Of Chance, enough has already been said. It is, however, worth our while to observe how constantly to the last Mr. Darwin was haunted by the consciousness that this was in reality the factor upon which his system must depend, and that it could not possibly account for much that he came across in nature. If, as he confessed, the sight of a peacock's tail-feather made him sick, it was just because its elaborate beauty, to which no commensurate advantage can be supposed to attach, forbade the notion that his theory could account for it. So, of another still more marvellous instance in which Nature exhibits artistic power, namely the ball-and-socket ornament on the wings of the Argus pheasant, he writes:[202] No one, I presume, will attribute this shading, which has excited the admiration of many experienced artists, to chance--to the fortuitous concourse of atoms of colouring matter. That these ornaments should have been formed through the selection of many successive variations, not one of which was originally intended to produce the ball-and-socket effect, seems as incredible as that one of Raphael's Madonnas should have been formed by the selection of chance daubs of paints made by a long succession of young artists, not one of whom intended at first to draw the human figure. [Illustration: 1. Basal portion of secondary wing-feather; nearest body, shewing first rudiment of "ocelli." 2. Portion of secondary wing-feather near body, shewing "elliptic" ornaments. 3. Part of secondary wing-feather, shewing developed "ocelli." Feathers from wing of Argus Pheasant, from Darwin's _Descent of Man_.] Nevertheless, Mr. Darwin proceeds to argue at considerable length that an explanation consistent with his theory is favoured by the occurrence on the same wings of designs exhibiting every stage of gradation from a mere spot to the finished ball-and-socket _ocellus_; in the same way as the tail feathers of a peacock advance from a mere sketch to the completed design. It is not easy, however, to understand in what way this is supposed to solve the difficulty and not vastly to increase it. That a finished artistic effect should be fortuitously produced at all would be incredible enough. That it should be worked up by Chance through a series of processes, each doing something towards its completion, is surely not less, but far more inconceivable.