Thus fertility, like parentage, has become a physiological character of species; and, though in the case of some domesticated animals, as pigeons, the extreme forms are more different from one another than many morphological species, yet they, apart from the historical evidence of their parentage, are held to be members of the same species, because they are all perfectly fertile one with another, and their offspring are also perfectly fertile.
Thirdly, it is a matter of experience that, as a general rule, and taking the whole cycle of forms through which a living being runs into account, offspring and parent are so similar that they belong to one and the same morphological species; and it is further in evidence that many species have endured for extremely long periods without any notable difference being discernible between ancestor and descendant. Moreover, in some cases, varieties are found to revert to the character of the species from which they have proceeded. The conclusion has been drawn that the character of species is physiologically fixed; that is to say, that, however long the process of generation may be continued, the individuals either retain the identical morphological character of the oldest ancestor, or, if they vary, the varieties remain fertile with one another.
Assuming that species have the physiological character thus enumerated, certain conclusions respecting the "origin of species" are inevitable. It is clear that no existing species can have arisen by the intercrossing of preëxisting species, or by the variation of preexisting species, but that every species must have existed from all eternity, or have come into existence suddenly in its present form, which is the objective fact denoted by what is termed creation.
At the dawn of modern biology, a century ago, no scientific evidence respecting the real history of life on the globe was extant, and, for any proof that existed to the contrary, species might have been of eternal duration. But philosophical speculation combined with theological dogma not only to favor the contrary opinion, but to lead the most philosophic naturalist of his day to embody the hypothesis of creation in a definition of species. "Totidem numeramus species quot in principio formæ sunt creatæ" (we reckon as many species as there were forms created in the beginning) is the well-known formula of Linnæus.
In practice Linnæus regarded species from a purely mythological point of view; in theory, he assumed the common ancestry and the limited variability of species, though he was disposed to allow more freedom in this direction than most of his successors. On the other hand, he seems to have attached comparatively little weight to the assumed sterility of hybrids, and to have held a sort of modified doctrine of evolution, supposing that existing species may have been produced by the interbreeding of comparatively few primordial forms.
