130 THE CHICAGO ACADEMY OF SCIENCES. 



number of new species may be thought to be ill advised. Our modern 

 conceptions of species, based on studies in ecology and evolution, have 

 made it plain, it would seem, that a species is simply an assemblage of 

 individuals which combine certain characteristics not shared by any 

 other similar group of individuals. With this concept in mind, it 

 is not difficult to comprehend that in a territory as large as North 

 America, with its diversity of environment, there should have been 

 evolved an hundred species and races of Lymnaeids. 



The question of species and varieties or subspecies has been de- 

 termined as follows : Names are admitted as specific where it is clearly 

 evident that no intermediate forms are now living; in other words, 

 there is a break in the line of evolution ; subspecies or races are ad- 

 mitted when the name covers a group of individuals combining certain 

 characteristics which intergrade more or less with what is believed to 

 be the parent species. In many cases there is apparently less difference 

 between some closely related species than between certain species and 

 races which seem to manifest wide differences (as cmarginata) , but 

 in these cases, although the differences are slight, they are uniform 

 and no intergradations occur. 



