CHAPTER V 



Geographical distribution of plants and animals. Relation 

 between organism and environment. Methods of, and 

 barriers to the spread of plants and animals. Plant and 

 animal societies. Life zones of North America. 



In surveying the organic world today one is struck by 

 the fact that the lower organisms as well as man are distrib- 

 uted in societies, each of which has its characteristic aspect. 

 From the wind-swept tundras of the north to the sunlit ever- 

 glades of the south, we may pass in review one succession 

 after another of plant and animal societies, each more or 

 less distinctive of the region in which it occurs; and if we 

 encompass the earth from east to west we encounter an even 

 greater diversity of living things, even though the physical 

 characters of every realm are more or less alike. And fur- 

 ther, if we survey the past history of life upon the earth, we 

 find an ever-shifting panorama, as fascinating in its chang- 

 ing scenes as is the restless sea of human life, whose ebb and 

 flow make history. And what the cause of all this change? 

 Where may we find a key to the checker-board puzzle of 

 the living world? How have arisen the organic societies of 

 past and present, and why their ceaseless succession like the 

 play of light and shade? While the immediate causes of the 

 movement and association of plants and animals upon the 

 earth is as yet in many instances obscure, we may seek for 

 the ultimate cause in the great principle of competition among 

 living things and in their adjustment to their environment. 

 Not alone does the fitness of the organism to its environment 

 determine its survival, but the fitness of the environment to 

 the organism and the ability of the latter to find its proper 

 place in nature. Many a square peg has gone down in the 

 battle of life because it failed to find a square hole, among 

 lower organisms as well as among men. 



The problem of geographical distribution then is that of 

 finding the place of origin of any given type of life, the rea- 

 sons for, and the routes and methods of its spread from its 

 center of origin, as well as the manner of its adaptation to 

 the surroundings, both biological and physical, in which it 

 lives today. 



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