360 The Theory of Natural Selection 



the bones were not correspondingly stronger. The organism 

 acts as a unit and speciaHzations in one part of the body can 

 be of advantage only in the presence of related specializations 

 in other parts. 



The theory of the struggle for existence has implied 

 too severe a contest with respect to most species and par- 

 ticularly in relation to detail of structure or function. It 

 is true that at any given time only a small fraction of the 

 progeny of a given species will survive, but the extermina- 

 tion is not generally of a kind that will insure a differential 

 survival of ** superior " individuals. Many organisms have 

 structures and instincts that are actually injurious to them, 

 yet these do not lead to their extermination. It is sufficient 

 only that the injuries are not fatal. The flight of the moth 

 to the candle will destroy annually many millions of moths, 

 but not enough to destroy the species having this instinct. 

 Many plants and animals are able to survive in spite of 

 obvious handicaps. 



Isolation 



The appearance of a favorable variation, aside from the 

 question of selection, can serve as the beginning of a new va- 

 riety only ( I ) if the quality in question is transmissible, and 

 (2) if the individual or individuals bearing the new char- 

 acter are protected from swamping by the vast number of 

 conformists. Many suggestions have been made to meet 

 this difficulty. Moritz Wagner considered an essential part 

 of the theory of natural selection the fact of isolation. This 

 makes it possible for the varying individuals to reproduce 

 their distinctive traits unaffected by the regressive effect of 

 the large numbers of the parental type. It has been shown 

 in fact that the differentiation of types does actually take 

 place wherever there is isolation. The studies of Gulick and 

 Crampton on snails (see page 190) furnish an excellent 

 example. Darwin himself had accumulated numerous ex- 

 amples of rabbits, dogs, cattle, and other animals, develop- 



