It is impossible here to do more than just touch the fringe of the difficult subject of colour-vision. And the only further fact that can here be noticed is that trichromatic colour-vision is apparently in us limited to the yellow-spot and its immediate neighbourhood. Around this is an area which is said to be bichromatic--all of us being, for this area, more or less green-blind. In the peripheral area around this, colour is indistinguishable, and we are only sensitive to light and shade. So far as the structure of the retina is concerned, we may notice in this connection that in the central region of most complete trichromatic vision there are cones only; around the yellow spot each cone is surrounded by a circle of rods; and further out into the peripheral region by two, three, or more circles of rods. Concerning the sense of sight in the lower mammals little need be said. In many cases the acuteness of vision is remarkable. Mr. Romanes's experiments on Sally, the bald-headed chimpanzee at Regent's Park, led him to conclude that she was colour-blind, but I question whether the experiments described quite justify this conclusion. Sir John Lubbock was unable to teach his intelligent dog Van to distinguish between coloured cards; but the failure was as complete when the cards were marked respectively with one, two, or three dark bands. We are not justified, therefore, in ascribing the failure to colour-blindness. The real failure, probably, was in each case to make the animal understand what was wanted. Bulls are, at any rate, credited with strong colour-antipathies, and insect-eating mammals are probably not defective in the colour-sense. It is said that nocturnal animals, such as mice, bats, and hedgehogs, have no retinal cones; and if the cones are associated with colour-vision, they may not improbably be unable to distinguish colours. Some moles are blind (e.g. the Cape golden mole). But the common European mole, though the eyes are exceedingly minute (1/25 of an inch in diameter), has the organ fairly developed, and is even said not to be very short-sighted. It is protected by long hairs when the animal is burrowing, and is only used when it comes to the surface of the ground.