142 HARSHBERGER: PHYTO-GEOGRAPHIC SKETCH 
formations recognized in this paper, as well as the roadside-plant 
formation, the pasture-field-plant formation, the ruderal-plant for- 
mation, part of the barren-plant formations and alluvial-soil-plant 
formations were covered by the original forest. This fact is men- 
tioned because it illustrates how man has altered the character of 
the original plant formations. By the removal of the original 
forest, by the introduction of various cultivated plants and weeds, 
the long-established balance produced by the competition of the 
native species is rudely disturbed and exotic species come into 
conflict with the resident species and even the character of the 
undisturbed formations is altered by the injection of new species 
into them. However, enough of the virgin forest remains to per- 
mit a phyto-geographic survey. The original forest was a meso- 
phytic one. It probably passed through various vicissitudes de- 
pendent upon the topographic changes, so that the xerophytic 
forest of the hillside was gradually replaced by a mesophytic for- 
est. The tendency has been in the entire region to the culmina- 
tion of the forest in the mesophytic type. The forest, of great 
original density, may be looked upon as the northeastern exten- 
sion of the forest found developed in its highest character in the 
region drained by the Tennessee river and its tributaries and by 
streams arising in the southern Allegheny mountains and flowing 
eastward into the Atlantic. Arbitrarily, a line drawn from a 
point where the Ohio joins the Mississippi river, east to the Cum- 
berland mountains and thence along the Allegheny mountains to 
the west branch of the Susquehanna river in Pennsylvania, then 
to the Blue Ridge and along it to the Schuylkill river, following 
the hills on the south side of the Great Valley to the Delaware 
river, represents the northern limit during glacial times of the 
forest which during the Miocene period extend north into the 
Arctic regions. 
The northeastern extension of the forest of glacial times was 
much poorer in species than the mixed deciduous forest farther 
south. This was probably due to the killing of the less hardy 
species by the glacial cold. Only those species remained in the 
area mentioned that were hardy. These hardy species, therefore, 
represent the main constituent species of the present-day forest 
with the possible introduction of a few additional species that ‘mi- 
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