252 TEE ORiaiN OF SPECIES 



distance from the concentrating apparatus, and an image 

 will be formed on it. 



In the great class of the Articulata, we may start 

 from an optic nerve simply coated with pigment, the 

 latter sometimes forming a sort of pupil, but destitute 

 of a lens or other optical contrivance. With insects it 

 is now known that the numerous facets on the cornea of 

 their great compound eyes form true lenses, and that the 

 cones include curiously modified nervous filaments. But 

 these organs in the Articulata are so much diversified 

 that Muller formerly made three main classes with seven 

 subdivisions, besides a fourth main class of aggregated 

 simple eyes. 



When we reflect on these facts, here given much too 

 briefly, with respect to the wide, diversified, and gradu- 

 ated range of structure in the eyes of the lower animals; 

 and when we bear in mind how small the number of all 

 living forms must be in comparison with those which 

 have become extinct, the difficulty ceases to be very 

 great in believing that natural selection may have con- 

 verted the simple apparatus of an optic nerve, coated 

 with pigment and invested by transparent membrane, into 

 an optical instrument as perfect as is possessed by any 

 member of the Articulate Class. 



He who will go thus far, ought not to hesitate to go 

 one step further, if he finds on finishing this volume that 

 large bodies of facts, otherwise inexplicable, can be ex- 

 plained by the theory of modification through natural 

 selection; he ought to admit that a structure even as 

 perfect as an eagle's eye might thus be formed, although 

 in this case he does not know the transitional states. It 

 has been objected that, in order to modify the eye and 



