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.
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