Page - 167 - in The Origin of Species

SPECIFIC CHARACTERS HIGHLYVARIABLE 167 the two sexes of the same species. Sir J. Lubbock has re- cently remarked, that several minute crustaceans offer ex- cellent illustrations of this law. "In Pontella, for instance, the sexual characters are afforded mainly by the anterior antennas and by the fifth pair of legs: the specific differences also are principally given by these organs." This relation has a clear meaning on my view: I look at all the species of the same genus as having as certainly descended from a common progenitor, as have the two sexes of any one spe- cies. Consequently, whatever part of the structure oi the common progenitor, or of its early descendants, became vari- ble, variations of this part would, it is highly probable, be taken advantage of by natural and sexual selection, in order to fit the several species to their several places in the econ- omy of nature, and likewise to fit the two sexes of the same species to each other, or to fit the males to struggle with other males for the possession of the females. Finally, then, I conclude that the greater variability of specific characters, or those which distinguish species from species, than of generic characters, or those which are pos- sessed by all the species;—that the frequent extreme varia- bility of any part which is developed in a species in an extra- ordinary manner in comparison with the same part in its congeners; and the slight degree of variability in a part, however extraordinarily it may be developed, if it be com- mon to a whole group of species;—that the great variability of secondary sexual characters, and their great difference in closely allied species;—that secondary sexual and ordinary specific differences are generally displayed in the same parts of the organisation,—are all principles closely connected to- gether. All being mainly due to the species of the same group being the descendants of a common progenitor, from whom they have inherited much in common,—to parts which have recently and largely varied being more likely still to go on varying than parts which have long been inherited and have not varied—to natural selection having more or less completely, according to the lapse of time, overmastered the tendency to reversion and to further variability, —to sexual selection being less rigid than ordinary selection, —and to