is, I think, to conclude against probability. Contrariwise, if we assume that the thickening 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 surface thus generated, rendering the parts less sensitive, would enable the bird in which they arose to give, without injury to itself, more violent blows and a greater number of them: so, in some cases, helping it to conquer and multiply. Among its descendants, inheriting the modification and the accompanying habit, the thickening would be further increased in the same way: survival of the fittest tending ever to accelerate the process. Presently the horny nodes so formed, hitherto defensive only in their effects, would, by their prominence, become offensive--would make the blows given more hurtful. And now natural selection, aiding more actively, would mould the nodes into spurs: the individuals in which the nodes were most pointed would be apt to survive and propagate; and the pointedness generation after generation thus increased, would end in the well-adapted shape we see. But if in these cases the differentiations which fit particular parts of the outer tissues to bear rough usage are caused mainly by the direct balancing of external actions by internal reactions, then we may suspect that the like is true of other modifications that occur where special strains and abrasions have to be met. Possibly it is true of sundry parts that are formed of hardened epidermis, such as the nails, claws, hoofs, and hollow horns of Mammals; “all of which,” says Prof. Huxley, “are constructed on essentially the same plan, being diverticula of the whole integument, the outer layer of whose ecderon has undergone horny metamorphosis.” Leaving open, however, the question what tegumentary structures are due to direct equilibration, furthered and controlled by indirect equilibration, it is tolerably clear that direct equilibration has been one of the factors.