188 DIFFICULTIES ON THEORY. Chap. VI. 



and must act by convergence ; and at their lower ends 

 there seems to be an imperfect vitreous substance. With 

 these facts, here far too briefly and imperfectly given, 

 which show that there is much graduated diversity in 

 the eyes of living crustaceans, and bearing in mind how 

 small the number of living animals is in proportion to 

 those which have become extinct, I can see no very 

 great difficulty (not more than in the case of many 

 other structures) in believing that natural selection 

 has converted the simple apparatus of an optic nerve 

 merely coated with pigment and invested by transparent 

 membrane, into an optical instrument as perfect as is 

 possessed by any member of the great Articulate class. 



He who will go thus far, if he find on finishing this 

 treatise that large bodies of facts, otherwise inexpli- 

 cable, can be explained by the theory of descent, ought 

 not to hesitate to go further, and to admit that a struc- 

 ture even as perfect as the eye of an eagle might be 

 formed by natural selection, although in this case he 

 does not know any of the transitional grades. His 

 reason ought to conquer his imagination ; though I 

 have felt the difficulty far too keenly to be surprised 

 at any degree of hesitation in extending the principle 

 of natural selection to such startling lengths. 



It is scarcely possible to avoid comparing the eye to 

 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 

 i may not tins inference be presumptuous? Have we 

 any right to assume that the Creator works by intel- 

 lectual powers like those of man ? If we must compare 

 the eye to an optical instrument, we ought in imagina- 

 tion to take a thick layer of transparent tissue, with a 

 nerve sensitive to light beneath, and then suppose every 



