Chap. VI. ORGANS OF EXTREME PERFECTION. 
187 
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 vibra¬ 
tions 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 condi¬ 
tion. 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 
and must act by convergence ; and at their lower ends 
there seems to be an imperfect vitreous substance. 
