We may also attribute to the inherited
effects of use the fact of the mouth in several kinds of flat-fish being
bent towards the lower surface, with the jaw bones stronger and more
effective on this, the eyeless side of the head, than on the other, for the
sake, as Dr. Traquair supposes, of feeding with ease on the ground.
Disuse, on the other hand, will account for the less developed condition of
the whole inferior half of the body, including the lateral fins; though
Yarrel thinks that the reduced size of these fins is advantageous to the
fish, as "there is so much less room for their action than with the larger
fins above." Perhaps the lesser number of teeth in the proportion of four
to seven in the upper halves of the two jaws of the plaice, to twenty-five
to thirty in the lower halves, may likewise be accounted for by disuse.
>From the colourless state of the ventral surface of most fishes and of many
other animals, we may reasonably suppose that the absence of colour in
flat-fish on the side, whether it be the right or left, which is under-
most, is due to the exclusion of light. But it cannot be supposed that the
peculiar speckled appearance of the upper side of the sole, so like the
sandy bed of the sea, or the power in some species, as recently shown by
Pouchet, of changing their colour in accordance with the surrounding
surface, or the presence of bony tubercles on the upper side of the turbot,
are due to the action of the light.
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