Seite - 314 - in The Origin of Species

314 ORIGIN OF SPECIES But it would be superfluous to discuss this question in de- tail; for with plants we have conclusive evidence that the sterility of crossed species must be due to some principle, quite independent of natural selection. Both Gartner and Kolreuter have proved that in genera including numerous species, a series can be formed from species which when crossed yield fewer and fewer seeds, to species which never produce a single seed, but yet are affected by the pollen of certain other species, for the germen swells. It is here mani- festly impossible to select the more sterile individuals, which have already ceased to yield seeds ; so that this acme of ster- ility, when the germen alone is affected, cannot have been gained through selection; and from the laws governing the various grades of sterility being so uniform throughout the animal and vegetable kingdoms, we may infer that the cause, whatever it may be, is the same or nearly the same in all cases. We will now look a little closer at the probable nature of the differences between species which induce sterility in first crosses and in hybrids. In the case of first crosses, the greater or less difficulty in effecting an union and in obtain- ing offspring apparently depends on several distinct causes. There must sometimes be a physical impossibility in the male element reaching the ovule, as would be the case with a plant having a pistil too long for the pollen-tubes to reach the ovarium. It has also been observed that when the pollen of one species is placed on the stigma of a distantly allied spe- cies, though the pollen-tubes protrude, they do not penetrate the stigmatic surface. Again, the male element may reach the female element but be incapable of causing an embryo to be developed, as seems to have been the case with some of Thu- ret's experiments on Fuci. No explanation can be given of these facts, anymore thanwhy certain trees cannot be grafted on others. Lastly anembryomay be developed, and then perish at an early period. This latter alternative has not been suf- ficiently attended to; but I believe, from observations com- municated to me by Mr. Hewitt, who has had great experi- ence in hybridising pheasants and fowls, that the early death of the embryo is a very frequent cause of sterility in first