HEEEDITY 



theless produced only when two independently 

 inherited factors are present together. A 

 character of this sort does not itself conform 

 with the simple Mendelian laws of inheritance, 

 but its factors do. Herein lies the explanation 

 of atavism or reversion, and the process by 

 which reversionary characters may be fixed. 



Atavism or reversion to an ancestral con- 

 dition is a phenomenon to which Darwin re- 

 peatedly called attention. He realized that it 

 is a phenomenon which general theories of 

 heredity must account for. He supposed that 

 the environment was chiefly responsible for the 

 reappearance in a species of a lost ancestral 

 condition, but that in certain cases the mere 

 act of crossing may reawaken slumbering an- 

 cestral traits. Thus he noticed that when 

 rabbits of various sorts are turned loose in a 

 warren together, they tend to revert to the 

 gray-coated condition of wild rabbits. And 

 when pigeons are crossed in captivity they 

 frequently revert to the plumage condition of 

 the wild rock pigeon, Columba livia. In plants, 

 too, Darwin recognized that crossing is a fre- 

 quent cause of reversion. The explanation 



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