g contest with this rude
world, and to win the advance they have gained. The reader will have
to tax his imagination to picture, it may be, a quarter of a million
species dwelling in the same field, each united with the other in the
method of exchange in such a way that the withdrawal of any one form
is likely in some measure to change the estate of every other. In some
cases this removal of one species means the loss of the life of many
and perhaps the better opportunity of other neighbors; again, the
influence on remoter members of the society may be so slight as to
escape detection. Yet it is doubtful if the slightest change in the
population of a biologic province can be brought about without some
effect upon all the members of the society. It is a vast, sensitive
thing, fit to be compared with the living body where every cell lives
in accord with every other of the frame.

So long as the organic hosts were in the prehuman stage the maintenance
of the accord was easily and naturally attained. Species arose and
perished, each in turn effecting a simple reconciliation with the
others, grasping only so much room and food as was necessary for its
proper support. But with the coming of man, the species which by its
swiftly progressive desires has become a host in itself, a disturbing
element was introduced into the old order. Man as a primitive savage
falls into the natural system without greatly disturbing it; but man as
a soil-tiller, in so far as he carries out his subjugative work,
utterly wrecks the ancient establishments of life. To attain his object
he has to banish from the soil nearly all the plants which originally
belonged upon it, and in their place, with or without intention, he
introduces species from other organic provinces. With the change in
plant-life necessarily goes a like, or even a greater, alteration in
the native animals. They are driven into the wilderness or, it may be,
extirpated. The reader who would obtain an idea of these changes will
do well to study the invasions of weeds or of those noxious insects
which in the economy of a civilized country may be likened to weeds.
These pests are in nearly all cases invaders which owe their successes
to the fact that our treatment of the regions they have entered has
opened vacancies in the once closed ranks of the indigenous host, into
which the foreigners are free to enter. In the fresh field they are not
likely to find enemies which by long training