43 8 Life and Immortality. 



change, as of climate for example. The proportional num- 

 ber of its inhabitants would almost immediately undergo a 

 change, and some of its species might become extinct. 

 From the complex and very intimate manner in which the 

 inhabitants of each country are bound together, we may 

 conclude that any change in the numerical proportion of 

 some of its inhabitants, independently of the change of 

 climate itself, would seriously affect the others. Were the 

 country open on its borders, new forms would certainly 

 immigrate, and this, too, would often seriously disturb the 

 relations of some of its former inhabitants. In the case, 

 however, of an island, or a country hemmed in by barriers, 

 into which new and better-adapted forms could not readily 

 enter, we would then meet with places in the economy of 

 nature which would assuredly be better filled up, if some of 

 the original occupants were in some manner modified, for 

 had the area been open to immigration, these same places 

 would have been seized by intruders. Thus, slight modi- 

 fications, which any way favored the individuals of a species, 

 would by better adapting them to changed conditions tend to 

 become preserved, and Natural Selection would there have 

 free scope for the work of improvement. Changes in the 

 conditions of life cause or excite a tendency to vary. In the 

 foregoing case the conditions are supposed to have changed, 

 and this would manifestly be favorable, by giving a better 

 chance of profitable variations occurring, to Natural Selection, 

 for unless such do occur, Natural Selection can do nothing. 

 As man, by adding up in any given direction individual dif- 

 ferences, can certainly produce a great result with his 

 domestic animals and plants, so could Natural Selection, but 

 far more easily from having an incomparably longer time for 

 its action. No great physical change, as of climate, nor any 

 unusual degree of isolation to check immigration, is actually 

 necessary, it would seem, to produce new and unoccupied 

 places for Natural Selection to fill up by modifying and 

 improving some of the varying inhabitants, for as all the 



