[CHAPTER I.]
Of THE NATURE OF ANIMALS.
As all our knowledge turns upon the relations by which one object differs from another, if there existed no brute animals, the nature of the human being would be still more incomprehensible. Having considered man in himself, ought we not to derive every assistance, by comparing him with the other parts of the animal creation? We will proceed then to examine the nature of animals, to compare their organization, to study their general economy, thereby to make particular applications, to mark resemblances, to reconcile the differences; and from the assemblage of those combinations, to distinguish the principal effects of the living mechanism, and to make a further progress in that important knowledge of which man is the object.
We will begin by reducing within its proper limits a subject which, at first view, appears to be immense. The properties of matter which animals possess in common with inanimate beings come not within our present consideration, and which we have already fully treated upon. For the same reason we shall reject such qualities as are found equally to belong to the vegetable and to the animal. As in the class of animals we comprehend a number of animated beings, whose organization is highly different from that of man, as well as from more perfect animals, so we shall wave the consideration of them, and confine ourselves to those animals which have evidently the greatest affinity to us.
But as the nature of man is superior to that of animals, so of that superiority we shall study to demonstrate the cause, in order that we may distinguish what is peculiar to man, from what belongs to him in common with other animals.
Previous to an examination of the minute parts of the animal machine, and their peculiar functions, let us view the general result of this mechanism, and, without at first reasoning upon causes, confine ourselves to an elucidation and description of effects.
An animal has two modes of existence; that of motion, or awake, and rest, or asleep; and which, while life lasts, succeed each other alternately. In the former, all the springs of the machine are in action; in the latter, there is only a part of them so, and this part acts as well while the animal is asleep as while it is awake, and is therefore absolutely necessary since the animal cannot exist without it. It is also independent of the other, as it acts of itself; the former, on the contrary, depends on the latter, as it cannot exercise itself alone. The one is a fundamental part of the animal economy, since it acts continually and without interruption; the other is less essential, since it acts but by internals.
The first division of the animal economy appears general and well founded. An animal when asleep is more easy to be examined than when awake and in motion. This difference is essential, and not a simple change of situation as in an inanimate body, which may be equally and indifferently at rest or in motion; for in either of these states it would perpetually remain, unless constrained to quit it by some external power or resistance. By its own powers the animal changes its condition; and naturally, and without constraint, it passes from repose to action, and from action to repose. The period for awaking returns as necessarily as that for sleep, and both arrive independent of any foreign cause; since in either state the animal cannot exist but for a certain time, and an uninterrupted continuity of either would be equally fatal, to life.
In the animal economy, therefore, we may distinguish two parts; the one acts perpetually without interruption, and the other acts only by intervals. The action of the heart and lungs in animals that breathe, and of the heart in the f[oe]tus, seem to constitute the former as does the action of the senses, and the movements of the members of the latter.
If we imagine beings endowed by nature with only the first part of this animal economy, though deprived of sense and progressive motion, would yet be animated, and differ in nothing from animals asleep. An oyster which appears to have no external sense or progressive motion, is a being formed to sleep for ever. In this sense a vegetable is merely a sleeping animal, and in general every organized being destitute of sense and motion may be compared to an animal doomed by Nature to a perpetual sleep.
In animals, then, sleep is not an accidental state, occasioned by the exertions of their functions while awake. It is, on the contrary, an essential mode of existence, which serves as a basis to an animal economy. By sleep our existence begins; the f[oe]tus sleeps continually, and the infant is more often asleep than awake. Sleep, therefore, which seems to be a state purely passive, resembling that of death, is, on the contrary, that which a living animal first experiences, and is the very foundation of life.