20 DULLETIX OF WISCONSIN NATURAL HISTORY SOCIETY. VOL. 2. NO. 1. 
They cannot properly be classed with forest associations, but must 
be considered as a distinct formation, and will not be considered 
here in detail. The remnants of the bottom wood group are in 
evident course of extinction. The old trees are gradually dying or 
being removed by the axe. Windfalls are exceedingly common in 
the thinned stands of elm. The young trees that are coming up 
are as apt to be hard maples and other trees from the mesophytic 
uplands as the progeny of the original occupants. 
Having thus briefly described the three mesophytic forest sub- 
associations of the region, the question arises as to their history. 
The hemi-hydrophytic sub-association is evidently conditioned 
directly by the physiography of the country. But the other two 
seem to' be far more directly the result of the struggle for life 
among species. 
To understand the mutual relations of these two types of forest 
it is necessary to throw a glance at a somewhat wider region than 
that under immediate consideration. The vegetation map of 
Eastern Wisconsin, accompanying the first Geological Survey of 
the State, shows that there is a broad belt to the west of Milwau- 
kee County, which is there denominated the Oak Group. To the 
east of this is a strip classified as the Oak and Maple Group, and 
still farther east comes the Maple and Beech Group. A visit to 
the forest anywhere within the Oak Group will show that various 
species of oak are almost the only trees found there. Their only 
associates are the hickories, only in some places the black cherry is 
found. These are precisely the species v/hich have been mentioned 
as the characteristic ones of the hemi-xerophytic sub-association. 
Perhaps it is possible to divide even this sub-association into seve- 
ral groups of more or less highly developed xerophily. At all 
events, the white and burr oak seem to have more pronounced 
xerophytic characters than the red and black oaks, and it is well 
known that groves of stunted burr oaks are not uncommon as far 
West as South Dakota, w^here conditions are decidedly xerophytic. 
A closer study of the oak forests will show that here and there, 
usually in a deep ravine or in the neighborhood of a lake or water 
course, there are a few individuals of basswood. The vicinity of 
such trees is usually distinguished also by the occurrence of 
strongly mesophytic herbs, such as Podophylhim peltafum, which 
are uncommon in the purely oak forests. Sometimes one will no- 
tice that the basswood and its minor companions grow exclusively 
on the southerly side of a ravine, while they are entirely absent 
on the northerly side. One remembers that these locations receive 
less sunlight than the northerly slopes. Their greater shade and 
consequently damp and cool condition retards the oxydization of 
