One might almost say that the plant is the framework, the foundation of the animal, and that to form the animal it sufficed to cover this foundation with a system of organs fitted to establish relations consists forms with the world outside. It follows of the succession substance of the animal form two quite distinct classes. One class in a continual into its own assimilation molecules that the functions and of excretion; through these functions the animal incessantly transsurrounding bodies, later to reject these molecules when they have become heterogeneous to it. Through this first class of functions the animal exists only within itself; through the other class it exists outside; it is an inhabitant of the world, and not, like the plant, of the place which saw its birth. The animal feels and perceives its surroundings, reflects its sensations, moves of its own will under their influence, and, as a rule, can communicate by its voice its desires and its fears, its pleasures or its pains. I call organic life the sum of the functions of the former class, for all organised creatures, plants or animals, possess them to a more or less marked degree, and organised structure is the sole condition necessary to their exercise. The combined functions of the second class form the ' animal' life named because it is the exclusive attribute of the animal kingdom.
Recherches Physiologiques sur la Vie et la Mort (1800), pp. 2-3