Origin of Species Variorum

Comparison with 1861

Text in this page (from paragraph 5200, sentence 310, word 6 to paragraph 5200, sentence 330, word 32) is not present in 1861

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 domesti- cation 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 given in this chapter do not seem to me opposed to, but even rather to support in some respects the view, that there is no fundamental distinction between species and varieties.

the sterility of the illegitimate unions of dimorphic and trimorphic plants and of their illegitimate progeny, perhaps 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 of the results of reciprocal crosses, clearly leads to the conclusion that the primary cause of the sterility of crossed species is confined to differences in their sexual elements. But why, in the case of distinct species, the sexual elements should so generally have become more or less modified, leading to their mutual infertility, we do not know; but it seems to stand in some close relation to species having been exposed for long periods of time to nearly uniform conditions of life. 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 it is remembered 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 of characters from both parent-forms. ↑ Finally, then, although we are as ignorant of the precise cause of the sterility of first crosses and of hybrids as we are why animals and plants removed from their natural conditions become sterile, yet the facts given in this chapter do not seem to me opposed to the belief that species aboriginally existed as varieties.