CHAP. IV. NATURAL SELECTION. 81 



bering that many more individuals are born than can 

 possibly survive) that individuals having any advantage, 

 however slight, over others, would have the best 

 chance of surviving and of procreating their kind? 

 On the other hand, we may feel sure that any varia- 

 tion in the least degree injurious would be rigidly 

 destroyed. This preservation of favourable variations 

 and the rejection of injurious variations, I call Natural 

 Selection. Variations neither useful nor injurious would 

 not be affected by natural selection, and would be left 

 a fluctuating element, as perhaps we see in the species 

 called polymorphic. 



We shall best understand the probable course of 

 natural selection by taking the case of a country under- 

 going some physical change, for instance, of climate. 

 The proportional numbers of its inhabitants would 

 almost immediately undergo a change, and some species 

 might become extinct. We may conclude, from what 

 we have seen of the intimate and complex manner in 

 which the inhabitants of each country are bound to- 

 gether, that any change in the numerical proportions of 

 some of the inhabitants, independently of the change 

 of climate itself, would most seriously affect many of 

 the others. If the country were open on its borders, 

 new forms would certainly immigrate, and this also 

 would seriously disturb the relations of some of the 

 former inhabitants. Let it be remembered how powerful 

 the influence of a single introduced tree or mammal has 

 been shown to be. But in the case of an island, or of 

 a country partly surrounded by barriers, into which new 

 and better adapted forms could not freely enter, we 

 should then have places in the economy of nature which 

 would assuredly be better filled up, if some of the ori- 

 ginal inhabitants were in some manner modified; for, 

 had the area been open to immigration, these same 



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