Nor is there the least reason to doubt
that each step in this scale might have been as serviceable to certain
ancient Cetaceans, with the functions of the parts slowly changing during
the progress of development, as are the gradations in the beaks of the
different existing members of the duck-family. We should bear in mind that
each species of duck is subjected to a severe struggle for existence, and
that the structure of every part of its frame must be well adapted to its
conditions of life.
The Pleuronectidae, or Flat-fish, are remarkable for their asymmetrical
bodies. They rest on one side--in the greater number of species on the
left, but in some on the right side; and occasionally reversed adult
specimens occur. The lower, or resting-surface, resembles at first sight
the ventral surface of an ordinary fish; it is of a white colour, less
developed in many ways than the upper side, with the lateral fins often of
smaller size. But the eyes offer the most remarkable peculiarity; for they
are both placed on the upper side of the head. During early youth,
however, they stand opposite to each other, and the whole body is then
symmetrical, with both sides equally coloured. Soon the eye proper to the
lower side begins to glide slowly round the head to the upper side; but
does not pass right through the skull, as was formerly thought to be the
case. It is obvious that unless the lower eye did thus travel round, it
could not be used by the fish while lying in its habitual position on one
side.
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