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 



