EVOLUTION 297 



have known for a long time that by careful selection, certain 

 chance differences or variations in a species of animal or plant 

 can be perpetuated. The criterion by which the breeder decides 

 which variations are desirable is in most cases utility. Thus the 

 breeder of horses for running races selects for sires stallions that 

 have established good records themselves or that are descended 

 from good racing stock. Improvements in animals from the 

 standpoint of their usefulness as food as in the case of the pig 

 (Fig. 174) have been brought about as the result of perpetuating 



Fig. 175. — Columba livia, the ancestor of 150 or more varieties of domestic 

 pigeons. (After Whitman, Carnegie Inst. Pub.) 



desirable qualities, that arose originally as chance variations. 

 In the case of the fancy breeds of dogs, characters other than 

 utilitarian ones, have become the standards of the various breeds. 

 In general the various breeds and varieties of our domestic 

 animals and plants have been developed by artificially controlled 

 breeding from relatively few primitive natural or wild species. 

 Anyone unacquainted with the history of the various breeds of 

 domestic pigeons would say that the breeds were at least as dis- 

 tinct as different species; yet, as Darwin first pointed out, they 

 have in all probability been derived by selective breeding from a 

 single wild species, Columba livia, the rock pigeon (Fig. 175). 

 A common breed, the fantail, possesses something like 40 tail 

 feathers, whereas the ordinary pigeon has but 16. In all prob- 



