306 ORIGIN OF SPECIES 



gard to hybrid animals, I have been surprised to find how 

 generally the same rules apply to both kingdoms. 



It has been already remarked, that the degree of fertility, 

 both 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 dif- 

 ferent 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 fer- 

 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 incipient fertilisa- 

 tion. 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 diifi- 

 cult to cross, and which rarely produce any offspring, are 

 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 con- 

 founded 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 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. 



