20 
INTRODUCTION. 
neither the one nor the other of these faculties, but are reduced to the simple function 
of vegetating. Although many plants retract their leaves when touched, and the roots 
direct themselves constantly towards moisture, the leaves towards air and light, 
and though some parts of vegetables appear even to exhibit oscillations without 
any perceptible external cause, still these various movements bear too little resem- 
blance to those of animals to enable us to recognize in them any proofs of perception 
or of will. 
The spontaneity of the movements of animals required essential modifications, even 
in their simply vegetative organs. Their roots not penetrating the ground, it was 
necessary that they should be able to place within themselves provisions of food, and 
to carry its reservoir along with them. Hence is derived the first character of animals, 
or their alimentary cavity, from which their nutritive fluid penetrates all other parts 
through pores or vessels, which are a sort of internal roots. 
The organization of this cavity and of its appurtenances required varying, according 
to the nature of the aliment, and the operations which it had to undergo before it 
could furnish juices proper for absorption : whilst the atmosphere and the earth supply 
to vegetables only juices ready prepared, and which can be absorbed immediately. 
The animal body, which abounds with functions more numerous and more varied 
than in the plant, required in consequence to have an organization much more com- 
plicated ; besides which, its parts not being capable of preserving a fixed relative posi- 
tion, there were no means by which the motion of their fluids could be produced by 
external causes, as it required to be independent: of heat and of the atmosphere : from 
this originates the second character of animals, or their circulatory system, which is 
less essential than the digestive, since it was unnecessary in the more simple animals. 
The animal functions required organic systems, not needed by vegetables, as that 
of the muscles for voluntary motion, and that of the nerves for sensibility ; and these 
two systems, like the rest, acting only through the motions and transformations of the 
fluids, it was necessary that these should be more numerous in animals, and that 
the chemical composition of the animal body should be more complicated than that of 
the plant : and so it is, for an additional substance (azote) enters into it as an essential 
element, while in plants it is a mere accidental junction with the three other general 
elements of organization, — oxygen, hydrogen, and carbon. This then is the third 
character of animals. 
The soil and the atmosphere supply to vegetables water for their nutrition, which is 
composed of oxygen and hydrogen, air, which contains oxygen and azote, and car- 
bonic acid, which is a combination of oxygen and carbon. To extract from these 
aliments their proper composition, it was necesary that they should retain the hydrogen 
and carbon, exhale the superfluous oxygen, and absorb little or no azote. Such, then, 
is the process of vegetable life, of which the essential function is the exhalation of 
oxygen, which is effected through the agency of light. 
Animals in addition derive nourishment, more or less immediately, from the vegetable 
itself, of which hydrogen and carbon form the principal constituents. To assimilate 
them to their own composition, they must get rid of the superfluous hydrogen, and 
especially of the superabundant carbon, and accumulate more azote ; this it is which 
is performed in respiration, by means of the oxygen of the atmosphere combining with 
the hydrogen and carbon of the blood, and being exhaled with them under the form of 
