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ORGANIC EVOLUTION—THE FACTORS
probable one, since it implies 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. 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, there remains only the inference that they have arisen by the transmission and accumulation of functional adaption.
"Another case which seems interpretable only in an analogous way, is that of the spurs which 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 many 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 on those points of 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 reaction of the blows, if not in inflicting the blows. But to conclude this is, I think, to conclude against probability. Contrariwise, if we assume that the thickenings of the epidermis produced by habitual rough usage is inheritable, the development of these structures presents no difficulty. The points of impact would become indurated in wings used for striking with unusual frequency. The callosities of the surface thus generated, rendering the parts less sensitive, would enable the bird in which they arose to give, without injury to itself, violent blows and a