But however that may be, it is certain from what we have said, that mules in general, which have always been accused of sterility, are nevertheless neither really nor universally so; and that this sterility is only manifested in that particular kind of mule proceeding from the connection of the ass and the horse; for the mule produced by the he-goat and the ewe, is as prolific as its parents, and most mules which proceed from different species of birds, are not barren; therefore it is only in the particular nature of the horse and ass, that we must seek for the causes of the infecundity of the mules produced by them; and instead of supposing barrenness a general and necessary defect in every mule, it, on the contrary, should be limited to that mule alone which proceeds from the ass and the horse, and this limitation should be further restricted, as these mules prove prolific in certain circumstances, especially when brought a degree nearer their original species. The mule, produced by the horse and the ass, has its organs of generation as complete as other animals; nothing seems wanting either in the male or female. The males have a great plenty of seminal liquor; and being never suffered to copulate, they are often so pressed for a discharge, that they frequently rest upon their bellies for that purpose. These animals are, therefore, provided with every thing necessary for the purpose of generation: they are even very ardent, and consequently, very indifferent in their choice. The males have nearly an equal vehement desire for the female mule, the she-ass, and the mare. There is, therefore, no difficulty in procuring the copulation, though it requires particular attention and care to render it prolific. A too strong ardour is often attended with sterility; and the female mule is at least as ardent as the she-ass. Now it is known that the latter rejects the seminal liquor of the male, and that to make her retain it, blows must be given, or cold water thrown over her crupper, to calm the convulsive emotions of desire which subsist after copulation, and which occasion this rejection. The she-ass, and the female mule, therefore, incline to sterility by their over-heat. The asses incline to it from another cause; for as they are originally natives of hot climates, cold opposes their generation, and this is the reason they are allowed to couple in summer only. If their union is permitted at any other time, and particularly in winter, it is seldom attended with impregnation. The season necessary to the success of their generation is as much so for the preservation of their production. If the young ass is not brought forth in warm weather it either languishes or dies; and as the time of the gestation with the ass is only once a year, she produces at the season she conceives: this sufficiently proves how necessary warmth is, not only for the fecundity but also for the life of these animals. This strong ardour of the