Then we have an optic nerve and pigment cells; then we find a
hollow filled with gelatinous substance of a convex form--the first
rudiment of a lens. Many of the succeeding steps are lost, as would
necessarily be the case, owing to the great advantage of each
modification which gave increased distinctness of vision, the creatures
possessing it inevitably surviving, while those below them became
extinct. But we can well understand how, after the first step was taken,
every variation tending to more complete vision would be preserved till
we reached the perfect eye of birds and mammals. Even this, as we know,
is not absolutely, but only relatively, perfect. Neither the chromatic
nor the spherical aberration is absolutely corrected; while long-and
short-sightedness, and the various diseases and imperfections to which
the eye is liable, may be looked upon as relics of the imperfect
condition from which the eye has been raised by variation and natural
selection.
These few examples of difficulties as to the origin of remarkable or
complex organs must suffice here; but the reader who wishes further
information on the matter may study carefully the whole of the sixth
and seventh chapters of the last edition of _The Origin of Species_, in
which these and many other cases are discussed in considerable detail.
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