ment of displacement in the subtile fluid of the nerves--which
the ancients called _animal spirits_; directs this fluid towards
that of its organs which any want impels to action; finally makes
this same fluid flow back into its habitual reservoirs when the
needs no longer require the organ to act.

"The inner feeling takes the place of the _will_; for it is now
important to consider that every animal which does not possess the
special organ in which or by which it executes thoughts, judgments,
etc., has in reality no will, does not make a choice, and
consequently cannot control the movements which its inner feeling
excites. _Instinct_ directs these actions, and we shall see that
this direction always results from emotions of the inner feeling, in
which intelligence has no part, and from the organization even which
the habits have modified, in such a manner that the needs of animals
which are in this category, being necessarily limited and always
the same in the same species, the inner feeling and, consequently,
the power of acting, always produces the same actions.

"It is not the same in animals which besides a nervous system have a
brain [the author meaning the higher vertebrates], and which make
comparisons, judgments, thoughts, etc. These same animals control
more or less their power of action according to the degree of
perfection of their brain; and although they are still strongly
subjected to the results of their habits, which have modified their
structure, they enjoy more or less freedom of the will, can choose,
and can vary their acts, or at least some of them."

Lamarck then treats of the consumption and exhaustion of the nervous
fluid in the production of animal movements, resulting in fatigue.

He next occupies himself with the origin of the inclination to the same
actions, and of instinct in animals.

"The cause of the well-known phenomenon which constrains almost all
animals to always perform the same acts, and that which gives rise
in man to a propensity (_penchant_) to repeat every action, becoming
habitual, assuredly merits investigation.

"The animals which are only 'sensible'[184]--namely, which possess
no brain, cannot think, reason, or perform intelligent acts, and
their perceptions being often very confused--do not reason and can
scarcely vary their actions. They are, then, invariably bound by
habits. Thus the insects, which o