In most
polymorphic genera some of the species have fixed and definite
characters. Genera which are polymorphic in one country seem to be,
with some few exceptions, polymorphic in other countries, and
likewise, judging from Brachiopod shells, at former periods of time.
These facts seem to be very perplexing, for they seem to show that
this kind of variability is independent of the conditions of life. I
am inclined to suspect that we see in these polymorphic genera
variations in points of structure which are of no service or
disservice to the species, and which consequently have not been seized
on and rendered definite by natural selection, as hereafter will be
explained.
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 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 in their own country for a long time; for as long, as
far as we know, as have good and true species. Practically, 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.
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