le 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 conclude
that varieties linking two other varieties together generally have
existed in lesser numbers than the forms which they connect, then 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 it is a far more important consideration, that during
the process of further modification, by which two varieties are supposed
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 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. Hence, the more
common forms, in the race for life, will tend to beat and supplant the
less common forms, for these will be more slowly modified and improved.
It is the same principle which, as I believe, accounts for the common
species in each country, as shown in the second chapter, presenting on
an average a greater number of well-marked varieties than do the rarer
species. I may illustrate what I mean by supposing three varieties of
sheep to be kept, one adapted to an extensive mountainous region; a
second to a comparatively narrow, hilly tract; and a third to the wide
plains at the base; and that the inhabitants are all trying with equal
steadiness and skill to improve their stocks by selection; the chances
in this case will be strongly in favour of the great holders on the
mountains or on the plains improvin