376 ELEMENTARY ZOOLOGY. 



the rivers, or from both, the land became populated by 

 animals which wandered away from the water, by burrow- 

 ing into the moist earth or otherwise, and gradually 

 acquiring the air-breathing habit. Last of all, the flying 

 types arose from those that had learned to live on the 

 land. 



371. What Determines the Distribution of a Species of 

 Animals over the Earth? If an animal is a marine animal 

 will it come to live in all the seas and be successful there? 

 If an animal lives on the land will it occupy all of the 

 land of the earth equally or even all the continent on 

 which it lives? In answering these questions the follow- 

 ing facts must be borne in mind : 



1. All animals tend, by means of their powers of 

 multiplication and motion, to scatter from the point at 

 which they may start and to possess the territory. Unless 

 something prevents, this would lead ultimately to the 

 whole earth being covered by all species. 



2 . All animals are specialists, in some degree, in the way 

 of habitat. They flourish less well in some localities than 

 in others, and less well than some other species will. 



3. A region, in which for any reason a given species 

 does not flourish, becomes to that extent a barrier, to 

 its scattering. To forms that frequent marshy land, 

 mountains and plains and rivers might prove a barrier, 

 keeping it out of all lands beyond. 



4. Animals that do succeed in getting over barriers 

 or getting into regions somewhat different from those 

 to which they are adjusted, must meet the competition 

 of the animals already on the ground, and they must 

 change in such a way as to become adjusted to the new 

 conditions of life. In doing this they may become so 



