If we contemplate a little the paths of Nature--if, for a time, we trace the beings in this Nature, under the different states through which, by reason of their properties, they are compelled to pass; we shall discover, that it is to motion, and motion only, that is to be ascribed all the changes, all the combinations, all the forms, in short, all the various modifications of matter. That it is by motion every thing that exists is produced, experiences change, expands, and is destroyed. It is motion that alters the aspect of beings; that adds to, or takes away from their properties; which obliges each of them, by a consequence of its nature, after having occupied a certain rank or order, to quit it, to occupy another, and to contribute to the generation, maintenance, and decomposition of other beings, totally different in their bulk, rank, and essence. In what experimental philosophers have styled the THREE ORDERS OF NATURE, that is to say, the _mineral_, the _vegetable_, and _animal_ worlds, they have established, by the aid of motion, a transmigration, an exchange, a continual circulation in the particles of matter. Nature has occasion in one place, for those particles which, for a time, she has placed in another. These particles, after having, by particular combinations, constituted beings endued with peculiar essences, with specific properties, with determinate modes of action, dissolve and separate with more or less facility; and combining in a new manner, they form new beings. The attentive observer sees this law execute itself, in a manner more or less prominent, through all the beings by which he is surrounded. He sees nature full of _erratic germe_, some of which expand themselves, whilst others wait until motion has placed them in their proper situation, in suitable wombs or matrices, in the necessary circumstances, to unfold, to increase, to render them more perceptible by the addition of other substances of matter analogous to their primitive being. In all this we see nothing but the effect of motion, necessarily guided, modified, accelerated or slackened, strengthened or weakened, by reason of the various properties that beings successively acquire and lose; which, every moment, infallibly produces alterations in bodies more or less marked. Indeed, these bodies cannot be, strictly speaking, the same in any two successive moments of their existence; they must, every instant, either acquire or lose: in short, they are obliged to undergo continual variations in their essences, in their properties, in their energies, in their masses, in their qualities, in their mode of existence.