d 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, 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 have a great advantage over the intermediate variety,
which exists in smaller numbers in a narrow and intermediate zone.
For forms existing in larger numbers will always have a better chance,
within any given period, of presenting further favourable variations for
natural selection to seize on, than will the rarer forms which exist in
lesser numbers. Hen