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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