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. Under genera, including the most
polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham
gives only 112,--a difference of 139 doubtful forms! Amongst 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 those 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, as geographical races! Many years ago,
when comparing, and seeing others compare, the birds from the separate
islands of the Galapagos Archipelago, both one with another, and with
those from the American mainland, I was much struck how entirely vague
and arbitrary is the distinction between species and varieties. On the
islets of the little Madeira group there are many insects which