192 ORIGIN OF SPECIES 



forming eye ; for we have only to place the naked extremity 

 of the optic nerve, which in some of the lower animals lies 

 deeply buried in the body, and in some near the surface, at 

 the right 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 opti- 

 cal contrivance. With insects it is now known that the nu- 

 merous facets on the cornea of their great compound eyes 

 form true lenses, and that the cones include curiously modi- 

 fied nervous filaments. But these organs in the Articulata 

 are so much diversified that Miiller 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 graduated 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 diffi- 

 culty ceases to be very great in believing that natural selec- 

 tion may have converted the simple apparatus of an optic 

 nerve, coated with pigment and invested by transparent mem- 

 brane, 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 explained 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 still preserve it as a perfect instrument, 

 many changes would have to be effected simultaneously, 

 which, it is assumed, could not be done through natural 

 selection ; but as I have attempted to show in my work on the 

 variation of domestic animals, it is not necessary to suppose 

 that the modifications were all simultaneous, if they were ex- 

 tremely slight and gradual. Different kinds of modification 

 would, also, serve for the same general purpose : as Mr. Wal- 



