438 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 
