"_The frequent use of an organ become constant by habit increases the faculties of this organ, even develops it, and enables it to acquire dimensions and a power of action which it does not possess in animals which exercise less._ "We have just said that the lack of employment of an organ which necessarily exists modifies it, impoverishes it, and ends by its disappearing entirely. "I shall now demonstrate that the continued employment of an organ, with the efforts made to draw out its powers under circumstances where it would be of service, strengthens, extends, and enlarges this organ, or creates a new one which can exercise the necessary functions. "The bird which necessity drives to the water to find there prey fitted for its sustenance, opens the digits of its feet when it wishes to strike the water and propel itself along its surface. The skin which unites these digits at their base, by these acts of spreading apart being unceasingly 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 (_Système des Animaux sans Vertèbres_, 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.