1484 THE STORY OF THE UNIVERSE 



their kind? On the other hand, we may feel sure 

 that any variation in the least degree injurious would 

 be rigidly destroyed. This preservation of favorable 

 individual differences and variations, and the destruc- 

 tion of those which are injurious, I have called Nat- 

 ural Selection, or the Survival of the Fittest 



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 proportional numbers of its inhabi- 

 tants will almost immediately undergo a change, and 

 some species will probably 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 together, that any change in the 

 numerical proportions of the inhabitants, indepen- 

 dently 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 inhabitants. Let it be remem- 

 bered how powerful the influence of a single intro- 

 duced tree or mammal has been shown to be. But 

 in the case of an island, or of a country partly sur- 

 rounded 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 

 cases, slight modifications, which in any way fa- 



