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 and significant as traces of gradual development, and the eye, which in the complexity of its highest form seemed, notwithstanding its defects, to demand such great intelligence to conceive and fashion it, becomes more intelligible when we can watch it a-building, and, as it were, see it put together bit by bit. I venture to quote here from a pamphlet of my own a very brief statement of the stages through which the eye has passed in its evolution: "The first definite eye-spot that we yet know of is a little colored speck at the base of the tentacles of some of the Hydromedusæ, jelly-fish in common parlance. They are only spots of pigment, and we should not know they were attempts at eyes were it not that some relations, the Discophora, have little refractive bodies in their pigment spots, and these refractive bodies resemble the crystalline cones of animals a little higher in the scale. In the next class (Vermes), including all worms, we find only pigment spots in the lowest; then pigment spots with a nerve-fibre ending in them; pigment spots with rod-shaped cells, with crystalline rods; pigment spots with crystalline cones. Next, the cones begin to be arranged radially; and in the Alciopidæ the eye has become a sphere with a lens and a vitreous body, layer of pigment, layer of rods, and optic nerve. To mark the evolution definitely in another way, we find the more highly developed eye of the adult appearing as a pigment spot in the embryo, so that both the evolution of the race and the evolution of the individual tell the same story. In the Echinoderma (sea-urchins, star-fishes) we find only pigment spots in the lower forms, but in the higher the rod-shaped cells, the transparent cones projecting from pigment cells. In the Arthropoda (lobsters, insects, etc.,) the advance continues from the Vermes. The retina is formed more definitely than in the Alciopidæ, and the eye becomes more complex. The compound eye is an attempt at grouping many cones together, and is found in the higher members of this sub-kingdom. In the lowest vertebrate, the Amphioxus, the eye is a mere pigment spot, but in the others the more complex forms are taken up and carried on to the comparative perfection of the mammalian eye" ("Eyes and Ears", pp. 9, 10). And be it noted that in the most complex and highly developed eye there is still the same relation of pigment layer, rod layer, cone layer,