370 LIFE OF THE PLEISTOCENE 



Valvaia tricarinata, Segmentina armigera, Planorbis trivolvis, Planorbis anlrosus, 

 Planorbis parvus, Galba caperata, Galba humilis modicella, Galba obrussa, and 

 Galba palustris. 



The land snails are perhaps, the best represented of the mollusks, when all 

 of the interglacial intervals are considered. Many species of some genera per- 

 sist thruout all of the intervals, as Vallonia, Strobilops, Bifidaria {Gastrocopta) 

 Succinea, Helicodiscus, Sphyradium, Pyramidula, Zonitoides, Vitrea, Euconulu* 

 and Polygyra. So far as known these species do not differ materially from the 

 same species as they exist today and it is inferred from this that conditions 

 during at least a part of these intervals did not differ from the conditions of 

 today in the same regions. 



It was at one time thot by many biologists that the Glacial Period was a 

 strong factor in the mutation of species and varieties. In 1894 Dr. Pilsbry 5 

 stated as his opinion that "one of the most potent causes of specific or varietal 

 differentiation has been the glacial epoch which undoubtedly caused a south- 

 ward movement of the entire northern fauna. Upon the recession of the ice 

 sheet the species thus driven south found themselves exposed to changing 

 climate and food-plants in their new home. Those following the retreat of 

 the ice found the topography, soil and drainage systems of their former area 

 in the north vastly changed. " While this factor has been found to have been 

 potent in changing some groups it seems to have had but little effect upon many 

 mollusks, the tables showing that a large number of the species, especially 

 the pulmonates, both land and fresh water, persist practically unchanged 

 from the Aftonian to the present time. The great differentiation of species, 

 especially among the land snails has taken place beyond the limits and infl- 

 ence of the great ice invasions. Too little is yet known concerning the vertical 

 distribution of the fresh water pulmonates, but the data at hand indicate that 

 but little change has occurred. It is believed by the writer, however, that the 

 great majority of the numerous species of the fresh water pulmonates have 

 been evolved since the retreat of the Wisconsin ice sheet. The species pre- 

 served in interglacial deposits show no change from the type of the recent forms 

 of the same species. Further research may carry many of the species into pre- 

 glacial time and may also add many of the supposed recent species to the list of 

 fossils. As previously explained (Chapter X, page 363 ) some of the loess 

 fossils attest a change to a drier climate during some of the intervals, a climate 

 similar to that of the southwest in Colorado and adjacent states, and at these 

 times a few peculiar species and races flourished in the loess-covered areas. 



II. INSECTA 



The insects are peculiar in that all but four species are extinct. The 

 majority of the species are from the Toronto deposits of the Sangamon Interval. 



1 Nautilus, VIII, p. 51. 



