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 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.
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