254 BOTANICAL GAZETTE | [OCTOBER 
land exterminated the plants of preglacial times without forcing 
a migration southward, except the northern herbs and trees left 
on the moraines, unglaciated islands, glacier margins, and nuna- 
taks of the great ice field.” 
4. During the late Pleistocene, and also during the inter- 
glacial period, the Scandinavian element of the boreal and north 
temperate floras was introduced by the migration of plants from 
Scandinavia by the high northern roadway through northern 
Europe and Asia to North America, and, as shown, these plants 
remained on the retreat of the glaciers in the far north, and on 
the alpine summits of the more elevated mountains, or were held 
trapped in the cool shade of the north temperate forest or 
isolated in cold sphagnum bogs. 
5. The plants of this north temperate region south of the 
terminal moraine during glacial times remained, on the retreat 
of the glaciers, in undisturbed possession of their original habi- 
tats. During the ice age they were still further reduced in 
numbers and influenced in their distribution by the physiographic 
forces constantly at work in the shaping of the continent. 
6. Upon the retreat of the ice-sheet the glaciated area was 
supplied with plants from two main sources: (a) the plants that 
had maintained themselves in the north during the ice age, and 
(2) the contingent of plants supplied by the territory to the south 
and east. 
7. Conclusively, therefore, the facts indicate the absence of a 
southern migration of plants.*° Rather, they point to the glaciers 
as the important factor in the isolation of such botanic regions 
as eastern North America and eastern Asia, which perforce show 
affinities in their floral make-up that can be explained civ, by 
reference to the principles aforementioned. 
*7 WRIGHT and UPHAM, Greenland icefields and life in the north Atlantic 197- 
198. 1896. 
RUSSELL, Glaciers of North gece 86 and 117. 1897; also in Annual Report 
US. ne Survey 13: 19-21. 1891- 
WricHT, G. FREDERICK, The ice age in North America 57-62. 1891; also 
American Gecteaks 8: 330, 
This southern migration seems to the writer well-nigh impossible, because it 
would mean a movement from an area of lower tension to one of higher tension. 
