18 



CHAPTER IV. 



CROSS-BREEDING AND ITS RESULTS. 



ANY two breeds of cattle are fertile 

 together, and, as a general rule, first 

 crosses hybrids combine the best char- 

 acteristics of both breeds, and for the 

 grazier's purpose are usually superior to 

 either parent. 



But the progeny of hybrids with hybrids, 

 which we call mongrels, are almost invariably 

 inferior in form, weak in constitution, and 

 frequently infertile. 



Without an admixture of pure blood mon- 

 grels certainly fail, for one reason or another, 

 to perpetuate their type beyond two or 

 three generations, even although parents and 

 progeny are carefully protected from any 

 struggle for their existence. 



Darwin was fully aware of the infertil- 

 ity of hybrids, both of animals and plants, 

 but does not seem to have fully realised 

 its significance in evolution. 



