172 ORGANS OF EXTREME PERFECTION. 



render their perception more easy. In this concentration 

 of the rays we gain the first and by far the most important 

 step toward the formation of a true, picture-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 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 com- 

 pound eyes form true lenses, and that the cones include 

 curiously modified 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. 



AVhen we refiect on these facts, here given much too 

 briefly, with respect to the wide, divei'sified, 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 

 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, manv 

 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 



