With the Pleuronectidae, on the
other hand, the older they grow the more habitually they rest on one side,
owing to the increasing flatness of their bodies, and a permanent effect is
thus produced on the form of the head, and on the position of the eyes.
Judging from analogy, the tendency to distortion would no doubt be
increased through the principle of inheritance. Schiodte believes, in
opposition to some other naturalists, that the Pleuronectidae are not quite
symmetrical even in the embryo; and if this be so, we could understand how
it is that certain species, while young, habitually fall over and rest on
the left side, and other species on the right side. Malm adds, in
confirmation of the above view, that the adult Trachypterus arcticus, which
is not a member of the Pleuronectidae, rests on its left side at the
bottom, and swims diagonally through the water; and in this fish, the two
sides of the head are said to be somewhat dissimilar. Our great authority
on Fishes, Dr. Gunther, concludes his abstract of Malm's paper, by
remarking that "the author gives a very simple explanation of the abnormal
condition of the Pleuronectoids."
We thus see that the first stages of the transit of the eye from one side
of the head to the other, which Mr. Mivart considers would be injurious,
may be attributed to the habit, no doubt beneficial to the individual and
to the species, of endeavouring to look upward with both eyes, while
resting on one side at the bottom.
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