CHAPTER II. VARIATION UNDER NATURE.
3. DOUBTFUL SPECIES.
The forms which possess in some considerable degree the character of
species, but which are so closely similar to other forms, or are so closely
linked to them by intermediate gradations, that naturalists do not like to
rank them as distinct species, are in several respects the most important
for us. We have every reason to believe that many of these doubtful and
closely allied forms have permanently retained their characters for a long
time; for as long, as far as we know, as have good and true species.
Practically, when a naturalist can unite by means of intermediate links any
two forms, he treats the one as a variety of the other, ranking the most
common, but sometimes the one first described as the species, and the other
as the variety. But cases of great difficulty, which I will not here
enumerate, sometimes arise 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
forms 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.