226 ORGANS OF EXTREME PERFECTION. [Chap. YI. 



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 diver- 

 sified 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 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 mem- 

 brane, into an optical instrument as perfect as is 

 possessed by anv 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 outdit 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 

 simultaneous, if they were extremely slight and gradual. 

 Different kinds of modification would, also, serve for the 



