Again, if we consider the difference of the temperament, disposition, and manners; in a word, the organization of these two animals, and, above all, the impossibility of mixing the breed, so as to make one common species, or even an intermediate species, which may be renewed; it appears a better founded opinion to think that these animals are of a species equally ancient, and originally as essentially different as they are at present. The ass differs materially from the horse in the smallness of the size, largeness of the head, length of the ears, hardness of the skin, nakedness of the tail, the form of the rump, and the dimensions of the neighbouring parts, the voice, the appetite, manner of drinking, &c. Can we then suppose that the horse and the ass came originally from the same stock? are they of the same family, or not? and have they not always been different animals?
This question of which philosophers will find the generality, difficulty, and consequences, and which we treat of in this article, because it here offers itself for the first time, appertains to the production of beings nearest to each other, and renders it necessary that we should consider nature under a new point of view. If from the immense variety of animated beings which people the universe, we chuse an animal, or even the body of man, to serve as a foundation to our knowledge, and to find out, by way of comparison, the other organized beings, we shall find that each possesses an independent existence, and that all vary, by different gradations, almost to infinity; there exists also, at the same time, a primitive and general design, which we may trace very far, and of which the gradations are much slower than those of the form, and other apparent relations, for, without mentioning the organs of digestion, circulation and generation, which appertain to all animals, and without which they could neither subsist nor reproduce, there is even in the parts which contribute most to the variety of the exterior form a prodigious resemblance, which necessarily calls to our minds an original design, upon which all seem to have been projected and executed. The body of a horse, for example, which, by a single glance of the eye, appears so different from the body of a man, when it is compared part by part, instead of surprising by the difference, only astonishes by the singular and almost perfect resemblance, in fact, take the skeleton of a man, bend downwards the bones of the pelvis, shorten those of the thighs, legs, and arms, lengthen those of the feet and hands, join the phalanges, lengthen the jaws, by shortening the frontal bone, and extend the spine of the back, this skeleton would cease to represent the remains of a human figure, and would be the skeleton of a horse; for it is easy to suppose, that in lengthening the spine of the back and jaws we augment, at the same time, the number of the vertebræ, ribs, and teeth; and it is only by the number of those bones, which may be looked upon as accessory, and, by the prolongation, the shortening, or junction, of the others, that the skeleton of a horse differs from that of the human body. We see in the description of the horse these facts too well established to doubt; but, to follow these relations still further, let us consider separately some essential parts of the structure; for example, we find ribs in all quadrupeds, in birds, and in fish; and we find the vestiges even in the shell of the turtle. Let us also consider, that the foot of a horse, so different in appearance from the hand of a man, is, notwithstanding composed of the same bones, and that we have, at the extremity of each of our fingers, the same little bone resembling a horse-shoe, which terminates the foot of that animal. From this we may judge if this hidden resemblance is not more marvellous than the apparent differences; if this constant conformity and design followed from man to quadrupeds, from quadrupeds to cetaceous animals, from cetaceous animals to birds, from birds to reptiles, from reptiles to fish, &c. in which the essential parts, as the heart, intestines, spine, senses, &c. are always found, does not imply, that, in creating animals the Supreme Being has followed but one idea, and varied it, at the same time, in every possible manner, that man may equally admire the magnificence, execution, and simplicity of the design.