HYBRIDISM 229 



the circumstances and rules governing the sterility of 

 first crosses and of hybrids. Our chief object will be to 

 see whether or not the rules indicate that species have 

 specially been endowed with this quality, in order to 

 prevent their crossing and blending together in utter 

 confusion. The following rules and conclusions are 

 chiefly drawn up from Gartner's admirable work on the 

 hybridisation of plants. I have taken much pains to 

 ascertain how far the rules apply to animals, and con- 

 sidering 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 to 

 exist ; 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 differ- 

 ent species of the same genus applied to the stigma of 

 some one species, 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 will produce. 

 So in hybrids themselves, there are some which never 

 have produced, and probably never would produce, 

 even with the pollen of either pure parent, 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 pro- 

 ducing a greater and greater number of seeds up to 

 perfect fertility. 



Hybrids from two species which are very difficult to 



