in the complexity of organisation of animals,
and in their faculties, seems to me easy to conceive; so, too, the means
which Nature has employed to diversify animals, and bring them to the
state in which we now see them, become easily determinable" (p. 168).

It is never made quite clear, we may note in passing, how far his second
and third laws tend to bring about an increase in complexity, in
addition to diversifying animals.[343]

"The function creates the organ," this would seem to be the kernel of
Lamarck's doctrine. But how does he reconcile this essentially
vitalistic conception with his strictly materialistic philosophy?

We have seen that irritability, the _sentiment interieur_, and
intelligence itself, are the effects of organisation. We are told
farther on that both the _sentiment_ and intelligence are caused by
nervous fluids. A great part of both the _Philosophie zoologique_ and
the introduction to the _Animaux sans Vertebres_ is given up to the
exposition of a materialistic psychology of animals and man, based
entirely upon this hypothesis of nervous fluids. Thus habits are due to
the fluids hollowing out definite paths for themselves.

The _sentiment interieur_ acts by directing the movements of the subtle
fluids of the body (which are themselves modifications of the nervous
fluids) upon the parts where a new organ is needed. But if it is itself
only a result of the movement of nervous fluids? Again, how can a need
be "felt" by a nervous fluid? This is an entirely psychological notion
and cannot be applied to a purely material system. Whence arises the
power of the _sentiment interieur_ to canalise the energies of the
organism, so to direct and co-ordinate them that they build up purposive
structures, or effect purposive actions (as in all instinctive
behaviour)? Either the _sentiment interieur_ is a psychological faculty,
or it is nothing.

There is no doubt that, as expressed by Lamarck, the conception conceals
a radical confusion of thought. It is not possible to be a
thorough-going materialist, and at the same time to believe that new
organs are formed in direct response to needs felt by the organism.
Lamarck could never resolve this antinomy, and his speculations were
thrown into confusion by it. To this cause is due the frequent obscurity
of his writings.

Should we be right in laying stress upon the psychological side of
Lamarck's theory, and disregarding the materialistic dress in which,
p