been
broken up into islands even during the later tertiary periods; and in such
islands distinct species might have been separately formed without the
possibility of intermediate varieties existing in the intermediate zones.
By changes in the form of the land and of climate, marine areas now
continuous must often have existed within recent times 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 {175} 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 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 sha