immediately provoked by the flowing there of subtile fluids, which
become the determining cause of movements which perform the action
in question. A multitude of observations support this fact, which
now no one would doubt.

"It results from this that multiplied repetitions of these acts of
organization strengthen, extend, develop, and even create the organs
which are there needed. It is only necessary to closely observe that
which is everywhere happening in this respect to firmly convince
ourselves of this cause of developments and organic changes.

"However, each change acquired in an organ by habitual use
sufficient to have formed (_opere_) it is preserved by generation,
if it is common to the individuals which unite in the reproduction
of their kind. Finally, this change propagates itself and is then
handed down (_se passe_) to all the individuals which succeed and
which are submitted to the same circumstances, without their having
been obliged to acquire it by the means which have really created
it.

"Besides, in the unions between the sexes the intermixtures between
individuals which have different qualities or forms are necessarily
opposed to the constant propagation of these qualities and forms. We
see that which in man, who is exposed to such different
circumstances which influence individuals, prevents the qualities of
accidental defects which they have happened to acquire from being
preserved and propagated by heredity (_generation_).

"You can now understand how, by such means and an inexhaustible
diversity of circumstances, nature, with sufficient length of time,
has been able to and should produce all these results.

"If I should choose here to pass in review all the classes, orders,
genera, and species of animals in existence I could make you see
that the structure of individuals and their organs, faculties, etc.,
is solely the result of circumstances to which each species and all
its races have been subjected by nature, and of habits that the
individuals of this species have been obliged to contract.

"The influences of localities and of temperatures are so striking
that naturalists have not hesitated to recognize the effects on the
structure, the developments, and the faculties of the living bodies
subject to them.

"We have long known that the animals inhabiting the torrid zone are
very different from those which live