Chap. vi. Organs of extreme Perfection. 145 



organs in the Articulata are so much diversified that Mttller 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 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 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 throng e. 

 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 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 kingdom, namely, the Vertebrata, we can start from an 

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

 transparent skin, furnished with a nerve and lined with pigment, 

 but destitute of any other apparatus. In fishes and reptiles, as 

 Owen has remarked, "the range of gradations of dioptric structures 

 is very great." It is a significant fact that even in man, according 

 to the high authority of Virchow, the beautiful crystalline lens is 

 iormed m the embryo by an accumulation of epidermic cells, lying 

 m a sack-like fold of the skin ; and the vitreous body is formed 



L 



