sforming external into
internal excitations.

To this second group of animals, possessing the _sentiment interieur_,
belong the higher Invertebrates, notably insects and molluscs. Only
animals possessed of a more or less centralised nervous system can
manifest this _sentiment_, or principle of (unconscious) reaction to
external stimuli.

The higher animals, or the four Vertebrate classes, form the group of
"intelligent animals." In virtue of their more complex organisation they
possess in addition to the _sentiment interieur_ the faculties of
intelligence and will.

Now, broadly put, Lamarck's theory of evolution is that new organs are
formed in direct reaction to needs (_besoins_) experienced by the
_sentiment interieur_. The _sentiment interieur_ is therefore the cause
not only of instinctive action but also of all morphogenetic processes.
Will and intelligence (which are confined to a relatively small number
of animals) have little or nothing to do directly with evolution.

To understand the working-out of Lamarck's evolution-theory we must
revert to his conception of the _Echelle des etres_. What he wrote in
the _Philosophie zoologique_ is here repeated in the work of 1816 with
little modification.

There is a real progression from the simpler to the more complex
organisations; Nature has gradually complicated her creatures by giving
them new organs and therefore new faculties.

It is interesting to note that Lamarck expressly refers to Bonnet (p.
110), but refuses to accept his view of an _Echelle_ extending down into
the inorganic. Like Bonnet, however, and like the German
transcendentalists, Lamarck makes man the goal of evolution (p. 116). He
makes it quite clear that his _Echelle_ is a functional one, for he
links Vertebrates to molluscs even while expressly admitting that they
are not connected by any structural intermediates (p. 123). He does not
fall into the error of the transcendentalists and assume that
Vertebrates and Invertebrates alike are formed upon one common plan of
structure.

The progression of organisation shown by the animal kingdom has not been
altogether regular and uninterrupted:--"The progression in complexity of
organisation shows here and there, in the general animal series,
anomalies induced by the influence of environment and by the influence
of the habits contracted" (_Phil. zool._, i., p. 145).

There are thus really two causes at work to produce the variety of
organisation