“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.” The following sketch that Darwin gives to show how he imagined the vertebrate eye to have been formed is very instructive, as illustrating how he supposed that natural selection acts:— “If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further we must suppose that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparent layers; and carefully preserving each which, under varied circumstances, in any way or in any degree, tends to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million; each to be preserved until a better one is produced, and then the old ones to be all destroyed. In living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man.” We may conclude in Darwin’s own words:— “To arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but I have felt the difficulty far too keenly to be surprised at others hesitating to extend the principle of natural selection to so startling a length.” The electric organs, present in several fish, offer a case of special difficulty to the selection theory. When well developed, as in the Torpedo and in Gymnotus, it is conceivable that it may serve as an organ of defence, but in other forms the shock is so weak that it is not to be supposed that it can have any such function. Romanes, who in many ways was one of the stanchest followers of Darwin, admits that, so far as he can see, the evolution of the electric organs cannot be explained by the selection theory. Darwin offers no explanation, but bases his defence on the grounds that we do not know of what use this organ can be to the animal.