578 THE AMERICAN NATURALIST [Vol. LI 



rived from studies of variation and geographical dis- 

 tribution and from the experimental study of evolution. 



ailceni (Brewster) Ridgway. 



If we look for a moment first at the complexities which 

 have been added to the original simple concept of species, 

 we find all grades and kinds of difference within the 

 species itself, such as subspecies, varieties, forms and 

 races, ending finally in the differences between single in- 

 dividuals. Some of these conceptions as ordinarily used 

 are also related to geographical distribution. To them 



