Examples of fertility in the hybrids of the ass and the horse have never been observed except in the _female mule_. There is not a single known example in the _male_. We meet with something analogous to this in birds, where the sterility of certain hybrids is less absolute. Thus vertebrata are similarly affected with plants; and in their case also the inequality between the two sexes can be explained by anatomical and microscopic examination. The male organs are generally but slightly developed, even the essential elements of the fertilising liquid undergo alteration. The female organs and elements, though modified, are relatively unaffected. There are some hybrids among animals, as among plants, which are not subject to the general law. Among birds in particular, a certain number, always however very limited, of more or less fertile hybrids have been obtained. But, with the males the faculty of reproduction is constantly weakened, and habitually disappears before the usual age; the female lays more rarely, and the eggs are fewer in number, and very often _clear_. This is an exact repetition of what took place in M. Naudin’s datura seeds, which he observed to become abortive or devoid of embryo. We must, moreover, exclude from the number of fertile hybrids a certain number of examples quoted by some authors, and which statements are proved by facts, now either better known or better appreciated, to have an erroneous foundation. Thus Hellenius thought he had crossed the Finnish ram with the Sardinian doe, but he had confounded the then little known moufflon with the roebuck. He thus obtained a mongrel, which having been crossed for two generations with the male parent, returned to the type of the latter. We have here evidently only a companion experiment to those of Koelreuter, which resulted in a reversion of the hybrid to the male type under a similar series of crossings. There are, however, some examples among birds and among mammalia of hybrids which have propagated _inter se_ for several generations, four or five at the most. The celebrated experiment of Buffon upon the crossing of the dog with the wolf in particular, belongs to this order of facts. It was unfortunately interrupted by the death of the great naturalist at the fourth generation. It is clear that there is nothing here which does not perfectly agree with our observations upon hybrid plants, which, although exceeding this number of generations, have never produced hybrid races.