and finally form organs.

Subtle fluids play a great part in Lamarck's biology: they take the
place of the soul or entelechy which the vitalists would postulate to
explain organic happenings. Lamarck seems in this to follow certain of
the old materialists, who conceived the soul to be formed of a matter
more subtle than the ordinary.[342]

In his second law Lamarck's essentially vitalistic attitude comes out
very clearly, for it states that a psychological moment enters into all
new production of form, that the ultimate cause of the development of
new form is the need felt by the organism. This need is of course not a
conscious one, it is a need perceived by the _sentiment interieur_.

In the large group of apathetic or insensitive animals, which do not
possess this faculty, needs cannot be experienced; accordingly new
organs are here formed directly and mechanically, by the movements of
the vital fluids set in action by excitations from without--the
evolution, like the behaviour, of these animals is due to the direct and
physical action of the environment. "But this is not the case with the
more highly organised animals which possess _feeling_. They experience
needs, and each need felt, acting upon their 'inner feeling,'
immediately directs the fluids and the forces to the part of the body
where action can satisfy the need. Now, if there exists at this point an
organ capable of performing the required action, it is quickly
stimulated to act; and if the organ does not exist and the need is
pressing and sustained, bit by bit the organ is produced and developed
in proportion to the continuity and the energy of its use" (p. 155).

In intelligent animals the _sentiment interieur_ may be moved by thought
or will.

As an example of the way in which the law works Lamarck takes the
hypothetical case of a gastropod mollusc, which as it creeps along
experiences dimly the need to feel the objects in front of it. It makes
an effort (unconscious, be it noted) to touch these objects with the
anterior portions of its head, and sends forward continually to these
parts a great volume of nervous and other fluids. From these efforts and
the repeated afflux of fluids there must result a development of the
nerves supplying these parts. And as, along with the nervous fluids,
nutritive juices constantly flow to the parts, there must result the
formation of two or four tentacles in the places to which these fluids
are directed. A curious