DIFFICULTIES ON THEORY 169 



cornea^ the inner one divided into facets, within each 

 of which there is a lens-shaped swelling. In other 

 crustaceans the transparent cones which are coated by 

 pigment, and which properly act only by excluding 

 lateral pencils of light, are convex at their upper ends 

 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 diver- 

 sity 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 inexplic- 

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

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

 structure 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 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, 



