ORGANS OF EXTREME PERFECTION. 173 



necessary to suppose that the modifications were all 

 simultaneous, if they were extremely slight and gradual. 

 Different kinds of modification would, also, serve for the 

 same general purpose: as Mr. Wallace has remarked, *' If 

 a len 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 regularity of curva- 

 ture will be an improvement. So the contraction 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 

 division of the animal kingdom, namely, the Vertebrata, 

 we can start from an eye so simple, that it consists, as in 

 the lancelet, of a little sack of transparent skin, furnished 

 with a nerve and lined with pigment, but destitute of any 

 other apparatus. In fishes and reptiles, as Owen has re- 

 marked, " The range of gradation 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 accumula- 

 tion 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 selec- 

 tion to so startling a length. 



It is scarcely possible to avoid comparing the eye with 

 a telescope. We know that this instrument has been per- 

 fected 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 



