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. Under genera, including the most polymorphic forms,
Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112--a
difference of 139 doubtful forms! Among animals which unite for each
birth, and which are highly locomotive, doubtful forms, ranked by one
zoologist as a species and by another as a variety, can rarely be found
within the same country, but are common in separated areas. How many of
the birds and insects in North America and Europe, which differ very
slightly from each other, have been ranked by one eminent naturalist as
undoubted species, and by another as varieties, or, as they are often
called, geographical races! Mr. Wallace, in several valuable papers on the