Now if we suppose this process, which in the
young is completed in a few days or weeks, to have been spread over
thousands of generations during the development of these fish, those
usually surviving whose eyes retained more and more of the position into
which the young fish tried to twist them, the change becomes
intelligible; though it still remains one of the most extraordinary
cases of degeneration, by which symmetry--which is so universal a
characteristic of the higher animals--is lost, in order that the
creature may be adapted to a new mode of life, whereby it is enabled the
better to escape danger and continue its existence.
The most difficult case of all, that of the eye--the thought of which
even to the last, Mr. Darwin says, "gave him a cold shiver"--is
nevertheless shown to be not unintelligible; granting of course the
sensitiveness to light of some forms of nervous tissue. For he shows
that there are, in several of the lower animals, rudiments of eyes,
consisting merely of pigment cells covered with a translucent skin,
which may possibly serve to distinguish light from darkness, but nothing
more.
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