It has been remarked, that studs, situated in dry and light countries, produce active, swift and vigorous horses, with nervous legs, and strong hoofs, while those which are bred in damp places, and in fat pasturage, have generally large heavy heads, thick legs, soft hoofs, and flat feet. This difference arises from the climate and food, which may be easily understood; but, what is more difficult to comprehend, and essential to be known, is, the necessity of always crossing, or mixing the breed of horses to prevent their degenerating. There is in nature a general prototype of each species, from which each individual is modelled, but which seems in procreation to be debased, or improved, according to its circumstances, insomuch, that in relation to certain qualities, there is a strange variety in the appearance of individuals, and at the same time a constant resemblance in the whole species. The first animal, the first horse, for example, has been the exterior and interior model, from which all horses that have existed, or shall exist, have been formed; but this model, of which we are only acquainted with copies, may have fallen off, or arrived at greater perfection, by multiplying and communicating its form. The original form subsists entire in each individual; but though there are millions of individuals, yet no two exactly resemble each other, nor, consequently, the model from which they are sprung. This difference, which proves how far Nature is from making any thing absolutely perfect, and how well she knows how to shade her works, is exactly the same in the human species, in all animals, and in all vegetables; and what is singular, the model of what is handsome and excellent is dispersed through all parts of the earth, and that in each climate there is a portion thereof, which perpetually degenerates, unless united with another portion taken from a distant country; so that to have good corn, beautiful flowers, &c. it is necessary to change the seeds, and that they never should be sown in the same ground where they grew. To have fine horses, dogs, &c. it is proper for the males and females to be of different countries. Without this being attended to, corn, flowers, and animals, will degenerate, or rather take so strong a tincture of the climate as to deform and bastardize the species; the form remains, but disfigured in all the lines which are not essential thereto; by mixing, on the contrary, the kinds, and above all, by crossing their breed with foreign species, their forms seem to become more perfect.