CHAPTEE IX 



ISOLATION 



EVERYONE is agreed that unless varieties are isolated 

 they will be in great danger of perishing through 

 free intercrossing with individuals which do not 

 possess the variation. There may be cases in which 

 the prepotency of an animal or plant secures for a 

 time the transmission of a variation, but this must 

 be exceptional. For it is very unlikely that pre- 

 potency and the occurrence of a favourable variation 

 should often occur together. 



Two processes are necessary for a variation to 

 succeed ; preservation and accumulation. Selection 

 does both; but isolation, pure and simple, includes 

 no cumulative principle. And yet there are many 

 characters which we cannot explain by selection ; 

 and in all of these we must, for the present, assume 

 that the cause of accumulation is definite variation. 



Useless Characters. These are characters which 

 are thought to be of no particular use to their pos- 

 sessors, neither have they ever been of any use to 

 ancestors of the present possesssors. Consequently 

 they could not have been preserved by selection. It 

 is highly probable that we call some characters use- 



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