ufficient to constitute containing parts, will be the
body most fitted to receive the first traces of organization and
life."

In the third part of the _Philosophie zoologique_ Lamarck considers the
physical causes of feeling--_i.e._, those which form the productive
force of actions, and those giving rise to intelligent acts. After
describing the nervous system and its functions, he discusses the
nervous fluid. His physiological views are based on those of Richerand's
_Physiologie_, which he at times quotes.

Lamarck's thoughts on the nature of the nervous fluid (_Recherches sur
le fluide nerveux_) are curious and illustrative of the gropings after
the truth of his age.

He claims that the supposed nervous fluid has much analogy to the
electric, that it is the _feu ethere_ "animalized by the circumstances
under which it occurs." In his _Recherches sur l'organisation des corps
vivans_ (1802) he states that, as the result of changes continually
undergone by the principal fluids of an animal, there is continually set
free in a state of _feu fixe_ a special fluid, which at the instant of
its disengagement occurs in the expansive state of the caloric, then
becomes gradually rarefied, and insensibly arrives at the state of an
extremely subtile fluid which then passes along the smallest nervous
ramifications in the substance of the nerve, which is a very good
conductor for it. On its side the brain sends back the subtile fluid in
question along the nerves to the different organs.

In the same work (1802) Lamarck defines thought as a physical act taking
place in the brain. "This act of thinking gives rise to different
displacements of the subtile nervous fluid and to different
accumulations of this fluid in the parts of the brain where the ideas
have been traced." There result from the flow of the fluid on the
conserved impressions of ideas, special movements which portions of this
fluid acquire with each impression, which give rise to compounds by
their union producing new impressions on the delicate organ which
receives them, and which constitute abstract ideas of all kinds, also
the different acts of thought.

All the acts which constitute thought are the comparisons of ideas, both
simple and complex, and the results of these comparisons are judgments.

He then discusses the influence of the nervous fluid on the muscles, and
also its influence considered as the cause of feeling (_sentiment_).
Finally he concludes