Origin of Species Variorum

Comparison with 1861

perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that nerves sensitive to touch may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound.

In looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and we are forced in each case to look to species of the same group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condi- tion. ↑ Amongst existing Vertebrata, we find but a small amount of gradation in the structure of the eye, and from fossil species we can learn nothing on this head. In this great class we should probably have to descend far beneath the lowest known fossiliferous stratum to discover the earlier stages, by which the eye has been perfected.

In the great kingdom of the Articulata, we can start from an optic nerve, simply coated with pigment, which sometimes forms a sort of pupil, but is destitute of a lens or any other optical mechanism. From this rudimentary eye, which can distinguish light from darkness, but nothing else, there is an advance towards perfection along two lines of structure, which Müller thought were fundamentally different; namely,— firstly, stemmata, or the so-called "simple eyes," which have a lens and cornea; and secondly, "compound eyes," which seem to act mainly by excluding all the rays from each point of the object viewed, except the pencil that comes in a line perpendicular to the convex retina. In compound eyes, besides endless differences in the form, proportion, number, and position of the transparent cones coated by pigment, and which act by exclusion, we have additions of a more or less perfect concentrating apparatus: thus in the eye of the Meloe the facets of the cornea are "slightly convex both externally and internally— that is, lens-shaped." In many crustaceans there are two corneæ— the external smooth, and the internal divided into facets— within the substance of which, as Milne Edwards says, "renflemens lenticulaires paraissent s'être développés;" and sometimes these lenses can be detached in a layer distinct from the cornea. The transparent cones coated with pigment, which were supposed by Müller to act solely by ex- cluding divergent pencils of light, usually adhere to the cornea, but not rarely they are separate from it, and have their free ends convex; and in this case they must act as converging lenses. Altogether so diversified is the structure of the compound eyes,

perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself .. originated; but I may remark that, as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility.

In searching for the gradations through which an organ in any species has been perfected, we ought to look exclusively to its lineal progenitors; but this is scarcely ever possible, and we are forced ... to look to other species and genera of the same group, that is to the collateral descendants from the same .. parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted ... in an unaltered or little altered condition. But the state of the same organ in distinct classes may incidentally throw light on the steps by which it has been perfected. .. .. .. .. ↑

The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells and covered by translucent skin, but without any lens or other refractive body. We may, however, according to M. Jourdain, descend even a step lower and find aggregates of pigment-cells, apparently serving as .. organs of vision, without any nerves, and resting merely on sarcodic tissue. .. .. .. .. .. Eyes of the above simple nature are not capable of distinct vision, and serve only to distinguish light from darkness. In certain star-fishes, small depressions in the layer of pigment which surrounds 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 their 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