Origin of Species Variorum

Comparison with 1869

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 there is some unknown bond connecting in all cases 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, drive us to conclude that in all cases the primary cause of sterility, both in the parents and in the offspring, is confined to differences in their reproductive systems. But why in numerous species, descended from a common parent-form, the reproductive system should in all have become more or less modified, leading to their mutual infertility, we do not know in the least; nor whether this has been effected directly, or in correlation with other structural and functional modifications. ↑

It is not surprising that the degree of difficulty in uniting two species, and the degree of sterility of their hybrid-offspring, should generally correspond, even if due to distinct causes; for both depend on the amount of difference of some kind 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 which are subjected to experiment; for systematic affinity attempts to express all kinds of resemblance between all species.

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 domesti- cation by the selection of mere external differences, and that they have not been long exposed to uniform conditions of life. Nor should it be forgotten that long-continued domestication apparently 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 of characters from both parent-forms. ↑ Finally, then, although we are profoundly ignorant of the precise cause of the sterility of first crosses and of hybrids, the facts

frequently affects pure species, when their natural conditions of life have been disturbed. This view is supported by a parallelism of another kind:— namely, that the crossing of forms only slightly different is favourable to the vigour and fertility of their offspring; and that slight changes in the conditions of life are apparently favourable to the vigour and fertility of all organic beings. ↑ It is not surprising that the degree of difficulty in uniting two species, and the degree of sterility of their hybrid-offspring, should generally correspond, though due to distinct causes; for both depend on the amount of difference of some kind between the species which are crossed. Nor is it surprising that the facility of effecting a first cross, the fertility of the hybrids produced from it, 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 which are subjected to experiment; for systematic affinity attempts to express all kinds of resemblance between all species. ↑

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 falsely stated, universally fertile. Nor is this nearly general 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 not of differences in the reproductive system. ↑ In all other respects, excluding fertility, there is a close general resemblance between hybrids and mongrels. Finally, then, although we are profoundly ignorant in every case of the precise cause of sterility, the facts briefly