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.