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 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,
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.
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