380 REVIEWS—ON THE ORIGIN OF SPECIES. 
apparently the normal structure. Fishes and other organisms, may, 
it is true, have lived at earlier periods than Geology indicates ; but 
that view, whether true or false, is purely hypothetical, is opposed to 
the results of actual observation, and cannot therefore be legitimately 
introduced into an argument of this kind. But we proceed to our 
quotation, the last that our decreasing space will allow us to give. 
“To suppose that the eye, with all its inimitable contrivances for adjusting the 
focus to different distances, for admitting different amounts of light, and for the 
correction of spherical and chromatic aberration could have been formed by 
natural selection, seems, I freely confess, absurd in the highest possible degree. 
Yet reason tells me, that if numerous gradations from a perfect and complex eye 
to one very imperfeet and simple, each grade being useful t» its possessor, can be 
shown to exist; if, further, the eye does vary ever so slightly, and the vari- 
ations be inherited, which is certainly the case; and if any variation or modifica- 
tion 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 insaperable by our imagination, can hardly be 
considered real. How a nerve comes to be sensitive to light, hardly concerns ug 
more than how life itself first originated ; but I 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 iu 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 hav- 
ing 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 thishead. 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 is a double cornea, the inner ones 
divided into facets, within reach of which there is alens-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. With these facts, here far too briefly and imperfectly 
given, which show that there is much graduated diversity in the eyes of living 
crustaceans, and bearing in mind how small the number of living animals is in 
