These then are the analogical reasons on which the above presumptions are founded. In the common course of nature, it is not the males but the females which constitute the unity of species. We know from the example of the sheep, which propagate alike with the ram, or the goat, that the female has much more influence than the male, on the specific qualities of the production, since the only issue from these two different males are lambs, that is, individuals which have a specific resemblance to the mother. Thus the mule resembles the mare more than she does the ass, and the bardeau more the she-ass than the horse; therefore _the mule ought to produce more certainly with the mare than with the she-ass, and the bardeau still more so with the she-ass than with the mare, so the horse and he-ass might possibly produce with both the she-mules_; because being females, though somewhat vitiated, each retains more specific qualities than the male-mules; _but the he-ass should produce with them more certainly than the horse_; because it is observed, that the he-ass possesses stronger prolific powers than the horse, even with the mare, for the first corrupts and totally destroys the generation of the latter. We may be convinced of this fact by first taking a stallion to a mare, and the next morning, or even some days after, serving her with a male-ass, and her production will always be mules, and not horses. This fact, of which every circumstance deserves attention, seems to indicate, that the ass and not the horse, is the stock, or principal root of the family, since the first predominates by its prolific powers over the latter even with its own female, especially as, if the ass is first given to the mare and the horse afterwards, the latter does not destroy the generation of the former, for even then the production is still a mule. On the other hand, the like effect does not happen when the he-ass precedes the horse, with the she-ass, for the latter never destroys the operation of the former. With respect to the copulation of mules among themselves, I have presumed it to be sterile, for we can expect nothing else from two natures already debased by generation, and which by their union cannot fail of being still more debased, than a production entirely vitiated, or absolutely none at all.