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.