1, that the thought alone of an action
which strongly interests it suffices to arouse the _inner feeling_
of an individual; 2, that a felt want can itself arouse the feeling
in question; 3, that every emotion of _inner feeling_, resulting
from a want which is aroused, directs at the same instant a mass of
nervous fluid to the points to be set in activity, that it also
creates a flow thither of the fluids of the body, and especially
nutrient ones; that, finally, it then places in activity the organs
already existing, or makes efforts for the formation of those which
would not have existed there, and which a continual want would
therefore render necessary--I should have had doubts as to the
reality of the law which I have just indicated.

"But, although it may be very difficult to verify this law by
observation, I have no doubt as to the grounds on which I base it,
the necessity of its existence being involved in that of the third
law, which is now well established.

"I conceive, for example, that a _gasteropod mollusc_, which, as it
crawls along, finds the need of feeling the bodies in front of it,
makes efforts to touch those bodies with some of the foremost parts
of its head, and sends to these every time supplies of nervous
fluids, as well as other fluids--I conceive, I say, that it must
result from this reiterated afflux towards the points in question
that the nerves which abut at these points will, by slow degrees, be
extended. Now, as in the same circumstances other fluids of the
animal flow also to the same places, and especially nourishing
fluids, it must follow that two or more tentacles will appear and
develop insensibly under those circumstances on the points referred
to.

"This is doubtless what has happened to all the races of
_Gasteropods_, whose wants have compelled them to adopt the habit of
feeling bodies with some part of their head.

"But if there occur, among the _Gasteropods_, any races which, by
the circumstances which concern their mode of existence or life, do
not experience such wants, then their head remains without
tentacles; it has even no projection, no traces of tentacles, and
this is what has happened in the case of _Bullaea_, _Bulla_, and
_Chiton_."

In the _Supplement a la Distribution generale des Animaux_
(Introduction, p. 342), concerning the real order of origin of the
invertebrate classes, Lamarck propose