ANIMAL HABITATS 23 



by the one are already fully occupied by the other 

 and since they are approximately equal in vigor 

 and abilities, neither species is able to invade the 

 territory occupied by the other. 



The relations existing between animal habitats 

 and the animals dwelling therein which have been 

 shown to exist for different continents also hold, 

 though less exactly, in habitats in different regions 

 of the same land mass, and may hold even in 

 similar habitats that lie adjacent to each other. 

 In Illinois, the pocket gopher, which lives almost 

 entirely underground, occurs to the south of the 

 Kankakee river and is again found in Wisconsin, 

 but in wholly similar habitats lying between, even 

 in those just north of the Kankakee river, the 

 animal is absent. The reasons for such erratic 

 distribution are not known but are apparently 

 related to the barriers furnished by the rivers of 

 the region which have not as yet been crossed by 

 the gopher. 



Animal habitats which are obviously units in 

 themselves, such as the forests, can be divided 

 readily enough into sub-habitats. In forests the 

 most obvious of these are arranged in different 

 levels usually called strata and the animals living 

 at these various levels may be spoken of as form- 

 ing stratal sub-communities. In the tropical 

 rain-forests where stratification is best developed 



