Fertility, and the number of succeeding generations is increased, when a superiority is given to one of the crossed species over the other. This fact has been recognised in plants, and we meet with it again in animals. By crossing and recrossing in a fixed manner the goat and the sheep, hybrids called _chabins_ are obtained which possess three-eighths of the paternal and five-eighths of the maternal blood. These animals produce a fleece much valued in South America, and are the source of real industry. They can be maintained for several generations, but at length all the crossings to which they owe their existence must be recommenced, they having returned to the parental types, ‘like plants,’ as M. Gay said. This proportion—three-eighths to five-eighths—appears to be very favourable to the maintenance of hybrid races; it is the proportion which characterises the famous leporides, the result of the crossing of the hare and the rabbit. But can these hybrids, of which so much has been said, maintain themselves without reverting to the parental types? M. Roux evidently believed it, and it is still asserted by M. Gayot. But the testimony of those who have established and impugned their assertions leaves scarcely any room for doubt. Isidore Geoffroy, who had at first believed in their fixity, and had spoken of it as a conquest, did not hesitate afterwards to admit the reversion. The fact has been established in the Jardin d’Acclimatation, and M. Roux himself, upon the assertion of M. Faivre, appears to have abandoned his previous assertions. The observations and experiments made by the Agricultural Society of Paris clearly show that the _leporides_, sent or presented by the breeders themselves, had entirely reverted to the rabbit type. Lastly, M. Sanson, discussing the anatomical side of the question, has arrived at the same conclusions. Moreover, whoever will credit the observations made by M. Naudin upon the Linariæ, will easily recognise the _reversion_ and the _disordered variation_ exhibited by the leporides of the Abbé Cagliari, who was the first to obtain a fertile crossing between the hare and the rabbit. These phenomena appear in an equally well marked manner in the result of the cross between the silkmoth (_Bombyx cynthia_) and the castor-oil silkmoth (_Bombyx arrindia_), obtained by M. Guérin Méneville. The hybrids of the first generation were almost exactly intermediate between the two species, and resembled each other. In the second this uniformity disappeared, in the third the dissimilarity had increased, some of the insects having reassumed all the characters of the paternal or maternal types. In the seventh generation this curious experiment was destroyed by ichneumons. But, as M. Valée, their intelligent breeder, told me, nearly all the moths had returned to the type of the Bombyx arrindia. The resemblance to what took place in the case of M. Naudin’s Linariæ is here complete.