NATURAL SELECTION. 75 



power or Deity; but who objects to an author speaking of 

 the attraction of gravity as ruling the movements of the 

 planets? Every one knows what is meant and is implied 

 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 pro- 

 portional numbers of its inhabitants will almost immedi- 

 ately undergo a change, and some species will probably be- 

 come extinct. We may conclude, from what we have seen 

 of the intimate and complex manner in which the inhabi- 

 tants of each country are bound together, that any change 

 in the numerical proportions of the inhabitants, independ- 

 ently 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 relations of some of the former inhab- 

 itants. 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 man- 

 ner modified; for, had the area been open to immigration, 

 these same places would have been seized on by intruders. 

 In such cases, slight modifications, which in- any way 

 favored the individuals of any species, by better adapting 

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

 served; and natural selection would have free scope for the 

 work of improvement. 



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

 chapter, that changes in the conditions of life give a ten- 

 dency to increased variability; and in the forgoing cases 

 the conditions have changed, and this would manifestly be 

 favorable to natural selection; by affording a better chance 



