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.
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