Hybrid (Gr.Hybrid 090020 ), an animal or plant produced by the sexual union of individuals belonging to two different species. As a rule, in nature sexual union takes place only between individuals of the same species, and the offspring accordingly presents the specific characters common to both its parents. It is in this way that the species is indefinitely maintained, with its distinctive characters, by the constant production of new individuals similar in appearance to the old and endowed with similar powers of reproduction. But union between a male and a female of different species, when fertile, produces an offspring which does not precisely resemble either of its parents, but presents a mixture in nearly equal proportions of their separate characters. Thus a mule, which is the most commonly known example of a hybrid, is neither a horse nor an ass, but something intermediate between the two, and is without the complete distinctive marks of any recognized animal species. One of the most important questions relating to hybridity is that of the possible fertility of sexual union between different species, and that of hybrids of the same or different kinds between themselves. In nature, the occurrence of hybridity is extremely rare.

This may be due to the more or less complete inaptitude of the male and female generative products to unite with each other in such a way as to produce a fertile result. Thus the germ and pollen of different flowers, or the ovum and spermatic fluid of different animals, may be incapable of fertilization, owing to peculiarities of their own internal constitution; and consequently their physical contact would produce no result. But there are other reasons upon which the non-occurrence of hybrids in nature may partly depend. Among animals there is an instinctive preference for sexual union with their own species rather than with others, and a similarity of habits, of locality, and general disposition, corroborates this preference, and alone makes it much more likely that sexual union, as a matter of fact, will take place between animals of the same species. A certain degree of similarity in the physical structure of the parents is essential to the fertility of their sexual union. Thus all the most frequent and most useful forms of hybridity occur between different species belonging to the same genus.

The horse, for example, will breed with the ass, the zebra, and the quagga; the dog has been certainly known to breed with the wolf, and probably with the fox; the goat with the sheep, the ram with the roe; and it has been comparatively easy to obtain hybrids from the union of the rabbit and the hare. But a cross union is not necessarily fertile, even between species of the same genus; between those of different genera it is still more exceptional; and it is doubtful whether hybridity, either natural or artificial, has ever occurred beyond these limits. The second question of interest relating to hybridity is that of the fertility of hybrids among themselves. As a rule it may be said that hybrids are not fertile. Thus the mule does not reproduce itself, but is only obtained by a repetition of the union of the assand the mare. The female mule will sometimes reproduce by union with either the horse or the ass; but in this case the offspring is no longer a mule, but reverts to the type of the original stock in precise proportion to the admixture of blood resulting from the union.

Notwithstanding, therefore, that the mule and its mode of production have been known from time immemorial, and notwithstanding the recognized usefulness of its qualities in some respects, we have never been able to obtain an independent and self-reproductive breed of mules; that is, the hybrid has never acquired the physiological characters of a natural species. - The terms hybrid and hybridization are often vaguely used as applied to plants, and many are called hybrids which are only crosses between varieties. The name hybrid should be restricted to plants resulting from the seeds of one species fertilized by the pollen of another species; those forms produced by cross breeding between varieties of the same species should never be called hybrids, but crosses. It is to be regretted that horticulturists generally ignore this distinction and use the terms hybrid and cross as synonymous. Hybrid plants sometimes occur in nature, and are frequently produced artificially. In hybridizing, it is necessary to prevent the flower used as the mother, or seed-bearer, from being fertilized by its own pollen both before and after the artificial application of the strange pollen; the operator is favored by the fact that pollen retains its vitality for some time after it is removed from the flower which produced it.

It is probable that with this, as with seeds, the duration of vitality varies in different species; at all events, it is known that some pollen will keep for weeks and even months. The flower selected as the seed-bearer is taken just as it is about to open and before any insects can have visited it; the envelopes are carefully opened or removed, and if a perfect flower its still unopened stamens are cut away with a delicate pair of scissors, the foreign pollen applied to the stigma with a small brush, and the flower or flowers enclosed in a bag of gauze to prevent the access of insects, which would probably bring pollen of the same kind to interfere with the action of the strange pollen. This is a brief outline of the process; there are details which can be learned by practice. It is not possible to know beforehand whether two species will hybridize; two species of a genus that seem to be the most nearly related will sometimes refuse to be hybridized, while other two that seem most unlike will readily form a union.

It makes a difference also which plant is chosen as the seed-bearer and which as the pollen-bearer; for instance, the pistil of A will refuse to be fertilized by the pollen of B, while the pistil of B will readily accept the pollen of A. Seeds from the flowers thus fertilized may produce plants quite intermediate between the two parents, or may more strongly resemble the one or the other. Sometimes a hybrid will have the leaves of one parent and the flowers and fruit of the other. By this means horticulturists have produced useful varieties of fruit, notably in grapes and strawberries, and some of the finest flowers are the result of hybridizing. Among hardy flowers, the rhododendrons and azaleas are striking examples of the improvement that may be effected in this manner; the fine rhododendrons are hybrids between the hardy R. Catawbiense of the southern Alleghanies and P. Ponticum, a greenhouse species from Asia Minor. It is a singular fact that the English hybrids, in which P. Catawbiense is the mother plant, are generally hardy, while the Belgian hybrids are very much less hardy for the reason that the Belgian florists use R. Ponticum as the seed-bearer. When a desirable form is obtained by hybridizing, it can be continued and multiplied indefinitely by means of layers, cuttings, or grafts.

Hybrid plants are sometimes fertile; the progeny from them shows a tendency to revert to the one or the other parent, and in a few generations all trace of the admixture is obliterated; sometimes the progeny is too weak to bear seeds, and thus becomes extinct. More generally hybrid plants are wholly or partly sterile; the degeneration shows itself most prominently in the anthers, which fail to produce pollen; the pistil in this case will be fertilized, if at all, by pollen from either parent, and thus a reversion of its progeny to a normal form assured; sometimes the pistils are abortive also. It will be seen that while hybrids may be produced among plants in a wild state, and are often produced in cultivation, there is abundant provision against the perpetuation of a race of monsters. - Another kind of hybrid in which fertilization plays no part has recently received the attention of vegetable physiologists. There are a number of well authenticated cases in which a graft or bud has so influenced the stock in which it was inserted that the stock, even below the point of union, put out branches partaking of the characters of both stock and scion. Some of these graft hybrids, as they are called, have been propagated.

An account of this kind of hybrids, as well as a very full resume of the whole subject of hybrids, will be found in Darwin's "Variation of Animals and Plants under Domestication." See also his "Origin of Species," and E. A. Carriere's Production et fixation des varietes dans les vegetaux (Paris, 1865).