They are lost or never gained by the males after emasculation. Generally they are not developed in the male during early youth, but appear a short time before the age for reproduction. This surprising uniformity in the laws regulating the differences between the sexes in so many and such widely separated classes, is intelligible if we admit the action of one common cause, namely Sexual Selection. This latter kind of selection is closely analogous to that which man unintentionally, yet effectually, brings to bear on his domesticated productions, when he preserves during a long period the most pleasing or useful individuals, without any wish to modify the breed. It appears that variations arising late in life are commonly transmitted to one and the same sex.
Variability is the necessary basis for the action of selection, and is wholly independent of it.
It follows from this, that variations of the same general nature have often been taken advantage of and accumulated through Sexual Selection in relation to the propagation of the species, as well as through Natural Selection in relation to the general purposes of life. Hence secondary sexual characters, when equally transmitted to both sexes can be distinguished from ordinary specific characters only by the light of analogy. The modifications acquired through Sexual Selection are often so strongly pronounced that the two sexes have frequently been ranked as distinct species, or even as distinct genera. The belief in the power of Sexual Selection rests chiefly on the following considerations. In innumerable instances these characters are fully developed only at maturity, and often during only a part of the year, which is always the breeding- season.