148 THE ORIGIN OF SPECIES 



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 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 or- 

 gans in the Articulata are so much diversified that Muller for- 

 merly 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 struc- 

 ture 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 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 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 pre- 

 serve 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 

 extremely slight and gradual. Different kinds of modification 

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

 has remarked, "If a len has too short or too long a focus, it may 

 be amended either by an alteration of curvature, or an alteration 

 of density; if the curvature be irregular, and the rays do not con- 

 verge to a point, then any increased regularity of curvature will 

 be an improvement. So the contraction of the iris and the muscular 

 movements of the eye are neither of them essential to vision, but 

 only improvements which might have been added and perfected 



