in the necessity -for moulting, which is indispensable to all Arthropods as long as they -continue to grow, but does not occur later. Every one who has followed -the growth of an insect or a crustacean knows well that the moultings -or ecdyses are often accompanied by great changes, and hardly ever -occur without some slight changes in the form of the body, especially -of the limbs, with their teeth, bristles, spines, and so on. These -new or transformed parts are formed before the throwing-off of the -old chitinous shell, and under its protection, and they are brought -about by an elaboration or transformation of the living soft matrix -of the skeleton, the hypodermis, which consists of cells, and is the -true skin. They must thus have arisen in the ancestors of our modern -Arthropods in the same way, that is, not by a gradual modification -_during_ use, but by a slight sudden transformation before use. The -steps in the transformation may have been very small, a bristle may -have become a little longer in the second stage of life than it was -in the first, or instead of five bristles a particular spot may bear -six in the second or third stage of life; but the variations in the -phyletic development must always be caused by germ-variations which -effect from within the variation in the relevant stage of development. -But the part which has varied can only function after it has become -firm and immodifiable. - -If these circumstances be kept clearly in mind, they furnish a quite -overwhelming mass of proof against the views of the Lamarckians. - -Furthermore, it is not even true that the thickest parts of the -external skeleton are those at which the muscles are inserted. The -wing-covers of beetles offer the best proof to the contrary, for -there are no muscles at all in them, yet they are, in many species, -the hardest and thickest part of the whole chitinous coat of mail. -The reason is not far to seek; they protect the wings and the soft -skin of the back, which lies concealed beneath them, and the muscles -are inserted in this!--a relation which can be explained only by its -suitability to the end, and not as due to any direct effect. - -When we remember the origin--which we have just described--of the -external skeleton from the soft layer of cells underneath it, the -thickness of the chitinous skeleton, which is very different at -different places in the same animal, but always adapted to its end, -furnishes a case of co-adaptation in parts which have a purely passive -function. The thickened part cannot be due to the insertion of a -muscle, but it is always there in advance, from internal causes, so -that the muscle finds sufficient resistance. Close to it there may lie, -perhaps, the edge of a segment, and at this spot the chitinous skeleton -becomes almost suddenly thinned to a joint membrane capable of being -bent or folded, not _because_ there was no pull from the muscles at -this spot, but in order that the two segments may be connected movably. -Thus, nowhere in the whole body of the Arthropod can the adaptation -of the skeleton, in regard to thickness and power of resistance, be -regulated by function itself, but only by processes of selection -which imparted to each spot the thickness it required, in order to be -effective in its function, whether that be offering resistance to the -strain of the muscles, or giving suppleness to a joint, or affording -the necessary hardness for biting the prey, or for boring into wood or -earth, or merely for protecting the animal from external injuries. - -There are, however, many individual functions of the Arthropods the -exercise of which depends on the simultaneous change of several -skeletal parts; as, for instance, many of the 'singing' or vocal -apparatuses in insects. In quite recent times such vocal organs have -been discovered in ants, in which they consist of a small striated -region on the surface of the third abdominal segment, and a sharp ridge -on the segment in front; the latter is rubbed against the former by the -movements of the two segments. Quite a similar 'stridulating organ' has -long been known in the bee-ant (_Mutilla_), and the whistling sound -produced by it is easily heard by our ears; moreover August Forel has -heard it in the large wood-ant (_Camponotus ligniperdus_), and has -described it as an 'alarm-signal,' which the animals give each other -on the approach of danger--an observation which has recently been -confirmed by Wasmann and extended by Robert Wroughton in regard to -Indian ants. All these arrangements for producing sound depend always -on two organs, of which one resembles the bow, the other the strings -of a violin; the one is of no value without the other, and they must -therefore have developed simultaneously, yet they cannot have arisen -through use, and the inheritance of the results of use, because they -are both dead chitinous parts, which are never strengthened by rubbing -against each other with the movements of the abdomen, but are rather -worn away. - -[Illustration: FIG. 91 (repeated). Hind-leg of a Grasshopper -(_Stenobothrus protorma_), after Graber. _fe_, femur. _ti_, tibia. -_ta_, tarsal joints. _schr_, the stridulating ridge.] - -The same is true of the chirping organs of grasshoppers, beetles, -and crickets; in all cases they consist of two different parts, -which together produce a sound, and which therefore must have arisen -simultaneously, and the origin of which cannot be referred to the -inheritance of the results of exercise, but rather to selection. It is -thus possible that co-adaptation of at least two parts may take place -even when the hypothetical Lamarckian principle is altogether excluded. - -When I say that we have here a case of two parts adapted to each -other, that is, strictly speaking, understating the case, for, in the -crickets and locusts, for instance, there is a whole series of peg-like -chitinous papillæ (Fig. 86), the so-called 'bridge,' each of which -must have arisen by itself through variation of the corresponding -spot of skin. At least I can see no ground for the assumption that -the chitinous surfaces on which the 'bridge' is now placed would -necessarily, from internal reasons, have varied precisely in the line -of the bridge as it has done. - -[Illustration: FIG. 102. Brush and comb on the leg of a Bee (_Nomada_). -_tib_, end of the tibia. _t^1_, first tarsal joint with the brush and -its comb (_tak_). Between these and the tibial spine (_tisp_) with its -lappet (_L_) the cross-section of an antenna (_At_) is indicated. Drawn -from a preparation by Dr. Petrunkewitsch.] - -Instructive examples of the co-adaptation of several parts to a common -action in organs which are not subject to the Lamarckian principle -are afforded by the diverse arrangements for cleaning the antennæ the -bearers of the smelling-organ which are so important to the life of -insects (Fig. 102). Here even the adaptation of an indented area on -the tibia of the anterior leg to the cylindrical form of the antenna -which passes through it, is sometimes so striking (Fig. 102, _tak_) -that it might be thought that it must have arisen through a gradual -wearing out; yet this is impossible, since we have to do with hard -dead chitinous surfaces, and moreover not with a solid mass, like a -hone, which is worn down by the knife, but with a hollow, thin-walled -tube. In ants, bees, and ichneumon-flies this minute, semi-circular -indentation contains small, pointed, triangular saw-teeth, closely -set like those of a comb (_tak_), and the apparatus is made usable -by the fact that a firm spine (_tisp_), fused to the end of the -tibia, overhangs the notch and presses the antenna towards it. In -many species this spine is double, or it is furnished with a thin -comb or lappet (Fig. 102, _L_), or with rows of teeth, or with short -bristles; in short, it may be equipped in the most different ways. Not -infrequently, as in wasps of the genera _Sphex_, _Scolia_, _Ammophila_, -the spine itself is also bent in a semicircle on the surface directed -towards the notch, and this may be effected in very different ways, -either by a bending of the whole thickness of the spine, or by the -presence of a comb which is concave on its inner surface. I should -never come to an end if I were to enumerate all the remarkable details -which may be found in the two main parts of this apparatus, and which -show very clearly how essential a co-operation of the two is in -fulfilling the function of cleaning the antennæ. This fitting together -of the two main parts cannot have been brought about in accordance with -the Lamarckian principle; the adaptation must therefore have come about -in some other way. - -The same thing is shown by the legs and other appendages of insects and -crustaceans, which are adapted for the most diverse functions, and the -individual sections of which must be correlated if the function is to -be possible. Let us consider only the claw structures in crustaceans -and scorpions. Here, too, it seems as if the outgrowth of the last -joint of the leg, which functions as the arm of the claw, must have -arisen as a direct effect of use, through the pressure of an object -held fast by the last joint, the movable half of the claw. Frequently, -moreover, tooth-like protuberances occur on the fixed blade of the -claw (Fig. 103). But how could these have arisen as a direct effect -of pressure, since they are always preformed during the soft state of -the appendage _before use_, and are only made use of after it is fully -hardened. The soft crustaceans, the so-called 'butter-crabs' which -have just cast their shells, creep carefully away and avoid using -their limbs until they have become hard again. Here, too, we have the -co-adaptation of two parts which vary independently, and which cannot -be affected by the Lamarckian principle. - -[Illustration: FIG. 103. Claw (_Sch_) on the leg of a 'Beach-fly,' an -Amphipod Crustacean (_Orchestia_). _I_, _II_, the two first joints. -_uA_, the lower blade of the claw, a non-mobile prolongation of the -penultimate joint. _oA_, the upper blade of the claw, the movable -last joint; the tubercles and indentations of the two blades fit one -another. After F. Müller.] - -But the appendages furnish more complex examples of mutual adaptation. -Thus the individual sections of the anterior leg of the mole-cricket -(_Gryllotalpa_) have varied greatly, yet quite differently, and the -whole together forms a most effective digging-tool. With it the animal -digs out the earth before it to right and to left, and to do this -it makes with both legs simultaneous outward movements, which are -otherwise quite unusual among insects, and does so with such strength -that Rösel von Rosenhof saw two bodies each weighing three pounds -pushed away in this manner. In this case four chief parts of the leg -(Fig. 104), the coxa (_cox_), the femur (_fe_), the tibia (_tib_), -and the tarsi (_tars_) are so adapted to each other in form, joints, -thickness of skeleton, and size, that they cannot have varied otherwise -than in relation to each other, but each piece has done so in an -individual manner. Most remarkable of all is the short broad tibia, -equipped with four large, hard teeth, which has to perform the digging -in the ground after the manner of a spade, while the disproportionately -thin and weak tarsal joints, the last of which bears two perfectly -straight spines instead of claws, are directed upwards, and do not -touch the ground, being no longer used for walking. Rösel supposed, -probably correctly, that they are used for cleaning the spade when it -becomes clogged up with earth, since the animal cannot clean it with -its mouth. These quite unusually formed parts of the limb cannot have -become what they are as the direct results of use, because, for one -thing, it would have been not their broad surfaces, but their narrow -edges, which would most easily cut through the earth, that would have -been directed outwards. The peculiar curving, first concave, then -convex, of the outer surface of the digging foot is exactly what is -best adapted for cutting into the earth and for the pushing aside which -follows, but it is not what it would have become if the chitin-wall had -yielded to the pressure of the earth and adapted itself to it. But, -as we are again dealing with the chitinous skeleton, there can be no -question of the direct effect of use, and, it seems to me, it must be -admitted that here we have a case of co-adaptation of at least seven -different parts, which have varied independently of each other, without -any assistance from the Lamarckian principle. - -[Illustration: FIG. 104. Digging leg of the Mole-cricket -(_Gryllotalpa_). _cox_, coxa attaching the limb to the thorax. _fe_, -the short broad femur. _tib_, the tibia forming a broad spade with six -large sharp teeth. _tars_, the tarsal joints, which are turned upwards -and cannot be used in locomotion. After Rösel.] - -But much more complicated cases than this might be cited, if we were in -a position to estimate exactly the functional value of the individual -parts of the wing-venation in the different insects, for it is well -known that this venation serves the systematist as a basis for the -definition of genera, especially in Lepidoptera and Hymenoptera. That -is to say, it varies from genus to genus in a characteristic manner, -obviously corresponding to the differences in the wing-form, and in -the flight itself. But, unfortunately, we are still far from being -able to make more than quite general hypotheses as to the meaning of -the lengthening and strengthening, or conversely, the degeneration or -elimination, of this or that vein. From extreme cases, however, as for -instance the rich venation in good fliers with large wings, and the -scanty venation in poor fliers with small wings, we learn at least so -much, that the degree and even the manner of venation bears a definite -relation to the function of the wing, and this we might have assumed. -But these wing-veins, in as far as they serve as a support for the -weak wing-membrane, are purely chitinous structures, skeletal parts -which are not even renewed from time to time like the skeletal parts -of the leg and many other parts of the insect. As they are laid down -at first in the pupa as soft strings of cells, so they remain, and -they only begin to be used when they are completely hardened. _They -can therefore never have been caused to vary through use in the course -of the phyletic development of species and genera_, and the Lamarckian -principle can have no part in their transformations. But if they follow -the most subtle changes, which we cannot precisely demonstrate, of the -whole wing-surface and in the mode of flight, as a man is followed by -his shadow, there must be some other principle which adapts the organ -to its function, and which is able continually to adapt the large -number of individual wing-veins in the manner most advantageous for the -general function. Here, therefore, we have a state of matters exactly -corresponding to that obtaining in the transformation of actively -functioning parts which form a system with common co-operative action, -as, for instance, in the case we first discussed, that of the stag's -antlers. - -Other even more complicated examples of harmonious adaptation of -passively functioning parts are afforded by the markings of animals, -such as those of the butterfly's wing. The colours have only a passive -rôle, whether they be due to pigments alone, or to structure, or to -both combined. When the coloration of a surface undergoes adaptive -variation, this cannot be due to any action of the colour, but must -depend on adaptation through selection. Yet it is well known that there -are many butterfly-wings whose surfaces exhibit different colours and -different shades of colour on their different parts, and that in such -a way that together they form a picture, that of a leaf, a piece of -bark, a stone overgrown with lichen, an eye, and so on. In such a case -the individual colour-spots stand in a particular, indirect relation -to each other; although they are independent of each other in their -variation, they are not indifferent and due to chance, for together -they produce a common picture; this is harmonious adaptation of many -parts, where the Lamarckian principle is absolutely excluded. - -It may, perhaps, be objected that this mimetic picture does not -arise all at once, but very slowly in the course of long series of -generations, and, indeed, of species. This must of course be so; the -simple beginnings are complicated and perfected through the course -of long ages. This is implied in the principle of selection as we -understand it. But does any one suppose that the gigantic antlers of -the giant-stag were developed in a few generations? In this case, too, -must not numerous races have succeeded each other before the primitive -antlers attained this enormous size? If this must be assumed there was -abundance of time for the adaptation, through _germinal_ variations, of -the secondarily varying parts, the muscles, tendons, nerves, and bones, -for all these parts function actively, and can without difficulty meet, -in the individual life, the increased claims made upon them by a slight -increase in the size of the antlers. For the certain and indubitable -consequence of exercise, of increased use, is the strengthening of the -functioning parts. - -Thus the appropriate germinal variation of the secondarily varying -parts may be delayed for a little without the individual being any -the less effective, or being obliged to succumb in the struggle for -existence. I do not, however, assert for a moment that the whole -explanation of the phenomena of co-adaptation is included in this; -on the contrary, I hope soon to be able to show that we may in such -cases assume a preponderance of variational tendencies in a favourable -direction, and that there is thus an indirect connexion between -the utility of a variation and its actual occurrence. In the first -place, however, I must refer to the other group of facts which I have -indicated, which show, likewise, that the simultaneous co-adaptation -of different parts may arise in certain circumstances, although the -Lamarckian principle be excluded. These are the facts presented to us -by the sterile forms of those insects, which, like bees, termites, and -ants, live together in large societies. - -Ants and bees are of special interest to us in this connexion, because -they have long been carefully watched by a number of distinguished -naturalists, and most of their vital functions have been precisely -studied. Ever since the days of 'Old Peter Huber' in Geneva there have -again and again been excellent observers who have devoted almost the -whole of their life-work and talents to the more complete study of -these wonderful animals. These insects are of interest to us here, -because, in the course of the social life, a type of individual has -arisen which diverges in structure in many parts of the body from -both the male and the female, although it is sterile and does not -reproduce, or does so in so few instances that the fact is of no moment -in considering the origin of the present bodily structure. As is well -known, these neuters, or better, workers, are, among ants and bees, -females which differ from the true females not only in their smaller -size and their infertility, but in many other points as well. Among -ants, for instance, they are absolutely wingless, and at the same time -they have a much smaller and differently formed thorax and a larger -head. But the most striking point is the difference in their instincts, -for while the females, concerned only with reproduction, pair and -lay eggs, it is the workers who feed and clean the helpless emerging -larvæ, and put them in places of safety, who carry the pupæ into the -warm sunshine, and afterwards back again to the sheltered nest, who -make this nest itself, and keep it in order, after having collected or -prepared the material for it; it is they alone who defend the colony -against the attacks of enemies, who undertake predatory expeditions, -attacking the nests of other ants, and engaging in obstinate combats -with them. - -How can all these peculiarities have arisen, since the workers do not -reproduce, or do so only exceptionally, and, in any case, are incapable -of pairing, and therefore--among bees at least--only produce male -offspring? Obviously it cannot have been through the transmission of -the effects of use and disuse, since they leave no offspring to which -anything could be transmitted. - -Herbert Spencer has attempted to maintain the position that the -characters of the workers of to-day already existed in the pre-social -state, that is, before the ants began to form colonies, and that, -therefore, they have not been newly evolved but only preserved. But, -even if this be conceded in regard to the care of the brood and the -building instinct, so much remains that could not have existed at that -stage, that the problem of the origin of these new characters remains -unsolved. The wings, for instance, among ants, can only have been lost -when females appeared which did not reproduce, for the pairing of ants -is associated with a nuptial flight high in the air. The wings are not -merely absent in the workers, they do not even develop in the pupæ; -they are, as Dewitz showed, present even now in the larva in the form -of imaginal disks, but from the pupa-stage onwards they degenerate, and -the segments of the thorax to which they are attached likewise appear -small and modified. A variation of the germ-plasm must therefore have -taken place, and to this is due the fact that the wing-primordia no -longer develop, and that the thorax has a different development from -what it had at the time when the animals were still fertile. - -It has indeed been said that there is no need for assuming a variation -of the germ-plasm, since the degeneration of the wing might be produced -by inferior nourishment. This opinion is based on the fact that, among -bees, the workers do actually arise from female larvæ which have -received a meagre diet poor in nitrogenous elements, while the same -female larvæ supplied with an abundant diet rich in nitrogen develop -into queens. - -But even though we may assume that there is a similar difference in the -mode of feeding among most ants, because the workers are considerably -smaller than the fertile females, it would be quite erroneous to -conclude that the difference between the two types rests solely on the -effect of differences in diet. The elimination of an individual organ -has never yet been determined by bad and scanty nourishment; it is the -whole animal with all its parts that degenerates and becomes small and -weakly. Often as caterpillars of different species have been placed -on starvation diet, whether for experimental purposes or to procure -very small butterflies, it has never yet happened that a single organ, -such as antenna, leg, or wing, has thereby been eliminated or caused -to degenerate. I have myself instituted many such experiments with the -maggots of the blue-bottle fly, by supplying them from their earliest -youth with just as little food as possible without actually starving -them to death, yet never have these larvæ given rise to flies in which -the wings were absent or rudimentary. - -Nor did these starved flies ever exhibit degenerate ovaries; they -were always completely developed and equipped with the full number -of ovarian-tubes. It was to decide this particular point that -these experiments were instituted, for my opponents maintained -that degeneration of the ovaries was a direct result of inferior -nourishment. But that is not the case. Special investigations in regard -to ants, undertaken at my request by Miss Elizabeth Bickford, showed -that the anatomical results reached by earlier investigators, like -Adlerz and Lespès, in regard to the degeneration of the ovaries in -workers, were absolutely correct, and that the 'degeneration' consists -not merely in the fact that the ovarian-tubes and ovum-primordia remain -small, but also in a diminution of the _number_ of ovarian-tubes (Fig. -105); the workers have always fewer ovarian-tubes than the females of -the same species, and--what is of especial importance--the reduction -in the number of ovarian-tubes has been effected to a different extent -in different species of ants. In the red wood-ant (_Formica rufa_) the -workers still possess from twelve to sixteen ovarian-tubes; in the -meadow-ant (_Formica pratensis_) only eight, six, or four; in _Lasius -fuliginosus_ there are usually only two (one on either side); and in -the little turf-ant (_Tetramorium cæspitum_) there are none at all. We -have here, therefore, a phylogenetic process of degeneration, which -has reached different degrees in the different species, and has only -been completed in one (_Tetramorium_). The case stands as I previously -stated it: 'The elimination of a typical organ is not an ontogenetic -process, but a phylogenetic one,' it depends not upon 'the mere -influences of nutrition which affect the development of the individual, -but always on variations in the germ-plasm, which, to all appearance, -can only come about in the course of a long series of generations'[14]. - -[14] _Aeussere Einflüsse als Entwickelungsreize_, Jena, 1894. - -[Illustration: FIG. 105. Ovary of a fertile Queen-Ant and ovaries of -a Worker. _Od_, oviduct. _A_, one ovary of _Myrmica lævinodis_ with -many ovarian-tubes, in each of which there is an almost ripe egg (_Ei_) -and a younger egg (_Ei´_). _B_, the ovaries of a Worker of _Lasius -fuliginosus_; each ovary has only one ovarian-tube, and no ripening -egg-cells. After Elizabeth Bickford.] - -Against this proposition an observation by O. vom Rath has been -cited. According to it, three drone larvæ which had been accidentally -fed by the workers with royal food exhibited striking retrogressive -peculiarities in their sexual organs. The testes contained only -immature sperms (just before emergence from the pupa), and the -copulatory organ was entirely wanting. That a certain degree of fatty -degeneration of the testes should be caused by the 'unusual fattening' -is not surprising, but it seems to me very questionable whether the -absence of the copulatory organ can be referred to the abnormal -diet; it ought to be definitely decided, by the investigation of -numerous cases, whether some abnormal peculiarity in the constitution -of the germ-plasm in these eggs was not the true cause. Hitherto, -unfortunately, I have not been able to procure the fresh material -necessary to decide this point[15]. From all this it must be evident -that we are not justified in regarding either the absence of wings or -the degeneration of the ovaries as a direct result of the inferior -nourishment supplied to the workers in the larval state: but should -any one still have doubt on this point I may mention that, among our -indigenous ants, there are two species in which the workers are just as -large as the fertile females, and that in tropical America a species -(_Myrmecocystus megalocola_) occurs in which the workers are larger -than the true females; this must mean that they have received more food -than the females, though perhaps not the same mixture of food. - -[15] Since completing my manuscript I note that the point was settled -three years ago, when Koshewnikow had the opportunity of investigating -drone-pupæ which were abnormally reared in royal cells, and therefore -fed with royal food. He found their sexual organs perfectly normal, and -agrees with me that the abnormalities in Vom Rath's case must have been -due to some other cause. (See the report by Von Adelung on the Russian -paper in _Zool. Centralblatt_, Sept. 10, 1901.) - -From all the facts we have discussed we can confidently conclude that -the differences in structure, which distinguish the workers from the -true females, do not depend upon the influence, in the individual -lifetime, of a poorer diet, but upon variation in the primary -constituents of the germ; we must conceive of the germ-plasm of ants as -containing, in addition to male and female ids, special ids of workers, -in which the determinants of wing and ovary are degenerate in some -degree, while the determinants of other parts, such as the brain, are -more highly developed. The manner of feeding, however, and perhaps the -mingling with the food of a special secretion of the salivary glands, -acts as a stimulus which determines whether one kind of id or another -is to be liberated, that is, to become active and to enter on the path -of development. - -A proof of this view is to be found, it seems to me, in the existence -of transition forms between workers and true females, which was first -brought to general knowledge by Forel. Perhaps it would be better to -call these 'mongrel forms' for their various parts do not maintain a -medium between the two types, but many parts follow the type of the -worker, and others that of the true female. Thus Forel twice found a -nest of the red wood-ant which contained a large number of these mixed -forms, all of which possessed the small head and large curved thorax of -the queen, but otherwise resembled the workers in size and appearance, -and also in the degeneration of the ovaries. Many of them were very -small, only 5 mm. in length, and had probably received very little -food, and, according to the theory of direct influence, these should -have been pure workers. That they possessed the head and thorax of a -queen is a proof that the characters of both forms of individual were -present in the germ-plasm as primary constituents, or indeed entire -ids. In normal circumstances only one kind of these ids would have -become active, either the worker-id or the queen-id, but in abnormal -circumstances they might both be liberated to activity simultaneously, -and then they would stamp one part of the body with the character of -a queen, another with that of a worker. Forel observed one of these -nests in two successive years, and both times found the mixed forms -in large numbers[16]. In the second year he found a great number of -newly-emerged individuals of this type. I have already inferred from -this observation that the mixed forms were probably in both years -the offspring of the same mother, and this may well have been the -case. My further conclusion, that the mixed forms must be due to some -abnormality in the constitution of the germ-plasm of the maternal eggs, -no longer appears to me so convincing as it did formerly, because, in -the interval, we have learnt, through that indefatigable investigator -of ants, Pater Wasmann, that there is another possible explanation of -these mixed forms; it, too, is based upon a hypothesis, but it is so -interesting that I must briefly outline it to you. - -[16] There are different kinds of 'mixed forms' among ants, which may -owe their origin to a variety of conditions, as Forel, Wasmann, and -Emery have shown in detail. - -Like Forel and myself, Pater Wasmann had supposed that the reason of -this kind of mixed form (the so-called pseudogynous worker) lay in an -abnormality of the constitution of the germ-plasm, but he now regards -it as the result of a change in the mode of rearing instituted by the -workers with respect to the constitutionally female or queen larvæ, -because there was a scarcity of workers. The hypothesis sounds very -daring, but it is well founded, at least in so far that there really -is a reason why a scarcity of females must occur at certain times in -some colonies of ants, and this might certainly determine the workers -in charge of the larvæ to feed females with worker food, so as to rear -them to render the necessary assistance. - -This reason lies in the occasional presence of a parasitic beetle, -_Lomechusa strumosa_, whose larvæ, curiously enough, are cared for and -fed by the ants as though they were their own, and in return they eat -up the larvæ of the ants, often destroying them in large numbers. -Wasmann informs us that the parasitic larvæ grow up just at the time at -which the ants are rearing their workers, and it is these, therefore, -which fall victims to the Lomechusa-larvæ, and the result is that a -scarcity of young workers must soon make itself felt. The workers -seek to make this good by rearing as workers all the larvæ previously -destined for queens. But this only succeeds partially, because the -development towards true females has already begun; thus mixed forms -arise. - -This explanation would be rather in the air if we did not know that, -among bees, such changes in the manner of rearing are by no means -uncommon. Indeed they occur regularly when the queen of a hive perishes -and no more 'female' eggs are in store; young worker larvæ are then fed -with royal food, and these develop into queens. There can thus be no -doubt that these insects have it in their power to liberate to activity -either the female ids or the worker ids by a specific mode of feeding, -and there is nothing contrary to reason in admitting the possibility -of an alternation of this influence in the course of development, for -something analogous occurs in regard to secondary sexual characters, -as, for instance, the appearance of male decorative colours in ducks -that have become sterile. - -But this change in the mode of rearing bee-larvæ gives rise to pure -queens and not to mixed forms, and we must therefore regard it as -undecided whether Wasmann's explanation is correct in this case, and -whether an abnormality in the constitution of the germ-plasm may not be -the true cause of this or other kinds of mixed forms among ants. In any -case the 'Lomechusa hypothesis' rests upon the assumption of different -kinds of ids in the germ-plasm, as Pater Wasmann expressly states, and -the differences between the worker and queen-ants have their cause in -this, and not directly in the kind of larval food. - -If there were not different ids corresponding to the different kinds -of individuals in the germ-plasm a kind of polymorphism might indeed -have arisen in the colony through differences in nutrition, but it -could not have been of the kind we now see--that is, a sharply defined -differentiation of persons, in adaptation to their different functions. -This presupposes elements in the germ which can vary slowly and -consistently in a definite direction without causing any change in the -rest of the germ. - -This state of affairs gives to the phyletic evolution of the workers -a great theoretical significance, for it proves that positive as -well as negative variations of the most diverse parts of the body, -that simultaneous and correlative variations of many parts, can take -place in the course of the phylogeny, without the co-operation of -the Lamarckian factor. I have not hitherto laid any special emphasis -upon the degree of differences occurring between workers and queens; -but I must now add that this may far exceed the degree that we are -familiar with in our common indigenous ants, both in regard to -instinct and to bodily form. Even in the red Amazon ant of Western -Switzerland, _Polyergus rufescens_, we find quite a new instinct[17], -that of carrying off the pupæ of other species of ants, not to devour, -but to introduce them to their own nest and thus secure 'slaves.' -For these workers of a strange species, which emerge in a strange -nest, naturally regard the place of their birth as their home, and -do there what instinct impels them, and what they would have done in -the nest of their parents: they feed the larvæ, fetch food, collect -building material, and so on. The domestic activity of the workers -of the master-species thus becomes superfluous, and they have ceased -to exercise it, and have now entirely lost the power of caring for -their brood, searching for food, and keeping up the nest. They have -even forgotten how to take food themselves, because they are always -fed by the 'slaves.' Forel informs us--and I have myself repeated the -experiment--that _Polyergus_ workers, which are shut up with a drop -of honey on the floor of their prison, will leave it, their favourite -food, untouched, and finally starve, unless one of their 'slaves' be -shut up with them. As soon as this happens, and the slave perceives the -honey, it partakes of it, and then the 'mistress' comes and strokes the -'slave' with her antennæ to signify her desires, whereupon the 'slave' -proceeds to feed her from its own crop. - -[17] 'New' in this sense, that the instinct is not exhibited by most -worker-ants, that it did not occur in the primaeval ancestors of modern -ants. It is, however, exhibited by a number of modern forms, and even -by some German species. - -But while the _Polyergus_ workers have forgotten their domestic habits, -and have even ceased to be able to recognize their food, remarkable -changes have taken place in their jaws; these have lost the blunt teeth -on the inner margin, which, in other species, serve for masticating -the food, for seizing building material, and for other domestic -occupations, and have become sharp weapons, bent in the form of a -sabre, very well suited for piercing the head of an enemy, but also -well adapted for carrying off the pupæ, because they can seize them -without doing them any injury. - -No one will doubt that the predatory expeditions of the Amazon ants, -and the slave-making habit, can only have developed after the habit -of living in large companies had long existed, and this case proves -that variations of instinct, as well as of bodily structure, can -take place even after the workers have long been sterile. The case -is the more instructive that it _seems_ as if it were due to the -transmission of a newly acquired and inherited habit of life, while -in point of fact these Amazon-workers can transmit nothing, because -they bear no offspring. But if old instincts can be lost, and new ones -acquired, when all possibility of inheritance is excluded, we see that -Nature has no need of the Lamarckian factor of modification for her -transformations and new adaptations. - -If we wish to understand clearly that, in these changes, we have -to do not merely with the alteration of a single part, but of many -parts which all work together, we have only to think of the still -more striking physical modifications which have taken place in many -tropical ants, and which have led to a dimorphism of the workers. -In many species, certainly, the only difference is in size, so that -one can distinguish between large workers and small, and the former -are sometimes five times as big as the latter. But even in the South -European _Pheidole megalocephala_, which is abundant in Italy, the -larger workers are also different in structure from the smaller, for -they have an enormous head with powerful jaws. They are usually known -as 'soldiers,' and are entrusted with the defence of the colony. -Emery directly observed in regard to _Colobopsis truncata_, an ant -which lives in the trunks of trees, that the soldiers, with their -enormous heads, occupied all the entrances to the nest, ready to seize -any intruder with their powerful jaws. In the Sauba ant (_Œcodoma -cephalotes_) Bates described three different types of worker, differing -in size, and although he was not able to determine with certainty what -the particular function of each was, there can be no doubt that they -have special offices, and that the differences in their structure are -adaptations to the differences in their functions. The same is true of -the Indian ant, _Pheidologeton diversus_, depicted in Fig. 106, whose -three forms of workers I owe to the kindness of Professor August Forel. - -If the increase in the size of the head and jaws must bring with it an -increase in the thickness of the skeleton of these parts, as well as -a strengthening of the musculature of the head, it follows that the -strain on the body must be greater, just as in the case of the increase -in the weight of the stag's antlers, so that the skeleton of the -thorax must likewise have become thicker and heavier, the muscles and -nerves of the legs stronger, the articulations of the joints capable -of greater resistance; in short, a whole series of variations of other -parts must have taken place simultaneously, if the primary variation -was to be of use, and not to lead to the destruction of its possessor. -Here again we have a proof that the co-adaptation of many parts can -take place without any intervention of the Lamarckian principle, and -that there must be some other factor which brings this about. - -[Illustration: FIG. 106. Three workers of the same species of Indian -Ant (_Pheidologeton diversus_), drawn from specimens supplied by -Prof. August Forel. _A_, the largest, _B_, the intermediate, _C_, the -smallest form.] - -Where, then, shall we look for this other factor, if not in the -processes of selection, in the selecting of the most suitable -variations among all those which occur? We are confronted with the -alternative of either working out a sufficient explanation with this -factor, or of giving up the attempt at explanation altogether. Yet the -application of the principle of selection in relation to the neuters -of colony-forming insects is by no means simple, for, as the workers -are sterile, a modification of them through processes of breeding -cannot begin directly with themselves. The workers which exhibit the -most suitable variations cannot be selected for breeding, but only -their parents, the sexual animals, and these according to whether -they produce better workers or worse. This is how Darwin looked at -the matter, and his view receives support from one peculiarity in -the composition of these animal colonies, whose significance becomes -apparent in relation to this problem. It has long been known that -in a bee-hive there are from 10,000 to 20,000 workers, but only one -true female, the so-called queen, and the meaning of this remarkable -arrangement probably is, that the adaptation of the workers through -natural selection becomes much more easily possible, _since the whole -number are the children of a single pair_. It is not the individual -workers, but the whole colony, that is, the whole progeny of the -queen, which is selected, according to the greater or less degree of -effectiveness displayed by the workers. Strictly speaking, it is the -single queen that is selected in relation to her power of producing -superior or inferior workers. A colony whose queen was unsatisfactory -in this respect could not hold its own in the struggle for existence, -and only the best colonies and the best hives would survive, that -is, through their descendants. If the hive contained a hundred -queens instead of a single queen the process of selection would be -much more complex and less clear, and it is even quite conceivable -that the production of specially modified workers, adapted to their -functions, or of two or three different kinds of workers, would not -have been possible at all. For it would not have helped much if one -out of a hundred females had produced workers of better structure; -only a majority of such females could give the colony any advantage as -compared with other colonies. - -It has not been definitely established whether, among ants, a single -female is in all cases the founder of the whole colony, but it is -certain that there are only a few females. In the tropical Termites we -know that the ovaries of the female attain to such a colossal size that -one female must certainly suffice for the necessities of the largest -colonies. Grassi has shown, indeed, that, as far as the South European -Termites are concerned, not only are there several females present, -but that even the workers frequently reproduce; but the Termites in -general are inhabitants of warm countries, and the few European species -probably hardly represent the original composition of these animal -colonies. But of the tropical species, which have as yet not been -sufficiently studied, we know at least the extraordinary dimensions -of the body, and the corresponding fertility of the queens, and we -conclude from this that only a few can be present in each termitary[18]. - -[18] Ingwe Sjostedt has recently established in Africa that it is -usually a single queen and a single king that found a termitary -(_Schwed. Akad. Abh._ Bd. 34, 1902). - -Now that we have discussed all these facts it will not be out of place -to summarize the results, in as far as they have any relation to the -acceptance or rejection of the theory of the inheritance of acquired -characters. - -No _direct_ proof of such transmission could be found; on the contrary, -it has been shown that all that has hitherto been advanced as such will -not stand the test of close examination; an inheritance of wounds and -mutilations does not exist, the transmission of traumatically induced -epilepsy is not only doubtful as regards its causes, but cannot even -be considered as the transmission of a particular morphological lesion. - -We may regard as _indirect_ proofs such facts as can only be explained -on the assumption of this mode of inheritance, and in this connexion -our opponents have cited especially the correspondence between -modifications acquired through use in the individual lifetime, -and worked out through histonal selection, with the phyletic -transformations of the same parts. But it has been shown that a number -of parts which do not function actively at all, but only passively, and -thus cannot be caused to change through use, like the hard skeletal -parts of the Arthropods, vary phyletically in the same certain and -direct course as those which function actively, so that we have every -ground for assuming that there are other factors operating in the -transformation of the active as well as of the passive parts. Finally, -we discussed the last and strongest argument which has been put forward -in favour of the Lamarckian principle, that of co-adaptation, that is, -the simultaneous adaptation of many parts co-operating in a common -action, and we were able to controvert this altogether by showing -that exactly similar phenomena of co-adaptation occur in systems -of passively functioning parts, and further, that they occur also -among the workers of ants and bees, that is, in animals which do not -reproduce, and which, therefore, cannot transmit the acquired results -of exercise during their life. - -We therefore reject--and are compelled to reject--the Lamarckian -principle, not only on the ground that it cannot be proved correct, but -also because the phenomena, to explain which it is used, occur also -under circumstances which absolutely exclude any possibility of the -co-operation of this principle. - - -_Supplementary note on the Transmissibility of Acquired Functional -Modifications._ - -I cannot conclude this section without some reference to the utterances -of some naturalists who have quite recently attempted to represent the -inheritance of functional modifications as a conceivable and even a -necessary assumption. - -I may name first Ludwig Zehnder, a physicist who has wandered into the -domain of biology. In regard to the very facts which I have adduced -as evidence _against_ the existence of such inheritance, he has -endeavoured to show how we might conceive of them as having by this -very means arisen[19]. - -[19] Zehnder, _Die Entstehung des Lebens_, Freiburg-i.-Br., 1899. - -He deals with the case of ants, that is to say, with the -differentiation of the sterile workers into several castes, in the -following interesting manner. - -The task of the workers is to procure all the food necessary for all -the individuals of the colony in quantity and quality corresponding -to the demand; failing this the whole colony would perish. Now the -different persons of the colony need different food, according to their -constitution and their functions. Soldiers, for instance, are more -powerful than ordinary workers, since they are adapted for fighting, -and they therefore require a different kind of food from the weaker -workers who are adapted only for other duties. Since the soldiers have -evolved from the latter by selection, what we may, for the sake of -brevity, call the soldier-food in the common stores of the ants would -be drawn upon more lavishly than before, and would therefore disappear -more quickly, and, whenever this occurred, those workers which had -already brought in this kind of food would be impelled to bring more -and more to satisfy the demand for it. But in order to do this they -would require to exert themselves more, and would therefore require a -larger quantity of food--not of course soldier-food, but the particular -kind which their particular qualities demanded. Probably this second -importation of food was undertaken by a second kind of worker, for, -according to Zehnder, each worker does not carry all the kinds of food; -they are divided into legions, each of which has its particular task of -food-collecting to fulfil. - -In the end the storehouse of the ant-colony must contain a provision -in which the different kinds of food are in exact proportion to the -necessities of the different kinds of persons in the colony. It must -alter in its composition again as soon as, in the course of time, one -or other kind of person acquires new characters, for these presuppose a -new kind of diet. - -But how are these acquired characters to be transmitted since neither -soldiers nor workers reproduce? Zehnder answers this by pointing out -that the sexual animals eat _all_ the kinds of nourishment which are -accumulated in the stores, that is to say, all the different kinds of -food exactly in the proportion in which they have been imported--the -proportion in which the different kinds of persons are represented in -the colony. Thus the kinds of nourishment which caused the appearance -of the newly acquired characters in the non-sexual animals also -reached the sexual animals and their sex-cells, and there gave rise to -substances which evoke the relevant qualities in their descendants, for -instance, in the soldiers, or in the still more modified workers, and -so on; and thus we have an 'inheritance of acquired characters.' - -This is certainly ingeniously and cleverly thought out, and it reads -even better and more smoothly in the original than in my brief summary, -but it will hardly be regarded as a refutation of my position; the -hypotheses are all too daring for that. We have no knowledge that -particular modifications in form can be produced and conditioned by -particular kinds of food, and, indeed, the contrary has been proved, -namely, that the two or three different castes of polymorphic species -have precisely similar diet. I need only recall the six forms of female -in _Papilio merope_, of which at least three have been obtained from -the same set of eggs, and by feeding with the same plant. - -It is true that there are ants which lay in stores of nourishment, -but these consist, for the most part, of one kind of seeds, or of -honey, not of different substances, and we have no knowledge that -the different persons use different food, or even that there is any -diversity in the mode of feeding the helpless larvæ. The feeding in -some species takes place from mouth to mouth, and therefore cannot be -precisely investigated, and we can only suppose from analogy with bees -that the larvæ of the males and females frequently receive not only -more abundant, but qualitatively different food. They are fed from the -crop unless the food consists of the pith of a tree in which the larvæ -are imbedded, as Dahl informs us is the case with some tropical ants of -the Bismarck Archipelago. - -But even if we assume that the soldiers take different food from the -ordinary workers, and different again from that of the sexual animals, -is it by virtue of the quality of their food that they have become -what they are? Have our breeds of pigeons or hens been produced by -different diet, or do we know anything in the whole range of animal -life of such a parallelism between food and bodily structure as Zehnder -here assumes? And if, in reality, let us say, the breeds of pigeon had -arisen through specific dieting, and we were to feed one pair with the -specific food-stuffs of three different breeds, would the descendants -of this pair exhibit the form of these three breeds? Or would they -exhibit them in precisely the proportion in which the food-stuffs had -been mixed? It seems to me that Zehnder's assumptions diverge so far -from what we are accustomed to regard as solid ground in biology that -they hardly require consideration, and yet he not only uses them for -the explanation of the case of the ants, but bases upon them the whole -of his theory of the inheritance of acquired characters. - -He considers that the results of use (that is, increased function) -are generally transmitted, because the increase in the organ which -is functioning more strongly changes the composition of the blood, -by withdrawing from it in a greater degree the specific substances -which the organ in question--a muscle, for instance--requires for its -activity. All parts of the animal are thereby affected and modified, -but especially those smallest vital units or 'fistellæ' (corresponding -to biophors) which preside over digestion, and of which there are -several sorts. Among them those work most arduously which have to -produce the specific substances which serve for the nutrition of the -muscles with increased function, because these are needed in larger -quantities. This kind of digestive 'fistella' therefore multiplies, -while other kinds, whose products are not required and therefore not -used up, cease to be so active, diminish in number, and in course of -time disappear. In this way the composition of the blood is altered, -and with it to a greater or less degree all the characters of the whole -organism. Of course the reproductive cells are also under the influence -of this change in the composition of the blood, because the different -nutritive substances are accumulated within them in an altered -proportion corresponding to the changed composition of the blood, the -nutritive substances for the muscles with increased function being -contained in it in a larger quantity, and thus the greater development -of the muscle will repeat itself in the progeny, that is to say, _the -acquired character is transmitted_. - -It is obvious that this is precisely the same line of argument as that -used in reference to the origin of the worker and soldier ants. The -different kinds of 'digestive fistellæ' correspond to the different -food-carrying workers, and the blood to the assumed storehouse -from which soldiers and workers select the food suitable for their -respective needs, while the sex-cells in the one case, the sexual -animals in the other, partake of all kinds exactly in the proportion -in which they are stored, and thus the organ which functions most -vigorously must be stronger in the offspring. - -How the minute quantity of nutritive material contained in the ovum, -still less in the sperm, is to effect the strengthening of the -particular muscles in the descendants is not stated; moreover, such -minimal quantities of food must soon be exhausted, and cannot possibly -increase. It would seem as if the muscles could not even begin by -being stronger, much less that they should remain so, if they were -not exercised equally vigorously by the descendants. If the specific -nutritive stuffs were 'fistellæ,' that is to say, were living units -capable of multiplication, one could understand it. But there can, -of course, be no possibility of a production of living units through -digestion; that can only give rise to digested substances. Or if the -alteration in the composition of the blood produced in the determinant -system of the germ-plasm just those variations requisite to bring -about a strengthening of the muscular system, it would remain to be -shown how this could happen, for the gist of the problem lies in this. -For muscles do not lie in the germ-plasm as miniature models of the -subsequent muscular system, and even if they did, would not all the -muscles, and not merely those which were no longer exercised, decrease -hereditarily when a particular group, like the muscles of the ear in -man, degenerates? Zehnder replies to this with the hypothesis that -the muscles are not all chemically alike, but that each possesses a -particular chemical formula, though they may all be very similar, and -that, therefore, the nutritive materials required by each must be -slightly different. In that case there would require to be, in the -ovum and sperm of man, in order that functional modifications might be -transmitted, as many special nutritive substances as there are muscles, -and, in addition to these, innumerable hosts of other kinds of specific -nutritive substances for all the other parts of the body, since all of -them can be strengthened by exercise and weakened by disuse. And even -if we suppose that all these millions of specific nutritive substances -are accommodated within the germ-cells, as Zehnder's theory requires, -they could not perform what Zehnder ascribes to them, for, as we have -already said, they cannot multiply in the manner of living units, and -so control the growing organism. The different specific nutritive -materials contained in the blood are just as powerless to perform the -task ascribed to them by Zehnder as the specific kinds of food in the -hypothetical storehouse of the ants are to give rise to the different -persons of the ant-colony. - -Zehnder also attempts to overthrow the arguments against the Lamarckian -principle which I based on the skeleton of Arthropods. - -It does not seem to him probable that the chitinous coat of mail can -be an absolutely dead structure, and he supposes that very delicate -nerve-fibrils penetrate into all its most minute parts, and so are -stimulated by 'every pressure and every strain' exerted on the -chitinous skeleton. They 'work' when they are stimulated, and in -doing so they use up 'their specific food-stuffs.' At places which -are frequently stimulated the corresponding nerves develop more than -elsewhere. The necessary specific food-stuffs for these particular -nerves therefore increase proportionately within the body, and also -in the reproductive cells. Accordingly, in the germ-cells there is an -increase of the aforesaid nervous substances, which in the offspring -become associated with the relevant part of the chitinous covering, and -induce in development the secretion of chitin at this part. At this -particular spot, then, the chitin will be specially thick. - -This clearly implies that each particular part of the skin has its -specific nutritive substances, necessitated by the nerves which -traverse it! Thus there must be as many nerve-nutritive substances -as there are skin-nerves, specific chemical combinations for every -part of the body which is capable of heritable variation. This is so -extraordinarily improbable that I need say nothing more about it. If -the Lamarckian principle requires this kind of hypothesis to bolster it -up, it is undoubtedly doomed. - -If we disregard altogether the positive aspect of Zehnder's hypothesis, -and assume that the skin-nerves are really stimulated through the -chitin by every strain and pressure to which a spot of skin is exposed, -and that they cause a correspondingly greater secretion of chitin, -which would then, according to the Lamarckian principle, be hereditary, -does this harmonize with what actually occurs in the development of -the skeleton as we know it in the case of Insects and Crustaceans? Not -at all! Can we suppose that the carapace of a crab or the enormously -hard wing-covers of a water-beetle are exposed to a continual pounding -and pressing and pushing? Exactly the contrary is the case. Every -assailant takes care not to grasp the animal where it is so well -protected, and seeks out the most vulnerable parts for its attacks. It -may be answered that, while this is certainly the case now, the animals -were badly protected when the ancestral forms were evolving. But that -they could not have become hard by dint of being frequently bitten or -otherwise wounded should be obvious from the fact that the whole of the -wing-covers and the whole of the carapace is uniformly covered with -thick chitin, while each wound would only stimulate particular spots; -and we should also have to admit that, since these parts of the skin -which are now so well protected are no longer seized and stimulated, -they would long ago have become thin again, according to the principle -of the degeneration of parts no longer used, or, in this case, no -longer stimulated. But there is no need for wasting time over such -quibbles, since there is a fact which absolutely contradicts Zehnder's -hypothesis. I mean the degeneration of the chitinous skeleton in those -Crustaceans and Insects which protect the abdomen within a shelter -like the hermit-crabs, the caddis-flies (Phryganidæ), (Fig. 107) and -the sack-carrying caterpillars of the Psychidæ among Lepidoptera. The -hermit-crabs, as is well known, squeeze their abdomen into a usually -spirally-coiled Gasteropod shell, and they always choose houses which -are wide enough to conceal the whole body up to the hard claws when -necessity arises. In this case there is surely a continual pressure on -the abdomen, which, being soft, must be squeezed very tightly every -time the animal retreats into its shell. One of my opponents has -described the disappearance of the tough integumentary skeleton from -the abdomen of these animals as an inherited result of this pressure, -and another regards it as the inherited result of the degeneration -of the muscles in this part of the body. But, according to Zehnder, -this continuous pressure, and the frequent rubbing up and down of -the abdomen on the inner surface of the Gasteropod's shell, would -undoubtedly have a stimulating effect on the skin-nerves, and would -therefore bring about a thickening of the chitinous cuticle. In regard -to the larval Phryganidæ and Psychidæ, the case would be the same, -though perhaps hardly to the same degree, for while these larvæ make -their own houses, and will therefore at least make them big enough to -begin with, the pressure and friction must increase with the growth of -the animal. - -[Illustration: FIG. 107. Larva of a Caddis-fly, after Rösel. _A_, -removed from its case, showing the hooks (_h_) which attach it thereto, -and the whitish abdomen, covered only by a thin cuticle. _B_, the same -larva, moving about with its case.] - -If the regulation of the strength of the integumentary skeleton be -referred to selection, we see at once why carapace and wing-covers -should be of equal thickness throughout their whole extent, and why -they do not disappear, although they do not function actively, and -are less stimulated than any other parts of the skeleton; and we also -understand why the abdomen of hermit-crabs and of larval Phryganidæ -and Psychidæ has become soft, whether it be exposed to pressure or -friction in a greater or a less degree. It no longer requires to be -hard, because it is protected by the house, and in the case of the -Pagurids it must not be hard because it could not then be readily -squeezed into the hard-walled and narrow recesses of the Gasteropod -shell; in this case there has therefore been positive selection. I have -not yet referred to the fact that the chitinous covering is certainly -not living, though it is not exactly dead; it is a secretion of the -epidermic cells, not a tissue, and we cannot suppose that there are -any nerve-endings in it. It is almost superfluous to say that the fact -that the skin is cast is in itself enough to make such an assumption -untenable, for the whole of the assumed delicate nervous network would -be shed at every moult and torn away from the nerves which lead to it. -As far as my knowledge goes, nothing of this kind occurs anywhere in -the whole range of the animal kingdom. - -Even if we assume, for the benefit of Zehnder's hypothesis, that -although there are no nerves in the chitin itself yet irritations -affecting the chitinous coat may be transmitted through it to the -delicate nerve-endings lying beneath it, this should take place in a -greater degree at the thin places of the skeleton than _at the thick -parts_! But this interpretation is again fallacious, for we see that -the tactile organs of Arthropods always break through the chitinous -cuticle and protrude beyond it in the form of setæ. - -Of the many other opponents of my views in regard to the -transmissibility of acquired functional modifications, I need only deal -in detail with Oscar Hertwig. - -He seeks for direct proofs of an inheritance of acquired characters, -and believes that he has found these in the hereditary transmission of -acquired immunity from certain diseases. He reminds us of Ehrlich's -well-known experiments on mice with ricin and abrin. - -Even small doses of these two poisons kill mice, but they are tolerated -in very minute doses, and if their administration be continued for some -time in such minute doses, the animals gradually acquire a high degree -of insensitiveness to these poisons; they become immune to ricin and -abrin. - -This immunity is transmitted from mother to young, but it only lasts -for a short time, about six to eight weeks after birth. Yet this is -regarded by Hertwig as an illustration of the transmission of an -acquired character, as an acquired modification of the cells of the -body, for he explains the immunity on the assumption that all the cells -of the body undergo a particular variation due to the influence of the -poison, and are thus, to a certain extent, modified in their nature, -and that the ovum also undergoes this variation and transmits it to the -young animal. The immunization might certainly come under the category -of functional modifications, and it might be thought that we have here -a case of transmission of such an acquired character. - -Against this, however, we have to put the fact that _the acquired -immunity is not transmitted from the father to the descendants_. -Hertwig attempts to explain this by saying that the short duration -of the experiments has only allowed the poison to affect the -cell-substance (cytoplasm) and not the nucleus, that is, the hereditary -substance of the sperm-cells, an assumption which has little -probability considering the intimate nutritive relations between the -cell-nucleus and the cytoplasm. I should be rather inclined to conclude -from the difference in the transmitting power of the sperm and of the -ovum, that this 'inheritance of immunity' does not depend, as Hertwig -supposes, on a modification of the cells to 'ricin immunity,' but, as -Ehrlich and the bacteriologists believe, on the production of so-called -'anti-toxins,' and that these anti-toxins are handed on to the embryo -not by the ovum itself, but by the interchange of blood between mother -and offspring which lasts throughout the whole embryonic period. It is -then self-evident why no transmission of immunity through the father -occurs. - -But it would lead me too far were I to attempt to refute all the -attempts that have been made to interpret individual cases as due to -the inheritance of acquired characters. I should, however, like to say -something as to the theoretical possibility of such an assumption. - -When we try to conceive how experiences and their consequences can -be entailed, how new acquirements of the 'personal part can have -representative effects on the germinal part,' we find ourselves -confronted with almost, if not entirely, insuperable difficulties. -How could it happen that the constant exercise of memory throughout -a lifetime, as, for instance, in the case of an actor, could -influence the germ-cells in such a way that in the offspring the same -brain-cells which preside over memory will likewise be more highly -developed--that is, capable of greater functional activity? We know -what Zehnder's answer to such a question would be; he would make the -blood the intermediary between the brain-cells and the germ-cells, but -we have seen that specific food-stuffs for each specific cell-group -cannot be assumed, and that, even if they could, they would not meet -the necessities of the case. Yet every one who does not regard the -germ-plasm as composed of determinants is constrained to make some such -assumption. But if we take our stand upon the theory of determinants, -it would be necessary to a transmission of acquired strength of -memory that the states of these brain-cells should be communicated by -the telegraphic path of the nerve-cells to the germ-cells, and should -there modify only the determinants of the brain-cells, and should do -so in such a way that, in the subsequent development of an embryo -from the germ-cell, the corresponding brain-cells should turn out to -be capable of increased functional activity. But as the determinants -are not miniature brain-cells, but only groups of biophors of unknown -constitution, and are assuredly different from those cells; as they -are not 'seed-grains' of the brain-cells, but only living germ-units -which, in co-operation with the rest exercise a decisive influence on -the memory-cells of the brain, I can only compare the assumption of the -transmission of the results of memory-exercise to the telegraphing of a -poem, which is handed in in German, but at the place of arrival appears -on the paper translated into Chinese. - -Nevertheless, as I have said before, I do not disagree with those who -say, with Oscar Hertwig, that the impossibility of forming a conception -of the physiological nexus involved in the assumed transmission does -not _ipso facto_ constrain us to conclude that the transmission -does not occur. I cannot, however, agree with Hertwig that the case -is exactly the same as in the 'converse process,' that is, 'in the -development of the given invisible primary constituents in the -inheritance of the cell into the visible characters of the personal -part.' Certainly no one can state with any definiteness how the germ -goes to work, so that from it there arises an eye or a brain with its -millionfold intricacies of nerve-paths, but although the process cannot -be understood in detail, it can in principle, and this is just what is -impossible in regard to the communication of functional modifications -to the germ. Moreover, in addition to this, there is the very important -difference that, in the one case, we know with certainty that the -process actually takes place, although we cannot understand its -mechanical sequence in detail, while in the other we cannot even prove -that the supposed process is a real one at all. From the fact that we -are unable to form clear conceptions of a hypothetical process, we are -not justified, it seems to me, in assuming it to be real, even though -we are aware of many other processes in nature which we are unable to -understand. - -Nor does Hertwig take up this position, for he is at pains to show -the mechanical possibility of the process of inheritance which he -assumes, and he bases this upon the suggestions made by Hering in his -famous work _Ueber das Gedächtniss als eine allgemeine Function der -organisirten Materie_ [_On memory as a general function of organized -matter_], 1870. As this essay probably contains the best that can be -said in favour of a transmission of functional modifications, and as -it also includes some indisputable truths, we may consider it in some -detail. - -Hering is undoubtedly right in regarding 'the phenomena of -consciousness as functions of the material changes of organic -substance, and conversely.' That is, he believes that every sensation, -every perception, every act of will arises from material changes in -the relevant nerve-substances. But we know that 'whole groups of -impressions, which our brain has received through the sense-organs, are -stored up in it, as if resting, and below the margin of consciousness, -to be reproduced when occasion arises, in correct order of space and -time, and with such vividness, that we may be deceived into regarding -as a present reality what has long ceased to be present.' There -must therefore remain in the nerve-substance a 'material impact,' a -modification of the molecular or atomic structure, which enables it 'to -ring out to-day the note that it gave forth yesterday if only it be -rightly struck.' - -Hering attributes a similar power of memory and reproduction to -the germ-substance; he believes that he is justified in making the -assumption that acquired characters can be inherited, although he -admits that it 'appears to him puzzling in the highest degree' -how characters which developed in the most diverse organs of the -mother-being can exert any influence on the germ. That he may be able -to assume this he points to the interconnexion of all organs by means -of the nervous system; it is this that makes it possible that 'the -fate of one reverberates in the other, and that, when excitement takes -place at any point, some echo of it, however dull, penetrates to the -remotest parts.' To the delicate-winged communication by means of the -nervous system, which unites all parts among themselves, must be added -the general communication by means of the circulation of the fluids of -the body. According to Hering's view, the germ experiences, in some -degree, in itself all that befalls the rest of the organs and parts -of the organism, and these experiences stamp themselves more or less -upon its substance, just as sense-impressions or perceptions stamp -themselves upon the nerve-substance of the brain, and these experiences -are reproduced during the development of the germ, just as the brain -brings memory-pictures back to consciousness. He says, 'If something in -the mother-organism has so changed its nature, through long habit or -exercise repeated a thousand times, that the germ-cell resting in it is -also penetrated by it in however weakened a fashion, when the latter -begins a new existence, expands, and increases to a new being whose -individual parts are still itself and flesh of its flesh, it reproduces -what it experienced as part of a great whole. This is just as wonderful -as when an old man suddenly remembers his earliest childhood, but it is -not more wonderful than this.' - -But I think it is more wonderful. There exist demonstrably in the -brain thousands upon thousands of nerve-elements, whose activity -is a definite and limited one, because each particular visual -impression, for instance, only excites to activity certain definite -nerve-elements, and can leave memory-pictures in these alone. According -to my conception of it, the germ-plasm is quite as complex in its -composition, and does not consist of homogeneous elements, but of -innumerable different kinds, which are not related to the parts of the -complete organism indiscriminately, but only to particular parts. But -is it allowable to assume that there are invisible nerve-connexions, -not only to every germ-cell, but also within the germ-plasm, to -every determinant, like the nerve-paths which lead from the eye -to the nerve-cells in the optic-area of the brain? For if it were -otherwise, how could we conceive of the modification of an organ--as, -for instance, the ear-muscles in Man--communicating itself to the -precise determinants of these muscles in the germ-plasm? I have often -been met with the reproach that my conception of the composition of -the germ-plasm is much too complex--but the complexity of Hering's -suggestion seems to me to go a long way beyond mine. - -Hering's ideas, which are not only ingenious but very stimulating, -might be accepted as the first indication of an understanding of the -assumed inheritance of functional modifications, if it could be proved -that such inheritance is a fact; but, as we have seen, that is not -the case. The assumption might be permitted, perhaps, if it could be -shown that certain groups of phenomena left no other possibility of -explanation open except this assumption, but that also, as far as I -can see, is not the case. Of course, others hold a different opinion, -but chiefly because they have rejected without much reflection the -sole explanation which presents itself for numerous phenomena--I -mean the processes which we are about to study under the name of -'germinal selection.' But, in any case, Hering's ideas seem to me very -valuable, because they make it apparent that, however much we know of -the organism, we only know it in a general way, and that numberless -delicate processes go on in it which leave no trace for our microscope, -and that we can only recognize the final results of numerous invisible -and often, in their subtlety, also unimaginable factors. This ought to -be taken to heart, especially by all those who speak of simplicity in -reference to the germ-plasm. So much at least is certain: If there were -any inheritance of functional modifications, we should have another -proof that the germ-plasm is composed of determinants, for without them -there could be no possibility that the 'experiences' of an individual -organ would be transmitted to the germ in the way that the Lamarckian -principle implies. Something, and that something material, must be -modified in the germ-plasm if the vigorous use of a group of muscles, -or of a gland, or of a nerve-cell, is to be communicated to the germ, -and not to the whole germ-plasm, but only to so much of it as is -necessary to cause variation in the corresponding group of cells in the -child. It may perhaps be said that this still does not necessitate the -assumption of special determinants for these cell-groups, and that one -might, with Herbert Spencer, conceive of the germ-plasm as consisting -of homogeneous units which vary in the development in accordance with -the diverse regularly alternating influences to which it is exposed -from step to step, and that, therefore, in each of these units of very -complex structure only a single molecule, or perhaps only a single -atom, would need to vary in order that, in the course of development, -the resulting cell-group should appear in the rudiment in somewhat -altered strength. - -But I do not believe that a chemical molecule, still less an atom, -is sufficient for this, for reasons which I have already stated--yet -we need not go into this now, but rather deduce the consequences of -this admission. It follows that the 'unit' is made up of numerous -'molecules' or 'atoms,' of which each, by dint of changes it has -undergone, causes particular parts of the body to vary in a definite -manner; in other words, we have here again a theory of determinants, -only they are on a much smaller scale, since each invisible little -vital particle or 'unit' contains all the determinants within itself, -while in my theory it is only the id, that is, the visible chromosome, -which includes the determinant complex. Such a theory would be far from -a simplification of mine, it would rather complicate it enormously, -and that without anything being gained. At most it would be made more -evident how inconceivably complex the nerve-paths must be which lead -from the part that has been modified by exercise to the germ-plasm, -and must also lead to all the innumerable 'molecules or atoms' of the -individual 'units.' But even on my theory of the composition of the -ids as aggregates of living determinants, such nervous transmission of -qualities would be a monstrosity which no one would accept, and I think -on this account that my argument as to the impossibility of conceiving -of the transmission of the modifications of the personal part to the -germinal part retains its force, notwithstanding Hering's interesting -analogy. - -If the transmission of functional modifications were an indisputable -fact, I repeat, we should have to give in, and then we might regard the -'memory of organized material' as affording a hint of the possibility -of the unimaginable process. But as long as the occurrence of this -transmission cannot be proved either directly or indirectly, such -a vague possibility of explanation need not induce us to assume an -unproved process. - - - - -LECTURE XXV - -GERMINAL SELECTION - - On what does disappearance after disuse depend, if not on - the Lamarckian principle?--Panmixia--Romanes--Fluctuations - in the determinant-system of the germ-plasm due to unequal - nutrition--Persistence of germinal variations in a definite - direction--The disappearance of non-functioning parts--Preponderance - of minus germinal variations--Law of the retrogression of - useless parts--Variation in an upward direction--Artificial - selection--Influence of the multiplicity of ids and of sexual - reproduction--Personal selection depends on the removal of certain - id-variants--Range of influence of germinal selection--Self-regulation - of the germ-plasm, which is striving towards stability--Ascending - variation-tendencies may persist to excess--Origin of secondary sexual - characters--Significance of purely morphological characters--The - markings of butterflies. - - -Now that we have recognized that the assumption of a transmission of -functional modifications is not justifiable, let us discuss some of -the many phenomena to explain which many people believe the Lamarckian -principle to be indispensable, and let us inquire whether we are in a -position to give any other explanation of these. How has it come about -that the effects of use and disuse _appear_ to be inherited? Can we -find a sufficient explanation in the principle of selection, and in the -natural selection of Darwin and Wallace? - -The answer to these two questions will be most quickly found if we -begin by seeking for an explanation of the disappearance of a part when -it ceases to be exercised. - -That this cannot lie in the Lamarckian principle we have already learnt -from the fact that passively functioning parts, such as superfluous -wing-veins, also disappear, and that the loss of the wings and -degeneration of the ovaries has taken place in worker ants, which can -transmit nothing because they do not reproduce. - -We might be inclined to regard this gradual disappearance and ultimate -elimination of a disused organ as a direct gain, on the ground that -the economy of material and space thus effected may be of decided -advantage to the individual animal and thereby also for the maintenance -of the species, and that those animals would have an advantage in the -struggle for existence in which the superfluous organ was reduced to -the smallest expression. But that would be far from supplying us with -a sufficient explanation of the phenomenon; the individual variations -in the size of an organ which is in process of degenerating are, -even in extreme cases, far too slight to have any selection-value, -and I cannot call to mind a single case in which the contrary could -be assumed with any degree of probability. What advantage can a newt -or a crustacean living in darkness derive from the fact that its eye -is smaller and more degenerate by one degree of variation than those -of its co-partners in the struggle for existence? Or, to use Herbert -Spencer's striking illustration, how could the balance between life and -death, in the case of a colossus like the Greenland whale, be turned -one way or another by the difference of a few inches in the length of -the hind-leg, as compared with his fellows, in whom the reduction of -the hind-limb may not have gone quite so far? Such a slight economy of -material is as nothing compared with the thousands of hundredweights -the animal weighs. As long as the limbs protrude beyond the surface -of the trunk they may prove an obstacle to rapid swimming, although -that could hardly make much difference, but as soon as the phyletic -evolution had proceeded so far that they were reduced to the extent of -sinking beneath the surface, they would no longer be a hindrance in -swimming, and their further reduction to their modern state of great -degeneration and absolute concealment within the flesh of the animal -cannot be referred even to negative selection. - -Years ago I endeavoured to explain the degeneration of disused parts -in terms of a process which I called Panmixia. Natural selection not -only effects adaptations, it also maintains the organ at the pitch -of perfection it has reached by a continual elimination of those -individuals in which the organ in question is less perfect. The longer -this conservative process of selection continues, the greater must be -the constancy of the organ produced by it, and deviations from the -perfect organ will be of less and less frequent occurrence as time goes -on. - -Now if this conservative action of natural selection secures the -maintenance of the parts and organs of a species at their maximum of -perfection, it follows that these will _fall below this maximum as soon -as the selection ceases to operate_. And it does cease as soon as an -organ ceases to be of use to its species, like the eye to the species -of crustacean which descends into the dark depths of our lakes, or to -the abyssal zones of the ocean, or into a subterranean cave-system. -In this case all selection of individuals ceases as far as the eye is -concerned; it has no importance in deciding survival in the struggle -for existence, because no individual is at a disadvantage through -its inferior eyes, for instance, by being in any way hindered in -procuring its food. Those with inferior organs of vision will, _ceteris -paribus_, produce as good offspring as those with better eyes, and the -consequence of this must be that there will be a general deterioration -of eyes, because the bad ones can be transmitted as well as the good, -and thus the selection of good eyes is made impossible. - -The mixture thus arising may be compared to a fine wine to which a -litre of vinegar has been added; the whole cask is ruined because the -vinegar mingles with every drop of the wine. As deviations from the -normal are always occurring in every part of every species, and among -them some that lessen the value of the organ, rarely perhaps at first, -but after a time in every generation, a sinking of the organ from -the highest point of possible perfection becomes inevitable as soon -as the organ becomes superfluous. The functional uselessness of the -organ must go on increasing the longer it is disused, as will readily -be admitted if it be remembered that only the most perfect adaptation -of all the separate parts of an organ can maintain its functional -capacity, that all the parts of an organ are subject to variation, and -that every deviation from the optimum implies a further deterioration -of the whole. An eye, for instance, can no longer vary in the direction -of 'better' if it has already reached the highest possible point of -perfection; every further variation must deteriorate it. - -Romanes gave expression to this idea, that the cessation of natural -selection alone must cause the degeneration of a part, a decade -before I did, but neither he nor the scientific world of his time -attached great importance to it, and it was forgotten again. This was -intelligible enough, for, at that time, the validity of the Lamarckian -principle had not been called in question, and therefore the need for -some other principle to explain the disappearance of disused parts had -not begun to be felt. - -I found myself in quite a different position. As my doubts regarding -the Lamarckian principle grew greater and greater, I was obliged to -seek for some other factor in modification, which should be sufficient -to effect the degeneration of a disused part, and for a time I thought -I had found this in panmixia, that is, in the mingling of all together, -well and less well equipped alike. This factor does certainly operate, -but the more I thought over it the clearer it became to me that -there must be some other factor at work as well, for while panmixia -might explain the deterioration of an organ, it could not explain -its decrease in size, its gradual wearing away, and ultimate total -disappearance. Yet this is the path followed, slowly indeed, but quite -surely, by all organs which have become useless. If panmixia alone -guided the deterioration of the organ, and it was thus only chance -variations which were inherited through panmixia and gradually diffused -over the whole species, how could it come about that all the variations -were in the direction of smaller size? Yet this is obviously the case. -Why should no variations in the direction of larger size occur? And -if this were so, why should a useless organ not be maintained at its -original size, if it be admitted that an increase in size would be -prevented by natural selection? But this never occurs, and diminution -in size is so absolutely the rule that the idea of a 'vestigial or -rudimentary' organ suggests a 'small organ' almost more than an -'imperfect' one. - -There must then be something else at work which causes the -minus-variations in a disused organ to preponderate persistently and -permanently over the plus-variations, and this something can lie -nowhere else than where the roots of all hereditary variations are -to be found--in the germ-plasm. This train of thought leads us to -the discovery of a process which we must call selection between the -elements of the germ-plasm, or, as I have named it shortly, _Germinal -Selection_. - -If the substance of the germ-plasm is--as we assumed--composed of -heterogeneous living particles, which have dissimilar rôles in the -building up of the organism, there must of necessity be among them -a definite labile state of equilibrium, which cannot be disturbed -without modifying in some way the structure of the organism itself -which arises from the germ-plasm. But if our further view be correct, -that these individual and different living units of the germ-plasm are -'determinants,' that is, are the primary constituents of particular -parts of the organism, in the sense that these parts could not arise if -their determinants were absent from the germ-plasm, and that they would -be different if the determinants were differently composed, we can draw -far-reaching deductions. - -It is true that we cannot learn _anything directly_ in regard to the -intimate structure of the germ-plasm, and even in regard to the vital -processes going on within it we can only guess a very little, but so -much we may say--that its living parts are nourished, and that they -multiply. But it follows from this that nourishment in a dissolved -state must penetrate between its vital particles, and that whether the -determinants grow, and at what rate they do so, depends mainly on the -amount of nourishment which reaches them. As long as the germ-cells -multiply by division the determinants have no other function but -to grow; a part of their substance undergoes oxidation and thereby -yields the supply of energy necessary to assimilation, that is, to the -formation of new living substance. - -If each kind of determinant always secured the same quantity of -nourishment, all would grow in the same degree, that is, in exact -proportion to their power of assimilation. But we know that in less -minute conditions which we can observe more directly, there is -nowhere absolute equality, that all vital processes are subject to -fluctuations; any little obstacles in the current of the nutritive -fluid, or in its composition, may cause poorer nutrition of one -part, better of another. We may therefore assume that there are -similar irregularities and differences in the minute and unobservable -conditions of the germ-plasm likewise, and the result must be a slight -shifting of the position of equilibrium as regards size and strength in -the determinant system; for the less well-nourished determinants will -grow more slowly, will fail to attain to the size and strength of their -neighbours, and will multiply more slowly. - -But the vigour of growth does not depend only on the influence of -nourishment; one cell grows quickly, another slowly in the same -nutritive fluid; it depends in great part on the cell's power -of assimilation. In the same way the assimilating power of the -determinants and their affinity for nourishment will vary with their -constitution, and a weaker determinant will remain smaller than a -stronger one, even when the stream of nourishment is the same. - -It seems to me that it is upon the unequal nutrition of the -determinants conditioned by the chances of the food-supply that -individual hereditary variability ultimately depends. If, for instance, -the determinant _A_ receives poorer nourishment at a particular time -than the determinant _B_, it will grow more slowly, remain weaker, and -then, when the germ-cells develop into an animal, the part to which it -gives rise will be weaker than it usually is in other individuals. - -These primary inequalities in the equipment of the determinants which -are caused by a passing inequality in the food-stream are, of course, -so slight that we are unable to observe their consequences. They must -persist for a considerable time before they become observable, but -they may persist for a long time, and their effect must then mount -up, because every diminution in the strength of the determinant also -signifies a lessened power of assimilation, and growth becomes slower -for the twofold reason that passive and active nutrition decrease -at the same time. In the less minute conditions observable in the -histological elements of the body we know that function strengthens -the organ, and that disuse weakens it, and we are justified in -applying this proposition also to these more intimate conditions and -minuter vital units. Thus, in the course of the multiplication of -the germ-cells, the less vigorously working determinant, _A_, will -gradually, but very slowly, become weaker, that is, of diminished -power of assimilation, presupposing of course that the intra-germinal -food-stream does not become stronger again at the same place--a -possibility to which I shall subsequently refer. But while one -determinant may be slowly becoming weaker, its neighbour, on the other -hand, may be varying on an ascending scale, just because the former is, -on account of its diminished power of assimilation, no longer able to -exhaust completely the food-stream which flows to it. - -The determinants are thus in constant motion, here ascending, there -descending, and it is in these fluctuations of the equilibrium of the -determinant-system that I see the roots of all hereditary variation, -while in the fact that the variation-directions of particular -determinants must continue the same without limit as long as they meet -with no obstacle lies the possibility of the adaptation of the organism -to changing conditions, the increase and transformation of one part, -the degeneration and disappearance of another, in short, the processes -of natural selection. The reason why such variation movements must -continue until they meet some resistance is that every chance upward -or downward movement--due, that is, to mere passive fluctuation in the -food-supply, at the same time strengthens or weakens the determinant, -and makes it either more or less capable of attracting nourishment to -itself; in the former case an increasingly strong stream of food will -be directed towards it, in the latter more and more of the available -food-supply will be withdrawn from it by its neighbour-determinants -on all sides; in the former the determinant will go on increasing -in strength as long as it can go on attracting more nourishment, in -the latter it will continue to become weaker until it disappears -altogether. To the ascending progression, as is evident, there are -limits set, not only by the amount of food which can circulate through -the whole id, but also by the neighbour determinants, which will sooner -or later resist the withdrawal of nourishment from them; but for the -descending progression there are no limits except total disappearance, -and this is actually reached in all cases in which the determinants are -related to a part which has become useless. But both these movements, -the upward and the downward alike, are quite independent of natural -selection, i.e. of personal selection; they are processes of a unique -kind which run their course purely in accordance with intra-germinal -laws. Whether a determinant 'ascends' or 'descends' depends solely -upon the play of forces within the germ-plasm, not upon whether the -direction of the variation in question is useful or prejudicial, or -on whether the organ in question, the determinate, is of value or -otherwise. In this fact lies the great importance of this play of -forces within the germ-plasm, that it gives rise to variations quite -independently of the relations of the organism to the external world. -In many cases, of course, personal selection intervenes, but even then -it cannot directly effect the rising or falling of _the individual_ -determinants--these are processes quite outside of its influence--but -it can, by eliminating the bearers of unfavourably varying -determinants, set a limit to further advance in such directions. This -we shall consider in more detail later on. Personal selection operates -by removing unfavourably varying individuals from the genealogical tree -of the species, but at the same time the determinants which are varying -unfavourably are also removed, and their variation is thus put a stop -to for all time. - -I have called these processes which are ceaselessly going on within the -germ-plasm, Germinal Selection, because they are analogous to those -processes of selection which we already know in connexion with the -larger vital units, cells, cell-groups and persons. If the germ-plasm -be a system of determinants, then the same laws of struggle for -existence in regard to food and multiplication must hold sway among its -parts which hold sway between all systems of vital units--among the -biophors which form the protoplasm of the cell-body, among the cells of -a tissue, among the tissues of an organ, among the organs themselves, -as well as among the individuals of a species and between species which -compete with one another. - -If this be the case, we have here ready to hand the explanation of -every heritable variation of a part, ascending and descending alike. -Let us consider for a little the latter category--that is, the -disappearance of functionless _or useless organs_. It is clear that, -from the moment in the life of a species that an organ, _N_, becomes -useless, natural selection withdraws her hand from it; individuals -with better or worse organs _N_ are now equally capable of life and -struggle, the state of panmixia is entered upon, and the organ _N_ -of necessity falls somewhat below its previously attained degree of -perfection. - -That this must be so will be admitted when it is remembered that each -organ of a species is only maintained at its highest level because -personal selection keeps ceaseless watch over it, and sets aside all -the less favourable variations by eliminating the individuals which -exhibit them. But this is no longer the case with a useless organ. When -a weaker variant of a disused organ arises through the intra-germinal -fluctuations of nutrition, this is transmitted to the descendants -just as well as the normally developed organ, and in the course of -generations will be inherited by a greater and greater number of -individuals, and must ultimately be inherited by all in some degree or -other. The objection has been urged from many sides that variations -upwards would be quite as likely to arise as those downwards, but -this is an error. Even if, at the beginning, the minus-variations -were rarer than the plus-variations, in the course of generations -the minus ones would preponderate because ascending variations of -disused organs are not indifferent for the organism but injurious to -it. Perhaps an increase in the size of the organ itself would do no -harm, but in that of its determinant it certainly would, because an -ascending determinant requires more nourishment than previously, and -withdraws it from its surroundings, and thus from the determinants in -its immediate neighbourhood; but these are those of functioning and -indispensable parts. Individuals in whose germ-plasm the determinants -of disused organs ascend, and thereby depress the determinants of -organs which are still active, are subject to personal selection, -and are eliminated. There thus remain only those with descending -determinants; in other words, the chance of variants in the direction -of weakness in useless determinants far outweighs that of variants in -the direction of increased strength; the latter will soon cease to -occur at all, for as soon as a determinant has fallen a little below -its normal level, it finds itself upon an inclined plane, along which -it glides very slowly but steadily downwards. This might be disputed if -it could be maintained that, at every stage of the descent, a change -of direction was possible. But this probably takes place rarely and -only in the case of individual ids, and will therefore not be permanent -because in general the stronger neighbour determinants will possess -themselves of the superfluous nourishment, and a lasting ascent will -thus be impossible to the weakened determinant. This is precisely what -I have called Germinal Selection. The determinant whose assimilating -power is weakened by ever so little is continually being robbed by -its neighbours of a part of the nourishment which flows towards it, -and must consequently become further weakened. As no more help will -be given to it by natural selection, since the organ is no longer of -any value to the species, the better among the weakened determinants -of _N_ are never selected out, and they must gradually give way in the -struggle with the neighbouring determinants which are necessary to the -species, becoming gradually weaker and ultimately disappearing. - -This process can, of course, no more be proved mathematically than -any other biological processes. No one who is unwilling to accept -germinal selection can be compelled to do so, as he might be to accept -the Pythagorean propositions. It is not built up from beneath upon -axioms, but is an attempt at an explanation of a fact established by -observation--the disappearance of disused parts. But when once the -inheritance of functional modifications has been demonstrated to be a -fallacy, and when it has been shown that, even with the assumption of -such inheritance, the disappearance of parts which are only _passively_ -useful, and of any parts whatever in sterile animal forms, remains -unexplained, he who rejects germinal selection must renounce all -attempt at explanation. It is the same as in the case of personal -selection. No one can demonstrate mathematically that any variation -possesses selection value, but whoever rejects personal selection gives -up hope of explaining adaptations, for these cannot be referred to -purely internal forces of development. - -The total disappearance of a part which has become useless takes place -with exceeding slowness; the whales, which have existed as such since -the beginning of the tertiary period, have even now not completely lost -their hind-limbs, but carry them about with them as rudiments in the -muscular mass of the trunk, and the birds, which are even older, still -show in their embryonic primordia the five fingers of their reptilian -forefathers, although even their bird-ancestors of the Jurassic period, -if we may argue from _Archæopteryx_, had only three fingers like our -modern birds. A long series of similar examples might be given, and -modern embryology in particular has contributed much that, like this -example of birds' fingers, points to a certain orderliness in the -disappearance of the individual parts of an organ which has become -superfluous. Parts which, in the complete animal, have disappeared -without leaving a trace, appear again in each embryonic primordium, and -disappear in the course of the ontogeny. Speaking metaphorically, we -might express this on the basis of the determinant theory, by saying -that the determinants, as they become weaker, can only control an -increasingly short period of the whole ontogeny of the organ, so that -ultimately nothing more than its first beginning comes into existence. -But this is only a metaphor; we cannot tell what really happens as -long as we are ignorant of the physiological rôle of the determinants, -and even of the laws governing the degeneration of a useless organ. -In respect of the latter, much might still be achieved if comparative -anatomy and embryology were studied with this definite end in view, and -perhaps we should even be able to draw more definite conclusions in -regard to the composition and activity of the determinants in the germ. - -In the meantime we must be content with the knowledge that, on the -determinant hypothesis, the disappearance of organs which have -become useless may be regarded as a process of intra-selection going -on between the 'primary constituents' (_Anlagen_) of the germ, and -depending on the same principle of the 'struggle of parts' which -William Roux introduced into science with such brilliant results. If a -struggle for food and space actually takes place, then every passive -weakening must lead to a permanent condition of weakness and a lasting -and irretrievable diminution in the size and strength of the primary -constituent concerned, unless personal selection intervenes, and -choosing out the strongest among these weakened primary constituents, -raises them again to their former level. But this never happens when -the organ has become useless. - -This explains why not only parts with active function, like limbs, -muscles, tendons, nerves, and glands, disappear when they cease to -function, but also passive parts like the colouring of the external -surfaces of animals, the lifeless skeletal parts of Arthropods and the -exact adaptation of their thickness to the dwindling function, the -disappearance of superfluous wing-veins, and of the hard chitinous -covering of the abdomen when it is concealed in a protecting house, -as in the case of hermit-crabs, Phryganidæ, and Psychidæ. Here too we -find a sufficient explanation of the fact that parts which have become -functionless, such as the wings of ants, can disappear even in the case -of sterile workers. - - * * * * * - -The principle of germinal selection, however, can only be understood -in its full significance if we take the positive aspect also into -consideration. We had reached the conclusion that because of the -fluctuations of the food-supply one set of the homologous determinants -represented in the various ids may vary in a minus direction, and -another set in a plus direction, and that this direction will be -adhered to as long as no intra-germinal obstacles come in the way. As -long as this does not happen the determinant concerned will pursue the -path of variation it has once struck out, and indeed the tendency will -be strengthened, because every passive variation, upwards or downwards, -results in a strengthening or weakening of the determinant's power of -assimilation. - -Let us take a case of positive variation of the determinants of an -organ _N_, which would be more useful to the species if it were -more highly developed than it had previously been. The variation in -an upward direction is at first purely passive, having arisen from -fluctuations in the food-supply, but it soon becomes active, since the -determinants that have become stronger will have a stronger affinity -for food and will attract more and more of the available supply. The -increased food-stream is thus maintained, and its gradual result is -such a strengthening of the determinants in the course of generations -of germ-cells, that the parts controlled by these determinants--the -determinates--must enter on a path of plus-variations. If to this -there be added personal selection, either natural or artificial, any -fluctuations of this primary constituent towards the minus side will be -effectually prevented, the direction of variation will remain positive, -and the continued intervention of personal selection may raise its -development to its possible maximum, that is, so far that further -development in the same direction would not make for greater fitness, -and personal selection must call a halt. This will always happen as -soon as further increase of the organ would be prejudicial to the -living power of the whole, and when the harmony of the bodily parts -would thereby be permanently disturbed. - -That variation in an upward direction really can persist for a long -time is shown by artificial selection as practised by Man in regard -to his domesticated animals and cultivated plants. At first general -variability, or at least variability in many directions, sets in as -a result of the greatly altered conditions of life; the ordinary -fluctuations of the determinants are intensified by the greater -fluctuations in the nutritive stream, and it becomes possible for Man -consciously or unconsciously to select for breeding whatever he prefers -among the chance variations that arise in individual parts or in whole -complexes of parts, and he may thus give rise to a long-continued, -often apparently unlimited, augmentation of variations in the same -direction, although he cannot exercise _any direct_ influence upon -the germ-plasm or its determinants. When a determinant has assumed -a certain variation-direction it will follow it up of itself, and -selection can do nothing more than secure it a free course by setting -aside variations in other directions by means of the elimination of -those that exhibit them. - -That artificial selection can cause the increase of a part has long -been established, but in what way this is possible, and how it can -be theoretically explained has hitherto been very obscure, for even -if we take the favourable case that both parents possess the desired -variation, it cannot be supposed that the characters of the parents -are, so to speak, added together in the child; all we can say is that -the probability that the children will also exhibit the character -in question--for instance, a long or crooked nose--becomes greater. -Certainly an increase of the character may result if in both parents -the determinants _K_ are present in excess as compared with the -heterodynamous determinants _K´_ and _K´´_, for in that case there -is an increased probability that, through reducing divisions and -amphimixis, there will again be a preponderance of the determinants -_K_ composing the germ-plasm of the child, and further, that these -determinants _K_ will dominate strongly as compared with the few -_K´_'s. It may thus happen that the long nose of the two parents -will give rise to a still longer nose in the child, or that parents -of considerable bodily size may have still bigger children, but such -increase would be confined to one generation, and would not lead to a -permanent increase of the character; permanent increase cannot depend -merely on the number of the determinants _K_ and on their supremacy -over their converse, the determinants _K´_; it must also depend on -their own variation, and this again can depend only on germinal -selection and not upon personal selection, although the former can be -materially assisted by the latter. - -That inheritance from both parents is only a secondary consideration -in regard to the increase of a part by artificial selection is made -evident by the fact that _many secondary sexual characters_ have been -modified, although the breeder selected only in regard to one parent. -Nevertheless in this very domain the greatest results have been -achieved; witness the Japanese breed of cocks with tail-feathers six -feet long. This astonishing result has been reached by the strictest -selection of the cocks in which the feathers were a little longer -than those of other cocks, and the increase in the length of feathers -depended--according to our theory--simply on the fact that, by the -selection of the determinants which were already varying in the -direction of increased length, this process of increase was guarded -from interruption by chance unfavourable conditions of nutrition. -The continuance of variation in the upward direction in which it had -already started is not effected directly by personal selection, but -is so indirectly, for without this constant fresh intervention of -selection the increase would be apt to come to a standstill, or the -variation might even take a contrary direction. There are two other -factors operative to which we have not yet given sufficient attention. -They are, the multiplicity of the ids in every germ-plasm, and sexual -reproduction. - -If--as we must assume--each germ-plasm is made up of several or many -ids, there must be several or many determinants of each part of the -organism, for each id contains potentially the whole organism, though -with some individuality of expression. The child is thus not determined -by the determinants of a single id, but by those of many ids, and the -variations of any part of the body do not depend on the variations of a -single determinant _X_, but on the co-operation of all the determinants -_X_ which are contained in the collective ids of the relevant -germ-plasm. Thus it is only when a majority of the determinants have -varied upwards or downwards that they dominate collectively the -development of the part _X´_ and cause it to be larger or smaller. - -We have assumed passive fluctuations in nutrition to be the first -cause in individual variation, and it is obvious that the action of -this first cause of dissimilarity must be greatly restricted by the -multiplicity of the ids and the corresponding homologous determinants. -For although passive fluctuations in nutrition should occur continually -in the case of all determinants, this would not imply that they would -follow the same direction in all the determinants _X_ of all ids, for -some determinants _X_ might vary upwards, and others downwards, and -these might counteract each other in ontogeny; so that in many cases -the fluctuations of the individual determinants will not be felt in -their products at all. But since there are--as we shall see later--only -two directions of variation, upwards and downwards, plus and minus, -it must also sometimes happen that a majority take one direction, and -this affords the basis on which germinal selection can build further, -and on which it is materially supported by reducing division and the -subsequent amphimixis. - -For reducing division removes half of the ids and thus of the -determinants from the mature germ-cell, and according as chance leaves -together or separates a majority of _X_-determinants varying in the -same direction, this particular germ-cell will contain the primary -constituents of a plus- or of a minus-variation of _X_, and it is -possible that the presence of a majority or a minority may be entirely -due to the reduction. The germ-plasm of the parent may contain, -for instance, the determinant _X_ in its twenty ids 12 times in -minus-variation form, 8 times in plus-variation form; and the reducing -division, according to our view, may separate these into two groups -of which one contains eight plus- and two minus-variations, the other -ten minus-variations, or the one six plus- and four minus-variations, -the other two plus- and eight minus-variations, and so on. Now every -germ-cell which contains a majority of plus- or minus-variations--and -this must be the case with most of them--may unite, if it attains to -amphimixis, with a germ-cell which also contains a majority of plus -or minus _X_-determinants, and if similar majorities let us say -plus--meet together, the plus-variation of _X_ must be all the more -sharply emphasized in the child. - -Thus, although the individual determinants _X_ may not be incited to -further variation by their co-operation with others varying in the -same direction, the collective effect of the plus-determinants will -be greater, and adherence to the same direction of variation in the -following generation will be assured, for if in the germ-plasm of the -parent there be, for instance, sixteen out of twenty determinants -possessing the plus-variation, a minus-majority can no longer result -from reducing division. - -It is upon this that the operation of natural selection, that is, -personal selection, must depend--that the germ-plasms in which the -favourable variation-direction is in the majority are selected for -breeding, for it is this and nothing else that natural selection does -when it selects the individuals which possess the preferred variations. -The ascending process is thus considerably advanced, because the -opposing determinants are more and more eliminated from the germ-plasm, -till the preferred variations of _X_ are left, and among these, as -ascent in the direction begun continues, the opposing variations -are again set aside by germinal selection, and so on. Reducing -divisions and amphimixis are thus powerful factors in furthering -the transformations of the forms of life, although they are not the -ultimate causes of these. - -Now that we have made ourselves familiar with the idea of germinal -selection we shall attempt to gain clearness as to what it can do, -and how far the sphere of its influence extends, and, in particular, -whether it can effect lasting transformations of species without the -co-operation of personal selection, and what kind of variations we may -ascribe to it alone. - -First, I must return for a moment to the question we have already -briefly discussed--whether the variation of a determinant upwards or -downwards must so continue without limit. We might be inclined to -think that the great constancy which many species exhibit was a plain -contradiction of this, for if every minute variation of a determinant -necessarily persisted without limit in the same direction, we should -expect to find all the parts of the organism in a state of continual -unrest, some varying upwards, some downwards, always ready to break the -type of the species. Must there not be some internal self-regulation -of the germ-plasm which makes it impossible that every variation -which crops up can persist unlimitedly? Must there not be some kind -of automatic control on the part of the germ-plasm, which is always -striving to re-establish the state of equilibrium that has once been -attained by the determinant system whenever it is disturbed? - -It is difficult to give any confident answer to this question. We -cannot reach clearness on this point through our present knowledge -of the germ-plasm, because we possess no insight into its structure; -we can only draw conclusions as to the processes in the germ-plasm -from the observed phenomena of variation and inheritance. But two -facts stand in direct antithesis to one another, first, the high -power of adaptation possessed by all species, and the undoubted -occurrence of unrestricted persistence in a given direction of -variation, as seen in artificial selection, and in the disappearance -of parts which have ceased to function; and, secondly, the great -constancy of old-established species which do indeed always exhibit a -certain degree of individual variability, but without showing marked -deviations as a frequent occurrence or in all possible directions, -as they certainly would if every determinant favoured by a chance -increase in the nutritive stream necessarily and irresistibly went on -varying further in the same direction. Or can the constancy of such -species be maintained solely by means of personal selection, which -is continually setting aside all the determinants which rise above -the selection-value by eliminating their possessors? I was for long -satisfied that this was the true solution of the difficulty, and even -now I do not doubt that personal selection does, in point of fact, -maintain the constancy of the species at a certain level, but I do not -believe that this is sufficient, but rather that it is necessary to -recognize an equalizing influence due to germinal selection, and to -attribute to this a share in maintaining the constancy of a species -which has long been well adapted. I am led to this assumption chiefly -by the phenomena of variation in Man, for we find in him a thousand -kinds of minute hereditary individual variations, of which not one is -likely to attain to selection value. Of course the constant recurrence -of reducing divisions prevents any particular id which contains a -varying determinant from being inherited through many generations; for -so many ids are being continually removed from the genealogical tree -by the constant rejection of the half of all ids of every germ-plasm, -that only a small part of the ancestral id remains in the grandchild, -great-grandchild, and so on. Certainly some of the ids of the ancestors -compose the germ-plasm of the descendants, and if all the determinants -of one of these ids had begun to vary persistently upwards or downwards -in an ancestor, then all the determinants of the relative id in the -descendants would possess the variation in an intensified degree; and -however slowly the variation advanced it would attain selection-value -in some one or other of the descendants, and would thus break -the previously stable type of the most perfectly adapted species. -The descendant in question would then succumb in the struggle for -existence. But as the number of the determinants in the germ-plasm is -probably much greater than that of the descendants of one generation, -every descendant would in the course of time deviate unfavourably in -some one character from the type of the species, and then either all -the descendants would be eliminated or the type would become unstable. -But neither of these things happens, and there are undoubtedly -species which remain constant for long periods of time, therefore the -assumption must be false and every variation of a determinant does not -of necessity go on in the same direction without limit. - -I therefore suppose that although slight variations are ceaselessly -taking place upwards or downwards in all determinants, even in constant -species, the majority of these turn again in the other direction before -they have attained to any important degree of increase, at least in -the germ-plasm of all species which have had a definitely established -equilibrium for thousands of generations. In such a germ-plasm, or -to speak more precisely, in the id of such a germ-plasm, marked -fluctuations in the nutritive stream will not be likely to occur as -long as the external conditions are unchanged, but slight fluctuations, -which will not be wanting even here, may often alternate and turn in -an opposite direction, and thus the upward movement of a determinant -may be transformed into a downward one. Every determinant is surrounded -by several others, and we can imagine that the regular nutritive -stream which we have assumed may be partially dammed up by a slight -enlargement of the determinant, and that this will drive the surplus -back again. But however we may picture these conditions, which are for -all time outside of the sphere of observation, the assumption of a -self-regulation of the germ-plasm, up to a certain degree, cannot be -regarded as inconceivable or unphysiological. - -But there are limits to this self-regulation; as soon as the increase -or decrease of a determinant attains a certain degree, as soon as it -has got beyond the first slight deviation, it overcomes all obstacles, -and goes on increasing in the direction in which it has started. -This must happen even in the case of old and constant species, and -frequently enough to admit of an apparent capacity for adaptation in -all directions. Every part of a species can vary beyond the usual -individual fluctuations, and as this is possible only by means of -intra-germinal processes, we must assume that even in the case of -germ-plasms which have long remained in a state of stable equilibrium -there may occasionally be marked fluctuations in the nutritive stream, -and thus more than usually pronounced variations of the determinants -affected by it will occur. These yield the material for new adaptations -if they are in the direction of fitness, or they are eliminated either -by the chances of reducing division or by personal selection if new -adaptations are not required. - -The old-established hereditary equilibrium of the germ-plasm must be -most easily disturbed when the species is in some way brought into new -conditions of existence, as, for instance, when plants or animals are -domesticated, and when in consequence, as we have already assumed, the -nutritive currents within the id gradually alter, quantitatively and -qualitatively; and on this account alone certain kinds of determinants -are favoured, while others are at a disadvantage. In this way there -arises the intensified general variability of domesticated animals -and cultivated plants which has been known since the time of Darwin. -Something analogous to this must occur in natural conditions, though -more slowly, when a species is subjected to a change of climatic -conditions, but we shall discuss this later on in more detail. - -We have thus arrived at the idea that the slight variations of the -determinants may be counteracted whether they be directed upwards or -downwards, and that in the case of so-called constant species they -do frequently equalize themselves; but that more marked variations, -produced by more pronounced nutritive fluctuations, may in a sense go -on without limit, and then can only be restricted and controlled by -personal selection, that is, by the removal of the ids concerned from -the genealogical lineage of the species. - -In one direction variation can be proved to go on without limit, and -that is downwards, as is proved by the fact of the disappearance of -_disused organs_, for here we have a variation-direction, which has -been followed to its utmost limit, and which is completely independent -of personal selection; it proceeds quite _uninterfered with_ by -personal selection, and is left entirely to itself. It is a significant -fact that the disappearance of the individual parts of a larger organ, -according to all the data that are as yet available, proceeds at a -very _unequal rate_, so that it evidently depends to a great extent on -chance whether a disused part begins to degenerate sooner or later. -Thus in one of the Crustaceans living in the darkness of the caves of -North America the optic lobes and optic nerves have disappeared, while -the retina of the eye, the lens, and the pigment have been retained, -and in others the reverse has taken place, and the nerve-centres -have persisted while the parts of the eye have been lost (Packard). -Variations of the relevant determinants towards the minus direction -may thus occur, sometimes sooner, sometimes later; but when once they -have started they proceed irresistibly, though with exceeding slowness. - -But variation in an upward direction also, when it has once been set -a-going, may in many cases go on unchecked until limits are set to -it by personal selection, when the excess of the organ would disturb -the harmony of the parts, or in any other way lessen the individual's -chances of survival in the struggle for existence. This is proved -especially by the phenomena of artificial selection, for almost all -the parts of fowls and pigeons have been caused to vary to excess by -breeding, and must thus have been, so to speak, capable of unlimited -increase; and yet, as we have seen, personal selection cannot directly -cause progress in any direction of variation; it can only secure a -free course by excluding from breeding the bearers of variations -with an opposite tendency. The beards of hens, the tail-feathers of -the long-tailed domestic cocks, the long and short, straight and -curved bills of pigeons, the enormously long ruffled feathers of the -Jacobin, the multiplication of the tail-feathers in the fan-tail, and -innumerable other breed-characters of these playthings of the breeder, -prove that when variation-tendencies of any part are once present, that -is, when they have arisen through germinal selection, they apparently -go on unchecked until their further development would permanently and -irretrievably destroy the harmony of the parts. As soon as this is -threatened the breed loses its power of survival, and Darwin in his -time cited the case of many extremely short-billed breeds of pigeon, -which require the aid of the breeder before they can emerge from the -hard-shelled egg, because their short and soft bills no longer allow -them to break their way out. Here the correlation between the hardness -of the egg-shell and that of the pigeon's bill has been disturbed, and -the breed can now only be kept in existence by artificial aid. - -There must be a possibility of something similar occurring in natural -conditions, and when it does the species concerned must die out. But in -the majority of cases the self-regulation which is afforded by personal -selection will be enough to force back an organ which is in the act -of increasing out of due proportion to within its proper limits. The -bearers of such excessively increased determinants succumb in the -struggle for existence, and the determinants are thus removed from the -genealogical lineage of the species. - -Having now established the fact that determinants can continue their -direction of variation without limit because of internal, that is -intra-germinal, reasons, we have come nearer an understanding of -many secondary sexual characters, whose resemblance to the excessive -developments artificially produced in our domestic poultry is so -very striking. Here, too, we shall have to regard germinal selection -as the root of the variations of plumage and other distinguishing -characters, which have evolved by intra-germinal augmentation into -the magnificently coloured crests, tufts, and collars, into the long -or graduated, multiplied or erectile tail-feathers of the birds of -Paradise, pheasants, and humming-birds. The conception of sexual -selection formulated by Darwin will be so far modified, that we are no -longer compelled to regard every minute step in this cumulative process -as due to the selection of the males by the females. A preference of -the finest males may still take place, and is probably general, since -only thus could the distinguishing male characters become common -property, that is, be transmitted to all or the majority of the ids -of the germ-plasm, but the increase of the individual determinants -which are in the act of varying goes on in each individual id, quite -independently of this personal selection. - -As it is not a single id with its determinant _a_ in ascending -variation that controls the organ _A_, but as it always requires -a majority of the ids _a_, this must be secured here by personal -selection just as it is in ordinary natural selection. If the -handsomest males are the successful competitors, then a majority of -the transformed ids _a´_ will be transmitted to a number of their -descendants, and the oftener this happens the larger will the majority -be, and the less becomes the danger that it will be dispersed again -by reducing division and amphimixis. Personal selection is thus in -no way rendered superfluous by germinal selection, only it does not -produce the augmentation of the distinguishing characters, but is -chiefly instrumental in fixing them in the germ-plasm; it collects, -so to speak, only the favourably varying ids, and, where complex -variations depending on the proper variation of many ids are concerned, -it combines these. How very great the influence of personal selection -may be in this case of secondary sexual characters we see clearly from -the soberly coloured mates of the brilliant males, for here natural -selection has been operative in conserving the coloration inherited -from remote ancestry. - -But if the question be asked, how _the first majority_ of determinants -varying in the same direction is brought about, there are two -possibilities: first, by chance, and secondly, by influences which -cause particular determinants of all the ids to vary in almost exactly -the same manner. We shall find illustrations of the latter among -climatic varieties; but the cases of the first kind are the more -important, for they form the foundation and the starting-point for -processes of selection of a higher order, for personal selection. It -might seem perplexing that processes of such importance should depend -ultimately upon chance; but when we remember that there are only two -directions of variation, namely a plus direction or a minus direction, -we recognize that the chance of a majority in one direction or another -is much greater than that of absolute equilibrium between the two, and -there is therefore a very strong probability that in many individuals -of the species either the upward or the downward movement of a -determinant _A_ will preponderate. - -Now as such variation movements, when they are of a certain strength, -increase automatically, we can easily see that they must gradually -attain to a level at which they acquire selection value, and how then, -by personal selection, the ids with favourably varying determinants may -be collected together. - -Of course it is not possible to state positively the time at which -in individual cases a variation acquires a biological significance, -that is, selection value. We can only say in a general way that, as -soon as it attains this, personal selection either in a positive or a -negative sense _must_ intervene; an injurious variation tends to the -elimination of its possessor, a useful one increases the probability of -its survival. - -There must, however, be for every variation a stage of development in -which it has as yet no decisive biological importance, and this stage -need not by any means be so insignificant that we cannot see it, or -can hardly do so: in other words, there are characters which have -arisen through germinal selection, which are of purely 'morphological -importance.' - -It has often been disputed whether there can be any such thing -as 'purely morphological characters,' which are indifferent as -far as the existence of the species is concerned. This question -used to be an important one, because the sphere of operation, and -therefore the importance of the Darwin-Wallace selection--personal -selection--depends on the answer, since this mode of selection only -begins when a character has some biological importance. But as soon as -we take germinal selection into consideration the question loses its -importance, because we now know that every variation is indifferent -to begin with, but every one can, under favourable circumstances, -be increased to such a pitch that it attains biological importance, -and that personal selection then takes over the task of carrying it -on, either in a positive or a negative sense. We may therefore leave -this disputed point alone just now, for while germinal selection -seems still far from being generally recognized, we have to remember -that we are not at all in a position to judge with any certainty as -to the biological value of a character. What labour and painstaking -investigation it has cost to give a verdict as to this even in a -few instances! Innumerable characters appear indifferent, and are -nevertheless adaptations. Darwin in his day pointed out the need for -caution in this matter, referring to the case of animal coloration as -an example; very little attention had been directed to it for a long -time because it had been believed to be without significance. And how -many diverse kinds of characters among animals and plants, which had -likewise been regarded as 'purely morphological,' have on more careful -investigation shown themselves of very great biological importance. -I need only refer to the shape, position, hair-arrangement, colour, -and lustre of flowers, and their relation to cross-fertilization by -means of insects, or to the thickness and shape of the leaves of -tropical trees with their coating of wax and their gutter-like outlets -for carrying off the tropical rain which falls in terrible downpour -(Haberlandt, Schimper), or to the limp, perpendicular drooping of the -tufts of the young and tender leaves of the same trees, which also -secures protection from being battered and torn by the rain. - -[Illustration: FIG. 107, _C._ Leptocephalus stage of an American Eel, -with seven pigment spots, of which three are on the left (_l_) and four -on the right (_r_) side. After Eigenmann.] - -There are even characters the biological use of which is unknown to -us, but in regard to which we can affirm that they have a use. Thus -Eigenmann described the larva of an American eel, which differs from -other so-called 'Leptocephali' in that a row of seven black spots runs -along its side. Apparently all these lie upon the side turned towards -us, but in reality they are distributed on both sides, three lying on -the left and four on the right, and so arranged that they look like a -single row of spots at regular intervals, for the flat little fish is -absolutely transparent. The habits of this larva are not yet known, but -we may conclude that this appearance of a simple row of spots must have -some value for the animal, for such a significant asymmetry could not -have arisen for purely internal reasons (Fig. 107, _C_). It is possible -that the fish is thus made to resemble parts of some marine alga, and -that it is thereby protected from many enemies; that there is not a -complete row upon each side may depend upon the fact that the two rows -would be visible at the same time, and that they would blur each other -in the eyes of the swimming enemy, and so destroy the resemblance of -the picture to its unknown model. - -But it cannot be denied that there are characters which have no special -biological significance. There are doubtless many such characters, -which stand beyond the threshold of good or bad, and which are -therefore not affected by personal selection; it is difficult and often -impossible to point these out with certainty. The shape of the human -nose and of the human ear, the colour of the hair and of the iris, may -be such indifferent characters whose peculiarities are to be referred -solely to germinal selection. On the other hand, I would not venture to -assert that the gay colouring and the complex markings on the wings of -our modern Lepidoptera are always and in all cases unimportant, even -when we cannot interpret their details either as protective, or as a -sign of nauseousness, or as mimetic. The usually very exact similarity -of the colour pattern in the individuals of each species seems to point -to the intervention of personal selection in some form or other, for in -what other way could such a large majority of variations in the same -direction have developed in the germ-plasm as this constancy of the -character indicates. - -We know, of course, that the colours of butterflies and moths can be -caused to vary through external and especially climatic influences, -but this would only account for simple modifications of colour, and -not for the origin of the complex colour patterns that actually -occur. I therefore believe with Darwin that sexual selection has had -much to do with this by giving a slight preference to the variations -produced by spontaneous germinal selection, and thus preventing the -majority of varied ids once acquired from being scattered again, but -always collecting more of them, and so securing free play for the -increase of the new character through intra-germinal processes. In -this way have arisen not only the brilliance of our Lycænidæ and of -the large Morphidæ of South America, but also many of the coloured -spots, streaks, bands, eyes, and other components which have gradually -in the course of time evolved into the complex colour pattern of -many of our modern butterflies. I should like to remind any one who -doubts this of a fact which corroborates the view that personal -selection has co-operated in the production of these colours--I -refer to the inconspicuous colouring of the females of many of these -brilliant males--while in contradistinction to these cases there are -other species in which both sexes are alike brilliant, so that it is -impossible that mere spontaneous germinal selection can have determined -that the females, because of their femaleness, should vary in a -different manner from the males. - -But while I believe that sexual selection in particular has had much to -do with producing the colours of Lepidoptera, the basis of all these -colour variations must still be looked for in germinal selection, and -we shall see later on how it is possible to think of the diversified -and often relatively abrupt transformations of marking as the resultant -of the co-operation of climatic influences with germinal selection. - -Of course there must also be unimportant changes in butterfly-markings -which depend solely on the internal play of forces in the determinant -system, and to this must be referred the markings of many of the -'variable' species whose variations are mere fluctuations in the -details of marking, which have therefore caused much trouble to the -systematists. Truly unimportant variations will rarely or never -combine into a 'constant' form, and the fact that there are species -which are 'variable' in such a high degree is enough to make us refer -their variations to their lack of importance, for if they possessed -any biological value the less valuable among them would gradually be -removed by selection. Perhaps the variable species of certain moths -like _Arctia caja_, and especially _Arctia plantaginis_, the little -'bear' of the Alps and Apennines, must be reckoned among these. -But from the fact that there are such fluctuations in the markings -of Lepidoptera, it seems to me that we must conclude that species -which show a high degree of constancy in their markings have been -influenced by selection, or by climatic influences which turned the -play of forces within the determinant system in the same direction in -all individuals. All these considerations and conclusions are quite -sound and serviceable theoretically, but they are difficult to apply -to individual cases, and where this is attempted it must be with the -greatest caution, and, if possible, on a basis of investigations -specially undertaken for the purpose; for how should we know whether -a species which to-day is highly variable may not a geological -epoch later become a very constant one? We must in any case assume -that marked fluctuations of characters are associated with many -transformations. - - - - -LECTURE XXVI - -GERMINAL SELECTION (_continued_) - - Germinal selection, spontaneous and induced--Climatic - forms of _Polyommatus phlæas_--Deformities--Excessive - augmentation of variations--Can it lead to the elimination - of a species?--Saltatory variations, copper-beech, weeping - trees--Origin of sexual distinguishing characters--Formation - of breeds among domesticated animals--Degenerate jaws--Human - teeth--Short-sightedness--Milk-glands--Small hands and feet--Ascending - variation--Talents, intellect--Combination of mental endowments--The - ultimate roots of heritable variation--There are only plus- and - minus-variations--Relations of the determinants to their - determinates--The play of forces in the determinant system of the - id--Germinal selection inhibited by personal selection--Objection on - the score of the minuteness of the substance of the germ-plasm. - - -Hitherto we have derived the variations of the determinants of the -germ-plasm, upon which we based the process of germinal selection, -from _chance local_ fluctuations in nutrition, such as must occur in -an individual id, independently of the nutrition of the other ids of -the same germ-plasm. But there are doubtless also influences which set -up similar nutritive changes in _all ids_, and by which, therefore, -all homologous determinants, in as far as they are sensitive to the -nutritive change in question, are affected in the same manner. To -this category belong changes in the external conditions of life, and -particularly climatic changes. It is, then, germinal selection alone -which brings about the presence of a majority of ids with determinants -varying in the same direction, and personal selection has no part -in the transformation of the species. Many years ago I instituted -experiments with a small butterfly, _Pararga egeria_, and these -showed that a heightened temperature so influenced the pupæ of this -form that the butterflies emerged with a different and deeper yellow -ground-colour, similar to that of the long-known southern variety -_Meione_. More thoroughly decisive, however, were the experiments on -_Polyommatus phlæas_, the small 'fire-butterfly,' which were carried -on in the eighties by Merrifield in England and by myself almost at -the same time. I shall discuss these later in more detail, and will -only say here that this butterfly, whose range extends from Lapland -to Sicily, occurs in two forms, the southern distinguished by a -'dusting' of deep black from the northern, in which the wing-surfaces -are of a pure red-gold. The experiments showed that the southern -form can be artificially produced by warmth, and the interpretation -must be that the direct influence of higher temperature affects the -quality of the nutritive fluids in the germ-plasm, and thereby at -the same time the determinants of one or more kinds of wing-scales -are caused to vary in all the ids in the same direction, in such a -fashion that they give rise to black scales instead of the former -red-gold ones. It is thus certain that there are external influences -which cause particular determinants to vary in a particular manner. -I call this form of germinal variation 'induced' germinal selection, -and contrast it with 'spontaneous' selection, which is caused, not by -extra-germinal influences, but by the chances of the intra-germinal -nutritive conditions, and which will, therefore, not readily occur at -the same time in all the ids of a germ-plasm, and so will not give rise -to variation of the same kind in the homologous determinants of all the -ids. - -The two processes must also be distinguished from each other in their -relation to personal selection, for induced germinal selection will -go on increasing until the maximum of variation corresponding to the -nature of the external influences and of the determinants concerned -is reached. Since _all_ the ids are equally affected and caused to -vary in the same way, personal selection has nothing to take hold of, -and the variation might go on intensifying even if it should become -biologically prejudicial. But it is quite otherwise with spontaneous -germinal selection, which has its roots not in all, but only in a -majority of the ids. Here the variation may go on increasing by -germinal selection alone, but only until it acquires a positive or -negative biological value, that is, until it becomes advantageous or -prejudicial to the life of the individual; then personal selection -intervenes and decides whether it is to go on increasing or not. -Spontaneous germinal selection can therefore only lead to the general -variation of a whole species when it is supplemented by some external -factor such as, especially, the utility of the variation. - -This does not imply, however, that indifferent variations of large -amount could not arise through spontaneous germinal selection, but -they would remain confined to a small number of individuals, and -would sooner or later disappear again. The congenital deformities of -Man may in part fall under this category. If, for instance, certain -determinants are, by reason of specially favourable local nutritive -conditions, maintained for a long time in progressive variation, they -will become so strong that the part which they determine will turn -out excessive, perhaps double. Hereditary polydactylism in Man may -perhaps be explained on this principle, and I had already referred -it to the more rapid growth and duplication of certain determinants -of the germ even before formulating the idea of germinal selection. -In this I was at one with the pathologist Ernst Ziegler, who had -designated polydactylism as a germ-variation, and in contrast to -others had not interpreted it in an atavistic sense, as a reversion to -unknown six-fingered ancestors. All excessive or defective hereditary -malformations may be referred to germinal selection alone, that is, to -the long-continued progressive or regressive variation of particular -determinant-groups in a majority of ids. - -The fact that, as far as our experience goes, superfluous fingers -are never inherited for more than five generations may be simply -explained, for there has been no reason for the intervention of -personal selection, either in the negative sense, for the six-fingered -state does not threaten life, nor in the positive, since it is not of -advantage. The deformity depends on spontaneous germinal variation, -which must have taken place in a majority of ids or it would not -have become manifest. But such a majority of 'polydactylous' ids is -liable to become scattered again in every new descendant, and to be -reduced again into a minority which can no longer make itself felt by -the chances of reducing division and the admixture of normal ids in -amphimixis. A polydactylous race of men could only arise through the -assistance of personal selection; in that case there would doubtless -be just as much chance of success in breeding a six-fingered race as -there was in breeding the crooked-legged Ancon sheep from a single ram -which was malformed in this manner. Without a gradual setting aside of -the germs with normal ids, that is, without personal selection, such -spontaneous deformities, and indeed all _spontaneous_ variations, must -fail of attaining to permanent mastery. - -This must frequently be the case in free nature also, but we shall have -to investigate later on, in the section devoted to the formation of -species, whether external circumstances (inbreeding) may not also occur -which make it possible for spontaneous variations to become constant -breed-characters, even although they remain neither good nor bad, and -are thus not subject to the action of personal selection. - -In general, however, amphigony with its reduction of the ids and its -constant mingling of strange ids will form the corrective to the -deviations which may arise through the processes of selection within -the id, and which lead to excessive or superfluous development of -certain structures, to a complete disturbance of the harmony of the -parts, and ultimately to the elimination of the species. - -It must be admitted, however, that Emery was probably right when he -directed attention to the possibility of a 'conflict between germinal -and personal selection.' It is quite conceivable that in cases of -useful variations, that is, of adaptations, the processes of selection -within the germ-plasm may lead to excessive developments, which -personal selection cannot control, because, on account of their earlier -usefulness, they have in the course of a series of generations and -species become fixed not only in a majority of ids, but in almost all -the ids of the collective germ-plasm of the species. In this case a -reversal must be difficult and slow, for the gathering together of ids -with relatively weaker determinants can only take place slowly, and -it is questionable whether the species would survive long enough for -the slow process to take effect. But, apart from the question of time, -such a reduction of an excessive development would sometimes be quite -impossible, for the simple reason that there is nothing for personal -selection to take hold of. - -Döderlein has pointed out that many characters go on increasing through -whole series of extinct species, and ultimately grow to such excess -that they bring about the destruction of the species, as, for instance, -the antlers of the giant stag or the sabre-like teeth of certain -carnivores in the diluvial period. I shall have to discuss this in more -detail in speaking of the extinction of species; it is enough to say -here that such long-continued augmentations in the same direction can -never be referred _solely_ to germinal selection, since it is hardly -conceivable that a species--much less a whole series of species--should -arise with injurious characters; they would have become extinct while -they were still in process of arising. Although we see that the -Irish stag, with his enormous antlers over ten feet across from tip -to tip, was heavily burdened, we are hardly justified in concluding -that the size and weight of the burden on his head tended to his -destruction from the first--for in that case the species would never -have developed at all--but it may well be that at some time or other -the life-conditions of the species altered in such a manner that the -heavy antlers became fatal to it. In this case the variation-direction -which had gained the mastery in all ids could no longer be sufficiently -held in check by personal selection, because the variations in the -contrary direction would be much too slight to attain to selective -value. Sudden, or at least rapidly occurring changes in the conditions -of life, such as the appearance of a powerful enemy, exclude all chance -of adaptation by the slow operation of personal selection. - -If we look into the matter more carefully, we see that it is not -strictly true to say that germinal selection alone brings about the -extinction of a species by cumulative augmentation of structures which -are already excessive; _it is the incapacity of personal selection -to keep pace with the more rapid changes in the conditions of life -and to reduce excessive developments to any considerable extent in a -short time_. This would always be possible in a long time, for the -determinants of the excessive organ _E_ can never be equally strong in -all the ids; they always fluctuate about a mean, however high this mean -may be. Here again it must still be possible that reducing-divisions -and amphimixis may lead to the formation of majorities of ids with -weaker _E_-determinants, and if sufficient time be allowed, artificial -selection could, by consistently selecting the individuals with, let -us say, weaker antlers, give rise to a descending variation-movement. -There are no variation-movements which cannot be checked; every -direction can be reversed, but time and something to take hold of -must be granted. That was wanting in the case of the giant stag, for -it would not have been saved even if its antlers had at once become a -couple of feet shorter, and germinal selection can hardly make so much -difference as that. - -Analogous to hereditary deformities, and of special interest in -connexion with the processes within the germ-plasm, are '_sports_' -variations of considerable magnitude which suddenly appear without our -being able to see any definite external reason for them. I have already -discussed these in detail in my _Germ-plasm_, and have shown how simply -these apparently capricious phenomena of heredity can be understood _in -principle_ from the standpoint of the germ-plasm theory. - -The chances of the transmission of the saltatory variation will be -greater or less according to whether the variation of the relevant -determinants involves a bare majority of ids or a large majority, for -the more ids that have varied, the greater is the probability that the -majority will be maintained throughout the course of ensuing reducing -divisions and amphimixis, that is, that the seeds of the plant will -reproduce the variation, and will not revert to the ancestral form. -Although one of the most satisfactory results of the id-theory lies -precisely in the interpretation of these conditions, I do not wish -to enter into the matter here, but will refer to the details in my -_Germ-plasm_, published in 1894, which I consider valid still. At that -time I had not formulated the idea of germinal selection, but the -explanation of the occurrence of such sport-variations which I gave was -based upon the assumption of nutritive fluctuations in the germ-plasm, -which gave rise to variations in certain determinants. There was still -lacking the recognition that the direction of variation once taken must -be adhered to until resistance was met with, and that the determinants -stand in nutritive correlation with one another, so that changes in -one determinant must re-act upon the neighbouring ones, as I shall -explain more fully afterwards. I also showed from definite cases that -such sports, though they are sudden--'saltatory'--in their mode of -occurrence, are long being prepared for by intimate processes in the -germ-plasm. This 'invisible prelude' of variation depends on germinal -selection. When a wild plant is sown in garden-ground it does not -require to vary at once; several, even many, generations may succeed -each other which show no sports; suddenly, however, sports appear, -at first singly, then, perhaps, in considerable numbers. It is not, -however, by any means always the case that considerable numbers occur, -for some varieties of our garden flowers have arisen only once, and -then have been propagated by seed; and such saltatory sports in plants -which are raised from seed are usually constant in their seed, and if -they are fertilized with their own pollen they breed true--a proof that -the same variations must have taken place in the relevant determinants -in a large majority of ids. - -In animals, it would appear, such saltatory variations occur much more -rarely than in plants; the case examined in detail by Darwin of the -'black-shouldered peacock' which suddenly appeared in a poultry-yard is -an example of this kind. Much more numerous, however, are the instances -among plants, and especially among plants which are under cultivation. -This indicates that we have here to do with the effect of external -conditions, of nutritive influences which cause the slow variation of -certain determinants, sometimes abetting and sometimes checking. As -soon as a majority of ids varied in this way comes to lie in a seed, a -sport springs up suddenly and apparently discontinuously--a plant with -differently coloured or shaped petals or leaves, with double flowers, -with degenerate stamens, or with some other distinguishing mark, and -these new characters persist if the variety is propagated without -inter-crossing. - -But it happens sometimes, though more rarely, that not the whole -plant but individual shoots may exhibit the variation. To this class -belong the 'bud-variations' of our forest trees, the copper-beeches, -copper-oaks, and copper-hazels, the various fasciated varieties of oak, -beech, maple, and birch, and the 'weeping trees'; also the numerous -varieties of potato, plantain, and sugar-cane. It seems that only a -few of these breed true when reproduced from seed, or in other words, -they usually exhibit reversions to the ancestral form: on the other -hand, in the weeping oak for instance, nearly all the seedlings exhibit -the character of the new variety, though 'in varying degrees.' The -records as to the transmissibility of bud-variations through seed are -probably not all to be relied upon, and new investigations are much -to be desired, but the fact that in many cases they may be propagated -not only by means of layers and cuttings but by seed also, is most -important in our present discussion, for it proves that here too the -varied determinants must be contained in a majority of ids. As it -is only a single shoot that exhibits the saltatory variation, only -the germ-plasm which was contained in the cells of this one shoot -can have varied, and it must have done so in so many ids that the -variation prevailed and found expression. But that, in this case also, -the variation does not appear in all, but only in a small majority -of ids, is proved by the frequent reversion of bud-varieties to the -ancestral form. I have already reported a case of this kind shown to -me by Professor Strasburger in the Botanic Gardens in Bonn, where a -hornbeam with deeply indented 'oak-leaves' had one branch which bore -quite normal hornbeam leaves. In my own garden there is an oak shrub -of the 'fern-leaved' variety, whose branches bear some leaves of the -ordinary form; variegated maples with almost white leaves often exhibit -in individual branches a reversion to the fresh green leaves of the -ancestral form. We see from this that what is so energetically disputed -by many must in reality occur--namely, differential or non-equivalent -nuclear division--for otherwise it would be unintelligible how -the ids of the new variety, if they once attain a majority in the -tree, could give place in an individual branch to a majority of the -ancestral ids. Only differential nuclear division, in the manner of a -reducing division, can be the cause of this. Of course this implies -only a dissimilar or differential distribution of the ids between the -two daughter-nuclei, not a splitting up of the individual ids into -non-equivalents. - -That in free Nature bud-variations left to themselves can ever become -permanent varieties is probably an unlikely assumption, because of the -inconstancy of their seeds which only breed true in rare cases; nor -is it likely that such variations as the copper-beech, the weeping -ash, and so on could hold their own in the struggle for existence -with the older species; but there is certainly nothing to prevent our -assuming that, in certain circumstances, saltatory variations, when -they have a germinal origin, may become persistent varieties and may -even lead to a splitting of the species. This may happen, for instance, -when the variations remain outside the limits of good and bad, and -thus are neither of advantage to the existence of the species nor a -drawback thereto. In the next chapter we shall discuss the influence of -isolation upon the formation of species, and it will be seen that in -certain conditions even indifferent variations may be preserved, and -that saltatory variations, as for instance in the evolution of species -of land-snails or butterflies, may have materially contributed to bring -this about. - -I should like to emphasize still more the part played by saltatory -variations arising from germinal selection in the origin of secondary -sexual characters. As soon as personal selection, whether sexual -or ordinary, prefers as useful in any sense a saltatory variation, -it is not only preserved and becomes a character of a variety, but -it may increase, and we have to ask whether such sudden variations -are frequently of a useful kind, especially when not individual -characters alone, but whole combinations of them are implicated. If -we may judge from the sports of the flowers and the leaves of plants, -transformations useful to the species as a whole rarely occur suddenly, -that is, they occur only in a few out of very numerous sports; they -are much more frequently indifferent, although quite visible and often -conspicuous variations. - -For this reason I am disposed to attribute to saltatory variations a -considerable share in the production of distinctive sexual characters. -From saltatory variations in flowers, fruits, and leaves we know that -these may be conspicuous enough even on their first appearance, and so -we are justified in finding in such variations the first beginnings of -many of the decorative distinguishing characters which occur in the -males of so many animals, especially butterflies and birds. As soon as -it is admitted that variations of considerable amount, which have been -slowly prepared in the germ-plasm by means of germinal selection, can -suddenly attain to expression, one of the objections against sexual -selection is disposed of, for conspicuous variations are necessary for -the operation of this kind of selection, since the changes in question -must attract the attention of the females if they are to be preferred. -Without such preference, even though it be not quite strict and -consistent, a long-continued augmentation of the decorative characters -is inconceivable. - -But as intra-germinal disturbances of the position of equilibrium in -the determinant system is at the root of the saltatory variations of -our cultivated plants, it must also have played a large share in the -evolution of breeds among our domesticated animals, which is therefore -by no means wholly due to artificial selection operating upon the -variation of individual characters. In all breeds in the formation of -which the production of more than a single definite character was -concerned, as, for instance, in the broad-nosed breeds of dog--bull-dog -and pug-dog--we may refer the peculiar variation of many parts to -disturbances of the equilibrium of the determinant system, which bring -to light, not suddenly as in the case of saltatory variations, but -gradually and increasingly, the curious complex of characters. Darwin -referred such transformations of the whole animal facies, where a -single varying character is deliberately selected, to correlation, and -by this he understood the mutual influence of the parts of an animal -upon one another. Such correlation certainly exists, as we have already -seen in discussing histonal selection, but here we have rather to do -with the correlation of the parts of the germ-plasm, with the effects -of germinal selection, which, affected by the artificial selection of -particular characters, gradually brings about a more marked disturbance -in the whole determinant system. - -In the evolution of our breeds of domesticated animals, germinal -selection in the negative sense must also have played a part--I mean -through the weakening and degeneration of individual determinants. -Only in this way, it seems to me, can we explain the tameness of our -domestic animals, dogs, cats, horses, &c., in which all the instincts -of wildness, fleeing from Man, the inclination to bite, and to attack, -have at least partly disappeared. It is, of course, very difficult to -estimate how much of this is to be ascribed to acquired habitude during -the individual lifetime. The case of the elephant might be cited in -evidence of tameness which arises in the individual lifetime, for all -tame elephants are caught wild, but it seems that captured young beasts -of prey, such as the fox, wolf, and wild cat, not to speak of lions and -tigers, never attain to the degree of tameness exhibited by many of our -domesticated dogs and cats. The very considerable differences in the -degree of tameness of dogs and cats go to show that the case is one of -instincts varying in different degree. - -If this be so, then the instinct of wildness, if I may express myself -so for the sake of brevity, has degenerated in consequence of its -superfluity, and through the process of germinal selection, which -allowed the determinants of the brain-parts concerned to set out on a -path of downward variation upon which they met with no resistance on -the part of personal selection. - -Herbert Spencer adduced against my position the case of the reduction -in the size of the jaws in many breeds of dog, especially in pugs and -other lap-dogs, which he regarded as evidence of the inheritance of -acquired characters. But this and analogous cases of the degeneration -of an organ during a long period in which the animal had been withdrawn -from the conditions of natural life is intelligible enough on the -assumption of persistent germinal selection aided by panmixia. The jaws -and teeth in these spoilt pets no longer require to be maintained at -the level of strength and sharpness essential to their ancestors which -depended on these characters, and so they fell below it, became smaller -and weaker, but could not disappear altogether, for the process of -degeneration was brought, or is being brought, to a standstill by the -intervention of personal selection. - -Even the lower jaw in Man is declared by many authors to be degenerate. -Collins found that the lower jaw of the modern Englishman was one-ninth -smaller than that of the ancient Briton, and one-half smaller than that -of the Australians; Flower showed that we are a microdont race like -the Egyptians, while the Chinese, Indians, Malays, and Negroes are -mesodont, and the Andamanese, Melanese, Australians, and Tasmanians are -macrodont. This does not of itself imply that we exhibit a degeneration -of dentition, though this conclusion is hinted at by other facts, -such as the variability of the wisdom-teeth. It need not surprise us, -indeed, that a retrogressive variation tendency should have started -in this case, for, with higher culture and more refined methods of -eating, the claims which personal selection was obliged to make on the -dentition have been greatly diminished, and germinal selection would -thus intervene. - -Every one knows how the quality of human teeth has deteriorated with -culture, and this not in the higher classes only, but even among the -peasantry, as Ammon has observed. The time is past when raw flesh -was a dainty, and when bad teeth meant poor nutrition, if not actual -starvation. Even nowadays famine plays a terrible and periodically -recurrent rôle as an eliminator among some negroid races. - -Many other organs in man have been reduced from their former pitch of -perfection through culture, and some of them are still in process of -dwindling. When I formulated the idea of panmixia and applied it to -explain cases which had previously been referred to the inheritance of -the results of disuse, I regarded the short-sightedness of civilized -Man from this point of view. My opinion aroused lively opposition at -the time, especially on the part of oculists, who very emphatically -referred the phenomenon to the inheritance of acquired shortsight, -and indeed regarded it as a proof of the transmission of functional -modifications. - -But, apart from the fact that the assumption of this mode of -inheritance must now be regarded not only as unproved, but as -contradicted by reliable data, panmixia, in conjunction with -the ceaseless fluctuations within the germ-plasm--germinal -selection--affords a better explanation than the other theory was ever -in a position to offer. At that time I pointed out that the survival -of the individual among civilized races had not for a very long time -depended on the perfection of his eyesight, as it does for instance -in the case of a hunting or warlike Indian, or of a beast of prey, -or of a herbivore persecuted by the beast of prey. And this is by no -means due solely to the invention of spectacles, but in a much greater -degree to the fact that every man no longer has to do everything, so -that numerous possibilities of gaining a livelihood remain open to the -less sharp-sighted; that is, the division of labour in human society -has made the survival of the short-sighted quite feasible. As soon as -this division of labour reached such a degree that the founding of a -family offered no greater difficulty to the short-sighted individual -than to one with normal sight, short-sightedness could no longer be -eliminated; and partly because of the mingling with normal sight, but -partly also because of the never-failing minus-fluctuations of the -germ-plasm determinants concerned, a variation in a downward direction -was bound to set in, and will continue until a limit is set to it by -personal selection. Meantime, we are obviously still in the midst of -the process of eye-deterioration; and the resistance to it is somewhat -inhibited in its operation, because although individuals with extremely -bad sight are for the most part hindered from gaining an independent -livelihood and having a family, this is certainly, thanks to our -mistaken humanity, not always the case. There are even instances of -marriage between two blind persons! - -As yet, however, the deterioration of eyes has not advanced very far; -not nearly all families are affected by it, and even in Germany, -the land of the 'longest school form' and of the greatest number -of spectacle-wearers, short-sight is still usually acquired by -individuals, although there must frequently be a more or less marked -predisposition to it. It is a common objection to this view that in -England, France, and Italy the percentage of short-sighted individuals -is much lower, and, in point of fact, one sees far fewer people wearing -spectacles in those countries. This, however, does not prove that a -similar deterioration of eyes has not begun there also, for how could -the small inherited beginnings be detected if they were not accentuated -by the spoiling of the eyesight in the lifetime of the individual by -much reading of bad print, and by writing with bent head, as is still -too often the case in many German schools. - -That our interpretation, through panmixia on a basis of germinal -selection, is the correct one, we infer also from the fact that -short-sightedness has been proved to be a frequent character even among -our domesticated animals, such as the dog and the horse. These animals -receive protection and maintenance from Man, and their survival and -reproduction no longer depend on the acuteness of their sight, and -thus the eye has fallen from its original perfection, just as in Man, -although in this case reading and writing play no part. - -A whole series of similar slight deteriorations of individual organs -and systems of organs might be enumerated, all of which have appeared -in consequence of long and intensive culture in Man. All these must -depend upon germinal selection, on a gradually progressive weakening -of the determinant-groups concerned, under the conditions of panmixia, -that is, in the absence of positive selection. - -To these must be added the deterioration of the mammary-glands and -breasts, and the inability to suckle the offspring which results -chiefly from this. Here we have a variational tendency which could not -appear in a people at a lower stage of culture, and it has not become -general in the lower classes of society among ourselves. - -The muscular weakness of the higher classes is another case in point, -and all gymnastics and sports will be of no avail as long as a relative -weakness of the muscles is not a hindrance to gaining a livelihood, and -having a family. Even universal conscription will do nothing to check -this falling off of the bodily strength. Certainly military service -strengthens thousands, and hundreds of thousands of individuals, but it -does not prevent the weaklings from multiplying, and thus reproducing -the race-deterioration. But it would indeed be well if only those who -had gone through a term of military service were allowed to beget -children. - -It is only among the peasantry, inasmuch as they really work and -do not merely look on as proprietors of the ground, that such a -deterioration of the general muscular strength could not become the -permanent variational tendency of the determinants concerned, because -among genuine peasants bodily strength is a condition of having and -supporting a family--at least on an average. - -The diminution in the firmness and thickness of the bones in the -higher classes, and many another mark of civilization, must be looked -at from the point of view of panmixia and germinal selection; perhaps -also the smaller hands and feet which frequently occur along with a -more graceful general build in the higher ranks of European peoples. -It would certainly not be surprising if in families which usually -intermarry, and which in no way depend for their material subsistence -on the possession of large and powerful hands and feet or bones -generally, a downward variation of the relevant germ-determinants -should have developed, but this could never overstep a certain limit, -because it would then be prejudicial even in civilized life. That we -must be very careful not to regard large hands and feet as the direct -result of hard physical toil was brought home to me by an observation -of Strasburger's. He was particularly struck by the fact that the -peasants of the high Tatra (Carpathians) were distinguished by the -smallness of their hands and feet. - -But while civilization has excited numerous downward variations in the -germ, it has, on the other hand, been the cause of numerous hereditary -improvements--variations in an upward direction. This opens up new -ground, for hitherto we have been confronted with the alternative of -either accepting the inheritance of acquired characters, and on this -basis referring the talents and mental endowments of civilized Man to -exercise continued throughout many generations, or of admitting an -increase of mental powers only in as far as they possess 'selection -value,' that is, as they may be decisive in the struggle for existence. -To these mental qualities belong cleverness and ingenuity in all -directions, courage, endurance, power of combination, inventive power, -with its roots in imagination and fertility of ideas, as well as desire -for achievement, and industry. Throughout the long history of human -civilization these mental qualities must have increased through the -struggle for existence, but how have the specific talents such as those -exhibited in music, painting, and mathematics come into existence? -And how have the moral virtues of civilized Man been evolved, and -particularly unselfishness? For it can hardly be maintained of any of -these endowments that they possess selection-value for the individual. - -It is not my intention to discuss these questions in detail; they are -too many-sided and of too much importance to be treated of merely in -passing; moreover, I gave expression years ago to my views on this -subject by dealing with one example--the musical sense in Man. I do -not believe that the musical sense had its beginnings in Man, or that -it has materially increased since the days of primitive Man, but in -conjunction with the higher psychical life of civilized peoples its -expressions and applications have risen to a higher level. It is, so to -speak, an instrument which has been transmitted to us from our animal -ancestors, and on which we have learnt to play better the more our mind -has developed; it is an unintended 'accessory effect' of the extremely -fine and highly developed organs of hearing with their nerve-centres -which our animal ancestors acquired in the struggle for existence, and -which played a much more important rôle in the preservation of life in -their case than it does in ours. The musical sense may be compared to -the hand, which was developed even among the apes, but which civilized -Man in modern times no longer uses merely to perform its original -function, grasping, but also for many other purposes, such as writing -and playing the piano. And just as the hand did not originate through -the necessities of the piano, neither did the extremely delicate sense -of hearing of the higher animals develop for the sake of music, but -rather that they might recognize their enemies, friends, and prey, in -darkness and mist, in the forest, on the heath, and at great distances. - -The case is probably the same with the rest of the special psychical -endowments or talents. I do not of course maintain that they, like -the musical sense, did not at some time play a rôle in the struggle -for existence and survival, and therefore could not increase, but -the increase was certainly not continuous, but much interrupted, -so that it would extend only to small groups of descendants, and -therefore could only contribute very slowly to the elevation of the -psychic capacities of a whole people. But in certain individuals -and families such augmentations would certainly take place through -germinal selection, and it seems to me probable that these would never -be wholly lost again, even if they appeared to be so, but would be -handed on, in id-minorities, through the chain of generations, and -would slightly raise the average of the talent in question, and might -even, under favourable circumstances, combine in the development of -a genius. We know how strongly hereditary such specific talents are; -let us suppose that the determinants of, say, the musical sense have, -by the intra-germinal chances of nutrition, been started on a path of -ascending variation; they will continue in this path until a halt is -called from some quarter or other. This can only happen if, in the -reducing division, or in amphimixis, the highly developed musical -determinants are wholly or partly eliminated, or are reduced to a -minority. As long as this does not happen the ascending variation will -go on, and then we may have the birth of a Mozart or of a Beethoven. -Personal selection will not interfere either in a positive or a -negative sense, since high development of the musical sense has no -effect either in advancing or retarding the struggle for existence; -the increase will therefore go on until the large majority of highly -developed musical determinants, which we must assume in the case of -a musical genius, is reduced, or even transformed into a minority, -through unfavourable reducing divisions of the germ-cells, and by -association with the germ-cells of less musical mates. - -The fact that highly developed specific talents have never been known -to be inherited through more than seven generations is quite in keeping -with this view. But even this persistence has been observed only in -the case of musical talent, and the long continuance of the inherited -talent may well be due, as Francis Galton suggests in his famous -statistical investigations into the phenomena of inheritance, to the -fact that musical men do not readily choose wives who are absolutely -lacking in this talent. It would be easy to rear an exceedingly -highly gifted musical group of families within the German nation, if -we could secure that only the highly-gifted musically should unite -in marriage--that is, if personal selection could play its part. In -another more general domain of mental endowment a case of this kind -has been recorded, for Galton tells us of three highly gifted English -families which intermarried for ten generations, and in that time -scarcely produced a descendant who did not deserve to be called a -distinguished man in some direction or other. - -Of course, such continued persistence, through a long series of -generations, of a high general mental level is more possible than the -transmission and increase of a specific talent, for in the former case -it is a question of a mixture of different high mental endowments, -of which not all need be developed in every individual, and yet the -individual need not fall to mediocrity if he possesses a combination -of other qualities. But in musical talent, on the other hand, the -falling from the height once attained takes place as soon as this one -character is no longer represented in a sufficiently strong majority -of determinants. Of course it would be a mistake to believe that the -talent of a Sebastian Bach or a Beethoven depended solely on the -highly developed musical sense; in them, as in all great artists, many -highly developed mental qualities must have combined with the musical -sense; a simpleton could never have written the Mass in B minor or the -Passion of St. Matthew even if he had possessed the musical genius of -Sebastian Bach. In this fact lies a further reason why genius is seldom -found at the same pitch in two successive generations; the combination -of mental characters always varies from father to son, and slight -displacements may give rise to very great differences in relation to -the manifestations of the specific talent. Under certain circumstances, -the weak development of a single trait of character, as, for instance, -power of action, or the excessive development of another, such as -indecision or desultoriness, may so nullify the existing favourable -combinations of mental characters, such as, let us say, musical sense, -inventive talent, depth of feeling, &c., that they bear no fruit -worth mentioning. And since as we have already seen, the different -mental qualities of the parents are to a certain extent separately -transmitted, that is, since they may appear in the children in the most -diverse combinations, we should rather be surprised that pronounced -talent in a specific direction can persist in a family for two and a -half centuries than that it should do so very rarely. For reducing -division is always combining the existing mental qualities anew, and -amphimixis is adding fresh ones to them. - -Thus germinal selection, that is, the free, spontaneous, but definitely -directed variation of individual groups of determinants, is at the -root of those striking individual peculiarities which we call specific -talents; but it can attain to the highest level only rarely and in -isolated cases, because these talents are not favoured by personal -selection, and therefore the excessively highly developed determinants -upon which they depend may be dispersed in the course of generations; -they may sink to smaller majorities, or even to minorities, in which -case they will no longer manifest themselves in visible mental -qualities. - -We deduced the process of germinal selection on the basis of the -assumption that the nutrition of all the parts and particles of -the body, therefore also of the determinants and biophors of the -germ-plasm, is subject to fluctuations. We regarded the resulting -variations of these last and smallest units of the germ-plasm as the -ultimate source of all hereditary variation, and therefore the basis of -all the transformations which the organic world has undergone in the -course of ages and is undergoing still. - -We have still to inquire whether we can give any more precise account -of the nature of these units of the germ-plasm. If I mistake not, -we may say at least so much, that all variations are, in ultimate -instance, quantitative, and that they depend on the increase or -decrease of the vital particles, or their constituents, the molecules. -For this reason I have hitherto always spoken of only two directions -of variation--a plus or a minus direction from the average. What -appears to us a qualitative variation is, in reality, nothing more -than a greater or a less, a different mingling of the constituents -which make up a higher unit, an unequal increase or decrease of these -constituents, the lower units. We speak of the simple growth of a cell -when its mass increases without any alteration in its composition, -that is, when the proportion of the component parts and chemical -combinations remains unchanged; but the cell changes its _constitution_ -when this proportion is disturbed, when, for instance, the red -pigment-granules which were formerly present but scarcely visible -increase so that the cell looks red. If there had previously been no -red granules present, they might have arisen through the breaking up -of certain other particles--of protoplasm, for instance, in the course -of metabolism, so that, among other substances, red granules of uric -acid or some other red stuff were produced. In this case also the -qualitative change would depend on an increase or decrease of certain -simpler molecules and atoms constituting the protoplasm-molecule. Thus, -in ultimate instance, all variations depend upon quantitative changes -of the constituents of which the varying part is composed. - -It might be objected to this argument that chemistry has made us -acquainted with isomeric combinations whose qualitative differences -do not depend upon a different _number_ of the molecules composing -them, but upon their different arrangement; it might be supposed that -something similar would occur also in morphological relations. And, in -point of fact, this seems to be the case. We may, for instance, imagine -one hundred hairs as being at one time equally distributed on the back -of a beetle, and at another standing close together and forming a kind -of brush, but although this brush would be a new character of the -beetle, yet its development would depend upon quantitative differences, -namely, on the fact that the same skin-area, which in the first case -bore perhaps only one hair, had in the second case a hundred. The -quantity of hair cells has notably increased upon this small area. In -the same way the characteristic striping of the zebra depends not on -a qualitative change in the skin as a whole, but upon an increased -deposit of black pigment in particular cells of the skin, therefore -on a quantitative change. In relation to the whole animal it is a -qualitative variation, as contrasted, for instance, with the horse, but -in respect of the constituent parts which give rise to the qualitative -variation it is purely quantitative. The character of the whole edifice -is changed when the proportion of the stones of which it consists are -altered. - -Thus the determinants of the germ may not only become larger or smaller -as a whole, but some kinds of the biophors of which they are made up -may increase more than others, under definite altered conditions, and -in that case the determinants themselves will vary qualitatively, so -that, from the changing numerical proportions of the different kinds of -biophors, a variation of the characters of the determinants can arise, -and consequently also qualitative variations of the organs controlled -by the determinants--the determinates. But, since nothing living can -be thought of as invariable, the biophors themselves may, on account -of nutritive fluctuations, grow unequally, and thereby vary in their -qualities. To follow this out in greater detail and attempt to guess -at the play of forces within the minutest life-complexes would at -present only be giving the rein to imagination, but in principle no -objection can be made to the assumption that every element of life down -to the very lowest and smallest can, by reason of inequalities in its -nutrition, be not only started on an ascending or descending movement -of uniform growth, but can also be caused to vary _qualitatively_, -that is, in its characters, because its component parts change their -proportions. - -Of course we know nothing definite or precise with regard to the units -of the germ-plasm, and we cannot tell what is necessary in order -that a determinant shall determine a part of the developing body in -this way or in that; thus we have no definite idea of the relations -subsisting between the variations of the determinants and those of -their determinates, but we know at least so much, that hereditary -variation of a part is only possible when a corresponding particle -in the germ-plasm varies; and we may at least assume that these -correspond to each other so far, that a greater development of the one -implies a greater development of the other, and that a reversal of -these relations is impossible. If the determinant _X_ disappears from -the germ-plasm the determinate _X´_ disappears from the soma. It is -therefore justifiable to infer from the degree of development of an -organ the strength of its determinant, and to assume that plus- and -minus-variations in both are correspondingly large. - -But in addition to the fluctuations in the equilibrium of the -germ-plasm which lie at the root of all hereditary variation, we have -to take into account something which we have already touched upon -briefly--the correlation of the determinants, the influencing of one -determinant by those round about it. I have spoken for the sake of -brevity of 'the determinant' of a part, although all the large and -more important parts must certainly be thought of as represented by -several or many, if not, indeed, by whole groups of determinants. -Although it is quite out of our power to follow the complex processes -of the mutual influences of the determinants upon each other, we can -say this at least, _that these influences must exist_, and we have -here a faint indication of what must occur in the case of spontaneous -variations within the germ-plasm. We must, in the first place, think -of the individual determinants as arranged in groups, so that, for -instance, the determinants of the right and left half of the body lie -together, and therefore are frequently affected together by influences -which cause variation, so that both vary in the same direction at -the same time. In point of fact, analogous deformities, such as -polydactylism of both right and left hands, and even of hands and feet -at once, do actually occur. That the right and left hands, the fore- -and hind-limbs, are represented in the germ by particular determinants, -may be inferred from their frequently different phyletic evolution into -different forms of hand and foot, e.g. into flipper and rudimentary -hind-leg in the whale, as well as from the cases of particulate -inheritance, which are rare, but which undoubtedly do occur, such -as when, in Man, there is a maternal blue eye on one side of the -head and a paternal brown eye on the other. But almost more striking -than the differences between these homologous or homotypic parts -are their points of resemblance, and these may probably be in part -referred to their disposition side by side and common history in the -germ-substance, although a far larger proportion of them are probably -due to their adaptation to similar functions, and are therefore to be -regarded as a phenomenon of convergence within the same organism. - -We have already seen that the first increase in the growth of one -determinant means a withdrawal of nourishment, however slight, from its -neighbours; this can, of course, be equalized again if the claims on -the common nutritive stream from another quarter are at the same time -diminished; but it is possible that the claims from another quarter -may also be increased, and the withdrawal will then be more marked, -and the determinants being thus injured from two directions at once -will sink downwards with greater rapidity. But it is also conceivable -that the majority of determinants of a part may vary upwards, and, -by their combined increased power of assimilation, direct towards -themselves such a greatly increased stream of nourishment that the -whole organ--for instance, a particular feather in a bird--varies in an -upward direction, and becomes larger and larger, as we see in the case -of many decorative feathers; or that certain determinants vary only as -far as some of their biophors are concerned, and similarly for their -determinates, as when a group of scales on a butterfly's wing that had -previously been black turn out a brilliant blue. It can probably also -happen that such variations within the determinants are transmitted to -neighbouring determinants because the nutritive conditions which caused -the first to vary have extended to those about them. The increase of -brightly coloured spots in birds and butterflies gives us ground for -concluding that there are processes of this kind within the germ-plasm. - -I will refrain from following this idea into greater detail, and -translating the observable relations and variations of the fully-formed -parts of the body into the language of the germ-plasm; but so much may -be taken as certain, that multitudinous inter-relations and influences -exist between the elements of the germ-plasm, and that one variation -brings another in its train, so that--usually at a very slow rate, -that is, in the course of generations and of species-forming, definite -variations occur from purely intra-germinal reasons--variations -which as far as they remain outside the limits of good or bad may of -themselves change the character of a species, but which when they are -seized upon by personal selection may, by sifting and combination of -the ids, be led on to still higher development. - -If we consider further that the variation of a part must depend -not only on the quality of the external stimulus but also upon the -constitution, the reacting power of the part, we shall understand that -similar nutritive variations may cause two different determinants to -vary in different ways, and when we reflect that every nutritive change -must extend from the point from which it started with diminishing -strength in a particular direction, we have a further factor in the -variation of determinants and one which influences even similar -determinants differently. - -Finally, if we remember that determinants of different constitution -will also extract different ingredients from the nutritive stream and -thus set up in it different kinds of chemical change, thus causing -an altered supply of nutritive substances to flow to the neighbour -determinants, we get some insight into a very complex and delicate -but perfectly definite set of processes, into a mechanism which we -can certainly only guess at, but whose results lie plainly before -us in the spontaneous variations of the organism. We understand in -principle the possibility of saltatory variation, as a more or less -widespread, more or less marked disturbance of the species-type in -this or that group of characters, and we may acknowledge that those -'kaleidoscopic variations' which Eimer supposed to be the sole basis -of the transformation of species, and which have been brought to the -foreground again quite recently by De Vries[20], are probably factors -in transmutation operative within a limited sphere. - -[20] See end of chap. xxxiii. - -But we must think of all these struggles and mutual influencings as -taking place on the smallest possible scale, so that it is only by -long summation that they can produce any visible effect, and we must -never forget the essential significance of the plurality of ids, for -these 'spontaneous' variations may take place in a different and quite -independent manner in each individual id. If this were not so no -intervention of personal selection would be possible, natural selection -would not exist, and the adaptation of the organism from the single -cell up to the whole would remain wholly unexplained. The whole crop of -spontaneous germ-variations, whenever it ceases to be 'indifferent,' -and becomes either 'good' or 'bad,' comes under the shears of personal -selection and under its almost sovereign sway. - -On the other hand, the sudden first appearance of a saltatory variation -takes place quite independently of personal selection, depending on -similar variations in a number of ids, which remain latent until they -have by the process of reducing division which precedes amphimixis, -chanced to attain a majority. In sudden bud-variations we may perhaps -suppose that reducing division occurring in some still unverified -abnormal manner is the reason why the germinal variation suddenly makes -itself visible--a supposition previously suggested as the explanation -of the reversion of these sports. - -The rarity of bud-variation is thus explained, while the greater -frequency of saltatory variations in plants propagated by seed may -be accounted for by the regular occurrence of reducing division in -sexual reproduction. But that the same or similar variations may -occur in several, it may be in many, ids at the same time must depend -upon similar general influences which affect the plant as a whole, as -happens through cultivation, manuring, and so on. I shall return to -this when discussing the influence of the environment. - -In some quarters this whole conception of germinal selection has been -characterized as the merest figment of imagination, condemned on this -ground alone, that it is based on the differences in nutrition between -such extremely minute quantities of substance as the chromosomes of -nuclear substance within the germ-cell. The quantity of substance -is certainly minute, but it needs nutriment none the less, and can -we believe that the stream of nourishment for all the invisibly -minute vital elements is exactly alike? It may be admitted that the -nourishment outside the ids is usually abundant, although undoubtedly -fluctuations occur in it also, but it certainly does not follow from -this that every vital unit within the id is similarly disposed in -relation to the nutritive supply, or has food in equal quantities at -its command, or even that each has as much as it can ever need. To make -an assertion like this seems to me much the same as if an inhabitant -of the moon, looking at this earth through an excellent telescope and -clearly descrying the city of Berlin with its thronging crowds and its -railways bringing in the necessaries of life from every side, should -conclude from this abundant provision that the greatest superfluity -prevailed within the town, and that every one of its inhabitants had as -much to live upon as he could possibly require. - -We certainly ought not to conclude from the fact that we cannot see -into the structure and requirements and methods of nutrition of a -very minute mass of substance that its nutrition cannot be unequal, -and that it cannot, by its inequalities, give rise to very material -differences, especially when we are dealing with a substance to which -we must attribute an extraordinarily complex organization built up of -enormous numbers of extremely minute particles. That this complexity -is undeniable is now admitted by many who formerly thought it possible -to believe in the simple structure of the germ-substance. How complex -not only the germ-substance but every cell of a higher organism is -in its structure, and how far below the limits of visibility its -differentiations and arrangements reach, is pressed upon our attention -by the most recent histological researches, such as those we owe to -Heidenhain, Boveri, and many others. The whole scientific world was -amazed when it came to know the mysterious nuclear spindle in the -seventies, and since then this has been quite thrown into the shade by -the discovery of the centrosphere, the centrosome, and more recently -even the centriole, and now we believe that these marvellous centres -of force may, or must, possess their own dividing apparatus! In the -face of discoveries like these no one is likely to be able to persist -in recognizing as existing only what is disclosed or even hinted at by -the most powerful lenses; no one can any longer doubt that far below -the limit of visibility organization is still at the basis of life, -and that it is dominated by orderly forces. To me, at least, it seems -more cogent to argue from the phenomena of heredity and variation to -an enormous mass of minute vital units crowded together in the narrow -space of the id, than to argue from the calculated size of atoms and -molecules to the number which we are justified in assuming to be -present in an id. In my book on the germ-plasm I made a calculation of -this kind, and I arrived at figures which seemed rather too small for -the requirements of the germ-plasm theory. This has been regarded as -a proof that I disregard the facts for the sake of my theory, but it -should rather be asked whether the size of the atoms and molecules is -a fact, and not rather the very questionable result of an uncertain -method of calculation. Undoubtedly modern chemistry has established -the _relative_ weight-proportions of the atoms and molecules with -admirable precision, but it can make only very uncertain statements -in regard to the _absolute_ size of the ultimate particles. It is -therefore admissible to assume that these have a still greater degree -of minuteness when the facts in another domain of science require this. - -We _must_ assume determinants, and consequently the germ-plasm must -have room for these; the variations of species can only be explained -through variations of the germ-plasm, for these alone give rise to -hereditary variation. It is upon this foundation that my germinal -selection is built up; whether I have in the main reached the truth the -future will show: but that I have not exhausted this new domain, but -only opened it up, I am very well aware. - - - - -LECTURE XXVII - -THE BIOGENETIC LAW - - Fritz Müller's ideas--Development of the Crustaceans--Of - the Daphnidæ--Of Sacculina--Of parasitic Copepods--Larvæ - of the higher Crustaceans--Change of phyletic stages in - Ontogeny--Haeckel's _Fundamental Biogenetic Law_--Palingenesis - and Cœnogenesis--Variation of phyletic forms by interpolation in - a lengthened Ontogeny--Justification of deductions from Ontogeny - to Phylogeny--Würtemberger's series of Ammonites--Phylogeny of the - markings in the caterpillars of the Sphingidæ--Condensation of - Phylogeny in Ontogeny--Example from the Crustaceans--Disappearance - of useless parts--The variation of homologous parts, according - to Emery--Germ-plasmic correlations--Harmony with the theory of - determinants--Multiplication of the determinants in the course of the - phylogeny. - - -What I propose to discuss in this lecture should have been considered -at an earlier stage, if we had pledged ourselves to adhere strictly -to the historical sequence of scientific discovery, for the phenomena -which we are about to deal with attained recognition shortly after -the revival of the evolution idea, and indeed they formed the first -important discovery which was made on the basis of the Darwinian -Doctrine of Descent. I have introduced them at this stage because -they have to do with phenomena of inheritance and modifications of -these, the understanding of which--in as far as we can as yet speak -of understanding at all--is only possible on the basis of a theory -of inheritance. Therefore, in order to examine these phenomena and -their causes, it was necessary first to submit a theory of heredity, -as I have done in the germ-plasm theory. We have to treat of the -connexion between the _development_ of many-celled individuals and -the _evolution_ of the species, between germinal history and racial -history, or, as we say with Haeckel, between ontogeny and phylogeny. - -Long before Darwin's day individual naturalists had observed that -certain stages in the development of the higher vertebrates, such as -birds and mammals, showed a likeness to fishes, and they had spoken -of a fish-like stage of the bird-embryo. The 'Natural Philosophers' -of the beginning of the nineteenth century, Oken, Treviranus, Meckel, -and others, had, on the basis of the transmutation theory of the time, -gone much further, and had professed to recognize in the embryonic -history of Man, for example, a repetition of the different animal -stages, from polyp and worm up to insect and mollusc. But von Baer -afterwards showed that such resemblances are never between different -types, but only between representatives of the same general type, -e.g. that of Vertebrata; and Johannes Müller maintained, from the -standpoint of the old Creation theory, that an 'expression of the -most general and simple plan of the Vertebrates' recurred in the -development of higher Vertebrates, giving as an instance that, at a -certain stage of embryogenesis, even in Man, gill-arches were laid down -and were subsequently absorbed. But why this 'plan' should have been -carried out where it was afterwards to be departed from remained quite -unintelligible. - -An answer to this question only became possible with the revival of -the Theory of Descent, and the first to throw light in this direction -was Fritz Müller, who, in his work _Für Darwin_, published in 1864, -interpreted the developmental history of the individual, 'the -ontogeny,' as a shortened and simplified repetition, a recapitulation, -so to speak, of the racial history of the species, the 'phylogeny.' But -at the same time he recognized quite clearly--what indeed was plain to -all eyes--that the 'racial history' cannot be simply read out of the -'germinal history,' but that the phylogeny is often 'blurred,' on the -one hand by the fusing and shortening of its stages, since development -is always 'striking out' a more direct course from the egg to the -perfect animal, while, on the other hand, it is frequently 'falsified' -by the struggle for existence which the free-living larvæ have to -maintain. - -For the establishment of these views Fritz Müller relied chiefly upon -larvæ, and in particular upon those of Crustaceans, and the facts, -which were in part new and in part interpreted in a new manner, were -so striking that it was impossible to deny their importance. In -particular, he drew attention to the fact that in several of the lower -orders of Crustaceans the most diverse species have a similar form when -they leave the egg, all of them being small, unsegmented larvæ, with -a frontal eye and a helmet-like upper lip, and with three pairs of -appendages, the two posterior pairs being two-branched swimming-legs -beset with bristles. In the size and form of the body, and especially -of the chitinous carapace, these larvæ differ in the various systematic -groups; thus, for instance, the larvæ of the Copepods are simply -oval, while those of the Cirrhipedes are produced anteriorly into two -horn-like processes, and so on, but in essentials they are all alike, -and for a long time these larval forms had been distinguished by the -special name of 'Nauplius' (Fig. 109). - -The development of the perfect animal begins with the longitudinal -growth of the Nauplius; the posterior end lengthens and becomes -segmented, between the anterior portion and the tail more segments are -interpolated, and on these new pairs of limbs may grow. The number -of these segments and limbs varies according to the group to which -the animal belongs. Thus the body of the perfect animal in the little -Cyprids always consists of eight segments, seven of which bear a pair -of limbs apiece; in the Branchiopods, on the other hand, the number -of segments varies from twenty to sixty, with ten to over forty pairs -of legs; in the Daphnids or water-fleas there are about ten segments, -with seven to ten pairs of limbs, and in the Copepods about seventeen -segments with eleven pairs of limbs. The difference between the orders -depends not only upon the differences in the number of segments and -limbs, but quite as much upon the form and development of the segments, -and above all of the limbs, and in this connexion it is worthy of note -that the additional limbs which grow out usually appear at first as -biramose swimming-legs, and are subsequently modified in form. Thus -the pairs of jaws, three in number, which appear in the Copepods are -developed from such swimming-legs, and so also is the second pair of -antennæ in the Copepods and the jaws of the Branchiopods, Cirrhipedes, -&c. - -[Illustration: FIG. 108. Nauplius larva of one of the lower -Crustaceans. After Fritz Müller. _Au_, the frontal eye; _I_, first pair -of limbs, corresponding to the future antennæ; _II_ and _III_, two -biramose swimming appendages.] - -If then we have before us in the 'germinal history' (ontogeny) a fairly -precise repetition of the 'racial history' (phylogeny), we may deduce -from this that the primitive forms of the Crustacean race were animals -which consisted of few segments, and that from these, in the course -of the earth's history, the very diverse modern groups of Crustaceans -have arisen, by the addition of new segments, and the adaptation of -the limbs upon them, which were at first biramose swimming-legs, to -different kinds of functions, one becoming an antenna, another a jaw -or a swimming-arm, a third, fourth, fifth, and so on, a jumping-leg, a -copulatory organ, an egg-bearer, a gill-bearer, or a tail-fin. - -[Illustration: FIG. 109. Metamorphosis of one of the higher Crustacea, -a Shrimp (_Peneus potimirim_), after Fritz Müller. _A_, the nauplius -larva with the three pairs of appendages: _I_, the antennæ; _II_ and -_III_, the biramose swimming-feet. _Au_, the single eye. _B_, first -Zoæa stage, with six pairs of appendages (_I-VI_). _Skn_, area where -new segments are being formed.] - -That the development has in general followed those lines is made clear -chiefly by the fact that the members of all these different orders of -Crustaceans still arise from nauplius larvæ, even in those cases in -which the perfect animal possesses a structure differing widely from -the usual Crustacean form. _All_ Crustaceans arise from the _nauplius -form_, even those of the higher orders, though they may not arise from -a nauplius _larva_. But this very circumstance, that in most of the -higher and many of the lower Crustaceans, the young animal, when it -emerges from the egg, already possesses more numerous segments and -limbs than a nauplius larva, again points to the connexion between -phylogeny and ontogeny, for in these cases the nauplius stage _is gone -through within the ovum_. The whole difference between this and the -forms we considered first lies in the fact that, in the latter, the -development is greatly shortened, condensed, as we might say, so that -the nauplius stage forms a part of the _embryonic_ development, and -that new segments and limbs develop in the embryo nauplius within the -egg, so that the young animal leaves the egg in a more advanced state, -nearer to that of the perfect animal, to which it can, therefore, -attain in a shorter time. - -[Illustration: FIG. 109. _C_, second Zoæa stage. The thorax is now -divided into cephalothorax (_Cph_) and abdomen (_Abd_); seven pairs of -appendages are developed, and five more (_VIII-XII_) are beginning to -appear. _Au_, paired eyes.] - -We should expect that this shortening of the larval period would be -associated with a prolongation of embryogenesis, especially in those -Crustaceans which possess a large number of segments and limbs, that -is--in the higher forms--and in the main this is the case. But there -are exceptions in two directions; in the first place there are some, -even among the lower Crustaceans, which leave the egg not as a nauplius -but in the perfect form of the adult, and secondly, there are, among -the higher Crustaceans, certain species which emerge from the egg not -in the more mature form but still in the primitive nauplius form. -Fritz Müller was the first to furnish an example of this last case, -a Brazilian shrimp, _Peneus potimirim_. Like the lowest Copepods or -Branchiopods, this species, which belongs to the highest order of -Crustaceans, goes through the whole long development, from the nauplius -through a series of higher larval forms up to the perfect animal, -and all _outside of the egg_, as an independent free-swimming larva -(Fig. 109, _A-E_). This is in sharp contrast to its near relative, the -freshwater crayfish, which goes through this whole development within -the egg, and emerges perfectly formed. - -[Illustration: FIG. 109. _D_, Mysis-stage. Thirteen pairs of appendages -are now formed: _I_ and _II_, antennæ; _III_, mandibles; _IV_ and -_V_, maxillæ; _VI-XIII_, swimming appendages with one branch or with -two. _Abd_, abdomen. _Sfl_, tail-fin. _E_, the fully-formed Shrimp, -with thirteen pairs of appendages on the cephalothorax (_Cph_); _I_ -and _II_, the two pairs of antennæ; then follow the maxillæ and -maxillipedes (_III-VIII_), the last of which is visible in the figure, -and the five pairs of walking-legs (_IX-XIII_) of which the third bears -a long chela. On the abdomen there are now six pairs of appendages -(_XIV-XIX_).] - -We see from this example that it is not some inward necessity which -thus, in the higher and more complicated organism, contracts the -ontogeny into the embryonic state, but that this depends upon external -adaptive factors. Here again we have adaptation, mainly to the -conditions of larval life. The elimination of the larvæ by enemies, -for instance, will, other things being equal, be so much the more -incisive the longer the larval development is protracted, but in that -case the general ratio of elimination of the species, and the degree -of fertility the species must possess in order to hold its own in the -struggle for existence, will also play a part in determining the mode -of development. For the higher the ratio of elimination the more eggs -the female must produce, and the more eggs that have to be produced the -smaller will be the quantity of nutritive material for the building -up of the young embryo which each egg can be furnished with. I know -of no records in regard to the eggs of that Brazilian shrimp in which -embryonic development ends with the nauplius stage, but we shall -certainly not be wrong in predicting that the eggs in this case will be -very small and very numerous, in contrast to those of the freshwater -crayfish, which are large and, as compared with others known to us, not -very numerous. - -It is a point of undeniable theoretical significance which the -life-histories of these Crustaceans disclose, that embryogenesis is -not condensed according to hidden internal laws when the structure -increases in complexity, but that the condensation of the ontogenetic -stages depends upon adaptation, and may be quite different in nearly -related species. It shows us anew that all biological occurrences are -dominated by the process of selection. - -I have already mentioned that exceptions to the usual mode of -development occur even among the lower Crustaceans, and I was thinking -at the time of the Daphnids, which leave the egg as fully formed little -animals, already equipped with all their segments and limbs. The -nauplius stage is passed through in the egg, and it is an interesting -indication that the ancestors of the modern species were in the way of -moulting, that this embryo nauplius moults within the egg by forming -a fine cuticle which is shed after a time. If it be asked why there -should be direct development in the case of these small and not very -complex water-fleas, while related species, the Branchiopods, which are -much richer in segments and in limbs, should emerge from the egg in -the form of a nauplius, and then pass through a longer larval period, -we may answer that the reason probably lies in the fact that, in the -former case, very few eggs are produced, sometimes only one, often two, -seldom more than a dozen, that these eggs can thus be relatively well -equipped with yolk, and that the formation of the little body which -bears only from seven to nine pairs of limbs can be easily completed -within this egg. Other things being equal, the direct development -would always be an advantage, because reproduction can begin sooner in -the young generation and the number of individuals will thus increase -more rapidly. And this is of particular importance in the case of the -water-fleas. - -But if it be asked, further, why so few eggs are produced in this -case, and whether these animals have no enemies, we must answer that, -on the contrary, they are preyed upon and eaten in thousands by -fishes and other freshwater animals, but that the drawback of the -scanty production of eggs is counteracted on the one hand by their -habit of reproducing parthenogenetically for the greater part of the -year, and on the other hand by their habit of concealing the eggs in a -special brood-chamber. This is the case not only in the summer eggs, -to which nourishment is conveyed in the brood-chamber from the blood -of the mother (Fig. 70), but also in the winter or 'lasting' eggs, -which receive within the chamber a protecting covering (the shell or -ephippium). - -[Illustration: FIG. 70 (repeated). Daphnella. _A_, summer ovum, with an -oil-globule (_Oe_). _B_, winter ovum.] - -In almost all the Daphnids the winter egg develops into a perfect -animal just like that to which the summer egg gives rise, although -it no longer receives any nourishment after it passes into the -brood-chamber. But it receives a larger supply of yolk on this account, -so that the nutritive provision within the egg is sufficient to develop -the perfect animal. There is only one exception to this, and it is -of special theoretical interest, because it shows more plainly than -any other fact that the greater or less degree of condensation in the -ontogeny depends upon the combined effect of the external conditions of -life. The largest of the Daphnidæ, _Leptodora hyalina_, a beautifully -transparent inhabitant of our lakes, which measures about a centimetre -in length (Fig. 110), also emerges from the summer egg as a perfect -animal, but from the winter egg, which floats freely in the water and -has only a small provision of yolk, it emerges as a nauplius, which -then undergoes larval metamorphosis before it becomes a perfect animal -(Fig. 111). - -[Illustration: FIG. 110. The largest of the Daphnids (_Leptodora -hyalina_), with summer ova (_Ei_) beneath the shell (_Sch_). _I-IX_, -the appendages. _II_, the oars (second antennæ) which always remain -biramose in Daphnids. _sb_, setæ. _ov_, ovaries. _Schl_, œsophagus. -_Ma_, stomach. _a_, anus. _H_, heart. _Au_, eye. _nG_, natural size.] - -Fritz Müller concluded from the repetition of the nauplius form in all -orders of Crustaceans that the primitive form of the Crustacean must -have been a nauplius, and that from it all the modern Crustaceans must -have evolved phyletically by the addition of segments varying in number -and differentiation. Now, however, it is doubted whether there ever -were nauplioid types capable of reproduction. But even if the nauplii -only _represent what have been the larval_ forms from very early times, -they are equally important in illustrating the relations between -ontogeny and phylogeny; they at any rate represent the primitive -pre-cambrian larval form from which all modern Crustaceans are derived. -This is borne out not only by the facts to which we have already -referred, but also by those Crustacean-groups which have diverged far -from the usual Crustacean habit and type. - -[Illustration: FIG. 111. Nauplius larva from the winter egg of -_Leptodora hyalina_; after Sars.] - -Thus the sessile Cirrhipedes, with their mollusc-like shells, their -soft, unsegmented bodies, degenerate heads, and their twelve vibratile -food-wafting limbs, emerge from the egg as nauplius larvæ. But the -remarkable parasites on the shore-crabs and the hermit-crab deviate -much further from the type of the rest of the Crustaceans, for they -hang like a sac or formless sausage-like soft mass to the abdomen of -their host, growing into it by fine, pale, root-like threads, through -which they suck up the blood of their hosts (Fig. 112, _C. Sacc._). -They possess neither head, nor thorax, nor abdomen, not even an -indication of segmentation, no limbs of any kind, neither antennæ, nor -mouth parts, nor swimming-legs. Nevertheless they are Crustaceans; -indeed, we can say with certainty that they belong to the order of -Cirrhipedes, for they leave the egg in the form of a nauplius larva -(_A_), with 'horns' on their carapace which no other forms except -themselves and the Cirrhipedes possess. That they are of the same -stock as these is also proved by their further development, for the -nauplius grows first, just as in the case of the Cirrhipedes proper, -into a 'Cypris-like larva' (_B_), so called because it bears a certain -resemblance to the Ostracods of the genus Cypris, and only from this -point do their paths of development diverge. The Cypris-like larva of -the true Cirrhipedes settles down somewhere, attached by its antennæ; -it grows, and its body becomes that of the perfect Cirrhipede; but the -Cypris-like larva of the Sacculinæ bores its way into the inside of a -crab or hermit-crab, at the same time losing its limbs, segmentation, -and its chitinous covering; and within the body of its host it is -transformed into the sac-like organism we have already described. After -a time it emerges again on the surface, and remains attached to the -abdomen of its host (Fig. 112, _C. Sacc._), drawing its nourishment -from the blood which it sucks up by means of its numerous delicate -roots (_W_, _W_). - -[Illustration: FIG. 112. Development of the parasitic Crustacean -_Sacculina carcini_, after Delage. _A_, Nauplius stage. _Au_, eye. -_I_, _II_, _III_, the three pairs of appendages. _B_, Cypris-stage. -_VI_-_XI_, the swimming appendages. _C_, mature animal (_Sacc_), -attached to its host, the shore-crab (_Carcinus mænus_), with a -feltwork of fine root-processes enveloping the crab's viscera. -_s_, stalk. _Sacc_, body of the parasite. _oe_, aperture of the -brood-cavity. _Abd_, abdomen of the crab with the anus (_a_).] - -From all this we may conclude that certain Cirrhipedes in times long -past adopted a parasitic habit in the Cypris-larva stage, and that they -gradually underwent adaptations to this mode of life, and that these -went further and further, until the animal was transformed into the -singular creature which we now see in the sexually mature form. - -The same is the case with the numerous fish-parasites of the order -Copepoda. They all leave the egg as nauplius larvæ, however greatly -they may be modified later on by adaptation to a parasitic habit, -and in them we can still observe, in the fully developed animals, -a whole series of grades of transformation. Thus many genera, like -_Ergasilus_, are distinguished from the free-swimming Copepods only -by the modification of their jaws into piercing and sucking organs, -and of a single pair of antennæ into hooks, by means of which they -attach themselves to the fish on which they feed. In other genera the -degeneration and modification go further; the antennæ, the eye, and -the appendages degenerate more or less, and very remarkable attaching -organs are sometimes developed, in the form of hooks or of knobbed -pincers, or of actual suckers. In several types the degeneration and -modification go so far that the segmentation of the body disappears, -and the animal looks more like an intestinal worm than like a -Crustacean (_Lernæocera_ and others). In all these forms adapted to a -parasitic mode of life it is always only the mature animal which has -been transformed in this manner, for previously it has gone through a -series of stages which are quite similar to those of the free-swimming -Copepods, beginning with the nauplius, and ending with the so-called -Cyclops stage, that is, a larval form which possesses antennæ, eyes, -and swimming-legs similar to our freshwater Copepods of the genus -_Cyclops_. - -Here again we see in the ontogeny the repetition of a series of -phyletic stages before the mature form is assumed. Why these stages -should have persisted it is easy enough to understand, for how could an -animal which emerged from the egg as a worm-shaped _Lernæocera_ find a -fresh fish which would serve it as host? Yet these parasites could not -possibly go on preying upon the same fish generation after generation. -To secure the existence of the species it was therefore indispensable -that the faculty of swimming should be retained at least in the young -stages; in other words, that the free-swimming ancestral stages should -be preserved in the ontogeny. In all these cases it is therefore beyond -doubt that the germinal history recapitulates a series of stages -comparable to those of the racial history, although not quite unchanged -but adapted to the modern conditions of life, for instance in having -shorter antennæ, smaller eyes, and with four instead of the usual five -swimming-legs. The search for a host does not seem to last long, for -fishes are usually found in large numbers together, and thus the young -parasitic Crustacean does not require to make a long journey before it -finds a refuge. - -It is noteworthy that the males of parasitic Crustaceans are not -only much smaller than the females (Fig. 113), but that they are -also much less modified, and resemble the ancestral free-swimming -Copepods to a much greater degree. They usually possess small but -well-developed swimming-legs, and by means of these they seek out the -female, dying after fertilization is accomplished. They are thus not -sessile parasites at all, and have therefore to go through the stages -of the free-swimming Copepods much more completely than the females, -whose task is to accumulate within themselves from the blood of the -fish as much material as possible for the forming of the eggs, and to -produce the largest possible number of these. These therefore greatly -surpass the free-swimming Copepods in fertility, as is evidenced by the -enormous egg-sacs they bear at the posterior end of the body (Fig. 113, -_ei_). - -Even among the higher Crustaceans, the so-called Malacostraca, the -germinal history not infrequently exhibits more or less of the racial -history in distinct recapitulation. - -[Illustration: FIG. 113. The two sexes of the parasitic Crustacean -_Chondracanthus gibbosus_, enlarged about six times; after Claus. -The main figure is that of the female, whose body bears quaint blunt -processes. At its genital aperture (♂) a dwarf male is situated. -_F_ and _F´_, the two pairs of appendages. _ei_, the long egg-sacs, -portions of which have been cut off in the figure.] - -It is true however, as we have already shown, that there are only a -few of the higher Crustaceans which emerge from the egg in the form -of a nauplius; in most of them this stage has been shunted backwards -in the ontogeny, and most of the crabs and hermit-crabs leave the egg -in a higher larval form, that of the so-called Zoæa (Fig. 114). This -term is applied to a larva which already exhibits two main divisions -of the body, a head and thorax portion (cephalothorax, _Cph_) and -an abdomen (_abd_). The cephalothorax is frequently equipped with -remarkable long spines (_st_), and it always bears from five to eight -pairs of limbs, anteriorly the antennæ (_I_ and _II_), then the -mandibles (_III_), further back swimming-legs (_IV_, _V_), and behind -these can be recognized the primordia of the other legs (_VI_-_XIII_), -which will grow freely out later on. Large facetted and stalked eyes -(_Au_) are borne on the head. This Zoæa form is not now found as a -mature Crustacean form, so we cannot maintain with any confidence -that it lived as a mature animal at an earlier period of the earth's -history, but a second still more complex larval form of the higher -Crustaceans is preserved for us in a group of marine Crustaceans, the -Schizopods. These are Crustaceans which, though small, approach in -external appearance our freshwater crayfish, only they have, instead -of the ten walking-legs, biramose swimming-legs, by means of which -they move freely in the water. The number of these branched legs is -even greater than ten, there are sixteen of them (Fig. 109 _D_, p. -164, _VI_-_XIII_). In the aquaria of the Zoological Station at Naples -one may often see these dainty little creatures swimming about in -large companies. Here they are of interest to us chiefly because their -structure occurs in the ontogeny of the highest Crustaceans, the -Decapods; that is, the phyletic stage represented by the Schizopods -appears as an ontogenetic stage, just before the final metamorphosis of -the larva to the perfect animal. This is the case in most of the marine -Decapods, in those forms which do not go through the whole course of -their development within the egg, but emerge as Zoæa larvæ, or even, -as in _Peneus potimirim_, as nauplii. In the last-named species (Fig. -109) the ontogeny contains at least three stages which must have -lived, perhaps not as mature forms, but as primitive larval forms, for -unthinkable ages--the stage of the nauplus (Fig. 109 _A_), that of the -Zoæa (Fig. 109 _B_ and _C_), and that of the Schizopod (Fig. 109, _D_); -from this last the fully developed Decapod Crustacean arises (Fig. 109, -_E_). - -We are, therefore, justified in saying that here the racial evolution -is recapitulated in the individual development, although condensed -and shortened in proportion as more numerous stages of the phyletic -development are gone through within the egg, for there the different -stages can succeed each other more rapidly and directly than in a -metamorphosis of the free-swimming larvæ, since these must procure -their own material for their further growth and their metamorphosis, -while the yolk of the egg supplies a store of material which is -sufficient for the production of a whole series of successive stages. - -[Illustration: FIG. 114. Zoæa-larva of a Crab, after R. Hertwig. -_I_-_V_, the already functional anterior appendages--antennæ, -mandibles, and swimming-legs. _VI_-_XIII_, rudiments of the posterior -appendages of the cephalothorax (_Cph_). _Abd_, the abdomen. _st_, -spine of the carapace. _Au_, eye. _H_, heart.] - -For this reason it inevitably resulted that the sharply defined -characters of the phyletic stages were more and more lost as soon as -they were transferred from larval stages to stages in embryogenesis. -For, in the first place, these sharply defined characters, such as -the spines of the Zoæa larva, or the swimming bristles of the 'oars,' -or the shape of thorax or abdomen characteristic of certain species, -are adapted to a free life, and would be valueless in an embryonic -stage; and secondly, in the transference of the free larval stages -to embryonic development the greatest possible condensation and -abbreviation of the stages must have been striven for, which could only -come about by a continual mutual adaptation of the embryonic parts to -one another, involving the suppression of everything superfluous. -Otherwise the transference of the free stages to the embryogenesis -would have brought no advantage, but rather a most prejudicial -protracting of the development. - -We must not, therefore, expect to find the stages of the phylogeny -occurring unaltered in every ontogeny in the way we have found the -nauplius, Zoæa, or Mysis stages in the larval development of the -Decapods. I have noticed already that in the water-fleas (Daphnidæ) and -other Crustaceans without metamorphosis the nauplius stage is still -passed through, but within the egg, and as an embryonic stage, and -this is quite true, but nevertheless it would hardly do to liberate a -nauplius like this from its shell and place it in the water, for the -influence of the water upon the delicate embryonic cells of its body -would soon cause it to swell, and would destroy it utterly. And, even -apart from this, it has no hard and resistant chitinous covering, no -fully-developed appendages, but only the stump-like blunt beginnings -of these without swimming-bristles and without muscles capable of -function, so that it could not even move. Nevertheless it is a nauplius -with all its typical distinctive characters, only it is not a perfect -nauplius capable of life, but rather a 'schema' of one, which must be -retained in the embryogenesis that it may give rise to the later stages. - -Shall we therefore say that the statement that phylogeny repeats itself -in ontogeny is false, that the nauplius stage within the embryo is -not a true nauplius at all? That would be pushing precision beyond -reasonable limits, and would obscure our insight into the causal -connexion between phylogeny and ontogeny, which, as we have seen, -undoubtedly exists. - -A few years after the appearance of Fritz Müller's work _Für -Darwin_, Haeckel elaborated Müller's idea, and applied it in a much -more comprehensive manner. He formulated it under the name of 'the -fundamental biogenetic law,' and then he used this 'law' to deduce -from the ontogeny of animals, and more particularly of Man, the -paths of evolution along which our modern species have passed in the -course of the earth's history. In doing so the greatest caution was -necessary, since ontogeny is not an actual unaltered recapitulation -of the phylogeny, but an 'abridged' and in most cases--in my own -belief, in all cases--_a greatly modified recapitulation_. Therefore -we cannot simply accept each ontogenetic stage as an ancestral stage, -but must take into consideration all the facts supplied to us by other -departments of biological inquiry which afford help in the decision -of such questions, especially those brought to light by comparative -morphology and by the whole range of comparative embryology. - -Haeckel was quite well aware of this difficulty, and repeatedly -emphasized it by laying stress on the fact that a 'blurring' of the -phyletic stages of development had arisen through the abridgement of -the phylogeny in the ontogeny, and a 'falsification' of it through -the secondary adaptation of individual ontogenetic stages to new -conditions of life. He therefore distinguished between 'Palingenesis,' -that is, simple though abridged repetition of the ancestral history, -and 'Cœnogenesis,' that is, modification of the racial history by -later adaptation of a few or many stages to new conditions of life. -As an example of cœnogenetic modification, I may cite the pupæ of -butterflies. Since these can neither feed nor move from one spot, they -can at no time have been mature forms, and cannot, therefore, represent -independent ancestors of our modern Lepidoptera; they have originated -through the constantly increasing difference between the structure of -the caterpillar and that of the moth or butterfly. Originally, that is, -among the oldest flying insects, the mature animal could be gradually -prepared within the larva as it grew, so that finally nothing was -necessary but a single moult to set free the wings, which had in the -meantime been growing underneath the skin, and to allow the perfect -insect to emerge, complete in all its parts. This is the case even now -with the grasshoppers and crickets. In these forms the larval mode of -life differs very little, if at all, from that of the perfect insect, -and the main difference between the two is the absence of wings in the -larva. But when the perfect insect adapted itself to conditions of life -quite different from the larval conditions, as was the case with the -nectar-sucking bees and butterflies adapted entirely for flight, while -the larvæ were still adapted exclusively to an abundant diet of leaves -and other parts of plants, and to a very inactive life upon plants, the -two stages of development ultimately diverged so widely in structure -that the transition from one to the other could no longer be made at -a single moulting, and a period of rest had to be interpolated, in -order that the transformation of the body could take place. In this -way arose the stage of the resting and fasting pupa, a 'cœnogenetic' -modification of the last larval stage, _not a recapitulation of an -ancestral form_, but a stage which has been interpolated, or better, -has 'interpolated itself' into the ontogeny on account of the widely -different adaptations of the early and the final stages. - -This is a perfectly clear idea, and Haeckel's distinction between -palingenesis and cœnogenesis is undoubtedly justified. - -But it is quite a different matter to be able to decide whether -a particular stage or organ has arisen palingenetically or -cœnogenetically with the same certainty as in the case of the -insect-pupa, or even with any degree of probability, and we must admit -that in very many cases, perhaps even in most cases, it is impossible. -This is so chiefly because pure palingenesis is hardly likely to occur -now; the ancestral stages were bound to be modified in any case if -they were to be compressed into the ever-shortening ontogeny of later -descendants, and particularly so if they were to be shunted back into -embryogenesis. In the latter case they would not only be materially -shortened, and, as I have already shown, modified by the mutual -adaptations of the different developing parts, but time-displacements -of embryonic parts and organs would be necessary, as has been very -clearly proved by the excellent recent investigations, which we owe -in particular to Oppel, Mehnert, and Keibel. A shunting forward or -backward of the individual organs takes place--conditioned apparently -by the decreasing or increasing importance of the organ in the finished -state; for in the course of the phylogeny everything may vary, and -not only may a new, somewhat modified, and often more complex stage -be added on at the end of the ontogeny, but each one of the preceding -stages may vary independently, whenever this is required by a change -in its relations to the other stages or organs. Adaptation is effected -at every stage and for every part by the process of selection, for all -parts of the same rank are ceaselessly struggling with one another, -from the lowest vital units, the biophors, up to the highest, the -persons. If we reflect that, in the course of the phylogeny of every -series of species, a number of organs always become superfluous and -begin to disappear in consequence, we can understand what great changes -must take place gradually as such a series of phyletic stages is -compressed into the ontogeny, for all organs which are no longer used -are gradually shifted further and further back in the ontogeny till -ultimately they disappear from it altogether. But, while the primary -constituents of these 'vestiges' play their part in ontogeny for a -shorter and shorter time, new acquisitions are being more and more -highly developed, and thus, in the course of the phylogeny, numerous -time-displacements of the parts and organs in ontogeny must result, so -that ultimately it is impossible to compare a particular stage in the -embryogenesis of a species with a particular ancestral form. _Only the -stages of individual organs can be thus compared and parallelized._ - -But we must not on that account 'empty out the child with the bath,' -and conclude that there is no such thing as a 'biogenetic law' or -recapitulation of the phylogeny in the ontogeny. Not only is there -such a recapitulation, but--as F. Müller and Haeckel have already -said--ontogeny is nothing but a recapitulation of the phylogeny, -only with innumerable subtractions and interpolations, additions -and displacements of the organ-stages both in time and place. It -would be a great mistake to conclude from the fact of these manifold -alterations that the whole proposition of the recapitulation of the -phylogeny in the ontogeny is erroneous, or at least valueless. If its -only use were to enable us to read the racial history of a species out -of its germinal history, it is intelligible enough that we might be -led to give it up in despair, but I think that the main thing is to -get some insight into the history of the ontogeny, and there can be -no doubt that this can have been built up on no other foundation than -upon the racial history. What is new could only have arisen from what -was already in existence, and everything in ontogeny, not only the -palingenetic stages which still represent in some measure the facies of -fully-formed ancestral stages, but also the cœnogenetic stages, like -the pupa-stage we have already discussed, have arisen historically, -nothing _de novo_, but all in connexion with what was already present. -But what was first present was in all cases the stages of the ancestral -forms. - -It is undoubtedly of the greatest value to be able to penetrate more -and more deeply into embryonic development, and to discover more -precisely the changes that have taken place throughout its course in -the originally existing material of ancestral forms. But it must not -be forgotten that, all transformations notwithstanding, so much of -the racial history is still very plainly indicated in the germinal -history, that this must always remain for us a most important source -from which to draw conclusions in regard to the phyletic development of -any animal group. I admit that these conclusions have sometimes been -drawn with too great confidence, but even if we cannot regard as well -founded Haeckel's view that in the ontogeny of Man there are fourteen -different ancestral stages recognizable, a protist stage, a gastræa -stage, a prochordate, an acranial, a cyclostome, a fish-stage, and so -on, we must recognize that the unicellular stage of ontogeny, with -which even now the development of every human being begins, undoubtedly -repeats the facies of an ancestor, although greatly altered; for we -must be descended from unicellular organisms. The essential part of -this ancestral stage is thus preserved in the ontogeny, and only what -is special and in some measure due to chance, that is, to adaptation to -special conditions of existence, has been modified. - -It has been supposed that the proposition that phylogeny is -recapitulated in the ontogeny is disproved, because the ontogenetic -stage must always contain within it the primordia of the later stages -which have been added since the corresponding phylogenetic stage. It -is certain that the egg-cell or the sperm-cell of Man contains, though -in a form not recognizable by us, all the determinants of the perfect -human body, but this neither affects its nature as a cell nor its -particular form as ovum or spermatozoon. It is essentials that are -important in this comparison, not accessories. Neither can I agree -with Hensen's argument when he says that the 'recapitulation-idea' is -erroneous, because the actual course of ontogeny is the 'best and only -possible one,' which, apart from previous history altogether, must of -necessity be followed. Certainly the actual course is the best, and -under the given circumstances the only possible one, but that does not -exclude recapitulation, on the contrary it implies it, for ontogeny -could at no time have arisen from a _tabula rasa_, but only from what -was historically existent. - -I do not propose to examine each of Haeckel's ancestral stages in -Man's pedigree, or to estimate the degree of probability with which -they may be deduced from the ontogeny; but that Man's ancestry does, -in a general way, include such a series of phyletic stages may be -admitted, even if we grant that many of these stages are now no longer -represented in the ontogeny as stages of the developing organism as -a whole, but only by stages of individual organs or group of organs. -Thus it may be disputed whether there is still a fish-stage in -human development, but it cannot be disputed that the rudiments of -'gill-arches' and 'gill-clefts,' which are peculiar to one stage of -human ontogeny, give us every ground for concluding that we possessed -fish-like ancestors. - -As we now know that the history of a given mode of embryogenesis -has involved numerous time-displacements of the organ-rudiments, we -must attach all the more weight to the developmental history of the -individual parts and characters, in which the phylogeny can often be -read more clearly than in the stages of the organism as a whole, and we -can probably find out important laws in this way. - -As far back as 1873 Würtemberger investigated the fossil ammonites with -special reference to this point. He was concerned even more at that -time with finding proofs of the theory of descent in general, and this -was the first case in which any one succeeded in demonstrating phyletic -transformation-series of species, deposited one above the other in a -corresponding series of geological strata, and connected by transition -forms lying between these. In studying this interesting material, of -which many examples were at his disposal, Würtemberger proved that the -variations which had taken place in the spirally coiled shell in the -course of ages appeared first on the last whorl, and then subsequently -extended to the one before this, and thence to the still younger whorls -of the shell. Meanwhile the last whorl not infrequently exhibited -another new character. Thus, for instance, protuberances on the shell -were shifted in the course of the phylogeny from the last convolution -to the second last, and later to the third last, and so on, while at -the same time the last convolution showed the protuberance changed into -spines. In other words, the new phyletic acquirements first appeared in -the mature animal (in the last-formed whorl or chamber of the shell), -but were subsequently shifted back in the ontogeny to younger stages in -proportion as new transformations of the mature animal appeared. Thus -there was, so to speak, a retraction of the phyletic acquisitions of -the mature animal deeper and deeper into the germinal history of the -species. - -About the same time--in the seventies--I obtained similar results -from living species when I was attempting to work out the ontogeny -of the markings on the external skin of the caterpillars of certain -butterflies, and I should like to submit a short account of these. - -In one of the early lectures we discussed the protective and defensive -colours of caterpillars in general, and those of caterpillars of the -Sphingidæ in particular. I showed that those naked caterpillars which -live on plants among the grass, or on the grass itself, are often -not only green, like fresh grass-stalks, or yellowish-grey, like dry -ones, but all the larger forms also exhibit light, usually white, -longitudinal lines, which, by mimicking the sharp light reflections on -the grass-stems, heighten the protective resemblance. - -We also spoke of the light transverse stripes, often marked with pink -or lilac-blue, of many of the large green caterpillars which live -on trees and bushes, and whose likeness to the leaves is heightened -by this imitation of the lateral veining of a leaf; and finally we -mentioned the warning coloration indicative of unpleasant or nauseous -taste, among which must be classed not only vivid contrasts of colour, -but also specially conspicuous elements of colour such as light -ring-spots upon a dark ground. These different colour schemes which -protect the caterpillars from their enemies are usually only to be -found in the adolescent caterpillar, not in the very small one which -has just emerged from the egg, and the development of the markings in -the individual life clearly shows that the phylogeny of the markings is -more or less obviously contained in the ontogeny. - -There are three different schemes of marking which occur in the -caterpillars of hawk-moths or Sphingidæ--longitudinal striping, -obliquely transverse striping, and spots. Longitudinal striping pure -and unmixed is now found only in a few species, for instance in the -caterpillar of the _Macroglossa stellatarum_ (Fig. 115), in which a -white longitudinal line, beginning at the tip of the tail, runs up each -side of the body to the head as a 'sub-dorsal stripe' (_sbd_). These, -with other two similar stripes, effectively secure the fairly large -caterpillar from discovery when it is among grass and herbs. - -[Illustration: FIG. 115. Caterpillar of the Humming bird Hawk-moth, -_Macroglossa stellatarum_. _sbd_, the sub-dorsal line.] - -Transverse striping occurs as the sole mode of marking in species -which live on bushes and trees whose leaves have strong lateral veins, -such as willows, poplars, oaks, privet, syringa, and so on, and these -markings associated with the leaf-green of their colouring protect them -most effectively from discovery. - -The third scheme of marking, namely by spots, occurs in various forms -in species of the genera _Deilephila_ and _Chærocampa_, and it varies -in its biological significance; in many species the spots serve as a -warning colour, by making the caterpillar conspicuous and easily seen -from a distance (_Deilephila galii_, Fig. 117); in others they imitate -the eyes of a larger animal, and have a 'terrifying' effect, as we have -already said (Fig. 4); in still other and rarer cases they heighten the -resemblance of the caterpillar to its food-plant by mimicking parts of -it, as, for instance, the red berries of the buckthorn (_Deilephila -hippophaës_, Fig. 8, _r_). - -[Illustration: FIG. 3 (repeated). Full-grown caterpillar of the Eyed -Hawk-moth, _Smerinthus ocellatus_. _sb_, the sub-dorsal stripe.] - -Thus all three modes of marking possess a biological value, and -protect the soft and easily wounded animal in some way, and, in the -case of at least two of them, it is clear that they must have arisen -at the very end of the caterpillar's development, since they can only -be effective as the animal is approaching full size, and would be -valueless in the very young caterpillar. The transverse striping only -makes the caterpillar like a leaf when the stripes bear about the same -relation to each other as those on the leaf, and eye-spots can only -scare away lizards and birds when they are of a certain size. Only -longitudinal striping is effective as a protection in the case of young -caterpillars, supposing, that is, that they live in or on the grass -(Fig. 116). - -[Illustration: FIG. 4 (repeated). Full-grown caterpillar of the -Elephant Hawk-moth, _Chærocampa elpenor_, in its 'terrifying attitude.'] - -[Illustration: FIG. 8 (repeated). Caterpillars of the Buckthorn -Hawk-moth, _Deilephila hippophaës_. _A_, Stage III. _B_, Stage V. _r_, -annular spots.] - -Let us consider the ontogeny of these different forms of markings, -beginning with the eye-spots. It appears that these develop from a -sub-dorsal stripe, which appears in the young caterpillar in the -second stage of its life, and from it, in the course of the further -development, two pairs of large eye-spots are formed. Even in the -young caterpillar, scarcely one centimetre in length (Fig. 116), it -can be observed that the fine, white sub-dorsal line takes a slight -curve upwards on the fourth and fifth segments (_C_), and on the -lower edge of these curves a black line is laid down (_D_). This is -then continued to the upper side (_E_), and encloses the piece of the -sub-dorsal stripe (_F_ and _G_), and thus there arises a white-centred, -black-framed spot which only requires to grow and to differentiate a -blackish shadow-centre, the pupil (_G_), to give the impression of a -large eye. This occurs as the caterpillar goes on growing, and after -the fourth moult or ecdysis the eyes have already some effect, as the -animal is six centimetres in length, but they become even more perfect -in the fifth and last stage. During this development of the eye-spot -the sub-dorsal stripe disappears completely from the greater part of -the caterpillar, persisting only on the first three segments (Fig. 116, -_B-F_). - -[Illustration: FIG. 116. Development of the eye-spots in the -caterpillar of the Elephant Hawk-moth (_Chærocampa elpenor_). _A_, -Stage I, still without marking, simply green. _B_, Stage II, with -sub-dorsal stripe (_sbd_). _C_, sub-dorsal line somewhat later, with -the first hint of the eye-spot (_Au_) on segments 4 and 5. _D_, -eye-spots in Stage III of the caterpillar, somewhat further developed -than in _E_, the third stage. _F_, Stage IV. _G_, the anterior eye-spot -at the same stage.] - -When we consider that this stripe in the little caterpillar a -centimetre long, which lives on the large leaves of the vine, or on -the obliquely ribbed willow-herb (_Epilobium hirsutum_), is quite -without protective value, its occurrence at that stage can only be -regarded as a phyletic reminiscence due to the fact that the ancestors -of these species of _Chærocampa_ possessed longitudinal stripes in the -adult state, probably because at that time they lived on plants among -the grass, and that, later, when the species changed their habitat to -plants with broad leaves which had arisen in the meantime, eye-spots -were developed in addition to the green or brown protective colouring -which they retained. Thus the modern development of these spots mirrors -their phyletic evolution very faithfully; on the two segments there -were formed, from pieces of the sub-dorsal line, first white spots -ringed round with black, then unmistakeable eyes with pupils (_C_, _D_, -_G_). This transformation can only have begun in the fairly well-grown -caterpillar, because it was only of any use to it; but later on it was -shunted further back in the ontogeny, from the sixth and fifth to the -fourth and third caterpillar stage, not in its complete development, -but in more and more incipient form; and nowadays the first traces of -eyes, as we have already seen, are visible in the course of the second -stage. The marking of the more remote ancestors, the longitudinal -striping, is now lost in proportion as the eye-spots develop, perhaps -because the former would take away from the full effect of the latter. -The longitudinal stripes are still quite plainly visible on the first -three segments, but these segments are drawn in and are scarcely -noticeable when the caterpillar assumes a defiant attitude (Fig. 4). - -In the case of marking with ring-spots, which is found especially in -species of the genus _Deilephila_, the ontogeny discloses that it has -developed phyletically from the sub-dorsal stripe; in the young stage -of this caterpillar also, the sole marking is longitudinal striping; -in _Deilephila zygophylli_, from the steppes of Southern Russia, -this persists apparently through all the stages, but in the others -it disappears almost completely in the later stages, but only on the -segments on which the spot-marking has developed from it. This happens -in a manner similar to that in which the eye-spot in _Chærocampa_ -arises, a piece of the white sub-dorsal stripe is enclosed above and -below by a semicircle of black, and later these semicircles unite, and -cut off the portion of the sub-dorsal line, and form a black spot with -a light centre within which a red spot frequently appears (Fig. 117, -_A_). - -[Illustration: FIG. 117. Caterpillar of the Bed-straw Hawk-moth -(_Deilephila galii_). _A_, Stage IV, sub-dorsal stripe still distinct, -the annular spots are still incompletely enclosed in it. _B_, -fully-formed caterpillar without trace of a sub-dorsal stripe, but with -ten annular spots.] - -In most species these ring-spots occur on many segments (10-12) (Fig. -117, _B_), and in cases where they are of importance in making the -caterpillar conspicuous and easily seen they sometimes form a double -row. But we know one species, _Deilephila hippophaës_, in which only -a single ring-spot exists, and it is a large brick-red spot on the -second last segment, mimicking the red berry of the buckthorn (Fig. -8, _A_ and _B_, _r_). But individuals also occur in which there are, -on the five or six segments in front, smaller ring-spots which become -less distinct the further forward they are, and in most caterpillars -it is possible, on careful examination, to recognize little red dots -on the faded sub-dorsal stripes of these segments (Fig. 8, _B_). We -might be disposed to think, on this account, that the ancestors of -_D. hippophaës_ bore rings on all the segments, and that these had -gradually become vestigial on the majority of them, because they had -lost their earlier biological importance, and now, by adaptation to the -buckthorn, could only be of use on the second last. But when we take -the ontogeny also into account we find in the young caterpillar only a -simple sub-dorsal line, upon which, in the third stage, the red spot of -the tail-horn segment appears (Fig. 8, _A_). - -No spots ever occur on the other segments at this stage; they only -appear in the last stage, but as they may be entirely wanting, they -must have arisen as the result of internal laws of correlation, that -is, they must be recapitulations of the hindmost spots which arose -in the phylogeny through natural selection. We may conclude this, at -least, if we believe in the truth of the fundamental proposition of the -biogenetic law, and admit that there is in the ontogeny some more or -less distinct recapitulation of the phylogeny. - -[Illustration: FIG. 118. Two stages in the life-history of the Spurge -Hawk-moth (_Deilephila euphorbiæ_). _A_, first stage, the caterpillar -dark blackish-green, without marking. _B_, second stage, the row of -spots is distinctly connected by a light streak, the vestige of the -sub-dorsal stripe.] - -This proposition may be recognized as true in the case of _Deilephila_ -also, if we compare the different species with one another as regards -their ontogeny. We find here too that not only the sub-dorsal, that is, -the phyletically oldest marking of the Sphingid caterpillars, occurs -everywhere in the young stages, but also that it is being shunted back -to younger and younger stages, in proportion to the degree of the -development of the spot-marking reached in the full-grown caterpillar. -Thus, for instance, in the caterpillar of _Deilephila euphorbiæ_ the -highest form of spot-marking is reached, and in this species the -sub-dorsal line is no longer the sole marking element at any stage. -Leaving out of the question the absolutely unmarked little caterpillar -which emerges from the egg (Fig. 118, _A_), there appears at once in -the second stage a series of ring-spots connected by a fine white -sub-dorsal line (Fig. 118, _B_). In the following stage, the third, -this sub-dorsal line disappears without leaving a trace, and there -remains only the spot-marking, which is subsequently duplicated. - -Let us compare with this the ontogeny of the bed-straw hawk-moth, -_Deilephila galii_ (Fig. 117). The full-grown caterpillar possesses -only a single row of ring-spots (_B_), and accordingly the young stages -of the caterpillar up to the fourth show a distinct sub-dorsal line -(_A_), although spots are seen upon it. A still earlier phyletic stage -of development is illustrated by _Deilephila livornica_, in which the -ring-spots are all connected by the sub-dorsal line. - -It can thus hardly be doubted that the biogenetic law is guiding us -aright when we conclude from a comparison of the ontogeny of the -different species of _Deilephila_, that the oldest ancestors of the -genus possessed only the longitudinal stripes, and that from these -small pieces were cut off as ring-spots, and that these were gradually -perfected and ultimately duplicated, while at the same time the -original marking, the longitudinal stripe, was shunted back further and -further in the young stages, until it finally disappeared altogether. - -Let us now refer for a moment to the third form of marking in the -caterpillars of the Sphingidæ--transverse striping. This has not arisen -out of the sub-dorsal line, but quite independently and at a later -date. This is proved with great certainty by the ontogeny of species -of the genus _Smerinthus_. The full-grown, and usually also the young -caterpillars, of these species have quite regularly the seven broad -oblique stripes which run in the direction of the tail-horn at equal -intervals on the lateral surfaces of the body (Fig. 3). They are -absent only from the three anterior segments, and upon these a part -of the older marking, the sub-dorsal stripe, has persisted. But we -find this fully developed in the youngest stages of other species. In -_Smerinthus populi_, the little caterpillar, which has no markings at -all when it leaves the egg, very soon shows the white sub-dorsal line, -and simultaneously with it the seven transverse stripes, which cut -obliquely through it; in the older caterpillars the sub-dorsal then -disappears (Fig. 119). - -When I was investigating these matters at the beginning of the -seventies I did not succeed in procuring eggs of the species of the -genus _Sphinx_, which likewise almost all exhibit the oblique striping -in their full-grown stages. But from what I knew of the ontogeny of -_Smerinthus_ species I was able to predict that, among the young -stages of _Sphinx_, there must be some with sub-dorsal lines. This -was confirmed later, for Poulton found in _Sphinx convolvuli_ that in -the first stage there are no oblique stripes, but only the sub-dorsal -stripe, while in _Sphinx ligustri_ both kinds of marking were present -at the same time. - -[Illustration: FIG. 119. Caterpillar of _Smerinthus populi_, the Poplar -Hawk-moth, at the end of the first stage, showing both the complete -sub-dorsal stripe and the oblique stripes.] - -From all these facts, which I have summarized as briefly as possible, -we see that the older phyletic characters are gradually crowded by the -newer into ever-younger stages in the ontogeny, until ultimately they -disappear altogether. We have now to ask to what this phenomenon is -due; is it a simple crowding out of the old and less advantageous by -the new and better characters as a result of natural selection, or is -there some other factor at work? It is clear in regard to these forms -of marking that they can have been developed at first only in the -almost full-grown larva by natural selection, because they are of use -only there, and that, at the same time, the old marking must have been -set aside through the influence of the same factor, in as far as it -prejudiced the effect of the new adaptation. This seems to be indicated -by the persistence of the sub-dorsal line on those segments which are -drawn in when _Chærocampa_ assumes a terrifying attitude, or which do -not bear oblique stripes in the leaf-like caterpillars, e.g. the three -anterior segments in the species of _Sphinx_ and _Smerinthus_. When -newly acquired schemes of marking like the eye-spots of _Chærocampa_ -are transmitted from the last stage to the stage before, this can be -explained by following the same train of thought, for the caterpillar -is already of sufficient size to be able to inspire terror with its -eyes; but in still younger stages the spots would not be likely to -have that effect, and yet they occur in quite small animals (20 mm.). -More obvious still is the uselessness of the oblique striping in the -young stages of the _Sphinx_ and _Smerinthus_ caterpillars, for in the -earliest stages of life the caterpillars are much too small to look -like a leaf, and the oblique stripes stand much closer together than -the lateral ribs of any leaf. Moreover, the little green caterpillars -require no further protection when they sit on the under side of a -leaf; they might then very easily be mistaken _in toto_ for a leaf-rib. -_Thus it is certainly not natural selection which effects the shunting -back of the new characters._ Nor can this be caused by the fact that -the new character can only be developed gradually and in several -stages, for the oblique striping at any rate arises in the ontogeny all -at once. There must therefore be some mechanical factor in development -to which is due the fact that characters acquired in the later stages -are gradually transferred to the younger stages. But this shifting -backwards can be checked by the agency of natural selection as soon as -it becomes disadvantageous for the stage concerned. - -It is in this way that I explain the fact that the majority of the -caterpillars of the Sphingidæ are absolutely without markings when -they emerge from the egg. Thus, for instance, the caterpillars of -_Chærocampa_ (Fig. 116, _A_), of _Macroglossa_ (Fig. 115), and of -_Deilephila_ (Fig. 118, _A_), as well as those of the _Smerinthus_ -species, are at first without stripe or mark of any kind; they are -of a pale green colour, almost transparent, and very difficult to -recognize when they sit upon a leaf. How very greatly the different -stages _can be_ independently adapted to the different conditions of -their life, when that is necessary for the preservation of the species, -is shown in the most striking manner by many species. Thus the little -green caterpillar of _Aglia tau_, when it leaves the egg, bears five -remarkable reddish rod-like thorns, which in form and colour resemble -the bud-scales of the young beech-buds among which they live, and which -disappear later on; the full-grown caterpillar shows nothing of these, -but is leaf-green, marked with oblique stripes. Even if the use of -these reddish thorns be other than I have indicated, we have in any -case to deal with a special adaptation of _one_, and that the first -caterpillar-stage, and what can happen at this stage is possible also -at every other. Nor is it only animals which undergo metamorphosis that -can exhibit independent phyletic variation at every stage, but those -also with direct development, and indeed, in the case of these, we may -assume adaptation of this kind at almost every stage in the history -of the organs, as we have already seen, because the great abridgement -of the phylogeny into the ontogeny necessitates a very precise -mutual adaptation of the organ-rudiments and of the diverse rates of -development. - -We have thus been led by the facts discussed--and numerous others -from other groups in the animal kingdom might be ranked along with -them--to two main propositions, which express the relation of phylogeny -to ontogeny. The first and fundamental proposition is the one already -formulated. The ontogeny arises from the phylogeny by a condensation of -its stages, which may be varied, shortened, thrown out, or compressed -by the interpolation of new stages. The second proposition refers to -individual parts, and may run as follows: As each stage can undergo -new adaptations by itself, so can every part, every organ; such new -adaptations very often show a tendency to be transferred to the -immediately antecedent stage in ontogeny. - -It is not my intention to formulate the laws of ontogeny just now, -otherwise many others might be added to these, such as that of the -regular transference of characters acquired at one end of a segmented -animal to the other segments: I must confine myself here to bringing -the two main propositions into harmony with the principles of our -theory of heredity. - -How phylogeny is condensed in ontogeny can be understood readily -enough in a general way, although we cannot profess to have any -insight into the detailed processes. The continuity of the germ-plasm -brings about inheritance, in that it is continually handing over to -the germ-plasm of the next generation the determinant-complex of the -preceding one. Every new adaptation at any stage whatever depends -on the variation of particular determinants within the germ-plasm, -and this in its turn depends on germinal selection, that is, on the -struggle of the different determinant-variants among themselves, and -on the variation in a definite direction which arises from this, as -we have already shown. A new kind of determinant can never arise of -itself, but always only from already existing determinants, and through -variation of these. But as spontaneous variation never causes all the -homologous determinants of a germ-plasm to vary in quite the same way, -but only a majority of them, there always remains a minority of the -old determinants, which may, under certain circumstances, predominate -again, as is proved by the aberrations in _Vanessa_ species due to -cold, and by many other kinds of reversion. - -But it is not this variation which leads to the prolongation of -ontogeny, and the repetition of the phyletic stages within it. In this -case it is rather that a new character takes the place of an old one, -not that it is added to it. A black spot may arise instead of a red -one, but not first a black spot and then a red one. Of course we still -know far too little in regard to the intimate succession of events in -the stages of ontogeny to be able to say definitely that, in such -apparently simple transformations, the older stage does not, in every -ontogeny, precede the more recent one as a preparation for it, though -it may be only for a brief and transient period. - -It is certain, however, that variations such as the addition of a new -stage in ontogeny are undergone, and that this implies the occurrence -of something really quite new. Therefore such a new stage can arise -only from the germ-plasm, by the duplication, and in part variation, of -the determinants of the preceding stage. If, for instance, the body of -a Crustacean be lengthened by a segment, this must be due to a process -of this kind, and in such a case it is intelligible enough that the new -segment can be formed in the ontogeny only after the development of the -older preceding one, for its determinants come from that, and are from -the beginning so arranged that they are only liberated to activity by -the formation of the preceding segment. - -Now, if in the course of the phylogeny numerous new segments were -added to the body of the Crustacean, the ontogeny would be materially -prolonged, and condensation would become necessary in the interests of -species-preservation. To bring this condensation about, whole series of -segments which were added successively in the phylogeny succeeded each -other with gradually increasing rapidity in the ontogeny, until finally -they appeared _simultaneously_: the determinants of the segments _n_, -_n_ + 1, _n_ + 2, ... _n_ + _x_ varied in regard to their liberating -stimuli, and were roused to activity no longer successively, but -simultaneously, in the cell complexes controlled by them. We have thus -recapitulation, but with abridgement and compression, of the phyletic -stages in the ontogeny. Thus in the nauplius of _Leptodora_ we see the -rudiments of five of the pairs of legs of the subsequent thorax (Fig. -111, _IV-VIII_), and in the Zoæa larva the rudiments of six thoracic -legs may be seen behind the already developed swimming-leg (Fig. 114, -_VI-XIII_). - -But in the course of the phylogeny a segment may also become -superfluous, and we know that it then degenerates and is ultimately -eliminated altogether. Thus in a parasitic Isopod, which lives -within other Crustaceans, a segment of the thorax is wanting in the -relatively well-developed larva, and in the Caprellidæ among the -Amphipod Crustaceans the whole abdomen of from six to seven segments -has degenerated to a narrow, rudimentary structure. In such cases -the gradual degeneration of the relative determinants has preceded -step for step the degeneration of the part itself, and when this is -complete the ontogeny shows nothing of what was previously present, -and so we may speak of a 'falsification' of the phylogeny. But that -the complete disappearance of the determinants only comes about with -extreme slowness, so that whole geological periods are sometimes not -enough for its accomplishment, we have already learnt from our study -of rudimentary organs, instances of which can be demonstrated in every -higher animal, bearing witness to the presence of the relevant organs -or structures in the ancestors of the species. - -We can infer with certainty, from the observational data at our -disposal, that the disappearance of useless parts is regulated by -definite laws; but it is too soon to attempt to formulate these laws, -or even to trace them back to their mechanical causes. As we have -already said, a much more comprehensive collection of facts, and -above all one which has been made on a definite plan, is a necessary -preliminary condition to this. But so much at least we may gather -from the facts before us, that the degeneration of an organ begins -at the final stage, and is transferred gradually backwards into the -embryogenesis. Thus the two fingers of birds which have disappeared -since Cretaceous times are still indicated in every bird-embryo, -though they subsequently degenerate. In various mammals 'pre-lacteal -tooth-germs' have been demonstrated in the jaws of embryos, which show -us that not only did ancestors exist whose dentition was the modern -'milk-teeth,' but that still more remote ancestors possessed another -set of teeth, which was crowded out by the 'milk-teeth'; thus the teeth -of the ancestors of the modern right whale (_Balæna mysticetus_) are -only represented in the embryo of to-day in the form of dental pits. -And, as we saw already, the Os centrale so characteristic of the wrist -of lower vertebrates only appears in Man at a very early embryonic -stage, and disappears again as such in the further course of the -embryogenesis. - -We may perhaps give a preliminary statement of this law as follows: It -is impossible that any part or organ should be removed suddenly from -the ontogeny without bringing the whole into disorder, and the least -serious disturbance of the course of development will undoubtedly be -caused if the final stage of the part in question become rudimentary -first. Only after this has happened, and the neighbouring parts have -adapted themselves to the disappearance, can this extend to the stages -immediately preceding it, so that these too degenerate, and allow -the surrounding parts to adapt themselves. The further back into the -ontogeny the disappearance extends the greater will be the number of -other structures affected in some way or other by the degeneration, and -these must not all be brought suddenly into new conditions, else the -whole course of development would suffer. Thus at first only those -determinants may disappear--and can disappear according to the laws -of germinal selection--which control the final form of the useless -organ, then those just preceding them, which controlled, let us say, -its size, and thus more and more of the previously active determinants -disappear, and hand in hand with this disappearance there is variation -of all the parts correlated with the dwindling condition of the organ, -so that their own development and that of the animal as a whole suffers -no injury. If it were otherwise, if when a part became useless its -collective determinants were all to disappear at the same time, the -whole ontogeny would totter, in fact it would be much as if a man who -wished to remove the breadth of a window from a house standing on -pillars were to begin by taking away the foundation pillar. - -It is, of course, to be understood that these processes go on so -exceedingly slowly that personal selection takes a share in them, at -least at the beginning. Later on, the further degeneration of a useless -organ or rudiment has no effect on the individual's power of life, and -therefore depends solely upon the struggle of the parts within the -germ-plasm (germinal selection). - -If we could see the determinants, and recognize directly their -arrangement in the germ-plasm and their importance in ontogeny, we -should doubtless understand many of the phenomena of ontogeny and their -relation to phylogeny which must otherwise remain a riddle, or demand -accessory hypotheses for their interpretation. Several years ago Emery -rightly pointed out that the phenomena of the variation of homologous -parts might be inferred by reasoning from the germ-plasm theory. If -one hand has six fingers instead of five, it not infrequently happens -that the other also exhibits a superfluity of fingers, and sometimes -the foot does so too. The phyletic modification of the limbs in the -Ungulates has taken place with striking uniformity in the fore and -hind extremities; no animal has ever been one-hoofed in front and -two-hoofed behind. Although I might suggest that this primarily depends -on adaptation to different conditions of the ground, and that the -Artiodactyls were evolved in relation to the soft marshy soil of the -forest, and the Perissodactyls for the steppes, it cannot be denied -that germinal conditions may have co-operated in bringing about this -uniformity of the direction of variation, especially as the whole -structure of the fore- and hind-limbs exhibits such marked similarity. -Emery is inclined to refer this to 'germ-plasmic correlations,' and -we have assumed from the very first that the different determinants -and groups of determinants do indeed stand in definite and close -relations to one another. But it seems to me premature to say anything -more precise and definite than that in the meantime. I should like, -however, to say that determinants or groups of determinants which -had in old ancestral germ-plasms to give rise to a series of quite -similar structures by multiplication during the ontogeny, and therefore -only needed to be present _singly_ in the germ-plasm, would, in -later descendants, have to shift their multiplication back into the -germ-plasm itself, if necessity required that the homologous parts -which they controlled should become _different_ from each other. -Then the previously single group of determinants in the germ-plasm -would have to become multiple. But as new determinants can only arise -from those which already exist, these new ones must have had their -place beside the old, and would therefore probably be exposed to -any intra-germinal causes of variation in common with them--that is -to say, they will tend to vary even later in a similar manner. For -instance, we might think of the segments of primitive Annelids, which -in form and contents are for the most part alike, as arising from one -germ-rudiment, from which, when, in the higher Annelids, the various -regions of the body had to take a different form, several primary -constituents of the germ-plasm separated themselves off; and in a -similar way the much higher and more complex differentiation of the -somatic segments in the Crustaceans must have been brought about. Thus -we understand how the determinant groups of the germ-plasm multiplied -according to the need for increasing differentiation, but remained in -intimate relation, which exposed them in some measure to a common fate, -that is, to common modifying influences, and in many cases determined -them to similar variation. - -But we cannot see directly into the germ-plasm, and are therefore -thrown back on the inductions we can make from the facts presented -to us by the phenomena of visible living organisms. As yet the -material for such inductions is scanty, because it has been got -together haphazard, and not collected on a definite plan. I therefore -refrain for the present from attempting any further elaboration of -my germ-plasm theory. It is only when an abundance of observation -material, collected according to a definite plan, lies at our disposal -that anything more in regard to the intimate structure of the -germ-plasm, or the mutual influences and relations of its determinants -and its modification in the course of phylogeny can be deduced with -any certainty. Meanwhile, we must content ourselves with having, -through the hypothesis of determinants, made intelligible at least -the one fundamental fact, how it is possible that in the course of -the phylogeny single parts and single stages can be thrown out or -interpolated, or even only caused to vary, without giving rise to -variation in all the rest of the parts and stages of the animal. -A theory of epigenesis cannot do this, for, if no representative -particles were contained in the germ-plasm, then every variation of -it would affect the whole course of development and every part of the -organism, and variations of individual parts arising from the germ -would be impossible. - - - - -LECTURE XXVIII - -THE GENERAL SIGNIFICANCE OF AMPHIMIXIS - - Twofold import of amphimixis--It conditions the continual changing - of individuality--Analogy from game of cards--The germ-plasm is at - once variable and persistent--The two roots of individual variation: - germinal selection and new combinations of the ids--'Harmonious' - adaptation conditions amphimixis--Difference between adaptation - and mere variation--Is a 'direct' use of amphimixis to be insisted - upon?--Ceaseless intervention of personal selection in the lineage of - the germ-plasm--Far-reaching effects of personal selection--Fixing - of the arrangements for amphimixis in the course of generations - of species--Increase of the constancy of a character with its - duration--Characters in the same species variable in different - degrees--The upper and under surfaces of Kallima--Wild plants brought - under cultivation do not at first vary--Amphimixis very ancient, - therefore very firmly established--Does amphimixis bring about - equalization (Hatschek, Haycraft, Quetelet)?--Galton's frequency - curves--Ammon's free scope for variations--De Vries' asymetrical - curves of frequency. - - -We have already made ourselves familiar with the process which in -unicellular organisms is called conjugation and in multicellular -organisms fertilization, and we have seen that its most obvious -significance lay in the fact that through it the germ-plasms of two -individuals are united. Since, according to our view, this germ-plasm -or idioplasm is the bearer of the hereditary tendencies of the -organism concerned, the mingling or amphimixis of two germ-plasms -brings together the hereditary tendencies of two individuals, and the -organism whose development is derived from this mingled germ-plasm -must therefore exhibit traits of both parents, and must to a certain -extent be made up of the traits of both. This is one result attained by -amphimixis. - -But we went further than this, and saw that there is a second result -implied in amphimixis, namely, that the individual character of -the germ-plasm is being continually altered by new combinations of -the ids contained in it. We inferred from what I believe to be the -demonstrated hypothesis that the germ-plasm is composed of ids, that -its reduction to half the original mass must mean a reduction of the -ids to half the number, and as the ids contain primary constituents -which are individually different, this must effect a new arrangement, -a new mingling of these individual peculiarities. The reduction of the -germ-plasm to half, that is, the diminution of the number of its ids -to half, is a phenomenon generally associated with amphimixis, and -has been established in the case of all animals which have hitherto -been investigated, and of all the most carefully studied plants, -and finally, it has been shown to be very probable in unicellular -organisms, for the processes of conjugation in Infusorians and many -other Protozoa include phenomena very similar to those of reducing -division in the higher animals. The prediction made on theoretical -grounds has here been verified by observation, and it is obvious that -the assumption of ids, that is, of units in the germ-plasm which -are handed on from one generation to the succeeding one, involves a -reduction of their number in each amphimixis. Without this the number -of ids would be doubled at each amphimixis, and would therefore -gradually amount to something enormous. We see therefore why this -normally recurrent reduction of ids before each amphimixis was -established in the course of evolution, and we see that it inevitably -involves that a new combination of ids should be associated with each -amphimixis. - -If nothing persists unless it be purposeful, that is, necessary, what -is the meaning of the fact that arrangements for amphimixis occur over -almost the whole known domain of life, from the very simple organisms -up to the highest, in unicellular and multicellular organisms, in -plants and animals alike? Why is it that this arrangement has been -departed from only in a few small groups of forms, while it occurs -everywhere else, in almost every generation, so indissolubly associated -with reproduction that it has even been regarded--with a lack of -clearness--as itself a form of reproduction, and is even now generally -called 'sexual reproduction'? And why is it that in many organisms, -especially lowly ones, it is not associated with every reproduction, -though it recurs at regular or irregular intervals? Such a universal -arrangement must undoubtedly be of fundamental importance, and we -have to ask wherein this importance lies. That is the problem to the -solution of which we must now apply ourselves. - -So much we may say at once: The significance of amphimixis cannot be -that of making multiplication possible, for multiplication may be -effected without amphimixis in the most diverse ways--by division of -the organism into two or more, by budding, and even by the production -of unicellular germs. Even though these last are usually in various -ways so organized that they must undergo amphimixis before they can -develop into new organisms, yet there are numerous germ-cells which -are not subject to this condition (e.g. spores), and there are--as we -have seen--many germ-cells, adapted for amphimixis, which always, or in -certain generations, or even only occasionally, emancipate themselves -from this condition under certain external influences: I refer to -egg-cells which develop parthenogenetically. - -If amphimixis is not a universal preliminary condition of reproduction, -wherein lies the necessity for its general occurrence among living -organisms? - -We have already learned that there are two results produced without -exception by amphimixis; one of these is the antecedent reduction -of the original number of ids by one half, and the consequent new -combination of ids which results from this; the other is the union -of two such halved germ-plasms from two different individuals. The -first we may, with Hartog, compare to the removal of half of a pack -of cards previously mixed, the second to the combination of two -such halves from different packs. The first process brings nothing -new into the complex of primary constituents, but rather removes a -part--larger or smaller--of its characters: not necessarily exactly -half of these, since each individual kind of id may be represented by -doubles or multiples. But the reduction simplifies the composition -of the germ-plasm, and might by itself, through the struggle of the -ids in ontogeny, lead to a resultant different from the parent, that -is, to a new individuality. Through the second process, however, new -individual traits are of necessity added, and make the resultants still -more markedly diverse, that is, if the ids of both parents attain to -expression in the struggle of ontogeny, and this, as we have already -seen, is usually the case, though not always and certainly not always -in all parts. Thus amphimixis, together with the preparatory reduction -of the ids, secures the constant recurrence of individual peculiarities -through the ceaseless new combinations of individual characters already -existing in the species. - -When sixteen years ago I first inquired into the actual and ultimate -significance of sexual reproduction, I thought I had found it in -this ceaseless production of new individualities. This seemed to me -a sufficient reason for the introduction of amphimixis into nature, -since the difference between individuals is the basis of the process -of selection, and thus the basis of all the transformations of -organisms, which we may refer to natural or sexual selection. Now -these differences of selection-value are--as I believed then, and do -still--not only by far the most frequent organic changes, but also the -most important, since they not only initiate, but control new lines of -evolution. Therefore I still regard amphimixis as the means by which a -continual new combination of variations is effected a process without -which the evolution of this world of organisms so endlessly diverse in -form and so inconceivably complex, could not have taken place. - -But I do not regard this amphimixis as the real root of variation -itself, for that must depend not on a mere exchange of ids, but rather -upon a variation of the ids. The ids of a worm of the primitive -world could not without variation now make up the germ-plasm of an -elephant, even if it be true that mammals are descended from worms. -The ids must have been meanwhile transformed times without number by -the modification, degeneration, and new formation of determinants. -Amphimixis, that is, the union of two germ-plasms, does not of itself -cause variation of the determinants, it only arranges the ids (the -ancestral plasms) in ever-new combinations. If the origin of variation -were limited to that alone, a transmutation of species and genera -would only be possible on a very limited scale; there could at most be -a narrow circle of variations, just as in the example already given -of the packs of cards; even if the taking away and mixing up of the -halves were repeated a thousand times, a definite though undoubtedly -large number of card-combinations would in the long run recur again and -again. But the case is different with the germ-plasm and amphimixis, -where there is an infinitely more varied series of results, because -the individual cards--the ids--are variable, even between one time of -sifting and shuffling and another, and therefore infinitely productive -of variety in the course of numerous repetitions of the shuffling. - -I have been frequently and persistently credited with maintaining that -the germ-plasm is invariable--a misunderstanding of my position, due -perhaps to a somewhat too brief and terse statement which I made at an -earlier period (1886). I had described the germ-plasm as 'a substance -of great power of persistence,' and as varying with difficulty and -slowly, basing this statement upon the age-long persistence of many -species in which the specific constitution of the germ-plasm must have -remained unchanged. The idea of 'germinal selection,' of a ceaseless -struggle between the 'primary constituents' of the germ, and of -the resulting continual slight and invisible rising and falling of -individual characters, had not yet dawned upon me, nor had I at that -time formulated the conception of 'determinants.' I was even doubtful -at that time whether development, heredity, and variation were not -interpretable on the assumption of an undifferentiated substance -without primary constituents. But at no time was I unaware that the -whole phyletic evolution of the organic world is only conceivable -on the assumption of continual variation of the germ-plasm, that it -actually depends upon this, even if these variations come about with -exceeding slowness, and are thus in a certain sense 'difficult.' - -Now that I understand these processes more clearly, my opinion is -that the roots of all heritable variation lie in the germ-plasm, -and furthermore, that the determinants are continually oscillating -hither and thither in response to very minute nutritive changes, and -are readily compelled to _variation in a definite direction_, which -may ultimately lead to considerable variations in the structure of -the species, if they are favoured by personal selection, or at least -if they are not suppressed by it as prejudicial. But selection is -continually keeping watch over both kinds of variation, and if the -conditions of life do not further the variation or do not even allow it -to persist, selection eliminates everything that lessens the purity of -the specific type, everything that transgresses the limits of utility, -or that might endanger the existence of the species. Thus we understand -how the germ-plasm may be variable, and yet at the same time remain -unvaried for thousands of years, how it is ready and able to furnish -any variation that is possible in a species if that is required by -external circumstances, and yet is able to preserve the characters of -the species in almost absolute constancy through whole geological ages; -in short, how it can be at once readily variable and yet slow to vary. - -The importance which amphimixis thus has in connexion with the -adaptation of organisms lies, if I mistake not, in the necessity -for co-adaptation, that is, in the fact that in almost all -adaptations it is not merely a question of the variation of a single -determinant, but of the correlated variations of many--often very -numerous--determinants, of 'harmonious adaptation,' as we have already -said. Many-sided adaptation of this kind seems to me impossible without -a continually recurrent sifting and recombining of the germ-plasms, and -this can only be effected by amphimixis. - -It may be objected that, apart from amphimixis, variation can be -brought about in many parts of an organism, as in purely asexual -reproduction. A plant, for instance, may vary when it is transferred -to a strange soil or climate; and even in that case the variations -seem to be harmonious, at least the harmony of the parts is so far -maintained that the plant continues to flourish, at any rate under -cultivation. A plant species may be incited by abundant nourishment -to gigantic growth, and caused to vary in many of its parts, and the -abundant food may even directly affect the germ-plasm so that all or -some of these variations may become hereditary; and yet this is far -from being a case of adaptation, it is merely a case of simultaneous -variations, and it is questionable whether they will make the continued -existence of the plant under the new conditions possible or not. It -might easily happen, for instance, that the plant, though it became -larger and bore more abundant blossoms, would be sterile, and therefore -unfitted for continued existence in a natural state. Variations are not -necessarily adaptations; the latter can never come about solely through -direct influence upon the germ-plasm. What direct influence upon the -germ-plasm could, for instance, make the hind-legs of a mammal long and -strong and the fore-legs short and weak? Obviously neither an increase -nor decrease in the food-supply, nor a higher or lower temperature--in -short, no direct influence, because all these affect the germ-plasm as -a whole, and therefore cannot possibly influence two homologous groups -of determinants in opposite directions. - -This, it seems to me, is only possible when amphimixis brings about -in one individual a favourable coincidence of the chance germinal -variations of the determinants of the fore- and hind-limbs; and just -as it is with the two variations in this simple hypothetical case, -so it will be in the actual processes of adaptation where there are -involved numerous--we know not how numerous--variations essential to a -'harmonious adaptation.' - -It need not be objected that the very number of variations necessary to -a 'harmonious adaptation' makes its occurrence impracticable; for it -is _the complete_ harmony of the parts that makes the adaptation, and -without this the individual was only imperfectly adapted, and therefore -incapable of survival. It is certainly not mathematically demonstrable -that this is the case, but as the whole process of transformation which -makes an old adaptation into a new one begins with minimal fluctuations -of the determinants, which must first be brought by germinal selection -to the level of selection-value, and must then be subject to personal -selection, so the whole process goes on so gradually and by such -small steps that the harmony of the parts is maintained by functional -adaptation during the individual life in a great number of individuals. -But these are just the individuals which survive in the struggle for -existence, and at the same time possess at every stage of the process -_the best combination of favourably varying determinants_. As these -favourable variations are, in consequence of germinal selection, not -mere isolated variations of fluctuating importance, but variations in -a _definite direction_, the whole process of variation must persist -in every single part in the direction imposed upon it by personal -selection. But since at every reducing division the ids of the -germ-cells are brought down to half their number, a possibility is -offered for gradually removing the unfavourable ids from the germ-plasm -of the species, since the descendants resulting from the most -unfavourable id-combinations always perish, and so from generation to -generation the germ-plasm gets rid of its unfavourably varying ids, and -the most propitious combinations afforded by amphimixis are preserved, -till ultimately there remain only those combinations which are varying -appropriately, or at least only those in which the appropriately -varying determinants are in the majority, and so have controlling -influence. - -Logically this deduction is undoubtedly indisputable, from the -standpoint of the germ-plasm theory; but whether it may be regarded as -a sufficient reason for the introduction of amphimixis, and for its -extremely tenacious persistence throughout the course of the long and -intricate phylogeny, cannot be maintained without special investigation. - -Against my position the objection has often been urged that an -arrangement cannot arise or be maintained through natural selection -unless it is _of direct use_ to the individual in which it occurs. -Sexual reproduction cannot therefore have been established simply -because it advances, or even because it makes possible the adaptations -of species, for these adaptations only came about occasionally, perhaps -once in a thousand generations or even less frequently; thus the -intervening generations could derive no advantage of any kind from the -arrangement in question, and therefore, according to the law of the -degeneration of unused characters, it must have long since been lost. I -mentioned this objection before, but was obliged to postpone a detailed -consideration of it until we had discussed germinal selection. - -We admit, of course, that characters are only preserved intact as -long as they are of advantage sufficient to turn the scale in favour -of their possessors, and that they begin to fall from their height of -perfection when that is no longer the case; we admit also that new -adaptations are not continually necessary, but are so only at intervals -of long series of generations, and yet the objection cited seems to me -baseless. - -Leaving out of account, for the moment, the first introduction of -amphimixis, let us deal with it as an existing occurrence, for the -tenacious persistence of which we wish to find reasons. - -Is it really the case that amphimixis is only of importance in -connexion with the new adaptation of a species, and that it has nothing -to do with the persistence of the species in the state of adaptation -already attained? According to the conception of the processes within -the germ-plasm which we have already stated, it is impossible that this -should be the case, for continual slight fluctuations are occurring in -the determinants in consequence of the fluctuations of the nutritive -stream, and these slight variations, plus or minus, do not in many -cases equalize one another or counteract one another by turning again -in a contrary direction; they go on increasing in the direction in -which they have begun. It is only when personal selection opposes them -that they come to a standstill, and this can only happen when they -attain to selection-value, that is to say, when they reach a level at -which they become disadvantageous in the struggle of persons. But as -germinal variations of this kind are continually occurring, personal -selection must keep continual watch over them, and eradicate them as -soon as they have attained selection-value. - -Therefore, when a species is most perfectly adapted to its conditions, -it would of necessity begin to degenerate if personal selection were -not continually guarding it, and setting aside everything that is in -excess or deficient as soon as it begins to be prejudicial. But the -adaptation of a species does not depend upon _one_ character persisting -at its normal level, but on the persistence of very many, and many of -these vary simultaneously upwards or downwards, and reach the limit of -selection-value at one time or another. If there were no amphimixis, -then either all individuals with any excessive variant would be at -once eliminated, or the species would go on deteriorating until this -excessive variant was so numerously and strongly represented in all -its individuals that it would perish through degeneration. But even -in the first of these cases the species would drift towards the fate -of extinction, because excessive variations do occur even in every -asexual generation, and would appear in an increasingly large number -of determinants if there were no possibility of rejecting them and -eliminating them from the lineage of the species. - -This is made possible through the periodic intervention of amphimixis; -it is actually effected thereby; and in this way alone the species -is kept at its high-water mark of adaptation. It is not necessary -to assume that every single determinant which is varying in an -unfavourable direction is at once eliminated as soon as it becomes -prejudicial, that is, reaches negative selection-value, or--to make -use of an expression introduced by Ammon--as soon as it oversteps the -boundaries of the 'playground of variations,' the limits within which -variations are neither favourable nor unfavourable. But _in the course -of generations_ they are unfailingly eliminated, especially when a -large number of unfavourably varying determinants are coincident in -the germ-plasm. Then the individuals which arise from a germ-plasm -thus composed must perish in the struggle for existence, and thus the -id-combinations with excessive determinants are eliminated from the -germinal constitution of the species. As this is repeated as often as -excesses of the ids occur, the species is kept pure. - -It might be objected that, through such a continual weeding-out of -rebellious determinants, the germ-plasm would become so constant in -its constitution that it would ultimately be secure from all such -aberrations of it on the part of its determinants, and therefore would -in time become quite incapable of diverging from its proper path at -all, and would thus no longer require this continual correction through -amphimixis. - -I do not wish to contradict this conclusion; indeed, I believe that the -constitution of the species becomes more and more constant in the way -I have indicated, and that an ever more perfect and stable equilibrium -of the whole determinant system is thus brought about. It follows that -in the course of generations the diverse determinants of the germ-plasm -will vary within a progressively shortened radius, and will thus more -and more rarely overstep the limits of the 'variation-playground'--and -yet I still believe that this justifiable conclusion tells in favour of -my interpretation of the utility of the persistence of amphigony once -introduced. - -Let it be remarked, in the first place, that it is by no means -essential to the preservation of a useful institution that it should -practically justify its utility in _every_ generation. Although, for -instance, the warm winter coat of a species of mammal may be necessary -to its survival, it does not disappear at once when a winter happens -to occur which is so warm that even individuals with poor pellage can -survive. Indeed, several such mild winters might occur in succession, -in which there was no weeding-out of the individuals with poor fur, and -yet the thickness of the winter fur of the species would not become -less fixed, just because this character no longer varies perceptibly -in an old-established species which has long been perfectly adapted, -and it could only be brought into a state of marked fluctuation very -slowly through direct influence on the germ-plasm, or through panmixia. -But exactly the same thing is true in regard to the determinants of the -reproductive cells, in respect of their adaptation to amphimixis, only -very much more emphatically. - -Before going further, I should like to show that the conclusion -we have just deduced from the theory, namely, that the equilibrium -of the determinant system of a species increases in stability with -the duration of its persistence, holds good not only for the whole -system, but for its individual parts, that is, for the individual -characters and adaptations. Experience teaches that characters are the -more exactly and constantly transmitted the older they are; generic -characters are more constant than species-characters, order-characters -more persistent than family-characters--this is implied even in their -name. But we are able to show even in relation to the characters of -a species that those which have been fixed for a long time are most -precisely and purely transmitted; that is, that their determinants are -least inclined to overstep the limits of the 'variation-playground' -either in an upward or downward direction. - -Two groups of facts prove this: first the observed fact that the very -different degree of variability which the different species exhibit is -by no means common to all the characters of the species in the same -measure; for individual characters may be variable or constant in very -different degrees. - -Many years ago[21] I drew attention to the fact that the different -stages in the life-history of insects, especially of Lepidoptera, -might be variable in quite different degrees. Thus, for instance, -the caterpillar might be very variable, and yet the butterfly which -arises from it might be extremely constant. I concluded from this--what -probably no one now will dispute--that the various stages may vary -phyletically independently of one another, that, for instance, the -caterpillar may adapt itself to a new manner of life, a new food-plant, -a new means of defence, while the butterfly, unaffected by this, goes -on quietly as it was before. Every new adaptation necessarily implies -variability, and so the stage which is in process of transformation -must have its period of variability, which gradually returns again to -greater constancy, and this the more completely the longer the series -of generations through which the weeding out of the less well-adapted -has endured. - -[21] _Studien zur Descendenztheorie_, Leipzig, 1876. - -But it is not only the individual stages of development that may be -unequally variable; the same is true of the characters of a species -which occur simultaneously. The most striking example of this known to -me is the leaf-butterfly, which I have already mentioned many times -in the course of these lectures--the Indian _Kallima paralecta_. -In this species the brown and red upper surface is almost alike in -colour and marking in all individuals, but the under surface, the -colour and marking of which is so deceptively mimetic of a leaf, is -variable to such a degree that it is difficult, among a large number -of specimens, to find even a few which are as like one another as are -the members of species in which the under side is constant. It need -not be urged that this is due to the complexity of the marking on the -under side. In many of our indigenous butterflies the under side is -just as complex in coloration and marking, and nevertheless it is very -constant, being almost identical in all individuals, as for instance -in _Vanessa cardui_. In _Kallima_ the great variability of the under -surface certainly depends not merely on the fact that the mimetic -character has been only recently acquired (phyletically speaking), but -chiefly on the fact that the dead leaves to which they approximate are -themselves very diverse in appearance, for many are dry, others moist -and covered with mould, and that the adaptations have therefore gone in -different directions, and as yet, at least, have neither combined to -form a single constant type, nor diverged to form two or three distinct -types. The various 'leaf-pictures' seem equally effective in concealing -the insects from their enemies, and thus there is still a continual -crossing and mingling of the different essays at leaf-picturing. - -A second group of facts, which indicates that old-established -characters have less tendency to overstep the limits of the neutral -'variation-playground,' is to be found in the experience of breeders, -and especially that of gardeners who have brought wild plants under -cultivation in order to procure varieties. - -It has been proved that the wild plants often exhibit no hereditary -variations for a long series of generations, notwithstanding the -greatly altered conditions of life, but that then a moment comes in -which isolated variations crop up, which may then be intensified by -the manipulations of the breeder to form sport-species with large -conspicuously coloured flowers, or with some other distinctive -character. Darwin called this a shattering of the constitution of the -plant; but the stable and slowly varying 'constitution' simply means -that in old-established and well-adapted species the determinants -possess only a very restricted 'variation-playground,' and because of -their firmly based harmonious correlation are not easily and never very -quickly induced to overstep its limits in any marked degree. - -Let us now apply all this to the institution of amphimixis and -amphigony, and it is immediately obvious that these determinants of the -germ-plasm which control the characters relating to sexual reproduction -must be _more stable and less variable than all others which a species -possesses, for they are infinitely older_. They are older than all -species-characters, older than the characters of the genus, of the -family, of the class, and indeed of the whole series or phylum to -which a higher animal, a vertebrate, for instance, belongs. We cannot -wonder, therefore, that amphigony has persisted through hundreds and -thousands of generations, even if it had not been reinforced in the -germ-plasm during this period by selection. We should rather wonder -that an institution so primaeval, and so firmly engrained in the -germ-plasm, can ever be departed from, even when its abandonment is to -the advantage of the species, as has happened in parthenogenesis. - -I have entered upon this long discussion because I believe that we -require to appreciate this power of persistence on the part of the -sexual determinants before we can explain the general occurrence of -amphigony. The occurrence of pure parthenogenesis, unaccompanied by -any degeneration of the species, can hardly be understood except on -the assumption that the constancy of the species, when it has once -been attained, may be preserved without the continual intervention of -amphimixis. How long it can be preserved is another question, which it -is difficult or impossible to answer, since species exhibiting _pure_ -parthenogenesis are rare, and since we cannot tell with certainty -how long it is since amphimixis ceased to occur in them. Generally -speaking, the answer in regard to the few species which have to be -taken into account in this connexion would be 'not long,' but whether -this 'not long' signifies hundreds of generations or thousands of -generations we must leave undecided. So much only we can say, that -in all species of animals in which the male sex has quite died out -or has dwindled to a minimal remnant, there are as yet no traces of -degeneration to be found, and that even organs which have fallen into -disuse and become functionless because amphigony has disappeared, are -nevertheless in several cases retained in perfect completeness. I shall -return to this subject later on, but in the meantime I wish to work -out our conception of the actual efficacy of amphigony or ordinary -sexual reproduction, and thereby increase our understanding of its -significance and power of persistence. - -We have seen that amphigony not only renders possible the novel -'harmonious adaptations' which are continually required, but that it -also leads, by a continual crossing of individuals, simultaneously with -the elimination of the less fit, to a gradually increasing constancy of -the species. This has been regarded by some writers as its sole effect; -thus recently by Hatschek, whose view has already been refuted. - -Haycraft also finds the significance of amphigony simply in the -equalizing or neutralizing of individual differences which it effects. -Quetelet and Galton have attempted to show that intercrossing leads to -a mean which then remains constant. Haycraft supposes that a species -can only remain constant if its individuals are being continually -intercrossed, and that otherwise they would diverge and take different -forms, because the 'protoplasm' has within itself the tendency to -continual variation. The transformation of species is effected by -means of this variation tendency, and the persistency and constancy -of species which are already adapted to the conditions of their life -are secured by the constant intercrossing of the individuals, and the -consequent neutralization of individual peculiarities. - -Although the cases already mentioned in which great constancy of -species is associated with purely parthenogenetic reproduction do -not tell in favour of the accuracy of the view just stated, yet -the fundamental idea, that amphigony is an essential factor in the -maintenance and even in the evolution of species, is undoubtedly sound. -We should certainly find neither genera nor species in Nature if -amphigony did not exist; but we cannot simply suppose that amphigony -and variation are, so to speak, antipodal forces, the former of which -secures the constancy of the species, the latter its transformation. -In my opinion, at all events, there is no such thing as a 'tendency' -of the protoplasm to vary, although there is a constant fluctuation of -the characters--dependent on the imperfect equality of the external -influences, especially of nutrition. This certainly results, as far -as it takes place within the germ-plasm, in a continual upward and -downward variation of the hereditary tendencies, and it would lead to -increasing dissimilarity of the individuals were it not that amphigony -is continually equalizing the differences by a constantly repeated -mingling of individuals. Quetelet and Galton have shown that the -tendency of this mingling is towards the establishment of a mean; the -characters of Man, such as bodily size, fluctuate about a mean, which -at the same time shows the maximum of frequency; and the frequency -curve of the various bodily sizes assumes a perfectly symmetrical -form, so that the average size is the most frequent, and deviations -from it upwards or downwards occur more rarely in proportion to the -amount of deviation, the largest and the smallest sizes occurring least -frequently. - -Thus an equalizing of variations by means of amphimixis really exists, -and the question we have to ask is, How does it come about? The case -is assuredly not the same as that in which equal quantities of red -and white wine are mixed to make a so-called 'Schiller.' This is -proved even by the fact that the mixture may turn out quite different -even when the wines--the two parents--are alike: for the children -of a pair are often dissimilar. And while the 'Schiller' cannot be -separated again into red wine and white, this happens often in sexual -reproduction, and sometimes to such an extent that the grandchild -exactly resembles one or other of the grand-parents, as is most clearly -proved in the case of plant-hybrids. - -There is thus a deep-seated difference, depending on the fact that -what is mingled in amphigony is not simple but composite, not a -simple uniform developmental tendency associated with a simple and -definite substance, but a combination of several or many developmental -tendencies, associated with several equivalent but different material -units. These units are the ids or ancestral plasms, and we have seen -how they are not only halved by reducing division, but are also -arranged in new combinations in amphimixis. - -These ids differ very little within the same germ-plasm; in species -which have long been established the majority probably only differ -in correspondence to the individual differences of the fully-formed -organisms, but they are only absolutely alike in the case of two -ids which have been formed by the division of a mother-id. Let us -disregard this for the moment, and assume that all the ids of a -germ-plasm are different: the germ-plasm of a father, _A_, will be -composed of ids _A_ 1-100, that of the mother, _B_, of the ids _B_ -1-100. But in each mature germ-cell of these two parents only fifty -ids are contained, and if we assume that the mingling of the ids is -controlled solely by chance, then in the various germ-cells _A_ × _B_ -the most diverse combinations of ids may be contained; for instance, -_A_ 1, 3, 5, 7, 9, 11, ...to 99, or _A_ 1-10 and 20-30, and 40-50, -and so on, and similarly in the germ-cells _B_. If all germ-cells -produced by _A_ and by _B_ attained to development, or even if all the -ova succeeded, the thousand or hundred thousand children of this pair -would necessarily exhibit every possible mingling of their characters, -and each in the same number according to the rules of probability -calculations. But it is well known _that this does not happen_; of -the thousands of human ova, for instance, which come to maturity in -the course of the life of a female individual more than ten rarely -develop, and more than thirty never, and these are determined solely -by chance and quite independently of the mixture of ids which they -contain. It is thus purely a matter of chance which of the complexes -of primary constituents contained in the germ-plasm of an individual -are transmitted to descendants, and it is also purely a matter of -chance which combination of ids comes to be developed. Therefore we may -say that no regular neutralizing of contrasts, either in the primary -constituents of the parents or as regards the differences in their -characters, can occur. In one case there is a blended inheritance; -in another the child takes after the father or after the mother; in a -third, and this probably occurs most frequently, the child resembles -the father in some characters and the mother in others. - -But how then does Galton's curve of frequency of variations come about? -Why does the mean of any character occur by far the most frequently, -and why does the frequency of a variation diminish regularly in -proportion to its approximation to either extreme? To this it is -answered: Because the process of mingling through amphigony goes on -through numerous generations, and thus an elimination of chance, and -the establishment of an average, must be brought about. - -But this does not quite suffice to explain matters, for experience -shows that asymmetrical frequency-curves of variations also occur, even -in species with sexual reproduction. As De Vries has recently shown, -there are also 'half-Galton curves,' that is, curves which suddenly -break off at their highest point. We must conclude from this that the -frequency of the different variations depends not only on their degree, -but also on the greater or less facility with which they arise from the -constitution of the species. - -This consideration can be readily elucidated with the help of -Ammon's exposition, and especially of his graphic representation -of the 'playground of variations.' If we think of the indifferent -variations occurring in any character of a species as arranged in -a series ascending from the smallest to the largest, this line may -be regarded as the abscissal-axis, and from it ordinates may be -drawn which express the frequency of the variation in question by -the differences in their length. If the tips of these ordinates -be united, we have the curve of frequency (Fig. 120, _A_), which -according to Galton ought to be symmetrical, and in most cases really -is so. Ammon calls the space between the smallest and the largest -variations the 'variation-playground,' that is, the playground within -which all variations are equally advantageous to the species. This -is not co-extensive with the variation-area, for there may be more -marked deviations below the beginning or above the upper end of the -variation-playground, but these, being disadvantageous, fall under -the shears of personal selection. The variation-playground may also -be called the area of indulgence of variation, because the variations -falling within it are spared from the eliminating activity of -selection, or the variation-area of survivors, because on an average -only those survive whose variations do not overstep the limits of this -area. - -[Illustration: FIG. 120. _A_, symmetrical, and _B_, asymmetrical -curve of frequency; after Ammon. _U_, minimal, _O_, maximal limit of -individual variation. _U-O_, the 'variation-playground.' _M_, the mean -of variation. _H_, the greatest frequency or mode of variation.] - -This implies that variations below _U_ (the lower limit of the area -of exemption) and above _O_ (the upper limit) can occur, but do not -survive and leave descendants, and we can therefore easily understand -why characters, of which different degrees arise with equal ease from -the constitution of the species, must gradually develop a symmetrical -curve of frequency because of the constant crossing. Obviously -those individuals which stand just upon the borders of admissible -variation will, other conditions being equal, leave behind them -fewer descendants than those which approximate to the middle of the -area of exemption; for as the characters concerned can vary in the -offspring in both directions, there will always be at the lower end -some of the descendants of a pair which will fall below the limits of -exemption, and at the upper end some which will rise above it. This -will happen even when pairing takes place between parents at the middle -or at the other end of the abscissa, for there are always cases of -the preponderance of one parent in heredity. A higher percentage of -the descendants of individuals on the borderline will therefore be -eliminated, and their frequency _must therefore be less_. Even if at -the beginning of the series of observations a condition obtained in -which all the ordinates of the area of exemption were equally high, -those nearest the boundaries would of necessity very soon become lower, -and this in proportion to their distance from the boundary, and the -frequency-curve, which at first would be a straight line (according -to our assumption, which of course does not tally with natural -conditions), would become a symmetrical curve, highest in the middle -and falling equally at either side. - -Ammon has worked out the hypotheses on which the curve of frequency -would become asymmetrical. Firstly, when the fertility is greater -towards the upper or lower limit of the area of exemption; secondly, -when germinal selection forces the variation in a particular direction, -upwards or downwards; and thirdly, 'when natural selection intervenes -diversely at the upper or lower limit.' Of these three possibilities -the first two must be acknowledged as quite probable, but the third, -it seems to me, could only cause a temporary asymmetry of the curve, -lasting, that is, only until a state of equilibrium has again been -reached; but that may in certain conditions take a long time. - -Asymmetrical curves of frequency (Fig. 120, _B_) therefore arise, for -instance, when the intra-germinal conditions (the 'constitution of the -species') more easily and therefore more frequently produce extreme -variations. In this case the area of exemption can only extend on one -side, and must remain in this state. In _Caltha palustris_, the marsh -marigold, we may find, according to De Vries, among a hundred flowers, -those with five, six, seven, and eight petals, in the following -proportions:-- - - Petals 5 6 7 8 - Number of flowers 72 21 6 1 - -and thus there is an asymmetrical curve of frequency. But if we take -the whole area of variation as the area of exemption, that is, if we -assume that it is indifferent for the species whether the flowers -have five, six, seven, or eight petals, the preponderance of the -five-petalled flowers may have its reason in the fact that it is much -easier for five than six or more petals to be produced because of the -internal structure of the whole plant. - -In this case the maximum of frequency lies at the lower limit of -variation, but it may also lie at the upper. Thus, according to De -Vries, the blossoms of _Weigelia_ vary, in regard to the number of -their petal-tips, in the following manner. Six-tipped corollas were not -found, and among 1,145 flowers there were the following proportions:-- - - Tips of the corolla 3 4 5 - Number of flowers 61 196 888 - -It is thus clear that amphimixis is an essential factor in the fixing -of forms, but that it certainly does not of itself determine these, and -that it is not always the average of the variations that is the most -frequent, but that the form of the curve of frequency is determined by -other factors also, namely, by germinal and personal selection and by -the directive control which these exert on variations. - -The equalizing effect of amphigony may perhaps be expressed -thus: In the case of every new adaptation there is at first a -large area of variation, but this gradually decreases owing to -a continual restriction on the part of natural selection, until -ultimately--when the highest degree of constancy of the character or -species has been attained--it only extends very little beyond the -'adaptation-playground' or the 'area of exemption.' - -One of the effects of amphimixis is thus to bring about an increasing -restriction of the area of variation, or, as we usually say, a -constancy of the facies of a given form, a condensation into a species. -How far this result is necessary or useful, and therefore how far -it may be regarded as accounting for the persistence of amphimixis, -we shall discuss in the chapter on the formation of species. My own -view is that even the fact that new adaptations are rendered possible -through amphimixis and amphigony, the mode of reproduction associated -with it, affords in itself a sufficient reason why amphimixis should -have been retained when once it had been introduced. - - - - -LECTURE XXIX - -THE GENERAL SIGNIFICANCE OF AMPHIMIXIS (_continued_) - - Association of amphimixis with reproduction--Origin of amphimixis--Its - lowest forms--Amphimixis in Coccidia--Chromosomes in unicellular - organisms--_Coccidium proprium_--'Amœba-nests' as a preliminary - stage to amphimixis--Plastogamy of the Myxomycetes--Result: a - strengthening of the power of adaptation--Strengthening of the power - of assimilation--Use of complete amphimixis--Proof of its constant - efficacy to be found in the rudimentary organs of Man--Allogamy--Means - taken to prevent the mingling of nearly related forms--Amphimixis is - not a 'formative' stimulus--Attraction of the germ-cells--Effects of - inbreeding compared with those of parthenogenesis--Nathusius's case of - injurious inbreeding--Hindrances to fertilization in the crossing of - species--Probable reason for the injurious effect of inbreeding. - - -We have endeavoured to understand why amphimixis should have been -established among the processes of life, and we have now to turn to the -question when and how, that is, in what form, it was first introduced. -But first I should like to refer for a little to the association of -amphimixis with reproduction, which we find in all multicellular -organisms, and among the higher types so unexceptionally that, until -not very long ago, amphimixis and reproduction were looked on as -one and the same thing, and all multiplication was believed to be -associated with 'fertilization.' We have seen that this is not the -case, that on the other hand the two processes are quite distinct, -and may be called contrasts rather than equivalents, for reproduction -always means an increase in the number of individuals, while amphimixis -implies--originally at least--their diminution by a half. - -Accordingly we found that, in unicellular organisms, amphimixis -is not associated with reproduction, but interpolated between the -divisions, and not even in such a manner that amphimixis precedes every -multiplication by division, but so that the conjugation of two animals -occurs only from time to time, after numerous divisions, sometimes -hundreds, have occurred. It is obvious that this must be so, since, if -amphimixis occurred regularly between every two divisions, no increase -in the number of individuals would be brought about, at least if the -fusion of the two conjugating individuals were complete. - -Why, then, is there such an intimate, and in the case of the higher -types, such an indissoluble, association between reproduction and -amphimixis that 'fertilization' appears to be a _sine qua non_ of -reproduction, and not very long ago seemed to us to be the 'quickening -of the ovum,' the 'burning spark' which causes the powder-barrel to -explode? - -The reason of this is not difficult to discover; it lies in the -structure of multicellular animals, and in their differentiation -according to the principle of division of labour, for since only -particular cells are capable of reproduction, that is, of giving -rise to the whole, it is in these necessarily that the process of -amphimixis has to occur if its significance lies in its effects on the -succeeding generations. It is true that in the lowest multicellular -organisms, such as the species of _Volvox_, there are, in addition -to the sex-cells, other reproductive cells quite similar to the ova, -whose development into a new colony takes place without amphimixis, -but the higher we ascend in the animal and plant series the rarer are -these 'asexual' germ-cells or 'spores,' and in the highest animal types -they are entirely absent and reproduction occurs only by means of the -'sex-cells.' - -I am inclined to look for the cause of this striking phenomenon mainly -in the fact that, if amphimixis had to be retained, this was effected -with increasingly great difficulty the more highly and complexly -differentiated the organisms became, and that more complicated -adaptations were therefore necessary in order that the union of the -two germ-cells might be rendered possible at all. There is first of -all the separation into two kinds of sex-cells, whose far-reaching -differentiations and precise adaptations to the most minute conditions -we have already discussed; then follow the innumerable adaptations -to bring about the meeting of the sex-cells, the arrangements for -copulation, and, finally, the instincts which draw the two sexes -together, the means of attraction which are employed, whether -decorative colours or attractive shapes, stimulating odours or musical -notes, in short, all the diverse and intricate arrangements, which -seem to be more subtly elaborated the higher the organism stands upon -the ladder of life. When we call to mind that sexual differentiations -finally go so far that they dominate the whole organism, alike in -its external appearance and in its internal nature, its feelings, -inclinations, instincts, its will and ability, as well as its structure -down to the finest nerve-elements, we can understand that a mode of -reproduction which demands such a composite disposition of details, -involving a moulding of the whole organism, so to speak, from birth -till death, must of necessity remain the only one, and that there -was no room for the persistence of any essentially different mode -of reproduction with quite different adaptations. Or, to speak -metaphorically, the power of adaptation which is innate in the organism -so exhausted itself in the establishment of this marvellous amphimixis -adjustment that the possibility of any other was totally excluded. - -It is true that it is only among the Vertebrates that we find -'the reproductive apparatus' so highly developed, but even among -Molluscs and Arthropods 'sexual' reproduction, that is, reproduction -associated with amphimixis, is the prevailing mode. In these, indeed, -parthenogenesis does occasionally occur, that is to say, sexually -differentiated female germ-cells are, by means of some slight -variations in the maturation of the egg, rendered capable of developing -without previous amphimixis, but this happens only in quite special -cases as an adaptation to quite special circumstances, and can only -be regarded as a temporary cessation of the association between -reproduction and amphimixis. In some cases it is a moiety of the ova -adapted for amphimixis which develop parthenogenetically, as it is the -same sexually differentiated animals, true females, which produce both -sorts, and this is often true to some extent when the differentiation -in the direction of parthenogenesis has advanced further, and the -ova have been separated into those requiring fertilization and those -which are parthenogenetic (e.g. the winter and the summer eggs of the -Daphnidæ). Parthenogenesis is not asexual but unisexual reproduction, -a mode of multiplication which shows us that even in highly -differentiated animals the apparently indissoluble association between -reproduction and amphimixis can be dissolved if circumstances require -it. - -But if amphimixis had to be retained in the higher animal forms--and we -have seen reasons why this must be--it could only be effected by means -of unicellular germs, for amphimixis is in essence a fusion of nuclei, -and this is the reason why 'vegetative' reproduction, so-called, -becomes less and less prominent in animals at least, and above the -level of the Arthropods disappears almost entirely. - -Let us now return to the question we asked at the beginning--When -and in what form was amphimixis first introduced into the world of -organisms? The best way to answer this is by observation. We must turn -to the lowest forms which now exhibit it, and see whether it occurs -in them in a simpler form, so that we may draw conclusions as to its -origin and its primitive significance, for it would be possible, _a -priori_, that this was something different from what it is now in -the relatively higher organisms, and that a change of function has -gradually come about. - -Assuredly the whole intricate complex of adaptations which is now -exhibited on the conjugation of the two sex-cells in animals and -plants, the differentiation of two kinds of 'sexually' antagonistic -cells, with all their special adaptations, the reduction of the -chromosomes, the institution of the karyo-kinetic apparatus, together -with the centrospheres and so on, cannot possibly have arisen all at -once by fortuitous variation, but can only have arisen gradually, -step by step, and as the result of 'innumerable external and internal -influences.' But why should not these arrangements, nowadays so -complex, have had a simple beginning? Why might not this beginning have -been the simple union of the protoplasmic bodies of two non-nucleated -Monera; followed, after the origin of nuclear substances, by the union -of these, and, finally, after the differentiation of a nucleus with -a definite number of chromosomes, with a dividing apparatus, with a -membrane, and so on, by complete amphimixis as we now know it? And how -many transition stages may not be added to fill up the gaps between -these three main stages? - -But how much we can actually prove in regard to these conceivable -preliminary stages of amphimixis is another matter. If we take a survey -of the observations that have been made up till now, we are confronted -at first by the undoubtedly striking fact that very little is known -about it as yet, for in fact the whole process is gone through even -in quite lowly forms of life in a manner very similar to that in the -higher forms. Amphimixis has been shown to be widespread even among -unicellular organisms, yet not in an _essentially_ simpler mode than -among multicellulars. We have seen that even in ciliated Infusorians -reducing division obtains, and that of the four nuclei which arise -from twofold division of the original nucleus three break up again, -and only the fourth, by a further division, separates into a male and -a female pronucleus, 'which then complete the amphimixis with the -corresponding pro-nuclei of another animal' (compare Fig. 85, 4-7, vol. -i. p. 321). This, and the existence of a dividing apparatus and of -chromosomes, make the process appear very little less complicated than -the fertilization of higher animals. The case is similar even in much -lower unicellular organisms, such as _Noctiluca_ (Fig. 83, vol. i. p. -317). In this form and in Rhizopods it is true that reducing divisions -have not yet been made out, but their occurrence in the lower Algæ -(_Basidiobolus_), and above all in those simple unicellular organisms -which give rise to malaria, and their allies, which live as 'Coccidia' -in the blood-cells and intestinal cells of animals, leads us to expect -that they may prove to be of general occurrence among unicellulars. - -In the Coccidia, which are extremely simple unicellular organisms, -equipped, however, with a nucleus, the adaptations relating to -amphimixis are more extensive and more complex than in the Rhizopods. -For while in the latter the two conjugating cells are absolutely alike -in external appearance, in the former the male cell is distinct from -the female, and indeed the differences are as marked as those that -usually occur in multicellular animals. - -[Illustration: FIG. 121. Life-cycle of _Coccidium lithobii_, a -cell-parasite of the centipede _Lithobius_; after Schaudinn. 1, a -'sporozoite'; 2, the same penetrating into an intestinal epithelial -cell; 3, the same growing into a 'schizont' capable of division; 4, the -same dividing, and 5, breaking up into numerous pieces which separate -from the 'residual body' in the centre, and either, as in 1, migrate -into epithelial cells and repeat the history, or pass on to the phase -of sexual reproduction. In the latter case, after eliminating a portion -of the nucleus (reduction) in 6 and 6 _a_, they form the 'macrogamete' -(the ovum); or within the mother-cell they produce microgametes (or -sperm-cells), 7 and 7 _a_. The penetration of a sperm-cell into an -egg-cell (amphimixis) is shown in 8, the fertilized egg-cell (9) -becomes the so-called oocyst or permanent spore, from which by repeated -division (10 and 11), new sporozoites, as in 1, arise, and begin the -cycle afresh.] - -We owe our present knowledge of these processes especially to Schuberg, -Schaudinn, and Siedlecki, and, because of their theoretical importance, -I should like to summarize the essential points. - -One of these Coccidia lives in the intestinal cells of a small -centipede, _Lithobius_; in Fig. 121 the parasite is shown as a -so-called 'Sporozoite,' that is, as a minute sickle-shaped cell, -which at first moves freely about the intestine of the host (1), but -then soon penetrates into an epithelial cell (2). There it grows to -a spherical shape (3), and then, after having devoured the cell, -it gives rise, by a peculiar process of division (Schizogony), to -a number of very minute nucleated pieces, again sickle-shaped, the -Schizonts, each of which bores its way into an epithelial cell as in -2, and follows the same path of development, so that a large number of -cells in the intestine of the same host are attacked in this manner. -But there is still another mode of reproduction, with which amphimixis -is associated, which leads directly to the formation of 'lasting' -germs which are enclosed in a capsule or cyst, reach the exterior with -the excrement of the host, and thus spread the infection to other -centipedes. The Schizonts which take this course develop into so-called -macro-gametes and microgametes, the former being the female, the latter -the male germ-cells. Then follows the penetration of a male gamete, -actively motile because of its two flagella, into the female gamete -(8). Amphimixis is accomplished, and the product of the fusion of the -two sex-cells (9) surrounds itself with a thinner cyst, within which -it multiplies by twofold division into four cells (10). These are the -'lasting' spores, which may dry up within the voided excrement of the -centipede (11), and if they be eaten by another animal of the species, -they infect it, for the sporozoites which have been formed by the -previous divisions creep out, and in form 1 begin the life-history anew. - -We have thus an alternation of four generations which are all -unicellular, and of which one series (1-5) shows multiplication by -fission, while the other (6-11) includes, besides multiplication -by fission and as a condition of this, the process of amphimixis. -Amphimixis _must_ occur in order that the formation of 'lasting' spores -and new sporozoites may result. We have thus a regular alternation -of 'asexual' and 'sexual' reproduction, and the latter shows great -resemblance to that of multicellular organisms. The macrogamete -corresponds to the ovum, the microgametes to the spermatozoa, and they -resemble these also in their greater numbers and in their structure. - -But the resemblance goes even further. The ovum is much larger than the -sperm-cell, and undergoes a kind of reduction of its nuclear substance; -shortly before fertilization the ovum-nucleus ('the germinal vesicle') -comes to the surface--just as in the case of animal ova--bursts, and -extrudes a part of its substance in the form of a sphere (Fig. 121, 6 -and 7). A reduction of the nuclear substance in the male cell has not -been demonstrated in all cases, but in one of the _Lithobius_-Coccidia, -_Adelea ovata_, the relatively large microgamete (the sperm-cell, Fig. -122, _Mi_) places itself close to one pole of the female macrogamete -(the egg-cell) and then divides twice in succession, so that four small -cells arise (Fig. 122, _A-C_); of these only one penetrates into the -egg-cell (_D_, ♂_K_) and unites with it, the other three come to nought -(_D_, _Mi_). What a surprising resemblance this bears to the twofold -division of the mother sperm-cell in multicellular animals, through -which the number of chromosomes is reduced to half! In the conjugation -itself the thread-like chromosomes of the female nucleus are plainly -recognizable, while those of the male remain coiled up (Fig. 122, _D_). - -That the nuclear substance can be separated into chromosomes (ids) even -in lowly unicellular organisms was probably first demonstrated by R. -Hertwig for _Actinosphærium_, a Heliozoon or freshwater sun-animalcule, -then by Lauterborn in regard to Diatoms, by Blochmann for an indigenous -Rhizopod, _Euglypha_, and by Ishikawa for the marine _Noctiluca_. Fresh -cases have been added in the last decade, so that we can now say that -a considerable number of unicellulars, from the ciliated Infusorians -and lower Algæ down to the Coccidia and Diatoms, exhibit a germ-plasm -composed of ids. These structures behave in the same way as those in -higher organisms, and Berger was able to demonstrate, in 1900, in the -case of a Radiolarian, their multiplication by spontaneous splitting. - -[Illustration: FIG. 122. Conjugation of a Coccidium (_Adelea ovata_), -after Schaudinn and Siedlecki. _A_, the microgamete (sperm-cell) -(_Mi_) has become closely apposed to the macrogamete (_Ma_). _B_, the -reduction division of the nucleus of the macrogamete has been effected; -_Rk_, directive corpuscles. In the microgamete the first division of -the nucleus has begun. _C_, four nuclei in the microgamete, of which -three come to nought. _D_, the fourth microgamete-nucleus (♂_K_) -has become apposed to the nucleus of the ovum, in which distinct -chromosomes are seen.] - -From our point of view all this cannot surprise us, since all these -organisms, though only single cells, possess great complexity of -structure; we need only call to mind the extremely fine differentiation -of structure in numerous ciliated Infusorians, such as _Stentor_, which -has already been mentioned, or the bell-animalcule (_Vorticella_) with -its long and peculiarly ciliated gullet, its retractile ciliated disk, -its muscular or myophane layer, its spirally retractile stalk with the -ribbon-like, rapidly acting muscular axis; or the regular geometrically -constructed flinty skeleton of the Radiolarians, with their radially -disposed sword-like or rod-like needles and their complex interlacing -lattice-work shells. In the latter case the complexity of the living -substance becomes visible only through its product, the shell, for the -protoplasm itself does not show any visible intricacy, and the same is -true of the Coccidium whose life-history we have just been tracing, -for in each of its stages it seems to be of very simple organization, -though the succession of numerous different forms shows that its -germ-substance must be composed of numerous determinants. - -We cannot doubt, however, that, in all unicellular organisms, the -protoplasm can be hardly less complicated as regards its minute -invisible structure, since otherwise it would be impossible that the -delicate vital processes which we observe in them should run their -course. In this I agree, at least in principle, with the beautiful -picture drawn by Ludwig Zehnder in his recent book[22] already -mentioned, though he reached it in quite a different way, namely, -by a purely synthetic method. He made the daring attempt to build -up the organic world from below, starting from atoms and molecules, -and ascending from these to the lowest vital units, our biophors, to -which he attributes a tubular shape and therefore calls fistellæ. He -imagines the cell to be made up of a large number, perhaps millions, -of different kinds of fistellæ, of which one presides over the power -of turgidity, another over endosmosis, a third over contraction, a -fourth over the conduction of stimuli, &c., so that there results a -high degree of cellular complexity, a composition out of numerous -kinds of biophors arranged on a definite architectural plan. All this -corresponds perfectly with the views I have so long championed, and -which alone make the existence of a nucleus intelligible, if it is -composed--as I assume--essentially of an accumulation of determinants, -that is, of hereditary substances. And that such a high degree -of complexity of structure is not a mere fanciful picture we see -occasionally even in the case of unicellular organisms. Thus, for -instance, in _Coccidium proprium_, parasitic in the newt (_Triton_), -the macrogamete or egg-cell (Fig. 123, _Ma_) before fertilization by -the sperm-cell or microgamete (Fig. 123, _Mi_) surrounds itself with a -capsule, at one pole of which a minute opening, the micropyle, remains -for the entrance of the male cell. This proves, it seems to me, that -this particular spot of the capsule is hereditarily determined, just -as much and just as definitely as the ray of the flint-skeleton of a -Radiolarian. But if any spot of the capsule can vary by itself alone, -may not numerous other points in the animal also be hereditarily -determinable? With such complexity of the invisible structure it -would not greatly surprise us if we should find amphimixis occurring -in all unicellular organisms, and in many of them at a high level of -elaboration. These apparently lowly and simple organisms are obviously -very far from being the lowliest and simplest, as we shall discover -later in a different connexion. But that amphimixis is found as a -periodically recurring process even among these, must depend upon the -fact that here too the preservation of the best-adapted structure, as -well as adaptability to new conditions, requires that the best variants -of many different parts of the cell should be brought together, -and since the hereditary substance lies in the ids of the nucleus, -the union of the ids of two unicellulars will make harmonious and -many-sided adaptation materially easier. It will thus give an advantage -in the struggle for existence, and we may therefore expect to find that -the nuclear substance in all unicellular organism is made up of ids. - -[22] Zehnder, _Die Entstehung des Lebens_, Freiburg-i.-Br., 1899. - -[Illustration: FIG. 123. Conjugation of _Coccidium proprium_, a -cellular parasite of the newt (_Triton_), after Siedlecki. _A_, a -microgamete (_Mi_) in the act of penetrating the shell of a macrogamete -(_Ma_) through the micropyle. _B_, the male and the female nuclear -constituents are uniting (♂ _chr_ and ♀ _chr_).] - -The observations hitherto made do not, however, appear to bear this -out, for in the lower Flagellata and Algæ the nuclear substance does -indeed consist of chromatin, but--as far as it can be made out--of a -compact unarranged mass of it. But even though deeper investigations -should succeed in demonstrating chromosomes in many of these, the -nucleus _must have arisen at some time_, and we must assume that it -did so through a more intimate union of previously loose aggregates of -determinants, which were gradually arranged and bound together by the -combining forces (affinities) we have assumed to obtain among them, -thus giving rise to the first chromosomes or ids which were complete in -themselves. Then came the multiplication of these ids by the process of -division, and only then was the state arrived at from which amphimixis, -as we now know it, could have arisen, namely, the existence of a -considerable number of identical ids, half of which could be exchanged -for the identical ids of another individual in conjugation. - -But as to our question, In what organisms did amphimixis first arise, -and how? there seems, from what we have already learned with regard -to the Coccidia, little prospect of our being able to give a definite -answer, for if amphimixis occurs even in these lowly organisms, and -occurs, too, in the same manner as in the higher unicellular organisms, -and not very much more simply than among the highest multicellular -organisms, we may conclude that the preliminary stages will now be very -difficult or impossible to detect, either because they are extinct, or -because they occur only in ultra-microscopic organisms. - -Nevertheless there do appear to be preliminary stages, and they are -exactly those which we should have assumed if we had been obliged to -construct them theoretically. - -The first phenomenon of this kind is the mere juxtaposition of two or -more unicellular organisms, without the occurrence of fusion. This was -probably first observed by Gruber in Amœbæ, and it was theoretically -interpreted at a later date by Rhumbler. As many as fifty Amœbæ gather -together to form a 'nest,' and remain closely apposed to each other for -a fortnight. Although no fusion took place, and there were no visible -results of this juxtaposition, it may be concluded that the animals had -some sort of attractive effect upon each other, and it may be supposed -that some sort of advantage must have been associated with this state -of quiet, close apposition against one another. Cytotropism, the -mutual attraction of similar cells, which Wilhelm Roux first observed -in the segmentation-cells of the frog's egg, seems to occur also in -unicellular organisms, and this may help us to understand how a fusion -of cell-bodies may have come about. - -Fusion of this kind was demonstrated in the Myxomycetes almost forty -years ago by De Bary, and it has been observed more recently in various -unicellular organisms, especially in Rhizopods and in Heliozoa. -These last often place themselves close together in pairs, threes, -or even more at a time, and then the delicate cell-bodies coalesce, -though no fusion of the nuclei takes place. With Hartog, we call this -process 'Plastogamy,' but we cannot agree with that observer when he -regards the importance of the process as consisting in the fact that -the nuclei thus come into contact with fresh cell-substance, after -having been surrounded for a very long time with the same cytoplasm. -If this were the import of amphimixis, then an _exchange of nuclei_ -would take place, and this we find nowhere even among the lowest forms -of life, for everywhere there is a union of the nuclear substance of -two individuals. But this is by the way! Further cases of plastogamy -have been observed in many of the limy-shelled Rhizopods. A union of -this kind does not usually lead to any visible consequences, but in -some Foraminifera a group of young animals is developed within the -cell-bodies by the division of the nuclei and the cell-body; thus -multiplication follows the fusion just as in perfect amphimixis, and -we may therefore assume that there is a causal connexion between the -two. In the slime-fungi, too, the union of several amœba-like cells -into a multi-nucleated plasmodium is followed later by the development -of numerous encapsuled spores, but only after the plasmodium, which to -begin with is microscopically small, has grown to a macroscopically -visible, reticulated mass (_Æthalium_) sometimes a foot in extent. In -this case the fungus, creeping slowly over its foundation of decaying -substance, takes up nourishment from it, and it is not possible to -tell whether the union of the amœbæ yields any further advantage than -that of facilitating the spreading over large uneven surfaces, and -through this, later, the development of large fruit-bodies. But in the -case of the Foraminifera the plastogamy has obviously another effect, -unknown and mysterious, which as yet no one has ever been able to -define precisely. Words like 'stimulus to growth,' 'stimulating of -the metabolism,' and even 'rejuvenation,' give no insight into what -happens, but that something happens, that through the fusion of two or -more unicellulars a stimulus is exerted, which reveals itself later in -increased rapidity of growth, we may, and indeed must assume, because -this process has become a permanent arrangement in so many unicellular -organisms. Only what is useful survives, and the uniting individuals -must derive some advantage from the process of fusion, and it remains -to be seen whether we can find out with any clearness what this -advantage may be. - -Till within a few decades ago it was believed that in this process -one individual devoured the other, but this can now no longer be -maintained. If any one still seriously considers this possible, -Schaudinn's observations would convince him of his error, for in -_Trichosphærium_, a marine, many-nucleated Rhizopod, he observed, on -the one hand, the union of two or more animals, i. e. plastogamy, and -on the other hand, the swallowing and digesting of a smaller member -of the species by a larger one--two processes which are absolutely -different, for in the first case the cell-bodies of the two animals -remain intact, while an animal that is eaten becomes surrounded by a -food-vacuole, and is dissolved and digested within it. In the former -case the vital units (biophors) of each animal obviously remain intact -and capable of function; in the second, those of the over-mastered -animal are at once dissolved and chemically broken up; as biophors, -therefore, they cease to exist. Whether one or the other process takes -place may perhaps depend on whether the two animals differ greatly in -size, so that the smaller can be quite surrounded by the larger. - -In a former lecture I have emphatically expressed my dissent from the -view which interprets amphimixis as a process of rejuvenation, meaning -thereby a necessary renewal of life, and I need not go into this again -in detail: for that the metabolism can continue through uncounted -generations without being artificially stimulated--that is, in any -other way than by nutrition--is proved by all those lowly organisms -which exhibit neither plastogamy nor complete amphimixis, and also by -the occurrence of purely parthenogenetic reproduction, &c. In what, -then, can the advantage lie which the conjugating unicellular organisms -derive from conjugation? Obviously not in that they impart to each -other what each already possessed, but only in the communication of -something special and individual, something that was peculiar to each, -and becomes common to both. - -Haberlandt believed that the development of auxo-spores in Diatoms -pointed towards the processes which form the deepest roots of -amphimixis. As is well known, the hard and unyielding flinty shell -of these lowly Algæ involves a diminution of the organism at every -division, so that the Diatoms become smaller and smaller as they go -on multiplying, and if that went on without limit they would come -rapidly to extinction. But a corrective is supplied in the periodical -occurrence of conjugation of two organisms which have already -materially diminished in size, and this is followed by the growth of -the two fused individuals to the original normal size of the species. - -It is, of course, obvious that in this case the union of two organisms -which have become too small may be of advantage in bringing them back -to the requisite normal size; but this is an isolated special case, -which certainly does not justify our regarding conjugation as a means -whereby diminished bodily size may be brought back to its normal -proportions. By far the greater number of unicellular organisms are not -permanently diminished in size by division, and even in the Diatoms the -mass of the two fused individuals does not amount to the normal size of -the species, so that even in this case there must be growth subsequent -to the conjugation before the normal is re-attained. It may be doubted, -therefore, whether the increase in mass is, even in the case cited, the -essential event in conjugation, and whether there are not other effects -which we cannot clearly recognize. Here, too, there must be differences -between the two conjugating individuals, as we have just seen, for if -they only communicated something similar to each other, the result -would be an increase only in their mass, not in their qualities. - -Although we cannot demonstrate differences of this kind in the case -of the lowly organisms with which we are now dealing, we may assume -their existence from analogy with the higher organisms. We know, -especially through G. Jäger, that in Man every individual has a -specific exhalation, his particular odour, and that in the secretions -of his glands there are incalculably minute differences in chemical -composition, which justify the conclusion that the living substance -of the secreting cells themselves exhibits such differences, and that -all the various kinds of cells in an individual are not absolutely -identical with the corresponding cells of another individual, but -that they are distinguished from them by minute yet constant chemical -differences. The assumption that differences of this kind exist even -in unicellulars, and in all lowly organisms generally, is not a merely -fanciful one, but has much probability. - -How far the combination of these individual differences of chemical, -and at the same time vital, organization is able to quicken, to -strengthen the metabolism, to bring about 'physiological regeneration,' -or whatever we may choose to call it, we do not yet understand. It has -been said that in plastogamy an exchange of 'substances' takes place; -that each gives to the other the substances which it possesses and -the other lacks, and that this causes an increase of vital energy. -But it is unlikely that we have here to do merely with chemical -substances, although these, of course, as the material basis of all -vital processes, are indispensable; it seems to me more probable that -the vital units (biophors) themselves in their specific individuality -must play the chief part. But even this is saying very little, for we -have not yet reached an understanding of these processes, and if we -were not forced by the fact of plastogamy to the conclusion that this -union must have some use, no one would have been likely to postulate it -as useful, still less as necessary. It has, of course, been frequently -suggested that multiplication by fission, if long-continued, results -in 'exhaustion,' and that this is corrected by amphimixis, but who -can tell why this 'exhaustion' might not be remedied, and even more -effectually remedied, by a fresh supply of fuel, that is, of food? -One might have thought that the vital processes would be thus more -readily recuperated than by the co-operative combination of two already -'exhausted' cells. Two exhausted horses may perhaps be able to pull the -load that one of them was no longer equal to, but in the case we are -considering it is the combined burdens of two units that have to be -borne, although each was no longer equal to its own share! That is more -than we can understand. - -Zehnder has recently defined the effect of amphimixis as a -'strengthening of the power of adaptation,' and he infers that the -'digestive fistellæ' (Biophors) of two individuals, which have -somewhat different powers of digestion, are, when they combine, able -to assimilate more kinds of food than either was able to assimilate -by itself. But I confess that I do not see how an advantage for the -whole would be gained through this alone, since half of the digestive -biophors would have to work for the nutrition of the mass of the -individual _A_, the other half of the differently constituted biophors -for that of the individual _B_, and the nutritive capacity would thus -remain exactly what it was before conjugation. Nevertheless I believe -that Zehnder was right in his supposition that conjugation is concerned -with strengthening the power of adaptation, and I have long maintained -and defended this interpretation with regard to true amphimixis in -nucleated organisms. In these cases it is quite obvious that the -communication of fresh ids to the germ-plasm implies an augmentation -of the variational tendencies, and thus an increase of the power -of adaptation. Under certain circumstances this may be of _direct_ -advantage to the individual which results from the amphimixis, but -in most cases the advantage will be only an indirect one, which may -not necessarily be apparent in the lifetime of this one individual, -but may become so only in the course of generations and with the aid -of selection. For amphimixis must bring together favourable as well -as unfavourable variations, and the advantage it has for the species -lies simply in the fact that the latter are weeded out in the struggle -for existence, and that by repetition of the process the unfavourable -variational tendencies are gradually eliminated more and more -completely from the germ-plasm of the species. - -But this cannot have been the efficient cause in the introduction of -amphimixis into the series of vital phenomena; the reason for this -must be found in some _direct_ advantage, such as that it improved and -increased the assimilating power, the growth, and the multiplication -of the particular individual, so that it gained an advantage over -individuals which had not entered into conjugation. This advantage must -exist, at least in the lower forms of conjugation, in pure plastogamy, -i. e. in the mere coalescence of the protoplasmic bodies. But, as it -seems to me, we have not yet clearly recognized what the advantage -precisely is; we do not yet see how such a mingling or combination -of two plasms should every time be of advantage for the combined -conjugate. If we assume with Zehnder that two kinds of 'nutritive' -biophors are brought together which differ slightly from each other -in digestive capacity, three cases may occur. Either the food _a_, -adequate for the animal _A_, is just as abundant as the food _b_, -suitable for the animal _B_, and then half the conjugated animal will -be nourished by means of the biophors _a_, the other half by means -of the biophors _b_, and the state of matters is the same as it was -before conjugation; or the food _b_ is more abundant than the food -_a_, or conversely, and then the biophors _b_ will have to take the -larger share in the nourishment of the conjugate _A_ + _B_, and they -will therefore multiply more rapidly and the biophors _a_ will decrease -relatively in number. Nutrition and growth will then go on more slowly -for a time, but will soon attain to their former intensity. The -combined individual _A_ + _B_ has then certainly gained an advantage -over the isolated animal _A_, and the living substance of _A_ which, -if left to itself would probably have perished, can continue to live -in combination with _B_. But in that case it is not obvious where -the advantage in the union can lie, as far as _B_ is concerned. An -advantage to _B_ only results if there be a combination not of _one_ -kind of biophor only, but of several or many kinds of biophors. If for -instance _A_, whose digestive biophors were weak, brought with it into -the partnership 'secretory' or nervous biophors stronger than those of -_B_, then there would be an advantage for both in the combination, and -it is thus that, in the meantime, I interpret the direct benefit which -results from pure plastogamy. This benefit must be the more important -and far-reaching the longer multiplication by fission continues without -the occurrence of conjugation. - -We thus reach what is perhaps a not wholly unsatisfactory conception of -amphimixis, in so far at least that we do not require to assume that -there has been a fundamental change in its significance between its -expression in the lowest organisms and in the higher and even highest -forms. Everywhere it is the same advantage: an increase in the power of -adaptation; but it sometimes finds expression directly in the product -of conjugation, sometimes only indirectly, sooner or later, among the -descendants of the product. - -How far below the Myxomycetes pure plastogamy reaches we do not know; -whether it also occurs among non-nucleated organisms (Haeckel's -Monera) we cannot tell from experience, since these assumed organisms -have not yet been observed with certainty. Perhaps they all lie -below the limits of visibility, and then we could never do more than -_suppose_ that plastogamic processes occur among them. Logically and -purely theoretically we may _suppose_ that amphimixis occurred first -between the plasmic bodies of non-nucleated Monera, then between the -cell-bodies of true cells, and finally between the nuclei of cells. - -Let us hold fast to what we have found to be probable, namely, that -the fusion of individually different simple organisms must or may -bring about a direct advantage--a stimulation of the metabolism, and -at the same time an improvement of the constitution in different -directions, and let us go on to the consideration of cell-fusion -combined with nuclear fusion, or complete amphimixis. In this something -is added which we can recognize as an important advantage, namely, -the combination of two hereditary substances, and thus the union of -two variation-complexes which, according to our view, is necessary if -transformation of species is to take place. In mere plastogamy such a -union of two hereditary masses could only take place in Monera, not in -nucleated organisms. If then there are really unicellular organisms -which exhibit plastogamy without karyogamy (certain Foraminifera), -we have a further proof that these processes of plasmic fusion imply -direct advantage, which is distinct from the indirect advantage lying -in the mingling of two different hereditary contributions, since -in these cases of plastogamy there is no demonstrable mingling of -hereditary bodies, no karyogamy. - -But as soon as karyogamy or nuclear fusion was associated with mere -plastogamy, complete amphimixis could never be lost again, because it -alone made it possible that there should be harmonious transformation -and adaptation in organisms which were becoming ever more complex; the -primary effect of the mingling would be more and more transcended, -since, without amphimixis, transmutation with harmonious adaptation -in all directions would be less and less possible as organisms became -more complex in structure. I have already referred to the manifold -details in the structure and development of the lowest organisms which -make this conclusion appear luminous to us, but we can also infer the -necessity for an unceasingly active selection, from a quite different -set of facts, namely, from what we know of rudimentary organs in Man. - -We may regard Mankind as a species which has its local races and -sub-races, but which is fixed in its essential characters, and only -fluctuates hither and thither in individual variation in each sub-race, -just like any other modern mammal, such as the marmot or the hare. -Nevertheless we know that Man, as regards certain fairly numerous -parts, is continually and persistently varying in a definite direction. -Wiedersheim, in his book _On the Structure of Man_[23], enumerates a -long series of parts and organs of the human body, which are in process -of gradual degeneration, and of which it may be predicted that they -will disappear from the human structure since they have lost functional -significance. Among these dwindling structures are the two last ribs, -the eleventh and twelfth, while the thirteenth has already disappeared, -and only occurs exceptionally as a small vestige in the adult human -being of to-day. The series includes also the seventh cervical rib, -the _os centrale_ of the wrist, the wisdom teeth, and the vermiform -appendix of the intestine. The last is much larger in many mammals, -and represents an important part of the digestive apparatus, but in -Man it has dwindled to an unimportant appendage, which is a source of -danger when foreign bodies (cherry stones and such like) lodge in it -and set up inflammation. The variations in its length warrant us in -concluding that it is still in process of degeneration; its average -length is about 8½ cm., but it varies from 2 cm. to 23 cm. in length, -and in about 25 per cent. of cases a partial or entire closing up of -its opening into the intestine may be observed. - -[23] _Ueber den Bau des Menschen_, 2nd ed., Freiburg-i.-Br., 1893. -Trans. London, 1896. - -Wiedersheim enumerates nearly a hundred parts thus in process of -degeneration: this means that nearly a hundred structures in Man -are at the present time in process of variation, and this could not -be so unless amphimixis were continually mingling the hereditary -contributions anew from generation to generation, so that the -minus-variations of the parts in question, starting from the germ-plasm -in which they arose at one time as chance variations, and confirmed in -their direction by means of germinal selection, are gradually being -transmitted to all the germ-plasms of the species. We thus see that -even in a period of species-life, which we may fairly call a period of -constancy, variations of a phyletic kind are continually in process, -which could not become general without the co-operation of amphimixis. - -Now, we have already seen that personal selection plays no part, or, at -least, no important part in such degenerations, because the variations -which are here concerned do not usually attain to selection value, -but it is just such variations proceeding with infinite slowness that -occur in functionally important organs likewise, and in the progressive -advance of which personal selection and mutual adaptation probably play -a part, so that in this way we can understand why the preservation of -amphigony by natural selection must be effected. It is impossible--for -obvious reasons--to name particular instances with certainty, as we -can do in the case of the rudimentary organs, but even on general -considerations we might expect that among the incipient variations -of the determinants of the germ-plasm there would be some which were -in an ascending direction, and that among these there would be some -which, advanced by germinal selection, would go on ascending until -they attained selection value. Wiedersheim reckons, for instance, -the gradually increasing differentiation of the cortical zone of the -human brain among the parts which are still in process of ascending -variation, and he is probably right in doing so. - -But if variations, so slow as to be unnoticeable, are still of abundant -occurrence in Man, we have no reason to doubt that similar processes -are going on in other animals; among the higher Vertebrates at least -there is hardly a species which does not exhibit regressive variations -even now, and in many cases progressive variations also are occurring, -although we cannot give definite proofs of this. - -The appearance of fixity which most species have is, therefore, -illusory; in reality they exhibit a slow flux, gradually setting aside -the superfluities they received from their ancestors, perfecting the -important parts to more precise adaptation and greater functional -capacity, and at the same time endeavouring to maintain all the parts -in constant harmony. We can understand that as long as this process of -gradual perfecting goes on, amphimixis will not readily be given up. -Those that retain it must always, in the long run, have the preference. -Moreover, as we have seen, _it cannot be given up_, when it has existed -through æons, because of the power of persistence which the germ-plasm -has gradually acquired in the course of such a long hereditary -succession. It could only be given up if an advantage decisive as to -survival were associated with its abandonment, such as can be actually -recognized in most cases of parthenogenesis, among animals at least. - -In my opinion this indirect effect of amphimixis, that is, the -increasing of the possibilities of adaptation by new combinations of -individual variational tendencies, is the main one, while the direct -nutritive effect of the two germ-cells upon one another is quite -subsidiary. In this opinion I find myself in opposition to the views of -many if not most naturalists, who assume that amphimixis has a direct, -sometimes, indeed, only a direct effect, and believe that they can -prove it by facts. - -In support of this position it has been pointed out that allogamy, that -is, the mingling of individuals of different ancestry, occurs even -among lowly unicellulars, and then higher up among most organisms; -but the question has not been asked whether this mutual attraction of -the unlike really expresses a primary characteristic of organisms, -and may not possibly be a secondary acquisition adapted to ensure the -occurrence of amphimixis. If we examine the facts we find that even in -the lowly Algæ, such as _Pandorina_ and _Ulothrix_, only the migratory -cells or swarm-spores of different cell-colonies conjugate with one -another, but not those of the same lineage, and this phenomenon may be -observed in many unicellular plants and animals. We are justified in -concluding from this that a fairly large degree of difference between -the conjugating gametes secures the best results, whether this result -is to be looked for in a 'rejuvenation' or in an increased adaptive -capacity; but it is erroneous to regard the stronger attraction -between individuals of different descent as a direct outcome of this. -To me, at least, it seems to be an adaptive arrangement. The whole -of the long and complex phylogenetic history of the sex-cells, the -gametes, shows clearly that we have here to deal with a succession of -adaptations, and that the degree of attraction which obtains between -gametes has gradually been increased and specialized in the course of -the phylogeny. I need only briefly recall what we have discussed in a -former lecture, that at first the copulating cells were exactly alike -in appearance and size, that then one kind of cell became rather larger -than the other, and that only gametes which were thus different in size -were mutually attractive--the micro-gametes and the macro-gametes, -or male and female germ-cells; we have seen that these differences -between the two became more and more accentuated, that the female -cell continued to grow larger than the male, and to accumulate more -and more nutritive material for the building up of the young organism -which arises from its union with the male cell, and that the male cells -became smaller, but more numerous, as was essential if their chances of -finding the often remote female cell were not to disappear altogether. -And besides, there are all the innumerable adaptations of the egg-cell -to the countless special circumstances which obtain in the different -groups, and the innumerable varieties in the form of the sperm-cell, -with all its delicate and complicated adaptations to the special -conditions under which the egg-cell can be reached and fertilized in -this or that group of organisms. Of a truth, he is past helping who -does not regard with wonder and admiration the adaptations which have -been worked out in this connexion in the course of evolution! But if -all these details are adaptations, so is the beginning of the whole -process of differentiation; allogamy, the attraction of conjugating -cells of different lineage, is not a primary outcome of individual -diversity; gametes of different descent did not strongly attract each -other of themselves, but they were equipped with a strong power of -mutual attraction, because the union of very different individualities -was the more advantageous. - -This is an important distinction, for the adaptation to allogamy is -widely distributed, and its latest manifestations have frequently -been misunderstood in the same way as its beginnings. The widespread -occurrence of allogamy has been interpreted as evidence in favour of -the rejuvenation theory, and the endeavour on Nature's part to secure -the union of the unlike has been associated with the hypothetical -'rejuvenating' power of amphimixis, and regarded as a direct and -inevitable outcome of this. That this view is erroneous we shall see -even more clearly from what follows. - -As among unicellular Algæ it is frequently only gametes of different -lineage which conjugate, so among animals and plants there are numerous -cases in which the union of nearly related gametes is more or less -strictly excluded, both by the prevention of self-fertilization -(autogamy) in hermaphrodites, or by the prevention of inbreeding, that -is, the continued pairing of near relatives. Now all the preventive -measures which effect this are of a secondary nature; they are -adaptations which result from the advantage involved in the mingling of -unrelated germ-plasms, even though it sometimes seems as if they were -an outcome of the primary nature of the germ-cells. - -The primary result of the mutual chemical influence of the two -germ-cells upon one another is--apart from the impulse to development -which the centrosphere of the sperm-cell supplies--as far as I see, -only the more favourable or the more unfavourable mingling of the -biophor- or determinant-variants, and the resulting increase or -decrease in adaptive capacity, which leads to the better thriving of -the offspring, or conversely to its degeneration. Everything else -is secondary and depends upon adaptation, effected in very diverse -ways, to secure the most favourable mingling of the germ-plasms for -the particular species concerned. Undoubtedly the parental ids united -through amphimixis have an effect upon each other, since throughout the -building up of the organism of the child the homologous determinants -struggle with one another for food, but they do not affect each other -in the way that many prominent physiological and medical writers -suppose, namely, that the union of the parental germ-plasms sets up a -'formative stimulus' which 'advances' or even 'greatly advances' the -process of development in the egg. - -Parthenogenetic development goes on just as rapidly, sometimes even -more rapidly than that of the fertilized ova of the same species! How -can the supposed 'formative stimulus' be so entirely dispensed with in -this case? - -Of course I am well aware that the two kinds of germ-cells have a -strong attraction for each other, and that the protoplasm of the ovum -actually exhibits tremulous movement when the spermatozoon penetrates -through the micropyle. I myself observed this in the case of the -lamprey (_Petromyzon_) when Calberla instituted his investigations on -the fertilization of that animal, but has that anything to do with -a formative stimulus? Is it anything more than the result of the -chemotactic stimulus exerted by the substance of the ovum upon that -of the spermatozoon and conversely? And have we any ground for seeing -anything more in this than an adaptation of the sex-cells to the -necessity of mutually finding each other out and thereafter combining? -Two quite different things are often confused with one another in this -connexion: the mutual attraction of the two kinds of sex-cells which -tends to secure their union, and the results of this union. A more -exact distinction is necessary between the effects and the advantages -which allogamy brings in its train and the means by which it is secured -in the different species. - -If amphimixis really set up a 'formative' stimulus, and if the amount -of this was regulated by the differences between the two parental -germ-plasms, then parthenogenesis, which implies the entire absence -of the mingling of two parental cells, would necessarily be even less -advantageous than amphimixis between near relatives; but this is not -the case. Continued inbreeding leads in many cases to the degeneration -of the descendants, and particularly to lessened fertility and even -to complete sterility. Thus in my prolonged breeding experiments with -white mice, which were later carried on by G. von Guaita, strict -inbreeding, effected throughout twenty-nine generations, resulted in -a gradually diminishing fertility, and similar observations have been -made by Ritzema Bos and others. But why does not the same thing happen -in pure parthenogenesis? My experiments in breeding parthenogenetic -Ostracods (_Cypris reptans_) shows that these crustaceans, in the -course of the eighty generations which I have observed till now[24], -have lost nothing of their prolific fertility and vital power; and -the same is true in free nature of the rose-gall wasp (_Rhodites -rosæ_), which enjoys the greatest fertility notwithstanding its purely -parthenogenetic reproduction, the females not infrequently laying -a hundred eggs in a single bud. How does it happen that 'the mutual -influence of two different hereditary substances which so powerfully -promotes individual development' can be here altogether dispensed with? -Only because it does not really exist, except in the imagination of my -opponents, still influenced by the old dynamic theory of fertilization. - -[24] The cultures were begun in 1884 and are still continued (March 6, -1902), still multiplying as abundantly as at the outset. I reckon that -there are on an average five generations in a year, which means about -eighty generations in sixteen years. - -But it may be asked, whence come the injurious results of inbreeding, -if not from the union of two nearly related germ-plasms? They certainly -do arise from that cause, but it is not through a 'formative stimulus,' -_too slight_ in this case, exercising a direct formative chemical -effect upon the two hereditary substances, but through the indirect -influences exerted by these _too similar_ hereditary contributions -during the development of the new individual. Lest it be imagined that -I am tilting against windmills, I will refer to one of the numerous -examples of the evil effects of inbreeding which have been submitted -to me as specially corroborative of the conception of amphimixis as a -'formative stimulus' whose strength depends upon the difference between -the germ-substances. The renowned breeder, Nathusius, allowed the -progeny of a sow of the large Yorkshire breed, imported from England -when with young, to reproduce by inbreeding for three generations. The -result was unfavourable, for the young were weakly in constitution -and were not prolific. One of the last female animals, for instance, -when paired with its own uncle--known to be fertile with sows of a -different breed--produced a litter of six, and a second litter of five -weakly piglings. But when Nathusius paired the same sow with a boar of -a small black breed, which boar had begotten seven to nine young when -paired with sows of his own breed, the sow of the large Yorkshire breed -produced in the first litter twenty-one and in the second eighteen -piglings. - -How could this really remarkable difference in the fertility of the -sow in question be the result of a formative stimulus, exercised by -the sperm-cells of the unrelated boar upon the ova of the female -animal? If the progeny of the sow had been more fertile than herself, -then we should have been at least logically justified in concluding -that this was the case, but it is not intelligible that the egg-cells -of this mother sow should be increased twice or three times because -they were fertilized by a new kind of sperm as they glided from the -ovary. The number of ova which are liberated from the ovary depends -in the first instance upon the number of mature ova contained in -it; and unless we are to make the highly improbable assumption that -the crossing with the strange boar had as an immediate result the -maturing of a large number of ova, we must look elsewhere than in the -ovary of the animal for the cause of this sudden fertility, possibly -in chance circumstances which we are unaware of and which make the -ovary occasionally more productive, possibly however in the fact that -inbreeding may have brought about various slight structural variations -in the animal, and among these some which made the fertilization of -the abundantly produced ova by the sperm of the related boar less -easy, and caused it to fail more frequently. As will be readily -understood, I cannot say anything definite on this point, but we know -that very slight variations in the sperm-cell or the ovum may make -fertilization difficult, or may even prevent it. I need only remind -you of the interesting experiments in hybridization which Pflüger -and Born made with Batrachians nearly thirty years ago, which showed -that in two nearly related species of frog the ova of the species A -were frequently fertilized by the sperms of the species B, but not -conversely, the ova of the species B by the sperms of A. This is the -case, for instance, with the green edible frog (_Rana esculenta_), -and the brown grass-frog (_Rana fusca_), and the reason of this -dissimilarity in the effectiveness of the sperm lies simply in 'rough -mechanical conditions,' in the width of the micropyle of the ovum, -and the thickness of the head of the spermatozoon. If each species -possesses a micropyle which is exactly wide enough to admit of the -passage of the spermatozoon of its own species, another species will -only be able to fertilize these eggs if the head of its spermatozoon -be not larger than that of the first species. Thus, as experiment has -proved, the spermatozoa of _Rana fusca_ fertilize the ova of almost -all other related species, for they have the thinnest head and it is -at the same time very pointed. In this case, therefore, it depends -upon the microscopic structure of the ovum whether fertilization can -take place or not, and we can imagine that similar or perhaps other -minute variations had taken place in the ova in the case of Nathusius's -sow, and that these made it difficult for the sperms of boars of the -same family to effect fertilization. These variations may have arisen -as a result of the continued inbreeding, because the same ids were -constantly being brought together in the fertilized ova, and thus any -unfavourable directions of variations which existed were strengthened. - -It seems to me that in this way alone can the injurious effect of -inbreeding be made intelligible. From both parents identical ids meet -in the fertilized ovum, in greater numbers the longer inbreeding -continues, for at the maturation of every germ-cell the number of -different ids is diminished by a few, and their number must therefore -gradually decrease, and it is conceivable that ultimately it may -sink to one kind of id, that is, that the germ-plasm may then consist -entirely of identical ids. If chance variations of certain determinants -in unfavourable directions occur in some of the ids composing the -germ-plasm, these are brought together in the offspring from both the -maternal and the paternal side, and will occur in an increasing number -of ids the longer the inbreeding has gone on, that is, the smaller the -number of different ids has become. The unfavourable variation-tendency -is therefore persisted in, and its influence upon the development of a -new descendant will be the greater the larger the number of identical -ids with these unfavourable variations. It is obvious that the crossing -of an animal, which is thus, so to speak, degenerating slightly, with -a member of an unrelated family must immediately have a good effect -upon the descendants, for in this way quite different ids with other -variations of their determinants are introduced into the inbred -germ-plasm which had become too monotonous. - -From this theoretical interpretation of the injurious consequences of -inbreeding we may at once infer that not every inbreeding necessarily -implies degeneration, for the occurrence of unfavourable variational -tendencies in the germ-plasm is presupposed as the starting-point of -degeneration, and if these do not exist there can be no degeneration. -This harmonizes with the fact that the evil effects of inbreeding are -observed _to vary greatly in amount, and may not occur at all_. But -they are greatest in breeds artificially selected by man, which have -long been under unnatural, directly influential conditions, and are -also removed from the purifying influence of natural selection. In such -cases, therefore, there is every probability that diverse unfavourable -variational tendencies in the determinants will occur. - -But how are we to understand the fact that pure parthenogenesis -may last through innumerable generations, and yet no degeneration -sets in? I believe very simply. In this case too, the same ids -which were peculiar to the mother of the race are contained in the -descendants, but they do not diminish in number, for in pure and -normal parthenogenesis, such as that of _Cypris reptans_, the second -maturation-division of the ovum does not take place, and this is -precisely the nuclear division which effects reduction. In addition, -the introduction of identical ids, which must take place in the case of -inbreeding at every amphimixis, does not occur, and, what is certainly -of great importance, all these cases are old species, living under -natural conditions--the same conditions under which they lived as -amphigonous species, and not newly formed breeds under artificial -conditions, as has probably always been the case in the experiments in -inbreeding. - -It is true that even in old species, living in a state of nature, -unfavourable variations may arise in the germ-plasm, and may go on -increasing during purely parthenogenetic multiplication, for the ids -with unfavourably varying determinants will no longer be set aside -by means of reducing division. But those individuals in which the -unfavourable variational tendency increases until it has attained -selection-value will be subject to selection and will be gradually -eliminated; indeed, the weeding out of the inferior individuals will be -more drastic here than where amphigony obtains, because in this case -all the offspring of one mother are nearly alike, so that the whole -progeny is exterminated if the mother varies unfavourably. - -On the other hand, a transformation in a favourable direction, an -adaptation to new conditions of life, as far at least as that implies -the simultaneous variation and harmonious co-adaptation of many parts, -cannot, as far as I can see, be effected in the course of purely -parthenogenetic reproduction, nor can a degeneration of complicated -parts which have become superfluous. For both these changes, in my -opinion, require that the ids of the germ-plasm should be frequently -mingled afresh, since apart from this there cannot be a harmonious -readjustment of complicated structures, nor can a uniform degeneration -affecting all parts set in. As an example of this last case we may take -that organ which became functionless in the purely parthenogenetic -species of Ostracods when amphigonous reproduction was given up--the -sperm-pocket or receptaculum of the female. All these species still -possess an unaltered receptaculum seminis, a large pear-shaped bladder -with a long, narrow, spirally coiled entrance-duct, very well adapted -for allowing the enormous spermatozoa of the males to make their way -in singly, and to arrange themselves within the receptacle side by -side in the most beautiful order, like a long ribbon, and finally to -migrate out again singly to fertilize the liberated ova. In _Cypris -reptans_ and several other species, however, no males have been found -in any of the places which have been carefully searched, and the -receptaculum of the female is always found to be empty. Nevertheless -it shows no hint of degeneration. It is possible enough that, as in -_Apus cancriformis_, which is of similar habit, the males have become -extinct in most colonies of these species, but that nevertheless they -do occur here and there from time to time in the area inhabited by the -species, and if this should prove to be the case, it would confirm the -conclusion, which is very probable on other grounds, that the pure -parthenogenesis of these species has not existed in most of their -habitats for a long time, speaking phylogenetically. For this reason we -must not over-estimate the significance of the complete persistence of -the receptaculum even with exclusively parthenogenetic reproduction. -It proves, however, that degeneration of a superfluous organ does -not necessarily set in even after hundreds of generations, and in -this fact there is certainly a corroboration of the view that it is -'chance' germinal variations which give the impulse to degeneration. -These first induce a downgrade variation through germinal selection, -and this, if it concerns an organ of no importance to the survival of -the species, is not hindered in its progress by personal selection. -Whether degeneration of the receptaculum would have occurred in -these parthenogenetic species if they had retained even a periodic -sexual reproduction, as is actually the case in the generations of -the alternately parthenogenetic and sexually reproducing Aphides, -we cannot decide, since we know nothing in either case as to the -length of time that parthenogenesis has prevailed among them, nor -have we any method of computing the number of generations that must -elapse before a superfluous organ begins to vacillate. We only know -that the parthenogenetic generations of Aphides no longer possess -a receptaculum, while other forms with alternating bi-sexual and -parthenogenetic modes of reproduction, which are in this respect -possibly more modern, e. g. some of the gall-wasps, possess one similar -to that of the Ostracods. - -[Illustration: FIG. 79 (repeated). The two maturation divisions of the -'drone eggs' (unfertilized eggs) of the bee, after Petrunkewitsch. -_Rsp_ 1, the first directive spindle. _K_ 1 and _K_ 2, the two -daughter-nuclei of the same. _Rsp_ 2, the second directive spindle. _K_ -3 and _K_ 4, the two daughter-nuclei. In the next stage _K_ 2 and _K_ 3 -unite to form the primitive sex-nucleus. Highly magnified.] - -I must refer to one other case of parthenogenesis, since it has been -hitherto regarded as a formidable puzzle for the germ-plasm theory, -and has only recently found its solution, I mean the facultative -parthenogenesis of the queen-bee. As the 'male' eggs of the bee remain -unfertilized, and yet undergo two reducing divisions, which must -diminish the number of ids in the ovum-nucleus by a half, the number of -ids in the germ-plasm of the bee must be steadily decreasing, and this -state of things has therefore been regarded by some English biologists -as convincing evidence of the untenability of the conception of ids and -of the whole germ-plasm theory. Apparently, indeed, it is contradictory -to the theory, and we must inquire whether the contradiction is merely -an _apparent_ one, disappearing when the facts are more precisely -known. It was mainly on this ground that I instituted the researches -carried out by Dr. Petrunkewitsch, the results of which I have already -in part communicated in a former lecture. These results confirmed -the previous conclusions that the 'male' eggs of the queen-bee -remain unfertilized, that two reducing divisions occur, and that in -consequence the ovum-nucleus only contains half the normal number of -chromosomes. That these increase again by division to the normal -number does not save the theory, for only _identical_ ids can arise in -this way, while the significance of the multiplicity of the ids lies -mainly in their difference. The halving of the number of ids in each -'male' ovum would necessarily lead, if not to a permanent diminution -in the number of ids, at least to a monotony of the germ-plasm, since -the number of _different_ ids would be steadily decreasing and the -number of _identical_ ids as steadily increasing. This too would be a -contradiction of the theory. But Dr. Petrunkewitsch's investigations -have shown that, of the four nuclei which are formed by the two -reducing divisions, the two middle ones (Fig. 79, _K_ 2 and _K_ 3) -recombine with one another, and fuse into a single nucleus, and that -from this copulation-nucleus in the course of development the primitive -germ-cells of the embryo arise. Now all the ids which were originally -present in the nucleus of the immature ovum may be reunited in this -'polar copulation-nucleus' if the two nuclei _K_ 2 and _K_ 3 turned -towards each other in Fig. 79 contain different ids. That this is the -case cannot of course be seen from the ids themselves, but it seems to -me extremely probable, since it is dissimilar poles of the two nuclear -spindles which here unite, namely, the lower pole (daughter-nucleus) of -the upper spindle and the upper pole of the lower spindle. In the first -directive or polar spindle there lay thirty-two chromosomes, which had -increased by duplication from sixteen, and of these sixteen passed over -into the first polar nucleus, while sixteen formed the basis of the -second directive spindle. These two sets of sixteen chromosomes must -have been quite similar, since the two sets arose by division of the -sixteen mother-chromosomes. Let us call the chromosomes _a_, _b_, _c_, -_d-q_, then similar sets of chromosomes must have been contained in the -two nuclear spindle figures depicted in Fig. 79 at the beginning of the -division, and eight of these went to each daughter-nucleus. Now, if -_a-k_ migrated to the upper pole of the spindle and _l-q_ to the lower -pole, then the union of _K_ 2 with _K_ 3 would bring together again -all the ids that had before been present. In consideration of this I -predicted to Dr. Petrunkewitsch that this copulation-product might be -the basis of the formation of the germ-cells in the drone-bee, and his -painstaking and difficult researches have confirmed this prediction, -strange though it may seem, that the male germ-cells have a different -origin from the female germ-cells. But this discovery gives a strong -support to the germ-plasm theory. It may, of course, be objected that -the assumed regular distribution of the ids in the two daughter-nuclei -cannot be proved, but we know already that this dividing apparatus -does _very exact_ work, and we are at liberty to assume it in an even -higher degree. Moreover, what other interpretation of the unexpected -development of the germ-cells discovered by Petrunkewitsch could be -given if this had to be rejected? A clearer proof of the individual -differences of ids and of their essential importance could not be -desired, than lies in the fact that in the 'male' eggs of the queen-bee -a different and novel mode of germ-cell formation is instituted, after -half the ids have been irrecoverably withdrawn from the ovum-nucleus. -We see from this that for individual development a duplication of -individual ids may suffice, but that for the further development of the -species a retention of the diversity of the ids is important. - - - - -LECTURE XXX - -INBREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION, AND THEIR -CONSEQUENCES - - The separation of the sexes exists even among the - Protozoa--Conditions determining the occurrence of - Hermaphroditism--Tape-worms, Cirrhipeds--Primordial males--Advantages - of parthenogenesis--Alternation with bi-sexual generations--In - Gall-wasps--In Aphides--Cross-fertilization secured in - plants--Self-fertilization is avoided whenever possible--The - mechanism of fertilization and the mingling of germ-plasms must - be clearly distinguished from one another--Cases of persistent - self-fertilization--The effects of inbreeding compared with those - of parthenogenesis--The effect of purely asexual reproduction--In - sea-wracks--In lichens and fungi--In cultivated plants--Degeneration - of the sex-organs--Summary. - - -We have seen that continued inbreeding must make the germ-plasm -monotonous, and therefore unplastic as regards the requirements of -adaptation. Accordingly, we found that the gametes of many unicellulars -are so constituted that they only possess a power of attraction for -gametes of a different lineage, not for those of their own stock. Among -multicellular organisms the most intense mode of inbreeding is to be -found in the uninterrupted self-fertilization of hermaphrodites: in -such cases the monotony of the germ-plasm must reach extreme expression -more readily than in the case of ordinary inbreeding. We can thus -understand why, in the scale of organisms, there is such an early -occurrence of gonochorism, the separation of the species into male and -female individuals. Even among unicellular plants or Protophytes this -occurs occasionally, as it does in the Vorticellids among Infusorians. - -In the Metazoa and Metaphyta the separation of the sexes finds emphatic -expression; it is absent from no important group, and in many, such -as, for instance, among the Vertebrates, it has become the absolutely -normal condition, with hardly any exception. But in many divisions of -the animal and plant kingdoms hermaphroditism also plays an important -part, as, for instance, in terrestrial snails and in flowering plants. - -Obviously the sexual adaptations of a species are definitely related -to the conditions of its life, and, though Nature's endeavour to -prevent inbreeding and to secure cross-fertilization is evidenced by -the occurrence of separate sexes in such a multitude of forms yet -in many cases gonochorism has been relinquished, and always where -this was necessitated by the conditions of life to which the group -concerned was subject. In such a case inbreeding is regulated as -far as possible, for instance, by an arrangement which ensures that -individuals shall be crossed at least from time to time. But cases of -exclusive and constant self-fertilization do also seem to occur, and -even these may be brought into harmony with our conception, according -to which cross-fertilization is an advantage, but only an advantage -which must be weighed against others, and which may eventually be given -up in favour of greater advantages. This occurrence of persistent -autogamy can no more be reconciled with the rejuvenation theory than -can continuous parthenogenesis, because, according to this theory, -the mingling of different individuals is a _sine qua non_, for the -continued life of the species. - -It is impossible for me here to discuss in detail all the deviations -from pure gonochorism or bi-sexuality which occur in nature, but I must -at least attempt to take a general survey, and to arrange the chief -phenomena of these various modes of 'sexual reproduction' in an orderly -scheme. I must take a survey of both plants and animals, but I shall -give the precedence to animals, as being to me more familiar ground. - -Where do we find, in the animal kingdom, that Nature has departed from -gonochorism, from the separation of the sexes, and for what reasons was -this departure necessary? And further, what means does Nature take to -compensate for this renunciation of the simplest method of securing the -continual cross-fertilization of individuals? - -Let us glance over the animal kingdom with special reference to these -questions: we find that hermaphroditism prevails chiefly among species -which at maturity have lost their power of free locomotion, and have -become sedentary, such as oysters, barnacles among Crustaceans, the -Bryozoa, and the sea-squirts (Ascidians) which are fixed to the rocks -at the bottom of the sea. For forms such as these it must often have -been advantageous that each individual could function both as male -and as female, especially when it was capable of self-fertilization, -since individuals which settled down singly, or in very small numbers -together, would not be lost as regards the persistence of the species. -The continuance of the species is thus better secured than it would -be by separation of the sexes, because in the latter case it might -frequently have happened that the animals which had settled beside -each other by chance were of the same sex, and would therefore remain -unfertile. But many of these species do not fertilize themselves, but -fertilize each other mutually; and this, too, carries a great advantage -with it, because in sedentary animals the sperms will fertilize -twice as many individuals, if each contains eggs, than if half were -exclusively male. It is thus to some extent an economy of sperms, but -at the same time also of ova, which is effected by hermaphroditism: -the result is that these valuable products are wasted as little as -possible. On this account we find that not only sedentary, but also -sluggish, slow-moving animals are equipped with male and female organs -of reproduction, as, for instance, all our terrestrial snails. They -fertilize each other mutually: when two meet it is always as males and -females, and notwithstanding the sluggishness of movement, it is not -likely to happen that a snail does not attain to reproduction because -it has not found a mate. The same is true of the earthworms, which -are likewise not adapted for making long journeys in search of the -opposite sex; they, and the leeches also, function as male and female -simultaneously, while their nearest relatives, the marine Chætopods, -are of separate sexes, which may be associated with their much greater -power of free movement in the water. - -In these cases self-fertilization is often absolutely excluded; it -may be physically impossible, and hermaphroditism therefore secures -cross-fertilization in such cases just as effectively as if the sexes -were separate. Similarly, in many hermaphrodite flowers, as we have -already seen, the pollen is so constituted and so placed within the -flower that it cannot of itself make its way to the stigma. In oysters, -for instance, the young animal is male, and liberates into the water an -enormous quantity of minute spermatozoa, and therewith fertilizes the -older individuals, functioning only as females, which have grown upon -the same bank. At a later stage of its development the oyster which was -male becomes female, and produces only ova. This state of affairs, of -which I shall shortly mention another case, has been called _temporary_ -hermaphroditism. In this case not only is self-fertilization excluded, -but close inbreeding also, since it is always a young generation -functioning simultaneously as males that mingles with an older -generation which has become female. - -It is quite otherwise with parasites which live singly within the body -of a host: for these it was indispensably necessary that they should -not only produce both kinds of germ-cells, but that they should unite -the two kinds in fertilization, and they therefore possess the power -of self-fertilization. Thus, in the urinary bladder of the frog, there -occurs a flat-worm (_Polystomum integerrimum_) which possesses special -organs for pairing with another individual, but which is also capable -of self-fertilization when, as frequently occurs, it has no companion -in its place of abode. But this self-fertilization is always liable to -be interrupted by cross-fertilization, for not infrequently there are -two, three, or even four such parasites within the bladder of a single -frog. - -In the tape-worms, too, cross-fertilization is not excluded, for there -are often two or more of these animals together in the intestine -of a host at the same time. But even where there is only one, -self-fertilization on the part of the joints, that is, the sexual -individuals, is prevented, and by the same device, metaphorically -speaking, as in the case of the oyster, for in each joint the male -elements mature first and the female elements afterwards. In certain -parasitic Isopods of the genus _Anilocra_ and related forms close -inbreeding is prevented in the same way--by a difference in the period -at which the two sets of gonads in the hermaphrodite individual become -mature (dichogamy). - -This is secured in a different way in Crustaceans which have grown to -maturity in a sedentary state, like the Cirrhipeds. These animals, -known as 'acorn-shells' and 'barnacles,' are sedentary, sometimes -on rocks and stones, sometimes on a movable object, the keel of a -ship, floating pieces of wood, cork, or cane, or sometimes attached -to turtles or whales, and although they generally occur in great -numbers together, they are probably only able to fertilize each -other occasionally, and are therefore essentially dependent upon -self-fertilization. But Charles Darwin discovered long ago that many -of them, notwithstanding their hermaphroditism, have males which are -small, dwarf-like, and very mobile organisms, destined only for a -very brief life. These seemed quite superfluous in association with -hermaphrodite animals, and they have therefore long been regarded as -vestigial males, as the last remnant, so to speak, of a past stage of -the modern Cirrhipeds, in which the sexes were separate. It is obvious, -however, that we must now attribute to them a deeper significance, -for these so-called 'primordial males,' although extremely transitory -creatures without mouth or intestine, represent a means of securing -the cross-fertilization of the species. What importance nature -attaches to their preservation is shown especially by the parasitic -Cirrhipeds which have been so carefully studied by Fritz Müller -and Yves Delage--those sac-like Rhizocephalidæ or root-crustaceans -which are altogether disfigured by parasitism. The fully developed -animals are hermaphrodite and live partly in, partly upon crabs and -hermit-crabs (Fig. 112, _C_, _Sacc_). These hermaphrodites indeed -fertilize themselves, but in their youth they are of distinct sexes, -and the females are so constituted that they lay eggs for the first -time just when the males of the current year are appearing. Thus the -first batch of eggs liberated by the females are fertilized by the -minute free-swimming 'primordial males,' but after that the females -themselves develop testes, and then fertilize themselves; the males -die very soon after copulation, and only appear the following year -in a new generation. They are therefore far from being mere historic -reminiscences, vestiges of the early history of the modern species, -for they are the instruments of a regular cross-fertilization of -the species, and therefore of a constant mingling of new ids in -the germ-plasm. This is not the place to discuss the marvellous -life-history of these parasites in detail; I can only say that when -we inquire into the whole story, and appreciate the difficulties -associated with the persistence of these 'primordial males,' we can no -longer doubt that crossing is an indispensable feature of amphimixis--a -feature which must at least occasionally occur if amphimixis is to -retain its significance. This is shown, it seems to me, especially by -these numerous instances of what we may call compulsory retention of -ephemeral males in hermaphrodite, self-fertilizing animals; it follows -also from the theory, for with continued self-fertilization all the ids -in the germ-plasm of an individual would tend to become identical, and -the mingling of two germ-plasms which contained _identical_ ids would, -at least according to the germ-plasm theory, have no meaning at all. - -[Illustration: FIG. 112 (repeated). Development of the parasitic -Crustacean _Sacculina carcini_, after Delage. _A_, Nauplius stage. -_Au_, eye. _I_, _II_, _III_, the three pairs of appendages. _B_, -Cypris stage. _VI-XI_, the swimming appendages. _C_, mature animal -(_Sacc_), attached to its host, the shore-crab (_Carcinus mænas_), -with a feltwork of fine root-processes enveloping the crab's viscera. -_S_, stalk. _Sacc_, body of the parasite. _oe_, aperture of the -brood-cavity. _Abd_, abdomen of the crab with the anus (_a_).] - -Thus we see that in the animal kingdom hermaphroditism is always -associated with cross-fertilization in some way or other, even though -the latter may occur rarely, being usually periodically interpolated, -and thus bringing new ids into the germ-plasm which is rapidly becoming -monotonous or uniform. Adaptations quite analogous to these are found -in relation to parthenogenesis, and it will repay us to give a brief -summary of these. - -Parthenogenesis effects a very considerable increase in the fertility -of a species, and in this increase the reason for its introduction -among natural phenomena obviously lies. By the occurrence of -parthenogenesis, the number of ova produced by a particular colony of -animals may be doubled, because each individual is a female, and as the -multiplication increases in geometrical ratio a few parthenogenetic -generations result in a number of descendants enormously in excess of -those produced by bi-sexual reproduction. We can therefore understand -why parthenogenesis should obtain among animals whose conditions of -life are favourable only for a short time, and are then uncertain and -dangerous for a long period. This is the case with the water-fleas, -the Daphnids (see Figs. 57 and 58), whose habitats--pools, ponds, and -marshes--often dry up altogether in summer, or freeze in winter, so -that it becomes almost if not quite impossible for the colonies to go -on living, and the preservation of the species can only be secured -by the production of hard-shelled 'lasting' eggs, which sink to the -bottom, dry up in the mud, or become frozen, or at least remain latent -in a sort of slumber. As soon as the favourable conditions reappear, -young animals which emerge from the eggs are all females and reproduce -parthenogenetically, so that after a few days there is a numerous -progeny swimming freely about, which in their turn are all females, -and reproduce after the same manner. In many Daphnids this goes on -for a series of generations, and there thus arises an enormous number -of animals, which may fill a marsh so densely that, by drawing a -fine net a few times through the water, one can draw out a veritable -animal soup. In our ponds and lakes these little Crustaceans form the -fundamental food of numerous fishes. But notwithstanding the enormous -havoc wrought among them by enemies, large numbers remain at the end -of a favourable season, and these produce the lasting eggs, _after -fertilization_. For shortly before the end of the season males appear -among the progeny of the hitherto purely parthenogenetic females. -Although each female will only produce a few of these 'lasting' eggs, -which require fertilization and are richly supplied with yolk, the -whole number in each colony is a very large one, because the number of -individuals is very large; and it must be so, since the eggs, though -secure against cold and desiccation, are very imperfectly protected -against the numerous enemies which may do them injury. - -Of course the number of individuals which form a colony may vary -greatly in the different species, and the same is true of the number -of parthenogenetic generations which precede the bi-sexual generation. -I have already shown in detail that this depends precisely on the -average duration of the favourable conditions, so that, for instance, -a species which lives in large lake-basins will produce many purely -parthenogenetic generations before the bi-sexual one, which only -appears towards autumn, while species which live in quickly-drying -pools have only a few parthenogenetic generations, and the true -puddle-dwellers give rise to males and sexual females along with the -parthenogenetic females as early as the second generation. - -We thus find in the Daphnids an alternation, regulated and made -normal by natural selection, of purely parthenogenetic with bi-sexual -generations, and the result is that the uniformity of the germ-plasm, -which is the necessary consequence of pure parthenogenesis, is -interrupted after a longer or shorter series of generations by -the occurrence of amphimixis. That the number of parthenogenetic -generations may be so varied, though with a definite norm for each -species, indicates again that amphimixis is not an absolute condition -of the maintenance of life, not an indispensable rejuvenation, designed -to counteract the exhaustion of vital force--whether this be meant -in a transcendental sense or otherwise--but that it is an important -advantage calculated to keep the species at its highest level, and that -its influence appears whether it occurs in the species regularly, or -frequently, or only rarely. - -This kind of alternation of generations, that is, the alternation -between unisexual (female) and bi-sexual generations, has been called -heterogony. In the Daphnids, certainly, a difference in form between -the parthenogenetic and the bi-sexual generation does not exist, -for the same females which produce eggs requiring fertilization can -also produce parthenogenetic ova, although these are very different -from each other, as we have already seen. The difference between -generations, therefore, does not lie in their structure, but in their -tendency to parthenogenetic or to amphigonous reproduction, and in the -absence or presence of male individuals. There are, however, other -cases of alternation of generations in which the different generations -diverge from each other in structure. One of the most remarkable of -these is that of the gall-wasps (Cynipidæ). In many of these little -Hymenoptera, which form galls on leaves, blossoms, buds, and roots, -especially of the oak, two generations occur annually, one in summer, -the other in early spring, or even in the middle of winter. The latter -consists of females only and reproduces parthenogenetically. We can -readily understand this from the point of view of adaptation to -particular conditions, since the young wasps which emerge from their -galls in winter, or in the middle of a raw spring, are exposed to many -dangers and are terribly decimated before they can succeed in laying -their eggs in the proper place on the plant. Moreover, much precious -time would be lost by the mutual search of the sexes for each other,--a -search which would often be entirely without result. Thus the wingless -female of _Biorhiza renum_ (Fig. 124, _A_), which is not unlike a plump -ant, attempts, without taking food, and often interrupted by a spell -of cold or a snowstorm, to reach a neighbouring oak-shrub, creeps up -on it, and lays its eggs in the heart of a winter bud, whose hard -protecting scales it laboriously perforates by means of its short, -thick, sharp ovipositor. - -[Illustration: FIG. 124. Alternation of generations in a Gall-wasp. -_A_, winter generation (_Biorhiza renum_). _B_ and _C_, summer -generations (_Trigonaspis crustalis_). _B_, male. _C_, female. After -Adler.] - -After it has succeeded in sinking its ovipositor into the heart of the -bud, it goes on working for hours, piercing the delicate tissue with a -multitude of fine canals, one close beside the other, and then deposits -an egg in each of these. The whole detailed piece of work requires, -according to Adler, uninterrupted active exertion for about three days, -even though in the end only two buds may be filled with eggs. If at -every egg-laying the arrival of a male had to be waited for, an even -larger number of females would fall victims to the unfavourable weather -and other dangers, while at the same time the number of emerging -females could be only half as large as it is. It is obvious that in -this case parthenogenesis is of very great advantage. - -In summer the climatic conditions are incomparably more favourable for -the gall-wasps, and accordingly we find that the summer generation -is bi-sexual, but, strangely enough, is so different from the winter -generation that the relationship of the two forms was for a long time -overlooked. The antennæ, the legs, and particularly the ovipositor, -the whole shape of the animal, its size, the length of the abdomen, -the structure of the thorax, and many other points are so different -that as long as the structural features afforded the only criterion -of relationship, the systematists quite naturally placed the winter -and summer forms in different genera. It was only when Dr. H. Adler -succeeded in breeding the one form from the other that people were -convinced that such marked differences in structure could be found -within the same life-cycle. - -[Illustration: FIG. 125. The two kinds of Galls formed by the species. -_A_, the many-chambered galls produced by the parthenogenetic -winter form, _Biorhiza renum_. _B_, those produced on oak-leaves by -_Trigonaspis crustalis_, the bi-sexual form. After Adler.] - -But we see here quite clearly why the two generations had to become so -different; simply because the winter generation had to adapt itself -to different conditions from the summer generation, above all as to -the laying of its eggs within the tissues of a plant of a different -constitution. In our example, the winter form _Biorhiza renum_ pierces -the terminal buds of the oak, and lays in each of them a large number -of eggs, sometimes as many as 300, so that a very large gall is -formed, in which a great many larvæ can find food, and grow on to the -pupa-stage. From this spongy gall, something like an inverted onion in -shape, and about the size of a walnut (Fig. 125, _A_), there emerge in -July the slender, delicately formed male and female gall-wasps which -were long known as _Trigonaspis crustalis_. Both males and females -are winged, and fly rapidly about in the air (Fig. 124, _B_ and _C_). -The sexes pair, and the females lay their eggs in the cell-layers -on the under side of an oak-leaf, on which arise small, wart-like, -kidney-shaped galls (Fig. 125, _B_) which fall to the ground in autumn, -and from which there emerge, in the middle of winter, the plump, -wingless females, to which, as we have already seen, the name _Biorhiza -renum_ was given. - -One generation, therefore, lays its eggs in the parenchyma of tender -leaves, and has only to pierce through a thin layer of plant-tissue, -while the other must penetrate deep down into the hard winter bud, -to be able to deposit its eggs in the proper place, and we therefore -find that in the two kinds of female the ovipositor differs in length, -thickness, and general structure, and so also does the whole complex -apparatus by which the ovipositor is moved. But these differences are -associated with the form of the abdomen, in which the ovipositor lies, -and with the strength and shape of the legs, which must be shorter -and stronger when the boring has to be performed through a hard -plant-tissue or to a considerable depth. We can readily understand how -numerous must be the secondary variations which a transformation of the -ovipositor brings in its train when we compare the ovipositor apparatus -in the two generations of one of these species (Fig. 126). - -[Illustration: FIG. 126. Ovipositor and ovum of the two generations -of the same species of Gall-wasp. _A_, those of the winter form, -_Neuroterus læviusculus_. _B_, those of the summer-form, _Spathegaster -albipes_. _st_, ovipositor. _ei_, ovum. Similarly magnified. After -Adler.] - -Figure 126 shows the ovipositor of another gall-wasp, of which the -winter form, _Neuroterus læviusculus_, also perforates the hard winter -buds of the oak, while the summer form, _Spathegaster albipes_, lays -its eggs in the tender young leaves of the same tree. The ovipositor of -the former is thin and long, that of the latter short and strong (Fig. -126, _A_ and _B_), and corresponding also to the depth at which the egg -must be sunk, or, so to speak, sown in the tissue of the plant, the egg -of the summer generation differs from that of the winter generation -by having a much shorter stalk (Fig. 126, _ei_). These little wasps -thus afford a beautiful example of the way in which even marked -changes in the conditions of life of a generation may be associated -with transformations in bodily structure, and we understand how it -was possible that by means of processes of selection the generations -which alternate periodically in the year should come to diverge very -considerably in structure. The example may also serve to illustrate how -diverse are the harmonious co-adaptations which such transformations -require, and how necessary, therefore, the continual re-combination of -the ids of the germ-plasm by means of amphimixis must be. We understand -why bi-sexual reproduction was only abandoned in one generation, and -that the one in which parthenogenesis was of considerable advantage. -But such transformations must have come about with extreme slowness, -since they were the result of climatic changes which only come about -very gradually. We thus come again to the same conclusion to which -we were led by our study of vestigial organs in Man, that numerous -species which appear to be at a standstill are continually working -towards their own improvement. But for this amphimixis is essential; -consequently the descendants which have arisen through amphimixis, and -whose ancestors have arisen in the same way, have an advantage over -those of parthenogenetic origin. On the whole, at least, this must be -so; in special cases it may be otherwise, namely, when the advantage -offered by parthenogenesis in respect to the maintenance of the species -preponderates over the advantage which amphimixis implies as regards -possibilities of transformation. - -As far as we have seen from the case of the gall-wasps, the absence of -amphimixis in every second generation implies no disadvantage in regard -to the capability for transformation which the species exhibits. As to -whether any disadvantage would ensue if the number of parthenogenetic -generations in the life-cycle were greater we can only guess, since no -case is known which enables us to decide this point, _pro_ or _con_, -with any certainty. The heterogony of the plant-lice, the Aphides, and -their relatives might be cited as against the probability, for in this -case a long series of parthenogenetic generations often alternates -with a single bi-sexual one, but the difference in structure is not so -great in this case, although it does exist, and moreover we can quite -well assume that the adaptation to parthenogenesis was effected at the -beginning of heterogony, when it still consisted of a cycle of only -two generations, and that further virgin generations were interpolated -subsequently. - -This assumption is supported by the fact that in some species of our -indigenous Ostracods, in _Cypris vidua_ and _Candona candens_, in -contrast to the Daphnids, several bi-sexual generations alternate -with one parthenogenetic generation. But in this case again there is -no difference whatever in the structure of the two generations, the -parthenogenetic generation being distinguished from the bi-sexual -generation simply by the absence of males. - -The alternation of generations in the plant-lice is particularly -instructive, because it emphatically indicates how much Nature is -concerned with the retention of amphimixis, and how little mere -multiplication has to do with this. This is especially striking in the -case of the bark-lice; for instance, in their notorious representative, -the vine-pest, _Phylloxera vastatrix_. - -[Illustration: FIG. 127. Life-cycle of the Vine-pest (_Phylloxera -vastatrix_), after Leuckart and Nitsche, and Ritter and Rübsamen. _A_, -the fertilized ovum. _B_, the resulting apterous and parthenogenetic -Phylloxera. _C_, its eggs, from which, as the uppermost arrow -indicates, there may arise similar apterous, parthenogenetic forms, or, -as the horizontal arrow indicates, winged forms (_D_), which produce -'female' and 'male' ova (_E^1_ and _E^2_); from these the sexual -generation arises, the female (_F^1_) and the male (_F^2_); the former -lays the fertilized ovum (_A_).] - -As in all plant-lice, the advantage for the sake of which sexual -reproduction was given up depends upon the fact that a practically -unlimited food supply is at the disposal of these parasites of the -vine, which can be made full use of during the proper season, and -which, since every animal is female and produces eggs, results in an -enormous increase in the number of individuals, and thus secures the -continuance of the species. These insects emerge in spring from small -fertilized eggs, which have lain dormant throughout the winter (Fig. -127, _A_), and they develop rapidly into wingless females (_B_), which, -sucking the juice of the vine, multiply by producing large numbers -of little white eggs (_C_). These develop without fertilization into -similar wingless females. Several generations of females succeed each -other, but then, usually from August onwards, differently formed winged -females (_D_) make their appearance, and these, flying from plant to -plant, effect the distribution of the species. But these, too, lay -parthenogenetic eggs (_E^1_ and _E^2_), and from these there emerge, -late in autumn, the members of the single bi-sexual generation, males -and females (_F^1_ and _F^2_), both very minute and wingless, without -a piercing proboscis, and thus incapable of taking food. These pair, -and the female lays a single egg (_A_) under the bark of the vine, -from which the leaves are now falling; this egg survives the winter, -and from it in the following April or May there emerges once more a -parthenogenetic female. - -It could hardly be more plainly shown than it is by this case that the -importance of amphimixis is something quite apart from reproduction -and multiplication, for here the number of individuals is not only not -increased by amphimixis, but is materially diminished, being indeed -lessened by a half. By the retention of amphimixis, the species gains -in this case no advantage _except_ the mingling of two germ-plasms. - -Something similar occurs in plants which exhibit alternation of -generations, for instance the ferns, in which the sexual generation, -the so-called prothallium or prothallus, contributes nothing to -the _multiplication_ of the plant, since only a single egg-cell -is developed; and the same is true of the mosses. In both cases -_multiplication_ depends solely on the asexual generation, which, as -the so-called 'moss-fruit' or 'fern-plant proper,' produces an enormous -number of spores, in addition to multiplying by runners. - -To sum up: we have seen that self-fertilization does occur in -hermaphrodite animals, where otherwise the species would be in -danger of extinction, but this is never the _sole_ and exclusive -mode of fertilization[25], for hermaphrodite species have always the -possibility of securing inter-crossing of individuals, and that in -various ways, whether by the intervention of 'primordial males' or -by an occasional or a periodic alternation of self-fertilization and -mutual fertilization. Pure parthenogenesis enduring through innumerable -generations does appear to occur, but in most cases unisexual -generations alternate with bi-sexual, so that a stereotyping of the -germ-plasm with complete uniformity of ids is obviated. - -[25] As to the cases Maupas has brought into notice, of permanent and -apparently exclusive self-fertilization in Rhabditidæ (round worms), -it seems fair to say that they have not been as yet sufficiently -investigated to admit of a secure appreciation of their value in their -theoretical bearings. Cf. _Arch. Zool. Exper._, 3rd ser., vol. viii, -1900. - -We must now briefly consider the higher plants with reference to the -maintenance of diversity in the germ-plasm through crossing. - -We saw in an earlier lecture that most flowers are hermaphrodite, -but that they do not fertilize themselves, and are adapted for -crossing, since the pollen of one flower is carried by insects to -the pistil of another, which cannot be reached by its own pollen, -either because it ripens too early or too late, or because the -stigma, notwithstanding its proximity, is so placed as to be out of -reach of the pollen from the adjacent stamens. I showed, following -the fundamental investigations of Sprengel, Charles Darwin, Hermann -Müller, and other successors of Darwin, that the flowers may in a sense -be regarded as the resultants of the insect-visits, since all their -accessory adaptations--large coloured petals, fragrance, nectar, and -even little minutiæ of colour and markings (honey-guides)--as well -as their detailed shape, as seen in 'landing stages,' corolla tubes, -and so on, are only intelligible when we refer their existence to -natural selection. We assume that each of these adaptations secured -some advantage for the species concerned, and that therefore their -first beginnings as slight germinal varieties were accepted, and were -brought gradually to their full expression by the united operation -of germinal and personal selection. This at least is how we should -express ourselves now that we have become acquainted with the factor -of germinal selection. The advantage secured by every such improvement -in a flower's means of attracting insects is obvious, as soon as it -is established that cross-fertilization is more advantageous for the -species than self-fertilization. - -We have discussed this already; we saw that experiments instituted -by Darwin proved that seedlings which had arisen through -cross-fertilization were superior to those arising through -self-fertilization, and that in many cases the mother-plant itself -produced fewer seeds when self-fertilized than when cross-fertilized. -This discovery afforded an explanation of the cross-fertilization -of flowers by insects which Sprengel had previously observed. We -understand how the flowers must have become so adapted through -processes of selection that they were unable to fertilize themselves, -but attracted insects, and, so to speak, compelled these to dust them -with pollen from another plant of the same species. We also understand -how self-fertilization remained possible for many flowers in the -event of cross-fertilization through insects not being effected, -since after a certain period of waiting, a curvature of the stamens -or the pistil may take place and lead to the stigma being dusted with -the pollen of the same flower. Obviously the development of _fewer_ -seeds is preferable to complete sterility. It is a well-known fact -that peculiar inconspicuous and closed flowers, designed solely for -self-fertilization, may occur along with the open flowers, as in the -case of the so-called cleistogamous flowers of the violet (_Viola_) -and the little dead-nettle (_Lamium amplexicaule_), and the phyletic -origin of these becomes intelligible as soon as it is established that -cross-fertilization is more advantageous than self-fertilization. - -Now, however, it seems as if the fundamental proposition of this theory -of flowers will have to be rejected. Not only do the cleistogamous -flowers just mentioned exhibit a great fertility, not at all less -than that of the open flowers of the same species which are adapted -for cross-fertilization, but there is a small number of plants which -produce seeds by _self-fertilization alone_. Thus in _Myrmecodia_ -cross-fertilization is absolutely prevented by the fact that the -flowers never open, and according to Charles Darwin _Ophrys apifera_ -also reproduces by self-fertilization alone, and is nevertheless a -thoroughly vigorous plant. There are several other cases of this sort, -and particularly among the orchids, though the whole of the structure -of their flowers is specially adapted for pollination by insects. -Many of them are only rarely visited by insects, some not at all, we -know not why, but it is readily intelligible that in such cases they -should have adapted themselves to self-fertilization wherever that was -possible. For this _no great_ variation was necessary; it was enough -that the pollinia, which formerly only became detached from their -attachment at a touch or a push from an insect, should free themselves -spontaneously. And this, according to Darwin, is what happens, -for instance, in _Ophrys scolopax_, which at Cannes is frequently -self-fertilizing. For the development of seed, however, it is not -enough that the pollen should reach the stigma; the pollen-grain has -to send out its tube and penetrate into the ovary, and in many orchids -this does not happen; they are infertile with their own pollen. Various -other plants are also non-fertile with their own pollen, for instance -the common corydalis, _Corydalis cava_, or the meadow cuckoo-flower, -_Cardamine pratensis_ (Hildebrand). - -How are we to reconcile these apparently absolutely contradictory -facts? On the one hand, the innumerable devices for securing crossing -lead us to conclude that it is necessary, or at least advantageous, -and on the other we find a small number of plants which reproduce -continually by self-fertilization and yet remain strong and vigorous. -And again there are many plants which yield seed when fertilized with -their own pollen, and others which remain absolutely sterile in the -same circumstances, yielding no seed or very little, and there is -indeed one on which its own pollen has the effect of a poison, for if -it reaches the stigma the flower dies. If there is anything injurious -in self-fertilization (Darwin), we can understand that it will be -avoided, but how can it be continued so long in many cases, and even -become in others the exclusive method of fertilization without visible -evil results? - -It seems to me that in these facts, established by observation, the -results of two quite different processes have been confused, and that -we can only gain clearness by studying them apart from one another; I -mean the processes involved in the mechanism of fertilization and those -involved in the mingling of the germ-plasms. - -In many cases self-fertilization is said to yield less seed and weaker -seedlings. Let us for the present take this statement as the basis of -our consideration; it does not seem to me conceivable, though here I -am not in agreement with views that have been expressed by others, -that both effects should depend upon the same causes, for the smaller -number of seeds cannot possibly depend upon the mingling of the two -parental germ-plasms, and thus not upon the process of amphimixis -itself, since the effect of the mingling does not make itself felt -until the organism of the offspring is being built up. Of course the -plant seed is the embryo of the young plant, but it will hardly be -thought probable that its development could be absolutely prevented -by the too close relationship of the two germ-cells, and thus the -number of the developing seeds cannot depend on the quality of the ids -co-operating in the segmentation-nucleus, but presumably on the number -of ova awaiting fertilization in the ovary, which are reached by a -pollen-tube and then by a paternal sex-nucleus. This again will depend -upon the impelling and attracting forces of the pollen-grain on the one -hand, and of the stigma and 'embryo-sac' of the flower on the other. In -other words, the fertility of a flower with its own pollen will depend -upon whether the two products of the flower are adapted for mutual -co-operation, and in what degree they are so. We are here dealing not -with the _primary_ reactions of the germ-plasms, which are as they are -and cannot be varied, but with secondary relations, which may be thus -or thus--in short, with _adaptations_. - -By what adaptations the pollen of a flower can be made ineffective -for that flower is a question which we must leave the botanists to -answer; in any case it must have been possible, and we see clearly -that it depends upon adaptation when we consider the numerous stages -which occur--from the rare case of the actually poisonous influence -of self-pollination already noticed, to complete sterility, and from -lessened fertility to greater or even perfect fertility. It is possible -that chemical products, secretions of the stigma or the pollen-grain, -or the so-called synergid-cells, have to do with this, or that the -size and therewith the penetrating power of the pollen-cell in -self-fertilization stand in inverse ratio to the length of the pistil, -as has been proved in regard to heterostylism by Strasburger; but in -any case it was possible for Nature, by means of slight variations in -the characters of the male and female parts of the flower, to diminish -the certainty of the meeting of the two germ-cells, even to the total -exclusion of the possibility of any union of these. - -If, then, self-fertilization had to be guarded against or at least -rendered difficult because its consequences were injurious, all -variations pointing in the direction of safeguarding would necessarily -be preserved and increased. In many cases variations in the structure -of the flower were sufficient; but when, as in _Corydalis cava_, the -pollen could not readily be prevented from falling upon the stigma, the -pollen might be made sterile as far as its own flower was concerned -by a process of selection, in which on an average those plants would -remain successful which produced the largest number of cross-fertilized -seeds, and in this case those which did so were those whose pollen -reacted most feebly to the stimulus of their own stigma. - -[Illustration: FIG. 128. Heterostylism (_Primula sinensis_), after -Noll. Two heterostylic flowers from different plants. _L_, the -long-styled form. _K_, the short-styled form. _G_, style. _S_, anthers. -_P_, _p_, pollen-grains. _N_, _n_, stigmatic papillæ of the long-styled -and short-styled forms respectively. _P_, _p_, _N_, _n_, magnified 110 -times.] - -That self-sterility in all these different degrees is not a primary -character of the species, but an adaptation to the advantages of -cross-fertilization, is apparent--if indeed it seems doubtful to any -one--especially from cases of heterostylism. I refer to the dimorphism -and trimorphism which Darwin discovered in many flowers, and which -shows itself in the fact that flowers otherwise almost exactly -alike, as, for instance, primroses, may exhibit a long style in some -individuals, and in others a short one (Fig. 128). At the same time, -there is a difference in the position of the stamens, which are placed -higher up in flowers with short styles, and much lower down in those -with long styles. Experiments have proved that the dusting of the -stigma has the best results if pollen from the short-styled reaches -the stigma of the long-styled form, or if pollen from the long-styled -form reaches the stigma of the short-styled. Thus we have again to deal -with an arrangement for crossing, an adaptation to the advantages of -cross-fertilization, and we can in this case see the reason why the -pollen has a different effect upon the two stigmas; the pollen-grains -of the flowers with short style are larger than those of the flowers -with long style, and as the length of the pollen-tube that can be sent -out must depend upon the mass of protoplasm within the pollen-grain, -it follows that the smaller pollen-grains will send out too short a -tube to reach through the long style to the embryo-sac. In addition to -this there is a difference in the papillæ of the stigmas, and it is -possible that these may form an obstacle to the penetrating of pollen -from a similar type. The process of selection which gives rise to such -arrangements as we find in Primulas may easily be imagined, as soon as -we are able to assume that cross-fertilization is more advantageous -than self-fertilization as regards progeny, that is, as regards the -continuance of the species. - -We have already seen that uninterrupted self-fertilization is unknown -among animals, but that it is not even very rare among plants, and -this emphatically corroborates our previous conclusion, that the -reason for which amphimixis was introduced as a normal event in nature -is not to be sought for in the necessity for a renewing of life, or -'rejuvenation.' It cannot be a necessity, but only an advantage, which -can in certain circumstances be dispensed with. - -Although it is obvious enough that continued inbreeding in its -most extreme form, self-fertilization, does not imply an absolute -abandonment of amphimixis, the adherents of the rejuvenescence theory -have regarded the unfavourable consequences of pure inbreeding as a -confirmation of their assumption, according to which amphimixis is -indispensable to the continuance of the life of the species, and it -is therefore an important fact, if it can be proved, that continued -self-fertilization can occur persistently, among plants at least, and -yet not cause any injurious results to the species. - -But how can this fact be understood from our point of view? How does it -happen that crossing is striven after in so many different ways and yet -so often given up again, and continued self-fertilization resorted to? - -To this it may be answered, in the first place, that it is not, -as far as we can see, for _internal_ reasons that persistent -self-fertilization becomes the rule; there is no peculiar condition -of the germ-plasm which makes it disadvantageous or superfluous -that the diversity of the id-combinations should be maintained; -self-fertilization is due to _external_ influences which bring it about -that the plant has only the alternative of producing no seeds at all -or of producing them by self-fertilization. In this connexion Darwin's -experiments with orchids are particularly noteworthy. - -In this very diversified order of plants there are numerous species -whose flowers are infertile with their own pollen, although it -does not reach the stigma in natural conditions, and therefore -there was no necessity--as far as we can see--for guarding against -self-fertilization by 'self-sterility.' These flowers are thus doubly -adapted, so to speak, for crossing by means of insects. But as regards -many of these, as well as many other modern orchids, insect-visits are -very rare, and in some cases do not occur at all, and therefore these -species cannot produce seed or can do so only exceptionally. - -This is true of most of the Epidendra of South America, and of -_Coryanthus triloba_ of New Zealand, two hundred blossoms of which only -yielded five seed-capsules, and also of our _Ophrys muscifera_ and _O. -aranifera_, the latter of which yielded only a single seed-capsule -from 3,000 flowers gathered in Liguria. We might expect that the -species in question must have become very rare, but this is not always -the case, since each of these capsules contains an enormous number -of seeds, sometimes many thousands. As soon as the visits of insects -cease altogether, the species must die out in the particular locality -concerned, unless it can revert to self-pollination and self-fertility. -There is a whole series of species in which the stigma of the flower -is sensitive to its own pollen, and in many of these an adaptation -to self-fertilization has actually been effected, for the pollinia -detach themselves from their anthers at maturity and fall upon the -stigma. I have already mentioned _Ophrys apifera_, which, according to -Charles Darwin, is no longer visited by insects, although its flowers -still possess the structure required for insect-fertilization. This -species has saved itself from extinction by the normal occurrence of -self-fertilization. - -This seems to me noteworthy in two respects. In the first place, it -shows that pure self-fertilization need not necessarily result in a -weakening of the species, and secondly, it affords a clear instance -of a species being transformed in one minute character only, all the -other characters remaining unaltered. In this case it was only the -pollinia that required to vary a little in their mode of attachment and -maturation, in order to effect the transformation of the flower for -self-fertilization, and in point of fact that is all that has varied. -The case is not relevant to our investigation at this moment, but -cases of the kind can so rarely be clearly demonstrated that I cannot -lose the opportunity of calling attention to it. The germ-plasm of -this _Ophrys_ must have varied at an earlier stage, for otherwise the -detachment of the pollinia would not have become normal and hereditary, -but it can only have varied to the extent that the structure of this -one small part of the flower was affected by the variation; something -must have varied in the germ-plasm that had no influence upon the other -parts of the flower, that is, solely the _determinants_ of the pollinia. - -Let us return after this digression to our previous train of thought; -we have to inquire how we can interpret the fact of continued -self-fertilization without any visible injurious results to the -species. If cross-fertilization be a material advantage as regards the -continuance of the species, how can it be transformed into its opposite -without evil effects? And there are no visible evil effects in _Ophrys -apifera_. It is indeed not so abundant as _Ophrys muscifera_, or other -allied species, but it certainly does not follow from that that it is -on the way to extinction; certainly no decrease either of vigour of -growth or of fertility can be observed. - -If we inquire from the standpoint of our theory, how the composition -of the germ-plasm must have altered through continual inbreeding, -we have already found the answer--that through the reduction of the -number of ids at the maturation of every germ-cell the diversity of the -germ-plasm would gradually be lessened, that the number of different -ids would thereby be lessened possibly even to the identity of the -whole of the ids. - -The consequences of such extreme uniformity of the germ-plasm would -not, according to our theory, necessarily be that the species would -be incapable of continued existence, but it would be that the species -would become incapable of adaptations in many directions. Adaptations -in one direction, such, for instance, as the variation in the mode of -attachment and detachment of the pollinia of an Orchid, would still be -possible. Thus a species which has long been perfectly adapted will be -able to make the transition to inbreeding without injury to its chances -of continued existence, if it be compelled by circumstances to do so. -Species, on the other hand, which are still undergoing considerable -transformations in many directions must be exposed by these to the -danger of degeneration, just as happens in the artificial experiments -with domesticated animals, whose secret weaknesses are greatly -exaggerated by inbreeding. - -We might be inclined to regard the effects of inbreeding as similar to -those of parthenogenesis; they are certainly analogous, for both modes -of reproduction must lead to a certain degree of uniformity in the -germ-plasm. But there seems to me to be a difference and one which is -not without importance. - -In parthenogenesis no amphimixis occurs, but neither does any reduction -of the number of the ids to one-half; all the ids present at the -beginning of parthenogenesis are retained; they are only no longer -mingled with strange ids. In inbreeding both amphimixis and reduction -take place, but the former soon ceases to convey any really strange -ids to the germ-plasm, but only the same as those which it already -contains, so that a rapidly increasing monotony of the germ-plasm -must result. To this must be added the possibility that among the -few ids which now--many times repeated--form the germ-plasm, some -must occur which exhibit unfavourable variational tendencies in -one or many determinants, and then the same thing will occur which -usually occurs in experimental inbreeding of domesticated animals, -namely, _degeneration of the progeny_. In parthenogenesis the case is -otherwise; unfavourable variational tendencies, as soon as they attain -selection-value, are, so to speak, eliminated root and branch, because -the individuals which exhibit them, and their whole lineage, are -exterminated, without their having any effect upon the other collateral -lines of descent. A purely parthenogenetic species will, therefore, not -degenerate as long as individuals of normal constitution are present, -for these reproduce with perfect purity. But if in later generations -unfavourable variational tendencies crop up in the germ-plasm through -germinal selection, the process of personal selection will be -reinforced on these or on their descendants, and it is conceivable, and -even probable, that in perfectly adapted species parthenogenesis may -last for a very long time without doing any injury to the constitution -of the species. - -The same is true of purely asexual reproduction, to the investigation -of which we shall now turn. - -Let us leave out of account the simplest animals (Monera) without -amphimixis, which we have already discussed. In simple animals -reproduction by budding or by fission is frequent, or it occurs in -alternation with sexual reproduction; in higher animals, Arthropods, -Mollusca, and Vertebrates, asexual reproduction is wholly absent. -In plants it plays an enormously greater part, and what is called -'vegetative reproduction,' which is purely asexual without any -amphimixis, is to be found in all groups of plants, especially in -the form of budding and spore-formation, besides which there is -multiplication by runners, rhizomes, tubers, bulbs, and bulbils. In -most cases there is, in addition to the purely asexual reproduction, -so-called sexual reproduction associated with amphimixis, and often -the sexual and asexual generations alternate with each other, so that -'alternation of generations' occurs, as is common in lower animals, -especially polyps, medusæ, and worms. - -But it sometimes happens among plants that the sexual reproduction is -absent, and that a species reproduces by the asexual mode only, and -this is the case which we must now consider more closely. - -Let us first of all seek to gain clearness as to the composition of -the germ-plasm in the case of purely asexual multiplication, and what -conclusions may be drawn from this, and then let us compare these -with the known observational data, and it will be apparent that in -individuals which have arisen by budding the complete germ-plasm -of the species must be contained; the number of ids will not only -remain the same in the bud as it was in the mother plant, but the -number of _different ids_ will not be diminished. The case is -analogous to that of pure parthenogenesis, in which the absence of -the second maturation-division of the ovum allows the germ-plasm to -retain the full complement of ids. Charles Darwin held that purely -asexual multiplication was 'closely analogous to long-continued -self-fertilization,' yet, as we have seen, according to our theory -there must be a not inconsiderable difference between the two -processes, depending on the fact that in exclusive self-fertilization -the number of different ids is continually decreasing, while in purely -asexual reproduction the germ-plasm loses nothing of the diversity of -its ids. If, therefore, the germ-plasm in purely asexual reproduction -no longer receives fresh ids through amphimixis, it at least loses none -of those it formerly possessed. Although we cannot consider it adapted -for entering upon new adaptations in many directions, yet we may expect -that the species will continue to reproduce unchanged for longer -than in the case of exclusive self-fertilization, the more so since -all unfavourable variational tendencies which crop up are eliminated -as soon as they attain to selection-value, and, as in the case of -parthenogenesis, they are eliminated without being mingled with other -lines of descent. - -Let us take, for instance, the purely asexual reproduction which -obtains in Algæ of the genus _Laminaria_, in regard to which it is -stated that it multiplies only through asexual swarm-spores. There -are quite a number of species of this large tangle, and if it should -be established that in all these the spore-cells really do not -conjugate, then the case would prove that the species of a genus can -maintain a well-defined existence for a long time after amphimixis has -been given up. But this would not be a proof of the possibility of -_species-formation_, for that the ancestral forms of the Laminarians -must have possessed amphigony may be assumed, since their nearest -relatives exhibit it still. It cannot be proved, but there seems -nothing against the assumption that these tangles have existed for a -long time under uniform conditions, and have become adapted to these -with a high degree of constancy. - -The conditions are similar in the marine Algæ of the genus _Caulerpa_, -the nearest relatives of which reproduce sexually, though they -themselves, as far as is known, reproduce only by spores. - -In the Lichens, which represent, as we have already seen, a -life-partnership between Fungi and Algæ, amphimixis appears not to -occur at all; the unicellular Alga reproduces by cell-division, the -Fungus by producing a great number of swarm-spores, which do not -conjugate with one another. As far as the Alga is concerned we might -perhaps suppose that the simplicity of its structure makes it possible -for it to dispense with a constant recombination of its few characters -to bring about the most favourable composition in its idioplasm; in -support of this we may note that even the life-long combination with -the Fungus has caused no visible variation in the Alga, as we must -conclude from the fact that these Algæ can also live independently, and -that the same species of Alga may combine with several different Fungi -to form different species of lichen, just as the same Fungus may also -form part of several species of lichen. We might also imagine that we -have here no more than a direct influence of the Alga and Fungus upon -one another, and that there is no adaptation to the new conditions of -life at all, yet that can hardly be seriously maintained in regard to -species which live under such definite and diverse conditions. It now -seems to be established--contrary to the older statements--that the -lichen-fungus only reproduces asexually, and in face of this it seems -to me that nothing remains except to make the assumption that lichens -formerly possessed sexual reproduction, but that they have lost it, -though whether all have done so is, perhaps, not yet quite certain. - -The same assumption must be made in regard to the Basidiomycetes among -the Fungi, and for most of the Ascomycetes, for in these groups of -Fungi sexual reproduction has only been demonstrated 'with certainty in -a few genera.' That in these cases also there has been a degeneration -of amphigony, until it has completely disappeared, seems probable from -the two other groups of Fungi, the Zygomycetes and Oomycetes, since in -these 'a reduction of sexuality amounting in some cases to complete -disappearance' can be demonstrated even in existing forms. But whether -it may be assumed that the Fungi which are now asexual are no longer -capable of new adaptations, and whether their parasitic habit may be -regarded as making up in some way for the lack of the remingling of -the germ-plasm, as the botanist Möbius supposes, I am not able to -decide. It is obvious that data in regard to amphimixis among the Fungi -are still incomplete, and recent investigations lead us to suspect -that sexual mingling may not be absent, but only disguised. Dangeard, -Harold Wager, and others have observed that a fusion of nuclei precedes -the formation of spores, and this may be regarded as amphimixis, -although the conjugating nuclei belong to cells of the same plant and -sometimes even to the same cell. But although we are here dealing -with a set of facts which cannot yet be satisfactorily formulated in -terms of our theory, it is nevertheless not contradictory to it that -amphimixis should be wholly absent in the higher Fungi. But the fact -would be contradictory to the unadulterated rejuvenescence-theory, for -if amphimixis were really a condition of the continuance of life, no -species--as we have already said--could continue to exist without it -for countless generations. - -[Illustration: FIG. 38 (repeated). A fragment of a Lichen (_Ephebe -kerneri_), magnified 450 times. _a_, the green alga-cells. _P_, the -fungoid filaments. After Kerner.] - -The same argument holds true for the higher plants, which have become -purely asexual under the influence of cultivation. I refer to many -of the well-marked varieties of our cultivated plants which multiply -exclusively, or almost exclusively, by means of tubers and slips, -as is the case with the potato, the manioc, the sugar-cane, the -arrowroot-plant (_Maranta arundinacea_), and others. All these facts -can easily be reconciled with our interpretation of the meaning of -amphimixis, although the attempt to range them as evidence against -our theory has more than once been made. We have thus arrived at the -conclusion that while many-sided adaptations, that is, variations -which transform the plant in accordance with the indirect influences -of new conditions of life, cannot be brought about without a -persistent mingling of germ-plasms, simple modifications may readily -appear although amphimixis is altogether absent. If a wild plant be -permanently transferred to a well-manured culture-bed, it is probable -that certain changes will occur in it, either gradually or at once. -But these are not adaptations; they are, so to speak, direct reactions -of the organism which do not even require selection to make them -increase, but depend upon the influencing of certain determinants of -the germ-plasm, and which, like all germinal variations, will follow -their course steadily until a halt is called either by germinal or by -personal selection. When a given plant is exposed to these new and -artificial conditions, the changes in question make their appearance -sooner or later, and follow their course, and go on increasing as long -as that is compatible with the harmony of the structure and functioning -of the plant, this depending, as in all individual development, on the -struggle between the parts, that is to say, on histonal selection. -Only in this respect is the utility or injuriousness of the change of -importance, for personal selection, the struggle between individuals, -does not affect plants which are under cultivation. - -That such modifications may increase and may persist through many -generations, even with asexual multiplication, depends upon the fact -that the budding cells contain germ-plasm, as well as the germ-cells, -and if particular determinants of the germ-plasm in general are caused -to vary by these new influences, the variation may be transmitted -from bud to bud, from shoot to shoot, and so go on increasing as long -as the new conditions persist, as well as in amphigonic (bisexual) -reproduction, where they are transmitted from germ-cell to germ-cell. -It is not inconceivable that an individual adaptation, that is to -say a useful adjustment, might be effected in the course of asexual -reproduction, although it is improbable that direct influences -would give rise to just those changes which would be useful under -the new conditions. But there are a number of cases which have been -interpreted in this way. In several of the cultivated plants named, -the reproductive organs have themselves degenerated, either only the -male, or only the female, or both at the same time; and some observers, -accepting the hypothesis of an inheritance of functional modifications, -have regarded this as the direct result of disuse during the long -period of asexual reproduction. - -Leaving out of account this erroneous presupposition, we may ask how -asexual reproduction, such as that of the potato by tubers instead of -by seed, which has gone on exclusively for several centuries, could -exercise any influence upon the flowers and seed-forming of this -species? In point of fact it has exercised none in most potatoes, for -the flowers and seeds are just as fertile now as they were when the -potato was first discovered. - -Whether the pollen of a flower is utilized in one or other of its -thousands of pollen-grains by reaching the stigma of another plant -of the same species, or whether all the pollen-grains are uselessly -scattered abroad, cannot possibly affect the flower so as to cause -degeneration; the theory of disuse cannot be applied in this case. -What is true of the potato holds good also of the manioc (_Manihot -utilissima_), but, on the other hand, many of the best varieties of -common fruits--pears, figs, grapes, pine-apples, and bananas--are -seedless. In _Maranta arundinacea_ 'the whole wonderful structure -of the flower has persisted, but the pollen-grains, that is the -germ-cells, are wanting.' Whether this implies a permanent degeneration -of the sexual organs, that is to say, one that is embodied in the -primary constituents of the species, or whether it is only the result -of over-abundant nourishment, or of other causes in the circumstances -affecting the particular plant, can only be decided by experiment. -Probably both occur. The common ivy, for instance, does not now blossom -in the northern parts of Sweden and Russia, but it does so still in -the southern provinces. If plants were brought to us from the most -northerly zone of distribution, they would in all probability flower -and bear fruit with us, and in that case the absence of bloom in these -plants must have been a direct effect of the cold climate. But it is -quite conceivable that cultivated plants have in many cases become -hereditarily infertile, when they are constantly propagated only by -means of buds, layering, and so on, not however because of any direct -effect of this mode of propagation, but through chance germinal -variations. For in regard to many of them man has lost all interest in -the flowers and fruit, as, for instance, in the case of the potato; in -other cases he is even interested in procuring seedless fruits. - -In the first case he will quite readily make use of plants with -imperfect flowers for propagating, if they are otherwise fit and -exhibit what he wants in other respects; in the second case, he will -give a preference to individuals with seedless fruits, and thus -increase and strengthen the tendency to degeneration of the seeds in -the race concerned. - -All these cases are quite in harmony with our conception of amphimixis, -which, now that we have investigated the facts throughout the animate -kingdom, we may sum up in the following propositions. In the whole -organic world, from unicellular organisms up to the highest plants -and animals, amphimixis now means an augmentation of the organism's -power of adaptation to the conditions of its life, since it is only -through amphimixis that simultaneous harmonious adaptation of many -parts becomes possible. It effects this by the mingling and constant -recombination of the germ-plasm ids of different individuals, and thus -gives the selection-processes the chance of favouring advantageous -variational tendencies and eliminating those which are unfavourable, -as well as of collecting and combining all the variations which are -necessary for the further evolution of the species. This indirect -influence of amphimixis on the capacity of organisms for surviving -and being transformed is the fundamental reason for its general -introduction and for its persistence through the whole known realm of -organisms from unicellulars upwards. - -The reason for its _first_ introduction among the lower forms of life -must have been a direct effect which had a favourable influence on the -metabolism, and this is so far coincident with the subsequent import of -amphimixis, inasmuch as it may be regarded not only as a heightening of -the power of adaptation, but as an immediate and direct increase and -extension of the power of assimilation. In any case, amphimixis is not -necessary to the actual preservation of life itself, but it does bring -about a wealth and diversity of organic architecture which without it -would have been unattainable. - -If amphimixis has been abandoned in the course of phylogeny by isolated -groups of organisms, this has happened because other advantages accrued -to them in consequence, which gave them greater security in the -struggle for existence; but it must be admitted that they thereby lost -their perfect power of adaptation, and that they have thus bartered -their future for the temporary securing of their existence. - -In addition to this variational influence, amphimixis has also played a -part in the evolution of sharply defined organic types, especially of -specific types; but of this we shall have more to say later on. - - - - -LECTURE XXXI - -THE INFLUENCES OF ENVIRONMENT - - Different modes and grades of selection--Changes due to the influences - of environment--Superfluity and lack of food--The horses and cattle - of the Falkland Islands--Angora animals--Protection against cold - in Arctic and marine mammals--Plant-galls--Nägeli's _Hieracium_ - experiments--Experiments with _Polyommatus phlæas_--Artificially - produced _Vanessa_-aberrations--Vöchting's experiments on the - influence of light in the production of flower-forms--Heliotropism - and other tropisms--Primary and secondary reactions of - organisms--Herbst's 'lithium larvæ'--Schmankewitsch's experiments - with _Artemia_--Poulton's caterpillars with facultative colour - adaptation--Colour-change in fishes, chamæleon, &c.--Actual scope of - those influences which directly produce organic changes. - - -Through a long series of lectures we have devoted our attention to -those phenomena which bear some relation to the processes of selection; -we have attempted to gain clearness in regard to the modes and stages -of these, and we reached the result that all variations which have -taken place in organisms since the first appearance of living matter -are directed by processes of selection, that is, their direction and -duration are determined by these processes, although they may have -their roots in external influences. But it is not to be supposed that -this guidance is due solely to that one kind of selection which, -with Darwin and Wallace, we designate 'natural selection'; on the -contrary, we must regard this as only one of the different modes of -the processes of selection, necessarily occurring between all living -units which are equivalent to one another, and which, therefore, must -maintain a continual struggle with one another for space and food. If -the expression 'natural selection' were not already so firmly fixed -in its meaning, I should propose that it should be employed in the -most general sense for all the processes of selection collectively, -but we must keep to its original meaning and use it only for personal -selection. - -We have seen that processes of selection take place even between the -elements of the germ-plasm in all organisms which possess a germ-plasm -as distinguished from the mass of the body, and that through these -processes there arise those hereditary individual variations which, -under some circumstances, form the basis of transformations in the -species. - -Obviously this may come about in a twofold manner: firstly, a -variation movement originating in the germ-plasm may go on increasing -till it attains to selection-value, and then 'personal selection' steps -in, and seeks to make it the common property of the species. But it -is obviously also conceivable that variational tendencies arising in -the germ-plasm may never attain to selection-value at all, and then -in most cases they will only continue to exist through a longer or -shorter series of generations as individual distinguishing characters, -without being transmitted to a larger number of individuals or becoming -a constant character of the species. Their persistence will depend -essentially on the chance of mingling with other individuals, and on -the halving of the germ-plasm which precedes sexual reproduction. -Sooner or later these individual peculiarities disappear again, as may -often be observed in the case of abnormalities or morbid tendencies in -man, in as far as these do not weaken vitality. In the latter case they -attain selection-value, though only negatively. - -But even quite indifferent germinal variations, which neither raise -nor lower the individual's power of survival, may, under some -circumstances, increase and lead to permanent variations of all the -individuals of a species, and this happens when they are conditioned by -external influences which affect all the individuals of a species, or -of the particular colony concerned, and it is this kind of organismal -change which we shall now study for a little in detail. - -The ordinary never-ceasing, always active germinal selection depends, -we must assume, upon intra-germinal fluctuations of nutrition, or -inequalities in the nutritive stream which circulates within the -germ-plasm. The variations which it produces may, therefore, be -different in each individual, since these fluctuations are a matter -of chance and may affect the determinants _A_ in one individual and -the determinants _B_, _C_, or _X_ in another, or alternating groups -of these. Or it may be that the homologous determinants _A_ may vary -in a plus direction in one individual, and in a minus direction in -another, while in a third they may remain unchanged, and although the -same direction of variation of a determinant _N_ may occur in many -individuals, it will certainly not do so in all, and still less will -it occur in all along with the same combination of fluctuations in the -rest of the determinants. It is only if this occurs that the variation -can become a specific character. - -We might expect on _a priori_ grounds that not only the chance -fluctuations of nutrition within the germ-plasm would cause its -elements to vary in this or that direction, but that there would also -be influences of a more general kind, especially those of nutrition and -climate, which would in the first place affect the body as a whole, but -with it also the germ-plasm, and which would therefore bring about -variations, either in all or only in certain determinants. In this case -all the individuals would vary in the same way, because all would be -similarly affected by the same causes of change. - -This is actually the case; it is indubitable that external influences, -such as those emanating from the environment or media in which species -live, are able to cause direct variation of the germ-plasm, that is, -permanent, because hereditary variations. We have already referred to -this process and called it 'induced germinal selection.' - -That such influences of environment may bring about changes in -_individual_ organisms is obvious enough; that, for instance, good -nutrition makes the body strong and vigorous, that too abundant food -makes it fat and causes degeneration, that insufficient food lessens -its stamina and vigour, are well-known facts. We have to inquire, on -the one hand, to what extent such influences are able to cause changes -in the individual body in the course of a lifetime, and, on the other -hand, more particularly, how far such changes or modifications of the -soma can call forth corresponding variations in the determinant system -of the germ-cells, and whether and under what circumstances they may be -transmitted; for where this is not the case there can be no permanent -hereditary variation of the whole species, and the variation will only -persist as long as the conditions which gave rise to it endure, and -will disappear again with these. - -The influence of nutrition as a cause of variation has often been -over-estimated. The old statement which has gone the round of the -textbooks since the time of John Hunter, that the stomach of carnivores -may be transformed by vegetable diet into a herbivore stomach, is -absolutely unproved. Brandes at least, who not only subjected all the -statements in the literature on this point to a critical investigation, -but also instituted experiments of his own, regards the statement as -altogether unfounded. All the 'cases' cited, in which the stomach of a -gull or of an owl fed on grain became transformed into an organ with -stronger muscles and covered with horny plates, depend, according to -Brandes, upon inexact observation. There can therefore be no question -of any inheritance of this fictitious stomach-transformation, and the -idea that such a fundamental histological adaptation as the alleged -transformation of the stomach of the grain-eating bird should arise as -a direct effect of the food is wholly without foundation. - -But it is quite otherwise with purely quantitative differences in -nutrition. That meagre diet influences individuals unfavourably is -indubitable, and we are certainly justified in considering whether -this may not have an effect on the germ-cells, and one which will -correspond to the changes induced on the body, so that if the -poor nutrition should last through many generations an hereditary -degeneration of the species would occur, which would not at once -disappear though the animals were transferred to more favourable -conditions. - -We certainly know nothing of how far the minuteness of the determinants -of the germ-plasm, the whole quantity of the germ-plasm, or the reduced -size of the germ-cell, may bear an internal relation to the smallness -of the animal which develops therefrom, but it surely cannot be -regarded as absurd to suppose that there is some such relation. There -are no experiments known to me which prove that meagre diet brings -about a progressive decrease in the size of the body. Carl von Voit -has observed that dogs of the same litter grew to very different sizes -of body according as they received abundant or scanty food, but it -would be difficult to make animals small through scantiness of food -and at the same time to keep them capable of reproduction, and thus -proofs of the inheritance of the dwarfing are lacking. Moreover, the -experiments which Nature herself has made are never quite convincing, -because we never can definitely exclude the indirect effect of altered -circumstances. The case of the feral horses of the Falkland Islands, so -often cited since the time of Darwin, which have become small 'through -the damp climate and scanty food,' seems to me, of all known cases of -the kind, the one we should most readily attribute to the direct effect -of continued scanty diet; but even here we cannot altogether exclude -the possibility of the co-operation of adaptations of some kind to the -very peculiar conditions of life in these islands, as far as the feral -horses are concerned. I have not been able to find any record of more -modern exact investigations either regarding these feral horses, or in -regard to the others which are reared in the Falklands under conditions -of domestication. Darwin himself, however, in the Journal of his famous -voyage tells us much that is interesting in regard to the mammals of -the Falkland Islands. Cattle and horses were brought there in 1764 by -the French, and have increased greatly in numbers since that time; they -roam about wild in large herds, and the cattle are strikingly large -and strong, while the horses both wild and tame are rather small, and -have lost so much of their original strength that they cannot be used -for catching wild cattle with the lasso, and horses have to be imported -from La Plata for this purpose. From this contrast between the horses -and the cattle we may at least conclude that it cannot be 'scanty food' -alone which causes the horses to become smaller, but that the climatic -conditions as a whole are concerned in the matter. Whether the total -amount of variation which has taken place in the horses which have -lived wild there for a hundred years would take place in the course of -a single life, or whether it is a cumulative phenomenon, has still to -be decided. - -Similar statements, for the most part still more uncertain, are made -in regard to changes in the hair of goats, sheep, cattle, cats, and -sheep-dogs, which are referred to climatic influence. The raw climate -of many highlands, like Tibet and Angora, is said to have directly -produced the long and fine-haired breeds. But there is a lack of proof -that adaptation or artificial selection did not also play a part, and -the fact that similar long-haired breeds have arisen among rabbits -and guinea-pigs in quite different places and under quite different -climatic conditions, but under the directing care of man, speaks in -favour of our supposition. But, on the other hand, it does not seem -impossible that the climate may have a variational influence upon -certain determinants of the germ-plasm, for we have already seen -that the influence of cultivation may incite plants and animals to -hereditary variations, and that slowly increasing disturbances in -the equilibrium of the determinant system may thereby be produced, -which may suddenly find marked expression as 'mutations.' But there -is little probability that _adaptations_, that is, transformations -corresponding to the altered climate, can arise in this way. The thick -fur of the Arctic mammals is assuredly not a direct effect of the cold, -although it has developed in all Arctic animals, not only in the modern -polar bears, foxes, and hares of the polar regions, but also in the -shaggy-haired mammoth of diluvial Siberia, whose tropical relatives of -to-day, the elephants, have an almost naked skin. Another interesting -case, recently brought to light, shows that a group of animals -which, in correspondence with their otherwise exclusively tropical -distribution, have only a moderately developed coat of hair, may, on -migrating to a cold country, grow as good a fur as the members of other -families. I refer to one of the higher apes, _Rhinopithecus roxellanæ_, -which live in companies in the forest on the high mountains of Tibet, -notwithstanding that the snow lies there for six months[26]. - -[26] See Milne-Edwards, _Recherches pour servir à l'histoire nat. d. -mammifères_, Paris, 1868-74. - -But we should assuredly make a mistake if we were to regard the thick -fur of these apes as a direct reaction of the organism to the cold. We -see at once that this cannot be the case if we compare them with marine -mammals, which differ just as much from one another in this respect -and yet are exposed to the same low temperature. The whale and the -dolphin are quite naked, absolutely hairless, but the seals possess a -thick hairy coat. This striking difference is obviously connected with -the mode of life; the whales remain always in the water, the seals -leave it often and therefore require the hairy coat, especially in -colder climates, since otherwise they would be too rapidly cooled by -the evaporation of the water from their bodies. For the whales, on the -other hand, even a very thick hairy coat would not have sufficed as a -protection against cold, since water is a much better conductor of heat -than air, and so it was necessary for them to become enveloped with -the well-known thick layer of blubber, a deposit of fat lying under -the skin, and this--after it was once developed--made the hairy coat -superfluous, so that it disappeared. The seals certainly also possess -a layer of fat under the skin, but it is only in the largest of them -that it affords sufficient protection against the cooling effect of -evaporation when they go upon land or on the ice, and it is therefore -only in these larger ones that the hairy coat has markedly degenerated, -as, for instance, in the walrus and the sea-lion; in all the smaller -seals, in which the mass of the body is much less, the hairy coat is -necessarily very thick and protected from soaking by being very oily, -because the layer of fat under the skin would not be sufficient to -prevent excessive cooling when on land. But the thick coat of hair is -no more _produced_ by the cold than is the layer of fat. As Kükenthal -has shown, all these characters are adaptations, and may depend here as -elsewhere upon natural selection and upon the 'fluctuating' variations -of the germ-plasm upon which that process is based. They are directed -by personal selection because there is the need for them, and they are -produced and augmented by germinal selection. - -In all these cases the _direct_ effect of external influences has -nothing to do with the matter, but in other cases that alone brings -about the whole change, which is then limited to the individual and -does not affect the species as a whole at all. - -Plant-galls afford striking illustration of the extraordinary changes -that may be brought about in an organism or in its parts by external -influence in the course of the individual life. All possibility of -adaptation on the part of the plant is excluded in this case. The -gall can only depend upon the direct influence of a stimulus, which -is exercised by the young animal, the larva, upon the cells which -surround it; and yet these cells vary to a considerable extent, become -filled with starch or form a woody layer, secrete special substances, -such as tannic acid, in large quantities, or develop hairs, moss-like -growths, pigments, and so on, which do not otherwise occur in that -particular part of the plant. Since Adler and Beyerinck have proved -that it is not a poison conveyed by the mother animal into the leaf or -bud when laying the eggs, which gives rise to the gall-formation, the -matter has become rather clearer. We can now understand that different -stimuli in succession affect the cells which enclose the larva, and -that the ordered succession of these and the exactly graded stimulation -incite the cells to activity in various ways, whether to mere growth -and multiplication in a given direction, or to the secretion of tannic -acid, or to the formation of wood, or to the deposition of reserve -material, and so on. Even the feeble movements of the young larva may -form a stimulus that increases with its growth; then the movements -made by the larva in feeding, and not least the different secretions -emanating from the salivary glands of the animal, which must contain -some substances capable of acting as stimuli and probably changing -in character as time goes on. All these factors must act as specific -stimuli to the plant-cells, influencing and modifying their processes -of growth and metabolism in one direction or another. In principle -at least, if not in detail, we understand the possibility that -through the ordered succession and exact balancing of these different -cell-stimuli the really marvellous structure of the gall may be brought -about as the product of the direct influence, exercised only once, -of the gall-insect upon the plant's parts. But the animal's power of -exercising such a succession of finely graded stimuli upon the plant -must be referred to long-continued processes of selection, and the -structure of the gall, which is adapted to its purpose down to the -minutest details, can thus be understood. The assumption of substances -which can act even in minute quantities as specific cell-stimuli, which -we require to make in this attempt to explain galls, is no longer -without corroboration since we find analogies in the Iodothyrin of -Baumann, the specific secretions of the thymus and the supra-renal -bodies in the higher animals, not to speak of the 'anti-toxins' of the -pathogenic bacteria, which are only known by their effects. - -The case of plant-galls is thus of great theoretical interest because -we can exclude all preparation of the plant-cells for the stimuli -exercised by the animal, since the gall is quite useless for the -plant, though many have endeavoured to discover some utility. We -have therefore here a clear case of modification due to the effect, -exercised once only, of external influences, an adaptation of the -animal to the mode of reaction of particular plant-tissues. - -It might be supposed that if any inheritance of somatogenic -modifications, any transmission of the acquirements of the personal -part to the germinal part, were possible at all, it would occur in this -case, for many species of gall-insects attack plants, particularly -oaks, in great numbers every year. It has actually been maintained -that galls may arise spontaneously, that is without the presence of a -gall-insect. But no proof of this has ever been found, and the fact -that no one has paid any attention to the assertion probably implies an -unconscious condemnation of the hypothesis of the transmissibility of -acquired characters. - -It has been proved by Nägeli's often discussed experiments on hawkweeds -(_Hieracium_) that much less specialized external influences can -give rise to changes which are not hereditary. The Alpine species of -hawkweed varied considerably in their whole habit in the rich soil of -the Botanic Gardens at Munich, but their descendants, when transferred -to a poor flinty soil, returned to the habit of the Alpine species. -The changes which occurred in garden soil were therefore somatic and, -as I have called them, 'transient,' and they did not depend upon -variations of the germ-plasm. It may be objected in regard to these -experiments that they were not continued long enough to prove that -hereditary variations would not also have cropped up in consequence of -the altered conditions. But in any case they prove that marked changes -in the whole body of the plant may occur without any obvious variation -of the germ-plasm. This does not mean, however, that the possibility of -variations of the germ-plasm through such direct external influences -is disputed. We must assume the occurrence of these on _a priori_ -grounds, if we refer--as we have done--individual hereditary variation -to fluctuations in the nutrition of the individual determinants of the -germ-plasm. It is probable that many general nutritive variations or -climatic factors affect the germ-plasm as well as the soma, and it is -by no means inconceivable that it is not all, but only certain definite -determinants that are caused to vary. - -A proof of this may be found in the results of experiments made upon -the little red-gold fire-butterfly (_Polyommatus phlæas_), to which -I have briefly referred in a former lecture. This little diurnal -butterfly of the family Lycænidæ has a wide distribution and occurs -in two climatic varieties. In the far north and also in the whole of -Germany the upper surface is red-gold with a narrow black outer margin, -but in the south of Europe the red-gold has been almost crowded out by -the black. I reared caterpillars in Germany from eggs of _P. phlæas_ -found at Naples and exposed them directly after they had entered on -the pupa-stage to a relatively low temperature (10° C.). Butterflies -emerged which were not quite so black as those of Naples, but -considerably darker than the German form. Conversely, German pupæ were -exposed to greater warmth (38° C.), and these gave rise to butterflies -which were rather less fiery gold and considerably blacker than the -ordinary German form. If I had to repeat these experiments I should -use a much lower temperature in the case of the cold experiments, -because we now know from the experiments of Standfuss, E. Fischer, and -Bachmetjeff, that most of the pupæ of diurnal butterflies can stand a -temperature below zero for a considerable time; probably the results -would be even more marked then. - -But even from the results of my former experiments we are justified -in concluding that the blackening of the upper surface of the wing is -really the direct result of the increased temperature during pupahood, -and that the pure red-gold results from the lowered temperature. -Similar experiments made by Merrifield with English _Phlæas_ pupæ agree -exactly with mine. But we may conclude further from these experiments -that both warmth and cold only give rise to slight variations in the -individual pupæ, and that the pure red-gold of the northern form -and the black of the southern are the result of a long process of -inheritance and accumulation, in which the germ-plasm has been caused -to vary in as far as the relevant determinants are concerned, so that -these yield the respective northern and southern forms even in less -extreme temperatures. - -As it is to be assumed that these determinants are present not -only in the primordium of the wing in the pupa, but also in the -germ-cells, both must be affected by the varying temperature, and, in -accordance with the continuity of the germ-plasm, each variation of -these determinants, however slight, would be continued in the next -generation. It is thus intelligible that somatic variations like the -blackening of the wings through warmth appear to be directly inherited -and accumulate in the course of generations; in reality, however, -it is not the somatic change itself which is transmitted, but the -corresponding variation evoked by the same external influence in the -relevant determinants of the germ-plasm within the germ-cells, in other -words, in the determinants of the following generation. - -This interpretation of these experiments, which I offered some years -ago, has been confirmed in several ways in regard to various other -diurnal Lepidoptera. By employing a temperature as low as 8° C. in -the case of fresh pupæ of various species of _Vanessa_ Standfuss -and Merrifield, and especially E. Fischer, succeeded in getting -great deviations in the marking and colour of the full-grown -insects,--so-called aberrations, such as had previously been found -only very rarely and singly under natural conditions. The deviations -from the normal must undoubtedly be ascribed to the effect of cold, -but it does not follow that they are new forms which have suddenly -sprung into existence, as many have assumed without further experiment. -Dixey, on the other hand, has attempted to establish, by a comparison -of the different species of _Vanessa_, the phyletic development of -their markings, and has found that these aberrations due to cold -are more or less complete reversions to earlier phyletic stages. As -regards the common small painted lady (_Vanessa cardui_), the small -tortoise-shell butterfly (_Vanessa urticæ_), the 'Admiral' (_Vanessa -atalanta_), the peacock (_Vanessa io_), and the large tortoise-shell -(_Vanessa polychioros_), I can agree with this interpretation, and I -do so the more readily because some years ago I suggested that the -alternation of differently coloured generations of seasonally dimorphic -Lepidoptera might be considered as a reversion. But this by no means -excludes the possibility that other than atavistic aberrations may be -produced by cold or heat. There is nothing against this theoretically. -Yet we must not, without due consideration, compare these abruptly -occurring variations to the sport-varieties of plants which we have -already discussed; there is an important difference between the two -sets of cases. In the Lepidoptera a single interference, lasting only -for a short time, modifies the wing-marking, but in the plant varieties -the visible appearance of the variation is preceded by a long period of -preparatory change within the germ-plasm. This period required for the -external influences to take effect was already recognized by Darwin, -and it has recently been named by De Vries the 'premutation period.' - -We may explain these remarkable aberrations theoretically in -the following way: The determinants of the wing-scales in the -wing-primordium of the young pupa are influenced by the cold in -different ways, some kinds of determinants being strengthened by it, -others markedly weakened, even crippled so to speak, and in this way -one colour-area spreads itself out more than is normal on the surface -of the wing, and another less, while a third is suppressed altogether. -That this disturbance of the equilibrium between the determinants -leads usually to the development of a phyletically older marking -pattern leads us to the conclusion that in the germ-plasm of the -modern species of _Vanessa_ a certain number of determinants of the -ancestors must be contained in addition to the modern ones. We might -even inquire whether these were not better able to endure cold than -their modern descendants, since their original possessors, the old -species of the Ice age, were accustomed to greater cold, but this idea -is contradicted by the experiments of E. Fischer, which go to show that -the same aberrations are evoked by abnormally high temperature. That -the old ancestral determinants are present in _different_ numbers in -the germ-plasm of the modern species, I am inclined to infer from the -fact that among a large number of experiments made by me in the course -of several years the aberrations have always occurred in very different -numbers in the different broods, although the greatest care was taken -to have the conditions as nearly alike as possible; absolutely alike, -of course, they never can be. - -But it would lead me too far if I were to enter on a detailed -discussion of these cases, which have not yet been fully worked up; -only one thing more need be mentioned, that is, that the aberrations -induced by cold are to a certain extent transmissible. Standfuss -first succeeded in making some aberrant specimens of _Vanessa urticæ_ -reproduce, and from their eggs he procured butterflies which showed -a much slighter deviation from the normal, which however was still -so decided that it could not be regarded as due to chance. I myself -succeeded in doing the same, but the deviation in this case was much -slighter. But that these observed cases are rightly referred to the -cold to which their parents had been subjected is proved by other -observations recently published by E. Fischer. These refer to one of -the Bombycidæ (_Arctia caja_), which flies by day, and accordingly -has a gay and very definite marking and coloration. A large number -of pupæ were exposed to cold at 8° C., and some of these resulted in -striking and very dark aberrant forms (Fig. 129, _A_). A pair of these -yielded fertilized eggs; in the progeny, which were reared at a normal -temperature, there were among the much more numerous normal forms a -few (17) which exhibited the aberration of the parents, though to a -considerably less degree (Fig. 129, _B_). - -This shows that the cold had affected not only the wing-primordia -of the parental pupæ, but the germ-plasm as well, and at the same -time that this latter variation was less marked than that of the -determinants of the wing-rudiments. This gives rise _to an appearance_ -of the transmission of acquired characters. - -In the case of many of these cold-aberrations in Lepidoptera the cold -gives rise to variations, but does so not by creating anything new, -but by giving the predominance to primary constituents which have long -been present, but are usually suppressed, and so it is also among the -plants. I have in mind, for instance, the interesting experiments of -Vöchting on the influence of light in the production of flowers in -phanerogams. These showed that the common balsam (_Impatiens noli me -tangere_) produces its familiar open flowers in a strong light, but -in weak light only bears small, closed, so-called 'cleistogamous' -flowers. But it would be utterly erroneous to suppose that the strong -or weak light is the real cause, the _causa materialis_, of these two -forms of flowers: the degree of illumination is merely the stimulus -which provokes one or other of the primary constituents to development, -both kinds being present in the constitution of the plant. As has long -been known, the balsam normally possesses two kinds of flowers, and -the slumbering primary constituents of these are so arranged that the -open flowers develop where there is a prospect of insect visits and -cross-fertilization, that is, in sunny weather or in a strong light, -while closed and inconspicuous flowers adapted for self-fertilization -develop in weak light, that is, in shady places and in concealed parts -of the plant, where insect visits are not to be expected. - -[Illustration: FIG. 129. _A_, an aberration of _Arctia caja_, produced -by low temperature. _B_, the most divergent member of its progeny. -After E. Fischer.] - -Among plants we find thousands of instances of such reactions of -the organism to external stimulus--reactions which are not of a -primary nature, that is, are not the inevitable consequences of -the plant's constitution, but which depend upon adaptations of the -special constitution of a species or group of species to the specific -conditions of its life. To this category belong all the phenomena of -heliotropism, geotropism, and chemotropism, which have been discovered -by the numerous and excellent observations of the plant physiologists. -That all these are adaptations and secondary reactions to stimuli is -proved by the fact that the same stimuli affect the homologous parts of -different species in very different, and often in opposite ways. For -instance, while the green shoots of most plants turn towards the light, -being positively heliotropic, the climbing shoots of the ivy and the -gourd are negatively heliotropic, which is an adaptation to climbing. -In this case the reason of the difference in the mode of reaction -must lie in the difference of constitution of the cellular substance -of the shoot, and since this may differentiate so very diversely in -its relation to light, the power of reaction which plant substance in -general has to light must not be regarded as a primary character, like -the specific gravity of a metal or the chemical affinities of oxygen -and hydrogen, but as adaptations of the living and varying substance -to the special conditions of life. And the origin of these adaptations -must depend upon processes of selection, and on these alone. This is -just the difference between living and non-living matter,--that the -former is variable to a high degree, the latter is not; it is the -fundamental difference upon which the whole possibility of the origin -of an animate world depends. - -Among animals also we must distinguish between the direct effects of -external influence to which the organism is not already adapted, and -those reactions which imply a previously established adjustment to the -stimulus. That is, we must distinguish between primary and secondary -reactions. - -For instance, Herbst made artificial sea-water in which the sodium was -partially replaced by lithium, and the eggs of sea-urchins developed -in this artificial sea-water into very divergent larvæ of peculiar -structure. We have here a primary reaction of the organism to changed -conditions of life--not an adaptation, not a prepared reaction. -Accordingly these 'lithium larvæ' eventually perished. - -The increasing blackness of _Polyommatus phlæas_, which we have already -discussed, must also be regarded as a primary reaction, but not so -the variations--often misinterpreted--of those species of _Artemia_ -which live in the brine-pools of the Crimea, in regard to which -Schmankewitsch showed that, when the amount of salt in the water is -diminished, they undergo certain changes which bring them nearer to -the fresh-water form _Branchipus_, while when the salt is increased -in amount they vary in the contrary direction. Probably these are -adaptations to the periodically changing salinity of their habitat. - -There can be no doubt of this in the case of the caterpillars of -different families, in regard to which Poulton showed that in their -early youth they possess the power of adapting themselves exactly -to the colour of their chance surroundings. It is obvious that the -protection which the caterpillar would gain from being coloured -_approximately_ like its surroundings would be insufficient, for -instance because the surroundings may be very diverse, since -the species lives upon different, variously coloured plants and -plant-parts. Thus a facultative adaptation arose. Selection gave rise -to an extraordinarily specialized susceptibility on the part of the -different cell elements of the skin to differences of light, and the -result of this is that the skin of the caterpillar invariably takes on -the colouring which is reflected upon it in the first few days of its -life from the plants and plant-parts by which it is surrounded. Thus -the caterpillars of one of the Geometridæ, _Amphidasis betularia_, take -on the colours of the twig between and upon which they sit, and they -can be made black, brown, white, or light green quite independently of -their food, according to the colour of the twigs (or paper) among which -they are reared. - -Colour-change in fishes, Amphibians, Reptiles, and Cephalopods, depends -upon much more complex adaptations. In their case a reflex-mechanism -is present which conducts the light-stimulus affecting the eye to the -brain, and there excites certain nerves of the skin; these in their -turn cause the movable cells of the skin which condition the colouring -to change and rearrange themselves in the manner necessary to bring -about the harmonization of colour. On this depends the colour-change of -the famous chamæleon, and also the scarcely less striking case of the -tree-frog, which is light green when it sits on trees, but dark brown -when it is kept in the dark. All these are secondary reactions of the -organism in which the external stimulus is, so to speak, made use of -to liberate adaptive variations, either permanently or transitorily. -In the caterpillars colour-changes are permanent, that is, it is only -the young caterpillar which takes on the colour of its surroundings; -later it does not change, even when it is exposed to different light, -or intentionally placed upon a food-plant of a different colour. -In fishes, frogs, and cuttlefishes, on the contrary, the reaction -of the colour-cells to light only lasts a little longer than the -light-stimulus, and it changes with it. The purposiveness of this -difference of reaction is obvious. - -We cannot say to what degree the direct influence of external -conditions is effectively operative on the germ-plasm, or how far, by -persistently repeated slight changes, the determinants and the parts -of the body determined by them may be made to vary in the course of -generations; that is to say, how large a part this direct influence -of climate and food may play in the transmutation of species. We can -give no answer from experience, because there is an entire lack of -perfectly satisfactory and clear experiments; we only know in a few -cases how great the variations are which can be brought about in the -body during the individual life by means of any of these factors. In -most cases it is uncertain whether actually hereditary effects play any -part, that is, whether the germ-plasm itself is affected. But if we -wish to be theoretically clear as to how far direct climatic effects -may go, we may say this, that they may operate as long as they cause -no disturbance in the life of the species concerned, for at the moment -that such a direct effect begins to be prejudicial to the species -personal selection will step in, and, by preferring the individuals -which react least strongly to the climatic stimulus, will inhibit the -variation. If in any case this should be physically impossible, the -species would die out in the climate in question. That a species of -plant or animal has climatic limits indicates that individuals which go -beyond these are exposed to influences which make life impossible and -which natural selection is unable to neutralize. We are here brought -face to face with one of the limits to the scope of natural selection. -There is no doubt that the influences of the environment must always -have a powerful effect upon the soma of the individual, but we have -seen, in the case of Alpine plants and of galls, how very far this -effect may go without leaving any trace in the germ-plasm. - - - - -LECTURE XXXII - -INFLUENCE OF ISOLATION ON THE FORMATION OF SPECIES - - Introduction--Isolated regions are rich in endemic species--Is - isolation a condition in the origin of species?--Moriz Wagner, - Romanes--'Amiktic' local forms, the butterflies of Sardinia, of the - Alps, and of the Arctic zone--Periods of constancy and periods of - variation in species--Amixia furthered by germinal selection--The - thrushes of the Galapagos Islands--The intervention of sexual - selection--Humming-birds--Central American thrushes--Weaver-birds - of South Africa--Papilionidæ of the Malay Archipelago--Natural - selection and isolation--Snails of the Sandwich Islands--Influences - of variational periods--Comparison with the edible snail and with the - snail fauna of Ireland and England--Changed conditions do not always - give rise to variation--Summary. - - -In an earlier lecture I endeavoured to show, by means of Darwinian -arguments and examples, how important for every species, in relation to -its transmutation, is the companionship of the other species which live -with it in the same area. We saw that the 'conditions of life' operated -as a determining factor in the composition of an animal and plant -association quite as momentously as any climatic conditions whatsoever, -and, indeed, Darwin rated the influences of vital association even -more highly, and attributed to them an even greater power of evoking -adaptation than he granted to the physical conditions of life. - -We are, therefore, prepared to recognize that even the transference of -a species to a different fauna or flora may cause it to vary, and this -occurs when the species gradually extends the area of its distribution, -so that it penetrates into regions which contain a materially different -association of forms of life. But these migrations are not necessarily -only gradual, that is, due to the slow extension of the original area -of distribution in the course of generations as the species increases -in numbers; they may also occur suddenly, when isolated individuals -or small companies of a species transcend in some unusual manner the -natural boundaries of the old area, and reach some distant new region -in which they are able to thrive. - -Species-colonies of this kind may be due to the agency of man, who has -spread many of his domesticated animals and plants widely over the -earth, but who has also intentionally or unintentionally forced many -wild animals and plants from their original area to distant parts -of the earth, as, for instance, when the English humble-bees were -imported into New Zealand with a view to securing the fertilization -of the clover; but such colonies also occur in thousands of cases -independently of man's agency, and the means by which they are brought -about are very diverse. Little singing-birds are sometimes driven -astray by storms, and carried far away across the sea, to find, if -fortune favours them, a new home on some remote oceanic island; -fresh-water snails, which have just emerged from the egg, creep on to -the broad, webbed feet or among the plumage of a wild duck or some -other migratory bird, and are carried by it far over land and sea, -and finally deposited in a distant marsh or lake. This must happen -not infrequently, as is evidenced by the wide distribution of our -Central European fresh-water snails towards the north and south. But -terrestrial snails can also, though more rarely, be borne in passive -migration far beyond limits which are apparently impassable, as is -evidenced by the presence of land-snails on remote oceanic islands. - -The Sandwich Islands are more than 4,000 kilometres from the continent -of America; they originated as volcanoes in the midst of the Pacific -Ocean; and yet they possess a rich fauna of terrestrial snails, -the beginnings of which can only have reached them by the chance -importation of individual snails carried by strayed land-birds. -Charles Darwin was the first who attempted to investigate the problem -of the colonization of oceanic islands by animal inhabitants, and the -chapter in _The Origin of Species_ which deals with the geographical -distribution of animals and plants still forms the basis of all the -investigations directed towards this point. We learn from these that -many land-animals, of which one would not expect it on _a priori_ -grounds, may be carried away by chance over the ocean, either, as in -the case of butterflies and other flying insects, and of birds and -bats, by being driven out of their course by the wind, or by being -concealed--either as eggs or as fully-formed animals--in the clefts of -driftwood, where they can resist for a considerable time the usually -destructive influence of salt water. Thus eggs of some of the lowest -Crustacea (Daphnidæ), which are contained in large numbers in the mud -of fresh water, may be transported with some of the mud on the feet -of birds, and this may happen also to encysted infusorians and other -unicellulars, and to the much more highly organized Rotifers as well. -In all these cases, and in many others, it may happen occasionally that -single individuals, or a few at a time, may be carried far afield, and -may reach regions from which their fellows of the same species are -entirely excluded. If they thrive there they may establish colonies -which will gradually spread all over the isolated area as far as it -affords favourable conditions of life. - -But oceanic islands are not the only cases of isolated regions; -mountains and mountain-ranges which rise in the midst of a plain also -form isolation-areas for mountain-dwelling plants or animals which -have not much power of migrating. In the same way marine animals -may be completely isolated from each other by land-barriers, as the -inhabitants of the Red Sea, for instance, are from those of the -Mediterranean, as has been clearly expounded by Darwin. The idea of an -isolated region is always a relative one, and the region which seems -absolutely insular for a terrestrial snail is not so at all for a -strong-flying sea-bird. There is no such thing as _absolute_ isolation -of any existing colony, for otherwise the colony could never have -reached the region; but the degree of isolation may be _absolute_ as -far as the time of our observation is concerned, if the transportation -of the species concerned occurs so rarely that we cannot observe it in -centuries, or perhaps in thousands of years, or if the extension of its -range could only take place through climatic or geological changes, -such as a subsidence of land-barriers between previously separated -portions of the sea, or, in the case of land animals such as snails, -the elevation of the sea-floor and the filling up of arms of the sea -which had separated two land-areas. But even the transportation of a -species by the accidental means already indicated will occur so rarely, -if the isolated insular region is very distant, that the isolation of -a colony by such a chance may be regarded as almost absolute as far as -the members of the same species in the original habitat are concerned. - -If we examine one of these insular regions with reference to the -animal inhabitants which live upon it in isolation, we are confronted -with the surprising fact that it harbours numerous so-called endemic -species, that is to say, species which occur nowhere else upon the -earth, and that these species are the more numerous the further the -island is removed from the nearest area of related species. It looks -at first sight quite as if isolation alone were a direct cause of the -transformation of species. - -The facts which seem to point in this direction are so numerous that I -can only select a few of them. The Sandwich Islands, to which we have -already referred, possess eighteen endemic land-birds, and no fewer -than 400 endemic terrestrial snails, all belonging to the family group -of Achatinellinæ, which occurs there alone. - -The Galapagos Islands lie 1,000 kilometres distant from the coast -of South America, and they too harbour twenty-one endemic species -of land-birds, among them a duck, a buzzard, and about a dozen -different but nearly related mocking-birds, each of which is found -in only one or two of the fifteen islands. The group of islands also -possesses peculiar reptiles, and they take their name from the gigantic -land-tortoises, sometimes 400 kilogrammes in weight, which in Tertiary -times inhabited also the continent of South America, but are now found -in the Galapagos Islands only. The islands also possess endemic lizards -of the genus _Tropidurus_, and although the lizards can no more have -been transported across the ocean than the tortoises, but corroborate -the conclusion drawn from geological data, that the islands were still -connected with the mainland in Tertiary times, the occurrence of a -particular species of _Tropidurus_ upon almost every one of the fifteen -islands testifies anew to the mysterious influence of isolation, for -most of these islands are quite isolated regions for the different -species of lizard, even more than for the mocking-birds, which have -also split up into a series of species. - -We are thus led to the hypothesis, which was first introduced into the -Evolution Theory by Darwin, that the prevention of constant crossing -of an isolated colony with the others of the same species from the -original habitat favours the origin of new endemic species, and his -conclusion is confirmed when we learn that islands like the Galapagos -group possess twenty-one endemic land-birds, but only two endemic -sea-birds out of eleven, for the latter traverse great stretches of -sea, and crossings with others of the same species on the neighbouring -continental coasts will often take place. The Bermuda Islands also -afford a proof that the development of endemic species is prevented by -regular crossing with other members of the species from the original -habitat, for although they are 1,200 kilometres distant from the -continent of North America--that is, further than the Galapagos Islands -from South America--they possess no endemic species of bird, and we may -undoubtedly associate this with the fact that the migratory birds from -the continent visit the Bermudas every year. - -Madeira also confirms our conclusion, for only one of the ninety-nine -species of bird occurring there can be regarded as endemic, and it has -often been observed that birds from the neighbouring African mainland -(only 240 kilometres distant) are driven across to Madeira. Terrestrial -snails, on the other hand, will seldom be carried to Madeira by birds, -and accordingly we find there an extraordinary number of endemic -terrestrial snails, namely, 109 species. - -Although these and similar facts indicate strongly that isolation -favours the evolution of new species, it would be erroneous to imagine -that every isolation of a species-colony conditions its transmutation -to a new species, or, as has been maintained, first by Moritz Wagner, -and later by Gulick and by Dixon, that isolation is a necessary -preliminary to the variation of species--that not selection but -isolation alone renders the transmutation of a species possible, and -thus admits of its segregation into several different groups of forms. -Romanes went so far as to regard the natural selection of Darwin and -Wallace as a sub-species of isolation, and isolation in its diverse -forms he regarded as the sole factor in the formation of species. He -assumed that it was only by the segregation of individuals which did -not vary that the constant reversion to the ancestral species could -be prevented, and he regarded the process of selection as essentially -resulting in the 'isolation' of the fittest through the elimination of -the less-fit. The idea is correct in so far, that selection undoubtedly -aids the favourable variation to conquest over the old forms, precisely -because the latter, being less favourably placed in the struggle for -existence, are gradually more completely overcome and weeded out, so -that a constant mingling of the new forms with the old is prevented, -just as it is by isolation of locality. Obviously the new and fitter -forms could not become dominant, could not even become permanent, -if they were always being mingled again with the old. But whether -it serves any useful purpose to bring this under the category of -'isolation,' and to say that mingling with the ancestral form during -transmutation is prevented by natural selection, in that favourably -varying individuals _are isolated_ by their superiority from the -inferior ones, that is, the non-varying individuals which are doomed -to elimination, is somewhat doubtful. For my part, I should prefer to -retain the original meaning of the word, and to call 'isolation' the -separation of a species-colony by spatial barriers. - -Whether this factor by itself prevents the mingling with the ancestral -form as effectually as selection does, and whether isolation alone and -by itself can lead to the evolution of new forms, or perhaps must lead -to them, must now be investigated. - -I look at this question from exactly the same point of view as I did -nearly thirty years ago, when in a short paper[27] I endeavoured to -show that, under favourable circumstances, an individual variation of a -species may become the origin of a local variety if it finds itself in -an isolated region. Suppose an island had no diurnal butterflies, until -one day a fertilized female of a species from the continent was driven -thither, found suitable conditions of life, laid its eggs, and became -the founder of a colony; the prevention of constant crossing between -this colony and the ancestral continental species would not in itself -be any reason why the colony should develop into a variety. But suppose -that the foundress of the colony diverged in some unimportant detail of -colouring, such as may at any time arise through germinal selection, -from the ancestral species; then this variation would be transmitted to -a portion of her progeny, and there would thus be a possibility that a -variety should establish itself upon the island which would be the mean -of the characters of the surviving progeny. The greater the divergence -was in the first progeny of the mother-colonist, and the stronger this -variational tendency was, the greater also would be the chance that it -would be transmitted further and become a characteristic aberration -from the marking of the original species. I then designated this effect -of isolation as due to _amixia_, that is, to the mere prevention of -crossing with the members of the same species in the original habitat. - -[27] _Ueber den Einfluss der Isolirung auf die Artbildung_, Leipzig, -1872. - -We have examples of this from the Mediterranean islands, Sardinia and -Corsica, which possess in common nine endemic varieties of butterflies, -most of which diverge from the species of the continent in a quite -inconsiderable degree, though quite definitely and constantly. Thus -there flies in these islands a variety (_Vanessa ichnusa_) of our -common little _Vanessa urticæ_ in which the two black spots on the -anterior wing exhibited by the original species are wanting. The large -tortoise-shell (_Vanessa polychloros_) also occurs there, but it has -not varied and still exhibits the black spots. Our little indigenous -butterfly (_Pararga megæra_), which is abundant on warm, stony slopes, -quarries, and roads, flies about in Sardinia, but as a variety -(_tigelius_), which is distinguished from the original species by the -absence of a black curved line on the posterior wings. - -That of two nearly related and similarly marked species, like the large -and small tortoise-shell, one should remain unvaried, while the other -has become a variety, shows us that amixia alone does not necessarily -lead to the evolution of varieties in every case. It might of course be -objected that one species may have migrated to the islands at a much -earlier period than the other, and that it might be a direct effect -of the climate which found expression in this way. But we have other -similar cases in which one of two species has varied in an isolated -region, while the other has not, and in regard to which we can prove -definitely that both were isolated at the same time. - -An instance of this kind is to be found in Arctic and Alpine -Lepidoptera, which inhabited the plains of Europe during the Glacial -period, and subsequently, when the climate became milder again, -migrated some to the north into countries within the Arctic zone, and -some to the south to the Alps to escape in their heights from the -increasing warmth. There are many diurnal Lepidoptera which now belong -to both regions, and of these some have remained exactly alike, so that -the Arctic form cannot be distinguished from the Alpine form; others -show slight differences, so that we can distinguish an Arctic and an -Alpine variety. To the former category belong, for instance, _Lycæna -donzelii_ and _Lycæna pheretes_, _Argynnis pales_, _Erebia manto_, -and others; to the second category belong, for instance, _Lycæna -orbitulus_, Prun., _Lycæna optilete_, _Argynnis thore_, and some -species of the genus _Erebia_. - -This cannot be an instance of the direct effect of general climatic -influences, for in that case all the nearly related species of a genus -would have varied or not varied; nor have we to do with adaptations, -for the differences in marking are seen on the upper surfaces of -the wing, which do not exhibit protective colouring, at least in -these Lepidoptera. It can only have been the prevention of crossing -that has fixed the existing variational tendencies in the isolated -colonies--variations which would have been swamped and obliterated if -there had been constant crossing with all the rest of the members of -the species. - -But there is another factor to be considered. Those Alpine Lepidoptera, -for instance, which have not remained exactly the same in the far -north, have formed local varieties in the rest of the area of their -distribution also, while species which have remained quite alike -in isolated regions, such as the Alps and the north, exhibit no -aberrations in other isolated regions, such as the Pyrenees, in -Labrador, or in the Altai. Thus one species must have had a tendency -in the Glacial period to form local varieties, and the other had not; -and I have already attempted to explain this on the hypothesis that the -former at the time of their migration and segregation into different -colonies were at a period of dominant variability, the latter at a -period of relatively great constancy. Leaving aside the question of -the causes of this phenomenon, we may take it as certain that there -are very variable and very constant species, and it is obvious that -colonies which are founded by a very variable species can hardly ever -remain exactly identical with the ancestral species; and that several -of them will turn out differently, even granting that the conditions of -life be exactly the same, for no colony will contain all the variants -of the species in the same proportion, but at most only a few of -them, and the result of mingling these must ultimately result in the -development of a somewhat different constant form in each colonial area. - -If we were to try to imitate this 'amixia' artificially we should -only require to take at random from the streets of a large town a -number of pregnant bitches, and place each of them upon an island -not previously inhabited by dogs, and then a different breed of dog -would arise upon each of these islands, even if the conditions of life -were exactly similar. But if, instead of these variable bitches, the -females of a Russian wolf were placed on the islands, the developing -wolf-colonies would differ as little from the ancestral species as the -various Russian wolves do from one another--similar climate and similar -conditions of life being presupposed. - -There is thus an evolution of varieties due to amixia alone, and we -shall not depreciate the significance of this if we consider that -individual variations are the outcome of the fluctuations in the -equilibrium of the determinant system of the germ-plasm, to which it -is always more or less subject, and that variations of the germ-plasm, -whether towards plus or minus, bear within themselves the tendency -to go on increasing in the direction in which they have begun, and -to become definite variational tendencies. In isolated regions such -variational tendencies must continue undisturbed for a long period, -because they run less risk of being suppressed by mingling with -markedly divergent germ-plasms. - -The probability that variational tendencies set up in some ids of the -germ-plasm by germinal selection will persist and increase is obviously -greater the more the germ-plasms combining in amphimixis resemble each -other. For instance, let us call the varying determinants _Dv_, and -assume as a favourable case that these are represented in three-fourths -of all the ids in the fertilized eggs of a butterfly-female which has -been driven astray on to an island, that is, that they are present -in twelve out of sixteen ids; then of 100 offspring of the first -generation it is possible that seventy-five or more will contain the -determinants _Dv_, some of them in a smaller number of ids, some in a -great number than the mother, according as the reducing division has -turned out. If the pairing of the second generation be favourable--and -this again is purely a matter of chance--a third generation must arise -which would contain the variants _Dv_ throughout, and thus the fixation -of this particular variation on this particular island would be begun. -In other words, the possibility would arise, that, if individuals with -a majority of _Dv_ ids predominated, they would gradually come to be -the only ones, since by continual crossing with the minority which -possessed only the determinants _D_, they would mingle the varied ids -with those of the descendants of these last, till ultimately germ-plasm -with only the old ids would no longer occur. - -In following out this process it is not necessary to assume that the -first immigrant possessed the variation _visibly_; if determinants -varying in a particular direction occurred in the majority of its ids, -these would, as a consequence of persistent germinal selection, go on -varying gradually until the externally visible variation appeared. This -would not have appeared at all if the animal concerned had remained -in the original habitat of its species, for there it would have been -surrounded by normal germ-plasms, and its direct descendants, even if -they had been as favourably situated for the origin of variations as -we have assumed, would not have reproduced only among themselves, and -therefore even in the next generation the number of _Dv_ ids would have -diminished. - -Obviously it is to a certain extent a matter of chance whether in the -isolated descendants the variation or the normal form remains the -victor, for it depends on the number of _Dv_ ids originally present in -the fertilized eggs, then on the chances of reducing divisions, and -finally on the chance which brings together for pairing individuals in -which the similarly varied ids preponderate. The probability of the -conquest of the variation will depend in the main on the strength of -the majority of the varied ids in the fertilized eggs of the parents; -if this be an overwhelming majority, then the chances of favourable -reducing divisions and pairings will also be great. The origin of a -pure amixia variety will thus depend upon the fact that _the same_ -variational tendency _Dv_ was present in a large number of the ids of -the ancestral germ-plasm. We need not wonder therefore that of the -numerous diurnal butterflies of Corsica and Sardinia only eight have -developed into endemic, probably 'amiktic,' varieties. - -But since we know that so many species in oceanic islands and other -isolated regions are endemic or autochthonous, i.e. of local origin, -there must obviously be some other factor in their evolution in -addition to the mere prevention of crossing with unvaried individuals -of the same species. The variational tendencies which have arisen in -the germ-plasm through germinal selection may--as we have already -seen--gain the ascendancy in various ways; first, by being favoured -by the climatic influences, then by being taken under the protection -of personal selection, whether in the form of natural or of sexual -selection. - -As the inhabitants of insular areas are not infrequently subject to -special climatic conditions, we may assume at the outset that many of -the 'endemic' species are climatic varieties, but in many cases this -explanation is insufficient. For instance, special local forms of -mocking-bird live on several of the Galapagos Islands, but this cannot -depend upon differences of climate, for the islands are only a few -kilometres apart, and resemble one another as regards the conditions -of life which they present. But as the differences between these -local forms show themselves especially in the male sex, as colour -variations of certain parts of the plumage, we must take account of -sexual selection, which, though with its basis in germinal selection, -has in many islands followed a path of its own. Sexual selection -operates especially in the case of sporadically occurring characters -which are in any way conspicuous. But it is just such variations as -these that are called into existence by germinal selection, whenever it -is allowed to continue its course undisturbed through a long series of -generations. Characters of this kind, such, for instance, as feathers -of abnormal structure or colour in a bird, or new colour spots in a -butterfly, make their appearance when a group of determinants has been -able to go on varying in the same direction for a long time unimpeded, -that is, without being eliminated as injurious by natural selection or -obliterated by crossing. This is very likely to happen in the case of -an isolated area, and as soon as the conspicuous character thus brought -about makes its appearance, sexual selection takes control of it, and -ensures that all the individuals, that is, all the germ-plasms which -possess it, have the preference in reproduction. - -I believe, therefore, that a large number of the endemic species of -birds and butterflies in isolated regions result from amixia based -upon germinal selection, whose results have been emphasized by sexual -selection. Experience corroborates this, as far as I can see, for many -of the endemic species of birds in the Galapagos and other islands -differ from one another solely or mainly in their colouring, and in -many it is especially the males which differ greatly. - -As to the humming-birds we may say, without going into details -regarding their sexual characters and their distribution, that the many -endemic species which inhabit the Alpine regions of isolated South -American volcanic mountains differ from one another chiefly in the -males and in the secondary sexual characters of these. The family of -humming-birds is characteristically Neotropical, that is, it has its -centre in the Tropics of the New World, and by far the greater number -of humming-bird species--there are about a hundred and fifty--occur -there only, while a few occur as migrants north of the Tropical zone, -and visit the United States as far north as Washington and New York. -We know that many of the most beautiful species have quite a small -area of distribution, that many are restricted to a single volcanic -mountain, living in the forests which clothe its sides. These species -are isolated there, for they do not migrate; apparently they cannot -endure the climate of the plains, but remain always in their mountain -forests. Without doubt they originated there, chiefly, I am inclined -to think, through the variation of the males due to sexual selection. -Any one who has seen Gould's magnificent collection of humming-birds -in the British Museum in London knows what a surprising diversity of -red, green, and blue metallic brilliance these birds display, what -contrasts are to be found in the diverse colour-schemes, and what -differences they exhibit in the length and form of the feathers of the -head, of the neck, of the breast, and especially of the tail. There -are wedge-shaped, evenly truncate, and deeply forked tails, some with -single long, barbless feathers, and so on. All these characters are -confined to the males, and are at most only hinted at in the female; -in no species does the female even remotely approach the male in -brilliance or decorativeness of plumage. - -I do not believe that so many species with very divergent plumage in -the males could have developed if they had all lived together on a -large connected area. But here, distributed over a large number of -isolated mountain forests, the decorative colouring or the distinctive -shape which chances to arise through germinal selection on any of these -terrestrial islands can go on increasing, undisturbed by crossing with -individuals of the ancestral species, and furthered, moreover, by -sexual selection. - -In this way, if I mistake not, numerous new species have arisen as a -result of isolation, and it is quite intelligible that several new -species may have arisen from one and the same ancestral species, as we -may see from the nearly related yet constantly different species of -mocking-bird on the different islands of the Galapagos group. - -A number of similar examples might be given from among birds. Thus -Dixon calls attention to the species of the thrush genus _Catharus_, -twelve of which live in the mountain forests of Mexico and of South -America as far as Bolivia, all differing only slightly from one another -and all locally separated. They came from the plains, migrated to the -highlands, were isolated there, and then no longer varied together all -in the same direction, but each isolated group evolved in a different -direction according to the occurrence of chance germinal variations: -one developed a chestnut-brown head, another a slate-grey mantle, a -third a brown-red mantle, and so on. From what we have already seen in -regard to the importance of sexual selection in evolving the plumage of -birds, it is probable that this factor has been operative in this case -also. - -Another example is afforded by the weaver-birds (_Ploceus_) of South -Africa, those ingenious singing-birds resembling blackbirds in size and -form, whose pouch-shaped nests, hanging freely from a branch, usually -over the water, and with their little openings on the under side, are -excellently protected from almost every form of persecution. These -birds have in South Africa split up into twenty or more species, but -the areas of each are not sharply isolated, and the division into -species cannot, therefore, be due to isolation. But it is not difficult -to guess upon what it depends, when we know that the males alone are -of a beautiful yellow and black colour, while the females are of a -greenish protective colouring all over. - -Thus, in my opinion, sexual selection plays a part more or less -important in the origin of the numerous endemic species of diurnal -Lepidoptera which are characteristic especially of the islands of the -Malay Archipelago, and which make the Lepidopteran fauna there so rich -in individuality. A large number, indeed the majority of the types of -Papilionidæ, have a peculiar species, a local form, on most of the -larger islands, which is sharply and definitely distinguished from -those of the other islands, usually in both sexes, but most markedly in -the much more brilliantly coloured males. - -Thus each of these types forms a group of species, each of which is -restricted to a particular locality, and has usually originated where -we now find it, although of course the diffusion of one of these -large strong-flying insects from one island to the other is in no way -excluded. As an example we may take the _Priamus_ group, the blackish -yellow _Helena_ group, the blue _Ulyssus_ group, and the predominantly -green _Peranthus_ group. - -If we inquire into the causes of this divergence of forms and their -condensation into numerous species, we shall find that their roots lie -in this case, as in that of all transformations, in germinal selection -and the variational tendencies resulting therefrom, but we must -_regard their fixation us the result of isolation_, which prevented -the variational tendencies which happened to develop on any one island -from being neutralized and swamped by mingling with the variations of -other islands. But that sexual selection took control of these striking -colour-variations and increased them still further is obvious from -the rarely absent dimorphism of the sexes. Even if the females do not -consciously select mates from among the males, they will more readily -accept as a mate the one among several suitors which excites them most -strongly. And that will be the one which exhibits the most brilliant -colours or exhales the most agreeable perfume, for we know from their -behaviour in regard to flowers how sensitive butterflies are to both -these influences. - -Although isolation has an important rôle in the formation of all -these species, it seems to me an exaggeration to maintain, as many -naturalists do, that the splitting up of a species is impossible -without isolation. Certainly the splitting up of species is, in -numerous cases, facilitated by isolation, and indeed could only have -been brought about in its present precision by that means, but it is -underestimating the power of natural selection not to credit it with -being able to adapt a species on one and the same area to _different_ -conditions of life, and we shall return to this point later on in a -different connexion. But in the meantime it must suffice to point out -that the polymorphism of the social insects affords a proof that a -species may break up into several forms in the same area through the -operation of natural selection alone. - -I am therefore of opinion, with Darwin and Wallace, that adaptation to -new conditions of life has, along with isolation, had a material share -in the evolution of the large number of endemic species of snail on the -oceanic islands. This brings us to the co-operation of natural selection -and isolation. If, thousands of years ago, by one of the rarest -chances, an _Achatina_-like snail was carried by birds to the Sandwich -Islands, it would spread slowly, at first unvaried, from the spot -where it arrived over the whole of the snailless island. But during -this process of diffusion it would frequently come in contact with -conditions of life which would not prevent it from penetrating further, -but to which it was imperfectly adapted, and in such places a process -of transformation would begin, which would consist in the fostering of -favourably varying individuals, and which would run its course quietly -by means of personal selection, based upon the never-ceasing germinal -selection, and unhindered by any occasional intrusion of still unvaried -members of the species from the original settlement on the island. -But these new conditions were not merely different from those of the -ancestral country; the island region itself presented very diverse -conditions, to which the snail immigrant had to adapt itself in the -course of time, as far as its constitution allowed. Terrestrial snails -are almost all limited to quite definite localities with quite definite -combinations of conditions; none of our indigenous species occurs -everywhere, but one species frequents the woods, another the fields; -one lives on the mountains, another in the valleys; one on gneiss -soil, another on limy soil, a third on rich humus, a fourth on poor -river-sand; one in clefts and hollows among damp moss, and another in -hot, dry banks of loess, and so on. Although we cannot see in the least -from the structure of the animal why this or that spot should be the -only suitable one for this or that species, we may say with certainty -that each species remains permanently in a particular place because -its body is most exactly adapted to the conditions of life there, and -therefore it remains victorious in the competition with other species -in that particular spot. - -In this way the immigrants to the Sandwich Islands must have adapted -themselves in the course of time to their increasingly specialized -habitats, and in doing so have divided up into increasingly numerous -forms, varieties, and species, and indeed into several genera. - -But this alone is not sufficient to explain the facts. According to -Gulick's valuable researches there live on one little island of the -Sandwich group no fewer than 200 species of Achatinellidæ, with 600-700 -varieties! This remarkable splitting up of an immigrant species is -regarded by him as a result of the isolation of each individual species -and variety, and I do not doubt that this is correct as far as a -portion of these forms is concerned, and that isolation plays a certain -part in regard to them all. Gulick, who lived a long time upon the -island, attempts to prove that the habitats of all these nearly related -varieties and species are really isolated as far as terrestrial snails -are concerned; that intermingling of the snails of one valley with -those of a neighbouring one is excluded, and that the varieties of the -species diverge more markedly from one another in proportion as their -habitats are distant. On the other hand, species of different genera -of Achatinellidæ often live together on the same area; but they do not -intermingle. - -Although Gulick's statements are worthy of all confidence, and though -his conclusions have great value as contributions to the theory of -evolution, I do not think that he has exhausted the problem of the -causes of this remarkable wealth of forms among the terrestrial snails -of oceanic islands. It is not that I doubt the relative and temporary -isolation of the snail-colonies at numerous localities in the island of -Oahu. But why have we not the same phenomenon in Germany, in England -or Ireland? Gulick anticipates this objection by pointing out the -peculiar habits of the Oahu snails. Many of the species there are -purely arboreal animals, living upon trees and never leaving them, even -during the breeding season, or in order to deposit eggs, for they bring -forth their young alive. Active migration from forest to forest seems -excluded by the fact that on the crests of the mountains there is a -less dense forest of different kinds of trees, and dry sunny air, which -could not be endured by the species of _Achatinella_ and _Bulimella_, -which love the moist shades of the tropical forests. Active migration -over the open grass-land at the mouths of the valleys is also excluded. - -It must be admitted that the isolation of these forest snails in their -valleys is for the time being very complete, and that intermingling -of two colonies which live in neighbouring valleys _does not occur -by active migration, within the span of one or several human -generations_. It will also be admitted that our terrestrial snails in -Central Europe are much less isolated in their different areas, that, -for instance, they could get from one side of a mountain to the other -by active migration; but we must nevertheless repeat the question: how -does it happen that in Oahu every forest, every mountain-crest, and so -on, has its own variety or species, while our snails are distributed -over wide stretches of country, frequently without even developing -sharply defined local varieties? The large vineyard or edible snail -(_Helix pomatia_) occurs from England to Turkey, that is, over a -distance of about 3,000 kilometres, and within this region it is found -in many places which might quite as well be considered isolated as -adjacent forest valleys in Oahu. It occurs also on the islands of the -Channel and of the Irish Sea, and lives there without intermingling -with the members of the species on the mainland. But even on the -Continent itself it would be possible to name hundreds of places in -which they are just as well protected from intermingling with those of -other areas as they are in Oahu. There too the snails must _somehow_ -have reached their present habitat some time or other, perhaps rather -in an indirect way, by means of other animals; but this is true also of -the snails of a continent, as we shall show more precisely later on. -In the meantime let us assume that this is so, and that the vineyard -snail (_Helix pomatia_), or some other widely distributed snail, -is relatively isolated. _Why then have not hundreds of well-marked -varieties evolved--a special one for each of the isolated areas?_ - -Obviously there must have been something in operation in the Sandwich -Islands which is absent from the continental habitats of _Helix -pomatia_, for this species shows fluctuations only in size, but is -otherwise the same everywhere, and the few local varieties of it which -occur are unimportant. I am inclined to believe that this 'something' -depends on two factors, and especially on the fact that the immigrant -snail enters upon a period of variability. This will be brought about -in the first place by the fact that the climate and other changes -in the conditions of life will call forth a gradually cumulative -disturbance in the equilibrium of the determinant system, and thus -a variability in various directions and in various combinations of -characters. To this must be added the operation of natural selection, -which attempts to adapt the immigrant to many new spheres of life, and -thus increases in diverse ways the variational tendencies afforded by -germinal selection. These two co-operating factors bring the species -into a state of flux or lability, just as a species becomes more -variable under domestication, likewise as a direct effect of change -of food and other conditions, such as the consciously or unconsciously -exercised processes of selection. It follows from this that, in the -gradual diffusion of snails all over the island, similar localities -would almost never be colonized by exactly similar immigrants, but -by individuals containing a different combination of the existing -variations, so that in the course of time different _constant forms_ -would be evolved through amixia in relatively isolated localities. - -But everything would be different in the diffusion of a new species -of snail in a region which was already fully or at least abundantly -occupied by snail-species. Let us leave out of account altogether -the first factor in variation, the changed climate, and we see that -a species in such circumstances would have no cause for variation, -because it would find no area unoccupied outside of the sphere to -which it was best adapted; it would therefore not be impelled to adapt -itself to any other, and in most cases could not do so, because in each -it would have to compete with another species superior to it because -already adapted. - -The case would be the same if an island were suddenly peopled with -the whole snail-fauna of a neighbouring continent, with which a land -connexion had arisen. If the island had previously been free from -snails, all the species of the mainland would be able to exist there in -so far as they were able to find suitable conditions of life, but each -species would speedily take complete possession of the area peculiarly -suited to it, so that none of their fellow migrants would be impelled, -or would even find it possible, to adapt themselves to new conditions -and thus to become variable and split up into varieties. If Ireland -were at present free from snails, and if a land connexion between it -and England came about, then the snail-fauna of England would probably -migrate quite unvaried to Ireland, and in point of fact the snail-fauna -of the two islands, which were formerly connected, is almost the same. -For the same reason the fauna of England, as far as terrestrial snails -are concerned, is almost the same as that of Germany. - -On the other hand, it may be almost regarded as a law that an -individual migrant to virgin territory must become variable. This could -not be better illustrated than by the geographical distribution of -terrestrial snails, which emphasizes the fact that a striking wealth of -endemic species is to be found on all oceanic islands. Moreover, the -fact that the number of these endemic species is greater in proportion -to the distance of the island from the continent, indicates that the -variability sets in more intensively and lasts longer in proportion to -the small number of species which become immigrants in the island, and -in proportion to the number of unoccupied areas which are open to the -descendants of the immigrant species. This is undoubtedly the reason -why the Sandwich Islands do not possess _a single species_ which occurs -elsewhere, and the segregation of the unknown ancestral form into many -species and several (four) sub-genera is also to be interpreted in the -same way. There was probably in this case only one immigrant species, -which found a free field, and adapted itself in its descendants to -all the conditions of snail-life which obtained there, and in doing -so split up into numerous and somewhat markedly divergent forms. But -the number of different forms is much greater than the number of -distinctive habitats, as Gulick indicates and substantiates in detail, -for similar areas, if they are relatively isolated from one another, -are inhabited not by the same forms, but by different though nearly -related varieties, and this depends on the fact that from the species -which was in process of varying a different combination of variations -would be sent out at different periods, and the temporary isolation -would result in the evolution of special local varieties. - -But I do not believe that this would continue for all time. I rather -think that these--let us say--representative varieties would diminish -in numbers in the course of a long period. For the isolation of single -valley-slopes or of particular woods is not permanent, individuals are -liable to be carried from one to another in the course of centuries as -they were at the beginning of the colonization of the isolated woods; -forests are cleared or displaced by geological changes, connexions are -formed between places, which were formerly separated, and in the course -of another geological period the number of representative varieties, -and probably even of species, will have diminished considerably,--the -former will have been fused together, the latter in part eliminated. -Even now Gulick speaks regretfully of the decimation of rare local -forms by their chief enemies, the mice. - -But even if the number of endemic forms in insular regions diminishes -from the time when they were first fully taken possession of, it -nevertheless remains a very high one, for even now Madeira possesses -104 endemic terrestrial snails, the Philippines have more than the -whole of India, and the Antilles as many as the whole American -continent. - -Many naturalists believe that each isolated variety must diverge -further and further from its nearest relatives as time goes on. -Although I entirely admit that this is possible, for I have endeavoured -to show that variational tendencies which have once arisen in the -germ-plasm go on in the same direction until they are brought to a full -stop in some way or other, yet I cannot admit that this must always -be so. The species which has been carried to a strange area need not -always contain particular variational tendencies in its germ-plasm, and -need not in every case be impelled to such variations by the influence -of new conditions. We know species which have made their way into new -regions, and, without varying at all, have held their own with, or -even proved superior to, the species which were already settled there. -Many cases of this kind are known, both among plants and animals; -these have been brought by man, intentionally or by chance, from one -continent to another, and have established themselves and spread over -the new area. I need only recall the evening primrose (_Œnothera -biennis_[28]), whose fatherland is Virginia, but whose beautiful big -yellow blossoms now display themselves beside nearly every river in -Germany, having migrated stream-upwards along the gravelly soil; or -the troublesome weed (_Erigeron canadense_), which is now scarcely -less common in our gardens than in those of Canada; or the sparrow -(_Passer domesticus_), which was introduced into the United States to -destroy the caterpillars, but which preferred instead to plunder the -rich stores of corn, and in consequence of these favourable conditions -increased to such an extent that it has now become a veritable pest, -all imaginable means for its extirpation having been tried--as yet, -however, with no great results. - -[28] This was written before the appearance of the researches which -De Vries has made on the variations of _Œnothera_ in Europe. Thus -the illustration may not be quite apposite, for it seems to remain -undetermined whether the 'mutations' which occur in Holland do not also -occasionally appear in America. See end of lecture xxxiii. - -In all these cases the migration is certainly of recent date, and it is -quite possible that, when a longer time has elapsed, some variations -will take place in the new home, but in any case these instances -prove that an immigrant species can spread over its new area without -immediately varying. - -Similarly, it must be admitted that species which have belonged to -two continents ever since Tertiary times need not have diverged since -that time, and we know, for instance, thirty-two species of nocturnal -Lepidoptera which are common to North America and to Europe and yet -exhibit no differences, while twenty-seven other nocturnal Lepidoptera -are, according to Grote, represented in America by 'vicarious' species, -that is, by species which have varied slightly in one or other of the -two areas, perhaps in both. - -To sum up: we must undoubtedly admit that isolation has a considerable -influence in the evolution of species, though only in association -with selection in its various grades and modes, especially germinal -selection, natural selection, and sexual selection. We can say -generally that each grade and mode of selection will more readily lead -to the transformation if it be combined with isolation. Thus germinal -selection may call forth slight divergences in colour and marking, -which will be permanent if the individuals concerned are in an isolated -region. In isolation these variations will increase undisturbed, and -in some circumstances will be intensified by sexual selection, so that -the male sex will vary alone in the first place, though the female -may follow, so that ultimately the whole species will be transformed. -Finally, the most marked effect of isolation is seen when individual -members of a species are transferred to virgin territory which offers -unoccupied areas, suitable not to one particular species alone, but to -many nearly related species, so that the immigrant colony can adapt -itself to all the different possibilities of life, and develop into a -whole circle of species. But we saw that such an aftergrowth of new -forms, whether varieties, species, or even genera, may far exceed the -number of different kinds of localities, if there be relative isolation -between the different groups of immigrants within the insular region, -as happens in the case of slow-moving animals like the terrestrial -snails, or of small singing-birds, to which each island of a little -archipelago is a relatively isolated region (Galapagos). - -We may thus fully recognize the importance of local isolation without -regarding the absence of crossing with the members of the species in -the original habitat as the sole cause of species-formation, without -setting 'isolation' in the place of the processes of selection. -These last, taken in the wide sense, always remain the indispensable -basis of all transformations, but they certainly do not operate -only in the form of personal selection, but, wherever indifferent -characters are concerned, in that of germinal selection. Here, too, -we see the possibility of reconciliation with those naturalists who -regard transformations as primarily dependent upon internal forces of -development. The fact is that _all variations depend upon internal -causes_, and their course must be guided by forces which work in an -orderly way. But the actual co-operation of all these forces and -variations is not predetermined, but depends to a certain extent upon -chance, for of the possible modes of evolution the one which gains -the upper hand in the play of forces at the moment is alone followed, -the better are everywhere preferred, from the most minute vital units -of the germ-plasm, up to the struggle between individuals and between -species. - - - - -LECTURE XXXIII - -ORIGIN OF THE SPECIFIC TYPE - - Transition species of Celebes snails, according to Sarasin--Possible - variations in the shell due to nutrition--Natural selection plays a - part--Germinal selection--Temporary transitions between species--The - fresh-water snails of Steinheim--How do sharply-defined species - arise?--Nägeli's Developmental Force--The species a complex of - adaptations--Adaptive differences between species--Adaptive nature of - specific characters--The case of Cetaceans--Of birds--Additional note: - the observations and theories of De Vries. - - -Our study of the influence which geographical isolation may have in -transforming old and giving rise to new forms of life has led us -naturally to a much more important problem, that of the origin of -species as more or less sharply defined groups of forms, and I wish -to make the transition to this problem by discussing another case of -species-splitting effected in association with, or, as is usually -said, _through_ isolation. The naturalists Paul and Fritz Sarasin, -well known through their excellent studies on many components of -the tropical fauna, have published in their latest work interesting -discoveries in regard to the terrestrial snails of Celebes. These -observations show that on this island a great transformation of snails -has taken place, even since the later Tertiary period. A large number -of new species of snail have arisen on this island since that time, -and this, as the authors show to be probable, in association with -the receding of the sea, that is, with the elevation of the island -further out of the water, and thus with the increase of its surface. -The modern terrestrial snails show chains of forms connected in many -ways so that a series of species is connected by transition forms, and -therefore does not really consist of separate species at all, although -the extremes would seem to be separate species if they were studied -by themselves without taking the transition forms into account. The -state of things is exactly as if a Tertiary snail had spread from any -small area over the whole island, and had been transformed slowly -and in a definite direction in accordance with its distance from its -starting-point. It is thus that we must interpret this discovery; -we have here, beside each other in space, and indeed often disposed -along geographical lines, the individual stages of a phyletic process -of transformation, which has reached different levels at different -places. One of the longest of these chains of forms is that of _Nanina -cincta_, which runs across the island from east to west, and, beginning -with the smallest and most delicate forms, ascends through many -intermediate stages to the giant form _N. limbifera_. Such chains of -forms have been previously recognized; thus Kobelt described one in the -case of the Sicilian land-snails of the genus _Iberus_, and other cases -are recorded in literature, but in all instances they refer to areas -which must be regarded as isolated for the snails, and which have been -colonized from a single starting-point. - -We have now to inquire whether and how we can explain the origin of -these chains of forms. The cousins Sarasin tell us how they at first -attempted to refer the differences between the individual links of such -a chain to the diverse influence of the external conditions of life, -but in vain; neither the height above sea-level nor the character of -the soil was sufficient, and natural selection was no more so; 'for -why should a high _Obba_-form twisted like a beehive be either better -or worse equipped for the struggle for existence than a smaller and -flatter one?' It is true that we do not understand why, but this does -not seem to me any reason to doubt that natural selection should be -regarded as one of the causes of the divergence of these species, -for we could not answer the same question in regard to any of the -other structural differences between two species of snail, for the -simple reason that we have far too little knowledge of the biological -value of the parts of a snail. Or could any one tell of what use it -would be to a snail-species to have the horns slightly longer, the -foot somewhat narrower, the radula beset with rather larger or more -numerous teeth? We might indeed imagine many ways in which it might -be of advantage, but we are not in a position to say definitely why, -for instance, longer horns should be better for one species than for -another, and yet we do not believe that the structure of snails is -less well adapted to the life of each species than that of any other -animals. The snail's structure is certainly built up of hundreds and -thousands of adaptations, like that of every other animal species, -but while in many others we can, at least in part, recognize the -adaptations as such, we cannot do so at all in regard to the snail. -Simroth has pointed out that the spiral asymmetrical shell bears a -relation to the one-sided opening of the genital organs, but that only -states the general reason for the coiling of the shell. In studying the -differences in the shell one is apt to think of its external appearance -alone, of the protection which it affords to the soft internal organs -of the easily wounded animal; perhaps also of the distribution of -weight, which must be different in a high tower-like structure and a -low flat spiral; possibly, too, of the varied obstacles and resistance -the snail has to encounter in creeping into clefts and holes, or among -a tangle of plants, according to the form of its shell; but is it not -also conceivable that the form of the shell has been determined by its -contents? As Rudolph Leuckart taught, the snail may be regarded as -composed of two parts, one of which is formed by the head and foot, -the other by the so-called 'visceral sac': the former may be called -the animal half, because it chiefly contains the dominant organs--the -nerve-centres, almost the whole mass of muscle, and the sense-organs; -the latter the vegetative half, since it contains the main mass of the -nutritive and reproductive systems--the stomach and intestine, the -large liver, the heart, the kidneys, the reproductive organs, and so -on. The vegetative half of the animal is always concealed within the -shell; would not therefore any great variation in the size of liver, -stomach, intestine, and so on, bring with it a variation in the size -and form of the shell, as well as in the expansion or contraction of -its coils? And might not such variations become necessary because of -some change in the food-supply? It is only a supposition, but it seems -to me very probable that becoming accustomed to a new diet, less easily -broken up and dissolved and of diminished nutritive value, would cause -modification not only of the radula and jaw-plate, but also of the -stomach and the liver, the intestine and the kidneys, whose activity -is closely associated. The stomach must become more voluminous, the -liver which yields the digestive fluid must become more massive, and -so forth. I will not follow this hypothetical example further, for I -merely wished to recall the fact that the snail shell, to the form -of which no biological significance can be commonly attributed, is -actually a sort of external cast of the visceral sac, and consequently -dependent on the variations to which that is liable in accordance -with the conditions of its life. To give precise proofs for such -processes is certainly not yet possible, for we do not even know with -certainty what the diet of the various species of snail is, much less -the difference between the modes of nutrition in two varieties, or the -nutritive value of the materials used, or the changes in secretion, -absorption, assimilation, and excretion which must be brought about by -these differences. But we can at least see that variations in nutrition -must be enough in themselves to give rise to new adaptations in the -size, constitution, and mutual adaptation of the internal vegetative -organs, and we cannot overlook the possibility that the form and -size of the vegetative half, and therefore the form and size of its -secretion, the shell, may also be caused to vary[29]. The fact that we -cannot recognize, for instance, the beehive shape of an _Obba_ as an -adaptation, is thus no proof that it is not one. But let us assume for -the moment that it is not, and that it cannot be referred to natural -selection any more than the other variations in the Celebes chain of -forms, and we may further admit that they cannot be referred to sexual -selection, still less to some 'inherent principle of perfecting,' not -only because there is no question of perfecting in the matter, but -because such a mystical principle is outside of the scope of natural -history and its principles of interpretation. - -[29] That this suggestion was not unjustified is evident from a -recent contribution by Simroth ('Ueber die Raublungenschnecken,' -_Naturwissenschaftliche Wochenschrift_, December 8 and 15, 1901). In -this paper the author, who is an expert as regards the biology of -Gastropods, shows that a change of diet may evoke many kinds of changes -in the structure of the food-canal, which may indirectly compel changes -in the shell. Thus in a small indigenous snail, _Daudebardia_, the -pharynx has grown so enormously in thickness and length in adaptation -to the predatory mode of life, that the head and the anterior part of -the body can no longer be retracted within the shelter of the shell. -For this reason, and also because of the snail's habit of following -earthworms into their burrows, the shell has been shunted far back and -obliquely downwards. It has at the same time markedly changed in its -shape, as may still be verified by comparing the form of the shell in -the young stages with that of the adult. - -But that transformations in a definite direction can and must arise -from fresh disturbances in the equilibrium of the determinant system, -that is from germinal selection, we have already shown. - -Even if the changes of form with which we are here dealing had really -no biological importance, they might quite well have been brought about -by germinal selection, and only one thing remains obscure, that is, -why the different stages on the path of distribution of a species are -at a different level of evolution and not all at the same level. Why -have not all been transformed? Why have some colonies remained near -the ancestral form, while others have varied only a little, and others -again a great deal? This cannot be explained by any assumption of an -internal power of development, and the explanation can only be found -in germinal selection associated with isolation, since the internal -processes in the germ-plasm can quite well run a different course in -different colonies. Nevertheless I am inclined to infer from these -differences in the individual colonies of these chains of forms, that -natural selection in the accepted sense has also played a part in the -evolution of these snail varieties. - -Such series of forms are especially interesting, because they show us -the process of species-formation in its different stages beside each -other in space, and thus simultaneously. They represent, so to speak, -a horizontal branch of the genealogical tree of the species, as the -Sarasins well express it, that is, a series of species arising from -each other, which do not break off, but are all capable of life at -the same time, and so exist simultaneously on different areas; they -are species adapted to different localities, not to different times. -The same is true of the snails of other isolated regions, except that -the chains of forms are usually not simple, but split up into several -chains of forms arising from one ancestral form, and under certain -circumstances each of these may break up into two or more diverging -series. The great number of related species in Madeira, or in the -Sandwich Islands, compels us to this assumption, although the branching -of the genealogical trees can no longer be demonstrated with certainty. - -This splitting up of forms into several series on a varied insular -region shows us once more that it is germinal selection alone which -forms the basis of all transformations, and that there is not, as -earlier naturalists, especially the botanists Nägeli and Askenasy, -maintained, any peculiar impelling Force of Development innate in -organisms. If there were such a force, a species would be obliged to -go on continuously in the same direction, exactly like the Sarasins' -chains of forms, but no breaking of the species into one or many forms -could occur. But this breaking up into series is easy to understand -when we take germinal selection into consideration, for the germ-plasm -contains many ids and determinants, and each of these can enter upon -new variations, so that one colony can vary in this direction, another -in that, and a great diversity of forms living in isolation must, or at -least may be the result, as we see in the case of the Sandwich Islands. - -Let us delay a moment over the Sarasins' case of the Celebes snails. We -are dealing here with series of forms in regard to which the ordinary -conception of species fails us, for they contain varieties whose -extremes are as far apart as distinct species usually are, which are -not, however, distinct, since they are connected with one another by -one and often by several transition forms. Thus we can only break them -up into two or more 'species' by an arbitrary division at one place or -other. The phenomenon itself is not new to us; we have seen that even -Lamarck and Treviranus made use of similar series of forms, connected -by transition stages, in their attack upon the old theory of creation, -and sought to prove by means of these that the idea of species is an -artificial one, read into nature by man, and not innate in nature, and -that the forms of life were only apparently fixed and sharply defined, -being in reality in process of slow transformation. Such beautiful -and convincing examples as we now possess were not available at that -time, but it might even then be said that it was the easier to make -a new species the fewer examples one had to deal with, and the more -difficult the more numerous these became, because with the number of -individuals, especially if they come from a wide area, the number and -diversity of the divergences increases also, so that in many cases, as -in that of the Celebes snails, it becomes impossible to draw a line -between the different species. - -There are, however, many animal and plant forms which do not show such -marked divergences, but rather exhibit a great harmony of individuals -even in detail, and the conception of species is more readily applied -to these. It would certainly be foolish to give it up, since we should -then lose all possibility of arriving at any sort of orientation -among the enormous wealth of forms in nature. But at the same time we -must not forget that these 'typical' species only appear so to our -short-sighted vision--short-sighted as far as time is concerned--and -that they are connected from long-past times with 'species' which -lived at an earlier date, by just such transition stages as connect -the Celebes species of to-day, which are all living at the same -time. The world of life on the earth only presents at any given time -a 'cross-section of its genealogical tree,' and according as its -branches grow out vertically or horizontally we receive an impression -of typical, sharply defined species or of circles or chains of forms. -In the first case the evolution of new species was associated with the -dying out of the horizontal branches, and the end-twigs of the branch -stand beside each other now apparently isolated and sharply defined; -in the other case only a portion of the ancestral species has been -transmuted, and the other part continues to live alongside of the -species derived from it, and perhaps repeats the process of giving off -a varied race of descendants. - -The last thirty years have yielded much palæontological evidence of -the successive stages of species-transformation. In quietly deposited -horizontal strata of the earth's crust, lying one above another, the -whole phyletic history of a group of snail-species has repeatedly -been found in historic order, the oldest in the deepest layer, the -youngest in the uppermost, and the numerous and often very divergent -'species' of a particular deposit are connected by transition forms in -the intermediate strata. From the point of view of time, therefore, -these are not 'typical' species, but circles of forms in a state of -variability. - -The most beautiful of such cases are the _Planorbis_ species from the -small lacustrine deposits of Steinheim in Swabia, the _Paludina_ strata -of Slavonia, and various groups of Ammonites. - -These cases have been described and discussed so often that I need only -refer to their most essential features. - -The _Planorbis_ strata of Steinheim were first investigated, from -the point of view of the theory of descent, by Hilgendorf (1866). -He described nineteen different varieties, which, as they are all -connected in chronological succession with each other, he grouped -together under the name of _Planorbis multiformis_. These little -freshwater snails are found in millions in the strata of the former -lake-basin of Steinheim, and they are arranged in so orderly and -regular a manner that two observers, working independently and at -different times, succeeded in building up the genealogical tree -in almost the same way. According to Alpheus Hyatt, the later -investigator, all the forms are derived from one ancestral form, -_Planorbis lævis_, from which four different series have descended, one -of them splitting up again into three subordinate series. - -All the individual members of these series are connected by -intermediate forms in such a manner that a long period of constancy of -forms seems to be succeeded by a shorter period of transformation, from -which again a relatively constant form arises. - -We see, therefore, that the idea of species is fully justified in -a certain sense; we find indeed at certain times a breaking up of -the fixed specific type, the species becomes variable, but soon the -medley of forms clears up again and a new constant form arises--_a new -species_, which remains the same for a long series of generations, -until ultimately it too begins to waver, and is transformed once -more. But if we were to place side by side the cross-sections of this -genealogical tree at different levels, we should only see several -well-defined species between which no intermediate forms could be -recognized; these would only be found in the intermediate strata. - -The problem we have now to discuss is, how it comes about that -relatively sharply defined species exist which are connected with -ancestral forms further back, but which form among themselves an -exclusive, more or less homogeneous, host of individuals. How does it -happen that we everywhere find a specific type, and not an endless -number of individual forms connected with one another in all directions? - -This would require no further explanation if a phyletic evolutionary -force impelled the forms of life to vary in a definite manner, and thus -to become transmuted into new forms in the course of generations. In -that case the whole genealogical tree of the organisms on the earth -must have been potentially contained in the lowest moneron, so that, -given time and the most indispensable general conditions of existence, -the living world just as we know it must have resulted. Nägeli was the -first to express this view, and he followed it out consistently, not -even hesitating to deny the existence of all processes of selection, -and to represent the whole of evolution as a process conditioned by -this phyletic force, which would have given rise to the world of -organisms which has actually arisen, even if the conditions of life at -the different periods of the earth's history had been other than they -were. I have always combated this idea, without however overlooking -that it is based upon facts which--at that time at any rate--gave it a -certain justification. We cannot pass it by without giving some other -interpretation of the facts. Following Nägeli, the botanist Askenasy -championed this view of 'variation in a different direction,' which -gives rise to new forms; and in more recent times Romanes, Henslow, -and Eimer expressed similar views, and--although they did not actually -dispute the existence of processes of selection--they attributed a much -less important rôle to them, and referred the phyletic genealogical -tree of organisms in the main to other and internal causes. - -Like Nägeli himself, his followers have laid stress upon the fact that -natural selection cannot be the cause of the evolution and succession -of particular species, because the differences which separate species -from species are not of an adaptive nature, and therefore cannot -depend upon selection; but if the step from one species to the next -succeeding one does not depend upon adaptation, then the greater steps -to genera, families, and orders cannot be referred to it either, -since these can only be thought of as depending upon a long-continued -splitting up of species. Genera, families, and all higher groups -must be recognized as conventional categories, not as real divisions -existing in nature itself. Even Treviranus and Lamarck maintained that -the differences between genera depended just as much upon our estimate, -our intellectual convenience, as do the differences between species. -All forms were originally connected, though they may not be so now, and -if the species are really not distinguished by adaptive characters, -then neither are any other grades of our classificatory system, -neither order nor classes, since they all depend originally on the -transmutation of species. It was therefore quite consistent of Nägeli -to seek the mainspring of organic evolution, not in adaptation, but in -an unknown evolutionary force. Thus he refused to recognize adaptation -as a consequence of selection, but regarded it, as Lamarck had done, -as the direct effect of external conditions, and as an entirely -subordinate factor in the transmutation of forms. - -Nägeli and his modern successors conceive of phyletic evolution as -depending upon definitely directed variation, resulting from internal -causes and occurring at definite times, which of necessity causes the -existing form to be transformed into a new one. To them the species -appears, so to speak, as a vital crystallization, or to use Herbert -Spencer's phraseology, as an equilibrium of living matter, which -becomes displaced from time to time, and passes over into a new state -of equilibrium, being transmuted into a new species, something like the -pictures in a kaleidoscope. The species is thus something conditioned -from within, which must be as it is and could not be otherwise, just -like a crystal which crystallizes in one particular system and not -in another; it must be just thus or it could not be at all. From the -point of view of this theory it would be easy to understand that the -thousands or millions of individuals composing a species all agree in -essentials--that a specific type exists. - -But this conception can hardly be entirely correct, although there is -some truth at its foundation, namely, that germinal variations which -arise independently are the basal roots of all transmutation. But the -species is not simply the result of these internal processes, it is -not even mainly so; it is not the result of an internal, definitely -directed developmental force, even if we attempt to think out such a -force in a purely scientific or mechanical, instead of a mystical, -sense. It seems clear to me that the species is not a life-crystal in -the sense that it must, like a rock-crystal, take form in a particular -way and in no other for purely internal reasons and by virtue of -its physical constitution; the species is essentially a complex of -adaptations, of modern adaptations which have been recently acquired, -and of inherited adaptations handed down from long ago--a complex which -might quite well have been other than it is, and indeed must have been -different if it had originated under the influence of other conditions -of life. - -But of course species are not exclusively complicated systems of -adaptations, for they are at the same time 'variation-complexes,' -the individual components of which are not all adaptive, since they -do not all reach the limits of the useful or the injurious. All -transformations arise from a basis of spontaneous chance variations, -just as all forest plants grow from the soil of the forest, but do not -all grow into trees, the adaptive forms which determine the essential -character of the forest; for many species remain small and low, like -the mosses, grasses, and herbs; and these too have a share, though a -subordinate one, in determining the character of the forest, which -depends definitely, though only partially, on the loftier growths. - -According to my view all adaptation depends on an alteration in -the equilibrium of the determinant system, such as must arise from -intra-germinal or even general fluctuations in the nutritive supply, -affecting larger or smaller groups of determinants and causing -variation in them to a greater or less degree. And these variations -may be in a quite definite direction, persisted in for internal -reasons, as we have already seen in the section dealing with germinal -selection. These variations are the building-stones out of which, -under the guidance of personal selection, a new specific type, that -is, a new complex of adaptations, can be established. In this type -many indifferent characters are involved, which are just as constant -characters of the species as the adaptations. - -The opponents of the selection theory have often urged against it -this constancy of indifferent characters, but as soon as we cease to -restrict the principle of selection to 'persons,' and extend it also -to the lower categories of vital units, the occurrence of indifferent -characters is easily understood. To illustrate characters of this kind, -Henslow has recently called attention to the species of gentian, whose -flowers have a corona split into five tips in some species and into -six or seven in others, and we cannot possibly ascribe any biological -significance to these specific characters. It is quite possible that -they possess none; but did not even Darwin express his belief that many -peculiarities of form 'are to be attributed to the laws of growth, and -to the mutual influence of parts,' forces which he rightly refrained -from including under 'natural selection' in his sense of the word, -but which we now regard as an expression of intra-selection or of -histonal selection? It is this, in our opinion, which brings about the -co-adaptation of the parts to form a harmonious whole, which admits of -the primary adaptations to the conditions of life being followed or -accompanied by correlative secondary variations, and which plays an -important part in directing the course of every individual development, -and is therefore uninterruptedly active within the organism. We cannot -analyse the factors precisely enough to be able to demonstrate in an -individual case why the corona should be divided into four in one -species of gentian and into five in another, but we can understand -in principle that all adaptations of a species which are not primary -are determined by the compelling influence of intra-selection. And -we need not now rest content even with that, for we know that this -intra-selection--as we have already seen--is active within the -germ-plasm, and it is only a logical consequence of the principle of -germinal selection to suppose that variations of definite determinants -due to personal selection may in the germ-plasm itself give rise -to correlative variations in determinants next to them or related -to them in any way, and that these may possess the same stability -as the primary variation. This seems to me a sufficient reason why -biologically unimportant characters may become constant characters of -the species. Correlation is not effected only in the perfect organism; -it exists at every period of its life, from the germ till death, and -what it brings about is quite as inevitable as what is evoked through -adaptation by means of personal selection. - -We can thus also understand that indifferent characters may be -contained not only in individual ids of the germ-plasm, but also -coincidentally in a great majority of them, as soon as we think of -them as dependent upon the characters established through personal -selection, for these must be contained in a majority of the ids. - -But there is still another reason why indifferent characters should -become stable, and that is the effect of general variational influences -on all the individuals of a species, as, for instance, in many climatic -varieties, and probably also in many cultivated varieties. - -But even when we have fully recognized that, from the arcana of the -germ-plasm, new minimal variations are continually cropping up, which -are biologically indifferent, and nevertheless become variational -tendencies, and may increase even to the extent of causing _visible_ -differences, and that therefore varieties of snails or of butterflies, -or of any animal or plant whatever, may originate through germinal -selection alone, it cannot for a moment be supposed that the -transmutation of species depends upon this process exclusively or even -preponderantly. This was Nägeli's mistake, and that of his followers as -well, that he ascribed to his 'principle of perfecting' the essential -rôle in directing the whole movement of evolution, while the general -structure of all species shows us that they are, so to speak, built -up of adaptations. But adaptations could not be--or could only be -fortuitously and exceptionally--the _direct_ result of an internal -power of development, since the very essence of adaptational changes is -that they are variations which bring the organism into harmony with the -conditions of its life. We are therefore forced either to underestimate -greatly the part played by adaptation in every organism--and that -is what Nägeli did--or to leave the standpoint of natural science -altogether, and assume a transcendental force which varied and -adapted the species of organisms _pari passu_ with the changes in the -conditions of life during the geological evolution of our earth. This -would be a sort of pre-established harmony, through which the two -clocks of evolution--that of the earth and that of organisms--kept -exact time, although they had quite different and independent works! - -But that the determining significance of adaptations in organic forms -is underestimated even now is evidenced by the continually repeated -statement that species differ, not in their adaptive characters, but in -purely morphological characters, whereas it is obvious that we are far -from being able to estimate the functions of a part with sufficient -precision to be able to say definitely whether the differences between -two nearly allied species are or are not adaptations to different -conditions. The same is true with regard to the other side of the -problem--the conditions of life. These are often to all appearances -identical in two allied species, but even where they are visibly -different it is often difficult to assert that the differences between -the two species can be interpreted with certainty as adaptations to -the specific conditions of life. At an earlier stage we discussed -the protective coloration of butterflies, and we saw that the forest -butterflies of the Tropics frequently mimicked a dry leaf on their -under surfaces. In the various regions of the extensive forest -districts of the Orinoco and the Amazon in South America there are -fifty species of the genus _Anæa_ alone, and in the resting pose all -these bear a most deceptive resemblance to a leaf, yet each of them -differs from the rest in the mingling of its colours, its brilliance, -and usually in markings when these are present. If we wished to be able -to decide whether these specific differences were of an adaptive nature -or not, we should first of all require to know in what kind of forest -two neighbouring species lived, and in what places, among what sort -of leaves, they were in the habit of settling. Even then we should at -best only know whether the species _A_ was better protected, as far as -our own eyes were concerned, among the leaves of the forest _A´_ than -the species _B_, and conversely; but we could not tell whether they -_required_ this protection, or whether the species _A_, if transferred -to the forest _B´_, would be more frequently discovered and destroyed -by its enemies than in its own forest-home, and that alone could -prove the difference to be biologically important, that is, to have -selection-value. The difficulty, indeed the impossibility, of arriving -at such decisions can perhaps be better illustrated by an example from -our indigenous fauna. No one doubts that the upper surface of the -anterior wing in the so-called banner-moth (_Catocala_) possesses a -very effective protective colouring; by day the moths rest with wings -spread out flat upon tree trunks, wooden fences, walls, &c., and they -are so excellently suited to their environment that they are usually -overlooked both by man and animals. But each of the twelve German -species of _Catocala_ has a special protective colouring; in _Catocala -fraxini_ it is a light grey, in _Catocala nupta_, a dark ash-grey, in -_Catocala elocata_ rather a yellowish-brown grey, in _Catocala sponsa_ -an olive brown, in _Catocala promissa_ a mingling of whitish-grey -and olive brown, and so on. All these colourings are protective; but -could any even of our most experienced and sharp-sighted entomologists -prove that each of these different shades of colour depends upon -adaptation to the usual resting-place of the particular species to -which it belongs? And yet it is on _a priori_ grounds highly probable -that this is the case. But even this would by no means dispose of the -whole problem, for each of these protective colour schemes is composed -of several, often many, tints; it must be so if they are to fulfil -their end at all, for a uniformly coloured wing would contrast with -the bark of every tree and with every wooden fence. The wing-surface -must therefore bear on a lighter background a number of lines and -streaks varying from brown to black, and usually running zigzag across -the wing; beside these are spots of lighter colour, which complete -the deceptive picture. This 'marking' of the wing is similar in all -twelve species, and yet in each it is different in detail. It is -constant in each, and thus is a specific character. But who would -venture to undertake the task of proving that each of these streaks, -spots, zigzag lines, &c., is or is not adaptive--that the details are -necessary adaptations to the resting-place which had become habitual -to the species, or, on the other hand, simply expressions of the -variational tendencies of the elements of marking, depending upon -germinal selection? This would be an impossible task, and yet we are -here dealing with a character which, _as a whole_, is undoubtedly -adaptive; in many of the differences between other species even that is -not certain. - -It seems to me, therefore, hardly reasonable to talk of the -'insufficiency of natural selection' because we are not able to -demonstrate that the minutiæ of specific characters are adaptational -results. Personal selection intervenes whenever the variations produced -by germinal selection attain to selection-value; and whether we can -determine the exact point at which this takes place in individual cases -is, as I have said before, theoretically quite indifferent. - -Moreover, there are cases in which we _can_ prove that specific -differences are of an adaptive nature. When, of two nearly related -species of frog, the spermatozoon of one possesses a thick head and -that of the other a thin head, and when at the same time the micropyle -through which alone the spermatozoon can make its way into the ovum is -wide in the first species and narrow in the second, we have before us a -specific character which is obviously adaptive. - -In order to gain clearness as to the significance of natural selection -in the restricted sense, that is of personal selection, it seems to -me much more important to study the different groups of animals and -plants with special reference to what they undoubtedly exhibit in the -way of adaptation. For that reason I discussed different groups of -adaptations in detail in some of the preceding lectures, although, or -rather because, they all teach us that every part of every species, -whether animal or plant, even every secretion, and indeed every habit, -every inherited instinct, is subject to adaptation to the conditions of -life. It seems difficult to refuse to admit that this is the natural -impression which this study conveys, and it is strengthened as our -knowledge increases; _that every essential part of a species is not -merely regulated by natural selection, but is originally produced -by it_, if not in the species under consideration at the time, then -in some ancestral species; and, further, that every part can adjust -itself in a high degree to the need for adaptation. It was not without -a purpose that I discussed the phenomenon of mimicry so fully, for -it, above all others, teaches us how great a power of adaptation the -organism possesses, and what insignificant and small parts may be -transformed, in a remarkable degree, in accordance with some actual -need. We saw that a butterfly might assume a colouring which diverged -entirely from that of its nearest relatives, but which caused it to -resemble an immune species of a different family, and thereby protected -it more effectively from persecution. Such a case can no more be due to -a dominating phyletic force than to a chance and sudden displacement -of the state of equilibrium of the determinant system; it can depend -only on natural selection, that is, on a sifting out of the diverse -variations offered by germinal selection, and the unhampered expression -and augmentation of those favoured. - -But it is not only these minute variations, insignificant in relation -to the whole structure of the animal, which can be determined by -natural selection. The same applies to the phyletic evolution as a -whole; even that is not directed by the assumed internal principle of -development. - -Adaptations, from their very nature, can only depend upon selection, -and not upon an internal principle of evolution, since that could -take no account whatever of external circumstances, but would cause -variations in the organism altogether independently of these. Thus, in -considering the origin of any of the larger groups of animals, we may -exclude a phyletic power as the guide of its evolution as soon as we -can prove that all its essential structural relations, as far as they -diverge from those of nearly related groups, are adaptations. We may -not be able to do this for nearly all of the animal groups, and it will -hardly be possible in regard to a single group of plants, because our -insight into the _biological_ significance of characters, which means -more than the functional significance of the individual parts, and -their correlation as parts of a whole, is seldom sufficiently intimate -or thorough. But among animals we can do this in regard to some groups; -one of these is the order of whales or Cetaceans. - -[Illustration: FIG. 130. Skeleton of a Greenland Whale with the contour -of the body. _Ok_, upper jaw. _Uk_, lower jaw. _Sch_, shoulder-blade. -_OA_, upper arm. _UA_, bones of fore-arm. _H_, hand. _Br_, vestige of -the pelvis. _Fr_, vestige of the femur. _Tr_, vestige of the lower part -of the leg. After Claus.] - -Cetaceans, as is well known, belong to the Mammalia, that is to -say, to a class whose structure was built for life on the land. The -ancestors of Cetaceans were similar to the other mammals, and possessed -a coat of hair and four legs, and a body the mass of which was so -distributed that it could be borne by those four legs. But all the -modern Cetaceans live in the sea, and they have therefore entirely -changed their bodily form; they have become spindle-shaped like fishes, -well adapted for cleaving the water, but incapable of moving upon land. -At the same time, their hind-legs have completely disappeared, and can -now be demonstrated only as rudiments within the mass of muscle (Fig. -130, _Br_, _Tr_, _Fr_), while the fore-legs have been transformed -into flippers, in which, however, the whole inherited, but greatly -shortened, skeleton of the mammalian arm is concealed (_OA_, _UA_, -_H_). The skin has lost its covering of hair so completely that in -some cases no traces of it are demonstrable except in the embryo. All -these changes are adaptations to an aquatic life, and could not have -been produced independently of the influence of external conditions. -But there is much more than this. A thick layer of blubber under the -skin gives this warm-blooded animal an effective protection against -being cooled down by the surrounding water, and at the same time gives -it the appropriate specific gravity for life in the sea; an enormous -tail-fin similar to that of fishes, but placed horizontally, forms the -chief organ of locomotion, and for this reason the hind-legs became -superfluous and degenerated. Similarly, the muscles of the ear have -also disappeared, for the hearing organ of this aquatic type is no -longer suited for receiving the sound-waves through an air-containing -trumpet, but receives them by a shorter route from the surrounding -water, directly through the bones of the skull. Remarkable changes -in the respiratory and circulatory organs make prolonged submersion -possible, and the displacement of the external nares from the snout to -the forehead enables the animal to draw breath when it comes up from -the depths to a frequently stormy surface. It would take a long time to -enumerate all that can be recognized as adaptive in these remarkable -aquatic mammals to a life in what to their ancestors would have been a -strange and hostile element. Let us study particularly the case of the -whalebone whales, for instance the Greenland whale, and we are at once -struck by the enormous size of the head, which makes up about a third -of the whole body (Fig. 130). Can this, which has such an important -effect in determining the whole type of animal, be an outcome of some -internal power of development? By no means! It is rather an adaptation -to the mode of nutrition peculiar to this swimming mammal, for it -does not, like dolphins and toothed whales, feed on large fishes and -Cephalopods, but on minute delicate molluscs--Pteropods and pelagic -Gastropods, on Salpæ, and the like, which often cover the surface -of the Arctic Ocean in endless shoals, sometimes extending for many -miles. To enable the whale to sustain life on such minute morsels it -was necessary that it should be able to swallow enormous quantities; -teeth were therefore useless, and they have become rudimentary, and can -only be demonstrated in the embryo as rudiments (dental germs) in the -jaw; but in place of these there hang from the roof of the mouth-cavity -great plates of 'whalebone,' a quite peculiar product of the mucous -membrane of the mouth, the ends of which are frayed into fibres, and -form a sieve-net for catching the little animals which are engulfed -with the sea-water. The mouth-cavity itself has become enormous, so -that great quantities of water at a time can be strained through the -net of whalebone-plates. - -When I mention that peculiar changes have occurred also in the internal -organs, that the lungs have elongated longitudinally and thus enable -the animal more readily to lie in the water in a horizontal position, -that peculiar arrangements exist within the nostrils and the larynx -which enable the animal to breathe and swallow simultaneously, and that -the diaphragm lies almost horizontally because of the length of the -lung, I think I have said enough to indicate that not only does almost -everything about the whale diverge from the usual mammalian type, but -that all these deviations are adaptations to an aquatic life. If -everything that is characteristic, that is, typical either of the order -or of the family to which animals belong, depends upon adaptation, -what room is left for the activity of an internal power of evolution? -How much is left of the whale when the adaptations are subtracted? -Nothing more than the general scheme of a mammal; but this was implicit -in their ancestors before the whales originated at all. But if what -makes whales what they are, that is, the whole 'scheme' of a whale, -has originated through adaptation, then the hypothetical evolutionary -power--wherever its seat may be--has had no share in the origin of this -group of animals. - -I said all this more than ten years ago, but the idea of an internal -directive evolutionary force is firmly rooted in many minds, and -new modifications of the idea are always cropping up, and of these -the most dangerous seem to me to be those which are not clear in -themselves, but suppose that the use of a shibboleth like 'organic -growth' means anything. That organic growth is at the base of the -phyletic evolution of organisms may be maintained from any scientific -standpoint whatsoever, from ours as well as from Nägeli's, for no one -is so extreme and one-sided as to regard the process of evolution as -due _solely_ to internal or _solely_ to external factors. The process -may thus always be compared to the growth of a plant, which likewise -depends on both internal and external influences. But that is saying -very little; we have still to show how much and how little is effected -by these internal and external factors, what their nature precisely -is, and what relation they bear to one another. There is thus a great -difference between believing, with Nägeli, that 'the animal and plant -kingdoms must have become very much what they actually are, even -had there been on the earth no adaptation to new conditions and no -competition in the struggle for existence,' and sharply emphasizing, in -accordance with the facts just discussed, that, in any case, a whole -order of mammals--the Cetaceans--could never have arisen at all if -there had been no adaptation. - -The same thing could be proved in regard to the class of Birds, for -in them too we are able to recognize so many adaptive features, that -we may say everything about them that makes them birds depends upon -adaptation to aerial life, from the articulations of the backbone -to the structure of the skull and the existence of a bill; from the -transformation of the fore-limbs into wings, and of the hind-limbs -to very original organs of locomotion on land or swimming organs in -water, to the structure of the bones, the position, size, and number -of the internal organs, down even to the microscopic structure of -numerous tissues and parts. What could be more characteristic of a -class of animals than feathers are of birds? They alone are enough -to distinguish the class from all other living classes; an animal -with feathers can now be nothing but a bird, and yet the feather is -a skin-structure which has arisen through adaptation, a reptilian -scale which has been so transformed that an organ of flight could -develop from its anterior extremity. We find it thus even in the two -impressions of the primitive bird _Archcæopteryx_, which have been -preserved for us in the Solenhofen slate since the Jurassic period in -the history of the earth. And into what detail does adaptation go in -the case of the feathers! Is not the whole structure, with its quill, -shaft, and vane, precisely adapted to its function, although that is -purely passive? What I have just said of the whole class of Birds holds -true for this individual structure, the feather; everything about it -is adaptation, and indeed illustrates adaptation in two directions, -for in the first place the feathers, by spreading a broad, light, -and yet resistant surface with which to beat the air, act as organs -of flight, while they are also the most effective warmth-retaining -covering conceivable. In both these directions their achievements -border on the marvellous. I need only recall the most recent discovery -in this domain, the proof recently given by the Viennese physiologist, -Sigmund Exner, that the feathers become positively electric in their -superficial layer, and negatively electric in their deeper layer, -whenever they rub against one another and strike the air. But they -are rubbed whenever the bird flies or moves, and the consequence -of the contrast in the electric charging of the two layers is that -the covering feathers are closely apposed over the down-feathers, -while, on the other hand, the similar charging of the down-feathers -makes them mutually repel each other, with the result that a layer -of air is retained between them, and thus there is between the skin -and the covering feathers a loose thicket of feathers uniformly -penetrated by air--the most effective warmth-preserver imaginable. -The electric characters of the feathers--and the same is true of the -hairs of animals--are thus not indifferent characters, but with an -appreciable biological importance, and the same is true of the almost -microscopical series of little hooklets which attach the barbs of the -covering feathers to one another, and thus form a relatively firm but -exceedingly light wing-surface which offers a strong resistance to -the air. But as we must regard these hooklets as adaptations, so must -we also regard the electrical characters of the feathers, and we must -think of them as having arisen through natural selection, as Exner -himself has insisted. - -If we are able to recognize all the more prominent features of -the organization in Cetaceans and Birds as due to adaptation, we -must conclude that, in the rest of the great groups of the animal -kingdom, the main and essential parts of the structure are adaptations -to the conditions of life, even although the relations between -external circumstances and internal organization are not so readily -recognizable. For if there were an internal evolutionary force at all, -we should be able to recognize its operation in the origin of the races -of Cetaceans and Birds; but if there be no such power, then even in -cases where the conditions of life are not so conspicuously divergent -as in Cetaceans and Birds, we must refer the typical structure of the -group to adaptation. Thus everything about organisms depends upon -adaptation, not only the main features of the organization, but the -little details in as far as they possess selection-value; it is only -what lies below this level that is determined by internal factors -alone, by germinal selection; but this is not an imperative force in -the sense in which the term is used by Nägeli and his successors, for -it is capable of being guided; it does not necessarily lead to an -invariable and predetermined goal, but it can be directed according to -circumstances into many different paths. But it is precisely this that -constitutes the main problem of the evolution theory--how development -due to internal causes can, at the same time, bring about adaptation to -external circumstances. - -This lecture had been transcribed so far, and was ready for the -press, when I received the first volume of a new work by De Vries, in -which that distinguished botanist develops new views in regard to the -transformation of species, based upon numerous experiments, carried on -for many years on the variation of plants. As not only his views, but -the interesting facts he sets forth, seem to contradict the conclusions -as to the transmutation of organisms which I have been endeavouring to -establish, I cannot refrain from saying something on the subject. - -De Vries does not believe that the transformation of species can -depend on the cumulative summation of minute 'individual' variations; -he distinguishes between 'variations' and 'mutations,' and attributes -only to the latter the power of changing the character of a species. -He regards the former as mere fluctuating deviations which may be -increased by artificial selection, and may even, with difficulty, -if carefully and purely bred for a long period, be made use of to -give character to a new breed, but which play no part at all in the -natural course of phylogeny. As regards phylogeny, he maintains that -only 'mutations' have any influence, that is, the larger or smaller -saltatory variations which crop up suddenly and which have from the -very first a tendency to be purely transmitted, that is, to breed true. - -The facts upon which these views are mainly based are observations -on and breeding experiments with a species of evening primrose -(_Œnothera_) which was found in quantities on a fallow potato-field -at Hilversum in Holland. It had been cultivated previously in a -neighbouring garden, and had sown itself thence in the field. The -numerous specimens of this _Œnothera lamarckiana_ growing there were -in a state of marked 'fluctuating' variability, but in addition there -grew among them two strongly divergent forms which must have arisen -from the others, and which led De Vries to bring the parent stock -under cultivation, in the hope that it would yield new forms, in the -Botanical Gardens at Amsterdam. This hope was fulfilled; in the second -cultivated generation there were, among the 15,000 plants, ten which -represented two divergent forms, and in the succeeding generations -these forms were repeated several times and in many cases, and five -other new forms cropped up, most of them in several specimens and in -different generations of the original stock. All these new forms, which -De Vries calls 'elementary species,' breed true, that is to say, when -they are fertilized with their own pollen they yield seed which gives -rise to the same 'elementary species.' The differences between the -new forms are usually manifold, and of the same kind as those between -the 'elementary' species of the wild Linnæan species. But, according -to De Vries, what we have been accustomed since the time of Linné to -call a 'species' is a collective category, whose components are these -'elementary' species which De Vries has observed in his experiments -with _Œnothera_. In other species, such as _Viola tricolor_ and _Draba -verna_, true-breeding varieties have long been known to botanists, and -these have been studied carefully and tested experimentally, especially -by A. Jordan, and more recently by De Bary. - -All 'species,' according to the Linnæan conception, consist, De Vries -maintains, of a larger or smaller number (in _Draba_ there are two -hundred) of these 'elementary' species, and these arise, as is proved -by the case of _Œnothera_, by saltatory or discontinuous 'variations' -which occur periodically and suddenly break up a species into many -new species, because the variations of the germ-plasm, which are for -a time merely latent, suddenly find expression in the descendants of -one individual or another. According to this view, species must be -the outcome of purely internal causes of development, which reveal -themselves as 'mutations,' that is as saltatory variations, which are -stable and transmissible from the very first, and among which the -struggle for existence decides which shall survive and which shall -be eliminated. For the mutations themselves occur in no particular -direction; they are sometimes advantageous, sometimes indifferent, -sometimes even injurious (for instance, when one sex is left out), and -so it is always only a fraction of the mutations, often only a few, -which prove themselves capable of permanent existence. Thus 'species do -not arise through the struggle for existence, but they are eliminated -by it' (p. 150); natural selection does nothing more than weed out -what is unfit for existence, it does not exercise any selective, in -the sense of directive, influence on the survivors. A difference in -the nature of variations was previously maintained by the American -palæontologist Scott, though for different reasons and also with a -different meaning. He believed that variations in a definite direction -were necessary to explain the direct course of development which many -animal groups, such as the horses and the ruminants, have actually -followed, and which he thought could not be ascribed to cumulative -adaptation to the conditions of life. The 'mutations' of De Vries are -not distinguished from the 'fluctuating' variations by following a -definite direction, but in that they are strictly heritable, that they -'breed true.' It is true that 'fluctuating' individual differences are -also transmissible, and can be increased by artificial selection, but -they lack one thing that would make them component parts of a natural -species, namely, constancy; they do not breed true, and are therefore -never independent of selection, but require to be continually selected -out afresh in order that they may be kept pure. They form 'breeds,' not -species, and if left to themselves they soon revert to the characters -of the parent species, as is well known of the numerous 'ennobled -races' among our cereals. De Vries therefore denies absolutely that a -new species could be developed by natural selection from 'fluctuating' -variations, and not alone because there is no constancy of character, -but also because the capacity of the character for being increased is -very limited. Usually nothing more can be achieved than doubling of -the original character, and then progress becomes more difficult and -finally ceases altogether. - -These are incisive conclusions, based upon an imposing array of weighty -facts. I readily admit that I have rarely read a scientific book -with as much interest as De Vries's _Mutationstheorie_. Nevertheless -I believe that one might be carried away too far by De Vries, for -he obviously overestimates the value of his facts, interesting and -important as these undoubtedly are, and under the influence of what -is new he overlooks what lies before him--the other aspect of the -transmutation of species, to which the attention of most observers -since Darwin and Wallace has been almost exclusively devoted--I mean -the origin of adaptations. Not that he does not mention these, he -assumes in regard to his mutations 'a selection working in a constant -direction,' and seeks to interpret them in terms of it, but as the -mutations occur from purely internal reasons--I mean without any -connexion with the necessity for a new adaptation--and occur only -in a small percentage of individuals, and in no definite direction, -they cannot possibly suffice to explain _adaptation_, which seems to -dominate the whole organic world. But this is precisely the point at -which many botanists cease to understand the zoologists, because among -plants there are fewer adaptations than among animals; or, in any -case, adaptations in plants are not so readily demonstrated as among -animals, which not infrequently seem to us to be entirely built up of -adaptations. - -In this book, and in this chapter itself, I have discussed adaptations -and their origin so much already that I need only refer to these pages -for convincing evidence that we cannot think of them as being brought -about by the accumulation and augmentation of individually occurring -saltatory 'mutations.' Not even if we assume that the leaps of mutation -can be increased in the course of generations; in short, even if we -say that mutations are all those variations which breed true and lead -to the development of species, while variations are those which do -not. This would only be playing with words, so let us say that the -fluctuating variations are really different in their nature, that is, -in their causes, from mutations. De Vries lays great stress on the -fact that these two kinds of variations must be sharply distinguished -from one another, and this may have been useful or necessary for the -first investigation of the facts before him, for we must first analyse -and then recombine, but that variations and mutations are in reality -different in nature can assuredly not be assumed, since innumerable -adaptations can only have arisen through the augmentation of individual -variations. These must therefore be able to become 'pure breeding,' -even although they may not have done so in the cases of artificial -selection which have hitherto been observed. How is it possible that -chance mutations, in no particular direction, occurring only rarely and -in a small percentage of individuals, can explain the origin of the -leaf-marking of a _Kallima_ or an _Anæa_--the shifting of the original -wing-nervures to form leaf-veins, and the exact correlation of these -veins across the surfaces of both pairs of wings? And even if we were -to admit that a mutation might have occurred which caused the veins of -the anterior and posterior wings to meet exactly by chance, that would -still not be a leaf-adaptation, for there would still be wanting the -instinct which compels the butterfly, when it settles down, to hold -the wings in such a position that the two pictures on the anterior -and posterior wings fit into each other. Correlated mutations of the -nervous system suited to this end are required, but that is too much to -attribute to happy chance! The same holds true in regard to the whole -leaf-picture on the two wings, for it could not possibly have arisen as -a whole by a sudden mutation. The whole litany of objections which have -been urged throughout several decades against the Darwin-Wallace theory -of natural selection, which were based on the improbability that chance -variations not in a definite direction should yield suitable material -for the necessary adaptations, may be urged much more strongly against -mutations, which make their appearance in much smaller numbers and with -less diversity. - -But it is--as we have already seen--in regard to the necessity which -exists almost everywhere for the co-adaptation of numerous variations -of the most different parts, that the 'mutation theory' breaks down -utterly. The kaleidoscopic picture, the mutation, is implicit from -the first, and must be accepted or rejected just as it is in the -struggle for existence; but harmonious adaptation requires a gradual, -simultaneous, or successive purposive variation of all the parts -concerned, and this can be secured only through the fluctuating -variations which are always occurring, and are increased by germinal -selection and guided by personal selection. - -Many naturalists, and especially many botanists, regard adaptation -as something secondary, something given to species by the way, to -improve the conditions of their existence, but not affecting their -nature--comparable perhaps to the clothing worn by man to protect -himself from cold; but that is hardly the real state of the matter. - -The deep-sea expedition conducted by Chun in 1898 and 1899 made many -interesting discoveries in regard to animals living in the depths of -the ocean, all of which exhibit peculiar adaptations to the special -conditions of their life, and especially to the darkness of the great -depths. One of the most striking of these discoveries was that of -the luminous organs which are found not in all but in a great many -animals living on the bottom of the abyssal area, and also among the -animals occurring at various levels above the floor of the abyss. -These are sometimes glands which secrete a luminous substance, but -sometimes complex organs, 'lanterns' which are controlled by the will -of the animal, and suddenly evolve a beam of light and project it in -a particular direction, like an electric searchlight. These organs -have a most complex structure, composed of nerves and lenses, which -focus the light, and on the whole are not unlike eyes. That this sort -of structure should have arisen all at once through a 'mutation' is -inconceivable; it can have originated only from simple beginnings by a -gradual increase of its structure along with continual strict selection -among the variations which cropped up. They all depend upon complicated -'harmonious' adaptation, and cannot possibly have been derived from -mutations, that is, from ready-made structural 'constellations,' unless -we are to call in the aid of the miraculous. But lanterns of this kind -are found in many different kinds of animals--in Schizopod Crustaceans, -in shrimps, in fishes of different genera and families. Many fishes -have long rows of luminous organs on the sides and on the belly, and -these probably serve to light up the sea-floor and facilitate the -finding of food; in others the luminous organs are placed upon the -snout just above the wide voracious mouth, and in that position they -have undoubtedly the significance attributed to them by Chun, namely, -that they attract small animals, just as the electric lamps allure all -sorts of nocturnal animals, and especially insects, in large numbers -to their destruction. But not fishes only, but molluscs, e.g. the -Cephalopods of the great depths, have developed luminous organs, and -one species of Cephalopod has about twenty large luminous organs, like -gleaming jewels, ultramarine, ruby-red, sky-blue and silvery, while -in another the whole surface of the belly is dotted over with little -pearl-like luminous organs. Even if we cannot be quite clear as to the -special use of these lanterns of deep-sea animals, there can be no -doubt that they are adaptations to the darkness of the great depths, -and when we find the _same_ adaptations (in a physiological sense) -in many animals belonging to the most diverse groups, there is no -possibility of referring them to sudden mutations which have arisen -all at once in these groups with no relation to utility, and yet have -not occurred in any animals living in the light. Only 'variations' -progressing and combining in the direction of utility can give us the -key to an explanation of the origin of such structures. - -The same is true of the eyes of deep-sea animals. It was believed at -one time that all the inhabitants of dark regions had lost their eyes. -This is the case with many cave animals and the inhabitants of the -lightless depths of our lakes, but in the abyssal zone of the sea it -is only some fishes and Crustaceans whose eyes have degenerated to -the vanishing point. Moreover, the disappearance apparently occurs in -species which are restricted to the ocean-floor in their search for -food, which therefore can make more use of their tactile organs than of -their eyes, for while the ocean-floor undoubtedly contains over wide -areas an abundance of food for these mud-eaters, it is only partly -illuminated, that is, only in places where there are luminous animals -such as polyp-colonies, &c. The fact that so many of the animals of the -great depths are luminous obviously conditions, not that most of the -immigrants into the abyssal zone should lose their eyes as useless, -but that they should adapt them to the light which is very weak in -comparison with that of the superficial layers. The eyes of deep-sea -fishes, for instance, are either enormously large, and therefore suited -for perceiving the faint light of the depths, or they have varied in -another and very characteristic manner: they have become elongated -into a cylinder, which projects far beyond the level of the head. It -looks almost as if the animals were looking through an opera-glass, and -Chun has called these eyes 'telescope-eyes.' A. Brauer has recently -shown what far-reaching variations of the original eye of fishes were -necessary in order to transform it into an organ for seeing in the -dark. These variations, however, have occurred in the eyes of the most -diverse animals in the deep sea, and not only do different families -of deep-sea fishes possess 'telescope-eyes,' but Crustaceans and -Cephalopods as well. Even our owls possess quite a similar structure, -although it does not project beyond the head in the same way. Here -again we have to deal with the phenomenon which Oscar Schmidt in -his time called _convergence_, that is, corresponding adaptations -to similar conditions in animal forms not genealogically connected -with one another. These telescope-eyes are not all descended from -one species which chanced in one of the 'mutation-periods' suddenly -to produce this combination of harmonious adaptations, but they have -risen independently through variation progressing step by step in -the direction of the required end, that is to say, through natural -selection based upon germinal selection. Only thus can their origin be -understood. - -But what is time of eyes adapted to darkness is true in some measure -of all eyes, for the eyes of animals are not mere decorative points -which might be present or absent; they cannot have arisen in any -animal whatever through sudden mutation--they have been laboriously -acquired with difficulty, by the slow increase of gradually perfecting -adaptations; they are parts which bear the most precise internal -correlation with the whole organization of the animal, and which can -only cease to exist when they become superfluous. Thus the origin of -eyes seems to me only conceivable on the basis of germinal selection -controlled towards what is purposeful by natural selection, that is to -say, on a basis of fluctuating variation, and not through chance. - -This is the case with all adaptations. Just as the eyes of animals -are adaptations which utilize the light-waves in the interest of the -organism and its survival, the same is true of all the sense-organs, -tactile organs, smelling and tracking organs, organs of hearing, and so -on. The animal cannot do without these; first the lower sense-organs -arose and then the higher; the increasingly high organization of the -animal conditioned this, and a multicellular animal without sensory -structures is inconceivable. The same may be said of the nervous system -as a whole, whose function it is to translate into action the stimuli -received through the sense-organs, whether directly or by means of -intervening nerve-cells, which form central organs of ever-increasing -complexity of composition. As telescope-eyes have evolved in some -groups of deep-sea animals, independently of one another, and certainly -not through the fortuitous occurrence of a mutation, but under the -compulsion of necessity in competition, so all the organs we have just -named, the whole nervous system with all its sense-organs, must have -arisen through the same factors of evolution in numerous independent -genealogical lines. And it must not be supposed that this is all; what -is true of the sense-organs--that they are necessities--is undoubtedly -true also of all parts and organs of the animal body, both as a whole -and in every detail. It cannot be demonstrated in all cases, but it -is nevertheless certain that this applies also to all the organs of -movement, digestion, and reproduction, to all animal groups and also -to the differences between them, even although these may not always be -obvious adaptations to the conditions of life. What part is left for -mutation to play if almost everything is an adaptation? Possibly the -specific differences; and these in point of fact cannot in many cases -be interpreted with certainty as adaptations, though this can hardly -be taken as a proof that they are not. Possibly also the geometrical -skeletons of many unicellulars, in which again we cannot recognize any -definite relation to the mode of life. It is easy enough to conceive -of the wondrously regular and often very complex siliceous skeleton -of the Radiolarians or Diatoms as due to saltatory mutations, and -'leaps' of considerable magnitude must certainly have been necessary -to produce some of the manifold transformations here as everywhere -else. But whether these are or are not without importance for the -life of the organisms, we are in the meantime quite unable to decide. -Here too it is well to be cautious in concluding that these organic -'crystallizations' are without importance, and therefore to infer that -they have arisen suddenly from purely internal causes. One of the -experts on Diatoms, F. Schütt, has shown us that differences in length -in the skeletal process of the Peridineæ have a definite relation to -their power of floating in the sea-water, that the long skeletal arms -or horns which these microscopic vegetable organisms extend into the -surrounding water form a float-apparatus, for their friction against -the particles of the water prevents sinking and enables them to -float for a considerable time at approximately the same level. These -skeletal forms are thus adaptations, and Chun has recently been able to -corroborate the conclusion that this adaptation is exactly regulated, -for the length of these horns varies with the specific gravity of -the different ocean-currents, species with 'monstrously long' horns -occurring, for instance, in the Gulf of Guinea, which is distinguished -by its low salinity and high temperature (Fig. 131, _A_), while in -the equatorial currents with higher salinity and cooler water, and -thus a higher specific gravity, there is a predominance of species -of Peridineæ with 'very short' processes and relatively undeveloped -float-apparatus (Fig. 131, _B_). It could be seen clearly in the course -of the voyage that the long-armed Peridineæ became more abundant as the -ship passed from the North Equatorial current into the Gulf of Guinea, -and that by and by they held the field altogether, but later, when the -'Valdivia' entered into the South Equatorial current, they disappeared -'all at once.' Thus in this case, in which the veil over the relations -between form and function in unicellular organisms has been lifted a -little, we recognize that the smallest parts of the cell-body obey -the laws of adaptation, and consistent thinking must lead us to the -conviction that even in the most lowly organisms the whole structure in -all its essential features depends upon adaptation. - -[Illustration: FIG. 131. Peridineæ: species of _Ceratium_. _A_, from -the Gulf of Guinea. _B_, from the South Equatorial currents. After -Chun.] - -If the horns of the Peridineæ grow to twelve times the usual length -in adaptation to life in sea-water with a salinity increased to the -extent of .002 per cent., then undoubtedly not only the protoplasmic -particles of the body which form the horns, but all the rest as well, -may be capable of adaptation; and if the _Peridinium_ protoplasm has -this power of adapting itself to the external conditions, then the -capacity for adaptation must be a general character of all unicellular -organisms, or rather of all living substance. As will be seen later -on, we shall be brought to the same conclusion by different lines of -evidence. But a recognition of this must greatly restrict the sphere of -operation which we can attribute to saltatory mutations in the sense -in which the term is used by De Vries, for adaptations from their very -nature cannot arise suddenly, but must originate gradually and step by -step, from 'variations' which combine with one another in a definite -direction under the influence of the indirect, that is, selective -influence of the conditions. - -According to the theory of De Vries it seems as if 'variations,' -augmented by selection, could never become constant, and that even the -degree to which they can be augmented is very limited. As far as this -last point is concerned, De Vries seems to me to overlook the fact -that every increase in a character must have limits set by the harmony -of the parts, which cannot be exceeded unless other parts are being -varied at the same time. Artificial selection, in fact, in many cases -reaches a limit which it cannot pass, because it has no control over -the unknown other parts which ought to be varied, in order that the -character desired may be increased still further. Natural selection -would in many cases be able to accomplish this, provided that the -variation is useful. But of what use is it to the beetroot when its -sugar-content is doubled, or to the Anderbeck oats to be highly prized -by man? And yet many individual characters have been very considerably -increased in domesticated animals by selection: of these we need only -call to mind the Japanese cock with tail-feathers twelve feet long. - -But undoubtedly these artificial variations do not usually 'breed true' -in the sense that De Vries's mutations of _Œnothera lamarckiana_ did, -that is to say, they only transmit their characters in purity with the -continual co-operation of artificial selection. This at least appears -to be the case, according to De Vries, in the ennobled cereal races, -which, if cultivated in quantities, rapidly degenerate. In many animal -breeds, however, this is not the case to the same degree; many, indeed -the majority, of the most distinct races of pigeon breed true, and only -degenerate when they are crossed with others. - -De Vries regards it as a mistake to believe that artificial selection, -persevered in for a long time, will succeed in producing a breed -which will--as he expresses it--be independent of further selection -and will maintain itself in purity. Experience cannot decide this, as -we have not command over the unlimited time necessary for selection, -but theoretically it is quite intelligible that a variation which had -arisen through selection would be more apt to breed true the longer -selection was practised, and there is nothing to prevent it becoming -ultimately quite as constant as a natural species. For, at the -beginning of breeding, we must assume that the variation is contained -in only a small number of ids; as the number of generations mounts up, -more and more numerous ids with this variation will go to make up the -germ-plasm, and the more the breed-ids preponderate the less likelihood -will there be that a reversion to the parent-form will be brought about -by the chances of reducing-division and amphimixis. That most if not -all breeds of pigeon still contain ids of the ancestral form in the -germ-plasm, although probably only a small number of them, we see from -the occasional reversion to the rock-dove which occurs when species -are repeatedly crossed, but that ancestral ids may also be contained -in the germ-plasm of long-established natural species is shown by the -occurrence of zebra-striping in horse-hybrids. We can understand why -these ancestral ids should not have been removed long ago from the -germ-plasm by natural selection, since they are not injurious and may -remain, so to speak, undetected. It is only when they have an injurious -effect by endangering the purity of the new species-type that they can -and must be eliminated by natural selection, and this does not cease -to operate, as the human breeder does, but continues without pause or -break. - -I therefore regard it as a mistake on the part of De Vries to exclude -fluctuating variation from a share in the transformation of organisms. -Indeed, I believe that it plays the largest part, because adaptations -cannot arise from mutations, or can only do so exceptionally, and -because whole families, orders, and even classes are based on -adaptations, especially as regards their chief characters. I need only -recall the various families of parasitic Crustaceans, the Cetaceans, -the birds, and the bats. None of these groups can have arisen through -saltatory, perhaps even retrogressive, 'mutation': they can only have -arisen through variation in a definite direction, which we can think of -only as due to the selection of the fluctuations of the determinants of -the germ-plasm which are continually presenting themselves. - -The difference between 'fluctuating' variability and 'mutation' seems -to me to lie in this: that the former has always its basis only in -a small majority of the ids of the germ-plasm, while the mutation -must be present in most of the ids if it is to be stably transmitted -from the very first. How that comes about we cannot tell, but we -may suppose that similar influences causing variation within the -germ-plasm may bring about variation of many ids in the same direction. -I need only recall what I have already said as to the origin of -saltatory variations, such as the copper-beech and similar cases. The -experiments made by De Vries seem to me to give a weighty support to -my interpretation of these phenomena. De Vries himself distinguishes a -'pre-mutation period,' just as I have assumed that the variations which -spring suddenly into expression have been in course of preparation -within the germ-plasm by means of germinal selection for a long time -beforehand. At first perhaps only in a few ids, but afterwards in -many, a new state of equilibrium of the determinant-system would be -established, which would remain invisible until the chance of reducing -division and amphimixis gave predominance to a decided majority of the -'mutation-ids.' In the experiments made by De Vries the same seven new -'species' were produced repeatedly and independently of one another in -different generations of _Œnothera lamarckiana_, and we thus see that -the same constellations (states of equilibrium) had developed in many -specimens of the parent plant, and that it depended on the proportion -in which the ids containing these were represented in the seed whether -one or another of the new 'species' was produced. - -My interpretation, according to which a larger or smaller number of -ids were the bearers of the new forms, receives further support from -the experiments, for the new species did not always breed true. Thus -De Vries found one species, _Œnothera scintillans_, which only yielded -35-40 per cent. of heirs, or in another group about 70 per cent.; the -other descendants belonged to the forms _lamarckiana_ or _oblonga_, but -the number of pure heirs could be increased by selection! - -I cannot devote sufficient space to go fully into these very -interesting experiments; but one point must still be referred to: -the parent form, _Œnothera lamarckiana_, was very variable from -the beginning, that is, it exhibited a high degree of fluctuating -variability. This tells in favour, on the one hand, of a deep-rooted -connexion between 'variation' and 'mutation,' and, on the other hand, -it indicates that saltatory variation may be excited by transference -to changed conditions of life--as Darwin in his day supposed, and as -I have endeavoured to show in the foregoing discussion. De Vries -assumes mutation-periods, I believe rightly; but they are not periods -prescribed, so to speak, from within, as those who believe in a -'phyletic force' must suppose; they are caused by the influences of the -environment which affect the nutritive stream within the germ-plasm, -and which, increasing latently, bring about in part mere variability, -in part mutations, just as I have indicated in the section on Isolation -(vol. ii. p. 280), and indeed, in one of my earliest contributions to -the theory of descent[30]. In that essay I suggested the conclusion -that periods of constancy alternate with periods of variability, -basing my opinion on general considerations, and on Hilgendorf's study -of the Steinheim snail-shells. According to De Vries's _Œnothera_ -experiments we may assume that periods of increased variability may -lead to the marked variations sometimes affecting several characters -simultaneously, and occurring in many ids, which have hitherto been -called 'saltatory' variations, and which we should perhaps do well to -call in future, with De Vries, _mutations_. - -[30] _Ueber den Einfluss der Isolirung auf die Artbildung_, Leipzig, -1872, p. 51. - -We cannot yet determine how far the influence of such mutations -reaches. I think it is plain that De Vries himself overestimated it, -but how many of the species-types which we find to-day depend upon -mere mutation can only be decided with any certainty after further -investigations. For the present it is well to be clear as to the -validity of the general conclusion, that all 'complex,' and especially -all 'harmonious,' adaptation, must depend, not upon 'mutation,' but -only upon 'variation' guided by selection. As species are essentially -complexes of adaptation, originating from a basis of previous complexes -of adaptation, there remains, as far as I can see, only the small field -of indifferent characters to be determined by mutations, unless indeed -we are to include under the term 'mutation' all variations which gain -stability, but this would be merely a play upon words. In my opinion -_there is no definable boundary line between variation and mutation, -and the difference between these two phenomena depends solely on -the number--larger or smaller--of ids which have varied in the same -direction_. - - - - -LECTURE XXXIV - -ORIGIN OF THE SPECIFIC TYPE (_continued_) - - Illustration of phyletic evolution by an analogy--Reconciliation - of Nägeli and Darwin--Unity of the specific type furthered by - climatic variation--By natural selection: illustration from aquatic - animals--Direct path of evolution--Natural selection works in - association with amphigony--Influence of isolation in defining the - specific type--Duration of the periods of constancy--The Siberian - pine-jays--Species are, so to speak, 'variable crystals'--Gradual - increase of constancy and decrease of reversions--Physiological - segregation of species through mutual sterility--Romanes's - physiological selection--Breeds of domestic animals mutually fertile, - presumably therefore 'amiktic' species also--Mutual fertility - in plant species--Mutual sterility certainly not a condition of - the splitting up of species--Splitting up of species without - amphigony--Lichens--Splitting up of species apart from isolation and - mutual sterility, _Lepus variabilis_. - - -Our train of thought in the last lecture brought us back again to the -so-called 'indifferent' characters, whose occurrence is so often used -as evidence against natural selection, as a proof that evolution is -guided essentially by internal forces alone. But this objection was -based on a fallacy, for the fact that the first small variations are -due to internal processes in the germ-plasm does not imply that the -whole further course of evolution is determined by these alone, any -more than the fact that a sleigh requires a push to start it on its -descent down an inclined plane would imply that its rapid descent is -due only to the force of the push, and not at the same time to the -attraction of gravity. But the analogy is not quite sound, for the -processes within the germ-plasm which condition and direct variation -do not merely give them the first shove off; they are associated with -every onward step in the evolutionary path of the species, they impel -it further and further, and without these continual impulses the -progressive movement would cease altogether. We have seen that, for -internal reasons, germinal selection continues to impel the varying -determinants further along the path on which they have started, and -thus gives them cumulative strength, and that it is in this way that -adaptations to the conditions of life are brought about. The evolution -of the character of a species may thus be compared to the course of -a sleigh upon a level snow-surface which it could traverse in any -direction, but it is moved only by the impulses received through -germinal selection. The conditions of life to which the varying parts -have to adapt themselves may be thought of as the distant goal, and -the processes in the germ-plasm which give rise to variation in a -definite direction may be compared to numerous human beings scattered -irregularly over the surface of the snow. If the sleigh receives from -one of these a push which chances to be in the direction of the goal, -it rushes on towards this and ultimately reaches it if the person -pushing continues to push in the same direction. So far, then, it -seems as if the transformation of the part concerned depended upon -germinal selection alone, but we must not forget that the germ-plasm -does not contain only one determinant for every part of the body, but -as many determinants as there are ids. We must therefore increase the -number of our sleighs, and now it is obvious that the pushers of the -sleighs, that is, germinal selection, may push one sleigh on toward -the goal, but others in the opposite or in any other direction. If we -assume that all the sleighs which have taken a wrong direction must -reach dangerous ground, and ultimately plunge into an abyss, but that -from a neighbouring point sleighs were being dispatched to replace all -that came to grief, that these in their turn might attempt to reach -the goal, it would ultimately come about that the requisite number -of sleighs would arrive at the goal--that is to say, that the new -adaptation would be attained. - -The abysses represent the elimination of the less favourable -variational tendencies, and the constant replacing of sleighs -represents the intermingling of fresh ids through amphimixis. If all -the sleighs run in the wrong direction they all come to grief, that -is, the individual concerned is eliminated with all the ids of its -germ-plasm--it disappears altogether from the ranks of the species. But -if only a portion of them run in the right direction, care is taken -that in following generations, that is, in the continuation of the -sleigh-race, this portion combines with those of another group which -are also running in the right direction--that is, with the half number -of ids from another germ-plasm in amphimixis. - -It is not possible to follow the analogy further, but perhaps it may -serve to illustrate how germinal selection may be the only impelling -force in the organisms, and yet only a small part of its results -are determined by itself, and by far the larger part by external -conditions. We understand how a variation in a definite direction can -exist, and yet it is not that which creates species, genera, orders, -and classes; it is the selection and combination of the variational -tendencies by the conditions of life, which occurs at every step. -There was no variational tendency leading from terrestrial mammals to -Cetaceans, but there was a variational tendency moving the nostrils -upwards towards the forehead, the hind-limbs towards diminution, -the lungs towards lengthening, the tail towards broadening out. But -each of these variational tendencies was always only one of several -possibilities, and that the particular path which led towards the -'goal' was followed was due to the fact that the others plunged into -the abyss to which the wrong paths led, that is to say, they were -weeded out by selection. Thus germinal selection offers a possibility -of reconciliation between Nägeli's and Darwin's interpretations, which -seem so directly contradictory; for the former referred everything to -the hypothesis of an internal evolutionary force, the latter rejected -this, and regarded natural selection as the main, if not the exclusive -factor in evolution. The internal struggles for food, which we have -assumed as occurring in the germ-plasm, represent an internal force, -though not in the sense of Nägeli, who thought of determining influence -operative from first to last, but still an impelling force, which -determines the direction of variation for the individual determinants, -and must therefore do the same for the whole evolution up to a certain -point; for it is only the _possible_ variations of the determinants in -a germ-plasm which can be chosen, selected, combined, and increased by -natural selection, and every germ-plasm cannot give rise to all sorts -of variations; the determinants contained in it condition what is -possible and what is not, and this is an important limitation to the -efficacy of natural selection, and to a certain extent also implies a -guiding and determining power on the part of the internal mainspring, -to wit, _germinal selection_. - -The essential difference between Darwin's view of the transformation -of forms and my own lies in the fact that Darwin conceived of natural -selection as working only with variations which are not only due to -chance themselves, but the intensification of which also depends in -its turn solely upon natural selection, while, according to my view, -natural selection works with variational tendencies which become -intensified through internal causes, and are simply accumulated by -natural selection in an ever-growing majority of ids in a germ-plasm -through the selection of individuals. - -This affects our view of the establishment of a specific type in so -far that my intra-germinal variational tendencies are not necessarily, -and not always due to chance, though they are so in most cases. If -certain determinants are impelled to vary in a particular direction -through climatic or any other influences, as we have seen to be the -case, for instance, with the climatic varieties of many Lepidoptera, -then the corresponding determinants in all the individuals must vary in -the same direction, and thus all the individuals of the species which -are subject to the same influence must undergo the same variation. -Transformations of this kind have exactly the appearance of resulting -from 'an internal evolutionary force,' such as Nägeli assumed, and the -unity of the specific type will not be disturbed by them. - -Nor will this occur, as far as I can see, if the transformation of -a species depends solely upon new adaptations and their internal -consequences, for if a particular organism has to adapt itself to -special new conditions, it will usually be able to do so only in _one_ -way, and thus natural selection will always allow _the same_ suitable -variational tendencies to survive and reproduce, so that the unity -of the specific type will not be permanently disturbed in this way -either. The more advantageous the new conditions of life prove, and the -more diverse the ways in which they can be utilized, the more rapidly -will the species first adapted to them multiply, and the more will -their descendants be impelled to adapt themselves specially to the -_different_ possibilities of utilizing the new situation, and thus, -from a parent species adapted in general to the new conditions, there -arise forms adapted to its more detailed possibilities. I must refer -again to the previous instance of the Cetaceans which originated from -vegetarian littoral, or fluviatile mammals, and have evolved since the -Triassic period into a very considerable number of species-groups. -All are alike in their _general_ adaptation, and these adaptations to -the conditions of life of aquatic animals--the fish-like form, the -flippers, the peculiarities of the respiratory organs and the organs of -hearing--when once acquired would not and could not be lost again; but -each of the modern groups of whales has its particular sphere of life, -which it effectively exploits by means of subordinate adaptations. -Thus there are the dolphins with their bill-like jaws and the two -rows of conical teeth, their active temperament, rapid movements, and -diet of fish; and the whalebone whales with their enormous gape, the -sieve-apparatus of whalebone-plates, and a diet of small molluscs and -the like. But each of these groups has split up into species, and if we -again regard the principle of adaptation as determining and directing -evolution, we are no nearer being able to prove the assumption in -regard to individual cases, for we know too little about the conditions -of life to be able to demonstrate that the peculiarities of structure -are actually adaptations to these. Theoretically, however, it is quite -easy to suppose that adaptation to a particular sphere of life was -the guiding factor in their evolution, and if this be so--as we have -already proved that it is in regard to the two chief groups and the -whole class--then the harmony of the structure must be due solely to -the continued selection of the fittest. We require no further principle -of explanation for the establishment of a specific type. - -This 'type' is thus not reached by any indefinite varying of the parent -form in all directions, but in general it is reached by the most direct -and shortest way. The parent form must indeed have become to some -extent fluctuating, since not only the variational tendencies 'aiming -at the goal,' but others as well, must have emerged in the germ-plasm; -but gradually these others would occur less and less frequently, being -always weeded out afresh by selection, until the great majority of the -individuals would follow the same path of evolution, under the guidance -of germinal selection, which continues to work in the direction that -has once been taken. After a short period of variation, which need not, -of course, involve the whole organism, but may refer only to certain -parts of it, a steady direct progress in the direction of the 'goal,' -that is, of perfect adaptation, will set in, as we have seen in the -case of the _Planorbis_ snails of Steinheim. - -We must not forget, however, that natural selection works essentially -upon a basis of sexual reproduction, which with its reduction of the -ids and its continually repeated mingling of germ-plasms, combines the -existing variational tendencies, and thus diffuses them more and more -uniformly among the individuals of a whole area of occupation. Sexual -reproduction, continual intermingling of the individuals selected -for breeding, is thus a very effective and important, if not an -indispensable, factor in the evolution of the specific type. - -But it is not only in the case of species transformations due to -new adaptations that sexual mingling operates; it does so also in -the case of variations due to purely intra-germinal causes. We have -already seen in discussing Isolation that isolated colonies may come -to have a peculiar character somewhat different from that of the -parent form, because they were dominated by some germinal variational -tendency which occurred only rarely in the home of the parent form, and -therefore never found expression there. On the isolated area this would -indeed be mingled with the rest of the existing germinal variational -tendencies, but the result of this mingling would be different, and the -further development of the tendency in question would probably not be -suppressed. - -We need not wonder, therefore, that specific types occur in such -varying degrees of definiteness. If a species is distributed over -a wide connected territory, sporadically, not uniformly, it will -depend partly upon the mutual degree of isolation of the sporadic -areas whether the individual colonies will exhibit the same specific -type or will diverge from one another. If the animal in question is -a slow-moving one like a snail, the intermingling from neighbouring -sporadic colonies will be much slower than in the case of a resident -bird such as a woodpecker. Many interesting results would undoubtedly -be gained if the numerous careful investigations into the geographical -distributions of species and their local races were studied with -special reference to this point, and much light would undoubtedly -be thrown upon the evolution of the specific type. But it would be -absolutely necessary to study carefully all the biological relations -of the animals concerned, to trace back the history of the species as -far as possible, and to decide the period of immigration, the mode and -direction of distribution, and so on. - -Nothing shows more plainly the enormous duration of the period of -constancy in species than the wide distribution of the same specific -type on scattered areas or even over different areas absolutely -isolated from one another. If, as we saw, the same diurnal butterflies -live in the Alps and the far North, they must have remained unvaried -since the Glacial period, for it was the close of that period that -brought them to their present habitats, and while other diurnal -butterflies now living on the Alps differ from their relatives in -the Arctic zone (Lapland, Siberia, and Labrador) in some unimportant -spot or line, and must therefore have diverged from one another in -the course of the long period since the Glacial epoch, they have -done so only to a minimal degree, and in characters which possibly -depend solely upon germinal selection and can hardly be regarded as -adaptations. - -I should like, however, to cite one of the few cases known to me in -which a slight deviation from the specific type undoubtedly depending -upon adaptation has occurred on an isolated region. The nut-jay -(_Caryocatactes nucifraga_) lives not only upon our Alps and in the -Black Forest, but also in the forests of Siberia, and the birds there -differ from those with us in small peculiarities of the bill, which -is longer and thinner in them, shorter and more powerful in ours. -Ornithologists associate this difference with the fact that in this -country the birds feed chiefly on hard hazel nuts, which they break -open with their bill, and on acorns, beechmast, and, in the Alps, -on cembra-cones, while in Siberia, where there are no hazel nuts, -they feed chiefly upon the seeds of the Siberian cedar, which are -concealed deep down in the cones. Thus we find that in Siberia the -bill is slender, and that the upper jaw protrudes awl-like beyond the -lower, for about 2.5 mm., and probably serves chiefly to pick out the -cedar nuts from behind the cone-scales. In the Alps the birds (var. -_pachyrhynchus_) break up the whole cone of the cembra-pine with their -thick, hard bill, and in the Upper Engadine, where the nut-jay is -abundant, I have often seen the ground underneath the cembra-pines -covered with the débris of its meal. In addition to these differences -between the two races, the Alpine form is stronger in build, the -Siberian form is daintier; in the former the white terminal band on -the tail is narrow (about 18 mm.), in the Siberian form it is broader -(about 27 mm.). - -Such cases of variation of individual parts in different areas seem to -me very important theoretically, because they furnish us with an answer -to the view which represents the species as a 'life-crystal,' which -must be as it is or not at all, and which therefore cannot vary as -regards its individual parts. The case of the nut-jay has the further -interest that it is one of the few in which we find the new adaptation -of a single character without variation of most of the other characters. - -It is only in an essentially different sense that we can compare the -species, like any other vital unit, to a crystal, in so far as its -parts are harmoniously related one to another, or, as I expressed -myself years ago, are in a state of equilibrium, which must be brought -about by means of intra-selection. This analogy, however, only applies -to the actual adjustment of the parts to a whole, and not to their -casual adjustment. Species are variable crystals; the constancy of a -species in all its parts must be regarded as something quite relative, -which may vary at any time, and which is sure to vary at some time -in the course of a long period. But the longer the adaptation of -a species to new conditions persists the more constant, _ceteris -paribus_, and the more slowly variable will it become, and this for -two reasons: first, because the determinants which are varying in a -suitable direction are being more and more strictly selected, more -and more precisely adapted, and are thus becoming more like each -other; and secondly, because, according to our theory, the homologous -determinants of all the ids do not vary in the required direction, and -a portion of the unvaried ancestral ids is always carried on through -the course of the phylogeny, and only gradually set aside by the -chances of reducing division. But the more completely these unvaried -ids are eliminated from the germ-plasm the less likely will they be -to find expression in reversions or in impurities of the new specific -characters. I may recall the reversions of the various breeds of -pigeons to the rock-dove, those of the white species of _Datura_ to the -blue form, and the _Hipparion_-like three-toed horse of Julius Cæsar, -and so on. The unvaried ancestral ids, which in these cases find only -quite exceptional expression, will make the new 'specific character' -fluctuating, as long as they are contained in the germ-plasm in -considerable numbers, but they must become more and more infrequent in -the germ-plasm as successive generations are passed through the sieve -of natural selection, and the oftener these germ-plasms, to which the -chances of reducing division and amphimixis have assigned a majority -of the old determinants, are expelled from the ranks of the species by -personal selection. The oftener this has occurred in a species the less -frequently will it recur, and the more constant, _ceteris paribus_, -will the 'type' of the species become. - -If we add to this idea the fact that adaptations take place very -slowly, and that every variation of the germ-plasm in an appropriate -direction has time to spread over countless hosts of individuals, we -gain some idea of the way in which new adaptations gradually bring -about the evolution of a more and more sharply-defined specific type. - -So far, however, we have only explained the morphological aspect of the -problem of the nature of species, but there is also a physiological -side, and for a long time this played an important part in the -definition of the conception of species. Until the time of Darwin it -was regarded as certain that species do not intermingle in the natural -state, and that, though they could be crossed in rare cases, the -progeny would be infertile. - -Although we now know that these statements are only relatively correct, -and that in particular there are many higher plants which yield -perfectly fertile hybrids, it is nevertheless a striking phenomenon -that among the higher animals, mammals, and birds the old law holds -good, and hybrids between two species are very rarely fertile. The two -products of crossing between the horse and the ass, the mule and the -hinny, are never fertile _inter se_, and very rarely with a member of -the parent stock. - -We have to ask, therefore, what is the reason of this mutual sterility -of species; whether it is a necessary outcome of the morphological -differences between the species, or only a chance accessory phenomenon, -or perhaps an absolutely necessary preliminary condition to the -establishment of species. - -The last was the view held by Romanes. He believed that a species could -only divide into two when it was separated into isolated groups either -geographically or physiologically, that is, when sexual segregation -in some form is established within the species, so that all the -individuals can no longer pair with one another, but groups arise which -are mutually sterile. It is only subsequently, he maintained, that -these groups come to differ from one another in structure. To this -hypothetical process he gave the name of 'physiological selection.' -This view depends--it seems to me--upon an underestimate of the power -of natural selection. Romanes believed that when a species began to -split up, even the adaptive variations would always disappear again -because of the continual crossing, and that only geographical isolation -or sexual alienation, that is, physiological selection, would be -able to prevent this. But even the fact that there are dimorphic and -polymorphic species proves sufficiently that adaptation to two or even -several sets of conditions can go on on the same area. In many ants we -find many kinds of individuals--the two sexual forms, the workers, and -the soldiers, and these last are undoubtedly distinguished by adaptive -characters which must be referred to selection. The same is true of -the caterpillars, whose coloration is adapted to their surroundings -in two different ways. If the individuals of one and the same species -can be broken up into two or more different forms and combinations -of adaptations, while they are mingling uninterruptedly with one -another, natural selection must undoubtedly be able, notwithstanding -the continual intermingling of divergent types, to discriminate between -them and to separate them sharply from one another. Assuredly then a -species can not only exhibit uniform variation on a single area, but -may also split up into two without the aid of physiological selection. -Theoretically it is indisputable that of two varieties which are -both equally well suited to the struggle for existence, a mixed form -arising through crossing may not be able to survive. Let us recall, for -instance, the caterpillars, of which some individuals are green and -some brown, and let us assume that the brown colour is as effective -a protection as the green, then the two forms would occur with equal -frequency; but though a mixed hybrid form which was adapted neither to -the green leaves nor to the brown might occasionally crop up, it would -always be eliminated. It would occur because the butterflies themselves -are alike, whether they owe their origin to green caterpillars or to -brown, and thus at first, at least, all sexual combinations would be -equally probable. - -I do not believe therefore in a 'physiological selection,' in Romanes's -sense, as an indispensable preliminary condition to the splitting of -species, but it is a different question whether the mutual sterility so -frequently observable between species has not conversely been produced -by natural selection in order to facilitate the separation of incipient -species. For there can be no doubt that the process of separating two -new forms, or even of separating one new form from an old one, would -be rendered materially easier if sexual antipathy or diminished -fertility of the crossings could be established simultaneously with the -other variations. This would be useful, since pure and well-defined -variations would be better adapted to their life-conditions than -hybrids, and would become increasingly so in the course of generations. -But as soon as it is useful it must actually come about, if that is -possible at all. It may be, however, as we have said before, that the -two divergent forms depend merely upon quantitative variations of the -already existing characters; sexual attraction, whether it depends -upon very delicate chemical substances, or on odours, or on mutual -complementary tensions unknown to us, will always fluctuate upwards and -downwards, and plus or minus determinants, which lie at the root of -these unknown characters in the germ-plasm, must continually present -themselves and form the starting-point for selection-processes of a -germinal and personal kind, which may bring about sexual antipathy and -mutual sterility between the varieties. I therefore consider Romanes's -idea correct in so far that separation between species is in many cases -accompanied by increasing sexual antipathy and mutual sterility. While -Romanes supposed that 'natural selection could in no case have been -the cause' of the sterility, I believe, on the contrary, that it could -only have been produced by natural selection; it arises simply, as -all adaptations do, through personal selection on a basis of germinal -selection, and it is not a preliminary condition of the separation -of species, but an adaptation for the purpose of making as pure and -clean a separation as possible. It is obviously an advantage for both -the divergent tendencies of variation that they should intermingle -as little as possible. This is corroborated by the fact that by no -means all the marked divergences of species are accompanied by sexual -alienation, and that the mutual sterility so frequently seen is not an -inevitable accompaniment of differences in the rest of the organism. - -That this is not the case is very clearly proved by our domesticated -animals. The differences in structure between the various breeds of -pigeon and poultry are very great, and breeds of dog also diverge -from one another very markedly, especially in shape and size of body. -Yet all these are fertile with one another, and they yield fertile -offspring. But they are products of artificial selection by man, and -he has no interest in making them mutually sterile, so that they have -not been selected with a view to sexual alienation, but in reference -to the other characters. The segregation of animal species into -several sub-species on the same area is probably usually accompanied -by sexual antipathy, since in this case it would be useful although -not indispensable. But the matter is different in the case of the -transformation of a colony upon a geographically isolated region. -'Amiktic' forms, such as _Vanessa ichnusa_ of Corsica, are hardly -likely to be sexually alienated from the parent form; we have here -to do only with the preponderance of a fortuitous and biologically -valueless variation and its consequent elevation to the rank of a -variety. The new form was not an adaptation, but only a variation, and -as it was of no use, it was not in a position to incite any process of -selection favouring its advancement. - -But even adaptive transformations on isolated regions from which the -parent species is excluded are not likely to develop rapidly any sexual -antipathy as regards the parent stock, and I should not be surprised if -experiments showed that there is perfect mutual fertility between, for -instance, many of the species of _Achatinella_ on the Sandwich Islands -or of _Nanina_ on Celebes, or between the species of thrushes on the -different islands of the Galapagos Archipelago, or between these and -the ancestral species on the adjacent continent, if that species is -still in existence. For there was no reason why sexual antipathy to -the parent form should have developed in any of these adaptation forms -which have arisen in isolation, and therefore it has probably not been -evolved. - -That our view of the mutual sterility between species, as an adaptation -to the utility of precise species-limitation, is the correct one is -evidenced not only by our domesticated races, but even more clearly by -plants, in regard to which it is particularly plain that the sexual -relations between two species are adaptational. We have already seen -in what a striking way the sensitiveness of the stigma of a flower is -regulated in reference to pollen from the same plant, that some species -are not fertilizable by their own pollen at all, that others yield very -little seed when self-fertilization is effected, and that others again -are quite fertile--as much so as with the pollen of another plant of -the same species. We regarded these gradations of sexual sensitiveness -as adaptations to the perfectly or only moderately well-assured visits -of insects, or to their entire absence. I wish to cite these cases as -well as the heterostylism of some flowers as evidence in support of -the conception of the mutual sterility of species which I have just -outlined. But this only in passing. The point to which I chiefly wish -to direct attention is the mutual fertility of many plant-species. -In lower as well as in higher plants fertile hybrids occur not -infrequently under natural conditions, and cultivated hybrids, such -as the new _Medicago media_, a form made by mingling two species of -clover, may go on reproducing with its own kind for a considerable -time. A number of Phanerogams yield fertile hybrids, and in Orchids -even species of different genera have been crossed and have yielded -offspring which was in some cases successfully crossed with a third -genus. - -If these facts prove anything it is that the factors which determine -the mutual sterility of species are quite distinct from their -morphological differences, in other words, from the diagnostic -characters of the specific type. For a long time the verdict on this -matter was too entirely based on observations made on animals, among -which mutual sterility arises relatively easily, even where it was not -intended (_sit venia verbo!_). Even the pairing, but still more the -period of maturity, the relations of maturity in ovum and sperm, and -even the most minute details in the structure of the sperm-cell, the -egg-shell, envelope, &c., have to be taken into account, and these may -bring about mutual or, as Born has shown, one-sided sterility. We know, -through the researches of Strasburger, that a great many Phanerogams, -when pollinated artificially from widely separated species of different -genera and families, will at least allow the pollen-tube to penetrate -down to the ovule, and that in many cases amphimixis actually results. -It follows that we must not lay too great stress upon the mutual -sterility which occurs almost without exception among the higher -animals, but must turn to the plants with greater confidence. - -Among plants there is very widely distributed mutual fertility between -species. I doubt, however, whether the observations on this point -are sufficient to warrant any certain conclusion in regard to the -importance of the phenomena in the formation of species. At least it -is not easy to see why the mutual sterility of many species of plants -should not have been necessary or useful in separating species, and why -it was not therefore evolved. We may point to the fact that animals -can move from place to place as the chief reason, and this factor -does undoubtedly play a part, but the widespread crossing of plants -by insects makes up to some extent, as far as sexual intermingling -is concerned, for their inability to move from place to place. I do -not know whether the species of orchid which are fertile with one -another belong to different countries, so that we may assume that they -originated in isolation, or whether fertile orchids from the same area -are fertilized by different insects and are thus sexually isolated. -This and many other things must be taken into consideration. Probably -these relations have not yet been adequately investigated; probably -what is known by some experts has not yet been made available to -all. Future investigations and studies must throw more light upon the -problem. - -In any case, however, we can see from the frequency of mutual fertility -among plants that mutual sterility is not a _conditio sine qua non_ -of the splitting up of species, and we must beware of laying too -great stress upon it even among animals. Germinal selection is a -process which not only forms the basis of all personal selection, but -which is also able to give rise of itself, without the usual aid of -sexual intermingling, to a new specific type. And we cannot with any -confidence dispute that, even without amphigony, a certain degree of -personal selection may not ensue solely on the basis of the favourable -variational departures originating in the germ-plasm. It would be -premature to express any definite views on this point as yet, but the -diverse cases of purely asexual or parthenogenetic reproduction in -groups of plants rich in species make this hypothesis seem probable. - -The most remarkable example of this is probably to be found in the -Lichens, the symbiotic nature of which we have already discussed, -in which--now at least--neither the Fungus nor the Alga associated -with it is known to exhibit sexual reproduction. If this is really -the case, then the existence of numerous and well-marked species of -lichens leads us to the hypothesis just expressed, and we must suppose -that the unity of the specific type is attained in this case solely -by a continual sifting of the useful from the useless variations of -the determinants, and through purely germinal intensification of the -surviving variational tendencies. - -Of course it is possible that the mutual adaptation of the Algæ and -Fungi in the evolution of species of Lichens took place very long ago, -at a time when sexual reproduction still existed, at least in one of -the associated organisms, the Fungus. The Ascomycetes, to which most -of the Lichens belong, do not at present usually exhibit the process -of amphimixis, as I have already noted; but it may perhaps be still -possible to decide whether they must have exhibited it, or at least -could have exhibited it at an earlier stage in their evolution. As the -group of Thallophytes is a very ancient one, it is not inconceivable -that the modern species of Lichens have existed for a long time, and -that they had their origin in the remote past with the assistance of -amphimixis. - -Nor need it be objected to this supposition that it has been found -possible to _make new Lichens_ by bringing together Fungi and Algæ -which had not previously been associated with one another; for in -the first place both were already adapted to partnership with other -species, and, moreover, so far no one has succeeded in rearing these -artificial lichens for any length of time, still less in seeing them -evolve into specific forms persistent in natural conditions. - -But if this supposition should prove to be not only improbable, but -actually erroneous, then the existence of Lichens would afford a clear -proof that the 'type' of the species does not depend essentially upon -the constant intermingling of individuals, but upon a process which -we may best designate _uniformity of adaptation_. We have simply to -suppose that under similar external influences similar variational -tendencies were started by germinal selection in all the individuals -of the two parent species of a lichen, and set a-going by germinal -selection, just as a warmer climate gives rise to a black variety -in the butterfly _Polyommatus phlæas_, because similar determinants -of the germ-plasm of all the individuals were impelled to vary in -the same manner and direction. This would then give rise to quite -definite variations, and since only the suitable variational tendencies -could survive, primitive though never complicated adaptations would -arise. But we cannot assume that the lichens are not adapted to the -conditions of their life as well as all other organisms. We cannot -judge how far even their shape is to be regarded as an adaptation, -whether the formation of encrusting growths, of tree-like forms, of -cup or bush-lichens, may not be regarded as adaptations towards a full -utilization of the conditions of their life--but even if this is not -the case, the formation of soredia remains an undoubted adaptation to -the symbiosis of those lichens which exhibit them. The soredia cannot -depend upon the direct effect of the conditions of life, for they are -reproductive bodies which did not exist before the existence of the -lichen, and only originated to facilitate their distribution. - -Thus there is still a great deal that is doubtful in our theories as to -the transformations of organisms, and much remains still to be done. -But even though we may doubt whether adaptations could come about in -multicellular organisms without amphigony, we may be quite certain of -the converse, that is, that the specific type can be changed in every -individual feature by natural selection on the basis of amphigony, -even as regards invisible features which only express themselves in -altered periods of growth. Even when there is no isolation whatever -and no mutual sterility, and when a mobile species is uniformly -distributed over a large area, a splitting up into races in regard to -one particular character may occur, _simply through adaptation_ to the -spatially different climatic conditions of the area inhabited. - -Early in these lectures we discussed the twofold protective value of -the coloration of the 'variable hare' (_Lepus variabilis_), which is -distributed over the Arctic zone of the Old and New World, and also -occurs in the higher regions of the Alps. Wherever there is a sharp -contrast between winter and summer the variable hare exhibits the -same specific type, being brown in summer and white in winter, but in -regard to this very character of colour-change it forms races to some -extent, for it is white for a longer or shorter time according to the -length of the winter--in Greenland for the whole twelve months of the -year, in Northern Norway only for eight or nine months, in the Alps -for six or seven months, but in the south of Sweden and in Ireland -not at all. There it remains brown in winter like our common hare -(_Lepus timidus_). This is not a question of the direct effect of cold; -if it were the species would become white in Southern Sweden also, -for there is no lack of severe cold there, but the ground is not so -uninterruptedly covered with snow, and so the white colour of the hare -would be as often, probably oftener, a danger than a safeguard, and -the more primitive double coloration has therefore been done away with -by natural selection. The change of colour is thus hereditarily fixed, -as is proved by the fact that the Alpine hare, if caught and kept in -the valleys below, puts on a white dress at the usual time, which the -common hare never does. - -As in Southern Sweden the winter coloration has been wholly eliminated, -so, conversely, from there to the Arctic zone the summer colouring -has been more and more crowded out, and in the Farthest North it has -totally disappeared from the characters of the species. We thus see -that wherever the species lives the double colouring is regulated, -as regards the duration of the winter coat, in exact harmony with -the external conditions. There is a pure white, a pure brown, and -a colour-changing race, and the latter is subdivided into two--one -wearing the winter dress for six, the other for eight months. Probably -these could be still further subdivided, if the different regions of -the Scandinavian Peninsula were investigated individually from south -to north. That the duration of the winter dress has its roots in -the germ-plasm, and does not depend solely on the earlier or later -period at which the cold sets in, is made clear by the two extreme -forms, the white and the brown _Lepus variabilis_, as well as by the -behaviour of captive animals. The familiar case of Ross's lemming, -which remained brown in the warm cabin, and then suddenly became -white when it was exposed to the cold of winter, only shows that the -cold acts as a liberating stimulus. The preparatory changes in the -pellage are already present, and the stimulus of cold brings them -rapidly to a climax. Here, therefore, the necessary variations of the -relevant germinal parts must have continually presented themselves for -selection, which is intelligible enough, since it is merely a question -of plus- or minus-variations. The fact that the six-months' dress can -be transformed into an eight-months' dress must have its cause in some -minute biological units of the germ-plasm; the determinants of the fur -must be able to vary in such a way that a longer or shorter duration of -the winter's coat is the result. The possibility of the whole variation -depends upon the continual fluctuations of all determinants, now -towards plus, now towards minus, and the necessity and inevitableness -of each adaptation to the duration of the winter lies in the unceasing -personal selection--the inexorable preferring of the better adapted. - - - - -LECTURE XXXV - -THE ORIGIN AND THE EXTINCTION OF SPECIES - -Adaptation does not depend upon chance--The case of eyes--Of -leaf-mimicry--All persistent change depends ultimately on -selection--Mutual sterility without great significance--Relative -isolation (_Lepus variabilis_)--Influence of hybridization--Decadence -of species--Differences in the duration of decadence--Natural -death of individuals--Extinction due to excessive variability -(Emery)?--_Machairodus_ as interpreted by Brandes--Lower types more -capable of adaptation than higher--Flightless birds--Disturbance of -insular fauna and flora by cultivation--The big game of Central Europe. - - -In the polar hare we have a case in which the adaptations to the life -conditions both of time and space are recognizable as the effect of -definite causes, and thus as a necessity; but the same must be true -everywhere even in regard to the most complex adaptations which seem -to depend entirely upon chance; everywhere adaptation results of -necessity--if it is possible at all with the given organization of the -species--as certainly as the adaptation dress of the hare depends on -the length of the winter, and in point of fact not less certainly than -the blue colour of starch on the addition of iodine. The most delicate -adaptations of the vertebrate eye to the task set for it by life in -various groups have been gradually brought about as the necessary -results of definite causes, just in the same way as the complex -protective markings and colouring on the wing of the _Kallima_ and -other leaf-mimicking butterflies. - -That adaptations can be regarded as mechanically necessitated is due -to the fact that in every process of adaptation the same direction of -variation on the part of the determinants concerned is guaranteed, -since personal selection eliminates those which vary in a wrong -direction, so that only those varying in a suitable direction survive, -and they then continue to vary in the same direction. But the greatest -difference between our conception of natural selection and that of -Darwin lies in this: that Darwin regarded its intervention as dependent -upon chance, while we consider it as necessary and conditioned by the -upward and downward intra-germinal fluctuation of the determinants. -Appropriate variational tendencies not only _may_ present themselves, -they _must_ do so, if the germ-plasm contains determinants at all -by whose fluctuations in a plus or minus direction the appropriate -variation is attainable. - -That a horse should grow wings is beyond the limits of the -possibilities of equine variation--there are no determinants which -could present variations directed towards this goal; but that any -multicellular animal which lives in the light should develop eyes lies -within the variational possibilities of its ectoderm determinants, and -in point of fact almost all such animals do possess eyes, and eyes, -too, whose functional capacity may be increased in any direction, and -which are adaptable and modifiable in any manner in accordance with -the requirements of the case. As soon as the determinants of the most -primitive eye came into existence, they formed the fundamental material -by whose plus- or minus-variations all the marvellous eye structures -might be brought about, which we find in the different groups of the -Metazoa, from a mere spot sensitive to light to a shadowy perception -of a moving body, and from that again to the distinct recognition -of a clear image, which we are aware of in our own eyes. And what -wonderful special adaptations of the eye to near and to distant vision, -to vision in the dusk and at night, or in the great ocean-depths, to -recognition of mere movement or the focussing of a clear image, have -been interpolated in the course of this evolution! - -All such adaptations are possible, because they can proceed from -variations of determinants which are in existence; and in the same -way it is possible, at every stage of the evolution of organisms, for -eyes to degenerate again, whether they have been high up or low down -in the scale of gradations of this perhaps the most delicate of all -our sense-organs. As soon as a species migrated permanently from the -light into perfect darkness its eyes began to degenerate. We know -blind flat worms, blind water-fleas and Isopods, also blind insects -and higher Crustaceans, and even blind fishes and amphibians, the eyes -of which are now to be found at very different levels of degeneration, -as Eigenmann has recently shown in regard to several species of -cave-dwelling salamanders of the State of Ohio. In all these cases -it is only necessary for the determinants of the eye to continue to -vary in the minus direction, and the disappearance of the eye must be -gradually brought about. - -We must picture upward development in quite a similar way. The forest -butterflies of the Tropics could not possibly all have their under -surfaces coloured like a leaf if the protective pattern depended solely -upon the chance of a useful variation presenting itself. It always -presented itself through the fluctuations of the determinants, and -thus the appropriate colourings were not merely able to develop, but -of necessity did so in gradually increasing perfection. If chance -played any part in the matter, it would be quite unintelligible why the -protective colouring should occur only where it acts as a protection, -and why, for instance, it should not appear sometimes upon the upper -surface of the butterfly wing, or upon the posterior wings which are -covered when the butterfly is at rest. We have already studied in -detail the precision with which the coloration is localized on minute -points and corners of the wing: this can only be understood if natural -selection works with the certainty of a perfect mechanism. Chance only -comes into the matter in so far as it depends upon chance whether the -relevant determinants in one id or another are to vary in the direction -of plus or minus; but as the germ-plasm contains many ids, and chance -may decide it differently in each of these, the presence of a majority -of determinants varying in a desirable direction does not depend upon -chance, for if they are not contained in one individual they are in -another. It is only necessary that they should be present in some, and -that these should be selected for reproduction. - -We must therefore regard natural selection, that is to say, _personal -selection_, as a mechanical process of development, which begins -with the same certainty and works 'in a straight line' towards its -'goal,' just as any principle of development might be supposed to do. -Fundamentally it is after all a purely internal force which gives -rise to evolution, the power of the most minute vital units to vary -under changing influences, and it is only the guidance of evolution -along particular paths that is essentially left to personal selection, -which brings together what is useful and thus determines the direction -of further evolution. If we bear in mind that even the minutest -variations of the biophors and determinants express nothing more or -less than reactions to changed external conditions in the direction of -adaptation, and that the same is true of each of the higher categories -of vital units, whether they be called cell, tissue, organ, person, -or corm, we see that the whole evolution of the forms of life upon -the earth depends upon adaptations following each other in unbroken -succession, and fitting into each other in the most complex way. The -whole evolution is made possible by the power of variation of the -living units of every grade, and called forth and directed by the -ceaseless changes of the external influences. I said years ago that -_everything_ in organic evolution depended upon selection, for every -lasting change in a vital unit means adaptation to changed external -influences, and implies a preference in favour of the parts of the unit -concerned, which are thereby more fitly disposed. - -In this sense we can also say that the species is a complex of -adaptations, for we have seen that it depends upon the co-operation -of different grades of selective processes, that in many cases it is -produced solely by germinal selection, but that in very many more -personal selection plays the chief part, whether in bringing about -sexual adaptations, or adaptations to the conditions of existence. - -When we have thus recognized that the origin of a variation in a -definite direction results as inevitably when it is called forth by the -indirect influence of conditions, that is, through the need for a new -adaptation, as when it is induced in the germ-plasm by direct causes -such as those of climate, we shall not be disposed to estimate very -highly the part played by mutual sterility in the origin of species. We -shall rather be inclined to assign it a rôle at a later stage, after -the separation of the forms has taken place, and this view is supported -by the fact of the mutual sterility of most nearly related species, and -by the theoretical consideration that the frequency of hybrids, even if -these are always eliminated in the struggle for existence, must signify -a loss for both the parent species. But no certain conclusion can be -based upon either of these arguments--not upon the theoretical one, -because here again we are unable to estimate the extent of this loss; -and not upon the argument from fact, because the results of experiments -in crossing animals have generally been overestimated, since we are apt -to regard the most nearly related animals that are at our disposal as -being very closely related. Thus, for instance, horse and ass, horse -and zebra are undoubtedly rightly included within the same genus, but -the fact that there are several species of zebra in Africa gives us an -idea of the number of transition stages that may have existed between -the horse and the zebra. Entomologists have sometimes reared hybrids -between the most nearly related indigenous species of hawk-moth of -the genus _Smerinthus_--hybrids of _Smerinthus ocellata_, the eyed -hawk-moth, and _Smerinthus populi_, the poplar hawk-moth. I have myself -made many experiments of this kind, and have often succeeded in getting -the two species to pair and even to deposit eggs, but I have never -seen a caterpillar emerge from them. The hybrids do occur, however, -and they have been repeatedly obtained by Standfuss. In external -appearance they are intermediate between the parent forms, but with -marked divergences, thus, for instance, the beautiful blue eye on the -posterior wing of _S. ocellata_ (Fig. 5, vol. i. p. 69) may have almost -disappeared or be only indicated. They are sterile. But we know three -species of _Smerinthus_ in North America, which are all much nearer to -_S. ocellata_ than _S. populi_ is, for they all possess the eye-spot -referred to, although it is less well developed. The proof that the -most nearly related species do not yield fertile descendants should be -sought for by crossing _Smerinthus ocellata_ with one of these American -species if it is to have any decisive value. - -Experiments of the same kind have been made by Standfuss with different -species of indigenous _Saturnia_, and these have shown not only that -crossing is possible, but that the hybrids are fertile in their turn. -These results are to be valued the more highly because it is well known -that Lepidoptera, and even the usually prolific silk-moths, do not -readily reproduce in captivity, even within the same species. We have -in _Saturnia pyri_, _spini_, and _carpini_ three well-marked distinct -species with no intermediate forms in nature, and with quite different -colouring in the caterpillars. That these should have been successfully -combined in a triple hybrid proves at least that sexual alienation -cannot have advanced far in this case. - -We must beware, however, of attributing too much to the constant -mutual crossing which occurs in a species living on a connected area -and of regarding its influence as irresistible. Undoubtedly it must -go far towards securing the uniformity of individuals, but not only -is it unable to achieve this, but it cannot successfully resist the -stronger influences making for variation which may be exerted upon a -part of the area of the species. We have already seen that it is quite -erroneous to suppose that every new adaptation must be lost sight of -again because of the continual crossing with other members of the -species upon the same area. Other things being equal, this depends -entirely upon the importance of the adaptation in question. Just as -climatic influences may be so strong that they entirely overcome the -influence of crossing, and give rise to a local race notwithstanding -imperfect geographical isolation, so the same may happen in the case of -adaptations. It is quite conceivable that the polar hare of Scandinavia -may have evolved a whole series of races, each of which is adapted -to the duration of the snow in its geographical range, although a -crossing of these quick-footed animals must frequently occur in the -course of time, even as regards forms from widely separated areas, and -although the whole region is inhabited without a break by the species, -so that a 'mingling' of the hares of all regions from south to north, -and conversely, may take place, and indeed must be continually taking -place, though of course very slowly. - -It is precisely this extreme slowness with which the intermingling -of racial characters take place that seems to me essential for the -production of local or, as in this case, regional races. It is not -difficult to calculate the rate of 'blood-distribution' if we assume -that the conditions for a rapid dissemination are as favourable as -possible. Let us assume that it takes place along a certain line--in -this case from south to north--and that the numerical strength of -the species remains constant, each pair of hares yielding a pair of -surviving offspring, which will attain to reproduction. Let us suppose -that one of these hares moves his home northwards to the extent of his -range, that is, as far as a hare is accustomed to range from his head -quarters, and that he pairs with one of the descendants on the next -stretch. - -Let us further suppose that this stretch is ten kilometres in extent, -and that the change of quarters take place once in each year, then the -blood of a South Scandinavian hare would have extended ten kilometres -further north in ten years, and in a hundred years 100 kilometres; it -would not, however, be quite pure, but mixed and thinned by crossing -with a hundred mates of different individual bloods, that is, thinned -to the extent of 2 to the 100^{th} power, that is, to less than a -millionth part. Thus even with these much too favourable assumptions -the influence of a region of hares 100 kilometres distant would be -actually _nil_ upon the inhabitants of a region which was in process -of new adaptation. That the assumptions are too favourable is quite -obvious, since every surviving hare would not be likely to move his -home, and probably the majority would remain in the old quarters -and find mates there. The blood-mingling would therefore take place -much more rarely, perhaps only once in ten years, and the wandering -descendants of the second generation might move southward, and so -neutralize the previous blood-mingling, and so on. But let us keep -to our favourable assumptions, and attempt to determine how strong -the assimilating influence of the blood-mingling from south to north -would be upon a point _A_. The blood of the nearest stretch diluted -to a half would affect the inhabitants of _A_ once in each year; the -second stretch would only contribute blood of 1/4 strength, the third -of 1/8, the fourth of 1/16, and the blood of the tenth would be diluted -to 1/1024. A region _B_, extending over twenty such stretches, or 200 -kilometres, would thus shelter within it a hare population of which -the centre would only be influenced from the periphery in vanishing -proportions. If the winter were of equal length over the whole area of -_B_, all the inhabitants would be tending to vary the period for which -the winter dress was worn in correspondence with the length of the -winter, and the centre of the region would be the less impeded in this -process because the more peripheral areas would also be approximating -to the same adaptation. But since even the admixture of 1/32 of strange -blood could have no hindering influence upon a variation, there would -remain a region of 2 × 5 = 10 stretches upon which the influence of -the non-varying regions would be without effect. There would therefore -arise a new race in relation to the duration of the winter dress, and -this would not cease abruptly, but would gradually pass over into the -neighbouring regions, which however would be pure at their centre, just -as is probably the case in reality, if we regard _B_ as any point in -the line of distribution from south to north. - -The harmony of the individuals within a species will therefore depend -in part upon the mingling of hereditary primary constituents associated -with reproduction, but in greater part upon adaptation to the same -conditions; it is _a similarity of adaptation_, and the strongest -influence which sexual reproduction exerts lies not in the mingling of -these hereditary constituents alone, but above all in the reduction -in the germ-plasm of the two parental hereditary contributions--a -reduction which results from and through the sexual intermingling. It -is only this that prevents these primary constituents from varying at -too unequal a rate in the transformations of species, and causes them -ultimately to resemble each other closely again. - -But while mutual sterility is not an absolutely necessary condition in -the separation of species, it would be going too far in the opposite -direction to regard mutual fertility as something general, or to -attribute to it a rôle in the origination of new species. - -Certain botanists, like Kerner von Marilaun, regard the mingling of -species as a means of forming new species with better adaptations; -they suppose that fertile hybrids may, in certain circumstances, crowd -out the parent species, and themselves become new species. It will be -admitted that such cases do occur, that, for instance, in the north of -Europe the hybrid between the large and the small water-lily, _Nuphar -luteum_ and _Nuphar pumilum_, to which the name _Nuphar intermedium_ -has been given, has driven both the parent species from the field, -because its seeds mature earlier, and it is therefore better adapted -to the short vegetative period of the north, but nevertheless we must -maintain that the evolution of species on the whole does not take -place through hybridization. Such cases are probably nothing more than -rare exceptions. This is corroborated by the entire insignificance of -hybridization in animals, among which species appear in the same way as -they do in plants, and where the mingling of two species occurs only -sporadically and in a few species, never to any very great extent. - -If species are complexes of adaptations, based in each case on the -given physical constitution of the parent species, then we can readily -understand the fact that they are in our experience not fixed or -eternal, but that they change in the course of the earth's history. -The numerous fossil remains in the various strata of the earth's crust -prove that this is true in a high degree, that in almost every one of -the more important geological strata new species occur, and that not -only species and genera, but families, orders, indeed whole classes -of animals, which lived at one time, have now completely disappeared -from the face of the earth. We can understand this phenomenon when we -reflect that the conditions of life have also been slowly changing -through the course of the earth's history, so that the old species had -only the alternative of dying out, or of becoming transformed into new -species. - -But simple as this conclusion is, it can hardly be deduced with -certainty from the occurrence and succession of the fossil species -alone. For instance, we should strive in vain to recognize the -cause which led one of those regularly arranged snail-species of -the Steinheim lake basin to become transformed into one or two new -species at a particular time, or to find the cause which moved those -curious tripartite Crustaceans of primitive times, the Trilobites, -which peopled the Silurian seas with such a wealth of forms, to -become suddenly scarce towards the end of the Silurian period, and -to disappear altogether in the succeeding period, the Devonian. The -famous geologist Neumayr sought to refer this striking phenomenon to -the fact that just at that time the Cephalopods, 'the most formidable -and savage marauders among the invertebrate marine fauna,' gained the -ascendancy, and it is quite possible that he was right in his surmise, -but who is to prove it? Can we decide even in the case of animals now -living whether the losses inflicted on a much persecuted species by an -abundant and greedy persecutor exceed the numbers of progeny, and are -therefore driving the species gradually towards extermination? Probable -as such a supposition appears, it cannot be accepted as proven. - -Since in many cases of the extinction of great animal-groups we cannot -even prove that there was a simultaneous ascendancy of powerful -enemies, other factors must be discovered to which the apparently -sudden disappearance may be attributed. Many naturalists have tried -to guess at internal reasons for extinction, and have adopted the -theory--associated with the tendency to assume mystical principles of -evolution--that species in dying out are obeying an internal necessity, -as if their birth and death were predestined, as it is in the case of -multicellular individuals, as if there were a _physiological death of -the species_ as there is of the multicellular individual. - -Neumayr showed, however, that the facts of palæontology afford no -support for this view. I need not repeat his arguments, but will simply -refer to his clear and concise exposition of the problem. It is obvious -that our theory of the extinction of species as due to external causes -cannot be rejected on the ground that our knowledge of the struggle -that species had to maintain for their existence in past times is -even mere imperfect than our knowledge of the struggle nowadays, and -that we are frequently unable to judge of it at all. But the facts of -geology are of value in another, quite different way. They reveal such -an extraordinary dissimilarity in the duration of species, and also -of the great groups of organisms, that the dissimilarity of itself -is sufficient to prevent our regarding the extinction of species as -regulated by _internal_ causes. Certain genera of Echinoderms, such as -starfish (_Astropecten_), lived in the Silurian times, and they are -represented nowadays in our seas by a number of species: and in the -same way the Cephalopod genus _Nautilus_ has maintained itself among -the living all through the enormous period from the Silurian sea to our -own day. Formerly the Nautilids formed a predatory horde that peopled -the seas, and, as we have seen, we may perhaps attribute to their -dominance the disappearance of an order of Crustaceans, the Trilobites, -which were equally abundant at that period. Now only a single species -of nautilus (_Nautilus pompilius_) lives on the coral reefs of the -southern seas. Similarly, the genus _Lingula_ of the nearly extinct -class of Brachiopods, somewhat mussel-like sessile marine animals, has -been preserved from the grey dawn of primitive times, with its records -in the oldest deposits, and is represented in the living world of -to-day by the so-called 'barnacle-goose' mussel, _Lingula anatina_. - -On the other hand, we know of numerous species which lasted for quite -a short time, such as, for instance, the individual members of the -series of Steinheim _Planorbis_ species, or of the Slavonic _Paludina_ -species. Not infrequently, too, genera make their appearance and -disappear again within the period of one and the same geological -stratum. - -These facts not only tell against an unknown vitalistic principle of -evolution, but in general against the idea of the determination of -the great paths of evolution by purely internal causes. If there were -a principle of evolution the dissimilarity in the duration of life -could not be so excessive; if there were a 'senile stage' of species -and a natural death of species comparable to the natural death of -individuals, it would not have been possible for most of the Nautilidæ -to have been restricted to the Silurian epoch, and yet for one species -to have continued to live till now; and if there were a 'tendency' -of species to vary persistently onwards, and to 'become further and -further removed from the primitive type,' as has been maintained, then -such ancient and primitive Cephalopod forms like the _Nautilus_-species -could not have persisted until now, but must long ago have Wen -transmuted into higher forms. The converse, however, is conceivable -enough, namely, that the great mass of the species of a group such as -the Nautilidæ were crowded out by superior rivals in the struggle for -existence, but that certain species were able to survive on specially -protected or otherwise favoured areas. We have a fine example of this -in the few still living species of the otherwise extinct class of -Ganoid fishes. During the Primary and Secondary epochs these Ganoids -peopled all the seas, but at the boundary between the Cretaceous and -the Tertiary period they retrograded considerably, simultaneously with -the great development of bony fishes or Teleosteans, and now they are -only represented by a dozen species distributed over the earth, and -most of these are purely river forms, while the others at least ascend -the rivers during the spawning season to secure the safety of their -progeny. For the rivers are sheltered areas as compared with the seas, -and large fishes like the Ganoids will be able there to hold their -own in the struggle better than they could in the incomparably more -abundantly peopled sea. - -Thus I can only regard it as playing with ideas to speak of birth, -blossoming, standstill, decay, and death of species in any other than a -figurative sense. Undoubtedly the life of the species may be compared -with that of the individual, and if the comparison be used only to make -clear the difference between the causes of the two kinds of phenomena, -there can be no objection to it, only we must beware of thinking we -have explained anything we do not know by comparing it with something -else that is also unknown. - -We have already shown that the natural death of multicellular organisms -is a phenomenon which first made its appearance with the separation -of the organism into somatic or body cells and reproductive or germ -cells, and that death is not an inevitable Nemesis of every life, for -unicellular organisms do not necessarily die, though they may be killed -by violence. These unicellular organisms have thus no natural death, -and we have to explain its occurrence among multicellular organisms -as an adaptation to the cellular differentiation, which makes the -unlimited continuance of the life of the whole organism unnecessary and -purposeless, and even prejudicial to the continuance of the species. -For the species it is enough if the germ-cells alone retain the -potential immortality of the unicellulars, while, on the other hand, -the high differentiation of the somatic cells necessarily involves -that they should wear themselves away in the performance of their -functions, and so become subject to death, or at least that they should -undergo such changes that they are no longer capable of functioning -properly, so that thus the organism as a whole loses the power of life. - -There can be no doubt whatever that death is virtually implied in -the very constitution of a multicellular organism, and is thus, so -to speak, a foreseen occurrence, the inevitable end of a development -which begins with the egg-cell and reaches its highest point with the -liberation of the germ-cells, that is, with reproduction, and then -enters on a longer or shorter period of decadence, leading to the -natural death of the individual. - -It is only by straining the analogy that this course of development -can be compared with the origin and transformation or extinction of -species. Not even the entirely external analogy of the blossoming -from a small beginning and the subsequent decay is always correct; -for in the fresh-water snails of Steinheim, at any rate, almost the -whole of the members of the species underwent a transformation at a -particular time, and became a new species, which was after a long -time retransformed without any appreciable decrease in the number of -individuals being observable. To speak of a 'senile stage' of the -species, of a stiffening of its form, of an incapacity for further -transformation, is to indulge in a play of fancy quite inadmissible in -the domain of natural science. - -It is admitted, however, that there is a correct idea at the base of -all this, for many species have not passed over into new forms, but -have simply died out because they were unable to adapt themselves -to changed conditions. This did not happen because they had become -incapable of variation, but because they could not produce variations -of sufficient magnitude, or variations of the kind required to enable -the species to remain an active competitor in the struggle for -existence. - -It obviously depends upon the coincidence of manifold circumstances, -whether an adaptation can be successfully effected or not. Above all, -it must be able to keep pace with the changes in the conditions of -life, for if these advance at a more rapid rate the organisms will -succumb in the midst of the attempt at adaptation. It is probably -in this way that the striking disappearance of the Trilobites is to -be explained, as Neumayr has pointed out, for the Nautilidæ, a new -group of enemies, multiplied so quickly at their expense that they -had not time to evolve any effective means of protection. It cannot -be maintained for a moment that every species is able to protect -itself against extermination by any other; the increased fertility, -the increased rapidity of locomotion, the increased intelligence -and similar qualities, may all be insufficient, and then extinction -follows; not, however, because the species has become 'senile,' but -because the variations possible to its organization do not suffice to -maintain it in the struggle. - -In discussing germinal selection I mentioned the view expressed -by Emery, that excessive variation in the same direction from -intra-germinal causes has not rarely been the cause of the extinction -of species. I also mentioned the very similar view of Döderlein, -who could not refer at that time to germinal selection, but assumed -internal compelling forces, which pressed a variation irresistibly -forward in the direction in which it had started, even beyond the -bounds of what is useful for the desired end, and which might thus -bring about the extinction of the species. I cannot entirely agree -with these views, as I have already indicated, because I do not -believe that the impulse to variation can ever become irresistible -and uncontrollable. If it could, then we should not see, as we do, -innumerable cases in which the augmentation or diminution of a part has -gone on precisely to the point at which it ceases to be purposeful. -Even the degeneration of organs only proceeds as far as is necessary -to accomplish a particular end, as we see plainly from the parasitic -Crustaceans of different orders. In many of these parasitic forms the -swimming legs degenerate, but in the female only, because these attach -themselves by suckers or in some other manner to their host, so that -they cannot let go again. But the males need their swimming legs to -seek out the females. The females too require them in their youth, -in order to seek out the fish from which they are to obtain their -food-supply, and thus the degeneration of the swimming legs has come to -a full stop exactly at the point where they cease to be of use; they -develop in early youth and degenerate later, when the animal becomes -sessile. In accordance with the law of biogenesis we may say that -while the degeneration is complete in the final stages of ontogeny, -its retrogression was not continued back to the germ, but only to the -young stages. From this it follows that the progress of a variation -may at any time have a goal fixed for it, and we have seen that this -is possible by means of personal selection, which accumulates the -never-failing fluctuations of the variation in the direction of plus or -of minus. In the individual id a determinant _X_ may perhaps decrease -and possibly also increase without limit, although we have no certain -knowledge in regard to the latter point, but as this determinant is -contained in all the ids, there are always plus and minus fluctuations -by means of which personal selection can operate. - -But of course it requires a certain amount of time for this, and in -the fact that this time is often not available lies, I think, the -reason why excessive differentiations have often led to the extinction -of a species, not because the increase of the excessive organ must go -on irresistibly, but because changes in the conditions have made the -exuberant organ inappropriate, and it could not degenerate quickly -enough to save the species from extinction. - -Brandes has recently given a beautiful illustration of this by -associating the existence of the remarkable sabre-toothed tigers -(_Machairodus_) with enormously long canine teeth, which lived in the -Diluvial period in South America, with the gigantic Armadillos which -lived there at the same time, whose bony armature two yards in height -now excites our admiration. He rightly points out that the dentition of -_Machairodus neogæus_ is by no means a typically perfect dentition for -a beast of prey, like that of the Indian tiger or the lion; as far as -incisors and molars are concerned it was much less effective than that -of these predatory animals, and the great length of the dagger-like -flattened canines, which protruded far beyond the mouth, entirely -prevented the bringing together of the teeth of the upper and lower jaw -after the fashion of a pair of pincers. He rightly infers from this -that this dentition was adapted to a specialized mode of nutrition, and -he regards the great mailed Armadillos, such as the three-yards-long -heavy Glyptodont of the Pampas, as the victims into which they were -wont to thrust their sabre-teeth in the region of the unprotected -neck, and thus to master the almost invulnerable creature, which was -invincible as far as all other predatory animals were concerned. Thus -the remarkable dentition is explained on the one hand, and on the other -the amazing extent and hardness of the victim's coat of mail. Thus, -too, we can understand why there should have been at that time a whole -series of cat-like animals with sabre-like teeth, in which the length -and sharpness of the teeth increased with the bodily size, for these -predatory animals corresponded to a whole series of Armadillos, whose -size was increasing, as was also the strength of their armour. - -Of course this interpretation is hypothetical, but it contains much -internal probability, so that it may be taken as a good illustration -of the reciprocal increase of adaptations between two animal groups. -We understand now why, on the one hand, this colossal tortoise-like -armour should have developed in a mammal, and, on the other hand, -why these enormously long sabre-teeth should have been evolved; we -also understand--and this is the point with which we are here chiefly -concerned--why these two 'excessive' developments should ultimately -lead to the destruction of their possessors. For a long period -the Armadillos were able to save themselves from extermination by -increasing their bodily size and the strength of their armour, and -they thus saved themselves from persecution on the part of beasts of -prey with smaller and weaker teeth. But the predatory animals followed -suit and lengthened their teeth and increased their bodily size, -until ultimately even the strongest armour of the victim afforded -no efficient protection, and the mighty Glyptodonts were by degrees -utterly exterminated. But then the death-knell of the _Machairodus_ -had also sounded, for he was so exactly adapted to this one kind of -diet that he could no longer overpower other victims and feed on their -flesh; the sabre-teeth prevented him from tearing his prey like other -predatory animals, he could probably only suck them. - -Even if this is a supposititious case, it serves to show that it was -not an internal principle of variation that caused the teeth of these -carnivores and the armour of their victims to increase so unlimitedly; -it was the necessity of adaptation. They did not perish because armour -and teeth increased so excessively, but because neither of these -adaptations could be neutralized all at once, and small variations were -of no use to them in their final struggle for survival. - -In a certain sense we may say that simpler, more lowly organisms are -more capable of adaptation than those which are highly differentiated -and adapted to specialized conditions in all parts of their bodies, -since from the former much that is new may arise in the course of time, -while very little and nothing very novel can spring from the latter. -From the simplest Protozoan the whole world of unicellular organisms -could arise, and also the much more diverse Metazoa; from the lower -marine worms there could arise not only many kinds of higher marine -worms--the segmented worms or Annelids--but also quite new groups of -animals, the Arthropods and the Vertebrates. It is hardly likely that a -new class of animals will evolve from our modern birds, because these -are already so perfectly adapted to their aerial life that they could -hardly adapt themselves to life on land or in the water sufficiently -well to be able to hold their own in regard to all the possibilities of -life with the rest of the dwellers on land or in water. We do indeed -know of birds which have returned entirely to a purely terrestrial -life--the ostriches, for instance--and of others which have adapted -themselves to a purely aquatic life, such as the penguins, but these -are small groups of species, and are hardly likely to increase. On -the contrary, we can prove that many have already succumbed in the -struggle with man, and we anticipate the extermination of others. But -the reason why they are so readily exterminated obviously lies in the -fact that they have surrendered the advantage given to them by their -bird-nature, by adapting themselves to terrestrial life, and that they -are not able to regain it, at least not in the short time that is at -their disposal if they are to be saved from extermination. The best -example of this is the Dodo (_Didus ineptus_). This remarkable-looking -bird, of about the size of a swan, lived in flocks upon the island of -Mauritius until about the end of the seventeenth century. It had small -wings with short quills which were useless for flight. As it could -neither escape by flight nor through the water, and could only move -clumsily and awkwardly upon land with its short legs and heavy body, it -was hopelessly doomed as soon as a stronger enemy made his appearance. -It fell a victim to the sailors who first landed on the island and -clubbed it with sticks in huge numbers. Until that event it was without -doubt excellently adapted to life on that fertile island, for on a -volcanic island in the middle of the ocean there were no large enemies, -and it was therefore not dependent on the power of flight for safety, -and could pick up abundant food from the ground. But when man suddenly -appeared on the scene and began to persecute it, it was not the 'senile -rigidity' of its organization that prevented it from making use of -its wings again; it was the slowness of variation and consequently -of selection, which is common to all species, which impelled it to -extinction. The same fate will probably overtake the Kiwi of New -Zealand (_Apteryx australis_) in the near future, for though it has -so far escaped the arrows of the aborigines, it is not likely in its -wingless condition to be able to hold out long against European guns, -unless close times and preserved forests are instituted for it, as they -have been for our chamois. - -Even sadder from the biologist's point of view than such extermination -of individual species through the vandalism and greed of our own -race is the disturbance of whole societies of animals and plants by -man that is going on or has been accomplished on most of the oceanic -islands, and we must briefly notice these cases while we are dealing -with the decadence of species. I refer to the crowding out of the -usually endemic animal and plant population on such islands through -cultivation. The first step in this work of 'cultivation' is always the -cutting down of the forests which for thousands of years have clothed -these islands as with a mantle of green, have regulated their rainfall, -secured their fertility, and allowed a medley of indigenous animals, -usually peculiar to the spot, to arise. We have already spoken of St. -Helena. The original and remarkable fauna and flora of this island had -for the most part disappeared 200 years ago, through the cutting down -of trees in the forests, and these were later wholly destroyed by the -introduction of goats, which devoured all the young trees as they grew. -But with the forests most of the indigenous insects and birds were -doomed to destruction, so that now there is not an indigenous bird or -butterfly to be found there; only a few terrestrial snails and beetles -of the original fauna still survive. - -But it is not only on islands that a large number of species have -been decimated or entirely exterminated by deforestation, by the -introduction of plants cultivated by man and of the 'weeds' associated -with these, and by the importation of domesticated animals. In Central -Europe not only have all the larger beasts of prey, like the bear, the -lynx, and the wolf, almost completely disappeared, but the reindeer, -the bison, the wild ox (Aurochs), and the elk have been exterminated -as wild animals, and in North America the buffalo will soon only exist -in preserved herds, if that is not already the case. Here, of course, -the direct interference of the all-too-powerful enemy, man, has played -the largest part in causing the disappearance of the species referred -to, but the process may give us an idea of the way in which a superior -animal enemy may be able gradually to exterminate a weaker species -where there is no attainable or even conceivable variation which -might preserve them from such a fate. Several of the mammals which I -have mentioned are not yet entirely exterminated; even the Aurochs -perhaps still exists in the pure white herds preserved in some British -parks; but there are more instances than that of the Dodo of the utter -extermination of a species through human agency within historic times. -It may be doubtful whether the sea-otter (_Enhydris marina_) has not -been already quite exterminated because of its precious fur, but it is -quite certain that the huge sea-cow (_Rhytina stelleri_), which lived -in large numbers in the Behring Straits at the end of the eighteenth -and the beginning of the nineteenth centuries, was completely -exterminated by sailors within a few decades. - -We may therefore gain from what is going on before our eyes, so to -speak, some sort of idea of the way in which the extermination of -species may go on even independently of man at the present time, and -how it must have gone on also in past ages of the earth's history. -Migrations of species have taken place ceaselessly, although very -slowly, for every species is endeavouring slowly to extend its range -and to take possession of new territories, and thus the fauna and -flora of any region must have changed in the course of time, new -species must have settled in it from time to time, and the conditions -of life must have changed, and in many cases this must have led to the -extermination of species, in the same way, though not so quickly, as -human interference now brings about their doom. - -This is true for plants as for animals. A good example, not indeed of -complete extermination, but of very considerable diminution in the -numbers of individuals of a plant-species by the advent of a species of -mammal, is communicated to us by Chun in regard to Kerguelen Land. A -flowering plant, the Kerguelen cabbage (_Pringlea antiscorbutica_), has -been greatly reduced in numbers since the thoughtless introduction of -rabbits to this uninhabited island (1874). While, in 1840, Captain Ross -used this plant in great quantities as a preventative against scurvy in -his crew, and even carried away stores to last for months, the Valdivia -Expedition in 1898 found rabbits in abundance, but the Kerguelen -cabbage had been entirely exterminated at every spot accessible to -these prolific and voracious rodents. It was only found growing upon -perpendicular cliffs or upon the islands lying out in the fiords. - -An avoidance of the threatened destruction of a species by its -adaptation to the new circumstances can only be possible when the -changes occur very slowly, and will therefore be more likely to be -achieved in the case of physical changes in the conditions of life, -such as climatic changes, a change in the mutual relations of land -and sea, and so on. But it appears that even climatic changes do -not evoke any variation and new adaptation as long as the species -can avoid the changes by migrating. The often quoted case of Alpine -and Arctic plants proves this at any rate, that those species which -inhabited the plateaus and highlands of Europe did not all vary to -suit the change when a warmer climate prevailed, but that in part at -least they followed the climate to which they were already adapted, -that is, that they migrated towards the north on the one hand and -higher up the Alps on the other. It cannot be denied that many of the -insects and plants did adapt themselves at that time to the warmer -climate, and became the modern species which now inhabit the plains, -for many related species occur on the Alps and in the plains, but -apparently many others simply made their escape from a climate which -no longer suited their requirements. Thus, as far as I am aware, there -is no species of _Primula_ in South or Central Germany which could be -derived from the beautiful red _Primula farinosa_ of the Alps, but this -species occurs also upon the old glacier-soil at the northern base of -the Alps, and in similar soil again in the north of Germany and on the -meadows of Holstein. Similar examples might be cited in regard to the -Alpine-Arctic butterflies. - -It is intelligible enough that we are still very far from being able -to give a precise account of the main changes in the plant and animal -world during the history of the earth in regard to the special causes -which have produced them. Possibly the future will throw more light -upon this subject by extending our knowledge of the fossil remains -of all countries. But so much at least we can say at present, that -there is no reason to refer the dying out of the earlier forms to -anything else than the changes in the conditions of life, the struggle -for existence, and the limitation of the power of transformation and -adaptation due to the organization which the species has already -attained; there is no trace of any such thing as a phyletic principle -of life in the vitalistic sense, as far as the decadence of species is -concerned. - - - - -LECTURE XXXVI - -SPONTANEOUS GENERATION AND EVOLUTION: CONCLUSION - - Spontaneous generation--Experimental tests impossible--Only the - lowest and smallest forms of life can be referred to spontaneous - generation--Chemical postulates for spontaneous generation--Empedocles - modernized--The locality of spontaneous generation--Progress of - organization--Direct and indirect influences causing variation--The - various modes of selection--Everything depends upon selection--Sinking - from heights of organization already attained--Paths of evolution--The - forces effecting it--Plasticity of living matter--Predetermination - of the animate world--Many-sided adaptation of each group--Aquatic - mammals and insects, parasites--Nägeli's variation in a definite - direction--Analogy of the traveller--Genealogical trees--The - diversity of forms of life is unlimited--The origin of the purposeful - apart from purposive forces working towards an end--The limits of - knowledge--Limitation of the human intelligence by selection--Human - genius--Conclusion. - - -We have now reached the end of our studies, and they have given us -satisfaction, at least in so far that they have brought us certainty -in regard to the chief and fundamental question which can be asked in -reference to the origin of the modern animate world of organisms. There -remains no doubt in our minds that the theory of descent is justified; -we know, just as surely as that the earth goes round the sun, that the -living world upon our earth was not created all at once and in the -state in which we know it, but that it has gradually evolved through -what, to our human estimate, seem enormously long periods of time. -This conclusion is now firmly established and will never again become -doubtful. The assumption, too, that the more lowly organisms formed -the beginnings of life, and that an ascent has taken place from the -lowest to the higher and highest, has become to our minds a probability -verging upon certainty. But there remains one point which we have not -yet touched upon--the problem of the origin of these first organisms. - -There are only two possibilities: either that they have been borne to -our earth from outside, from somewhere else in the universe, or that -they have originated upon our earth itself through what is called -'spontaneous generation'--_generatio spontanea_. - -The idea that very lowly living organisms might have been concealed -within the clefts and crevices of meteorites, and might thus have -fallen upon our earth and so have formed the first germs of life, was -first formulated by that chemical genius, Justus Liebig. It seems -certain that the state of glowing heat in which meteorites are, when -they come into our atmosphere, only affects the outer crust of these -cosmic fragments, and that living germs, which might be concealed in -the depths of their crevices and fissures, might therefore remain -alive, but nevertheless it is undoubtedly impossible that any germ -should reach us alive in this way, because it could neither endure -the excessive cold nor the absolute desiccation to which it would be -exposed in cosmic space, which contains absolutely no water. This could -not be endured even for a few days, much less for immeasurable periods -of time. - -But we have to take account, too, of an entirely general reason, -which lies in the fact that all life is transient, that it can be -annihilated, and is not merely mortal! Everything that is distinctively -organic may be destroyed to the extent of becoming inorganic. Not -only may the phenomena of life disappear, and the living body as such -cease to be, but the organic compounds which form the physical basis -of all life are ceaselessly breaking up, and they fall back by stages -to the level of the inorganic. It seems to me that we must necessarily -conclude from this that the basis of Liebig's idea was incorrect, that -is, the assumption that 'organic substances are everlasting and have -existed from the first just in the same way as inorganic substances.' -This is obviously not the case, for a thing that has an end cannot be -everlasting; it must have had a beginning too, and consequently organic -combinations are not everlasting, but are transitory; they come and -go, they arise wherever the conditions suitable for them occur, and -they break up into simpler combinations when these conditions cease -to be present. It is only the elements which are eternal, not their -combinations, for these are subject to more or less rapid continual -change, whether they have arisen outside of organisms or within them. - -It seems to me that these considerations destroy the foundations of -the hypothesis of the cosmic origin of life on our earth; in any case -they leave the hypothesis without great significance; for if we could -even admit the possibility of a transference of living organisms from -space, the question would only be pushed a little further back by the -assumption, and not solved, for the organisms thus brought in must have -had their origin on some other planet, since they are, _ex hypothesi_, -not everlasting. - -Thus we are directed to our earth itself as the place of the origin -of the tellurian world of life, and I see no possibility of avoiding -the assumption of _spontaneous generation_. It is for me a logical -necessity. - -Even about the middle of the nineteenth century there was acute -discussion in regard to the occurrence of spontaneous generation. In -the French Academy especially Pouchet brought forward arguments in -favour of it, and Pasteur against it. Pouchet observed that living -organisms made their appearance in infusions of hay and other vegetable -material in which any possible living germs had presumably been -destroyed by prolonged boiling. Living organisms, Algæ, and Infusorians -appeared, notwithstanding the fact that the glass bottles in which -they were kept were hermetically sealed. But Pasteur showed that the -air contains numerous living germs of lowly organisms in its so-called -motes, and that, if these were first removed, Pouchet's infusion would -not exhibit any signs of life. He caused the air, which was continually -passed through the tubes, to stream first along the heated barrel of -a gun, and so destroyed these germs, and no organisms were obtained -in the infusions. He showed that the air is teeming with germs by an -experiment with boiled infusions which were allowed to lie undisturbed -for a considerable time in bottles with open necks, one on the roof -of the Institute at Paris, the other on the top of the Puy de Dôme in -Auvergne, which was at that time still the highest mountain in France. -In the Parisian experiment, organisms appeared in the bottles in a very -few days, while in those exposed to the pure air at the mountain-top -none were seen, even after months had elapsed. - -Strangely enough, these and similar experiments were at the time -regarded as conclusive proof against the existence of spontaneous -generation, though it is obvious enough that the first living being -on this earth cannot have sprung from hay, or from any other organic -substance, since that would presuppose what we are attempting to -explain. After the fiery earth had so far cooled that its outermost -layer had hardened to a firm crust, and after water had condensed to a -liquid form, there could at first only have been inorganic substances -in existence. In order to prove spontaneous generation, therefore, -it would be necessary to try to find out from what mingling of -inorganic combinations organisms could arise; to prove that spontaneous -generation could never have been possible is out of the question. - -It would be impossible to prove by experiment that spontaneous -generation could _never_ have taken place; because each negative -experiment would only prove that life does not arise _under the -conditions of the experiment_. But this by no means excludes the -possibility that it might arise under other conditions. - -Up till now all attempts to discover these conditions have been futile, -and I do not believe that they will ever be successful, not because the -conditions must be so peculiar in nature that we cannot reproduce them, -but above all, because we should not be able to perceive the results -of a successful experiment. I shall be able to prove this convincingly -without difficulty. - -If we ask ourselves the question how the living beings which might have -arisen through spontaneous generation must be constituted, and on the -other hand, in regard to what kinds of living forms we can maintain -with certainty that they _could not_ have arisen thus, it is obvious -that we must place on the latter list all organisms which presuppose -the existence of others, from which they have been derived. But to -this category belong all the organisms which possess a germ-plasm, -an idioplasm that we conceive of as composed of primary constituents -(_Anlagen_) which have gradually been evolved and accumulated through -a long series of ancestors. Thus not only all multicellular animals -and plants which reproduce by means of germ-cells, buds, and so forth, -but also all unicellular organisms, must be placed in this class. For -these last--as we have seen--possess in their nucleus a substance made -up of primary constituents, without which the mutilated body is unable -to make good its loss, in short, an idioplasm. That this plays the same -rôle in unicellular as in multicellular organisms we can infer with -the greatest certainty from the process of amphimixis, which runs its -course in an analogous way in both cases. - -Thus, even though we did not know what Ehrenberg demonstrated in the -third decade of last century, that Infusorians in an encapsuled state -can be blown about everywhere, and can even be carried across the -ocean in the dust of the trade-winds, to re-awaken to life wherever -they fall into fresh water, we should still not have remained at the -standpoint of Leuwenhoek, who regarded Infusorians as having arisen -through spontaneous generation. They cannot arise in this way, nor can -they have done so at any time, because they contain a substance made -up of primary constituents, which can only be of historic origin, and -cannot therefore have arisen suddenly after the manner of a chemical -combination. - -The same is true of all the unicellular organisms, even of those -which are much more simple in structure than the Infusorians, whose -differentiation into cortical and medullary substances, oral and anal -openings, complex arrangements of cilia and much else, betokens a high -degree of differentiation in the cell. But even the Amœba is only -apparently simple, for otherwise it could not send out processes and -retract them again, creep in a particular direction, encyst itself, -and so on, for all this presupposes a differentiation of its particles -in different directions, and a definite arrangement of them; and -there is in addition the marvellous dividing-apparatus of the nucleus -which is not wanting even in the Amœba. All this again points to a -historic evolution, a gradual acquiring and an orderly arrangement of -differentiations, and such an organism cannot have arisen suddenly like -a crystal or a chemical combination. - -Thus we are driven back to the lowest known organisms, and the question -now before us is whether these smallest living organisms, which are -only visible under the highest powers of the microscope, may be -referred to spontaneous generation. But here too the answer is, No; for -although there is no nucleus to be found, and no substance which we -can affirm with any certainty to be composed of primary constituents -or idioplasm, we do find distinct traces of a previous history, and -not the absolutely simple structure of homogeneous living particles, -unarranged in any orderly way, which is all that could be derived from -spontaneous generation. It has been shown quite recently that the -typhus bacillus possesses an extremely delicate much-branched tuft -of flagella, which gives it a tremulous motion, and in the cholera -bacillus cortical and medullary substances can be distinguished. Thus -even here there is differentiation according to the principle of -division of labour, and how numerous must be the minute vital particles -of which a substance consists when it can form such fine threads as the -flagella just mentioned! Nägeli, who elaborated an analogous train of -thought in regard to spontaneous generation, calculated the number of -these smallest vital particles (his 'micellæ') which must be contained -in a 'moneron' of 0.6 mm. diameter, if we take its dry substance at -10 per cent., and he arrived at the amazing figure of 100 billions of -vital particles. Even if we suppose the diameter of such an organism -to be 0.0006 mm., it would still be composed, according to this -calculation, of a million of these vital particles. - -We have reached, in the course of these lectures, the conviction -that minute living units form the basis of all organisms, namely, -our 'life-bearers' or 'biophors.' These must be present in countless -multitudes, and in a great number of varieties in the different forms -of life, but all agree in this, that they are simple, that is, they are -not composed in their turn of living particles, but only of molecules, -whose chemical constitution, combination, and arrangement are such as -to give rise to the phenomena of life. But they may vary, and on this -power depends the possibility of their differentiation, which has -taken place in more and more diverse ways in the course of phylogeny. -They, too, arise in the existing organism, like all vital units, only -by multiplication of the biophors already present, but they do not -necessarily presuppose a historic origin; it is conceivable of them, -at least as far as their first and simplest forms are concerned, that -they may have arisen some time or other through spontaneous generation. -In regard to them alone is the possibility of origin through purely -chemico-physical causes, without the co-operation of life already -existing, admissible. It is only in regard to them that spontaneous -generation is not inconceivable. - -We must, therefore, assume that, at some time or other in the history -of the earth, the conditions necessary to the development of these -invisible little living particles must have existed, and that the whole -subsequent development of the organic world must have depended upon an -aggregation of these biophors into larger complexes, and upon their -differentiation within these complexes. - -We shall never be able, then, directly to observe spontaneous -generation, for the simple reason that the smallest and lowest living -particles which could arise through it, the Biophoridæ, are so -extremely far below the limits of visibility, that there is no hope -of our ever being able to perceive them, even if we should succeed in -producing them by spontaneous generation. - -I do not propose to discuss the chemical problem raised by the possible -occurrence of spontaneous generation. We have already seen that dead -protoplasm, in addition to water, salts, phosphorus, sulphur, and some -other elements, chiefly and invariably contains albumen; an albuminoid -substance must, therefore, have arisen from inorganic combinations. -No one will maintain that this is impossible, for we continually see -albuminoid substances produced in plants from inorganic substances, -compounds of carbon and nitrogen; but under what conditions this would -be possible in free nature, that is, outside of organisms, cannot as -yet be determined. Possibly we may some time succeed in procuring -albumen from inorganic substances in the laboratory, and if that -happens the theory of spontaneous generation will rest upon a firmer -basis, but it will not have been experimentally proved even then. -For while dead albumen is certainly nearly allied to living matter, -it is precisely _life_ that it lacks, and as yet we do not know what -kinds of chemical difference prevail between the dead proteid and the -living; indeed we must honestly confess that it is a mere assumption -when we take for granted that there are only chemico-physical -differences between the two. It cannot be proved, in the meantime, -that there is not another unknown power in the living protoplasm, -a 'vitalistic principle,' a 'life-force,' on the activity of which -these specific phenomena of life, and particularly the continually -repeated alternation of disruption and reconstruction of the living -substance, dissimilation and assimilation, growth and multiplication, -depend. It is just as difficult to prove the converse, that it is -impossible that chemico-physical forces alone should have called forth -life in a chemical substance of very special composition. Although -no one has ever succeeded, in spite of many attempts, in thinking -out a combination of chemical substances which--as this wonderful -living substance does--on the one hand undergoes combustion with -oxygen and, on the other hand, renews itself again with 'nutritive' -material, yet we cannot infer from this the impossibility of a purely -chemico-physical basis of life, but must rather hold fast to it until -it is shown that it is not sufficient to explain the facts, thus -following the fundamental rule that natural science must not assume -unknown forces until the known ones are _proved_ insufficient. If -we were to do otherwise we should have to renounce all hope of ever -penetrating deeper into the phenomena. And we have no need to do -this, for in a general way we can quite well believe that an organic -substance of exactly proportioned composition exists, in which the -fundamental phenomena of all life--combustion with simultaneous -renewal--must take place under certain conditions by virtue of its -composition. - -How, and under what external conditions, such a substance first arose -upon the earth, from and of what materials it was formed, cannot -be answered with any certainty in the meantime. Who knows whether -the fantastic ideas of Empedocles in an altered form would not be -justified here? I mean that, at the time of the first origin of -life, the conditions necessary for many kinds of complex chemical -combinations may have been present simultaneously on the earth, and -that, out of a manifold variety of such substances, only those survived -which possessed that marvellous composition which conditioned their -continual combustion, but also their ceaseless reconstruction by -multiplication. According to Empedocles, there arose from chaos only -parts of animals--heads without bodies, arms without trunks, eyes -without faces, and so on--and these whirled about in wild confusion -and flew together as chance directed them. But those only survived -which had united rightly with others so as to form a whole, capable of -life. Translated into the language of our time, that would mean what I -have just said--that, of a large number of organic combinations which -arose, only a few, perhaps one, would possess the marvellously adjusted -composition which resulted in life, and with it self-maintenance and -multiplication; and that would be the first instance of selection! - -But let us leave these imaginings, and wait to see whether the chemists -will not possibly be able to furnish us with a starting-point for a -more concrete picture of the first origin of life. In the meantime, we -must confess that we find ourselves confronted with deep darkness. - -The question as to the 'Where' of spontaneous generation must also be -left without any definite answer. Some have supposed that life began -in the depths of the sea, others on the shore, and others in the air. -But who is to divine this, when we cannot even name theoretically the -conditions and the materials out of which albuminoid-like substances -might be built up in the laboratory? Nägeli's hypothesis still seems -to me to have the greatest probability. According to his theory, the -first living particles originated not in a free mass of water, but in -the reticulated superficial layer of a fine porous substance (clay or -sand), where the molecular forces of solid, fluid, and gaseous bodies -were able to co-operate. - -Only so much is certain, that wherever life may first have arisen upon -this earth, it can have done so only in the form of the very simple -and very minute vital units, which even now we only infer to be parts -of the living body, but which must first have arisen as independent -organisms, the 'Biophoridæ.' As these, according to our theory, -possessed the character of life, they must have possessed above all the -capacity of assimilating in the sense in which the plants assimilate, -that is, of renewing their bodily substance continually from inorganic -substances, of growing, and of reproducing. They need not on that -account have possessed the chemical constitution of chlorophyll, -although the capacity of assimilation in green plants depends upon this -substance, for we know colourless fungi, which, notwithstanding the -absence of chlorophyll, are able to build up the substance of their -body from compounds of carbon and nitrogen. - -The first advance to a higher stage of life must have been brought -about by multiplication, since accumulations of Biophoridæ, -unintegrated but connected masses, would be formed. - -In this way the threshold of microscopical visibility would gradually -be reached and crossed, but--to argue from the modern Baccilli--long -before that time a differentiation of the biophors on the principle -of division of labour would have taken place within a colony of -Biophoridæ. This first step towards higher organization must probably -have taken enormous periods of time, for before any differentiation -could occur and bring any advantage the unintegrated aggregates of -Biophors must first have become orderly, and have formed themselves -into a stable association with definite form and definite structure, -somewhat analogous to the spherical cell-colonies of _Magosphæra_ or -_Pandorina_. Only then was the further step made of a differentiation -of the individual biophors forming the colony, and this is comparable -to the species of _Volvox_ among the lower Algæ. The gradual ascent of -these colonies of biophors must, then, be referred to the principles -to which we attribute the ascent of the higher forms of life to -ever-higher and ever-new differentiations; the principles of division -of labour and selection. - -These differentiated colonies of biophors have brought us nearer to the -lowest known organisms, among which there are some whose existence we -can only infer from their pathological effects, since we have not been -able to make them visible. The bacillus of measles has never yet been -seen, but we cannot doubt its existence, and we must assume that there -are bacilli of such exceeding smallness that we shall never be able -to see them, even with the most improved methods of staining and the -strongest lenses. - -These non-nucleated Monera lead on to the stage of nucleus-formation, -and this at once implies the cell. As, on our view, the nucleus is -primarily a storehouse of 'primary constituents' (_Anlagen_), its -origin must have begun at the moment at which the differentiation of -the cell-body reached such a degree of differentiation of its parts -that a mechanical division into two halves was no longer possible, and -that the two products of division, if they were each to develop to -a new and intact whole, required a reserve of primordia (_Anlagen_) -to give rise to the missing parts. As this higher differentiation -would bring about a superiority over the lower forms of life, in that -they would make possible the utilization of new conditions of life, -but on the other hand could only survive if the differentiation of a -reserve of primary constituents, that is, a nucleus, were introduced -at the same time, the development of the nucleus can be ranged under -the principle of utility to which we traced back the evolution of all -higher and more differentiated forms of life. But it would scarcely -be profitable to try to follow out in detail the first steps in the -progress of organization under the control of selective processes, -since we know far too little about the life of the simplest organisms -to be able to judge how far their differentiations are of use in -improving their capacity for life. - -That would be a bold undertaking even in regard to unicellular -organisms, and it is only in the case of multicellular organisms that -we can speak with greater certainty and really recognize the changing -of the external conditions, in the most general and comprehensive -sense, as the fundamental cause of the lasting variations of organic -forms. We can here distinguish with certainty between the direct and -the indirect effect of external influences, and we see how these -sources of variation interact upon each other. The lowest and deepest -root of variation is without doubt the direct effect of changed -conditions. Without this the indirect effect would have had no lever -with which to work, for the primitive beginnings of variation would be -absent, and an accumulation of these through personal selection could -not take place. It is a primitive character of living substance to be -variable, that is, to be able to respond to some extent to changed -external conditions, and to vary in accordance with them, or--as we -might also say--to be able to exist in many very similar but not -identical combinations of substances, and we must imagine that even -the first biophors which arose through spontaneous generation were -different according to the conditions under which, and the substances -from which, they originated. And from each of these slightly different -beginnings there must, in the course of multiplication by fission, -have been produced a whole genealogical tree of divergent variations -of the primitive Biophoridæ, since it is inconceivable that all the -descendants would remain constantly under the same conditions of -life under which they originated. For every persistent change in -the conditions of existence, and especially of nutrition, must have -involved a variation in the constitution of the organism, whose vital -processes, and especially the repair of its body, depended on these -conditions. - -But the external influences to which the descendants of a particular -form of life were subject never remained permanently the same. Not -only did the surface of the earth and its climatic conditions change -in the course of time with the cooling of the earth, but mountains -arose and were levelled again, old land-surfaces sank out of sight or -emerged again, and so on; all that, of course, played its part in the -transformation of the forms of life, but did so to any considerable -extent only at a later stage, when there were already highly -differentiated organisms. These unknown primitive beginnings of life -must have been forced to diverge into different variations through the -different conditions of the same place in which they lived. - -Let us think of the simplest microscopic Monera on the mud of the -sea-coast, equipped with the faculty of plant-like assimilation, and we -shall see that their unlimited multiplication would cause differences -in nutrition, for those lying uppermost would be in a stronger light -than those below, and would, therefore, be better nourished, and, -consequently, would transmit the variations thus induced to their -progeny which arose by fission. Thus it is conceivable that even the -more or less favourable position as regards light would bring about the -origin of two different races from the same parent form, and as it is -conceivable in the case of light, so is it also in regard to all the -influences which cause variation in the organism. - -We have already seen that variations in the lowest (non-nucleated) -forms of life caused by the direct influence of the vital processes -may be directly transmitted to the descendants, but that in all -those whose bodies have already differentiated into a germ- or -idioplasmic-substance, in contrast to a somatic substance in the more -restricted sense, this hereditary transmission is only possible in the -case of the variations of the germ-plasm, and _hereditary_ variations -of the species can only arise by the circuitous route of influencing -the germ-plasm. The body (soma) can be caused to change by external -influences, by the use or disuse of an organ, but variations of this -kind are not transmitted; they do not become a lasting possession of -the species, but cease with the individual; they are transient changes. - -Thus it was only through those external influences--including -those from the soma of the organism itself--which affected the -germ-substance, either as a whole or in certain of its primary -constituents, that hereditarily transmissible variations of the -organism arose, and we have already discussed in detail how particular -variational tendencies may arise through the struggle of the parts -within the germ-plasm, which may give an advantage to certain groups of -primary constituents. And these tendencies are of themselves sufficient -to cause the specific type to vary further and further in given -directions. - -Nevertheless, the infinite diversity of the forms of life could never -have been brought about in this way alone, if there had not been -another--the _indirect_--effect of the changeful external influences. - -This is due to the fact that the variations of direct origin sooner -or later obtain an influence in determining the viability of their -possessors, either increasing or diminishing it. It is this, in -association with the unlimited multiplication of individuals, which -gives a basis to the principle of transformation, which it is the -immortal merit of Charles Darwin and Alfred Russel Wallace to have -introduced into science: _the principle of selection_. We have seen -that this principle may have a much more comprehensive meaning than -was attributed to it by either of these two naturalists; that there -is not merely a struggle between individuals which brings about their -adaptation to their environment, by preserving those which vary in -the most favourable way and rejecting those which vary unfavourably, -but that there is an analogous struggle between the parts of these -individuals, which, as Wilhelm Roux showed, effects the adaptation -of the parts to their functions, and that this struggle must be -assumed to occur even between the determinants and biophors of the -germ-plasm. There is thus a germinal selection, a competition between -the smaller and larger particles of the germ-plasm for space and -food, and that it is through this struggle that there arise those -definitely and purposefully directed variations of the individual, -which are transmissible because they have their seat in the immortal -germ-plasm, and without which an adaptation of individuals in the sense -and to the extent in which we actually observe it would be altogether -inconceivable. I have endeavoured to show that the whole evolution -of the living world is guided essentially by processes of selection, -in as far as adaptations of the parts to one another, and of the -whole to the conditions of life, cannot be conceived of as possible -except through these, and that all fluctuations of the organism, -from the very lowest up to the highest, are forced into particular -paths by this principle, by 'the survival of the fittest.' This ends -the whole dispute as to whether there are indifferent 'characters' -which have no influence on the existence of the species, for even the -characters most indifferent for the 'person' would not exist unless -the germinal constituents (determinants) which condition them had -been victorious in the struggle for existence over others of their -kind, and even the 'indifferent' characters, which depend solely -upon climatic or other external influences, owe their existence to -processes of germinal selection, for those elements of the determinants -concerned were victorious which throve best under such influences. -But should variations thus produced by external influence increase so -far that they become prejudicial to the survival of their bearers, -then they are either set aside by personal selection or, if that be -no longer possible, they lead to the extinction of the species. Thus -the multitude of small individual variations, which probably occur in -every species, but which strike us most in Man--the differences in the -development of mouth, nose, and eyes, in the hair, in the colour of -skin, &c., as far as they are without significance in the struggle for -existence--depend upon processes of germinal selection, which permitted -the greater development of one group of determinants, or of one kind of -biophor in one case, of another in another. The proportionate strength -of the elements of the germ-plasm is not readily lost at once, but is -handed on to successive generations, and thus even these 'indifferent' -characters are transmitted. - -It is obvious that, if the principle of selection operates in nature -at all, it must do so wherever living units struggle together for the -same requirements of life, for space and food, and these units need not -be persons, but may represent every category of vital units, from the -smallest invisible units up to the largest. For in all these cases the -conditions of the selection-process are given: individual variability, -nutrition, and multiplication, transmission of the advantage -attained, and, on the other hand, limitation of the conditions of -existence--especially food and space. The resulting struggle for -existence must, in every category of vital units, be most acute -between the individual members of each category, as Darwin emphasized -in the case of species from the very first, and persistent variations -of a species of living units can only be brought about by this kind -of struggle. Strictly speaking, therefore, we should distinguish as -many kinds of selection-processes as there are categories of living -units, and these could not be sharply separated from one another, -apart from the fact that we have to infer many of them, and cannot -recognize their gradations. Here, as everywhere else, we must break up -the continuity of nature into artificial groups, and it seems best to -assume and distinguish between four main grades of selective processes -corresponding to the most outstanding and significant categories of -vital units, namely: Germinal, Histonal, Personal, and Cormal Selection. - -Histonal Selection includes all the processes of selection which take -place between the elements of the body (soma), as distinguished from -the germ-plasm, of the Metazoa and Metaphyta, not only between the -'tissues' in the stricter sense, but also between the parts of the -tissues, that is, the lower vital units of which they are composed, and -which Wilhelm Roux, when he published his _Kampf der Teile_ ('Struggle -of the Parts'), called 'molecules.' It occurs between all the parts -of the tissues down to the lowest vital units, the biophors. We must -also reckon under histonal selection the processes of selection -which take place between the elements of the simplest organisms, and -through which these have gradually attained to greater complexity of -structure and increased functional capacity. As long as no special -hereditary substance had been differentiated, variations which arose -in the simplest organisms through selection-processes of this kind -were necessarily transmitted to the descendants, but after this -differentiation had taken place this could no longer occur--'acquired' -modifications of the soma were no longer transmitted, and the -importance of histonal selection was limited to the individual. But -this form of selection must be of the greatest importance in regard to -the adaptations of the parts which develop from the ovum, especially -during the course of development, and it is also indispensable all -through life in maintaining the equilibrium of the parts, and their -adaptation to the varying degree of function required from them (use -and disuse). But its influence does not reach directly beyond the -life of the individual, since it can only give rise to 'transient' -modifications, that is, to changes which cease with the individual life. - -In contrast to this is Germinal Selection, which depends upon the -struggle of the parts of the germ-plasm, and thus only occurs in -organisms with differentiation of somatoplasm and germ-plasm, -especially in all Metazoa and Metaphyta--forming in these the basis of -all hereditary variations. But not every individual variation to which -germinal selection gives rise persists and spreads gradually over the -whole species, for, apart from the cases we have already mentioned, in -which indifferent variations favoured by external circumstances gain -the victory, this happens only if the variations in question are of use -to their bearer, the individual. Any variation whatever may arise in a -particular individual purely through germinal selection, but it is only -the higher form of selection--Personal Selection--that decides whether -the variation is to persist and to spread to many descendants so that -it ultimately becomes the common property of the species. Germinal and -personal selection are thus continually interacting, so that germinal -selection continually presents hereditary variations, and personal -selection rejects those that are detrimental and accepts those that -are useful. I will not repeat any exposition of the marvellous way in -which personal selection reacts upon germinal selection, and prevents -it from continuing to offer unfavourable variations, and compels it to -give rise to what is favourable in ever-increasing potency. Although -it apparently selects only the best-adapted _persons_ for breeding, -it really selects the favourable id-combinations of the germ-plasm, -that is, those which contain the greatest number of favourably varying -determinants. We saw that this depends upon the multiplicity of ids -in the germ-plasm, since every primary constituent of the body is -represented in the germ-plasm, not once only, but many times, and it is -always half of the homologous determinants contained in the germ-plasm -of an individual which reach each of its germ-cells, always, moreover, -in a different combination. Thus, with the rejection of an individual -by personal selection, a particular combination of ids, a particular -kind of germ-plasm is in reality removed, and thus prevented from -having any further influence upon the evolution of the species. By -this means germinal selection itself is ultimately influenced, because -only those ids remain unrejected in the germ-plasm whose determinants -are varying in directions useful to the species. Thus there comes about -what until recently was believed to be impossible: the conditions -of life give rise to useful directions of variation, not directly, -certainly, but indirectly. - -We may distinguish as a fourth grade of selection Cormal Selection, -that is, the process of selection which effects the adaptation of -animal and plant stocks or corms, and which depends on the struggle of -the colonies among themselves. This differs from personal selection -only in that it decides, not the fitness of the individual person, but -that of the stock as a whole. It is a matter of indifference whether -the stocks concerned are stocks in the actual material sense, or only -in the metaphorical sense of sharing the common life of a large family -separated by division of labour. In both cases, in the polyp-stock as -well as in the termite or ant-colony, the collective germ-plasm, with -all its different personal forms, is what is rejected or accepted. The -distinction between this cormal selection and personal selection is, -therefore, no very deep one, because here too it is in the long run the -two sexual animals which are selected, not indeed only in reference -to their visible features, but also in reference to their invisible -characters, those, namely, which determine in their germ-plasm the -constitution of their neuter progeny or, in the case of polyps, their -asexually reproducing descendants. - -We venture to maintain that everything in the world of organisms that -has permanence and significance depends upon adaptation, and has arisen -through a sifting of the variations which presented themselves, that -is, through selection. Everything is adaptation, from the smallest -and simplest up to the largest and most complex, for if it were not -it could not endure, but would perish. The principle which Empedocles -announced, in his own peculiar and fantastic way, is the dominating -one, and I must insist upon what has so often been objected to as an -exaggeration--that everything depends upon adaptation and is governed -by processes of selection. From the first beginnings of life, up to -its highest point, only what is purposeful has arisen, because the -living units at every grade are continually being sifted according to -their utility, and the ceaseless struggle for existence is continually -producing and favouring the fittest. Upon this depends not only the -infinite diversity of the forms of life, but also, and chiefly, the -closely associated progress of organization. - -It cannot be proved in regard to each individual case, but it can be -shown in the main that attaining a higher stage in organization also -implies a predominance in the struggle for existence, because it opens -up new possibilities of life, adaptations to situations not previously -utilizable, sources of food, or places of refuge. Thus a number of the -lower vertebrates ascended from the water to the land, and adapted -themselves to life on dry land or in the air, first as clumsily moving -salamanders, but later as actively leaping frogs; thus, too, other -descendants of the fishes gained a sufficient carrying power of limbs -to raise the lightened body from the ground, and so attained to the -rapid walk of the lizards, the lightning-like leaps of the arboreal -agamas, the brief swooping of the flying-dragons, and ultimately -the continuous flight which we find in the flying Saurians and the -primitive birds of the Jurassic period, and in the birds and bats of -our own day. - -It is obvious that each of these groups, as it originated, conquered -a new domain of life, and in many cases this was such a vast one, and -contained so many special possibilities, that numerous subordinate -adaptations took place, and the group broke up into many species and -genera, even into families and orders. All this did not come about -because of some definitely directed principle of evolution of a -mysterious nature, which impelled them to vary in this direction and in -no other, but solely through the rivalry of all the forms of life and -living units, with their enormous and ceaseless multiplication, in the -struggle for existence. They were, and they are still, forced to adapt -themselves to every new possibility of life attainable to them; they -are able to do this because of the power of the lowest vital units of -the germ to develop numerous variations; and they are obliged to do it -because, of the endless number of descendants from every grade of vital -unit, it is only the fittest which survive. - -Thus higher types branched off from the lower from time to time, -although the parent type did not necessarily disappear; indeed it -could not have disappeared as long as the conditions of its life -endured; it was only the superfluous members of the parent form that -adapted themselves to new conditions, and as, in many cases, these -required a higher organization, there arose a semblance of general -upward development which simulated a principle of evolution always -upwards. But we know that, at many points on this long road, there -were stations where individual groups stopped short and dropped back -again to lower stages of organization. This kind of retreat was almost -invariably caused by a parasitic habit of life, and in many cases this -degeneration has gone so far that it is difficult to recognize the -relationship of the parasite to the free-living ancestors and nearest -relatives. Many parasitic Crustaceans, such as the Rhizocephalids, -lack almost all the typical characteristics of the crustacean body, -and dispense not only with segmentation, with head and limbs, but also -with stomach and intestine. As we have seen, they feed like the lower -fungi, by sucking up the juices of their hosts, by means of root-like -outgrowths from the place where the mouth used to be. Nevertheless, -their relationship to the Cirrhipedes can be proved from their larval -stages. There are, however, parasites in the kidneys of cuttlefish--the -Dicyemidæ--in regard to which naturalists are even now undecided -whether they ought to form a lowly class by themselves between -unicellular animals and Metazoa, or whether they have degenerated, by -reason of their parasitism, from the flat worms to a simplicity of -structure elsewhere unknown. They consist only of a few external cells, -which enclose a single large internal cell, possess no organs of any -kind, neither mouth nor intestine, neither nervous system nor special -reproductive organs. But although degeneration cannot be proved in this -case, it can be in hundreds of other cases with absolute certainty, -as, for instance, in the Crustacea belonging to the order of Copepods, -which are parasitic upon fishes, in which we find all possible stages -of degeneration, according to the degree of parasitism, that is, to -the greater or less degree of dependence upon the host; for organs -degenerate and disappear in exact proportion to the need for them, and -they thus show us that degeneration also is under the domination of -adaptation. - -Thus retrogressive evolution also is based upon the power of the living -units to respond to changing influences by variation, and upon the -survival of the fittest. - -The roots of all the transformations of organisms, then, lie in changes -of external conditions. Let us suppose for a moment that these might -have remained absolutely alike from the epoch of spontaneous generation -onwards, then no variation of any kind and no evolution would have -taken place. But as this is inconceivable, since even the mere growth -of the first living substance must have exposed the different kinds -of biophors composing it to different influences, variation was -inevitable, and so also was its result--the evolution of an animate -world of organisms. - -External influences had a twofold effect at every stage upon every -grade of vital unit, namely, that of directly causing variation and -that of selecting or eliminating. Not only the biophors, but every -stage of their combinations, the histological elements, chlorophyll -bodies, muscle-disks, cells, organs, individuals, and colonies, can -not only be caused to vary by the external influences to which they -are subjected, but can be guided by these along particular paths of -variation, so that among the variations which crop up some are better -adapted to the conditions than others, and these thrive better, and -thus alone form the basis of further evolution. In this way definite -tendencies of evolution are produced, which do not move blindly and -rigidly onwards like a locomotive which is bound once for all to the -railroad, but rather in exact response to the external conditions, -like an untrammelled pedestrian who makes his way, over hill and dale, -wherever it suits him best. - -The ultimate forces operative in bringing about this many-sided -evolution are the known--and although we do not recognize it as yet, -perhaps the unknown--chemico-physical forces which certainly work only -according to laws; and that they are able to accomplish such marvellous -results is due to the fact that they are associated in peculiar and -often very complex different kinds of combinations, and thus conform -to the same sort of regulated arrangements as those which condition -the operations of any machine made by man. All complex effects depend -upon a co-operation of forces. This is seen, to begin with, in the -chemical combinations whose characters depend entirely upon the number -and arrangement of the elementary substances of which they consist; the -atoms of carbon, hydrogen, and oxygen, which compose sugar, can also -combine to form carbonic acid gas and water, or alcohol and carbonic -acid gas; and the same thing is true if we ascend from the most complex -but still inanimate organic molecules to those chemical combinations -which, in a still higher form, condition the phenomena of life, to the -lowest living units, the biophors. Not only do these last differ in -having life, but they themselves may appear in numerous combinations, -and can combine among themselves to form higher units, whose characters -and effectiveness will depend upon these combinations. Just as man may -adjust various metallic structures, such as wheels, plates, cylinders, -and mainsprings in the combination which we call a watch, and which -measures time for us, so the biophors of different kinds in the living -body may form combinations of a second, third, &c., degree, which -perform the different functions essential to life, and by virtue of -their specific, definite combination of elementary forces. - -But if it be asked, what replaces human intelligence in these -purposeful combinations of primary forces, we can only answer that -there is here a self-regulation depending upon the characters of the -primary vital parts, and this means that these last are caused to vary -by external influences and are selected by external influences, that -is, are chosen for survival or excluded from it. Thus combinations of -living units must always result which are appropriate to the situation -at the moment, for no others can survive, although, as we have seen, -they must arise. This is our view of the causes of the evolution of the -world of organisms; the living substance may be compared to a plastic -mass which is poured out over a wide plain, and in its ceaseless -flowing adapts itself to every unevenness, flows into every hole, -covers every stone or post, leaving an exact model of it, and all this -simply by virtue of its constitution, which is at first fluid and then -becomes solid, and of the form of the surface over which it flows. - -But it is not merely the surface in our analogy which determines -the form of the organic world; we must take account not only of the -external conditions of existence, but also of the constitution of -the flowing mass, the living substance itself, at every stage of its -evolution. The combination of living units which forms the organism is -different at each stage, and it is upon this that its further evolution -depends; this difference determines what its further evolution _may_ -be, but the conditions of life determine what it _must_ be in a -particular case. Thus, in a certain sense, it was with the first -biophors, originating through spontaneous generation, that the whole -of the organic world was determined, for their origin involved not -only the physical constitution by which the variations of the organism -were limited, but also the external conditions, with their changes up -till now, to which organisms had to adapt themselves. There can be -no doubt that on another planet with other conditions of life other -organisms would have arisen, and would have succeeded each other in -diverse series. On the planet Mars, for instance, with its entirely -different conditions as regards the proportions in weight and volume of -the chemical elements and their combinations, living substance, if it -could arise at all, would occur in a different chemical composition, -and thus be equipped with different characters, and without doubt -also with quite different possibilities of further development and -transformation. The highly evolved world of organisms which we may -suppose to exist upon Mars, chiefly on the ground of the presence -of the remarkable straight canals discovered by Schiaparelli, must -therefore be thought of as very different from the terrestrial living -world. - -But upon the earth things could not have been very different from -what they actually are, even if we allow a good deal to chance and -assume that the form of seas and continents might have been quite -different, the folding of the surface into mountains and valleys, and -the formation of rents and fissures, with the volcanoes that burst -from them, need not have turned out exactly as it has done. In that -case many species would never have arisen, but others would have taken -their place; on the whole, the same types of species-groups would -have succeeded each other in the history of the earth. Let us suppose -that the Sandwich Islands, like many other submarine volcanoes, had -never risen above the surface of the sea, then the endemic species of -snails, birds, and plants which now live there could not have arisen, -and if the volcanic group of the Galapagos Islands had arisen from -the sea not in their actual situation, but forty degrees further -south or north, or 1,000 kilometres further west, then it would have -received other colonists, and probably fewer of them, and a different -company of endemic species would be found there now. But there would -be terrestrial snails and land-birds none the less, and on the whole -we may say that both the extinct and the living groups of organisms -would have arisen even with different formations of land and sea, of -heights and depths, of climatic changes, of elevations and depressions -of the earth's crust, at least in so far as they are adaptations to -the more general conditions of life and not to specialized ones. The -great adaptation to swimming in the sea, for instance, must have taken -place in any case; swimming worms, swimming polyps (Medusæ), swimming -vertebrates, would have arisen; terrestrial animals would have evolved -also, on the one hand from an ancestry of worms in the form of jointed -animals and land or freshwater worms, and again from an ancestry of -fishes. Aerial animals would also undoubtedly have evolved even if the -lands had been quite differently formed and bounded, and I know of no -reason why the adaptation to flight should not have been attempted in -as many different ways as it has actually been by so many different -groups--the insects, the reptiles (the flying Saurians of the Jurassic -period), the extinct _Archæopteryx_, the birds, and the bats among -mammals. - -We can trace plainly in every group the attempt not only to spread -itself out as far as possible over as much of the surface of the earth -as is accessible to it, but also to adapt itself to all possible -conditions of life, as far as the capacity for adaptation suffices. -This is very obvious from the fact that such varied groups have striven -to rise from life on the earth to life in the air, and have succeeded -more or less perfectly, and we can see the same thing in all manner of -groups. Almost everywhere we find species and groups of species which -emancipate themselves from the general conditions of life in their -class, and adapt themselves to very different conditions, to which -the structure of the class as a whole does not seem in the least -suited. Thus the mammals are lung-breathers, and their extremities -are obviously adapted for locomotion on the solid earth, yet several -groups have returned to aquatic life, as, for instance, the family of -otters and the orders of seals and whales. Thus among insects which -are adapted for direct air-breathing, certain families and stages -of development have returned to aquatic life, and have developed -breathing-tubes by means of which they can suck in air from the surface -of the water into their tracheal system, or so-called tracheal gills, -into which the air from the water diffuses. But the most convincing -proof of the organism's power of adaptation is to be found in the -fact that the possibility of living parasitically within other -animals is taken advantage of in the fullest manner, and by the most -diverse groups, and that their bodies exhibit the most marvellous and -far-reaching adaptations to the special conditions prevailing within -the bodies of other animals. We have already referred to the high -degree reached by these adaptive changes, how the parasite may depart -entirely from the type of its family or order, so that its relationship -is difficult to recognize. Not only have numerous species of flat worms -and round worms done this, but we find numerous parasites among the -great class of Crustaceans; there are some among spiders, insects, -medusoids, and snails, and there are even isolated cases among fishes. - -If we consider the number of obstacles that have to be overcome in -existence within other animals, and how difficult and how much a matter -of chance it must be even to reach to such a place as, for instance, -the intestine, the liver, the lungs, or even the brain or the blood of -another animal, and when, on the other hand, we know how exactly things -are now regulated for every parasitic species so that its existence is -secured notwithstanding its dependence upon chance, we must undoubtedly -form a high estimate of the plasticity of the forms of life and their -adaptability. And this impression will only be strengthened when we -remember that the majority of internal parasites do not pass directly -from one host to another, but do so only through their descendants, and -that these descendants, too, must undergo the most far-reaching and -often unexpected adaptations in relation to their distribution, their -penetration into a new host, and their migrations and change of form -within it, if the existence of the species is to be secured. - -We are tempted to study these relations more closely; but it is now -time to sum up, and we must no longer lose ourselves in wealth of -detail. Moreover, the life-history of many parasites, and of the -tape-worm in particular, is widely known, and any one can easily fill -up the story, of which we have given a mere outline. I simply wish to -point out that in parasitic animals there is a vast range of forms of -life in which the most precise adaptation to the conditions occurs -in almost every organ, and certainly at every stage of life, in the -most conspicuous and distinct manner. In the earlier part of these -lectures we gained from the study of the diverse protective means by -which plants and animals secure their existence the impression that -whatever is suited to its end (_Das Zweckmässige_) does not depend -upon chance for its origin, but that every adaptation which lies at -all within the possibilities of a species will arise if there is any -occasion for it. This impression is notably strengthened when we think -of the life-history of parasites, and we shall find that our view -of adaptations as arising, not through the selection of indefinite -variations, but through that of variations in a definite direction, -will be confirmed. Adaptations so diverse, and succeeding one another -in such an unfailing order as those in the life-history of a tape-worm, -a liver-fluke, or a _Sacculina_, cannot possibly depend upon pure -chance. - -Nevertheless, chance does play a part in adaptations and -species-transformations, and that not only in relation to the -fundamental processes within the germ-plasm, but also in connexion -with the higher stages of the processes of selection, as I have -already briefly indicated. After the publication of my hypothesis of -germinal selection it was triumphantly pointed out that I had at last -been obliged to admit a phyletic evolutionary force, the 'definitely -directed' variation of Nägeli and Askenazy. This reproach--if to -allow oneself to be convinced be a reproach--is based upon a serious -misunderstanding. My 'variation in a definite direction' does not refer -to the evolution of the organic world as a whole. I do not suppose, -as Nägeli did, that this would have turned out essentially as it has -actually done, even although the conditions of life or their succession -upon the earth had been totally different; I believe that the organic -world, its classes and orders, its families and species, would have -differed from those that have actually existed, both in succession and -appearance, in proportion as the conditions of life were different. -My 'variation in a definite direction' is not predetermined from the -beginning, is not, so to speak, exclusive, but is many-sided; each -determinant of a germ-plasm may vary in a plus or minus direction, and -may continue under certain circumstances in the direction once begun, -but its components, the different biophors, may do the same, and so -likewise may the groups, larger and smaller, of biophors which form -the primordia (_Anlagen_) of the organs within the germ-plasm. Thus -an enormously large number of variational tendencies is available for -every part of the complete organism, and as soon as a variation would -be of advantage it arises--given that it is within the possibilities -of the physical constitution of the species. It occurs because its -potentialities are already present, but it persists and follows a -definite course because this is the one that is favoured. In other -words, it is primarily fixed by germinal selection alone, but is then -preferred by personal selection above the variants running parallel -with it. In my opinion the definite direction of the chance germinal -variations is determined only by the advantage which it affords to -the species with regard to its capacity for existence. But according -to Nägeli the direction of a variation is quite independent of its -utility, which may or may not exist. From Nägeli's point of view we -could never understand the all-prevailing adaptation, but if the -utility of a variant is itself sufficient to raise it to the level of a -persistent variational tendency, then we understand it. - -Years ago (1883) I compared the species to a wanderer who has before -him a vast immeasurable land, through which he is at liberty to choose -whatever path he prefers, and in which he may sojourn wherever and -for as long as he pleases. But although he may go or stay entirely -of his own free will, yet at all times his going or staying will be -determined--it must be so and cannot be otherwise--by two factors: -first, by the paths available at each place--the variations which crop -up--and secondly, by the prospects each of these available paths open -up to him. He is striving after a restful place of abode which shall -afford him comfortable subsistence, his former home having been spoilt -for him by increasing expensiveness or too great competition. Even the -direction of his first journey will not depend upon chance, since of -the many paths available he will, and must, choose that which leads to -a habitable and not too crowded spot. If this has been reached--that is -to say, if the species has adapted itself to the new conditions--the -colonist sets up his abode there, and remains as long as a comfortable -existence and a competence are secure; but if these fail him, if grain -becomes scarce, or if prices rise, or if a dangerous epidemic breaks -out, then he makes up his mind to wander anew, and once more he will -choose, among the many available paths, that which offers him the -prospect of the speediest and most certain exit from the threatened -region, and leads him to another where he may live without risk. There, -too, he will remain as long as he is comfortable and not exposed to -want or danger, for the species as a whole only becomes transformed -when it must. And so it will go on _ad infinitum_; the traveller will, -when he is scared away from one dwelling-place, be able to continue his -journey in many directions, but he will always select the one path -which offers him the best prospects of a comfortable settlement, and -will follow it only to the nearest suitable place of abode, and never -further. The transformation of a species only goes on until it has -again completely adapted itself. In this way he will in the course of -years have traversed a large number of different places which, taken -together, may lie in a strange and unintelligible course, but this -course has nevertheless not arisen through a mere whim, but through -the twofold necessity of starting from a given spot--that in which he -had previously lived--the constitution of the species, and secondly of -choosing the most promising among the many available paths. - -But chance does play a part in determining the route of the traveller, -for on it depends the nature of the conditions in the surroundings of -his previous dwelling-place, when he is forced to make another move; -for these conditions change, colonies are extended or depopulated, -a town previously cheap becomes dear, competition increases or -decreases, disease breaks out or disappears; in short, the chances of a -pleasureable sojourn in a particular place may alter and determine the -wanderer who is on the point of leaving his place of abode to take a -different direction from that which he would probably have chosen, say, -ten years earlier. - -The analogy might be carried further, as, for instance, to illustrate -the possibility of a splitting up of the species; we may suppose that -instead of one wanderer there is a pair, who found a family at their -first halting-place. Children and grandchildren grow up in numbers and -food becomes scarce. One part of the descendants still finds enough -to live upon, but the rest set out to look for a new habitation. In -this case, too, many paths, sidewards or backwards, stand open to the -wanderers, but only those paths will be actually and successfully -followed by any company of them which will lead to a habitable place -where settlement is possible. If some of the descendants follow paths -with no such prospect they will soon turn back or will succumb to the -perils of the journey. - -It seems to me that the contrast between this and Nägeli's view of -the transmutation of species is obvious enough. According to him the -wanderer is not free to choose his path, but goes on and on along a -definite railway-line that only diverges here and there, and it cannot -be foreseen whether the track leads to paradisaic dwellings or to -barren wastes--the travellers must just make the best of what they -find. They carry a marvellous travelling outfit with them--a sort of -_Tischlein, deck' dich_--the Lamarckian principle, but the magic power -of this is very doubtful, and it will hardly suffice to guard them -against the heat of the deserts, the frost of the Arctic regions, or -the malaria of the marshes into which their locomotive blindly carries -them. - -According to my view, the traveler--that is, the species--has always -a large choice of paths, and is able, even while he is on the way, -to discern whether he has chosen a right or a wrong one; moreover, -in most cases, one or, it may be, a number of the paths lead to the -desired dwelling-place. But it also undoubtedly happens that, after -long wandering and when many regions have been traversed, a company -may finally arrive at a place which is quite habitable and inviting at -first sight, but which is surrounded on several sides by the sea or by -a rushing stream. As long as the soil remains fertile and the climate -healthy all goes well, but when matters change in this respect, and -perhaps the only way back lies through marshes and desert land and is -therefore impassable, then the colony will gradually die out--that is -the death of the species. - -But let us now leave our parable and inquire what paths the organic -world has actually taken in its transformations, in what succession -the individual forms of life have evolved from one another; in short, -how the actual genealogical tree of this earth's animate population is -really constructed in detail. To this I can only reply that we have -many well-grounded suppositions, but only real certainty in regard -to isolated cases. Thus the genealogical tree of the horse has been -traced far back, and a great deal is known of the phylogeny of several -Gastropods and Cephalopods, but in regard to the genealogical tree -of organisms as a whole we can only make guesses, many of which are -probable, but are never quite certain. The palæontological records -which the earth's crust has preserved for us for all the ages are much -too incomplete to admit of any certainty. Many naturalists, notably -Ernst Haeckel, have done good service in this direction, for from -what we know of palæontology, embryology, and morphology, they have -constructed genealogical trees of the different groups of organisms, -which are intended to show us the actual succession of animal and plant -forms. But, interesting as these attempts are, they cannot for the -most part be anything more than guesswork, and I need not, therefore, -state or discuss them here in any detail, since they can afford us no -aid in regard to the problem of the origin of species with which these -lectures are concerned. In regard to the animal world at least--and the -case of plants is probably very similar--the record of fossil forms -fails us at an early stage. Thus the oldest and deepest strata in which -fossils can be demonstrated, the Cambrian formation, already contains -Crustaceans, animals at a relatively high stage of organization, which -must have been preceded by a very long series of ancestors of which -no trace has been preserved. The whole basal portion of the animal -genealogical tree, from the lowest forms of life at least up to these -primitive Crustaceans, the Trilobites, lies buried in the deepest -sedimentary rocks raised from the sea-floor, the crystalline schists, -in which it is unrecognizable. Enormous pressure and, probably also, -high temperature have destroyed the solid parts as far as there were -any, and the soft parts have only left an occasional impression even in -the higher strata. - -Thus enormous periods of time must have elapsed from the beginning -of life to the laying down of that deepest 'Palæozoic' formation, -the Cambrian, for not only does the whole chain which leads from the -Biophoridæ to the origin of the first unicellulars fall within this -period, as well as the evolution of these unicellulars themselves -into their different classes, and their integration into the first -multicellulars, but also the evolution of these last into all the main -branches of the animal kingdom as it is now, into Sponges, Starfishes, -and their allies, Molluscs, Brachiopods, and Crustaceans, for all these -branches appear even in the Cambrian formation, and we may conclude -that the worms also, most of which are soft and not likely to be -preserved, were abundantly present at that time, since jointed animals -like the Crustaceans can only have arisen from worms. Moreover, we have -every reason for the assumption that Cœlenterates also, that is to say -polyps and medusoids, lived in the Cambrian seas, because their near -relatives with a solid skeleton, the corals, are represented in the -formation next above, the Silurian. The same is true of the fishes, -of which the first undoubtedly recognizable remains, the spines of -sharks, have been found in the Silurian. These two presuppose a long -preparatory history, and thus we come to the conclusion already stated, -that all the branches of the animal kingdom were already in existence -when the earth's crust shut up within itself the first records -available for us of the ancestors of our modern world of organisms. - -Of course at that time the higher branches had only been represented -by their lower classes, and this is true especially of vertebrates, -so that, from the laying down of the Cambrian strata to the modern -world of organisms, a very considerable increase of complexity in -structure and an infinite diversifying of new groups must have taken -place. Amphibians do not appear to have been present in Cambrian times; -reptiles are represented in the Carboniferous strata, but only appear -in abundance in Secondary times; birds appear first in the Jurassic, -but in a very different guise (_Archæopteryx_) from the modern forms, -covered indeed with feathers, but still possessing a reptilian tail; -later they occur as toothed birds in the Cretaceous, and in Tertiary -times they have their present form. The development of mammals must -have run almost parallel with that of birds, that is, from the -beginning of Secondary times onwards, and their highest and last member -appears, as far as is known to research, only in post-Glacial times, in -the Diluvial deposits. - -To the types which have arisen since the Cambrian period belongs the -class of Insects with its twelve orders and its enormous wealth of -known species, now reckoned at 200,000. They are demonstrable first in -the Devonian, and then in the Carboniferous period, in forms, just as -our theory requires, with _biting_ mouth-organs; it is not until the -Cretaceous strata that insects with purely suctorial mouth-organs--bees -and butterflies--occur, as it was also at that time that the flowers, -which have evolved in mutual adaptation with insects, first appeared. - -The number of fossil species hitherto described is reckoned at about -80,000--certainly only a mere fragment of the wealth of forms of -life which have arisen on our earth throughout this long period, and -which must have passed away again; for very few _species_ outlive a -geological epoch, and even genera appear only for a longer or shorter -time, and then disappear for ever. But even of many of the older -classes, such, for instance, as the Cystoids among the Echinoderms of -the Silurian seas, no living representative remains; and in the same -way, the Ichthyosaurs or fish-lizards of the Secondary times have -completely disappeared from our modern fauna, and many other animal -types, like the class of Brachiopods and the hard-scaled Ganoid fishes, -have almost died out and are represented only by a few species in -specially sheltered places, such as the great depths of the sea, or in -rivers. - -Thus an incredible wealth of animal and plant species was potentially -contained in these simplest and lowest 'Biophorids' which lay far -below the limits of microscopic visibility--an indefinitely greater -wealth than has actually arisen, for that is only a small part of -what was possible, and of what would have arisen had the changes of -life-conditions and life-possibilities followed a different course. The -greater the complexity of the structure of an organism is, the more -numerous are the parts of it which are capable of variation, and the -different directions in which it can adapt itself to new conditions; -and it will hardly be disputed that _potentially_ the first Biophorids -contained an absolutely inexhaustible wealth of forms of life, and -not merely those which have actually been evolved. If this were not -so, Man could not still call forth new animal and plant forms, as he -is continually doing among our domesticated animals and cultivated -plants, just as the chemist is continually 'creating' new combinations -in the laboratory which have probably never yet occurred or been -formed on the earth. But just as the chemist does not really 'create' -these combinations, but only brings the necessary elements and their -forces together in such a combination that they must unite to form the -desired new body, so the breeder only guides the variational tendencies -contained in the germ-plasm, and consciously combines them to procure a -new race. And what the breeder does within the narrow limits of human -power is being accomplished in free nature, through the conditions -which allow only what is fit to survive and reproduce, and thus bring -about the wonderful result--as though it were guided by a superior -intelligence--the adaptation of species to their environment. - -Thus in our time the great riddle has been solved--the riddle of the -origin of what is suited to its purpose, without the co-operation of -purposive forces. Although we cannot demonstrate and follow out the -particular processes of transformation and adaptation in all their -phases with mathematical certainty, we can understand the principle, -and we see the factors through the co-operation of which the result -must be brought about. It has lately become the fashion, at least among -the younger school of biologists, to attach small value to natural -selection, if not, indeed, to regard it as a superseded formula; -mathematical proofs are demanded or, at any rate, desired. I do not -believe that we shall ever arrive at giving such proofs, but we shall -undoubtedly succeed in clearing up much that now remains obscure, and -in essentially modifying and correcting many of the theories we have -formed in regard to this question. But what has been already gained -must certainly be regarded as an enormous advance on the knowledge of -fifty years ago. We now _know_ that the modern world of organisms has -been evolved, and we can form an idea, though still only an imperfect -one, how and through the co-operation of what factors it could and must -have evolved. - -When I say _must_, this refers only to the course of evolution from -a given beginning; but as to this beginning itself, the spontaneous -generation of the lowest Biophorids from inorganic material, we are far -from having understood it as a necessary outcome of its causes. And if -we have assumed it as a reasonable postulate, we by no means seek to -conceal that this assumption is far from implying an understanding of -what the process of biogenesis was. I do not merely mean that we do -not know under what external conditions the origin of living matter, -even in the smallest quantity, can take place; I mean, especially, -that we do not understand how this one substance should suddenly -reveal qualities which have never been detected in any other chemical -combination whatever--the circulation of matter, metabolism, growth, -sensation, will, and movement. But we may confidently say that we shall -never be able fully to understand these specific phenomena of life, as -indeed how should we, since nothing analogous to them is known to us, -and since understanding always presupposes a comparison with something -known. Even although we assume that we might succeed in understanding -the mere chemistry of life, as is not inconceivable, I mean the -_perpetuum mobile_ of dissimilation and assimilation, the so-called -'animal' functions of the living substance would remain uncomprehended: -Sensation, Will, Thought. We understand in some measure how the kidneys -secrete urine, or the liver bile; we can also--given the sensitiveness -to stimulus of the living substance--understand how a sense-impression -may be conveyed by the nerves to the brain, carried along certain -reflex paths to motor nerves and give rise to movement of the muscles, -but how the activity of certain brain-elements can give rise to a -thought _which cannot be compared with anything material_, which is -nevertheless able to react upon the material parts of our body, and, as -Will, to give rise to movement--that we attempt in vain to understand. -Of course the dependence of thinking and willing upon a material -substratum is clear enough, and it can be demonstrated with certainty -in many directions, and thus materialism is so far justified in drawing -parallels between the brain and thought on the one hand, and the -kidneys and urine on the other, but this is by no means to say that we -have understood how Thought and Will have come to be. In recent times -it has often been pointed out that the physical functions of the body -increase very gradually with the successive stages of the organization, -and from the lowest beginnings ascend slowly to the intelligence of -Man, in exact correspondence with the height of organization that has -been reached by the species; that they begin so imperceptibly among -the lower animal forms that we cannot tell exactly where the beginning -is; and it has been rightly concluded from this that the elements of -the Psyche do not originate in the histological parts of the nervous -system, but are peculiar to all living matter, and it has further been -inferred that even inorganic material may contain them, although in an -unrecognizable expression, and that their emergence in living matter -is, so to speak, only a phenomenon of summation. If we are right in our -assumption of a spontaneous generation it can hardly be otherwise, but -saying this does not mean that we have understood Spirit, but at most -secures us the advantage and the right of looking at this world, as far -as we know it, as a unity. This is the standpoint of Monism. - -The psychical phenomena, which we know from ourselves, and can assume -among animals with greater certainty the nearer they stand to us, -occupy a domain by themselves, and such a vast and complex one that -there can be no question of bringing it within the scope of our present -studies, and the same is true of the phyletic development of Man. But -we must at least take up a position in regard to these problems, and -there can be no question that Man has evolved from animal ancestors, -whose nearest relatives were the Anthropoid Apes. Not many years ago -bony remains of a human skeleton, or at least of some form very near -to modern Man, were found in the Diluvial deposits of Java, and this -has been designated _Pithecanthropus erectus_, and perhaps rightly -regarded as a transition form between Apes and Man. It is possible that -more may yet be discovered; but even if that is not so, the conclusion -that Man had his origin from animal forefathers must be regarded as -inevitable and fully established. We do not draw conclusions with our -eyes, but with our reasoning powers, and if the whole of the rest of -living nature proclaims with one accord from all sides the evolution -of the world of organisms, we cannot assume that the process stopped -short of Man. But it follows also that the _factors_ which brought -about the development of Man from his Simian ancestry must be the same -as those which have brought about the whole of evolution: change of -external influences in its direct and indirect effects, and, besides -this, germinal variational tendencies and their selection. And in -this connexion I should like to draw attention to a point which has, -perhaps, as yet received too little attention. - -Selection only gives rise to what is suited to its end; _beyond -that it can call forth nothing_, as we have already emphasized on -several occasions. I need only recall the protective leaf-marking of -butterflies, which is never a botanically exact copy of a leaf, with -all its lateral veins, but is comparable rather to an impressionist -painting, in which it is not the reproduction of every detail that is -of importance, but the total impression which it makes at a certain -distance. If we apply this to the organs and capacities of Man, we -shall only expect to find these developed as far as their development -is of value for the preservation of his existence and no further. But -this may perhaps seem a contradiction of what observation teaches us, -that, for instance, our eyes can see to the infinite distance of the -fixed stars, although this can be of no importance in relation to -the struggle for existence. But this intensity of the power of vision -has obviously not been acquired for the investigation of the starry -heavens, but was of the greatest value in securing the existence of -many of our animal ancestors, and was not less important for our own. -In the same way our finely evolved musical ear might be regarded as a -perfecting of the hearing apparatus far beyond the degree necessary to -existence, but this is not really the case: our musical ear, too, has -been inherited from our animal ancestors, and to them, as to primitive -Man, it was a necessity of existence. It was quite necessary for the -animals to distinguish the higher and lower notes of a long scale, -sharply and certainly, in order to be able to evade an approaching -enemy, or to recognize prey from afar. That we are able to make music -is, so to speak, only an unintentional accessory power of the hearing -organs, which were originally developed only for the preservation of -existence, just as the human hand did not become what it is _in order -to play the piano_, but to touch and seize, to make tools, and so on. - -_Must this, then, be true also of the human mind?_ Can it, too, only -be developed as far as its development is of advantage to Man's -power of survival? I believe that this is certainly the case in a -general way; the intellectual powers which are the common property -of the human race will never rise beyond these limits, but this is -not to say that certain individuals may not be more highly endowed. -The possibility of a higher development of certain mental powers or -of their combinations--whether it be intelligence, will, feeling, -inventive power, or a talent for mathematics, music or painting--may -be inferred with certainty from our own principles; for not only may -the variational tendencies of individual groups of determinants in the -germ-plasm be continued for a series of generations without becoming -injurious, that is to say, without being put a stop to by personal -selection, but sexual intermingling always opens up the possibility -that some predominantly developed intellectual tendencies (_Anlagen_) -may combine in one way or another, and so give rise to individuals of -great mental superiority, in whatever direction. In this way, it seems -to me, the geniuses of humanity have arisen--a Plato, a Shakespeare, a -Goethe, a Beethoven. But they do not last; they do not transmit their -greatness; if they leave descendants at all, these never inherit the -_whole_ greatness of their father, and we can easily understand this, -since the greatness does not depend upon a single character, but upon -a particular combination of many high mental qualities (_Anlagen_). -Geniuses, therefore, probably never raise the average of the race -through their descendants; they raise the intellectual average only -through their own performances, by increasing the knowledge and power -handed on by tradition from generation to generation. But the raising -of the average of mental capacity, which has undoubtedly taken place -to a considerable degree from the Australasian aborigines to the -civilized peoples of antiquity and of our own day, can only depend on -the struggle for existence between individuals and races. - -But if the human mind has been raised to its present level through -the same slow process of selection by means of which all evolution -has been directed and raised to the height necessary for the 'desired -end,' we must see in this a definite indication that even the greatest -mind among us can never see beyond the conditions which limit our -capacity for existence, and that now and for all time we cannot hope -to understand what is supernatural. We can recognize the stars in the -heavens, it is true, and after thousands of years of work we have -succeeded in determining their distance, their size, and gravity, as -well as their movements and the materials of which they are composed, -but we have been able to do all this with a thinking power created -for the conditions of human existence upon the earth, that is to say, -developed by them, just as we do not only grasp with our hands, but -may also play the piano with them. But all that involves a higher -thinking power that would enable us to recognize the pseudo-ideas of -everlastingness and infinity, the limits of causality, in short, all -that we do not know but regard as at best a riddle, will always remain -sealed to us, because our intelligence did not, and does not, require -this power to maintain our capacity for existence. - -I say this in particular to those who imagine they have summed up the -whole situation when they admit that much is still lacking to complete -knowledge, say, to a true understanding of the powers of Nature or -of the Psyche, but who do not feel that in spite of all our very -considerably increased knowledge we stand before the world as a whole -as before a great riddle. But I say it also to those who fear that the -doctrine of evolution will be the overthrow of their faith. Let them -not forget that truth can only be harmful, and may even be destructive, -when we have only half grasped it, or when we try to evade it. If -we follow it unafraid, we shall come now and in the future to the -conclusion that a limit is set to our knowledge by our own minds, and -that beyond this limit begins the region of faith, and this each must -fashion for himself as suits his nature. In regard to ultimate things -Goethe has given us the true formula, when the 'Nature-spirit' calls -to Faust, 'Du gleichst dem Geist, den Du begreifst, nicht mir!' For all -time Man must repeat this to himself, but the need for an ethical view -of the world, a religion, will remain, though even this must change in -its expression according to the advance of our knowledge of the world. - -But we must not conclude these lectures in a spirit of mere -resignation. Although we must content ourselves without being able to -penetrate the arcana of this wonderful world, we must remain conscious, -at the same time, that these unfathomable depths exist, and that we -may 'still verehren was unerforschlich ist' (Goethe). But the other -half of the world, I mean the part which is accessible to us, discloses -to us such an inexhaustible wealth of phenomena, and such a deep and -unfailing enjoyment in its beauty and the harmonious interaction of the -innumerable wheels of its marvellous mechanism, that the investigation -of it is quite worthy to fill our lives. And we need have no fear -that there will ever be any lack of new questions and new problems to -solve. Even if Mankind could continue for centuries quietly working -on in the manifold and restless manner that has, for the first time -in the history of human thought, characterized the century just gone, -each new solution would raise new questions above and below, in the -immeasurable space of the firmament, as in the world of microscopical -or ultramicroscopical minuteness, new insight would be gained, -new satisfaction won, and our enthusiasm over the marvel of this -world-mechanism, so extraordinarily complex yet so beautifully simple -in its operation, will never be extinguished, but will always flame up -anew to warm and illumine our lives. - - - - -INDEX - -[References to vol. ii have the volume prefixed.] - - - Accessory idioplasm, 383. - - Acræides, immunity of, 100. - - Adaptation, in leaf butterflies, ii. 346; - of the sperm-cells to fertilization, 278, 279; - facultative, ii. 278; - functional, 244; - harmonious, ii. 80, 197; - not chance but necessity, ii. 346; - all evolution depends upon, ii. 347. - - Affinities, vital, within the 'person,' ii. 36; - within the id, 374. - - Agassiz, L., immutability of the species, 16. - - Alcoholism, ii. 68. - - Aldrovandi, 13. - - Amixia, ii. 285, 286. - - Ammon, O., the variation-playground, ii. 199, 202. - - Amœba 'nests,' ii. 219. - - Amphigony, 267; - as a factor in maintaining species, ii. 204. - - Amphimixis, general significance of, ii. 192; - antiquity of, ii. 202; - Ammon's playground of variations, ii. 206; - Amœba 'nests' as a preliminary stage, ii. 219; - beginnings of, ii. 213; - parthenogenesis as self-fertilization, ii. 233; - in Coccidium, ii. 214, 216; - chromosomes in Protozoa, ii. 216; - the 'cycle' idea, 326; - increased stability due to, ii. 200; - continued inbreeding, ii. 231; - 'formative' stimulus, ii. 229; - Galton's curves of frequency, ii. 206; - in relation to rudimentary organs, ii. 226; - immediate consequences of, ii. 224; - plastogamy as a preliminary stage of, ii. 222; - alters individuality, ii. 192; - and natural death, 335; - direct advantages of, ii. 198; - origin of, ii. 211; - association of, with reproduction, ii. 210; - increases power of adaptation, ii. 223; - preliminary stages of, ii. 213; - not a rejuvenescence in the sense of preserving life, ii. 221. - - Ancestral plasm, ids of, ii. 38. - - Ants, several kinds of ids in the germ-plasm of, 390; - harmonious adaptation of sterile forms, ii. 89; - degeneration of wings and ovaries in the workers, ii. 90; - transition forms between females and workers, ii. 92; - Wasmann's explanation of these, ii. 93; - _Polyergus rufescens_, ii. 95; - dimorphism of workers, ii. 96; - number of queens, ii. 98. - - Apes, furred, in Tibet, ii. 269. - - Arctic animals, sympathetic colouring in, 62. - - Aristotle, 10. - - Assimilation, ii. 371. - - Auerbach, spindle-figure of the dividing cell-nucleus, 289. - - Autotomy, self-amputation, ii. 18. - - - Baer, K. E. von, development of the chick in the egg, 25. - - Barfurth, on the segmentation of the egg in the sea-urchin, 408. - - Bates, discovery of mimicry, 91; - on the Sauba ant, ii. 96. - - Beccari, _Amblyornis inornata_, 223. - - Bees, harmonious adaptation in the workers, ii. 89; - influence of nutrition on the degeneration of the ovaries, ii. 92; - importance of the fact that there is only one queen, ii. 97. - - Belt, plants and ants, 171. - - Beneden, E. van, fertilization of the ovum of _Ascaris_, 295; - deutoplasm, 282; - theory of mitotic cell-division, 291. - - Bickford, Elizabeth, experiments on regeneration, ii. 90. - - Binswanger, on artificial epilepsy in guinea-pigs, ii. 68. - - Biogenetic Law, Fritz Müller's view, ii. 160; - crustacean larvæ, ii. 161; - Haeckel's views, ii. 173; - markings of the caterpillars of the Sphingidæ, ii. 177; - shunting back of the stages in the ontogeny, ii. 177. - - Biophors, the smallest vital units, 369; - struggle of the, ii. 52; - spontaneous generation of, 369. - - Birds, adaptation in, ii. 315. - - Blochmann, on the directive corpuscles in parthenogenetic ova, 304; - on the development of the ovum of the bee, 336; - on chromosomes in unicellulars, ii. 217. - - Blumenbach, 'nisus formativus,' 352; - inheritance of mutilations, ii. 66. - - Bois-Reymond, doubts as to the inheritance of functional - modifications, 242. - - Bonnet, preformation theory, 350, 351. - - Bordage, regeneration, ii. 20. - - Borgert, proof of the splitting of the chromosomes in the division of - unicellulars, ii. 216. - - Boveri, fertilization of non-nucleated pieces of ovum with nucleus of - another species, 341. - - Brandes, on the extinction of _Machairodus_ species and the giant - armadillos, ii. 358, 359; - on the supposed transformation of the stomach in birds as a result - of nutrition, 267. - - Brown-Séquard, artificial epilepsy in guinea-pigs, ii. 67. - - Brücke, Ernst, organization of the living substance, 368. - - Budding and division, ii. 1. - - Bütschli, theories of amphimixis, 330; - discovery of the spindle-figure in nuclear division, 289. - - Burdach, inheritance of mutilations, ii. 65. - - Buttel-Reepen, Hugo von, on fertilization in the bee ovum, 306. - - Butterflies, their enemies, 98; - aggressive colourings, 68, 70; - aberrations due to cold, ii. 274; - transmissibility of these, 275; - endemic species, 285; - polar and Alpine species, 285; - species of the Malay region, 291. - - Butterflies, protective coloration in, 74. - - - Cænogenesis, ii. 173. - - Calkins, conjugation of infusorians, 329. - - Caterpillars, protective coloration in, 67. - - _Catocala_, adaptive coloration in the various species, ii. 310. - - Cell-division, integral and differential, 374; - differential in Ctenophores, 408; - proofs of differential, 377. - - Centrospheres, 289, 309. - - Ceratium, ii. 326. - - Chance, elimination sometimes due to, 44, 47. - - Characters, purely morphological, ii. 133. - - Child, determination of, at fertilization, ii. 46. - - Chromatin, the hereditary substance, 287; - grounds for the belief, 337-43. - - Chromosomes, their occurrence in unicellulars, ii. 217; - simple and plurivalent (-idants), 349, 350; - individuality of, 349; - number of, in different species, 291; - indications of complexity of their structure, 292; - reasons for their existence, 303. - - Chun, segmentation of the ovum in Ctenophores, 408; - Kerguelen cabbage and rabbits, ii. 362; - deep-sea investigation, ii. 322. - - Cirrhipeds, ii. 241. - - Climate, influence of, in causing variation, ii. 269. - - Climatic varieties, ii. 269, 272. - - Coadaptation, ii. 80; - in crustaceans, ii. 81; - in the markings of butterflies, ii. 87; - in the forelegs of the mole-cricket, ii. 86. - - Cold aberrations in butterflies, transmissibility of, ii. 275. - - Coloration, animal, its biological import, 58; - sympathetic in butterflies, 74; - in moths, 76; - of animals in green surrounding, 64; - of eggs, 60; - of nocturnal animals, of polar animals, 64; - water animals, 63. - - Coloration, shunting backwards of, in the ontogeny, 73. - - Colour-adaptation, double, 64, 73; - colour change in fishes, amphibians, reptiles and - Cephalopoda, ii. 278. - - Combinations of determinants, ii. 40. - - Conjugation, in Protozoa, 317; - in _Paramæcium_, 319. - - Conklin, on the behaviour of the centrosphere in the ovum - of _Crepidula_, 309, ii. 41. - - Connective tissue of vertebrates, 386. - - Constancy and variability, periods of, ii. 294, 295; - degree of constancy of a character increases with its age, ii. 200. - - Convergence, ii. 323. - - Cope, supposed palæontological proofs for the Lamarckian - principle, ii. 77. - - Copernicus, 13. - - Copulation of _Coccidium proprium_, ii. 217. - - Correlation of the parts of the body, 41; - of determinants of the germ-plasm, ii. 153. - - Correns on Xenia, ii. 59. - - Corsica, endemic butterflies of, ii. 285. - - Crampton, segmentation in a marine snail, _Ilyanassa_, 409. - - Crystal animals, sympathetic colouring, 63. - - Cultivated plants, asexual reproduction in, ii. 261. - - Cuvier, 16; - his dispute with St.-Hilaire, 24. - - - Dahl, the ants of the Bismarck Archipelago, ii. 101. - - Danaides, immune butterflies, 94. - - _Danais erippus_ and _Limenitis archippus_ (mimicry), 113, 114. - - Darwin, Charles, first appearance of _The Origin of Species_, 28; - story of his life, 29. - - Darwin, Erasmus, theory of evolution, 17. - - Darwin and Nägeli, ii. 322. - - Darwinian theory, dependence of the frequency of species on - enemies, 47; - on external circumstances, 45; - correlation of parts, 41; - races of pigeons, 34; - of domesticated animals, 31; - geometrical ratio of increase, 46; - struggle for existence, 47; - struggle between individuals of the same species, 52; - artificial selection, 39; - natural selection, 42; - affects all parts and stages, 54; - variation, 43; - summary, 55; - origin of flowers, 182; - pangenesis, ii. 62. - - Death, natural, 260. - - Degeneration of a typical organ not an ontogenetic but a phylogenetic - process, ii. 91; - of disused parts, ii. 116. - - Delage, the germ-substance, 401; - 'a portmanteau theory,' ii. 3; - experiments with sea-urchins, 342. - - Desert animals, sympathetic colouring in, 62. - - Determinants, active and passive state, 380; - controlling the cells, 381; - proofs of their existence, 361, 371, 408; - in limbs of Arthropods, 361; - liberation of, 382; - size and number, 369. - - Determinates, 355. - - Deutoplasm, 280. - - Dewitz, degeneration of wings in the ontogeny of worker-ants, ii. 90. - - Diatoms, ii. 324. - - Dimorphism, sexual, its idioplasmatic cause, 388. - - Disappearance of disused parts, ii. 135; - unequal rate of, ii. 129. - - Dividing apparatus of the ovum, 288, 308. - - Division, proof of differential nuclear division (_Phylloxera_), 377; - multiplication by division, ii. 1. - - Dixon, isolation as a condition of species formation, ii. 284. - - Döderlein, increase of characters in diluvial forms, ii. 139. - - Dog, breeds of, 31; - attachment to man, ii. 73. - - Driesch, 'prospective' importance of a cell, 378, 408. - - Dzierzon, discovery of parthenogenesis in bees, 303. - - - Echinoderms, mesoderm cells of, 386, 387. - - Ectocarpus, 334. - - Egg-cell, form and structure, 280; - its migrations, 281. - - Ehrlich, experiments with ricin and abrin, ii. 106. - - Eigenmann, on blind cave-salamanders, ii. 347; - on species of Leptocephalus, ii. 133. - - Eisig, on symbiosis, 162. - - Elimination, ratio of, 47. - - _Elymnias_, a genus of mimetic butterflies, 103. - - Emery, on extinction of species, ii. 357; - on _Colobopsis truncata_, ii. 96; - on germinal selection, ii. 139; - 'mixed' forms in ants, ii. 93; - variation of homologous parts, ii. 189. - - Empedocles, 9; - ii. 370, 378. - - Endemic species, ii. 283. - - Endres, 'prospective' significance of the blastomeres of the ovum of - the frog, 407. - - Epigenesis and evolution, 350. - - Epilepsy, artificial, in guinea-pigs, ii. 67. - - Equilibrium between species of a region, 49. - - Evolution, phyletic, ii. 332; - paths of, ii. 381; - forces of, ii. 381; - mechanism of, 353; - facts of, 406. - - Evolution, progressive, attempt of species to extend its - range, ii. 383; - unlimited diversity of forms of life, ii. 391; - parable of the traveller, ii. 386. - - Evolution theory, general meaning of, 6; - 'prospective' import of the cell, 378. - - Exner, electric adaptation of the fur of mammals and feathers of - birds, ii. 316; - vision of insects, 216. - - Eye-spots, 69; - ii. 179. - - - Falkland Islands, influence of climate on cattle and horses, ii. 268. - - Feathers, regarded as an adaptation, ii. 316. - - Fertilization, process of, 286; - in lichens, 313; - in _Ascaris_, 296; - in the sea-urchin ovum, 293; - in Phanerogams, 313; - in higher plants, ii. 251; - importance of the chromatin, 290; - conjugation, 317; - the centrosphere the dividing apparatus of the cell, 289; - chromatin the hereditary substance, 287; - differentiation of individuals among the Protozoa, 322; - number of chromosomes reduced to half, 297; - rôle of the centrosphere, 308; - summary of process of fertilization, 343. - - Fischel, segmentation, of the Ctenophore ovum, 408; - regeneration of the lens in Triton, ii. 20. - - Fischer, E., experiments with butterfly pupæ in low - temperature, ii. 275. - - Flowers, origin of, 179; - adaptation to insects, 189; - in _Aristolochia_, _Pinguicula_, and _Daphne_, 186; - colour as an attraction to insects, 195; - collecting apparatus of bee, 193; - cross-fertilization, means for securing, 182; - in _Salvia_, 183; - lousewort, 184; - flowers adapted to fly-visits, 185; - orchids, 187; - deceptive flowers, _Cypripedium_, 200; - fertilization of Yucca, 202; - imperfection of adaptation a proof of origin through selection, 204; - mouth-parts of insects, 189; - bee, 172; - butterfly, 193; - cockroach, 191; - wind-pollination, 182. - - Forel, Auguste, alarm-signals in ants, ii. 83. - - Fraisse, on regeneration, ii. 30. - - Function, passively functioning parts in relation to the Lamarckian - principle, ii. 77; - harmonious adaptation in these, ii. 81. - - Fungi, reproduction of, ii. 267. - - Fur of mammals, adaptation to the conditions of life, ii. 269. - - - Galapagos Islands, fauna of, ii. 283, 292. - - Galileo, Galilei, 13. - - Galls, plant, 385; - ii. 271. - - Gall-wasps, reproduction of, ii. 245. - - Galton, Francis, on continuity of the germ-plasm, 411; - on inheritance of talents, ii. 150; - curves of frequency, ii. 206; - doubt of the Lamarckian principle, 242. - - Genius, human, ii. 394. - - Germ-cells, and somatic cells, 411; - development of, 410; - their mutual attraction, ii. 230. - - Germinal infection, ii. 69. - - Germinal Selection, ii. 113; - influenced by personal selection, ii. 155; - relation of determinants to determinates, ii. 153; - combination of mental gifts, ii. 150; - influence of amphimixis, ii. 125; - influence of the multiplicity of ids, ii. 124; - objections on the score of smallness of the substance of the - germ-plasm, ii. 156; - degeneration of a species through cultivation, ii. 144; - there are only plus and minus variations, ii. 151; - excessive increase of variations, ii. 139; - basis of sexual characters, ii. 135; - its sphere of operation, ii. 127; - small hands and feet in the higher classes, ii. 147; - climatic forms, ii. 134; - bud-variations, ii. 141; - play of forces in the determinant system, ii. 154; - artificial selection, ii. 123; - short-sight, ii. 146; - milk-glands, ii. 147; - deformities, ii. 137; - muscular weakness in the higher classes of men, ii. 147; - positive variation, ii. 122; - regulated by personal selection, ii. 131; - source of purely morphological characters, ii. 132; - disappearance of disused parts, ii. 119, 129; - self-regulation of the germ-plasm, ii. 128; - specific talents, ii. 149; - sport-variations, ii. 140; - spontaneous and induced, ii. 137; - excessive increase of a variation tendency, ii. 130; - preponderance of panmixia, ii. 120; - origin of secondary sexual characters, ii. 143. - - Germinal vesicle, 295. - - Germ-plasm, conception of, 410; - continuity of, 411; - at once variable and persistent, ii. 220; - disintegration of, in ontogeny, 379; - nutritive variations within the, 379; - structure of the, 373; - variation of, due to environment, ii. 267; - to nutrition, ii. 268. - - Germ-plasm theory, 345; - accessory idioplasm, 383; - active and passive state of determinants, 379; - connective tissue-cells, 386; - determinants and determinates, 355; - lithium-larvæ, 383; - ids, conception of, 349; - idants, 349; - male end female ids, 389; - mesoderm cells of sea-urchin, 387; - plant-galls, 385; - polymorphism, 390; - proofs of existence of determinants (_Lycæna agestis_, insect - metamorphosis, &c.), 356; - sexual dimorphism, 388. - - Germ-tracks, 411. - - Gesner's _Book of Animals_, 13. - - Godelmann, regeneration of Phasmids, ii. 28 _n._ - - Goebel, 269. - - Goethe, archetypal animal and plant, 18. - - Green animals, 64. - - Gruber, A., regeneration experiments on the Protozoa, 340. - - Guignard, fertilization of Phanerogams, 315. - - Gulick, snails in the Sandwich Islands. ii. 329. - - - Haase, Erich, on Pharmacopagæ, 101; - on mimicry, 104. - - Haberlandt, protection of leaves, ii. 133; - Auxo-spores, ii. 221. - - Haeckel, Ernst, fundamental biogenetic law, ii. 173; - monogony and amphigony, 267; - palingenesis and cœnogenesis, ii. 173; - genealogical trees, ii. 388. - - Häcker, Valentin, importance of the nucleolus, 287; - separateness of paternal and maternal nuclear substance during - development, ii. 42; - process of nuclear division, 291. - - Hahnel, observations on the enemies of butterflies, 154; - lizards and birds as enemies of butterflies, 97, 98. - - Haller, 267. - - Harmony, pre-established, apparently existing in development, ii. 309. - - Hartog, views on amphimixis, 334; - ii. 194. - - Haycraft, on the equalizing effect of amphigony, ii. 203. - - Heidenhain, theory of mitotic division, 291. - - Heider, on the intimate processes of segmentation of the ovum, - 'regulation' and 'mosaic' ova, 409. - - Heliconiidæ, first example of immune butterflies, 91. - - Henslow, on purely morphological specific differences, ii. 308. - - Herbst, lithium-larvæ, 383; - ii. 277. - - Hereditary sequence, alternation of, ii. 50. - - Hering, his reasons for assuming the inheritance of functional - modifications, ii. 110. - - Hermaphroditism in flowers, ii. 250; - in animals, ii. 239; - advantages of, ii. 239. - - Herrich-Schäfer, on mimicry, 105. - - Hertwig, O., fertilization of sea-urchin eggs, 293; - theory of development, 354; - differential cell-division, 376; - inheritance of functional modifications, ii. 106; - maturation divisions of the sperm-cells, 300. - - Hertwig, R., chromosomes in Actinosphærium, ii. 216. - - Heterogony, ii. 244. - - Heteromorphosis, Loeb on, ii. 7. - - Heterostylism, ii. 254. - - Heterotopia, 365, 367. - - Hirasé, fertilization of Phanerogams, 313. - - Histonal selection, 240; - and personal selection, 280. - - Hübner, O., experiments on regeneration in _Volvox_, ii. 4. - - Humming-birds, species fixed by isolation, ii. 290. - - Hyatt, Alpheus, the snail-strata of Steinheim, ii. 305. - - Hybrids, ii. 60; - of pigeons, 34; - plant, ii. 57. - - Hydra, regeneration in, ii. 4. - - Hydroid polyps, development of germ-cells in, 411. - - - Idants, 349. - - Ids, 349; - male and female, 389; - mimicry a proof of the existence of, 390. - - Immortality, potential, of the Protozoa, 260. - - Immunity of butterflies, 99. - - Imperfection of adaptation, 203. - - Inbreeding, evil consequences of, ii. 231. - - Infection of the germ, ii. 69. - - Infusorians, experiments of Maupas on, 328; - Calkins on, 329; - differentiation of nucleus into macro-and micro-nucleus a means of - compelling conjugation, 334. - - Inheritance, of acquired characters, ii. 62 (_see_ also Lamarckian - principle); - of functional modifications, ii. 64; - of mutilations disproved, ii. 65; - from parent to child, ii. 38; - hereditary substance, 288, 341; - preponderance of one parent, ii. 47; - alternation in ontogeny, ii. 48. - - Instinct, 141, ii. 70; - will and, 152. - - Instincts, aberrant, 149; - attachment of dog, ii. 73; - change of, in _Eristalis_, &c., 150; - egg-laying of butterfly, 159; - exercised only once, 155; - ii. 75; - 'feigning death,' 145; - imperfectly adapted, 152; - inheritance of, ii. 72; - masking of crabs, 145; - material basis of, 142; - monophagy of caterpillars, 146; - new in domesticated animals, ii. 73; - nutritive, 146; - in Ephemerids and sea-cucumbers, 148; - in predatory fishes, 149; - origin of, ii. 70; - pupation of butterflies, 156; - self-preservation of, 144; - wild animals on lonely islands, ii. 73. - - Intra-selection (histonal selection), 240. - - Ischikawa, on chromosomes in unicellulars, ii. 216; - on the conjugation of _Noctiluca_, 317; - ii. 42. - - Island faunas, ii. 283. - - Isolated regions, ii. 284. - - Isolation, favours species-formation, ii. 383; - relative, ii. 350; - snails on the Sandwich Islands, ii. 292. - - - Jäger, G., on the continuity of the germ-plasm, 411. - - Japanese cock, 356. - - - Kaleidoscope, transformation resembles a, ii. 307. - - Kallima, mimicry of leaf, 83, 236, 237. - - Karyokinesis, 290. - - Kathariner, birds as enemies of butterflies, 97. - - Kennel, birds as enemies of butterflies, 97. - - Kerner von Marilaun, Alpine plants, 122; - influence of hybridization on the formation of new species, ii. 352. - - Knowledge, limits of, ii. 392. - - Köhler, on scent-scales in the Lycænidæ, 370. - - Koshewnikow, on the influence of royal food on drone-larvæ, ii. 92. - - Kükenthal, on the fur of aquatic mammals, ii. 270. - - - Lamarck, theory of development, 21; - on limits of genera and species, ii. 306. - - Lamarckian principle, ii. 62; - Lamarck regarded inheritance of functional modifications as a matter - of course, 241; - cleaning apparatus of bees, ii. 84; - claw of crustacean, ii. 85; - Darwin's attitude to, 242; - facts (foreleg of mole, cricket, &c.), ii. 86; - Galton's attitude to, 242; - Hering's view, ii. 109; - O. Hertwig's view, ii. 106; - neuters among ants and bees, ii. 89; - phyletic development, ii. 77; - skeleton of Arthropods, ii. 82; - stridulating organs, ii. 83; - theoretical impossibility of, ii. 107; - variation of passive parts, ii. 77; - venation of butterfly's wing, ii. 87; - Zehnder's defence of, ii. 99. - - _Lathræa_, 135. - - Lauterborn, on amphimixis in diatoms, ii. 216. - - Leaf-imitation, in Locustidæ, 88; - in moths, 87; - in butterflies, 83, 357-61; - in Anæa species, ii. 310. - - _Lepus variabilis_, 62; - ii. 344, 350. - - Leeuwenhoek, first use of the microscope, 14. - - Leuckart, _Trichosomum crassicauda_, with dwarf males, 227; - structure of snails, ii. 301. - - Leuckart and von Siebold, 333. - - Leydig, regeneration of the lizard's tail, ii. 30. - - Liberation of the determinants in ontogeny, 382-6; - quality of nutrition as a liberating stimulus in bees and - ants, ii. 92. - - Liebig, theory of the origin of life, ii. 365. - - Limits of knowledge determined by selection, ii. 394. - - Linné, conception of species, 14. - - Lloyd Morgan, artificially induced instincts, ii. 72. - - Loeb, experiments on regeneration, ii. 6, 7; - the cell-nucleus as an organ for oxidation, ii. 31. - - Luminous organs in deep-sea animals, ii. 321. - - - MacCullock, autotomy, ii. 19. - - _Machairodus_, ii. 358. - - Mammals, adaptation to aquatic life, ii. 333. - - Maturation divisions, ii. 40; - in plants, 315; - in the ovum, 298; - in the sperm, 301; - influence of, ii. 44. - - Maupas, intimate processes of conjugation, 319; - conjugation of Infusorians, 329. - - Medium, influence of, ii. 267. - - Mendel's Law, ii. 57. - - Merogony, fertilization of non-nucleated pieces of ovum, 343. - - Merrifield, temperature-experiments with _Polyommatus - phlæas_, ii. 273; - cold experiments with _Vanessa_, ii. 274. - - Meyer, Hermann, architecture of the bone spongiosa, 246. - - Mimicry, 91; - in beetles, bees, ants, &c., 116; - in butterflies does not affect caterpillar or pupa, 104; - in both sexes, 96; - in vertebrates, 117; - degree of resemblance to model, 104; - _Elymnias undularis_, 106; - _Papilio merope_, 108; - _P. turnus_, 110; - same effect produced in different ways, 105; - several imitators of one immune species, 101; - species of genera which need protection imitate different immune - models, 102; - 'rings' of mimetic species, 112; - rarity of mimetic species, 108; - wide divergence of mimetic species from their congeners, 115. - - Mitosis, 288. - - Möbius, 296. - - Monism, 393. - - Monogony, 266. - - Montgomery, on reduction of the chromosomes, ii. 43. - - Morgan, experiments on regeneration, ii. 15. - - Morphological characters, dependent on germinal selection, ii. 132; - discussion as to indifferent characters, ii. 132, 309. - - Mortality of multicellular organisms, 260; - causes of this, 263. - - Morton, Thomas, on degeneration in the children of alcoholics, ii. 69. - - Moths, protective coloration in, 80. - - Müller, Fritz, scent-scales, 217; - on mimicry, 111; - plants and ants, 171; - relation between ontogeny and phylogeny, ii. 160. - - Müller, Johannes, the vision of insects, 216. - - Musical sense in man, ii. 148. - - Mutation theory of de Vries, ii. 317. - - Mutilations, supposed inheritance of, ii. 65. - - Mutual sterility, of no great importance in connexion with lasting - variation, 349. - - - Nägeli, Carl von, on the definite directions of - variations, ii. 306, 385; - objection to origin of flowers through selection, 198; - on the difference in size between egg and sperm, 337; - his _Hieracium_ experiments, ii. 272; - Nägeli's view and Darwin's reconciled through germinal - selection, ii. 334; - number of smallest vital units in a 'moneron,' ii. 368. - - Nathusius, inbreeding experiments, ii. 231. - - Natural Selection, not directly observable, 58; - under the influence of isolation, ii. 292. - - Neo-Lamarckism, 243. - - Neotaxis, ii. 40. - - Nerve-tracks in relation to instincts, ii. 71. - - Normal number of a species, 45. - - _Notodonta_, protective coloration in, 80. - - Nuclear division, process of, 289; - integral and differential, 374, 377. - - Nussbaum, M., regeneration-experiments in Protozoa, 340; - on the continuity of the germ-cells, 411; - infection of the ovum in Hydra, ii. 68. - - Nutrition, influence of, on variation, ii. 267; - relation between nutrition and the number in a species, 45. - - - Oken's 'Naturphilosophie,' 21. - - Omnipotence of selection, ii. 348. - - Ontogenesis, relation to phylogenesis, ii. 159; - shunting back of the phyletic stages in embryogenesis, ii. 176; - condensation of phylogeny in ontogeny, ii. 186. - - Orchids, fertilization of, ii. 256. - - Organs, rudimentary, ii. 226. - - Origin of flowers, _see_ Flowers. - - Osborn, supposed palæontological proofs for the Lamarckian - principle, ii. 77. - - Ovaries, 282. - - Ovogenic determinants, 388. - - Ovum, maturation of, 295. - - - Packard, disappearance of useless parts, 129. - - Palingenesis, ii. 173. - - _Pandorina_, reproduction of, 257, 293. - - Pangenesis, ii. 62. - - Panmixia, ii. 114. - - _Papilio meriones_, 108, 427; - _P. turnus_, 110. - - Parasites, power of adaptation in, ii. 384. - - Parthenogenesis, discovery of, 303; - exceptional and artificial, 307; - facultative in bees, ii. 235; - receptaculum seminis in Cypris-species without males, 326, ii. 234; - advantages of, ii. 243; - its effects compared with those of inbreeding, ii. 233; - alternation of, with bisexual generations (heterogony), ii. 243. - - Personal selection, indirect effects of, ii. 200. - - Petrunkewitsch, A., maturing divisions in the ovum of the - bee, 306, 336. - - Pfeffer, rôle of malic acid in the fertilization of ferns, 273. - - Pflüger and Born, experiments in hybridization, ii. 232. - - Phasmids, regeneration in, ii. 17. - - _Phylloxera_, reproduction in, ii. 249. - - Phylogenetic variation of butterfly and caterpillar independent of - each other, 362. - - Phylogeny, condensation of, in ontogeny, ii. 186. - - _Physiologus_, 11. - - Pictet, turban eyes in male Ephemerids, 229. - - Pigeons, breeds of, 34. - - Plants, fertilization of the higher, ii. 250; - carnivorous, 132; - _Aldrovandia_, 138; - _Dionæa_, 138; - _Drosera_, 136; - _Lathræa_, 135; - _Nepenthes_, 134; - _Pinguicula_, 135; - _Utricularia_, 133. - - Plant-galls, ii. 270. - - Plastogamy a preliminary stage to fertilization, ii. 220. - - Pliny, 11. - - Polar bodies, 294. - - Polymorphism, its idioplasmic roots, 390. - - _Polyommatus phlæas_, dimorphism of caterpillars, 363; - climatic varieties, ii. 272. - - Postgeneration (Roux), 407. - - Pouchet, spontaneous generation, ii. 366. - - Poulton, on facultative colour adaptation in caterpillars, ii. 278; - on mimicry, 105. - - Prediction on the basis of the evolution theory, 3. - - Preformation and Epigenesis, 351. - - Primordial males among Cirrhipeds, ii. 242. - - Protective arrangements in plants, 119; - Alpine plants, 126; - chemical substances, 128; - ethereal oils, 128; - hairs, 122; - poisons, 120; - Raphides, 129; - 'Prigana scrub,' 126; - against small enemies, 127; - Tragacanth, 124. - - Protective colouring, rôle of light in, 78; - _Kallima_, 83; - _Notodonta_, 80; - _Xylina_, 82. - - Protective marking in caterpillars, 67. - - Protozoa, chromosomes in, ii. 216. - - - Quetelet, amphigony preserves the mean of the species, ii. 204. - - - Races, development of, depending on adaptation, ii. 335; - dependent on germinal selection, ii. 144. - - Radiolarians, skeleton of, ii. 324. - - Rand, experiments on regeneration in Hydra, ii. 5. - - Rath, O. von, on the influence of royal food on drone-larvæ, ii. 91. - - Ray, John, conception of 'species,' 14. - - Reactions, primary and secondary, ii. 277. - - Reducing divisions, _see_ Maturation divisions. - - Regeneration, ii. 1; - atavistic, ii. 30; - autotomy, ii. 16; - in birds, ii. 14; - in Hydra, ii. 4; - in Hydroid polyps, ii. 9; - in plants, ii. 9, 32; - in Planarians, ii. 6, 13; - in starfishes, ii. 30; - in Vertebrates, ii. 10; - of the lens in Triton, ii. 19; - a phenomenon of adaptation, ii. 9; - nuclear substance the first organ of, ii. 31; - phyletic origin of, ii. 23; - disappearance of the power of, ii. 16; - and budding, ii. 31; - relation of, to liability of part to injury, ii. 7; - not always purposive, ii. 25. - - Reinke, objections to the 'machine theory' of life, 402; - on regeneration, 32. - - Rejuvenescence, theory of, 325-8. - - Reproduction, adaptation of the germ-cells, 277; - asexual, ii. 259; - structure of the ovum, 280; - of the bird's egg, 285; - zoosperm, 273; - in Amœbæ, 253; - in Infusorians, 254; - in _Pandorina morum_, 257, 269; - in fungi, 267; - by means of germ-cells, 266; - differentiation of germ-cells into male and female, 267; - by division, 264; - two kinds of eggs in same species, 282; - nutritive ovum cells, 283; - introduction of death into the living world, 261; - contrast between reproductive and body cells in the Metazoa, 256; - budding and division in the Metazoa, 264; - potential immortality of the Protozoa, 260; - sperm and ovum in Algæ, 272; - in _Volvox_, 265, 271; - zoosperms of Ostracods, 275; - different kinds of spermatozoa, 278. - - Reproductive cells, development of, 410; - in Diptera, 411; - in Hydroid polyps, 413. - - Reversion, ii. 53; - in doves, ii. 55; - in the horse, ii. 55. - - Riley, fertilization of the Yucca by a moth, 202. - - Ritzema Bos, experiments on mice, ii. 65, 66. - - Romanes, isolation theory, ii. 284; - physiological selection, ii. 337; - panmixia, ii. 115. - - Rosenthal, experiments with mice, ii. 65, 66. - - Roux, Wilhelm, Mosaic theory, 379; - struggle of the parts, 244; - postgeneration, 407. - - Rückert, the nuclear substances in Copepods, ii. 42. - - Rudimentary organs in man, ii. 226. - - - St.-Hilaire, unity of type, 18. - - Samassa, segmentation of the frog's egg, 407. - - Sarasin, snails of Celebes, ii. 299. - - _Saturnia_, pupation of, 158. - - Schaudinn, fertilization in Coccidia, ii. 214; - maturing division in Sun-animalcule, 318. - - Schimper, plants and ants, 171. - - Schleiden and Schwann, discovery of the cell, 26. - - Schmankewitsch, experiments with _Artemia_, ii. 277. - - Schmidt, Oscar, ii. 324. - - Schneider, discovery of the 'spindle-figure' of nuclear division, 289. - - Schütt, Diatoms, ii. 325. - - Schwarz, Ostracods, 276. - - Segmentation-cells in animal ova, their prospective importance, 406. - - Seitz, a case of mimicry, 114. - - Selection-processes, grades of, ii. 265; - evolution guided by, ii. 298. - - Selection, sexual, 210-39; - absence of secondary sexual characters in the lower animals, 231; - adaptations for seizing the females, 229; - choice on the part of the females, 214; - odours and scent-scales, 217; - song of cicadas and birds, 221; - superfluity of males, 213; - weapons for the struggle for mates, 228; - summary, 238. - - Selection value, ii. 132, 311. - - Self-fertilization in plants, ii. 252; - continued influence of, ii. 257; - alternation of self- with cross-fertilization, ii. 241. - - Self-preservation, instinct of, 144. - - Sex-cells, mutual attraction of, ii. 228. - - Sex, determination of, 377; - ii. 44. - - Sexual characters, secondary, have their roots in germinal - selection, ii. 130, 143, 289-91, 378. - - Sexual selection, _see_ Selection, sexual. - - Sexual selection through isolation, ii. 289. - - Short-sight, ii. 146. - - Siedlecky, copulation in _Coccidium proprium_, ii. 218. - - Simroth, ii. 302. - - Slevogt, on birds as enemies of butterflies, 97. - - Sluiter, on symbiosis, 167. - - Smerinthus, markings of the caterpillars, ii. 177, 184. - - Snail-strata of Steinheim, ii. 305. - - Sommer, on artificial epilepsy in guinea-pigs, ii. 68. - - Special investigation, period of, 25. - - Species, the, a complex of adaptations and variations, ii. 307. - - Species-colonies, ii. 280. - - Species, extinction of, ii. 357; - dying out of the large animals of Central Europe, ii. 361; - extinction due to cultivation, ii. 360; - to unlimited variation, ii. 357; - _Machairodus_, ii. 358; - lower types more capable of adaptation than higher, ii. 359; - extinction of flightless birds, ii. 360. - - Species-formation, ii. 299; - favoured by isolation, ii. 284; - snails of Celebes, ii. 219; - without amphigony in lichens, ii. 343; - without isolation in _Lepus variabilis_, ii. 344; - Peridineæ, ii. 325; - protective coloration in butterflies, ii. 310; - the Steinheim snail-strata, ii. 315; - telescope eyes in deep-sea animals, ii. 323; - typical species, ii. 304; - variation in definite directions, ii. 306; - the bird as a complex of adaptations, ii. 316; - the whale as a complex of adaptations, ii. 313; - mutual fertility between many plant-species, ii. 340. - - Species, variable and constant, ii. 286. - - Specific type, its occurrence favoured by germinal - variation, ii. 333, 334; - by natural selection, ii. 334; - origin of the, ii. 299, 332-5. - - Spencer, Herbert, germinal substance composed of homogeneous - particles, 355; - on 'units,' the smallest vital particles, 369; - protective adaptations in plants to be referred to - selection, ii. 77. - - Spermaries, 282. - - Spermatozoa, _see_ Zoosperms. - - Sperm-cells, 272. - - Spermogenic determinants, 388. - - Sphingidæ, caterpillars of the, biological value of their - markings, 73; - ontogeny and phylogeny of the markings, ii. 177. - - _Sphinx convolvuli_, double adaptation of the caterpillar, 71, 72; - _S. euphorbiæ_, var. _Nicæa_, purely local form of caterpillar, 362. - - Spontaneous generation, 410; - conditions necessary, ii. 370; - only possible as regards invisible minute organisms, ii. 369; - the 'where' of, ii. 371; - impossibility of proving or disproving it experimentally, ii. 366. - - Sprengel, fertilization of flowers, 180. - - Standfuss, cold experiments with butterfly pupæ, ii. 275. - - Steinheim snail-strata, ii. 305. - - Steller's sea-cow (_Rhytina stelleri_), ii. 74. - - Stick-insects, 88. - - Strasburger, fertilization of Phanerogams, 314. - - Stuhlmann, zoosperms in Ostracods, 276. - - Swammerdam, 14. - - Symbiosis, candelabra trees and ants, 171; - hermit-crabs and Hydroid polyps, 163; - hermit-crabs and sea-anemones, 162; - origin of symbiosis, 176; - lichens, 173; - fishes and sea-anemones, 167; - green Amœbæ, 170; - green fresh-water polyp (_Hydra viridis_), 168; - _Nostoc_ and _Azolla_, 177; - sea-anemones and yellow Algæ, 171; - root-fungi, 175. - - - Talents, specific, of man referred to germinal selection, ii. 149; - depend on a combination of mental gifts, ii. 150. - - Tichomiroff, artificial parthenogenesis, 307, 333. - - Thorn-bugs, 89. - - Transparent winged butterflies, 106. - - Treviranus, as founder of the evolution theory, 18; - on generic differences, ii. 306. - - Trimen, observations on the immunity of the Acræidæ, 100. - - Tropism in plants, ii. 276. - - Twins, identical, ii. 44. - - - _Vanessa_, endemic species of, with protective colouring, 75. - - Variability, fluctuating, ii. 327. - - Variation, all ultimately quantitative, ii. 151; - in a definite direction, ii. 118; - double roots of, ii. 195; - ascending, ii. 122; - sports or saltatory variations, ii. 140; - roots of hereditary, ii. 118. - - Variation of individual characters, ii. 336; - not always due to adaptation, ii. 197. - - Variation, periods of, ii. 294. - - Vital force, ii. 369. - - Vitalism, ii. 369. - - Virchow, Rudolf, on the inheritance of mutilations, ii. 65. - - Vöchting, influence of light on the production of flowers, ii. 276; - on regeneration, ii. 32. - - Voigt, Walter, experiments in regeneration, ii. 6; - on Planarians, ii. 25. - - Voit, Carl von, influence of nutrition on bodily size, ii. 268. - - Volvocineæ, reproduction in, 257. - - Vries, de, asymmetrical curves of frequency, ii. 234; - theory of mutations, ii. 317; - Pangen theory, 380. - - - Wagner, Franz von, regeneration in _Lumbriculus_, ii. 27. - - Wagner, Moriz, on the influence of isolation, ii. 284. - - Wahl, Bruno, on the development of _Eristalis_, 399. - - Wallace, on the immunity of Heliconiidæ, 99; - on the causes of the coloration of butterflies, 211. - - Wasmann, Erich, on transition forms in ants, ii. 93; - on sounds produced by ants, ii. 83. - - Weaver birds, ii. 290. - - Whales, their origin through adaptation, ii. 313. - - Wheeler, rôle of the centrosphere in the ovum, 309. - - Wiedersheim, rudimentary organs in man, ii. 226. - - Wiesner, the smallest vital particles, 369. - - Wing-primordia in insects, 364. - - Winkler, Hans, experiments on artificial parthenogenesis, 307, 333; - on merogony, 343. - - Wolff, G., regeneration of the lens in Triton, ii. 19. - - Wolff, K. v., the founder of the epigenetic theory of evolution, 352. - - Wroughton, Robert, production of sounds by Indian ants, ii. 95. - - Würtemberger, form-series of ammonites, ii. 176. - - - Xenia, ii. 58. - - _Xylina_, protective colouring of, 82. - - - Yolk of egg, 282. - - - Zehnder, the living substance made up of fistellæ, ii. 217; - polymorphism in ants, ii. 99; - on the Lamarckian principle, ii. 99-106; - on the skeleton of Arthropods, ii. 103; - effect of amphimixis, ii. 223. - - Ziegler, Ernst, on deformities, ii. 138. - - Ziegler, H. E., experiments on merogony in sea-urchin ova, 342. - - Zoja, experiments with the ova of Medusæ, 407. - - Zoosperms, 273, 278, 279. - - - - - OXFORD: HORACE HART - PRINTER TO THE UNIVERSITY - - - - -STANDARD SCIENTIFIC WORKS. - - -HABIT AND INSTINCT: - -_A STUDY IN HEREDITY_. - -BY PROFESSOR C. LLOYD MORGAN, LL.D., F.R.S., - -Principal of University College, Bristol. - - Demy 8vo. With Photogravure Frontispiece. 16_s._ - - -ANIMAL BEHAVIOUR. - -BY PROFESSOR C. LLOYD MORGAN, LL.D., F.R.S., - - Large Crown 8vo. With numerous Illustrations. 10_s._ 6_d._ - - -THE CHANCES OF DEATH, - -_AND OTHER STUDIES IN EVOLUTION_. - -BY KARL PEARSON, F.R.S., - -Professor of Applied Mathematics in University College, London, and -formerly Fellow of King's College, Cambridge. - - Two volumes, Demy 8vo. 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