nerally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it
is quite remarkable how abruptly, as Alph. De Candolle has observed, a
common alpine species disappears. The same fact has been noticed by E.
Forbes in sounding the depths of the sea with the dredge. To those who
look at climate and the physical conditions of life as the all-important
elements of distribution, these facts ought to cause surprise, as
climate and height or depth graduate away insensibly. But when we
bear in mind that almost every species, even in its metropolis, would
increase immensely in numbers, were it not for other competing species;
that nearly all either prey on or serve as prey for others; in short,
that each organic being is either directly or indirectly related in the
most important manner to other organic beings--we see that the range
of the inhabitants of any country by no means exclusively depends
on insensibly changing physical conditions, but in large part on
the presence of other species, on which it lives, or by which it is
destroyed, or with which it comes into competition; and as these species
are already defined objects, not blending one into another by insensible
gradations, the range of any one species, depending as it does on the
range of others, will tend to be sharply defined. Moreover, each species
on the confines of its range, where it exists in lessened numbers, will,
during fluctuations in the number of its enemies or of its prey, or in
the nature of the seasons, be extremely liable to utter extermination;
and thus its geographical range will come to be still more sharply
defined.

As allied or representative species, when inhabiting a continuous area,
are generally distributed in such a manner that each has a wide range,
with a comparatively narrow neutral territory between them, in which
they become rather suddenly rarer and rarer; then, as varieties do not
essentially differ from species, the same rule will probably apply to
both; and if we take a varying species inhabiting a very large area,
we shall have to adapt two varieties to two large areas, and a third
variety to a narrow intermediate zone. The intermediate variety,
consequently, will exist in lesser numbers from inhabiting a narrow and
lesser area; and practically, as far as I can make out, this rule holds
good with varieties in a state of nature. I have met with striking
instances of the ru