HYBRIDISM 19 



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 circumstances; it depends in part upon the con- 

 stitution of the individuals which happen to have been 

 chosen for the experiment. 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 

 resemblance between species in structure and constitution. 

 r^ow the fertility of first crosses, and of the hybrids pro- 

 duced 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 facil- 

 ity 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 species 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 instance, the many species of Nico- 

 tiana have been more largely crossed than the species of 

 almost any other genus; but Gartner found that N. acu- 



