SOURCES OF CHANGE 75 



concern both heritage and environment. There is 

 some difference in the various types of change. 

 Species will always continue to display a capacity 

 for individual adjustment to fluctuating environ- 

 ment and there is no reason to suppose that the 

 resulting conditions in individuals must become 

 fixed characters of the species nor that they may 

 never become so fixed. These changes must take 

 their place with the purely hereditary characters 

 as possible materials of evolution and from the en- 

 tire assemblage our conclusions regarding the origin 

 of species must be drawn. 



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