1» THE ORIGIN OF SPECIES 



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 tliat 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 facil- 

 ity, and produce numerous hybrid-offspring, yet these 

 hybrirds are remarkably sterile. On the other hand, there 

 are species which can be crossed very rarely, or with ex- 

 treme 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, 



