372 E. R. HALL 



poses new unsolved problems; (2) the geographic distributions of 

 many kinds of organisms in the temperate region of North America 

 are explained by Quaternary climates — climates of the immediate 

 past geologically speaking; and (3) shifts of geographic range now in 

 progress are primarily northward. 



On the second point, it seems that the alternation of dry and moist 

 periods in the central part of what is now the United States left their 

 marks on contemporary organisms. These alternations of climate 

 are thought to have been associated with recessions and extensions 

 of glaciers in the northern part of North America. There is reason 

 to suppose that each of several mammalian stocks now separated 

 into two species (eastern and western) formerly ranged as one species 

 across the United States from the Atlantic to the Pacific. For ex- 

 ample, the Eastern Cottontail of the eastern region that supports 

 shrubs and trees and Nuttall's Cottontail of the western region that 

 supports shrubs and trees probably owe their existence as separate 

 species to a period of aridity, south of the glacial front, so marked 

 that the common stock withdrew from most of the region that we 

 now designate as the Great Plains. In the now still arid Great Plains, 

 albeit less arid than at some times in the past, the two stocks have 

 reinvaded the region but only by following the few ribbonlike 

 riparian plant associations that extend from west to east across the 

 grassy plains. Where the two stocks of cottontails now meet, they 

 do not intergrade (crossbreed), having evolved, while separated from 

 each other, along different physiological and physical lines. The 

 Shrews of the Sorex vagrans group recently reported on by Dr. James 

 S. Findley {Univ. Kansas PubL, Mus. Nat. Hist., 9: 1-68, 18 figures, 

 December 10, 1955) provide a second example of the effect of a 

 period of aridity, in this instance in the Great Basin of the western 

 part of the United States. While the two stocks of Sorex vagrans — 

 east and west — were separated, evolution did not proceed quite so 

 far as in the Cottontails and so the two stocks of shrews crossbreed 

 at a few of the places where their geographic ranges ultimately met 

 again. At other places they do not crossbreed. Indeed the geographic 

 ranges broadly overlap and provide one of the few examples in 

 mammals of two subspecies of the same species occurring together 

 over a considerable geographic region. 



Incidentally, it seems to me, that the effectiveness of the grass- 

 lands of the Great Plains, extending from Mexico to Canada, in 



