OP FIRST CEOSSBS and OF HYBRIDS. 385 



knowledge is in regard to hybrid animals, I have been sv^r- 

 prised 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 hybi'ids, 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 cer- 

 tain 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 pro- 

 duced, 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 diffi- 

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

 generally very sterile; but the parallelism between the 

 difiiculty 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 difiiculty, 

 but the hybrids, when at last produced, are very fertile. 

 Even within the limits of the same genus, for instance ip 

 Dianthus, these two opposite cases occur. 



