HYBRID (Gr. hybrid, from hybris, extravagance, licentiousness) is the term applied by naturalists to the offspring of different but generally nearly allied species of animals and plants, and must be distinguished from the word mongrel, which is applied to the offspring of different varieties of the same species.
M. Broca, whose memoir on hybridity of animals is the most complete that has yet appeared, remarks that this condition may he (1) natural, (2) excited (praroquee), or (3) artificial. The first variety is such as occurs spontaneously amongst animals in their wild state; the second includes those cases in which domesticated animals, which would not naturally cross with one another, do so under the influence of man, and in opposi tion to their natural instincts; while the third variety is due to the artificial admixture of the male and female generative elements, and as far as is yet known occurs only in fishes, and in the Vegetable kingdom. The second variety is by far the most common and the most important.
When the male of the species A can impregnate the female of the species B, it may happen that the process can be inverted, and that the male B can impregnate the female A. In other cases, however, while the male A can readily impregnate the female B, the male B cannot impregnate the female A. In the first case, the hybridity is termed bilateral; in the second, unilateral. The former is rare, and even when it does occur, the cross in one direction is more common and more productive than in the other. Thus, the ordinary mule, the offspring of the male ass and the mare, is much more readily obtained, and, physiologically, is less imperfect than the corresponding animal, the hinny, which occasionally results from the union of the stallion and female ass. See MULE, HINNY. Our domestic sheep and goats afford an example of the latter (unilateral) kind of hybridity. The union of the he-goat and the ewe is frequently productive, while the union of the ram with the is always unproductive.
In the present state of our knowledge, it. is impossible to predicate in what cases the crossing of different species will be productive, and in what cases it will be barren.
While some closely allied species do not admit of a cross, other species, far more removed from one another, not only yield hybrids, but even fruitful hybrids. There is, however, a limit, beyond which the chance of offspring becomes reduced to zero, and, according to Broca: "If the crossing of animals of different genera is now an incontes table fact, there is no authentic evidence that offspring has resulted from the crossing of animals of different orders." Cases have been referred to, as showing that animals of different orders may cross, but none of them are satisfactorily established. The strongest apparent case of hybridity between different orders is that of the jumarts, which were said to result from the union of the bull and the mare, or of the stallion and the cow. These jumarts were believed in from the time of Columella to that of Buffon, who fully investigated the subject, and found that they were merely hinnies—the offspring of the stallion and the she-ass. Among mammals, hybrids have been obtained between the different species of the genus eqaus. So far as experiments go, the horse, the ass, the zebra, the quagga, etc., breed freely inter se, but the degrees of fertility among their offspring have not been fully determined. The dog has been made to breed with the wolf and the fox, the lion with the tiger, the he-goat with the female sheep, the ram with the female roe-deer (corms capreolus), and the hare with the rabbit. (See Professor Owen's article, "Hybrid," in Brande's Dictionary of Science, Literature, and Art.) A case was recorded some time ago in The Field newspaper, in which a prolific union took place between a mastiff dog and a lioness that had been brought up together.
Among birds, hybridity is not uncommon. The swan will breed with the goose, the grouse with the blackeock, the pheasant with the common fowl,'the goldfinch with the canary, etc. reptiles, hybrid offspring has been observed between the toad and the frog. Among fishes, hybrids have been obtained by artificial impregnation between different species of the genus cyprinus.