CH. x.] NATURAL SELECTION. 9 



which has elapsed since men began to observe nature sys 

 tematically, is but an infinitesimal portion of the period 

 requisite for any fundamental alteration in the characteristics 

 of a species. But there are innumerable cases in which 

 species are made to change rapidly through the deliberate 

 intervention of man. In the course of a few thousand years, 

 a great number of varieties of plants and animals have been 

 produced under domestication, many of which differ so widely 

 from their parent-forms that, if found in a state of nature, 

 they would be unhesitatingly classified as distinct species, 

 and sometimes as distinct genera. Modifications in the 

 specific characters of domesticated organisms are the only 

 ones which take place so rapidly that we can actually observe 

 them ; and it therefore becomes highly important to inquire 

 what is the agency which produces these modifications. 



That agency is neither more nor less than selection, taking 

 advantage of that slight but universal variation in organisms 

 implied by the fact that no two individuals in any species 

 are exactly alike. If man, for example, wishes to produce a 

 breed of fleet race-horses, he has only to take a score of 

 horses and select from these the fleetest to pair together: 

 from among the offspring of these fleet pairs he must again 

 select the fleetest ; and thus, in a few generations, he will 

 obtain horses whose average speed far exceeds that of the 

 fleetest of their undoniesticated ancestors. It is in this and 

 no other way that our breeds of race-horses have been pro 

 duced. In this way too have been produced the fine wools 

 of which our clothing is made. By selecting, generation after 

 generation, the sheep with the finest and longest wool, a breed 

 of sheep is ultimately reared with wool almost generically 

 different from that of the undomesticated 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 



