2o6 FOOT-NOTES TO EVOLUTION. 



On the theory that each species occupies those places 

 best suited to its life, this fact would represent a great 

 oversight on the part of Mother Nature. But with this 

 is the curious fact that the Yellowstone itself, both 

 above and below its falls, is well stocked with trout and 

 with no other fish. This is an anomaly of distribution, 

 but this anomaly disappears when we examine the con- 

 tinental divide at the head of the Yellowstone. At 



^ ^ one point, as Dr. Barton W. Evermann, 



1 wo-ocean Pass. t-. ^i- t^ t , • 



Dr. Oliver P. Jenkms, and others have 



shown, the Two-ocean Pass, only about an eighth of a 



mile of wet meadow and marsh, separates the drainage 



of the Yellowstone from that of the Columbia. From 



the Columbia the Yellowstone has therefore received its 



trout. No doubt every anomaly of distribution would 



become perfectly simple could we only know all the 



facts concerned in the case. 



The laws of geographical distribu- 

 Laws of distribu- ^- r ■ , , 



... tion 01 animals reduce themselves to 

 tipn of animals. 



these very simple propositions: 

 Every species of animal may be found in any part of 

 the earth, unless : 



1. It has been unable to reach that region, through 

 barriers of some sort, or, 



2. Having reached it, it is unable to maintain itself, 

 in competition with other forms, or on account of the 

 conditions of environment, or else, 



3. Having maintained itself, it has become so altered 

 through natural selection as to become a species distinct 

 from its ancestors. 



The primary barriers to distribution are the heights 

 of the land and the depths of the sea — physical obstacles 

 not to be crossed. Next in importance is the barrier of 

 climate. With some forms of life this is absolute, for the 

 palm and the banana are the index of the torrid zone as 



