76 



Mil 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. Jt is noteworthy that the glaciated territory now 

 jinx luces 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 Lymnaeid fauna 

 shows that sixty species and varieties inhabit the glaciated 1 portion of 

 the country, while but fifteen inhabit the unglaciated portion. Twenty- 

 six species are common to both areas. 



Fig. 7. 



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 Lymnaeid 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 



including the local mountain glaciers of the Rocky Mountains. 



