Influence of circumstances on the Actions of Animals. 1057 
of that organ, develops it, and makes it acquire dimensions and 
force of action which it has not in animals which exercise it less, 
One comes to see that the disuse of an organ which has existed, 
modifies, impoverishes, and finally obliterates it. I will now 
demonstrate that the continual employment of an organ, with 
the efforts made in keeping its place in the circumstances under 
which it exists, strengthens, extends, and increases that organ, or 
creates new ones which are able to exercise the functions which 
have become necessary. 
The bird that hunger (necessity) attracts to water to find there the 
prey on which it lives, separates its toes and its feet when it strikes 
the water, and moves on its surface. The skin which unites these 
toes at their base, acquire the habit of extending themselves by these 
perpetual spreadings; thus, after a time, the large membranes are 
formed which we see uniting the toes of ducks, geese, ete. The 
same efforts made to swim, that is to say, to push the water in order 
to advance and move in that liquid, have developed the same mem- 
brane which is between the toes of frogs, sea-turtles, the otter, the 
beaver, ete. On the contrary, the bird that its manner of life 
habituates to roost in trees, and who proceeds from individuals who 
have all contracted that habit, has necessarily the toes more elon- 
gated and shaped otherwise than those of aquatic animals which I 
have cited. Their nails, after a time, become long, sharp, and 
curved in a hook by holding so often the limbs on which the animal 
rests. So it is known of river birds who do not swim, and who 
only desire to approach the borders of the water to find their prey, are 
continually exposed to being forced into the mud. Now this bird, 
wishing to act so that its body may not be wet, makes great 
efforts to extend and elongate its feet. It follows from the continued 
habits which this bird, and all those of its race have contracted 
in continually extending and elongating its legs, that the individ- 
uals of this race are found elevated on stilts, and have also gradu- 
ally obtained long boots. That is to say, they are denuded of 
eathers as far as the thighs and often beyond (Systeme des Animaus 
sans Vertebres, p. 14). It is known also that the same bird, wishing 
to pick without wetting its body, is obliged to make continuous efforts 
to elongate its neck, Now, to the continuance of these habitual 
efforts in this individual, and those of its race, they owe that, after 
