AL THE AMERICAN NATURALIST. (VoL. XXXIIL. 
and the struggle for existence is largely between individuals of 
the same species. In the tension zone, however, the struggle 
is not only between individuals of the same species, but also 
between individuals and aggregations of different species. The 
elements in the deciduous flora are always ready to seize upon 
any advantage which will give them a foothold further south, 
while the elements in the coniferous flora are always ready 
for an opposite move. The former is a southward-moving, the 
latter a northward-moving flora. In consequence of these con- 
ditions, the area between their borders is in a state of unstable 
equilibrium, so far as its vegetation is concerned. The tension 
is constant, and any change in or interference with the environ- 
ment releases the tension and causes a disturbance of the rela- 
tions between the two floras until new conditions have become 
established. Each such disturbance is marked to a greater or 
less extent by changes of species, and lines of limitation thus 
vary from time to time. | 
The influence of civilization has been the most marked factor 
in this connection. Fire and cultivation have caused, directly 
and indirectly, great changes in the interrelations of species. 
In places certain species have been removed for economic pur- 
poses, and other species, less valuable, allowed to remain undis- 
turbed; and this gives at once a direct advantage to the latter 
in the subsequent struggle for place. In other localities all 
species have been removed, and such a change wrought in the 
environment that foreign species, better able to adapt them- 
selves to the new conditions, become established there. 
Fire is probably the most destructive agency, as it not only 
destroys the living vegetation above ground, but often also the 
roots and seeds beneath. It is also to be borne in mind that 
in poor soil, such as obtains in the pine barrens, the ruin is 
more complete than in richer soil. In the latter case the soil 
will soon recuperate and be in a condition to start and nourish 
a new growth by reason of the diverse elements contained in it; 
_ but in the case of a sand or gravel soil, almost entirely com- 
posed of quartz, practically all the plant food is concentrated in 
a the surface layer of humus, which has been slowly built up by 
ae the growth and decay of the vegetation itself, and when this is | 
