188 DARWINISM AND HUMAN LIFE 



the survival or non-survival of their possessors." ' 

 As Darwin says, in a notable passage : " Battle 

 within battle must be continually recurring, with 

 varying success ; and yet, in the long run, the 

 forces are so nicely balanced that the merest trifle 

 would give the victory to one organic being over 

 another." 



(3) The theory continues, that, if variations occur 

 in the direction of increased fitness, and if the 

 variations are heritable, and if there is discriminate 

 ehmination with reference to these variations, 

 then the possessors of the fitter variations must be 

 favoured with longer life and larger families — ^with 

 survival, in short. And if this is kept up con- 

 sistently, then new adaptations, and, with the help 

 of isolation, new species, will arise. Those mem- 

 bers of a species that are handicapped will become 

 a minority and eventually their type wiU be 

 eliminated. Those that have varied so as to be in 

 any appreciable way favoured will become the 

 majority, and eventually the type, of the species. 



A little reflection will show that there are two 

 main modes of natural selection. It may produce 

 its effects by the discriminate elimination of the 

 less fit, or by the increased and more effective 

 reproductivity incident on the success of the more 

 fit. These two modes are sometimes distinguished 

 as Lethal and Eeproductive Selection respectively. 

 In both cases the fitter members of a generation 

 contribute more than the less fit to the next generar 

 tion. If we regard sexual selection as a special 

 case of natural selection, which seems the clearest 

 view, we have to include extreme cases like that 



1 " Evidence of Natural Selection," by E. S. Russell, in Riviata 

 di Scienza (1908), vol, iii. 



