DIFFICULTIES OF THE THEORY 253 



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 5 



suppose that the modifications were all simultaneous, >^ 

 if they were extremely slight and gradual. DifiEerent ^^ 

 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 n^ 

 amended either by an alteration of curvature or an al- 

 teration 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 per- 

 fected at any stage of the construction of the instru- 

 ment." Within the highest division of the animal king- 

 dom, 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 

 formed in the embryo by an accumulation of epidermic 

 cells, lying in a sack-like fold of the skin; and the vit- 

 reous body is formed from embryonic sub-cutaneous tis- 

 sue. To arrive, however, at a just conclusion regarding 

 the formation of the eye, with all its marvellous yet not 



