But cases of great difficulty, which I
will not here enumerate, sometimes occur in deciding whether or not to
rank one form as a variety of another, even when they are closely
connected by intermediate links; nor will the commonly-assumed hybrid
nature of the intermediate links always remove the difficulty. In very
many cases, however, one form is ranked as a variety of another, not
because the intermediate links have actually been found, but because
analogy leads the observer to suppose either that they do now
somewhere exist, or may formerly have existed; and here a wide door
for the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a species or
a variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as
species by at least some competent judges.
That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France or of
the United States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H. C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for
me 182 British plants, which are generally considered as varieties,
but which have all been ranked by botanists as species; and in making
this list he has omitted many trifling varieties, but which
nevertheless have been ranked by some botanists as species, and he has
entirely omitted several highly polymorphic genera.
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