758 METASPERMAE OF THE MINNESOTA VALLEY. 
genus Carex is one of the apparent causes of the variation from 
the general percentages, but it is interesting tosee that itis 
the ‘‘mean” taxonomic group—that of the Archichlamydeae 
which suffers by reduction, to the greatest degree. While multi- 
plicity of species is often a sign of comparative newness in a 
genus this multiplication may arise either in older or newer 
families. The monocotyledonous and metachlamydeous herbs, 
in their relation to the general and special tensions, have been 
explained in outline above. A similar explanation must be 
offered of the reduction of the Archichlamydeae from the domin- 
ant family position. At once in the oldest and in the youngest of 
the three taxonomic groups have been working the causes which 
tend to multiciplicity of genera in excess of reduction, for these 
two groups are peculiarly exposed under the law of ejection. 
Hence they become relatively plastic and specific modifications 
are frequent. The third group, however, undergoes the series 
of changes which tend to reduction of species in excess of mul- 
tiplication, and, with this reduction, the tendency is towards 
greater solidarity of formations and movement toward the shrub- 
by or arboreal habit. Thus in the percentages of the dominant 
families further evidence concerning the difference in meaning 
of Archichlamydeae, Metachlamydeae and Monocotyledones, in 
the distribution over a limited area, is discovered. 
VI. CONCLUSION. 
The statistical investigation of the Metaspermae indigenous 
to the valley of the Minnesota having now been completed as 
far as the limits of this work may permit, it remains to offer 
some brief summary and explanation of the more important 
facts believed to have been determined in the preceding pages. 
It has been shown that while the valley of the Minnesota is 
geographically central in the North American continent, it is 
by no means botanically central, but on the contrary, strongly 
southern and eastern, This particularly important fact needs 
explanation. Upon examination it would appear that two sets 
of factors must be conceived as having interacted to bring 
about this result. These factors may be grouped as physical 
(in the narrow sense) and biological. Under the first head it 
must be observed that while geographically central, the Min- 
nesota valley is not central in point of elevation, climate, pre- 
vailing winds, and drainage. The line of mean elevation lies 
to the west of the valley, the continental climatic mean, so far 
as concerns temperature, lies to the north of it. The winds of 
