cH. xvm The Evolution of Evolution Theories 301 



The secondary factors of evolution may be ranked under 

 two heads : — 



I. Natural Selection, or the survival of the fittest in the 

 struggle for existence, and 2. Isolation, or the various means 

 by which species tend to be separated into sections which 

 do not interbreed. 



Natural selection is a phrase descriptive of the course of 

 nature, of the survival of the fit and the elimination of the 

 unfit in the struggle for existence. It involves on the one 

 hand the survival, i.e. the nutritive and reproductive success 

 of the variations fittest to survive in given conditions, and 

 on the other hand the destruction or elimination of forms 

 less fit. Suitable variations pay ; nature or natural selection 

 justifies and fosters them. Maternal sacrifice or cunning 

 cruelty, the milk of animal kindness or teeth strong to 

 rend, distribution in space or rate of reproduction, are all 

 affected by natural selection. But it is another thing to say 

 that all the adaptations and well-endowed species that we 

 know have been produced by the action of natural selection 

 on fortuitous, indefinite variations. This is what Samuel 

 Butler calls the " accredited fallacy.'' 



Secondly, there seem to be a great many ways by which 

 a species may be divided into two sections which do not 

 interbreed, and if this isolation be common it must help 

 greatly in divergent evolution. 



Thus Romanes, who has been the chief exponent of the 

 importance of isolation, on which Gulick has also insisted, 

 says : " Without isolation, or the prevention of free inter- 

 crossing, organic evolution is in no case possible. It is 

 isolation that has been ' the exclusive means of modifica- 

 tion,' or more correctly, the universal condition to it. 

 Heredity and variability Iseing given, the whole theory of 

 organic evolution becomes a theory of the causes and con- 

 ditions which lead to isolation." 



