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.