Mr. Thayer has pursued this subject during the past seven years, and last autumn he gave some interesting demonstrations in the Zoological Gardens in London. He showed a model of a white egret, which was but little noticeable when standing up clear against a bright, white-clouded sky. The long plumes on the wings, developed in the breeding season, were shown (by putting them on and taking them off) to assist in causing invisibility, since they made the side of the body flat and concealed the shadow on its rounded underside. A similar bird-model marked with strong black on the neck and legs—the rest being white—refused (so to speak) to shape itself as a bird at all, and looked at a distance of twenty yards like a bit of rock or stump of wood with a twig and dead leaf attached. The effect of different tones of brown cardboard cut into the form of a butterfly, when seen on different backgrounds, was shown; but the most interesting experiment was made with a black-green piece of cloth cut to the shape of a butterfly and fastened on to a sheet of dead-black cloth in the open air, in the presence of white cloud light of moderate brilliancy. At five yards one could see the outline of the dark-green butterfly-shaped piece; at fifteen yards one could just distinguish the edge separating the dark-green piece from the black cloth. Now Mr. Thayer stuck in the middle of the dark-green butterfly-wing a small circle of pure white (about one-third of an inch across). The effect was entirely to obliterate the previously visible edge; one could no longer see the dark-green area at all—one only saw a white spot on a continuous dark ground, the dark-green and the black were merged into one. That is no doubt due to the powerful stimulation of the sensitive “retina” of the eye by the white light of the spot; the feeble stimulation by the dark-green and black, though these remain physically as distinct from one another as before, ceases to affect the brain, which is, as it were, entirely occupied with the strong white spot. This, according to Mr. Thayer, is the value to butterflies and other animals of a violently contrasted white spot or band on a dark general colouring. The fringe of white dots and connected white flakes nearer the centre of the wing—common on the wings of butterflies—has, similarly, the result of rendering the wing-outline imperceptible and the butterfly invisible. Many such relations of colour spots and bands, as well as of dark and light markings, have been elucidated by Mr. Thayer, and will be illustrated by coloured drawings in the book which he is preparing on the subject.