RELATIONSHIPS OF METASPERMAE. 591 



C. Southern realm. 



U) 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 or C. In 

 Realm C, only the fourth region is comparatively uninfluenced 

 by the plants of Realm B. 



The region of the Minnesota 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- 



