The phenomena of hybridity furnish another ground of argument in favor of the transmutation of species, and of course in favor of the law hypothesis; for that hybrids are sometimes the result of the union of different species will not be denied. There is, however, a natural repugnance to union between different species; and in a state of nature this can very rarely be overcome. But domestication changes and almost obliterates many natural instincts, and hence hybridity is far more common among domesticated animals and plants. As a general fact, also, the hybrid offspring is incapable of propagating its own race, without union with one of the original species by which it was produced; and this inability to continue this mixed race has been generally regarded among naturalists as the best characteristic of species. Some, however, attempt to show that some hybrid races do continue from generation to generation to propagate their kind. But in most cases the hybrid race ere long runs out, and there is always a strong tendency to revert to the original stock; and were it not for the influence of man, probably such a thing as hybridity would scarcely ever have been heard of. Nature seems to have established strong barriers around species, so that an identity should be preserved; and even if we admit the possibility of their coalescence in some cases, yet we have evidence that almost always they are preserved distinct from century to century; and the same is true even of the more prominent varieties, for we find not only the same species, but the same varieties of animals and plants, preserved some three thousand years in the Egyptian catacombs, that are now alive in the same country. How idle, then, to suppose that the laws of hybridity will account for such radical and entire transmutations as this hypothesis supposes! To accomplish this, it would need as strong a tendency in nature to a union of species, genera, and families, as now exists against it.
But a special appeal has been made on this subject to geology. The history of organic remains, it is thought, corresponds to what we might expect, if the hypothesis of development is true. In the oldest rocks we find chiefly the more simple invertebrate animals, and the vertebrated tribes appear at first in the form of fish, then of reptiles, then of birds, then of mammals, and last of all of man. What better confirmation could we wish than this gradually expanding series? True, all the great classes of organic beings, vegetable and animal, are found nearly at the earliest epoch, and continue through the entire series of rocks. But we have only to suppose a distinct stirps for each of the classes, and that the developments took place along parallel lines, in order to harmonize the facts with the hypothesis.