440 ANIMAL STUDIES 



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 bar- 

 riers may be. Were it not for these barriers, each type or 

 species would become cosmopolitan or universal. Man is 

 preeminently a barrier-crossing animal ; hence he is found 

 in all regions where human life is possible. The different 

 races of men, however, find checks and barriers entirely 

 similar in nature to those experienced by the lower animals, 

 and the race peculiarities are wholly similar to characters 

 acquired by new species under adaptation to changed con- 

 ditions. The degree of hindrance offered by any barrier 

 differs with the nature of the species trying to surmount it. 

 That which constitutes an impassable obstacle to one form 

 may be a great aid to another. The river which blocks the 

 monkey or the cat is the highway of the fish or the turtle. 

 The waterfall which limits the ascent of the fish is the 

 chosen home of the ouzel. The mountain barrier which 

 the bobolink or the prairie-dog does not cross may be the 

 center of distribution of the chief hare or the arctic blue- 

 bird. 



349. Fauna andfaunal areas. The term fauna is applied 

 to the animals of any region considered collectively. Thus 

 the fauna of Illinois comprises the entire list of animals 

 found naturally in that State. It includes the aboriginal 

 men, the black bear, the fox, and all its animal life down 

 to the Amceba. The relation of the fauna of one region 

 to that of another depends on the ease with which bar- 

 riers may be crossed. Thus the fauna of Illinois differs 

 little from that of Indiana or Iowa, because the State con- 

 tains no barriers that animals may not readily pass. On 

 the other hand, the fauna of California or Colorado differs 

 materially from that of adjoining regions, because a moun- 

 tainous country is full of barriers which obstruct the diffu- 

 sion of life. Distinctness is in direct proportion to isola- 



