ISO ORGANS OF EXTKEME PERFECTION Chap. VI. 



mucli diversified that Milller formerly made three main chxsses 

 of compound eyes -with seven subdivisions, l^esidcs a fourth 

 main chiss of ag-j^regated simple eyes. 



AVhcn we reflect on these facts, here f^ivcn too briefly, with 

 respect to the wide, diversified, and graduated range of struct- 

 in-e in the eyes of the lower animals ; and when we bear in 

 mind how small the number of all the forms now living must 

 he in comparison with those which have become extinct, the 

 ditliculty ceases to be very great in believing that natural se- 

 lection 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 

 an}' member of the great 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 bod- 

 ies of facts, otherwise inexplicable, can be explained by the 

 theory of descent with modification ; he ought to admit that a 

 structure even as perfect as an eagle's eye might be formed by 

 natural selection, although in tliis case he does not know the 

 transitional states. It has been objected that, in order to mod- 

 ify 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 do- 

 mestic animals, it is not necessary to suppose that all the modi- 

 fications were simultaneous, if tliey were extremely slight and 

 gradual. Even in the most highly-organized division of the 

 animal kingdom, namely, the Vcrtebrata, 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 both fishes 

 and reptiles, as Owen has remarked, " the range of gradations 

 of dioptric structures is very great." It is a significant fiict 

 that even in man, according to tlie high authority of Virchow, 

 tlic beautiful crj^stalline lens is formed in tlie embryo by an 

 accumulation of epidermic cells, lying in a sack-like fold of the 

 skin ; and tlie vitreous body is formed from embryonic subcu- 

 taneous tissue?. It is indeed indispensable, in order to arrive at 

 a just conclusion regarding the formation of the eye, with all 

 its marvellously perfect characters, that the reason should con- 

 quer the imagination ; but I have felt this difficulty far too 

 keenly to be surprised at any degree of hesitation in extend 

 ing the principle of natural selection to so startling a length. 



