ceasingly repeated contracts the habit of
extending; so that after a while the broad membranes which connect
the digits of ducks, geese, etc., are formed as we see them. The
same efforts made in swimming--_i.e._, in pushing back the water, in
order to advance and to move in this liquid--have likewise extended
the membrane situated between the digits of the frogs, the
sea-turtles, the otter, beaver, etc.

"On the contrary, the bird whose mode of life habituates it to perch
on trees, and which is born of individuals who have all contracted
this habit, has necessarily the digits of the feet longer and shaped
in another way than those of the aquatic animals which I have just
mentioned. Its claws, after a while, became elongated, pointed, and
curved or hook-like in order to grasp the branches on which the
animal often rests.

"Likewise we see that the shore bird, which is not inclined to swim,
and which moreover has need of approaching the edge of the water to
find there its prey, is in continual danger of sinking in the mud.
Now, this bird, wishing to act so that its body shall not fall into
the water, makes every effort to extend and elongate its legs. It
results from this that the long-continued habit that this bird and
the others of its race contract, of extending and continually
elongating their legs, is the _cause_ of the individuals of this
race being raised as if on stilts, having gradually acquired long,
naked legs, which are denuded of feathers up to the thighs and often
above them (_Systeme des Animaux sans Vertebres_, p. 16).

"We also perceive that the same bird, wishing to catch fish without
wetting its body, is obliged to make continual efforts to lengthen
its neck. Now, the results of these habitual efforts in this
individual and in those of its race have enabled them, after a
time, to singularly elongate them--as, indeed, is proved by the long
neck of all shore birds.

"If any swimming birds, such as the swan and the goose, whose legs
are short, nevertheless have a very long neck, it is because these
birds in swimming on the surface of the water have the habit of
plunging their head down as far as they can, to catch aquatic larvae
and different animalcules for food, and because they make no effort
to lengthen their legs.

"When an animal to satisfy its wants makes repeated efforts to
elongate its tongue, it will acquire a consi