INTROGRESSION AND EVOLUTION 65 



have been encouraged in much the same ways as when man 

 upsets the ordinary balance of nature. 



It is probable that the same kind of phenomenon took 

 place in the eastern United States after the last glaciation. 

 ^\^lenever the retreat of the continental ice was rapid, large 

 areas must have been open for colonization, and sometimes 

 at least they must have presented the invaders with new sets 

 of soil types and habitats different from those previously 

 knowTi. WTien the ice front advanced again it may very 

 likely have left isolated pockets of vegetation well behind the 

 readvancing front. If these areas were small, the ^'Sewall 

 Wright effect" would have produced local differentiation 

 within the pocket so that at the next time of retreat there 

 would be opportunities for these new highly localized va- 

 rieties to introgress into the main body of the species. The 

 distribution and differentiation of the northern blue flags 

 {Iris versicolor and Iris virginica) suggest that a considerable 

 area in the interior of the lower peninsula of Michigan may 

 have been isolated for quite a time in this fashion. W. H. 

 Camp has already given an informal report (1943) on his 

 studies of hybridization in North American beeches (Fagus) 

 which demonstrate the effect of the various retreats and ad- 

 vances of the ice front on introgression in that genus. With 

 a series of studies on different genera we should be able to 

 approach the subject experimentally rather than dog- 

 matically. 



It seems probable that a somewhat similar mass introgres- 

 sion may have taken place in the northern and eastern 

 Ozarks in post-glacial times. During the xerothermic period 

 when the prairie grasslands extended much farther east than 

 they do now, many of our common woodland species of 

 eastern North America must have existed in the Ozarks in 

 small, isolated refuges. Today, in much the same way, small 

 patches of isolated woodland are to be found in sheltered 

 canyons in western Oklahoma. \\Tien the climate was 

 distinctly hotter and drier than it is now, the central Ozarks 

 in southern ^Missouri must have had a climate more like that 



