Seite - 192 - in The Origin of Species

192 ORIGIN OF SPECIES forming eye ; for we have only to place the naked extremity of the optic nerve, which in some of the lower animals lies deeply buried in the body, and in some near the surface, at the right distance from the concentrating apparatus, and an image will be formed on it. In the great class of the Articulata, we may start from an optic nerve simply coated with pigment, the latter sometimes forming a sort of pupil, but destitute of a lens or other opti- cal contrivance. With insects it is now known that the nu- merous facets on the cornea of their great compound eyes form true lenses, and that the cones include curiously modi- fied nervous filaments. But these organs in the Articulata are so much diversified that Miiller formerly made three main classes with seven subdivisions, besides a fourth main class of aggregated simple eyes. When we reflect on these facts, here given much too briefly, with respect to the wide, diversified, and graduated range of structure in the eyes of the lower animals ; and when we bear in mind how small the number of all living forms must be in comparison with those which have become extinct, the diffi- culty ceases to be very great in believing that natural selec- tion may have converted the simple apparatus of an optic nerve, coated with pigment and invested by transparent mem- brane, into an optical instrument as perfect as is possessed by any member of the Articulate Class. He who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this volume that large bodies of facts, otherwise inexplicable, can be explained by the theory of modification through natural selection ; he ought to admit that a structure even as perfect as an eagle's eye might thus be formed, although in this case he does not know the transitional states. It has been objected that in order to modify the eye and still preserve it as a perfect instrument, many changes would have to be effected simultaneously, which, it is assumed, could not be done through natural selection ; but as I have attempted to show in my work on the variation of domestic animals, it is not necessary to suppose that the modifications were all simultaneous, if they were ex- tremely slight and gradual. Different kinds of modification would, also, serve for the same general purpose : as Mr. Wal-