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Why have not the bowels of the earth preserved the monuments of so strange a genealogy, if it be not because the species of former times were as constant as ours; or, at least, because the catastrophe which destroyed them, had not left them sufficient time for undergoing the variation alleged?

In order to reply to those naturalists who acknowledge that the varieties of animals are restrained within certain limits fixed by nature, it would be necessary to examine how far these limits extend. This is a very curious inquiry,—highly interesting in itself, under a variety of relations, and yet one that has been hitherto very little attended to.

Before entering upon this inquiry, it is proper to define what is meant by a species, so that the definition may serve to regulate the employment of the term. A species, therefore, may be defined, as comprehending the individuals which descend from each other, or from common parents, and those which resemble them as much as they resemble each other. Thus, we consider as varieties of a species, only the races more or less different which may have sprung from it by generation. Our observations, therefore, regarding the differences between the ancestors and descendants, afford us the only certain rule by which we can judge on this subject; all other considerations leading to hypothetical conclusions destitute of proof. Now, considering the varieties in this view, we observe that the differences which constitute it, depend upon determinate circumstances, and that their extent increases in proportion to the intensity of these circumstances.