These and kindred speculations have produced some rather curious discussions, which not infrequently end in conundrums. If we speak of a case of adaptation as a condition, a fact, we easily run the risk of getting into confusion about cause and effect. For example: Is the stag swift because he has long and slender legs, or are his legs long because he is swift? In reality, swiftness and length of legs are cause and effect in one. His legs have been so modified as to make him swift, because he has put them continuously to whatever was his full speed, which in his thick-footed ancestors was probably a very slow one. The above question reads, therefore, more sensibly as follows: Has the stag become swift because his legs have become long and slender, or have his legs become long and slender because he has attained swiftness? Now, we see that both halves of the double question are practically the same and instantly suggest the answer.
A fundamental difference between artificial machines and living organisms is that the former are worn out by use, while the latter not only repair the loss caused by use, but are also stimulated to further increase. On the other hand, organs which are not put into function, or are not used, degenerate. The various cells of the organ react upon external stimuli by increased activity. Why this should be so is another question—perhaps because those which do not would soon be not fit to survive. Each cell has a function; the more specialized the more intense it is. Every external stimulus, every contact with the outer surroundings, is an insult, necessarily of detrimental effect, as it disturbs the equilibrium of the cell body. It must, therefore, be of advantage to the cells' well-being to return as soon as possible to the status quo ante, and this can only be done by increased activity.
In the present state of our knowledge, we can approach only the simplest cases of acquisition of characteristics. Mostly they are so complicated, subject to so many unthought-of conditions, that we do not know from which end to approach the problem. Frequently the supposed use of certain obvious features is the merest guesswork. This applies especially to features to which we are not accustomed (although wrongly so) to assign a function—for example, coloration. A green tree-frog will with predilection rest on green leaves. The advantages of concealment are obvious, and in this case he 'adapts himself' to the surroundings by making for green localities: if he did not he would be eaten up sooner than his more circumspect comrades. But this making for, and sitting in, the green has not necessarily made him of that colour. Extreme advocates of one view would argue as follows: Once upon a time there were among the offspring of ancestral tree-frogs some which, among other colours, exhibited green, not much, perhaps not even perceptible to our eyes. The occurrence of this colour, according to them, was spontaneous, a freak—as if in reality there were anything spontaneous in the sense of being causeless. The descendants of these more greenish creatures, provided they did not pair with frogs of the ordinary set, became still greener (by accumulative inheritance), and so on, until the green was pronounced sufficient to be of advantage when competition could set in.
With this view there is always the difficulty of understanding how the initial very small changes can be useful, unless we have to deal with extremely simple organisms. Is it likely in the case of our frogs that an almost imperceptible variation in colour makes them more fit to live? We have to assume that 'luck' or chance kept them for generations out of harm's reach, until the accumulation of green, hitherto quite ineffective, neither harmful nor useful, became strong enough to be effective. Such cases undoubtedly happen.
But we can also argue out this problem in a somewhat different way, which goes nearer to the root of the whole process. The original slight, imperceptible change in pigmentation is not a spontaneous freak; it was caused by the direct influence of the surroundings in which the particular frogs happened to live, be this factor light or temperature or food. Thus it stands to reason that the offspring, living under similar conditions, will be acted upon in the same way. That factor which has added green to the parents will add green to the children, until by accumulative inheritance a more decidedly green race is produced.
The offspring of green plants do not become green when grown in the dark; the young plants inherit not the green, but the capacity of becoming green when acted upon by sunlight. This as an instance of direct influence of the surroundings on a substance (chlorophyll), which has not yet performed a function. But the kittens of a pair of black cats produce black hair before they are born, and we have no reason to doubt that the black pigment in their tegumentary structures is ultimately referable to the action of the sunlight. In many instances creatures living for generations in darkness become white, pigmentless, and they regain it when exposed to light. For example, the white, colourless Proteus from the caves of Adelsberg becomes clouded grey, and ultimately jet black, when kept in a tank whence light is not strictly excluded.
Blindness is a very general characteristic of creatures which dwell in darkness. There are all stages between total blindness and weak eyes. Now, do these blind creatures live in darkness because they are blind, or have they become first weak-eyed and then blind because of the continuous disuse of their eyes? The former explanation has actually been suggested! Individuals not smitten, but spontaneously, as a freak, born with sore eyes, have crept into the darkness for relief and have produced a blind race! To carry such a notion to the bitter end leads to absurdities. Anyhow, it is not understandable where the benefit of losing the eyesight arises. It can be explained only by continued disuse: witness Spalax typhlus, the blind mole, and, above all, the Endoparasites.
Let us now take an example to explain the influence of a tangible external stimulus. Repeated pressure produces callosities. Although they are not exactly beneficial in the shape of corns on our toes, they are so on our hands. At any rate, the morphologist can trace the development of the footpads, nails, hoofs, and horns, step by step from small beginnings. The cells of the Malpighian stratum, of the inner, active portion of our epidermis, are excited to extra activity, and by continually producing more horn cells than peel off the surface of the skin in the normal process of wear and tear cause the formation of the pad. It need scarcely be mentioned that hypertrophic growths are not necessarily useful; they are often harmful, and in that case pathological.
Lastly, a few words about the very difficult question of teleology. In trying to explain Evolution in a mechanical—sometimes called monistic, but in reality natural—way, we exclude anything like a set purpose, a goal, or ideal, a final condition which the organism strives to attain. Unknown, however, to many morphologists, especially embryologists, their writings are full of this teleological notion. Indeed, there are many cases in which an organism becomes changed, and quickly, too, in a way which cannot but be called reasonable. It starts modifications, be they outgrowths, alterations in shape or colour, or the making good of injuries received, which by 'short-cuts' produce the only advantageous result that can reasonably satisfy the new requirement or altered circumstances.
Trees growing in precarious positions, after part of the supporting rock has slipped away, throw out new roots, and rearrange some of the old ones in the only way which could save the tree. In animals which have lost part of a limb the wound closes up, and what is left is turned into a serviceable stump—for example, in water-tortoises (creatures in which reproduction of lost limbs does not happen). In frogs and newts the lost part is reproduced, not correctly, but in a good semblance. Tortoises which have had their shell smashed can throw off an astonishingly large portion and renew the bone as well as the overlapping scutes; but this mending is not neatly done. It serves the requirement, but it is patchwork; the new shell is such as no tortoise ever possessed before.