that they are, in some degree, for her
inexhaustible.

"The principal circumstances arise from the influence of climate;
from those of different temperatures of the atmosphere, and from all
the environing media; from that of the diversity of different
localities and their situation; from that of habits, the ordinary
movements, the most frequent actions; finally, from that of means of
preservation, of mode of living, of defence, of reproduction, etc.

"Moreover, owing to these diverse influences, the faculties increase
and become stronger by use, become differentiated by the new habits
preserved for long ages, and insensibly the organization, the
consistence--in a word, the nature and condition of parts, as also
of the organs--participate in the results of all these influences,
become preserved, and are propagated by generation.

"These truths, which are only the results of the two natural laws
above stated, are in every case completely confirmed by facts; they
clearly indicate the course of nature in all the diversity of its
products.

"But instead of contenting ourselves with generalities which might
be considered as hypothetical, let us directly examine the facts,
and consider, in the animals, the result of the use or disuse of
their organs on the organs themselves, according to the habits that
each race has been compelled to contract.

"I shall now attempt to prove that the constant lack of exercise of
organs at first diminishes their faculties, gradually impoverishes
them, and ends by making them disappear, or even causing them to be
atrophied, if this lack of use is perpetuated for a very long time
through successive generations of animals of the same race.

"I shall next prove that, on the contrary, the habit of exercising
an organ, in every animal which has not attained the limit of the
diminution of its faculties, not only perfects and increases the
faculties of this organ, but, besides, enables it to acquire
developments and dimensions which insensibly change it; so that with
time it renders it very different from the same organ in another
animal which exercises it much less.

"_The lack of use of an organ, become constant by the habits formed,
gradually impoverishes this organ, and ends by causing it to
disappear and even to destroy it._

"As such a proposition can only be admitted on proof, and not by its
simple announceme