600 METASPERMAE OF THE MINNESOTA VALLEY. 



of the variety of floral elements met with in a morning walk 

 over any portion of the basin. 



The influence of these minor tensions is most interestingly 

 portrayed in the modifications through them of the general lat 

 eral tension line. For example the irregular contour of the 

 limiting line between the forest and the prairie is due in part 

 to the presence of minor tensions, either positive or negative, 

 along the general line, and by means of these minor lines the 

 exact outline of the forest edge is, in part, determined. In ex- 

 plaining the contour of the forest line it is apparent then that 

 we must consider a number of different forces acting both di- 

 rectly and indirectly, in varying degrees of 'directness or indi- 

 rectness. The more direct influence of the relative humidity, 

 elevations, soil compositions, exposure to light, etc , are accom- 

 panied by those indirect influences which appear most dis- 

 tinctly in the tensions. As in the Minnesota valley from New 

 Ulrn to Montevideo the south bluffs are more densely wooded 

 than the north apparently because their exposure to the de- 

 siccating action of the sun's rays is less so in less extended 

 areas one may recognise the effects of the minor tensions in de- 

 termining the physiognomy of smaller and still smaller areas. 

 This group of tensions may then, for each degree, be reduced 

 to more and more special cases, and ultimately appears in the 

 form of mutual competition between adjacent individuals of the 

 same, or different species, or even between differently situated 

 organs of the same individual. By synthesis of competitions, 

 together with progressive alterations of climate, topography, 

 distance and the rest, the tensions may be considered to arise; 

 and by analysis of the various degrees of tension we come back 

 to individual competitions and to more and more definite geo- 

 graphical influences. 



General division of the world into botanical realms. 

 From the considerations given it will be seen that a yet more 

 general division than that of Drude may be proposed. The 

 two great realms are: 



(A.) The Central Realm. 

 (B.) The Distal Realm. 



The valley of the Minnesota is upon ore of the transition lines 

 between these two principal realms. 



OUTLINES OF METASPERMIC HISTORY IN THE NORTHERN 

 HEMISPHERE. 



Emergence of metaspermic forms. Leaving aside the prob- 

 able origin of metaspermic plants in point of development from 



