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_Species which vary little._
The preceding statements, as to the great amount of variation occurring
in animals and plants, do not prove that all species vary to the same
extent, or even vary at all, but, merely, that a considerable number of
species in every class, order, and family do so vary. It will have been
observed that the examples of great variability have all been taken from
common species, or species which have a wide range and are abundant in
individuals. Now Mr. Darwin concludes, from an elaborate examination of
the floras and faunas of several distinct regions, that common, wide
ranging species, as a rule, vary most, while those that are confined to
special districts and are therefore comparatively limited in number of
individuals vary least. By a similar comparison it is shown that species
of large genera vary more than species of small genera. These facts
explain, to some extent, why the opinion has been so prevalent that
variation is very limited in amount and exceptional in character. For
naturalists of the old school, and all mere collectors, were interested
in species in proportion to their rarity, and would often have in their
collections a larger number of specimens of a rare species than of a
species that was very common.
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