HYBRIDITY. The phenomenon of the sex ual crossing of two individuals belonging to dis tinct species. A hybrid is the product of this crossing, and is contrasted with a mongrel, or the product of individuals belonging to distinct races or varieties of the same species. The importance of hybridity is threefold: (1) A Test of Speeics.—Hybridity has been used by Cuvier and others as a test of species, as contrasted with varieties. It has been assert ed that hybrids are sterile, whereas mongrels are fertile. So long as infertility of descendants is used as a criterion of species. this tenet is un assailable; the difficulty arises from the fact that the strict application of the criterion leads us to deny specific distinctions to animals that are commonly regarded as such—e.g. the dog and wolf. (log and jackal, the hare and rabbit, vari ous species of Bovida or oxen, sheep and goat, and, among plants, various forms of Rhododen dron, Gladiolus, Dianthus, Nieotiana, etc. Focke says concerning plants: "Many hybrids, especial ly those between unlike lines of descent, are in fertile, most show a diminished fertility, some a nearly normal fertility." It may be concluded, therefore, that the Cuvierian definition of spe cies will not hold. While fertile hybrids are thus not common, the capacity for hybridization in the first generation is widespread.
The question arises, Why are hybrids so often sterile ? Various hypotheses have been proposed. but none can be said to be proved. Darwin's hypothesis is thus summarized: "The sterility of first crosses and of their hybrid progeny has not been acquired through natural selection. In the ease of first crosses it seems to depend on several circumstances; in some instances on the early death of the embryo. In the ease of hy brids, it apparently depends on their whole or ganization having been disturbed by being com pounded from two distinct forms, the sterility being closely allied to that which so frequently affects pure species when exposed to new and un natural conditions of life. lie who will explain these latter cases will be able to explain the sterility of hybrids. Wallace has a somewhat different view. A new species arises in con• ucetion with some slightly varying conditions of ekviromment. If these are sufficient to create a change in coloration or form, they will, by virtue of correlation. affect also the germ cells, and may give rise to a certain amount of in fertility. Now the infertility will be beneficial whenever new species arise in the same area with the parent form. because it would result in a destruction of both of the adapted forms. Con sequently, whenever two species which are at the same time adapted and infertile arise, they will retain their respective mlapted conditions and will survive. Finally, Catchpool and Romanes
have independently suggested an explanation that is worthy of consideration—it is that the sterility of species is not something acquired, hut is primary. Whenever from any cause two lots of individuals of one species become partly in fertile inh r se, those lots are, as it were, seg regated. Each of them can develop its own way and give rise to a distinct, species. If the species were not thus sterile at the beginning. differen tiation would he difficult.
(2) nelation 10 hateritonee.—Ilybridity is im portant for the study of the laws of inheritance. :since the parents are more unlike, the nature of their combinations is still more interesting than in the case of ordinary sexual reproduction. Hybrid: are particularly apt to show a reversion to an ancestral condition, especially when op f,osing characters are intermingled. The folloy in concerning hybrids have been ed by Focke: (a) "All the individuals formed by the crossing of two pure species or races are. if they have been produced and grown under the same conditions, exactly like each other as a rule. or they differ hardly more than specimens of one and the same species are apt to do." This propo sition is subject to many exceptions. In certain crosses the female element is predominant in the progeny; in others the male. Especially in cases of alternative heritage (see HEREDITY) the hybrids may he of two types, the one resem bling more the one parent. the other type the other parent. (b) "The characteristics of the two crosses may be different from those of the parent species. They differ most from both parent species in size and luxuriance. as well as in fertility." In the case of alternative heritage an interesting rule was worked out by Mendel. He finds thnt one of the species is prepotent(p), the other subpotential(s). The hybrids will he either of the p or the a species, and the relative proportion will be p 75 per cunt_ a 25 per cent. If. how, self-fertilization occur, 50 per cent. of toe plants will produce either p or s in the proportions 75 per cent. p. 25 per cent. 5. All the descendants of p and s breed true. The consequenee of Mendel's 'law of dichotomy' in hybrids is that the proportion of the pure races is constantly increasing in the successive genera tions deseended from a hybrid. "Malformations and curious forms are notch more common. espe cially in the flower parts of hybrids, than in in dividuals of a pure descent." Double flowers ap pear to he formed espeeially- easily in hybrids. Hybrids are frequently infertile, because the pol len is imperfectly formed.