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Updated: September 12, 2025
These two methods of subdivision seem in the main, and notwithstanding their relatively imperfect application in many single examples, to correspond with two really distinct cases. The derivative varieties are distinguished from the parent-species by some single, but striking mark, and often this attribute manifests itself as the loss of some apparent quality.
The fore-limbs, for instance, which served as legs in the parent-species, may have become, by a long course of modification, adapted in one descendant to act as hands, in another as paddles, in another as wings; and on the above two principles namely of each successive modification supervening at a rather late age, and being inherited at a corresponding late age the fore-limbs in the embryos of the several descendants of the parent-species will still resemble each other closely, for they will not have been modified.
Thus it is, as I believe, that two or more genera are produced by descent, with modification, from two or more species of the same genus. And the two or more parent-species are supposed to have descended from some one species of an earlier genus.
The subject is in many ways important for us, more especially as the sterility of species when first crossed, and that of their hybrid offspring, cannot have been acquired, as I shall show, by the preservation of successive profitable degrees of sterility. It is an incidental result of differences in the reproductive systems of the parent-species.
So in hybrids themselves, there are some which never have produced, and probably never would produce, even with the pollen of either pure parent, a single fertile seed: but in some of these cases a first trace of fertility may be detected, by the pollen of one of the pure parent-species causing the flower of the hybrid to wither earlier than it otherwise would have done; and the early withering of the flower is well known to be a sign of incipient fertilisation.
Otherwise expressed, the system is built up of species, and varieties are only local and lateral, but never of real importance for the whole structure. Heretofore we have generally assumed, that varieties differ from the parent-species in a single character only, or at least that only one need be considered. We now come to the study of those varieties, which differ in more than one character.
Thus it is, as I believe, that two or more genera are produced by descent with modification, from two or more species of the same genus. And the two or more parent-species are supposed to have descended from some one species of an earlier genus.
Now let us apply these facts and the above two principles which latter, though not proved true, can be shown to be in some degree probable to species in a state of nature. Let us take a genus of birds, descended on my theory from some one parent-species, and of which the several new species have become modified through natural selection in accordance with their diverse habits.
Thus in all the many breeds in which the adult male differs greatly in colour from the female, as well as from the wild parent-species, he differs also from the young male, so that the newly- acquired characters must have appeared at a rather late period of life.
Hybrids are seldom raised by experimentalists in great numbers; and as the parent-species, or other allied hybrids, generally grow in the same garden, the visits of insects must be carefully prevented during the flowering season: hence hybrids will generally be fertilised during each generation by their own individual pollen; and I am convinced that this would be injurious to their fertility, already lessened by their hybrid origin.
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