rms and so many different habits of which the known
animals offer us examples, it is necessary to consider that
circumstances infinitely diversified, but all slowly changing, into
which the animals of each race are successively thrown, have caused,
for each of them, new wants and necessarily changes in their habits.
Moreover, this truth, which cannot be denied, being once recognized,
it will be easy to see how the new needs have been able to be
satisfied, and the new habits formed, if any attention be given to
the two following laws of nature, which observation always confirms:

"_First Law._

"In every animal which has not exceeded the term of its development,
the more frequent and sustained use of any organ gradually
strengthens this organ, develops and enlarges it, and gives it a
strength proportioned to the length of time of such use; while the
constant lack of use of such an organ imperceptibly weakens it,
causes it to become reduced, progressively diminishes its faculties,
and ends in its disappearance.

"_Second Law._

"Everything which nature has caused individuals to acquire or lose
by the influence of the circumstances to which their race may be for
a long time exposed, and consequently by the influence of the
predominant use of such an organ, or by that of the constant lack of
use of such part, it preserves by heredity (_generation_) and passes
on to the new individuals which descend from it, provided that the
changes thus acquired are common to both sexes, or to those which
have given origin to these new individuals.

"These are the two fundamental truths which can be misunderstood
only by those who have never observed or followed nature in its
operations, or only by those who allow themselves to fall into the
error which I have combated.

"Naturalists having observed that the forms of the parts of animals
compared with the uses of these parts are always in perfect accord,
have thought that the forms and conditions of parts have caused the
function; but this is a mistake, for it is easy to demonstrate by
observation that it is, on the contrary, the needs and uses of
organs which have developed these same parts, which have even given
origin to them where they did not exist, and which consequently have
given rise to the condition in which we observe them in each animal.

"If this were not so, it would have been necessary fo