SG NATURAL SELECTION. Chap. IV. 



slii^ht iihysical chanire, for instance, of climate. The propor- 

 tional numbers of its inhabitants -would almost immediately 

 undergo a change, and some species might become extinct. 

 AVe may conclude, from what avc have seen of the intimate 

 and complex manner in which the inhabitants of each country 

 arc boinid together, that any change in the numerical jiropor- 

 tions of some of the inhabitants, independently of the change 

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

 country were open on its borders, new fonns would certainly 

 innnigrate, and this also would often seriously disturb the re- 

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

 bered how powerful the influence of a single introduced tree 

 or mammal has been sliOAvn 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 Natvire Avhich would 

 assuredly be better fdled up, if some of the original inhabitants 

 were in some manner modified ; for, had the area been ojien to 

 immigration, these same places would have been seized by in- 

 truders. In such cases, slight modifications, which in any way 

 favored the individuals of any species, by better adajiting them 

 to their altered conditions, would tend to be prcsen-ed ; and 

 natural selection would have free scope for the Avork of im- 

 provement. 



We have reason to believe, as stated in the first chapter, 

 that changes in the conditions of life cause or excite a ten- 

 dency to vary ; and in the foregoing case the conditions are 

 supposed to have changed, and this would manifestly be favor- 

 able to natural selection, by giving a better chance of profit- 

 able variations occurring; and unless such do occur, natural 

 selection can do nothing. Under the term of "variations," it 

 must never be forgotten that mere individual diflerences are 

 always included. As man can certainly ]noducc a great result 

 with his domestic animals and plants by adding up in any 

 given direction individual differences, so could natural selection, 

 but far more easily, from having incomparably longer time for 

 action. Nor do I believe that any great physical change, as 

 of climate, or any imusual degree of isolation to check immi- 

 gration, is actually necessary to ])roduce new and unoccujiied 

 places for natural selection to fill up by modifying and im])rf)v- 

 ing some of the A-arying inhabitants. For as all the inhabit- 

 ants of each country are struggling together Avith nic(^ly-bal- 

 anccd forces, extremely-slight modifications in the structure oi 



