would have served exclusively as
a sifting apparatus. From this stage, in which the lamellae would be
two-thirds of the length of the plates of baleen in the Balaenoptera
rostrata, gradations, which may be observed in still-existing Cetaceans,
lead us onward to the enormous plates of baleen in the Greenland whale.
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. 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 le