NATURAL SELECTION 95 



of the planets? Every one knows what is meant and is im- 

 plied by such metaphorical expressions ; and they are almost 

 necessary for brevity. So again it is difficult to avoid per- 

 sonifying the word Nature; but I mean by Nature, only the 

 aggregate action and product of many natural laws, and by 

 laws the sequence of events as ascertained by us. With a 

 little familiarity such superficial objections will be forgotten. 



We shall best understand the probable course of natural 

 selection by taking the case of a country undergoing some 

 slight physical change, for instance, of climate. The propor- 

 tional numbers of its inhabitants will almost immediately un- 

 dergo a change, and some species will probably become ex- 

 tinct. We may conclude, from what we have seen of the in- 

 timate and complex manner in which the inhabitants of each 

 country are bound together, that any change in the numerical 

 proportions of the inhabitants, independently of the change 

 of climate itself, would seriously affect the others. If the 

 country were open on its borders, new forms would certainly 

 immigrate, and this would likewise seriously disturb the rela- 

 tions of some of the former inhabitants. Let it be remem- 

 bered 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 are'a 

 been open to immigration, these same places would have been 

 seized on by intruders. In such cases, slight modifications, 

 which in any way favoured the individuals of any species, 

 by better adapting them to their altered conditions, would 

 tend to be preserved; and natural selection would have free 

 scope for the work of improvement. 



We have good reason to believe, as shown in the first chap- 

 ter, that changes in the conditions of life give a tendency to 

 increased variability; and in the foregoing cases the con- 

 ditions have changed, and this would manifestly be favour- 

 able to natural selection, by affording a better chance of the 

 occurrence of profitable variations. Unless such occur, nat- 

 ural selection can do nothing. Under the term of "vari- 



