The lower eye would, also, have been liable to be abraded by the
sandy bottom. That the Pleuronectidae are admirably adapted by their
flattened and asymmetrical structure for their habits of life, is manifest
from several species, such as soles, flounders, etc., being extremely
common. The chief advantages thus gained seem to be protection from their
enemies, and facility for feeding on the ground. The different members,
however, of the family present, as Schiodte remarks, "a long series of
forms exhibiting a gradual transition from Hippoglossus pinguis, which does
not in any considerable degree alter the shape in which it leaves the ovum,
to the soles, which are entirely thrown to one side."
Mr. Mivart has taken up this case, and remarks that a sudden spontaneous
transformation in the position of the eyes is hardly conceivable, in which
I quite agree with him. He then adds: "If the transit was gradual, then
how such transit of one eye a minute fraction of the journey towards the
other side of the head could benefit the individual is, indeed, far from
clear. It seems, even, that such an incipient transformation must rather
have been injurious." But he might have found an answer to this objection
in the excellent observations published in 1867 by Malm. The
Pleuronectidae, while very young and still symmetrical, with their eyes
standing on opposite sides of the head, cannot long retain a vertical
position, owing to the excessive depth of their bodies, the small size of
their lateral fins, and to their being destitute of a swim-bladder.
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