Part 25

If we look backwards down the ladder of animal forms we find a progressive degradation in the organisation of the creatures comprised; the organisation of their bodies becomes simpler, the number of their faculties less. This well-recognised fact throws a light upon the order in which nature has produced the animals; but it leaves unexplained the fact that this gradation, though sustained, is irregular. The reason will become clear if we consider the effects produced by the infinite diversity of conditions in different parts of the globe upon the general form, the limbs, and the very organisation of the animals in question.

It will, in fact, be evident that the state in which we find all animals is the product, on the one hand, of the growing composition of the organisation which tends to form a regular gradation; and that, for the rest, it results from a multitude of circumstances which tend continually to destroy the regularity of the gradation in the increasingly composite nature of the organism.

Not that circumstances can effect any modification directly. But changed circumstances produce changed wants, changed wants changed actions. If the new wants become constant the animals acquire new habits, which are no less constant than the wants which gave rise to them. And such new habits will necessitate the use of one member rather than another, or even the cessation of the use of a member which has lost its utility.

We will look at some familiar examples of either case. Among vegetables, which have no actions, and therefore no habits properly so called, great differences in the development of the parts do none the less arise as a consequence of changed circumstances; and these differences cause the development of certain of them, while they attenuate others and cause them to disappear. But all this is caused by changes in the nutrition of the plant, in its absorptions and transpirations, in the quantity of heat and light, of air and moisture, which it habitually receives; and, lastly, by the superiority which certain of its vital movements may assert over the others. There may arise between individuals of the same species, of which some are placed in favourable, others amid unfavourable, conditions, a difference which by degrees becomes very notable.

Suppose that circumstances keep certain individuals in an ill-nourished or languid state. Their internal organisation will at length be modified, and these individuals will engender offspring which will perpetuate the modifications thus acquired, and thus will in the end give place to a race quite distinct from that of which the individual members come together always under circumstances favourable to their development.

For instance, if a seed of some meadow flower is carried to dry and stony ground, where it is exposed to the winds and there germinates, the consequence will be that the plant and its immediate offspring, being always ill-nourished, will give rise to a race really different from that which lives in the field; yet this, none the less, will be its progenitor. The individuals of this race will be dwarfed; and their organs, some being increased at the expense of the rest, will show distinctive proportions. What nature does in a long time we do every day ourselves. Every botanist knows that the vegetables transplanted to our gardens out of their native soil undergo such changes as render them at last unrecognisable.

Consider, again, the varieties among our domestic fowls and pigeons, all of them brought into existence by being raised in diverse circumstances and different countries, and such as might be sought in vain in a state of nature. It is matter of common knowledge that if we raise a bird in a cage, and keep it there for five or six years, it will be unable to fly if restored to liberty. There has, indeed, been no change as yet in the form of its members; but if for a long series of generations individuals of the same race had been kept caged for a considerable time, there is no room for doubt that the very form of their limbs would little by little have undergone notable alteration. Much more would this be the case if their captivity had been accompanied by a marked change of climate, and if these individuals had by degrees accustomed themselves to other sorts of food and to other measures for acquiring it. Such circumstances, taken constantly together, would have formed insensibly a new and clearly denned race.

The following example shows, in regard to plants, how the change of some important circumstance may tend to change the various parts of these living bodies.

So long as the ranunculus aquatilis, the water buttercup, is under water its leaves are all finely indented, and the divisions are furnished with capillaries; but as soon as the stalk of the plant reaches the surface the leaves, which develop in the air, are broadened out, rounded, and simply lobed. If the plant manages to spring up in a soil that is merely moist, and not covered with water, the stems will be short, and none of the leaves will show these indentations and capillaries. You have then the ranunculus hederaceus, which botanists regard as a distinct species.

Among animals changes take place more slowly, and it is therefore more difficult to determine their cause. The strongest influence, no doubt, is that of environment. Places far apart are different, and--which is too commonly ignored--a given place changes its climate and quality with time, though so slowly in respect of human life that we attribute to it perfect stability. Hence it arises that we have not only extreme changes, but also shadowy ones between the extremes.

Everywhere the order of things changes so gradually that man cannot observe the change directly, and the animal tribes in every place preserve their habits for a long time; whence arises the apparent constancy of what we call species--a constancy which has given birth in us to the idea that these races are as old as nature.

But the surface of the habitable globe varies in nature, situation, and climate, in every variety of degrees. The naturalist will perceive that just in proportion as the environment is notably changed will the species change their characters.

It must always be recognised:

(1) That every considerable and constant change in the environment of a race of animals works a real change in their wants.

(2) That every change in their wants necessitates new actions to supply them, and consequently new habits.

(3) That every new want calling for new actions for its satisfaction affects the animal in one of two ways. Either it has to make more frequent use of some particular member, and this will develop the part and cause it to increase in size; or it must employ new members which will grow in the animal insensibly in response to the inward yearning to satisfy these wants. And this I will presently prove from known facts.

How the new wants have been able to attain satisfaction, and how the new habits have been acquired, it will be easy to see if regard be had to the two following laws, which observation has always confirmed.

FIRST LAW.--In every animal which has not arrived at the term of its developments, the more frequent and sustained use of any organ strengthens, develops, and enlarges that organ, and gives it a power commensurate with the duration of this employment of it. On the other hand, constant disuse of such organ weakens it by degrees, causes it to deteriorate, and progressively diminishes its faculties, so that in the end it disappears.

SECOND LAW.--All qualities naturally acquired by individuals as the result of circumstances to which their race is exposed for a considerable time, or as a consequence of a predominant employment or the disuse of a certain organ, nature preserves to individual offspring; provided that the acquired modifications are common to the two sexes, or, at least, to both parents of the individual offspring.

Naturalists have observed that the members of animals are adapted to their use, and thence have concluded hitherto that the formation of the members has led to their appropriate employment. Now, this is an error. For observation plainly shows that, on the contrary, the development of the members has been caused by their need and use; that these have caused them to come into existence where they were wanting.

But let us examine the facts which bear upon the effects of employment or disuse of organs resulting from the habits which a race has been compelled to form.

II.--The Penalties of Disuse

Permanent disuse of an organ as a consequence of acquired habits gradually impoverishes it, and in the end causes it to disappear, or even annihilates it altogether.

Thus vertebrates, which, in spite of innumerable particular distinctions, are alike in the plan of their organisation, are generally armed with teeth. Yet those of them which by circumstances have acquired the habit of swallowing their prey without mastication have been liable to leave their teeth undeveloped. Consequently, the teeth have either remained hidden between the bony plates of the jaws, or have even been, in the course of time, annihilated.

The whale was supposed to have no teeth at all till M. Geoffrey found them hidden in the jaws of the foetus. He has also found in birds the groove in which teeth might be placed, but without any trace of the teeth themselves. A similar case to that of the whale is the ant-eater (nyomecophaga), which has long given up the practice of mastication.

Eyes in the head are an essential part of the organisation of vertebrates. Yet the mole, which habitually makes no use of the sense of sight, has eyes so small that they can hardly be seen; and the aspalax, whose habits-resemble a mole's, has totally lost its sight, and shows but vestiges of eyes. So also the proteus, which inhabits dark caves under water.

In such cases, since the animals in question belong to a type of which eyes are an essential part, it is clear that the impoverishment, and even the total disappearance, of these organs are the results of long continued disuse.

With hearing, the case is otherwise. Sound traverses everything. Therefore, wherever an animal dwells it may exercise this faculty. And so no vertebrate lacks it, and we never find it re-appearing in any of the lower ranges. Sight disappears, re-appears, and disappears again, according as circumstances deny or permit its exercise.