226 LAWS GOVERNING THE STERILITY [CHAP. IX. 



of first crosses and of hybrids, graduates from zero to perfect 

 fertility. It is surprising in how many curious ways this 

 gradation can be shown ; but only the barest outline of the facts 

 can here be given. When pollen from a plant of one family is 

 placed on the stigma of a plant of a distinct family, it exerts no 

 more influence than so much inorganic dust. From this absolute 

 zero of fertility, the pollen of different species applied to the 

 stigma of some one species of the 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 Verbascum, can be united with unusual facility, and 

 produce numerous hybrid-offspring, yet these hybrids are re- 

 markably feterile. 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 general resem 

 bianco between species in structure and constitution. Now the 



