apparently forgets that the callosities on the hands of men who do hard manual labour, and on the feet of men who go barefoot, are never reproduced in their infants, who were born as soft-handed and soft-soled as the infants of those who do no labour, nor go barefoot. As regards the Chaja screamer, Mr. Spencer, like Mr. Cunninghame when writing on the evolution of horns, does not correctly state the case when he says, "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." His words imply that the thickenings were "occasional" variations, and that, if we hold that spontaneous variations are alone transmitted, we must believe that the evolution of spurs in the Chaja screamer was due to the increased rate of survival conferred by these rare variations—for rare they must have been to have occurred "symmetrically," "at points required." But, as already stated, evolution does not proceed on lines of rare variations, which are necessarily swamped during the process of interbreeding, but on lines of normal variations. Every screamer must have varied above or below a certain mean as regards the thickness and denseness of the tissues "at the points required," and fortunate variations in this case must have been particularly important, since unfavourable variations must have led, in a prolonged combat or in a series of combats, to such a denudation of the epidermis as rendered a renewal of combat painful, and thereby caused a corresponding disinclination to fight for food or females. Fitness in
