en transformation
of the eye from one side to the other was inconceivable, while, if the
transit were gradual the first step could be of no use, since this would
not remove the eye from the lower side. But, as Mr. Darwin shows by
reference to the researches of Malm and others, the young of these fish
are quite symmetrical, and during their growth exhibit to us the whole
process of change. This begins by the fish (owing to the increasing
depth of the body) being unable to maintain the vertical position, so
that it falls on one side. It then twists the lower eye as much as
possible towards the upper side; and, the whole bony structure of the
head being at this time soft and flexible, the constant repetition of
this effort causes the eye gradually to move round the head till it
comes to the upper side. 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. 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