Page - 307 - in The Origin of Species

LAWS GOVERNING THE STERILITY 307 The fertility, both of first crosses and of hybrids, is more easily affected by unfavorable conditions, than is that of pure species. But the fertility of first crosses is likewise in- nately variable ; for it is not always the same in degree when the same two species are crossed under the same circum- stances ; it depends in part upon the constitution of the in- dividuals which happen to have been chosen for the experi- ment. So it is with hybrids, for their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the same capsule and exposed to the same conditions. By the term systematic affinity is meant, the general re- semblance between species in structure and constitution. Now the fertility of first crosses, and of the hybrids produced from them, is largely governed by their systematic affinity. This is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility. But the correspondence between systematic affinity and the facility of crossing is by no means strict. A multitude of cases could be given of very closely allied species which will not unite, or only with extreme difficulty ; and on the other hand of very distinct species which unite with the utmost facility. In the same family there may be a genus, as Dianthus, in which very man}^ species can most readily be crossed ; and another genus, as Silene, in which the most per- severing efforts have failed to produce between extremely close species a single hybrid. Even within the limits of the same genus, we meet with this same difference ; for instance, the many species of Nicotiana have been more largely crossed than the species of almost any other genus ; but Gartner found that N. acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be fertilised by no less than eight other species of Nicotiana. Many analogous facts could be given. No one has been able to point out what kind or what amount of difference, in any recognisable character, is suf- ficient to prevent two species crossing. It can be shown that plants most widely different in habit and general appearance, and having strongly marked differences in every part of the