a term derived from the Greek ὑβρίς, a mule, is used by naturalists to denote the offspring of two animals or plants of different species. Of these the best known and most useful is the progeny of the horse and the ass, which has from remote antiquity been largely employed in the East as a beast of burden, and is well known in Europe as the most sure-footed of domestic quadrupeds. The male parent may belong to either species; but the offspring of the mare and the he-ass is superior in size and strength to that of the stallion and the she-ass. To the former Buffon restricts the name of mulet or mule, and names the latter bardesca. Hybrids have also been produced between the dog, the wolf, the jackal, and the fox; between the lion and the tiger; between the sheep and the goat. These may be considered as hybrids between allied species, or may be termed specific hybrids. But hybrids are known also between the stallion and the cow, the cow and the stag, the antelope and the goat; and even between the bull and the sheep, animals belonging to different genera, which may be termed generic hybrids.

Other classes of animals afford instances of hybrid progeny. Specific hybrids occur among birds; as between the hooded and carrion crows, the goldfinch and the canary, the latter and the linnet, the wood-grouse and the blackcock, the goose and the swan; and it is said that generic hybrids have occurred between the pheasant and the Tringa pugnax or reeve. Among fishes, hybrids might have been supposed to be more common than among the classes already noticed; because their spawn must be frequently mixed; yet such are not very common, though they are found among Salmonidae, Cyprinid, and a few other genera. Among reptiles, hybrids have occurred between the toad and the frog; but Spallanzani was unable to obtain them between the frog and the newt, or between the latter and the toad. In the division of insects, hybrids, both specific and generic, are not unknown; as among different species of Papilionidae and Chrysomelidae; and they are said to have arisen from the congress of Melalontha agricola with Cetonia hispa, of Cantharis melanura with Elater niger.

In the vegetable kingdom, although the well-known experiment of grafting different plants on the same stock, when each produces its own fruit, would show that the food of plants is nearly similar; and although the pollen of one flower is apt to be carried to the pistil of a different flower by the winds, or by insects, yet hybrid plants are rarer in a state of nature than we might have expected. But in our conservatories, where many different plants are in juxtaposition, and more especially if artificial impregnation be adopted, hybrid plants are produced. This is well known to florists, who have thus obtained varieties of the rose, calcareous, &c. But the comparative rarity of hybrids among native plants shows that there is some natural impediment to the production of hybrids in the vegetable kingdom.

With regard to animals, nothing is better established than that individuals of different species, when left to themselves, do not voluntarily copulate. Thus, the stallion will not cover the ass if a mare be present; and to effect copulation of the mare with the he-ass, she must be blindfolded. Yet we have seen the stallion without repugnance cover the female mule when in heat, and the she-mule readily receive the ass. The hybrids between the zebra and the ass are readily produced also, if the natural mates of either be not present. The curious experiments of Buffon prove, that individuals of different species are not naturally inclined to copulate. He reared in the same kennel the young of both sexes of the dog, the fox, and the wolf; and when young they played familiarly together; but the females of each species, when in heat, exhibited insurmountable repugnance to receive the males of the other species; and mortal combats, instead of copulation, took place between different sexes of the different species. John Hunter also found that when he had a she-wolf lined by a greyhound, it was necessary to hold her fast; and, although during copulation she remained pretty quiet, immediately after she attempted to attack the greyhound.

It is well known, that hybrid animals are generally barren; and this seems a provision of nature to prevent the confusion of species. There are a few instances on record of common mules producing young. Both the male and female mule have perfect generative organs, and are uncommonly salacious; yet their congress with each other is almost always sterile. It has, however, been long known, that the mule can engender with the mare, as is mentioned by Aristotle, who states also, that the she-mule conceives, but very seldom brings her fetus to perfection. We have seen a foal that was dropt by a female mule; but the circumstance is very rare. The fertility of certain other hybrids with males of the pure species is noticed by Buffon, in his Dissertation on Mules, and in the celebrated experiments of John Hunter. Hunter submitted the female hybrid between a jackal and a dog to a male terrier, and the hybrid between a dog and a wolf to a male greyhound; and puppies in both instances were produced. Hence he inferred, that the dog, the jackal, and the wolf, belong to the same species. This conclusion is perhaps not so decisive as that great anatomist supposed: for may it not be merely the effort of nature to return to the pure breed? Yet this argument, with the total want of any wild prototype of the domestic dog, unless we derive it from the wolf, the jackal, or the fox, would favour the idea of their original identity. The difference in form and size is not a strong objection to this view; for it is certainly not greater than between the mastiff and the greyhound, the water-spaniel and the lap-dog; of the specific identity of which we entertain no doubts. In a state of nature, however, hybrids are seldom if ever produced in the animal kingdom, even among nearly allied species, and nature seems to abhor such intermixtures.