Page - 166 - in The Origin of Species

166 ORIGIN OF SPECIES since the period when the species branched off from a com- mon progenitor, it is probable that they should still often be in some degree variable,—at least more variable than those parts of the organisation which have for a very long period remained constant. Secondary Sexual Characters Variable.—I think it will be admitted by naturalists, without my entering on details, that secondary sexual characters are highly variable. It will also be admitted that species of the same group differ from each other more widely in their secondary sexual characters, than in other parts of their organisation: compare, for instance, the amount of difference between the males of gallinaceous birds, in which secondary sexual characters are strongly dis- played, with the amount of difference between the females. The cause of the original variability of these characters is not manifest; but we can see why they should not have been rendered as constant and uniform as others, for they are accumulated by sexual selection, which is less rigid in its ac- tion than ordinary selection, as it does not entail death, but only gives fewer offspring to the less favoured males. What- ever the cause may be of the variability of secondary sexual characters, as they are highly variable, sexual selection will have had a wide scope for action, and may thus have suc- ceeded in giving to the species of the same group a greater amount of difference in these than in other respects. It is a remarkable fact, that the secondary differences be- tween the two sexes of the same species are generally dis- played in the very same parts of the organisation in which the species of the same genus differ from each other. Of this fact I will give in illustration the two first instances which happen to stand on my list; and as the differences in these cases are of a very unusual nature, the relation can hardly be accidental. The same number of joints in the tarsi is a character common to very large groups of beetles, but in the Engidas, as Westwood has remarked, the number varies greatly; and the number likewise differs in the two sexes of the same species. Again in the fossorial hymenoptera, the neuration of the wings is a character of the highest impor- tance, because common to large groups ; but in certain genera the neuration differs in the different species, and likewise in