(1) Its front limbs, which it uses very little, resting on them only in the instant during which it quits its erect posture, have never acquired a development in proportion to the other parts; they have remained thin, little, and weak.
(2) The hind legs, almost continually in action, whether to bear the weight of the whole body or to execute its leaps, have, on the contrary, obtained a considerable development; they are very big and very strong.
(3) Finally, the tail, which we observe to be actively employed, both to support the animal's weight and to execute its principal movements, has acquired at its base a thickness and a strength that are extremely remarkable.
When the will determines an animal to a certain action, the organs concerned are forthwith stimulated by a flow of subtle fluids, which are the determining cause of organic changes and developments. And multiplied repetitions of such acts strengthen, extend, and even call into being the organs necessary to them. Now, every change in an organ which has been acquired by habitual use sufficient to originate it is reproduced in the offspring if it is common to both the individuals which have come together for the reproduction of their species. In the end, this change is propagated and passes to all the individuals which come after and are submitted to the same conditions, without its being necessary that they should acquire it in the original manner.
For the rest, in the union of disparate couples, the disparity is necessarily opposed to the constant propagation of such qualities and outward forms. This is why man, who is exposed to such diversity of conditions, does not preserve and propagate the qualities or the accidental defects which he has been in the way of acquiring. Such peculiarities will be produced only in case two individuals who share them unite; these will produce offspring bearing similar characteristics, and, if successive generations restrict themselves to similar unions, a distinct race will then be formed. But perpetual intermixture will cause all characters acquired through particular circumstances to disappear. If it were not for the distances which separate the races of men, such intermixture would quickly obliterate all national distinctions.
IV.—The Conclusion
Here, then, is the conclusion to which we have come. It is a fact that every genus and species of animal has its characteristic habits combined with an organisation perfectly in harmony with them. From the consideration of this fact one of two conclusions must follow, and that though neither of them can be proved.
(1) The conclusion admitted hitherto—that nature (or its Author) in creating the animals has foreseen all the possible sets of circumstances in which they would have to live, has given to each species a constant organisation, and has shaped its parts in a determined and invariable way so that every species is compelled to live in the districts and the climates where it is actually formed, and to keep the habits by which it is actually known.
(2) My own conclusion—that nature has produced in succession all the animal species, beginning with the more imperfect, or the simpler, and ending with the more perfect; that in so doing it has gradually complicated their organisation; and that of these animals, dispersed over the habitable globe, every species has acquired, under the influence of the circumstances amid which it is found, the habits and modifications of form which we associate with it.
To prove that the second of these hypotheses is unfounded, it will be necessary, first, to prove that the surface of the globe never varies in character, in exposure, situation, whether elevated or sheltered, climate, etc.; and, secondly, to prove that no part of the animal world undergoes, even in the course of long periods of time, any modification through change of circumstances, or as a consequence of a changed manner of life and action.