ON SPECIES AND VARIETY. 183 



intermixtures of which the various species are susceptible, 

 but which they would never produce if left to themselves. 



The degrees of these variations are proportional to the 

 intensity of the causes that produce them, namely, the 

 slavery or subjection under which these animals are to 

 man. They do not proceed far in half-domesticated 

 species. 



In the domesticated herbivorous quadrupeds, which 

 man transports into all kinds of climates, and subjects to 

 various kinds of management, both in regard to labour 

 and nourishment, he procures certainly more considerable 

 variations, but still they are all merely superficial : greater 

 or less size j longer or shorter horns, or even the want of 

 these entirely ; a hump of fat, larger or smaller, on the 

 shoulder ; these form the chief differences among parti- 

 cular races of the Bos Taurus, or domestic Black Cattle ; 

 and these differences continue long in such breeds as 

 have been transported to great distances from the 

 countries in which they were originally produced, when 

 proper care is taken to prevent crossing. 



Nature appears also to have guarded against the 

 alterations of species which might proceed from mixture 

 of breeds, by influencing the various species of animals 

 with mutual aversion. Hence all the cunning and all 

 the force that man is able to exert is necessary to 

 accomplish such unions, even between species that have 

 the nearest resemblance. And when the mule-breeds 

 that are thus produced by these forced conjunctions 

 happen to be fruitful, which is seldom the case, this 

 fecundity never continues beyond a few generations, and 

 would not probably proceed so far, without a continuance 

 of the same causes which excited it at first. 



