Page - 191 - in The Origin of Species

ORGANS OF EXTREME PERFECTION 191 ations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under chang- ing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be consid- ered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life it- self originated ; but I may remark that, as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it docs not seem impossible that certain sen- sitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensi- bility. 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 pos- sible, 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 incident- ally 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, appar- ently 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 sur- face, 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 fomiation of a true, picture-