242 LAWS GOVERNING THS STERILITY [Chap. IX, 



fertility, the pollen of different specks applied to the stigma of 

 some one species of ths same genus, yields a perfect gradation in 

 the number of seeds produced, up to nearly complete or even quite 

 complete fertility ; and, as we have seen, in certain abnormal cases, 

 even to an excess of fertility, beyond that which the plant's own 

 pollen produces. So in hybrids themselves, there are some which 

 never have produced, and probably never would produce, even with 

 the pollen of the pure parents, a single fertile seed : but in some of 

 these cases a first trace of fertility 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 incipient 

 fertilisation. From this extreme degree of sterility we have self- 

 fertilised hybrids producing a greater and greater number of seeds 

 up to perfect fertility. 



The hybrids raised from two species which are very difficult 

 to cross, and which rarely produce any offspring, are generally very 

 sterile ; but the parallelism between the difficulty 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 Yerbascum, 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 within 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 unfavourable 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 

 constitution 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 genera] resemblance 

 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 sys- 

 tematists in distinct families ; and on the other hand, by very 



