Chap. IX.] DEGEEES OF STERILITY. 11 



these two forms must be regarded as good and distinct 

 species. The same remarks may be extended to the two 

 chief races of the pig. We mnst, 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 

 intercrossing 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 con- 

 sidered 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 ad- 

 mirable work on the hybridisation 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 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 



