74 ON THE ORIGIN OF SPECIES 



doubt (remembering 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 hanc^ we may feel sure that any 

 variation 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 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 

 original inhabitants were in some manner modified ; 

 for, had the area been open to immigration, these same 

 places would have been seized on by intruders. In such 

 case, every slight modification, which in the course of 

 ages chanced to arise, and which in any way favoured 

 the individuals of any of the species, by better adapting 

 them to their altered conditions, would tend to be pre- 



