12 LAWS GOVERNING THE STERILITY [Chap. IX. 



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 off- 

 spring, 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- 



