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Hybridism, the crossing of distinct species of plants or animals, the term mongrel being used for the result of crossing two varieties of one species. The subject is of practical importance, especially in horticulture, as a means of introducing new forms; but of still more importance theoretically in relation to the theory of evolution. The principles of hybridism are much the same in the animal and the vegetable kingdoms.

There is no known authentic case of the fertile crossing of two species belonging to distinct families; but there are numerous cases, especially among plants, of such fertile crossing of species belonging to absolutely distinct genera. They are known as bigeners or bigeneric hybrids. Among orchids, for instance, it often happens that a plant belonging to genus a when pollinated by one belonging to genus b will produce seed; that not only will this seed grow, but that the plants thus produced may be pollinated by either of the parent forms, or even by some third species belonging to a third genus, c; and so on. Pollen from a plant of a distinct family, however, when placed on a stigma is as inoperative as so much mere dust.

It has been urged as an objection to the theory of evolution that distinct species are not, as a rule, capable of producing hybrids, and that such hybrids, when produced, are themselves sterile - the case of the mule, the domesticated hybrid of the horse and the ass, being a familiar' instance in point. It is argued that specific types have in this way been preserved immutable from the beginning. To this it may be replied: - (1) that, though the theory of evolution implies mutability of type, it, does not suppose such mutability to have resulted from hybridism; (2) that the argument is mainly a circular fallacy, forms that are mutually sterile being, for that reason, ranked as distinct species, and then the conclusion drawn from them that species are mutually sterile; (3) that a distinction must be drawn between the sterility of the two species when crossed and that of their hybrid offspring, the two things by no means always varying together: and (4) that in this matter of sterility there is not the universal distinction, which is alleged, between the natural species of a genus in a wild state and the domesticated varieties of one species; but that the difference is only one of degree, cases occurring of every degree of fertility in the latter.

The facts of hybridism are often very remarkable. There is fairly conclusive evidence that our domestic races of dog have sprung from more than one wild species; but they are all now freely fertile among themselves. The hare and the rabbit have bred together, producing offspring extremely fertile with either parent form; but it is by no means a general rule'that reciprocal crossings are equally fertile, i.e. that, if the male of species A produces fertfle offspring with the female of species B, the male of B will do so with the female of A. Many closely-allied species cannot be crossed, or can only be so with difficulty, whilst others far more divergent in character may cross freely. Annual plants have been crossed by perennials, deciduous trees by evergreens, plants flowering at one season by those flowering at another, and natives of one country by those of another differing in climate. On the other hand, whilst several species of passion-flower (q.v.) can be readily crossed, they cannot be fertilised by pollen of their own species.

Darwin suggested, as an explanation of the admittedly frequent sterility of domesticated hybrids, the consideration that the reproductive system of an organism is the most delicately susceptible to slight changes in conditions such as, domestication produces. It should, however, be borne in mind that in not a few cases domestication has induced, not sterility, but extreme fertility in hybrids.