DEGREES OF STERILITY. 267 



the two chief races of the pig. "We must, therefore, either 

 give up the belief of the universal sterility of species when 

 crossed ; or we must look at this sterility in animals, not 

 as an indelible characteristic, but as one capable of being 

 removed by domestication. 



Finally, considering all the ascertained facts on the inter- 

 crossing of plants and animals, it may be concluded that 

 some degree of sterility, both in first crosses and in hybrids, 

 is an extremely general result ; but that it cannot, under 

 our present state of knowledge, be considered as absolutely 

 universal. 



LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND 



OF HYBRIDS. 



We will now consider a little more in detail the laws 

 governing the sterility of first crosses and of hybrids. Our 

 chief object will be to see whether or not these laws indicate 

 that species have been specially endowed with this quality, 

 in order to prevent their crossing and blending together in 

 utter confusion. The following conclusions are drawn up 

 chiefly from Gartner's admirable work on the hybridization 

 of plants. I have taken much pains to ascertain how far 

 they apply to animals, and, considering how scanty our 

 knowledge is in regard to hybrid animals, I have been sur- 

 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 hybrids, graduates from zero to 

 perfect fertilit}^. 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 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* 



