Already reference has been more than once made to those thickenings that occur where the skin is exposed to unusual pressure and friction. Are these adaptations inheritable? and may they, by accumulation through many generations, produce permanent dermal structures fitted to permanent or frequently-recurring stress? Take, for instance, the callosities on the knuckles of the _Gorilla_, which are adapted to its habit of partially supporting itself on its closed hands when moving along the ground. Shall we suppose that these defensive thickenings are produced afresh in each individual by the direct actions; or that they are inherited modifications caused by such direct actions; or that they are wholly due to the natural selection of spontaneous variations? The last supposition does not seem a probable one. Such thickenings, if spontaneous, would be no more likely to occur on the knuckles than on any other of the hundred equal areas forming the skin-surface at large; and the chances against their simultaneous occurrence on all eight knuckles would be incalculable. Moreover, the implication would be that those slight extra thicknesses of skin on the knuckles, with which we must suppose the selection to have commenced, were so advantageous as to cause survivals of the individuals having them, in presence of other superiorities possessed by other individuals. Then that survivals so caused, if they ever occurred at all, should have occurred with the frequency requisite to establish and increase the variation, is hardly supposable. And if we reject, as also unlikely, the reproduction of these callosities _de novo_ in each individual (for this would imply that after a thousand generations each young gorilla began with knuckles having skin no thicker than elsewhere), there remains only the inference that they have arisen by the transmission and accumulation of functional adaptations. Another case which seems interpretable only in an analogous way, is that of the spurs that are developed on the wings of certain birds--on those of the Chaja screamer for example. These are weapons of offence and defence. It is a familiar fact that some birds strike with their wings, often giving severe blows; and in the birds named, the blows are made more formidable by the horny, dagger-shaped growths standing out from those points on the wings which deliver them. Are these spurs directly or indirectly adaptive? To conclude that natural selection of spontaneous variations has caused them, is to conclude that, without any local stimulus, thickenings of the skin occurred symmetrically on the two wings at the places required; that such thickenings, so localized, happened to arise in birds given to using their wings in fight; and that on their first appearance the thickenings were decided enough to give appreciable advantages to the individuals distinguished by them--advantages in bearing the reactions of the blows if not in inflicting the blows. But to conclude this