it has resulted that the fore legs have become longer than the
hind legs, and that the neck has become so elongated that the giraffe,
without standing on its hind legs, can raise its head to a height of
nearly twenty feet. Observation of all animals will furnish similar
examples.

None, perhaps, is more striking than that of the kangaroo. This animal,
which carries its young in an abdominal pouch, has acquired the habit of
carrying itself upright upon its hind legs and tail, and of moving from
place to place in a series of leaps, during which, in order not to hurt
its little ones, it preserves its upright posture. Observe the result.

(1) Its front limbs, which it uses very little, resting on them only in
the instant during which it quits its erect posture, have never acquired
a development in proportion to the other parts; they have remained thin,
little, and weak.

(2) The hind legs, almost continually in action, whether to bear the
weight of the whole body or to execute its leaps, have, on the contrary,
obtained a considerable development; they are very big and very strong.

(3) Finally, the tail, which we observe to be actively employed, both to
support the animal's weight and to execute its principal movements, has
acquired at its base a thickness and a strength that are extremely
remarkable.

When the will determines an animal to a certain action, the organs
concerned are forthwith stimulated by a flow of subtle fluids, which are
the determining cause of organic changes and developments. And
multiplied repetitions of such acts strengthen, extend, and even call
into being the organs necessary to them. Now, every change in an organ
which has been acquired by habitual use sufficient to originate it is
reproduced in the offspring if it is common to both the individuals
which have come together for the reproduction of their species. In the
end, this change is propagated and passes to all the individuals which
come after and are submitted to the same conditions, without its being
necessary that they should acquire it in the original manner.

For the rest, in the union of disparate couples, the disparity is
necessarily opposed to the constant propagation of such qualities and
outward forms. This is why man, who is exposed to such diversity of
conditions, does not preserve and propagate the qualities or the
accidental defects which he has been in the way of acquiring. Such
peculiarities will be produced only in case