RELATIONSHIPS OF METASPERMAE, 591 
C. Southern realm, 
(1) Andes region. 
(2) South African region. 
(3) Australian region. 
(4) Antarctic region. 
None of these regions are very sharply limited but are de- 
fined so as to indicate the transitions. Of Realm A, the last 
four regions are tropical as well as northern in their character. 
In all regions of Realm B there are to be observed, especially 
at higher altitudes, elements transitional between A orC. In 
Realm C, only the fourth region is comparatively uninfluenced 
by the plants of Realm B. 
The region of the Minuesota valley, according to the classifi- 
cation of Drude, lies partly in the Middle North American 
region and partly in the transitional region between the Middle 
North American and the Northern. Its principal characters 
are derived from the commingling of a group of north bound 
generally endemic plants with a south-bound group of less 
generally endemic plants. Its old-world character is given 
rather by the influence of the Northern region—and this in- 
fluence is most distinctly felt in the upper latitudes of the 
valley—than by the southern. As will be shown later, the 
species of plants common to the Minnesota valley and to the 
old world are generally more northern than southern in their 
North American distribution. 
General position of the Minnesota valley as a botanical 
district. From the different classifications given it will be 
seen that the Minnesota valley, in the botanical sense, is first 
of all, northern-extratropical; second, North American; third, 
middle North American. This order corresponds with the 
geographical order. It will now be necessary to note the gen- 
eral methods by which the melange of plants found growing 
in such a region becomes possible. 
Greater compositeness of the Northern realm. In any of 
the classifications of the botanical regions of the earth it will 
be noted that a greater homogeneity is to be seen in the regions 
of the northern hemisphere than in those of the southern, The 
reasons for this difference are both geographical and geologi- 
cal. The Antarctic region consists of a series of isolated areas 
such as Kerguelen, New Zealand, Patagonia and the lower 
Cape of Good Hope district. The Arctic region on the other 
hand consists of a compact circle of land surrounding the un- 
known polar area and broken only by narrow inlets such as 
Berings straits or Davis strait. Spitzbergen is the only rela- 
