s
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. 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 proportion to those
which have become extinct, I can see no very great difficulty (not more
than in the case of many other structures) in believing that natural
selection has converted the simple apparatus of an optic nerve merely
coated with pigment and invested by transparent membrane, into an
optical instrument as perfect as is possessed by any member of the great
Articulate class.

He who will go thus far, if he find on finishing this treatise that
large bodies of facts, otherwise inexplicable, can be explained by the
theory of descent, ought not to hesitate to go further, and to admit
that a structure even as perfect as the eye of an eagle might be formed
by natural selection, although in this case he does not know any of the
transitional grades. His reason ought to conquer his imagination; though
I have felt the difficulty far too keenly to be surprised at any degree
of hesitation in extending the principle of natural selection to such
startling lengths.

It is scarcely possible to avoid comparing the eye to a telescope.
We know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that the
eye has been formed by a somewhat analogous process. But may not this
inference be presumptuous? Have we any right to assume that the Creator
works by intellectual powers like those of man? If we must compare the
eye to an optical instrument, we ought in imagination to take a thick
layer of transparent tissue, with a nerve sensitive to light beneath,
and then suppose every part of this layer to be continually changing
slowly in density, so as to separate i