Species and Species Change 



133 



gradually and are the accumulation of small changes similar to 

 those occurring contemporaneously. 



An example is the shifting of ranges which accompanied the 

 glacial advances and dissipations during the Pleistocene. When 

 glaciers and presumably cooler climates moved southward, the 

 ranges of affected species changed radically before them; many 

 species lived in southern areas inimical to them today. When 

 warmer climates and the ice margin moved northward, the ranges 

 of many species enlarged northward also (Fig. 55) (Deevey, 1949; 



Distribution of living 

 •;•.• Discus cronkhitei 



Fig. 55. Changes in the distribution of North American Mollusca during the 

 Pleistocene. During one of the advanced glacial stages, prior to 12,000 years 

 ago, the snail Discus cronkhitei was widespread in Kansas (black area on map). 

 Now it is known only much to the north and east or at high elevations in the 

 mountains. (After Leonard.) 



