NATURAL SELECTION 239 



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 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 difliculty 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 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 fur- 

 ther, 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 simulta- 

 neous, 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 lens 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 

 converge 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 at any 

 stage of the construction of the instrument.'' Within the highest 

 division of the animal kingdon, namely, the Vertebrata, we can start 

 from an eye so simple, that it consists, as in the lancelet, of a little 



