76 THE CHICAGO ACADEMY OF SCIENCES. 
until scarcely a body of water in this area can be found which does 
not contain one or more species. The diversity of habitat thus formed 
has resulted in a considerable increase in the number of species since 
the Glacial Period. It is noteworthy that the glaciated territory now 
produces the larger number of species, and this numerical superiority 
is easily understood when the characteristics of the two regions—the 
glaciated and the unglaciated—are compared. The latter consists of 
regular river valleys and tributaries formed by simple erosion (figure 
6), while the former is made up of rivers, lakes and swamps, forming 
a perfect network (figure 7). Comparison of the figures will render 
this statement clear. A comparison of the existing Lymnezid fauna 
shows that sixty species and varieties inhabit the glaciated’ portion of 
the country, while but fifteen inhabit the unglaciated portion. Twenty- 
six species are common to both areas. 

EirGeie 
Portion of Vilas and Oneida counties, Wisconsin, showing topographic 
conditions in a typical glaciated country. (From map published by Chicago and 
Northwestern Railroad Company.) 
The courses of many streams were changed during the several ice 
invasions, and it is probable that many streams were captured and 
beheaded, especially in the upper Mississippi Valley, as suggested by 
General Warren a number of years ago, and the Lymnzid fauna was 
doubtless thus enabled to enter more northern drainages. Interesting 
cases of change of drainage are found in Tight’s paper (referred to in 
1Including the local mountain glaciers of the Rocky Mountains, 
