Chap. VI.] ORGANS OP EXTREME PERFECTION. 227 



general purpose: as Mr. Wallace 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 regularity 

 of curvature will be an improvement. So the con- 

 traction 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 per- 

 fected at any stage of the construction of the instru- 

 ment." 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 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, ac- 

 cording to the high authority of Virchow, the beautiful 

 crystalline lens is formed in the embryo by an accumu- 

 lation of epidermic cells, lying in a sack-like fold of 

 the skin; and the vitreous body is formed from embry- 

 onic 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 



