18 THE WANDERINGS OF ANIMALS [OH. 



investigates the prevailing local physical conditions 

 and how these have influenced the faunas. The 

 cultivation of this field promises fair to throw much 

 light upon Nature's way of making species, and it is 

 all the more enchanting because here the animals, 

 whether recent or extinct, are brought before us as 

 living fellow creatures. The main purpose of an 

 organism is to live. If i purpose ' be objected to, let 

 us say it is its business, that with which it is busy. 



But the broadest of land connexions is of no avail 

 to the dwellers of forests if it is a desert, or even a 

 prairie. What is attractive to some is repellant to 

 others. It is the environment, the condition of the 

 home, the oikos, which decides the tenants, hence the 

 importance of oecology. 



CHAPTER II 



FEATURES OF ENVIRONMENT 



Tropical Forests. 



NECESSARY conditions for the production of a 

 typical tropical forest are moisture and heat. The 

 mean temperature is that of the tropics, say 80 F. 

 rarely sinking below 70 or exceeding 90. The 

 moisture must be due to rain with an annual 

 minimum of at least 60 inches, a fair average being 



