GENERAL PRINCIPLES OF BREEDING. 38 



subjected the variations are not perpetuated; 

 while, on the other hand, if 'they are in con- 

 formity to the existing wants or conditions, 

 thereby better fitting the individual to succeed 

 in the struggle for existence, natural selection 

 and a survival of the fittest will tend to perpet- 

 uate them. 



It is evident, therefore, that the laws of 

 heredity tend to reproduce in the progeny the 

 character of the ancestors, and that when the 

 ancestry is of a fixed and uniform type the 

 maxim that "like produces like" admits of few 

 exceptions. Yet there are exceptions even here, 

 as we have seen in the case of sports; and the 

 modifications produced by changed conditions 

 of life, adaptation to new uses and new modes 

 of subsistence, tend to vary what, under the 

 operation of the unrestricted laws of heredity, 

 would fix a given type and leave the breeder's 

 art powerless to effect change or improve- 

 ment. 



Heredity, which makes of every individual 

 the sum, or aggregation, of that which has lived 

 before him, is essentially a conservative force, 

 and opposes all changes, all progress, all im- 

 provement; but evolution, which compels he- 

 redity to give way to internal and external 

 causes, and modifies both the physical and 

 mental organism, places in the breeder's hands 

 the means of effecting wonderful changes. 



