f the
composition of organization in each animal.

"In each locality where animals can live, the circumstances which
establish there an order of things remain for a long time the same,
and really change there only with a slowness so great that man
cannot directly notice them. He is obliged to consult monuments to
recognize that in each one of these places the order of things that
he discovers there has not always been the same, and to perceive
that it will change more.

"The races of animals which live in each of these places should,
then, retain their customary habits there also for a long time;
hence to us seems an apparent constancy of races which we call
_species_--constancy which has originated among us the idea that
these races are as ancient as nature.

"But in the different points of the earth's surface which can be
inhabited, nature and the situation of the places and climates
constitute there, for the animals as for the plants, _different
circumstances_ of all sorts of degrees. The animals which inhabit
these different places should then differ from each other, not only
on account of the state of nature of the organization in each race,
but, besides, by reason of the habits that the individuals of each
race there are forced to have; so, in proportion as he traverses the
larger parts of the earth's surface the observing naturalist sees
circumstances changing in a manner somewhat noticeable; he
constantly sees that the species change proportionately in their
characters.

"Now, the true order of things necessary to consider in all this
consists in recognizing:

"1. That every slight change maintained under the circumstances
where occur each race of animals, brings about in them a real change
in their wants.

"2. That every change in the wants of animals necessitates in them
other movements (_actions_) to satisfy the new needs, and
consequently other habits.

"3. That every new want necessitating new actions to satisfy it,
demands of the animal which feels it both the more frequent use of
such of its parts of which before it made less use, which develops
and considerably enlarges them, and the use of new parts which
necessity has caused to insensibly develop in it by the effects of
its inner feelings; which I shall constantly prove by known facts.

"Thus, to arrive at a knowledge of the true causes of so many
different fo