Page - 301 - in The Origin of Species

DEGREES OF STERILITY 301 often transmitted in an augmented degree to the offspring; and both sexual elements in hybrid plants are already affected in some degree. But I believe that their fertility has been diminished in nearly all these cases by an independent cause, namely, by too close interbreeding. I have made so many experiments and collected so many facts, showing on the one hand that an occasional cross with a distinct individual or variety increases the vigour and fertility of the offspring, and on the other hand that very close interbreeding lessens their vigour and fertility, that I cannot doubt the correctness of this conclusion. Hybrids are seldom raised by experimental- ists in great nurribers ; 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, if left to themselves, will generally be fertilised during each generation by pollen from the same flower; and this would probably be injurious to their fertility, already lessened by their hybrid origin. I am strengthened in this conviction by a remarkable statement repeatedly made by Gartner, namely, that if even the less fertile hybrids be artificially fertilised with hybrid pollen of the same kind, their fertility, nothwithstanding the frequent ill effects from manip- ulation, sometimes decidedly increases, and goes on increas- ing. Now, in the process of artificial fertilisation, pollen is as often taken by chance (as I know from my own experi- ence) from the anthers of another flower, as from the anthers of the flower itself which is to be fertilised; so that a cross between two flowers, though probably often on the same plant, would be thus effected. Moreover, whenever compli- cated experiments are in progress, so careful an observer as Gartner would have castrated his hybrids, and this would have ensured in each generation a cross with pollen from a distinct flower, either from the same plant or from another plant of the same hybrid nature. And thus, the strange fact of an increase of fertility in the successive generations of artificially fertilised hybrids, in contrast with those spon- taneously self-fertilised, may, as I believe, be accounted for by too close interbreeding having been avoided. Now let us turn to the results arrived at by a third most experienced hybridiscr, namely, the Hon. and Rev. W. Iler-