40 



and the eventual return of this material to the ocean must 

 have had a very considerable effect, acting over long periods 

 of time and presumably accounting for some of the secondary 

 changes in level, all of which must have modified the areas 

 inhabited by intertidal forms. 



According to Flint (1957), based on Carbon-14 findings, the 

 main continental glacier was advancing across the region of 

 the Great Lakes about 25,000 years ago, reaching the crest of 

 its advance at about 18,000 years. After several fluctuations it 

 began its final retreat. About 8,000-8,500 years ago the Great 

 Lakes were almost free of ice, and the glacier continued its 

 long shift to the northward. 



Although the glacial history of the western branch of the 

 continental glacier, known as the Cordilleran sheet, is less 

 completely established, its main features presumably followed 

 the pattern of the larger ice-sheet. For many thousands of 

 years an immense area, extending through much of Alaska, 

 British Columbia and Washington almost to the Columbia 

 River, was covered by a vast mass of ice, estimated to have 

 been at least 6,000 feet thick over much of British Columbia, 

 3,000 feet over the Puget Sound valley between the Olympic 

 Mountains and the Cascades. 



From the above interpretation it will be seen that for a vasf" 

 period of time the habitats of an immense number of our pres- 

 ent local races of Thais must have been enclosed in ice or ren- 

 dered uninhabitable by arctic conditions. However, to the 

 north and to the southward of the glaciated area colonies of 

 Thais were in existence, but the main groups of breeding popu- 

 lations were separated by wide spaces of inhospitable terrain 

 and tended to diverge. Eventually, as the glacial conditions 

 receded, areas suitable for colonies of Thais were uncovered, 

 and eventually received migratory populations of the snaiU. 

 When populations that had been separated for great periods 

 of time and bearing differing genetic values merged, extensive 

 variation was initiated, as is indicated by the heterogeneous 

 character of the populations present in this intermediate zone. 



