Chap. IT. 
DOUBTFUL SPECIES. 
47 
Those forms which possess in some considerable 
degree the character of species, but which are so closely 
similar to some 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 re¬ 
spects 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 in 
their own country for a long time; for as long, as far 
as we know, as have good and true species. Practi¬ 
cally, when a naturalist can unite two forms together 
by others having intermediate characters, 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 diffi¬ 
culty, 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 con¬ 
nected 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 natural¬ 
ists 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 com¬ 
petent judges. 
