As in the beginning every species was created, the first individual of each has served for a model to their descendants. The body of each animal or vegetable is a mould, to which are assimilated indifferently the organic particles of all animals or vegetables which have been destroyed by death, or consumed by time. The brute particles, of which part of their composition was formed, returned to the common mass of inanimate matter; but the organic particles, whose existence is permanent, are again resumed by organized bodies: they are extracted at first from the earth by vegetables, and then absorbed by animals who feed thereon; and thus serve for the support, growth, and expansion of both. By this constant and perpetual circulation from body to body, they serve to animate all organized beings. These living substances in quantity are always the same, and differ only in form and appearance. In fertile ages, and when population is the greatest, the whole surface of the earth seems to be covered with men, domestic animals, and useful plants. But in the times of famine and depopulation, the ferocious animals, poisonous insects, parasitical plants, and useless herbs, resume, in their turn, dominion over the earth. To man these changes are material, but to Nature they are perfectly indifferent. The silk worm so inestimable to the former, is to the latter only a caterpillar of the mulberry tree. Though this caterpillar, which so materially assists in the supply of our luxuries, should disappear; though the plants, from which our domestic animals procure their nourishment, should be devoured by other caterpillars; though still others should destroy the substance of our corn before the harvest; in short, though man and the larger animals should be starved by the inferior tribes, Nature would not be less abundant nor less alive; she never protects one at the expence of another, but especially supports the whole. As to individuals she is regardless of number; she considers them only as successive images of the same impression; as passing shadows of which the species is the substance.
In earth, air, and water, then, there exists a certain quantity of organic matter which cannot be destroyed, but which is constantly assimilated in a certain number of moulds, that are perpetually undergoing destruction and renewal: these moulds, or rather individuals, tho’ varying in number in every species, are nevertheless always the same, that is, proportioned to the quantity of living matter; and this appears to be absolutely the case, for if there were any redundance of this matter, or if it were not at all times fully occupied by the individuals of the species which exist, it would, most assuredly, form itself into new species, for being alive it would not remain without action; and once uniting with brute matter is sufficient to form organized bodies; and it is by this constant combination, and invariable proportion, that Nature preserves her form and consistence.
The laws of Nature, both with respect to the number of species and of their support and equilibrium, being fixed and constant, she would invariably have the same appearance, and be in all climes absolutely the same, if her complexion did not so completely vary in almost every individual form. The figure of each species is an impression, in which the principal characters are so strongly engraven as never to be effaced; but the accessory parts and shades are so greatly varied that no two individuals have a perfect resemblance to each other; and in all species there are a number of varieties. The human species, which has such superior pretensions, varies from white to black, from small to great, &c. The Laplander, the Patagonian, the Hottentot, the European, the American, and the Negro, though the offspring of the same parents, have by no means the resemblance of brothers.
It is evident, therefore, that every species is subject to individual differences, but that each of them does not equally possess the constant varieties which are perpetuated through successive generations; the more dignified the species, the less changeable is its figure, and the less are the varieties of it. The multiplication of animals being inversely in proportion to their magnitude, as the possibility of variation must be in exact proportion to the numbers they produce, there consequently must be more varieties among the small than the large animals; and also, for the same reason, there will be a greater number of species which seem to approach each other; for the unity of the species in the large animals is more fixed, and the nature of their separation more extended. What a number of various and similar species surround those of the squirrel, the rat, and other small quadrupeds, while the massy elephant stands alone, without a compeer, and at the head of the whole.
The brute matter, of which the body of the earth is principally composed, is a substance that has not undergone many alterations, though the whole has more than once been disturbed and put in motion by the hand of Nature. The globe of the earth has been penetrated by fire, and afterwards covered and disordered by water. The sand, which occupies the interior parts of the earth, is a vitrified matter; and the layers of clay, by which its surface is covered, are nothing but the same sand having been decomposed by the operation of the waters. Granite, free-stone, flint, nay, all metals, are composed of this same vitrified matter, whose particles have been condensed or separated, according to the laws of their affinity. These substances are totally destitute of animation; they exist, and will continue to do so, independently of animals and vegetables. There are, however, many other substances, which, although they have the appearance of being equally inanimate, originate from organized bodies; and of this description are marble, lime-stone, chalk, and marl; they being composed of the fragments of shells, and of those small animals which by transforming the water of the sea into stone, produce coral, and all the madrepores, whose varieties are numberless, and whose quantity are almost immense. Pit-coal, turf, and many other substances found in the upper strata, are also of this nature, they being only the residue of vegetables which have been more or less corrupted or consumed. Besides these, there are other substances which have been produced by the second action of fire upon original matter; these are but few in number, and consist of such as pumice-stones, sulphur, the scoria of iron, asbestos, and lava. To one or other of these three great combinations may be referred all the relations of brute matter, and all the substances of the mineral kingdom.
The laws of affinity, by which the various particles of these different substances separate from each other, in order to unite among themselves and form homogeneous masses, are perfectly similar to that general law by which the celestial bodies act upon each other; in both cases their exertions are the same. Globules of water, of sand, or of metal, have the same influence, and act upon each other as the earth acts upon the moon; and if the laws of affinity have hitherto been deemed different from those of gravity, it is because the subject has been considered in a very confined point of view. The mutual action of celestial bodies is very little influenced by figure; their distance from each other is so very great, that this is necessarily the case; but when they are not far asunder, then the effect of figure is considerable. For instance, if the earth and moon, instead of spherical figures, were both short cylinders, and exactly equal throughout in their diameters, their reciprocal action would be very little varied from what it is at present, because the distances of all their parts from each other would be very little changed. But if these two globes were cylinders of great extent, and approached near to each other, the law of their reciprocal action would seem to be different, inasmuch as the distances of their parts would be greatly varied; and hence whenever figure becomes a principle in distance the law will appear to vary, although in fact it is always the same.
The human intellect guided by this principle, may advance one step further in penetrating into the operations of nature. The figure of the constituent particles of bodies still remains unknown; we cannot entertain the smallest doubt that water, air, earth, metals, and all homogeneous particles, are composed of elementary particles, which are perfectly similar, although we are still ignorant of their figure. By the aid of calculation this at present unknown field of knowledge may be disclosed by posterity, and the figure of the elementary bodies be ascertained with tolerable precision. They may take the principle we have established as the basis of their enquiry; namely, “that all matter is attracted in the inverse ratio of the square of the distance; and this law seems to admit of no variation in particular attractions but what arises from the figure of the constituent particles of each substance, because this figure enters as an element or principle into the distance;” and having once discovered, by repeated experiments, the law of attraction in any particular substance, they may then, by the aid of calculation, be able to trace the figure of its constituent particles. To render this point more clear, let us suppose, that by placing mercury on a perfectly polished surface, repeated experiments prove that this fluid metal is always attracted in the inverse ratio of the cube of the distance; it will then become necessary to investigate what figure gives this expression; and this figure will be certainly that of the constituent particles of mercury. If it should appear, by such experiments, that the attraction of mercury was in the inverse ratio of the square of the distance, it would be clearly demonstrated that its constituent particles were spherical, because a sphere is the only figure which observes this law, and at whatever distance globes are placed the law of their attraction is always the same.
Newton had some idea that chemical affinities (which are nothing more in fact than these particular attractions which we have mentioned) were produced by the same kind of laws as those of gravitation; but he does not appear to have perceived that all those particular laws were merely simple modifications of the general one, and that their apparent difference arose solely from the circumstance of the figure of the atoms, which attract each other, having, when at small distances, a greater influence upon the force of this law than the mass of matter.