168 ON THE ORIGIN OF SPECIES 



reason tells me, that if numerous gradations from a 

 perfect and complex eye to one very imperfect and 

 simple, each grade being useful to its possessor, can 

 be shown to exist ; if further, the eye does vary 

 ever so slightly, and the variations be inherited, 

 which is certainly the case ; and if any variation or 

 modification in the organ be ever useful to an animal 

 under changing conditions of life, then the difficulty 

 of believing that a perfect and complex eye could be 

 formed by natural selection, though insuperable by 

 our imagination, can hardly be considered real. How 

 a nerve comes to be sensitive to light, hardly concerns 

 us more than how life itself first originated ; but I may 

 remark that several facts make me suspect that any 

 sensitive nerve may be rendered sensitive to light, and 

 likewise to those coarser vibrations of the air which 

 produce sound. 



In looking for the gradations by which an organ in 

 any species has been perfected, we ought to look 

 exclusively to its lineal ancestors ; but this is scarcely 

 ever possible, and we are forced in each case to look to 

 species of the same group, that is to the collateral 

 descendants from the same original parent-form, in 

 order to see what gradations are possible, and for the 

 chance of some gradations having been transmitted 

 from the earlier stages of descent, in an unaltered or 

 little altered condition. Amongst existing Vertebrata, 

 we find but a small amount of gradation in the 

 structure of the eye, and from fossil species we can 

 learn nothing on this head. In this great class we 

 should probably have to descend far beneath the 

 lowest known fossiliferous stratum to discover the 

 earlier stages, by which the eye has been perfected. 



In the Articulata we can commence a series with an 

 optic nerve merely coated with pigment, and without 

 any other mechanism ; and from this low stage, 

 numerous gradations of structure, branching off in two 

 fundamentally different lines, can be shown to exist, 

 until we reach a moderately high stage of perfection. 

 In certain crustaceans, for instance, there i3 a double 



