The environment affects the shape and organization of animals, that is
to say that when the environment becomes very different, it produces in
course of time corresponding modifications in the shape and organization
If a new environment, which has become permanent for some race of
animals, induces new habits in these animals, that is to say, leads them
into new activities which become habitual, the result will be the use of
some one part in preference to some other part, and in some cases the
total disuse of some part no longer necessary.
Nothing of all this can be considered as hypothesis or private opinion;
on the contrary, they are truths which, in order to be made clear, only
require attention and the observation of facts.
Snakes have adopted the habit of crawling on the ground and hiding in
the grass; so that their body, as a result of continually repeated efforts
at elongation for the purpose of passing through narrow spaces, has
acquired a considerable length, quite out of proportion to its size. Now,
legs would have been quite useless to these animals and consequently
unused. Long legs would have interfered with their need of crawling, and
very short legs would have been incapable of moving their body, since they
could only have had four. The disuse of these parts thus became permanent
in the various races of these animals, and resulted in the complete
disappearance of these same parts, although legs really belong to the
plan or organization of the animals of this class.
The frequent use of any organ, when confirmed by habit, increases the
functions of that organ, leads to its development, and endows it with a
size and power that it does not possess in animals which exercise it less.
We have seen that the disuse of any organ modifies, reduces, and finally
extinguishes it. I shall now prove that the constant use of any organ,
accompanied by efforts to get the most out of it, strengthens and enlarges
that organ, or creates new ones to carry on the functions that have become
The bird which is drawn to the water by its need of finding there the
prey on which it lives, separates the digits of its feet in trying to
strike the water and move about on the surface. The skin which unites
these digits at their base acquires the habit of being stretched by
these continually repeated separations of the digits; thus in course of
time there are formed large webs which unite the digits of ducks, geese,
etc. as we actually find them.
It is interesting to observe the result of habit in the peculiar
shape and size of the giraffe; this animal, the largest of the mammals,
is known to live in the interior of Africa in places where the soil is
nearly always arid and barren, so that it is obliged to browse on the
leaves of trees and to make constant efforts to reach them. From this
habit long maintained in all its race, it has resulted that the animal's
fore-legs have become longer than its hind legs, and that its neck is
lengthened to such a degree that the giraffe, without standing up on its
hind legs, attains a height of six meters (nearly twenty feet).
Philosophic Zoologique. Paris 1809.
Translated by H. Elliott, Macmillan Company,