NATUR.\L SELECTION 249 



sack of transparent skin, furnished with a nerve and lined with pig- 

 ment, but destitute of any other apparatus. 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- 

 Uke 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 starthng 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 Creator 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 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 thick- 

 nesses, placed at different distances from each other, and with the sur- 

 faces 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 dis- 

 tincter 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 beUeve that a Uving optical instrument 

 might thus be formed as superior to one of glass, as the works of the 

 Creator are to those of man ? 



