Ciiap. VI. ORGANS OF EXTREME PERFECTION. 187 



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 exclu- 

 sively to its lineal ancestors ; but this is scarcely ever 

 possible, and we are forced in each case to look to spe- 

 cies of the same group, that is to the collateral descend- 

 ants 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, nume- 

 rous gradations of structure, branching off in two fun- 

 damentally different lines, can be shown to exist, until 

 we reach a moderately high stage of perfection. In 

 certain crustaceans, for instance, there is a double 

 cornea, the inner one divided into facets, within each 

 of which there is a lens-shaped swelling. In other 

 crustaceans the transparent cones which are coated by 

 pigment, and which properly act only by excluding 

 lateral pencils of light, are convex at their upper ends 



