84 ISOLATION AS A FACTOR 



ble of enduring for a long period and of spread- 

 ing widely," But he regards past isolation as a 

 factor in this case also, for he says : — 



" Moreover, great areas, though now continuous 

 owing to oscillations of level, will often have recently 

 existed in a broken condition so that the good effects 

 of isolation will generally to a certain extent have 

 concurred. 



" In isolation in a small area, conditions will tend 

 to be uniform, so that natural selection will tend to 

 modify all the varying species throughout the area in 

 the same manner in reference to the same conditions." 



He goes on to show that in isolation, inter- 

 crossing with outside individuals will be pre- 

 vented; that individuals will be freed from outside 

 competition, a condition favorable or " giving 

 time " for " improvement," that is, for adaptive 

 divergence. 



It will be noticed that Darwin uses the word 

 " isolation " in its literal meaning of island-resi- 

 dence, and that he does not extend it to include 

 segregation or separation by barriers. Yet a 

 mountain lake or a river basin may be just as 

 much isolated in a biological sense for its water 

 animals as an island is for its land inhabitants. 

 Darwin makes no effort to separate two sets of 

 facts. The one is that a great continent or a 

 great sea or a great river will contain at any 

 point more species than a small continent, a small 

 sea, or a small river basin. This is because the 

 large area offers freer access for many different 

 types of organisms. Its less perfect barriers 



