286 LAWS GOYERNme THE STERILITY 



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 

 innately variable; for it is not always the same in degree 

 when the same two species are crossed under the same cir- 

 cumstances; it depends in part upon the constitution of 

 the individuals which happen to have been chosen for the 

 experiment. So it is with hybrids, for their degree of fer- 

 tility is often found to differ greatly in the several indi- 

 viduals raised from seed out of the same capsule and ex- 

 posed 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 pro- 

 duced from them, if largely governed by their systematic 

 affinity. This is clearly shown by hybrids never having 

 been raised between species ranked by systematists in dis- 

 tinct families; and on the other hand, by very closely 

 allied species generally uniting with facility. But the cor- 

 respondence 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 many siDecies can most readily be crossed; and 

 another genus, as Silene, in which the most persevering 

 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 in- 

 stance, 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 par- 

 ticularly distinct species, obstinately failed to fertilize, or 

 to be fertilized, by no less than eight other species of Nico- 

 tiana. Many analogous facts could be given. 



No one has been able to point out what kind or what 

 amount of difference, in any recognizable character, is suf- 

 ficient to prevent two species crossing. It can be shown 

 that plants most widely different in habit and general ap- 

 pearance, and having strongly marked differences in every 

 part of the flower, even in the pollen, in the fruit, and iii 



