s
in a far less continuous and uniform condition than at present. But I
will pass over this way of escaping from the difficulty; for I believe
that many perfectly defined species have been formed on strictly
continuous areas; though I do not doubt that the formerly broken
condition of areas now continuous has played an important part in the
formation of new species, more especially with freely-crossing and
wandering animals.

In looking at species as they are now distributed over a wide area,
we generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally 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
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 must 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 depends, or by which it is
destroyed, or with which it comes into competition; and as these species
are already defined objects (however they may have become so), 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 seasons, be extremely
liable to utter extermination; and thus its geographical range will come
to be still more sharply defined.

If I am right in believing that allie