LYMNID4 OF NORTH AMERICA. 69 
physical and organic. An incidental result of this wholesale migration 
was an unwonted commingling of plants and animals, for every ag- 
gressive form pushed forward in the van of the advancing zone, and 
hence came into new organic environment, while every laggard form 
fell behind, and was overtaken by the less reluctant migrants.” 
POST-GLACIAL DISPERSAL. 
To understand the extent of the post-glacial dispersal of the 
Lymneas and their reoccupancy of the territory from which they had 
been driven by the ice sheet, it will be necessary to bear in mind the 
condition of the North American continent at the end of the maximum 
extension of the ice sheet (figure 2). A study of the map shows that 
there were probably four main areas which could supply the biota for 
this purpose. These were: (1) that part of the United States lying 
south of Illinois and Ohio, west of the Alleghany Mountains, and of 
the Missouri River Valley, and east of the Rocky Mountains, including 
the lower drainage area of the Mississippi Valley and the adjacent 
prairies and plains; (2) an area south of British Columbia and Assini- 
boia, including Montana, Idaho and Washington and embracing the 
upper drainage areas of the Missouri and the Columbia rivers; (3) the 
whole of northern and western Alaska, embracing the Yukon River 
Valley, besides other smaller stream valleys; and (4) the driftless area 
in southern Wisconsin bordering the Mississippi River. A possible 
fifth region of survival is found in New Jersey and Pennsylvania east 
of the Appalachian chain, and south of New York. 
From these five centers, the Lymnzid fauna has been dispersed 
until at the present time there is scarcely a part of the glaciated terri- 
tory which does not support one or more species of this interesting 
family. In fact, the metropolis of the family is in the territory imme- 
diately surrounding the Great Lakes, and from here the largest number 
of species has been recorded. (See page 62.) 
In considering the routes by which the Lymneas have reoccupied 
the englaciated territory, it must be remembered that, unlike the land 
forms, the post-glacial dispersal of which has been so ably discussed 
by Dr. C. C. Adams (Biol. Bull. IX, No. 1, 1905), the Lymnzeas were 
more or less dependent upon the natural drainage of rivers and streams, 
although other agencies probably contributed to this means; but it is 
to the natural progress of locomotion that we must look for the spread- 
*There is reason to believe that favorable conditions for the survival of 
many boreal mollusks existed in Greenland, Newfoundland, Anticosti and other 
Places along the Atlantic coast, and also along the Pacific coast. (Vide Scharff, 
1907, and Adams, 1905.) 
