Origin of Species Variorum

Comparison with 1872

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 their great compound eyes form true lenses, and that the cones include curiously modified nervous filaments. ↑ Altogether so diversified is the structure of the compound eyes, that Müller makes three main classes, with no less than seven sub-divisions of structure; he makes a fourth main class, namely, "aggregates" of stemmata, and he adds that "this is the transition-form between the mosaic-like compound eyes unprovided with a concentrating apparatus, and organs of vision with such an apparatus." With 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 living animals is in proportion to

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. When we reflect on these facts, here given 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