LAMARCK’S THEORY OF DESCENT 331 
category, being necessarily limited and always the 
same in the same species, the inner feeling and, con- 
sequently, the power of acting, always produces the 
same actions. 
“It is not the same in animals which besides a 
nervous system have a brain [the author meaning 
the higher vertebrates], and which make compari- 
sons, judgments, thoughts, etc. These same animals 
control more or less their power of action according 
to the degree of perfection of their brain; and al- 
though they are still strongly subjected to the results 
of their habits, which have modified their structure, 
they enjoy more or less freedom of the will, can 
choose, and can vary their acts, or at least some of 
them.” 
Lamarck then treats of the consumption and ex- 
haustion of the nervous fluid in the production of 
animal movements, resulting in fatigue. 
He next occupies himself with the origin of the 
inclination to the same actions, and of instinct in 
animals. 
‘« The cause of the well-known phenomenon which 
constrains almost all animals to always perform the 
same acts, and that which gives rise in man to a pro- 
pensity (penxchant) to repeat every action, becoming 
habitual, assuredly merits investigation. 
“ The animals which are only ‘ sensible’ *—namely, 
which possess no brain, cannot think, reason, or per- 
form intelligent acts, and their perceptions being 
often very confused—do not reason and can scarcely 
vary their actions. They are, then, invariably bound 
by habits. Thus the insects, which of all animals 
endowed with feeling have the least perfect nervous 
* Lamarck’s division of Azizmaux sensibles comprises the insects, 
arachnids, crustacea, annelids, cirripedes, and molluscs. 
