878 THE AMERICAN NATURALIST.. [Vor. XXXIX. 
so that comparisons with the conditions in two or more centers 
may be necessary to interpret fully the local data. 
The geographic point of view is of the greatest importance in 
the study of regions intermediate between the great plant for- 
mations. In such localities there is a mingling of both species 
and societies which have spread from very different distribution 
centers. The local order of succession is usually a mixture of 
two or more orders, characteristic of as many centers. For 
example, in northern Michigan the successive societies (Whit- 
ford, :01) in the development of the forests, are: (1) xerophi- 
lous herbs, (2) the heath, (3) the coniferous forest, and (4) the 
maple-beech climax forest. While there can be little doubt but 
that this is the correct interpretation of the forest relationships 
in this region, there is a notable difference between these socie- 
ties. The first three may be found anywhere in that region, 
while the last occurs only in areas favored by a rich soil, the 
climatic influence of the lakes, and the possibilities of migration 
from the southeastern deciduous forest. The first three socie- 
ties are stages in the development of the climax forest of the 
Northeastern Conifer forest center, the last is the attenuated 
border of the climax stage of the southeastern Deciduous forest 
center. The first three stages are largely physiographic, while 
the last is also geographic, for it marks not only the succession 
of one society over another, but also the invasion and succession 
of one forest center over another. 
In northern Pennsylvania and part of the mountains of New 
England, it appears from preliminary work that the successional 
relationship of the societies can only be traced by taking into 
account the fact that the societies of northern slopes and certain 
other edaphic situations are related to the Northeastern Conifer 
forest and form an order of succession distinct from that to 
which the societies of southern slopes and other favored situa- 
tions belong. 
Where best developed in the lower Ohio basin and Piedmont 
plateau, the climax stage of the Deciduous forest center (Fig. 3) 
is made up of many species of which the dominant are Quercus 
alba, Magnolia acuminata, Acer saccharum, Fagus americana, 
Liriodendron tulipifera, Fraxinus americana, Quercus rubra, and 
