Variation and Natural Selection. 97 



variations will give a new mean nearer the plus or advan- 

 tageous end of the scale than the old mean. By how 

 much the favourable variations tend to raise the mean 

 standard, by so much will the race tend to advance. 

 But in this process I see no reason why the neutral 

 variations should be eliminated, except in so far as, in 

 the keen struggle for existence, they become relatively 

 unfavourable. 



It is clear, however, that the intercrossing and inter- 

 breeding which occurs between average individuals on the 

 one hand, and those possessing favourable variations on 

 the other, while it tends gradually to raise the mean 

 standard, tends also at the same time to reduce the advan- 

 tageous variations towards the mean. It must tend to check 

 advance by leaps and bounds, and to justify the adage, 

 Natura nil facit per saltum. At the same time, it will 

 probably have a greater tendency to reduce to a mean level 

 neutral variations indefinite in direction than advantageous 

 variations definite in direction. Still, it is a most im- 

 portant factor, and one not to be neglected. It tends to 

 uniformity in the species, and checks individualism. It 

 may act as a salutary brake on what we may figuratively 

 term hasty and ill-advised attempts at progress. And at 

 the same time, it favours repeated new experiments in the 

 combination of variations, occasionally, we may suppose, 

 with happy results. 



But it does more than this. It tends to check, and, if 

 the offspring always possessed the blended character of both 

 parents, would be absolutely fatal to, divergence of character 

 within the interbreeding members of a species. And yet 

 no fact is more striking than this divergence of character. 

 It is seen in the diversified products of human selection ; 

 for example, among pigeons. It is seen in the freedom of 

 nature. Mr. Wallace gives many examples. "Among 

 our native species," he says, "we see it well marked in the 

 different species of titmice, pipits, and chats. The great 

 titmouse, by its larger size and stronger bill, is adapted to 

 feed on larger insects, and is even said sometimes to kill 



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