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 

 Lymnseas 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. 1 



From these five centers, the Lymnseid 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 Lymnaeas 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 Lymnseas 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.) 



