Natural Selection 



Darwin laid such great stress. Natural selection, 

 although a most important factor in evolution, is 

 not an indispensable one. Evolution is possible 

 without natural selection. 



Let us suppose that there is no such thing as 

 natural selection ; that the numbers of existing 

 species are kept constant by the elimination of 

 all individuals born in excess of the number 

 required to maintain the species at the existing 

 figure, and that the elimination of the surplus is 

 effected, not by natural selection, but by chance, 

 by the drawing of lots. Under such circum- 

 stances there may be evolution, existing species 

 may undergo change, but the evolution will be 

 determined solely by the lines along which 

 variations occur. 



If mutations take place along certain fixed 

 lines, and tend to accumulate in the given 

 directions, evolution will proceed along these 

 lines quite independently of the utility to the 

 organism of the mutations that occur. An un- 

 favourable mutation will have precisely the same 

 chance of survival as a favourable one. 



If, on the other hand, mutations occur in- 

 discriminately on all sides of the mean, then 

 those mutations which happen to occur most 

 frequently will have the best chance of survival, 

 and they will mark the lines of evolution. But 

 suppose that no mutation occurs more frequently 

 than the others. Under such circumstances there 



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