1908] HARV EY—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 :02), 
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.2 To the east and 
southeast the deciduous forest type becomes increasingly character- 
'stc, while to the west and southwest the plain or prairie type gradu- 
y 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 
cg diagonally across natural drainage lines, following the upland 
s. 
Forest invasion of the prairie 
: sige the arborescent elements of the southeastern biota, migrating 
P the valleys of the Mississippi and of the Missouri and its tribu- 
ecard (99, p. 82) says: “There are 66 or 67 species of native trees in Ne- 
a » and 56 Or 57 CMILLAN 
92, Pp. 653, 
a 
show indubitahl tern origin. Of the native trees of South Dakota at least 75 per cent. 
‘ i le Southeastern affinities. e presence of such genera as Opuntia, 
Strongly a Mentzelia, Croton, Bouteloua, Bulbilis, Lygodesmia, and Aplopappus 
peaks the southwestern alliance. 
