268 LAWS GOVERNING THE STERILITY 



tility may be detected, by the pollen of one of the pure 

 parent-species causing the flower of the hybrid to wither 

 earlier than it otherwise would have done ; and the early 

 withering of the flower is well known to be a sign of incip- 

 ient fertilization. From this extreme degree of sterility we 

 have self-fertilized hybrids producing a greater and greater 

 number of seeds up to perfect fertility. 



The hybrids raised from two species which are very diffi- 

 cult to cross, and which rarely produce any offspring, ar© 

 generally very sterile ; but the parallelism between the diffi- 

 culty of making a first cross, and the sterility of the hybrids 

 thus produced — two classes of facts which are generally 

 confounded together — is by no means strict. There are 

 many cases, in which two pure species, as in the genus Ver- 

 bascum, can be united with unusual facility, and produce 

 numerous hybrid offspring, yet these hybrids are remarkably 

 sterile. On the other hand, there are species which can be 

 crossed very rarely, or with extreme difficulty, but the 

 hybrids, when at last produced, are very fertile. Even with- 

 in the limits of the same genus, for instance in Dianthus, 

 these two opposite cases occur. 



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 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 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 dis- 

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

 species generally uniting with facility. But the correspond- 

 ence 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 dia- 



