Origin of Species Variorum

In searching for the gradations through which any 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 in each case to look to other species and genera 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 condition. ↑ But the state of the same organ even in the other main divisions of the organic world 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, 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 an organ of vision, but which rest merely on sarcodic tissue not furnished with any nerve. Eyes of the above simple nature are not capable of distinct vision, but serve merely 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, 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