arieties
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 ought to
be reversed. But there is also reason to believe, that those 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 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 spe