In another case which has been adduced as one of special difficulty, a
more complete explanation is given. Soles, turbots, and other flatfish
are, as is well known, unsymmetrical. They live and move on their sides,
the under side being usually differently coloured from that which is
kept uppermost. Now the eyes of these fish are curiously distorted in
order that both eyes may be on the upper side, where alone they would be
of any use. It was objected by Mr. Mivart that a sudden 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.
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