162 ORGANS OF EXTREME PERFECTION. 



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 concentrat- 

 ing 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 some- 

 times forming a sort of pupil, but destitute of 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 modi- 

 fied nervous filaments. But these organs in the Articulata 

 are so much diversified that Mtiller 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 difficulty ceases to be very great in believing 

 that natural selection may have converted the simple appara- 

 tus 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 Articulata 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 simul- 

 taneously, 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 neces- 

 sary to suppose that the modifications were all simultaneous, 

 if they were extremely slight and gradual. Different kinds 

 of modification would, also, serve for the same general pur- 

 pose : as Mr. Wallace has remarked, " If a len has too short 

 or too long a focus, it may be amended either by an altera- 

 tion of curvature, or an alteration of density ; if the curva- 

 ture be irregular, and the rays do not converge to a point, 

 then any increased regularity of curvature will be an inv 



