It should be understood that the rule by no means applies to
any part, however unusually developed, unless it be unusually
developed in comparison with the same part in closely allied species.
Thus, the bat's wing is a most abnormal structure in the class
mammalia; but the rule would not here apply, because there is a whole
group of bats having wings; it would apply only if some one species of
bat had its wings developed in some 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, applies 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 as females more rarely offer remarkable
secondary sexual characters, it applies more rarely to them. 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; and I may here add, that I particularly
attended to Mr. Waterhouse's remark, whilst investigating this Order,
and I am fully convinced that the rule almost invariably holds good
with cirripedes.
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