THE DISTRIBUTION OF ANIMALS 443 



With a change of environing conditions, certain species 

 are unable to maintain themselves. With the destruction 

 of the forests going on in parts of our country the great 

 host of wood-creatures, the bears, squirrels, the wood- 

 birds and insects, can no longer maintain themselves, and 

 grow rare and disappear. Man often also influences the 

 status of a species by checking its increase either by 

 actual slaughter, as with the bison and passenger-pigeon, 

 or by making adverse changes in its environment, as by 

 destroying forests, or putting the plains under cultivation. 



In the discussion of " species-forming " (see p. 408) it 

 was shown that adaptation may lead to the altering of 

 species, and to the formation of new ones (under the in- 

 fluence of natural selection). With the gradual change of 

 conditions, or with the facing of new conditions because 

 of an unusual migration to or invasion of new territory, 

 those individuals of the species exposed to the new con- 

 ditions must adapt themselves in structure and habit in 

 order to meet successfully the new demands. By the 

 cumulative action of natural selection these adaptive 

 changes are emphasized ; and this emphasis may come to 

 be so pronounced that the part of the species represented 

 in this newly acquired territory, if isolated from the orig- 

 inal stock, is so altered as to be quite distinct in appear- 

 ance from the old. If these changed individuals are also 

 physiologically distinct from the old stock, i.e. are 

 unable to mate with them, a new species is established. 

 As already mentioned, the peopling of islands from main- 

 lands is an excellent and readily observable example of 

 the phenomena referred to in the third law of distribution. 



