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northern plants and northward for the southern plants. A line or 
area of tension is thus formed, and if the world in general is con- 
sidered, two main tension areas or zones will be apparent, one to 
the north and another to the south of the equator. In these 
zones of tension, where the competition between northern and 
southern forms is greatest, transitional floras are to be found. In 
the Minnesota valley the author notes the fact, which has received 
attention in other localities, that the southern or north-bound 
plants are largely endemic, or in other words American in type, 
while the northern or south-bound ones are generally less so, in 
other words foreign in type. 
The division of the earth into natural vegetation regions has 
been attempted by several authorities, notably by Griesebach, 
Engler and Drude, all of whose regional classifications are given 
in full and discussed in so far as North America is concerned. 
The author’s conclusions are that there isa greater homogeneity in 
the regions of the northern than in those of the southern hemi- 
sphere, the reasons being both geographical and geological. 
Geographically the facilities for migration and commingling are 
much more favorable in the northern than they are in the south- 
ern hemisphere, which latter consists of a series of isolated areas, 
whereas the former is practically a compact circle of land sur- 
rounding the pole, broken only by narrow bodies or arms of 
water. Geologically all the evidence is in favor of a closer rela- 
tionship in recent times between Asia and North America than 
between the latter and Europe, and modern biological researches 
strongly emphasize the evidence. It has long been a well recog- 
nized fact that the plants of Greenland are remarkably distinct 
from those of Scandinavia, whereas the floras of Alaska and North- 
eastern Asia show but slight differences one to the other. The 
glaciation of the Ice Age also had a profound influence in modi- 
fying the distribution of the plants which previous to that time oc- 
cupied the Arctic and sub-Arctic regions. In North America, 
where the mountain chains extend north and south, these plants 
could migrate or remain undisturbed to the south of the ice sheet; 
whereas in the old world, where the mountain systems are largely 
east and west, such plants would. be cut off from any means of 
southward migration and would necessarily perish. «Decimation 
