thout 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. {188} 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 chang