Origin of Species Variorum

But why, in the case of species, the sexual elements should so generally have become more or less modified, leading to their mutual infertility, we do not know. ... ↑

It is not surprising that the .. difficulty in crossing any two species, and the sterility of .. their hybrid-offspring, should in most cases correspond, even if due to distinct causes; for both depend on the amount of difference ... between the species which are crossed. Nor is it surprising that the facility of effecting a first cross, and the fertility of the hybrids thus produced, and the capacity of being grafted together—though this latter capacity evidently depends on widely different circumstances—should all run, to a certain extent, parallel with the systematic affinity of the forms .. subjected to experiment; for systematic affinity includes resemblances of all kinds. .. .. .. .. ..

First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not, as is so often stated, invariably fertile. Nor is this almost universal and perfect fertility surprising, when we remember how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under domestication by the selection of mere external differences, and that they have not been long exposed to uniform conditions of life. It should also be especially kept in mind, that long-continued domestication .. tends to eliminate sterility, and is therefore little likely to induce this same quality. Independently of the question of fertility, in all other respects there is the closest general resemblance between hybrids and mongrels, in their variability, in their power of absorbing each other by repeated crosses, and in their inheritance