Moreover, the species of the larger genera are related to each other, in
the same manner as the varieties of any one species are related to each
other. No naturalist pretends that all the species of a genus are equally
distinct from each other; they may generally be divided into sub-genera, or
sections, or lesser groups. As Fries has well remarked, little groups of
species are generally clustered like satellites around other species. And
what are varieties but groups of forms, unequally related to each other,
and clustered round certain forms--that is, round their parent-species.
Undoubtedly there is one most important point of difference between
varieties and species, namely, that the amount of difference between
varieties, when compared with each other or with their parent-species, is
much less than that between the species of the same genus. But when we
come to discuss the principle, as I call it, of divergence of character, we
shall see how this may be explained, and how the lesser differences between
varieties tend to increase into the greater differences between species.
There is one other point which is worth notice. Varieties generally have
much restricted ranges. This statement is indeed scarcely more than a
truism, for if a variety were found to have a wider range than that of its
supposed parent-species, their denominations would be reversed. But there
is reason to believe that the species which are very closely allied to
other species, and in so far resemble varieties, often have much restricted
ranges.
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