Chapter XVI: Part 16

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.

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.