ined thin, very small, and weak;

"2. The hind legs, almost continually in action, both for supporting
the body and for leaping, have, on the contrary, obtained a
considerable development, and have become very large and strong;

"3. Finally, the tail, which we see is of much use in supporting the
animal and in the performance of its principal movements, has
acquired at its base a thickness and a strength extremely
remarkable.

"These well-known facts are assuredly well calculated to prove what
results from the habitual use in the animals of any organ or part;
and if, when there is observed in an animal an organ especially well
developed, strong, and powerful, it is supposed that its habitual
use has not produced it, that its continual disuse will make it lose
nothing, and, finally, that this organ has always been such since
the creation of the species to which this animal belongs, I will ask
why our domestic ducks cannot fly like wild ducks--in a word, I
might cite a multitude of examples which prove the differences in us
resulting from the exercise or lack of use of such of our organs,
although these differences might not be maintained in the
individuals which follow them genetically, for then their products
would be still more considerable.

"I shall prove, in the second part, that when the will urges an
animal to any action, the organs which should execute this action
are immediately provoked by the affluence of subtile fluids (the
nervous fluid), which then become the determining cause which calls
for the action in question. A multitude of observations prove this
fact, which is now indisputable.

"It results that the multiplied repetitions of these acts of
organization strengthen, extend, develop, and even create the organs
which are necessary. It is only necessary attentively to observe
that which is everywhere occurring to convince ourselves of the
well-grounded basis of this cause of organic developments and
changes.

"Moreover, every change acquired in an organ by a habit of use
sufficient to have produced it is then preserved by heredity
(_generation_) if it is common to the individuals which, in
fecundation, unite in the reproduction of their species. Finally,
this change is propagated, and thus is transmitted to all the
individuals which succeed and which are submitted to the same
circumstances, unless they have been obliged