Origin of Species Variorum

sterile when crossed; and bearing in mind that in reciprocal crosses between the same two species there is sometimes the widest difference in the resulting degrees of sterility, we must give up the belief that natural selection has come into play; and we are driven to our former proposition, that the sterility of first crosses, and indirectly of hybrids, is simply incidental on unknown differences in the reproductive systems of the parent-species.

We may now try and look a little closer at the probable nature of these differences, which induce sterility in first crosses, as well as in hybrids. Pure species and hybrids differ, as already remarked, in the state of their reproductive organs; but from what will presently follow on reciprocally dimorphic and trimorphic plants, it would appear as if some unknown bond or law existed, which causes the young from a union not fully fertile to be themselves more or less infertile. ↑

In the case of first crosses between pure species, the greater or less difficulty in effecting an union and in obtaining 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 pollen of one species is placed on the stigma of a distantly allied species, 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 Thuret's experiments on Fuci. No explanation can be given of these facts, any more than why certain trees cannot be grafted on others. Lastly, an embryo may be developed, and then perish at an early