440 ELEMENTARY ZOOLOGY 



The third great barrier, the ocean, is perhaps the most 

 obvious of all in its influence. It is only in rare cases 

 that any land-animal can independently cross a great 

 ocean. Thus the land-animals of Australia differ from 

 those of all other countries, and those of Africa and South 

 America have developed almost independently of one 

 another. The ocean is, as already mentioned, also a 

 barrier for fresh-water aquatic animals, arid even marine 

 fishes which live normally in shallow waters along the 

 shore rarely venture across the great depths of mid-ocean. 



The obstacles or barriers met with determine the limits 

 of a species. Each species broadens its range as far as it 

 can. It attempts unwittingly, through natural processes 

 of increase, to overcome the obstacles of ocean or river, 

 of mountain or plain, of woodland or prairie or desert, of 

 cold or heat, of lack of food or abundance of enemies 

 whatever the barriers may be. The degree of hindrance 

 offered by any barrier differs with the nature of the animal 

 trying to pass it. That which forms an impassable 

 obstacle to one species may be a great aid to the spread 

 of another. ' * The river which blocks the monkey or the 

 cat is the highway of the fish and turtle. The waterfall 

 which limits the ascent of the trout is the chosen home of 

 the ouzel." 



Faunae and zoogeographic areas. The term fauna is 

 applied to the animals of any region considered collect- 

 ively. Thus the fauna of Illinois includes the entire list 

 of animals found naturally in that State. The fauna of a 

 schoolyard comprises all the animals found living naturally 

 in the yard. The fauna of a pond includes all the animal 

 inhabitants of the pond. (Flora is used similarly of all 

 the plants in a given region.) The relation of one fauna 

 to another depends on the character and effectiveness of 

 the barriers between, and the physical character of the 

 two regions. Thus the fauna of Illinois differs but little 



