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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