ADAPTATION AND DISTRIBUTION 451 



would ultimately occupy all suitable areas unless there 

 were some actual barrier to prevent their reaching them. 



Adaptation and Distribution. — Another fact of 

 great significance is that plants and animals do not oc- 

 cupy all the areas which they can readily reach. A study 

 of any area of a few square miles will ordinarily show 

 that there are several different assemblages of plants 

 within it, each occupying a habitat of a special sort. The 

 plants in each of these, for the most part, could readily 

 reach any of the others, and likewise in most cases they 

 do so but do not establish themselves, because the en- 

 vironment is not suitable. For illustration, it is not 

 unusual to find a lake, a stream, a plain, and a mountain 

 within close proximity but without any tendency for the 

 plants of the lake to spread to the mountain top. While 

 the mountain and the plain are not so different as the 

 mountain and the lake, yet each is likely to have a dis- 

 tinctive assemblage of plants and to show little tendency 

 for this difference to be lessened by migration from one 

 habitat to the other. This sort of phenomenon shows 

 that some plants are adapted to live under one set of 

 conditions and not under others. The same thing is 

 true of animals but to a lesser extent on account of their 

 greater independence of certain features of the environ- 

 ment. A polar bear would probably find no insuperable 

 physical obstacle to prevent his migration from Alaska 

 to Panama but he would almost certainly find the condi- 

 tions there so little adapted to his fur-clad body and 

 habits of life that he would lead a sad life of no long 

 duration. Nor would the cougar of the Mexican jungle 

 find himself better situated should he perchance migrate 

 to the polar bear's arctic home. 



Floras and Faunas. — In order to enable us to speak 

 accurately the biologist has coined two words to express 

 the assemblage of plants or animals characteristic of a 

 particular region. All the plants found native in Maine 

 or California, the United States or Europe constitute 



