GEOGRAPHICAL DISTRIBUTION OF ANIMALS 437 



map is very instructive, and it at once raises a series of 

 questions as to the reasons for each of the facts in geo- 

 graphical distribution, for it is the duty of science to sup- 

 pose that none of these facts is arbitrary or meaningless. 

 Each fact has some good cause behind it. 



344. Laws of distribution. The laws governing the dis- 

 tribution of animals are reducible to three very simple 

 propositions. Every species of animal is found in every part 

 of the earth having conditions suitable for its maintenance, 

 unless 



(a) Its individuals have been unable to reach this re- 

 gion, through barriers of some sort ; or 



(b) Having reached it, the species is unable to maintain 

 itself, through lack of capacity for adaptation, through 

 severity of competition with other forms, or through de- 

 structive conditions of environment ; or 



(c) Having entered and maintained itself, it has become 

 so altered in the process of adaptation as to become a spe- 

 cies distinct from the original type. 



345. Species debarred by barriers. As examples of the 

 first class we may take the absence of kingbirds or meadow- 

 larks or coyotes in Europe, the absence of the lion and 

 tiger in South America, the absence of the civet-cat in New 

 York, and that of the bobolink or the Chinese flying-fox in 

 California. In each of these cases there is no evident rea- 

 son why the species in question should not maintain itself 

 if once introduced. The fact that it does not exist is, in 

 general, an evidence that it has never passed the barriers 

 which separate the region in question from its original 

 home. 



Local illustrations of the same kind may be found in 

 most mountainous regions. In the Yosemite Valley in 

 California, for example, the trout ascend the Merced Eiver 

 to the base of a vertical fall. They can not rise above this, 

 and so the streams and lakes above this fall are destitute 

 of fish. 



