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 Lymneids. 
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 emarginata), but 
in these cases, although the differences are slight, they are uniform 
and no intergradations occur. 
