over 14.3 provinces. So that the acknowledged varieties have very
nearly the same restricted average range, as have those very 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.
Finally, then, varieties have the same general characters as species,
for they cannot be distinguished from species, except, firstly, by the
discovery of intermediate linking forms, and the occurrence of such
links cannot affect the actual characters of the forms which they
connect; and except, secondly, by a certain amount of
difference, for two forms, if differing very little, are generally
ranked as varieties, notwithstanding that intermediate linking forms
have not been discovered; but the amount of difference considered
necessary to give to two forms the rank of species is quite indefinite.
In genera having more than the average number of species in any
country, the species of these genera have more than the average number
of varieties. In large genera the species are apt to be closely, but
unequally, allied together, forming little clusters round certain
species. Species very closely allied to other species apparently have
restricted ranges. In all these several respects the species of large
genera present a strong analogy with varieties. And we can clearly
understand these analogies, if species have once existed as varieties,
and have thus originated: whereas, these analogies are utterly
inexplicable if each species has been independently created.
We have, also, seen that it is the most flourishing and dominant
species of the larger genera which on an average vary most; and
varieties, as we shall hereafter see, tend to become converted into new
and distinct species. The larger genera thus tend to become larger; and
throughout nature the forms of life which are now dominant tend to
become still more dominant by leaving many modified and dominant
descendants. But by steps hereafter to be explained, the larger genera
also tend to break up into smaller genera. And thus, the forms of life
throughout the universe become divided into groups subordinate to
groups.
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