276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



of shells of the Pleistocene age, he found among Illinois deposits some 

 species which are identical with forms dwelling in the same area 

 today but he also found some shells representing species the only living 

 varieties of which are today found at latitudes considerably farther 

 north. A knowledge of the habits and living conditions of present- 

 day forms has been helpful in interpretation of the conditions under 

 which similar forms of past ages probably existed. 



When the Chicago Drainage Canal was constructed, Baker (1930a) 

 was able to study the evidence of the molluscan life in old glacial 

 Lake Chicago, the forerunner of Lake Michigan, and could demon- 

 strate fluctuations in lake level, changes from swampy to lake condi- 

 tions, and from deep to shallow water. 



The glacial advances and retreats of the Pleistocene Epoch pro- 

 foundly affected all life, including the terrestrial as well as the fresh- 

 water Mollusca (Baker, 1930b). Following an advance of the ice 

 sheet, all life was killed or was driven southward, causing a com- 

 mingling of arctic, subarctic, and temperate life south of the border 

 of the continental ice sheet. During the warmer interglacial inter- 

 vals, this life again returned to the territory left bare by the retreat- 

 ing ice just as rapidly as the vegetation essential to its existence became 

 established. In the series of repeated invasions by the glaciers, there 

 were successive decimations and repopulations in the same area. The 

 reoccupied territory, left bare by the retreating ice, was usually 

 entirely different from that previously existing. Such radical 

 changes had profound influence upon the kind of plants and animals 

 that could become successfully established in the area. Changed 

 climatic factors also contributed to the altered conditions for life. 

 Under the combined action of these factors, some species became ex- 

 terminated while others were stimulated to undergo variations which 

 led to the production of new species. For the glaciated part of the 

 United States, it is estimated that 74 percent of the mammals of pre- 

 glacial times are extinct. It should be remembered that 1 to 5 million 

 years are estimated as the period in which this was accomplished. Of 

 the fresh-water, gill-bearing snails, 13.8 percent are extinct; of the 

 fresh-water pulmonates, 8.3 percent, and of land snails, 11.1 percent. 

 By way of summary for the entire area, about 7.8 percent of all 

 animal species became extinct. 



In acid waters, snails and mussels are almost wholly lacking and the 

 small numbers of unthrifty individuals which are encountered in 

 either natural or artifically produced acid waters have either extremely 

 thin shells or, if the acidity is due to recent pollution, the limy mate- 

 rial is eaten away wherever the organic covering has been broken by 

 natural abrasion or by accident. 



