"_We may easily err in attributing importance to characters, and in believing that they have been developed through natural selection._ We must by no means overlook the effects of the definite action of changed conditions of life,--of so-called spontaneous variations, which seem to depend in a quite subordinate degree on the nature of the conditions,--of the tendency to reversion to long-lost characters,--of the complex laws of growth, such as of correlation[155], compensation, of pressure of one part on another, &c., and finally of sexual selection, by which characters of use to one sex are often gained and then transmitted more or less perfectly to the other sex, though of no use to this sex. But structures thus indirectly gained, _although at first of no advantage to a species_, may subsequently have been taken advantage of by its modified descendants, under new conditions of life and newly acquired habits[156]." [155] It must be observed that Darwin uses this word, not as Mr. Wallace always uses it (viz. as if correlation can only be with regard to adaptive characters), but in the wider sense that any change in one part of an organism--whether or not it happens to be an adaptive change--is apt to induce changes in other parts. [156] _Origin of Species_, pp. 157-8. It appeared--and still appears--to me, that where so many causes are expressly assigned as producing useless _specific_ characters, and that some of them (such as climatic influences and independent variability) must be highly general in their action, I was justified in representing it as Darwin's opinion that "a large proportional number of specific characters" are useless to the _species_ presenting them, although afterwards they may sometimes become of use to genera, families, &c. Moreover, this passage goes on to point out that specific characters which at first sight appear to be obviously useful, are sometimes found by fuller knowledge to be really useless--a consideration which is the exact inverse of the argument from ignorance as used by Mr. Wallace, and serves still further to show that in Darwin's opinion utility is by no means an invariable, still less a "necessary," mark of specific character. The following are some of the instances which he gives. "The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition, and no doubt they may facilitate, or be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has _arisen from the laws of growth_, and has been taken advantage of in the parturition of the higher animals[157]."