NATURAL SELECTION 



ticated race. In this and no other way have the 

 different races of dogs — the greyhound, the 

 mastiff, the terrier, the pointer, and the white- 

 haired Eskimo — been artificially developed 

 from two or three closely allied varieties of the 

 wolf and jackal. The mastiff and bloodhound 

 are more than ten times as large as the terrier, 

 and, if found in a state of nature, they would 

 perhaps be classed in distinct genera, like the 

 leopard and panther, whose differences are hardly 

 more striking. Yet the ancestral races from which 

 these dogs have been reared differed but slightly 

 from each other. The different breeds of dogs 

 vary in the number of their toes, teeth, and 

 vertebrae, in the number and disposition of 

 their mammae, in the shape of their zygomatic 

 arches, and in the position of their occiputs ; 

 although dogs have not been selected with re- 

 ference to these peculiarities, about which un- 

 instructed men neither know nor care, but only 

 with reference to their speed, fleetness, strength, 

 or sagacity. In the case of domestic pigeons, 

 where man has been to a great extent actuated 

 by pure fancy in his selections, the divergences 

 are still more remarkable. All domestic pigeons 

 are descended from a single species of wild 

 pigeon — yet their differences, even in bony 

 structure, in the, internal organs, and in mental 

 disposition, are such as characterize distinct 

 genera, and to describe them cojnpletely would 



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