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.
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.