i9o8] HARVEY— PRAIRIE-GRASS FORMATION 85 



falling temperature of the Pleistocene. In this glacial movement, 

 the plains region, unable to support tree growth, acted as an entering 

 wedge, causing an east and west divergence. One wing of the mi- 

 gration, dominantly coniferous, followed the Rocky Mountains 

 southward; the other, typically deciduous, sought the Mississippi 

 and its tributary valleys as a migration track; while the prairie moved 

 directly toward the Gulf. At the time of maximum ice advance the 

 descendants of the Tertiary forest were mobilized in the southern 

 Appalachians about the Chattanooga region as a center (Adams :o2), 

 while the prairie formation concentrated in the southwestern United 

 States, with a possible center in the region of northeastern Texas 

 and eastern Oklahoma and southern Kansas. 



With final glacial retreat from this region and subsidence of the 

 glacial sea, migration tension was removed and distribution tension 

 became active. The life waves now in succession rolled northward. 

 The content of our flora demands a consideration of the third wave 

 only. A study of the floristics shows indisputably the commingling 

 of forms of diverse geographical affinity. An unmistakable floristic 

 relation, in many cases specific, exists with a southwestern and 

 southeastern center of post-glacial dispersal.^ To the east and 

 southeast the deciduous forest type becomes increasingly characteristic, 

 while to the west and southwest the plain or prairie type gradu- 

 ally predominates; the region thus lies in the western border of the 

 tension zone in which migration from these two competing centers 

 of distribution meet. From the southeast the dispersal route has 

 been up the Missouri valley; while the northwestern migration has 

 spread diagonally across natural drainage lines, following the upland 

 plains. 



Forest invasion of the prairie 



When the arborescent elements of the southeastern biota, migrating 

 up the valleys of the Mississippi and of the Missouri and its tribu- 



2 Bessy ('99, p. 82) says: "There are 66 or 67 species of native trees in Ne- 

 braska, and 56 or 57 have advanced to the state from the southeast." Macmillan 

 ('92, pp. 653, 721) has shown that 66 per cent, of genera indigenous to the Minnesota 

 valley are eastern and 62 per cent, southern, while 32. 5 per cent, of the total species 

 are of southeastern origin. Of the native trees of South Dakota at least 75 per cent, 

 show indubitable southeastern affinities. The presence of such genera as Opuntia, 

 Cactus, Yucca, Mentzelia, Croton, Bouteloua, Bulbilis, Lygodesmra, and Aplopappus 

 strongly bespeaks the southwestern alliance. 



