Origin of Species Variorum

of one species to unite with another is incidental on unknown differences in their reproductive systems. There is no more reason to think that species have been specially endowed with various degrees of sterility to prevent their crossing and blending in nature, than to think that trees have been specially endowed with various and somewhat analogous degrees of difficulty in being grafted together in order to prevent their inarching in our forests.

The sterility of first crosses and of their hybrid progeny has not, as far as we can judge, been acquired through natural selection. In the case of first crosses it seems to depend on several circumstances; in some instances in chief part on the early death of the embryo. .. .. .. .. .. .. .. .. .. .. .. .. .. ↑ In the case of hybrids, it perhaps depends on their whole organisation having been disturbed by being compounded from two distinct forms; the sterility being closely allied to that .. which so frequently affects pure species, when exposed to unnatural conditions of life. .. .. .. This view is supported by a parallelism of another kind: namely, that, first, the crossing of forms only slightly differentiated favours the vigour and fertility of their offspring, whilst close interbreeding is injurious; and secondly, that slight changes in the conditions of life apparently add to the vigour and fertility of all organic beings, whilst greater changes are often injurious. But the facts given on the sterility of the illegitimate unions of dimorphic and trimorphic plants and of their illegitimate progeny, render it probable that .. some unknown bond in all cases connects the degree of fertility of first unions with that of their offspring. The consideration of these facts on dimorphism, as well as the results of reciprocal crosses, clearly leads to the conclusion that the primary cause of the sterility is confined to dif- ferences in the sexual elements.