CHAPTER II
VARIATION UNDER NATURE
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 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.
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. For
instance, Mr. H. C. Watson has marked for me in the well-sifted London
Catalogue of Plants (4th edition) 63 plants which are therein ranked
as species, but which he considers as so closely allied to other
species as to be of doubtful value: these 63 reputed species range
on an average over 6.9 of the provinces into which Mr. Watson has
divided Great Britain. Now, in this same Catalogue, 53 acknowledged
varieties are recorded, and these range over 7.7 provinces; whereas,
the species to which these varieties belong range over 14.3 provinces.
So that the acknowledged varieties have nearly the same, restricted
average range, as have the closely allied forms, marked for me by
Mr. Watson as doubtful species, but which are almost universally
ranked by British botanists as good and true species.
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