"Against this it is alleged that we have no reason to believe species to have changed within any known era. The skeletons of some Egyptian birds, preserved two or three thousand years ago, differ in no particular from the same kind of creatures at the present day. But this is what we should expect, inasmuch as the position and climate of Egypt itself do not appear to have changed. If the conditions of life have not varied, why should the species subjected to those conditions have done so? Moreover, birds can move about freely, and if one place does not suit them they can find another that does. All that these Egyptian mummies really prove is, that there were animals in Egypt two or three thousand years ago which are like the animals of to-day; but how short a space is two or three thousand years, as compared with the time which Nature has had at her disposal! A time infinitely great quâ man, is still infinitely short quâ Nature.[241]
"If, however, we turn to animals under confinement, we find immediate proof that the most startling changes are capable of being produced after some generations of changed habits. In the sixth chapter we shall have occasion to observe the power of changed conditions [circonstances] to develop new desires in animals, and to induce new courses of action; we shall see the power which these new actions will have, after a certain amount of repetition, to engender new habits and tendencies; and we shall also note the effects of use and disuse in either fortifying and developing an organ, or in diminishing it and causing it to disappear. With plants under domestication, we shall find corresponding phenomena. Species will thus appear to be unchangeable for comparatively short periods only."[242]
It is interesting to see that Mr. Darwin lays no less stress on the study of animals and plants under domestication than Buffon, Dr. Darwin, and Lamarck. Indeed, all four writers appear to have been in great measure led to their conclusions by this very study. "At the commencement of my investigations," writes Mr. Darwin, "it seemed to me probable that a careful study of domesticated animals and of cultivated plants would offer the best chance of making out this obscure problem. Nor have I been disappointed; in this and in all other perplexing cases, I have invariably found that our knowledge, imperfect though it be, of variation under domestication, afforded the best and safest clue. I may venture to express my conviction of the high value of such studies, though they have been very commonly neglected by naturalists."[243]
In justice to the three writers whom I have named, it should be borne in mind that they also ventured to express their conviction of the high value of these studies. Buffon, indeed, as we have seen, gives animals under domestication the foremost place in his work. He does not treat of wild animals till he has said all he has to say upon our most important domesticated breeds,—on whose descent from one or two wild stocks he is never weary of insisting. It was doubtless because of the opportunities they afforded him for demonstrating the plasticity of living organism that the most important position in his work was assigned to them.
Lamarck professes himself unable to make up his mind about extinct species; how far, that is to say, whole breeds must be considered as having died out, or how far the difference between so many now living and fossil forms is due to the fact that our living species are modified descendants of the fossil ones. Such large parts of the globe were still practically unknown in Lamarck's time, and the recent discovery of the ornithorhynchus has raised such hopes as to what might yet be found in Australia, that he was inclined to think that only such creatures as man found hurtful to him, as, for example, the megatherium and the mastodon, had become truly extinct, nor was he, it would seem, without a hope that these would yet one day be discovered. The climatic and geological changes that have occurred in past ages, would, he believed, account for all the difference which we observe between living and fossil forms, inasmuch as they would have changed the conditions under which animals lived, and therefore their habits and organs would have become correspondingly modified. He therefore rather wondered to find so much, than so little, resemblance between existing and fossil forms.
Buffon took a juster view of this matter; it will be remembered that he concluded his remarks upon the mammoth by saying that many species had doubtless disappeared without leaving any living descendants, while others had left descendants which had become modified.
Lamarck anticipated Lyell in supposing geological changes to have been due almost entirely to the continued operation of the causes which we observe daily at work in nature: thus he writes:—
"Every observer knows that the surface of the earth has changed; every valley has been exalted, the crooked has been made straight, and the rough places plain; not even is climate itself stable. Hence changed conditions; and these involve changed needs and habits of life; if such changes can give rise to modifications or developments, it is clear that every living body must vary, especially in its outward character, though the variation can only be perceptible after several generations.
"It is not surprising then that so few living species should be represented in the geologic record. It is surprising rather that we should find any living species represented at all.[244]
"Catastrophes have indeed been supposed, and they are an easy way of getting out of the difficulty; but unfortunately, they are not supported by evidence. Local catastrophes have undoubtedly occurred, as earthquakes and volcanic eruptions, of which the effects can be sufficiently seen; but why suppose any universal catastrophe, when the ordinary progress of nature suffices to account for the phenomena? Nature is never brusque. She proceeds slowly step by step, and this with occasional local catastrophes will remove all our difficulties."[245]