246 THE ORIGIN OF SPECIES 



certain how far they apply to animals, and, considering how 

 scanty our knowledge is in regard 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 different species applied to the stigma of some one 

 species of the same genus, yields a perfect gradation in the num- 

 ber of seeds produced, up to nearly complete or even quite com- 

 plete 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 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 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 

 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 fertihty, 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 



