NATURAL SELECTION 83 



would require to endow them with properties which 

 at present they do not possess. 



Man in forming a new variety uses, and can 

 only use, individual variations. By his power of 

 selecting and isolating the individuals suited to his 

 purpose he is enabled to modify and vary his breeds 

 in many directions. But one thing he cannot do. 

 He cannot give his modifications specific fixity, so as 

 to prevent his modified animals from crossing, when 

 opportunity offers, with other breeds of the same 

 species. Thus, when he ceases to select his individuals, 

 and to isolate the breed, permitting its individuals to 

 have free intercourse with individuals of other breeds, 

 that breed is doomed to speedy extinction ; for Nature 

 has not made the possession of similar variations a 

 means of attracting the sexes to each other. This 

 fact, when we consider that the maintenance of any 

 type depends upon the constant copulation of like with 

 like throughout successive generations, is pregnantly 

 suggestive of the fact that Nature does not intend 

 individual variations to possess any evolutional value. 

 In modifying her specific forms, Nature, as we shall 

 see, employs only specific characters that is, characters 

 that belong to all the individuals of the species. She 

 therefore secures the results which she contemplates 

 by the unbroken and uninterrupted copulation of like 

 with like. 



Before proceeding to a more detailed examination 

 of the properties and functions of the individual 

 variation as shown in domestication, which provides 



