CHAPTER II. VARIATION UNDER NATURE.
6. MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE VARIETIES IN BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH OTHER, AND IN HAVING RESTRICTED RANGES.
There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and when intermediate links have not been found between doubtful
forms, naturalists are compelled to come to a determination by the amount
of difference between them, judging by analogy whether or not the amount
suffices to raise one or both to the rank of species. Hence the amount of
difference is one very important criterion in settling whether two forms
should be ranked as species or varieties. Now Fries has remarked in regard
to plants, and Westwood in regard to insects, that in large genera the
amount of difference between the species is often exceedingly small. I
have endeavoured to test this numerically by averages, and, as far as my
imperfect results go, they confirm the view. I have also consulted some
sagacious and experienced observers, and, after deliberation, they concur
in this view. In this respect, therefore, the species of the larger genera
resemble varieties, more than do the species of the smaller genera. Or the
case may be put in another way, and it may be said, that in the larger
genera, in which a number of varieties or incipient species greater than
the average are now manufacturing, many of the species already manufactured
still to a certain extent resemble varieties, for they differ from each
other by a less than the usual amount of difference.
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.