Origin of Species Variorum

Comparison with 1866

the nerve are filled, as described by the author just quoted, with transparent gelatinous matter, and this projects outwardly with a convex surface, like the cornea in the higher animals. He suggests that this structure serves not to form an image, but only to concentrate the luminous rays and render their perception more perfect. In this concentration of the rays we gain the first and by far the most important step towards the formation of a true or picture-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 must be formed on it. ↑

In the great class of the Articulata, if we look for gradations, we may start from an optic nerve simply coated with pigment, which sometimes forms a sort of pupil, but is destitute of a lens or other optical contrivance. From this point we have to make a rather wider stride than in the case of the above-mentioned star-fish, and we come to certain Crustaceans in which the eyes are covered by a double cornea,—the external membrane smooth and the internal one divided into facets,—within the substance of which, as Milne Edwards states, " renflemens lenticulaires paraissent s'être développés; " and these lenses can sometimes be detached in a layer distinct from the cornea. With insects it is now known that the numerous cones surrounded by pigment, which form the great compound eyes, are filled with transparent refractive matter, and these cones produce images; but in addition, in certain beetles the facets of the cornea are slightly convex both externally and internally,—that is, are lens-shaped. Altogether so diversified is the structure of the eye in the Articulata that Müller makes three main classes, with seven subdivisions, of compound eyes, and he adds a fourth main class of aggregated simple-eyes. ↑ ↑ With these facts, here far too briefly and imperfectly, with respect to the wide, diversified, and graduated range of structure in the eyes of the existing Articulata; and when we bear in mind how small the number of all living forms must be in comparison with those which have become extinct, I can see no very great (not more so than in the case of many other structures) in believing that natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the great Articulate class.

the nerve are filled, as described by the author just quoted, with transparent gelatinous matter, projecting with a convex surface, like the cornea in the higher animals. He suggests that this .. serves not to form an image, but only to concentrate the luminous rays and render their perception more easy. In this concentration of the rays we gain the first and by far the most important step towards the formation of a true, .. picture-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 optical contrivance. With insects it is now known that the numerous facets on the cornea of the great compound eyes form true lenses, and that the cones include curiously modified nervous filaments. But these organs in the Articulata are so much diversified that Müller formerly made three main classes ... of compound eyes with seven subdivisions, besides a fourth main class of aggregated simple eyes. When we reflect on these facts, here given 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 the forms now living must be in comparison with those which have become extinct, the difficulty ceases to be very great ... in believing that natural selection may have converted the simple apparatus of an optic nerve, .. coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the great Articulate Class.