Seite - 193 - in The Origin of Species

^ORGANS OF EXTREME PERFECTION 193 lace has remarked, '"if a lens has too short or too long a focus, it may be amended either by an alteration of curvature, or an alteration of density; if the curvature be irregular, and the rays do not converge to a point, then any increased regu- larity of curvature will be an improvement. So the contrac- tion of the iris and the muscular movements of the eye are neither of them essential to vision, but only improvements which might have been added and perfected at any stage of the construction of the instrument." Within the highest di- vision of the animal kingdom, namely, the Vertebrata. we can start from an eye so simple, that it consists, as in the lance- let, of a little sack of transparent skin, furnished with a nerve and lined with pigment, but destitute of any other ap- paratus. In fishes and reptiles, as Owen has remarked, "the range of gradations of dioptric structures is very great." It is a significant fact that even in man, according to the high authority of Virchow, the beautiful crystalline lens is formed in the embryo by an accumulation of epidermic cells, lying in a sack-like fold of the skin ; and the vitreous body is formed from embryonic sub-cutaneous tissue. 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. It is scarcely possible to avoid comparing the eye with a telescope. We know that this instrument has been perfected by the long-continued efforts of the highest human intellects ; and we naturally infer that the eye has been formed by a somewhat analogous process. But may not this inference be presumptuous? Have we any right to assume that the Cre- ator works by intellectual powers like those of man? 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 be- neath, and then suppose every part of this layer to be con- tinually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at dillor- ent distances from each other, and with tlie surfaces of each G—lie XI