The most constant and invariable thing in Nature is the image or model allotted to each particular species, both in animals and vegetables; what is most variable is the substance of which they are composed. Matter, in general, seems to receive all forms with indifference, and to be capable of all configurations; the organic and living particles of this matter pass from vegetables into animals, without suffering dissolution or alteration, and equally form the living substance of herbs, trees, flesh, or bones. It may seem from this first glance that matter can never predominate over form, and that no sort of nourishment taken by the animal, provided he can draw out the organic particles, and assimilate them to himself by nutrition, can occasion any change upon his form, and can have no effect but that of supporting, or adding to the growth of his body. Of this we have a proof in those animals which live solely upon herbage, who, though a substance widely different from their own bodies, draw from it every thing necessary to constitute flesh and blood, and will even exceed in bulk those who feed upon animal food. In taking a more particular view of Nature we find this is not always the case. Height, for example, which is one of the attributes of form, varies in every species according to the difference of climate; as do the quantity and quality of the flesh, two other attributes of form, according to the different kinds of food. This organic matter, therefore, which the animal assimilates to its body by nutrition, is not absolutely indifferent to the reception of this or that modification: it is not deprived of its original figure; it continues to act in its own form, and though this action be almost imperceptible, yet, in process of time, it necessarily produces very sensible effects. The stag, who inhabits the forests, and lives only upon wood, produces a species of trees, which is nothing more than the superabundant part of his food. The beaver which inhabits the water, and feeds upon fish, has a tail covered with scales; and the flesh of the otter, as well as of most aquatic fowls, is of a fishy nature. It may therefore be presumed, that animals which live constantly upon one kind of food will, in time, imbibe a tincture of its aliment; and however strong the original impression of nature may be, a kind of transformation will take place by an assimilation contrary to the first. In this case the nourishment no longer assimilates entirely to the form of the animal, but the animal assimilates in part to the form of the nourishment, as is seen in the horns of the stag and the tail of the beaver.