Though the species of animals are all separated by an interval which Nature cannot overleap, yet some resemble others in so many respects that there seems only a necessary space to draw a line of separation. When we compare these neighbouring species, and consider them relatively to ourselves, some appear to be of the greatest utility, and others seem to be only auxiliary species, which might in many respects serve in the place of the former. Thus the ass might nearly supply the place of the horse, and the goat that of the sheep. The goat, like the sheep, furnishes both milk and suet in great abundance. Their hair, though coarser than wool, can serve the purpose of making very good cloth; their skins are more valuable than those of the sheep; and the flesh of a young kid nearly resembles that of lamb. These auxiliary species are wilder and more robust than the principals. The ass and the goat do not require near so much care as the horse and the sheep, for they every where find means of support, and browze equally on the most coarse as on the most delicate plants; they are less affected by the influence of the climate, and can do better without the aid of man; the less dependence they have on us, the more they seem to belong to Nature; and instead of considering these subordinate species as degenerations of the principal species; instead of looking on the ass as a degenerated horse; it might with more reason be said, the horse is an ass brought to perfection, and that the sheep is a more delicate kind of goat, which we have taken care of, brought to perfection, and propagated for our own use; and, in general, that the most perfect species, especially among domestic animals, take their origin from those wild and less perfect kinds which resemble them the most, as the powers of Nature are greatly augmented when united to those of man. Although the goat is a distinct species, and possibly further removed from the sheep than the ass is from the horse, yet the buck will as willingly couple with the ewe as the he-ass with the mare; the ram with the she-goat in the same manner as the horse with the she-ass. But though these couplings happen very frequently, and are sometimes prolific, yet no intermediate species has been formed between the goat and the sheep. The two species are distinct, remaining at the same distance from each other; no change has been effected by the intermixture, no new or middle race has arisen therefrom; at most they have only produced individual differences, which have no influence on the unity of each primitive species, but, on the contrary, confirm the reality of their different characteristics.