GEOGRAPHICAL DISTRIBUTION OP ANIMALS 283 



To processes of this kind, on a larger or smaller scale, 

 the variety in the animal life of the globe is very largely 

 due. Isolation and adaptation give the clew to the forma- 

 tion of a very large proportion of the " new species " in 

 any group. 



153. Effect of barriers. It will be thus seen that geo- 

 graphical distribution is primarily dependent on barriers or 

 checks to the movement of animals. The obstacles met 

 in the spread of animals determine the limits of the spe- 

 cies. 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 bar- 

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

 species would become cosmopolitan or universal. Man is 

 pre-eminently 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. 



154. Relation of species to habitat. The habitat of a 

 species of animal is the jegion in which it is found in a 

 state of Nature. It is currently believed that the habitat 

 of any creature is the region for which it is best adapted. 



