front of an aquarium. The long rubber tube was set in action as a siphon
and the sheet of rubber placed against the glass. As long as water was
running through the siphon the sheet of rubber remained pressed against
the glass and supported. As soon as the current of water was stopped the
apparatus fell to the bottom of the tank. In this model water passed out
from beneath the rubber through the glass tube attached to the siphon and
passed in by the opposite glass tube, and at the sides of it. The latter
tube represented the respiratory channel of the fish, and the space
between tube and rubber represented the spaces between the head of the
fish and the vertical surface to which it clung.

In the fish the marginal fins not only extend to the base of the tail, but
are broader at the posterior end than elsewhere, whereas in other
Flat-fishes the posterior part of the marginal fins are the narrowest
parts. The shape of the fins and the breadth of the body posteriorly,
then, are adaptations which have a definite function, that of enabling the
fish to adhere to vertical surfaces. But, on the other hand, the extension
of the marginal fins in a transverse direction beneath the tail has no use
in the process of adhesion, nor has any other use been found for it. It is
a generic character, so far as we know, without utility. On the other
hand, it is very probable that this subcaudal extension of the fins is
merely a result of the posterior extension and enlargement of these fins
which has taken place in the evolution of the adaptation. If the
Lamarckian explanation of adaptation were true, it would be possible to
understand that the constant movements of the fins and muscles by which
the adhesion was effected caused a longitudinal growth of the fins in
excess of the length actually required, and that this extra growth
extended on to the body beneath the tail, although the small flaps on the
lower side were not necessary to the new function which the fins
performed.