Thus, the wing of the bat is a most abnormal
structure in the class of mammals; but the rule would not apply here,
because the whole group of bats possesses wings; it would apply only if
some one species had wings developed in a remarkable manner in comparison
with the other species of the same genus. The rule applies very strongly
in the case of secondary sexual characters, when displayed in any unusual
manner. The term, secondary sexual characters, used by Hunter, relates to
characters which are attached to one sex, but are not directly connected
with the act of reproduction. The rule applies to males and females; but
more rarely to females, as they seldom offer remarkable secondary sexual
characters. The rule being so plainly applicable in the case of secondary
sexual characters, may be due to the great variability of these characters,
whether or not displayed in any unusual manner--of which fact I think there
can be little doubt. But that our rule is not confined to secondary sexual
characters is clearly shown in the case of hermaphrodite cirripedes; I
particularly attended to Mr. Waterhouse's remark, whilst investigating this
order, and I am fully convinced that the rule almost always holds good. I
shall, in a future work, give a list of all the more remarkable cases. I
will here give only one, as it illustrates the rule in its largest
application. The opercular valves of sessile cirripedes (rock barnacles)
are, in every sense of the word, very important structures, and they differ
extremely little even in distinct genera; but in the several species of one
genus, Pyrgoma, these valves present a marvellous amount of
diversification; the homologous valves in the different species being
sometimes wholly unlike in shape; and the amount of variation in the
individuals of the same species is so great that it is no exaggeration to
state that the varieties of the same species differ more from each other in
the characters derived from these important organs, than do the species
belonging to other distinct genera.
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