600 METASPERMAE OF THE MINNESOTA VALLEY. 
of the variety of floral elements met with ina morning walk 
over any portion of the basin. 
The influence of these minor tensions is most interestingly 
portrayed in the moditications 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 
Ulm 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 one 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 
