The Inzards of Kansas 69 
there have been two great centers of biological dispersal 
for the United States. They are the ‘‘Southeast and the 
Southwest, the former moist, the latter arid.’? Ruthven 
(1908) wrote that ‘‘Most of the forms which inhabit the 
prairie region either extend into the eastern forest re- 
gion or into the plains region, or rarely both, few being 
confined to the prairie region. . . . There is a great 
difference in the extent to which the forms of eastern 
North America push westward, or the plains forms push 
eastward into the prairie region before becoming modi- 
fied or checked.’’ Thus it is found that dispersal after 
the glacial period has been from the southeast and south- 
west toward the central region of the United States in 
which Kansas is located. Since the two regions are op- 
posed to each other, the line of their meeting must be 
somewhere in the central region, the distribution of each 
Species being limited by its ability to withstand condi- 
tions more typical of its opposite region of dispersal, 
and less typical of its own region. The difference in the 
extent of the toleration of each species for the opposite 
region varies, as data on Kansas lizards show, and no 
distinct line can be drawn to mark the point at which 
the species with their center of distribution in the east- 
ern region cease to exist and where the species with their 
center of distribution in the western region begin to ap- 
pear. Thus, at the line of meeting there is an overlap- 
ping between the two groups, and it is in such a region 
that the factors limiting distribution are to be sought. 
If moisture were uniform at the point and temperature 
varied, temperature would be a controlling factor. If 
the formations of the area were geologically varied some 
of them would probably be preferred by certain lizards 
and some by others. Pearse (1926) has stated that ‘‘The 
distribution of animals often has direct relation to the 
