rply 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 allied or representative species, when
inhabiting a continuous area, are generally so distributed 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, {176} the same rule will probably
apply to both; and if we in imagination adapt a varying species to 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 rule in the case of varieties intermediate between
well-marked varieties in the genus Balanus. And it would appear from
information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that
generally when varieties intermediate between two other forms occur, they
are much rarer numerically than the forms which they connect. Now, if we
may trust these facts and inferences, and therefore conclude that varieties
linking two other varieties together have generally existed in lesser
numbers than the forms which they connect, then, I think, we can understand
why intermediate varieties should not endure for very long periods;--why as
a general rule they should be exterminated and disappear, sooner than the
forms which they originally linked together.

For any form existing in lesser numbers would, as already remarked, run a
greater chance of being exterminated than one existing in large numbers;
and in this particular case the intermediate form would be eminently liable
to the inroads of closely allied forms existing on both sides of it. But a
far more important consideration, as I believe, is that, during the process
of further modification, by which two varieties are supposed on my theory
to be converted and perfected into two distinct species, the two which
exist in larger numbers from inhabiting larger areas, will