Chap. YL] ORGANS OF EXTREME PERFECTION. 227 



same 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 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 remarked, "the range of gradations of dioptric 

 structures i? very great." It is a significant fact that 

 even in man, according to the high authority of 

 Yirchow, 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 



