HYBRIDISM. This term has its origin in a Greek word meaning an insult or outrage : a hybrid or mongrel used to he regarded as an outrage on nature—being the offspring of wide crosses, e.g., the mating of , individuals belonging to different species; a mongrel, the offspring of the crossing of different varie ties within a species. But since Mendel worked and wrote hy bridity has come to possess a very definite meaning. Any indi vidual that is heterozygous for one or more of the hereditary characters it displays is in respect of that character or those char acters a hybrid, for hybridity is but another name for heterozygo sis. (See HEREDITY.) The degree of hybridity exhibited by the offspring is determined by the extent to which the hereditary con stitutions of the two parents differ one from another.
Many species are mutually sterile in matings. Many inter specific hybrids are remarkable in that among them one sex is absent, rare or infecund. Many hybrids are manifestly better equipped in their general characterizations in relation to some particular task or destiny than are the parental pure-bred forms.
The mating of individuals differing markedly one from the other in their characterizations, e.g., belonging to different species, is commonly impossible or unproductive. Two species are normally debarred from producing fertile offspring by any of the following means :—Differences in breeding season or breeding place; psy chological repugnance of one from, or failure to be stimulated by the other's recognition marks, scents, peculiarities in courtship be haviour, or mechanical stimuli; mechanical disharmony of the cop ulatory apparatus; disharmonies in fertilization and development.
These, limiting interbreed ing between allied species, are illustrated in the records of Lo Bianco (1909) relating to the period of sexual maturity of Mol lusca and Crustacea in the Gulf of Naples.
These are well illustrated by the case of parasites. The same host species may be occupied by several parasite species but the total range of each parasite, as far as it is known, differs. Another illustration is provided by the ob servations of Johansen on the distribution of animals in Rander's Fjord in relation to salinity.
It is uncertain whether unwilling ness to mate with an individual of another species is a widespread phenomenon. The records of hybridization in captivity make it evident that sexual intercourse is by no means uncommon. Pair ing inclination is not a fair index of morphological affinity nor is disinclination to mate necessarily an index of mutual sterility as can be shown by artificial fertilization. Repugnance has been re corded in mammals (the guinea-pig, Detlefsen 1914) ; in birds (Columba, Staples Brown) ; in Pulmonate Gastropoda (Cepea, Diver) ; and in insects (Morrison 1924) ; but these instances are by no means so numerous as are records of complaisance. The ex tent to which any particular kind of behaviour as a means of sexual excitation is definitely repugnant to individuals of a different spe cies is most difficult to ascertain. Doubtless the role of special scent glands, special markings and mechanical stimuli as means of isolation is important in certain groups. In Lepidoptera the scent glands act as a bar to coitus (Petersen, 1904). The olfactory stimuli are important in Drosophila (Sturtevant, 1921). There are specific differences in the flash emitted by fireflies (McDer mott, 1917), which renders interspecific matings unlikely. Mechanical Disability.—Complete mechanical disability ob tains in those cases in which one form liberates its gametes into a fluid medium, having no differentiated external reproductive or gans whilst the other practises, and is equipped for, internal fer tilization. In cases in which, in both forms, internal fertilization is normal, disharmonies in the methods of sexual intercourse, in the shape and size of the intromittent organ and the vagina, in the construction of special devices for the conveyance of the spermatozoa, can lead to mutual sterility.
Even though the gametes are brought together, fertilization may still be impossible and consequent development may remain im perfect or incomplete. Sterility may then be due to inability on the part of the sperm to penetrate the ovum, failure of cleavage at various stages on the part of the fertilized ovum (Newman, 1915), cessation of, or disharmony in, development of the embryo (Bon nier, 1924), low vitality of the hybrid form (Harrison, 192o), disharmony in the development of the reproductive organs of an otherwise normal hybrid (Goodspeed, 1917), or aberrations in the chromosomal distributive mechanism.
The classical experiments of Godlewski and Loeb on artificial fertilization have shown that in certain instances of failure on the part of the sperm to penetrate the egg fertilization can be in duced by rendering the surrounding medium more alkaline.
In many hybrids gametogenesis—the production of functional ova and sperms—is incomplete and imperfect. This may be due to actual disorganization of the nuclear mechanism, taking the form of abnormal synapsis (Smith and Thomas, 1914) ; failure on the part of the chromosomes to conjugate (Harrison and Don caster, 1914) (Federlay, 1914) ; polyploidy (Gates, 1921). The fertility or otherwise of hybrids would seem to be determined by the presence or absence of a balance between the chromosomes, and not by their actual number.
The cause of the beneficial result of the meeting of differently constituted germplasms, of the marked gen eral vigour of hybrids, or heterosis as it is technically called, has been extensively studied. It has been suggested that this heterosis is due either to a physiological stimulus, the effect of which is revealed in an increase in cell division, or else to the pooling of appropriate hereditary factors contributed to the offspring of the two parents. East and Jones (1921) have produced a mass of evidence which supports this latter interpretation. Heterosis is well exemplified in the common mule, which though sterile, is hardier, stronger and larger than either parent. In general, first generation hybrids are an improvement on either parent, but usually the increased vigor is shown at its best only in the first generation after the cross.
Hybrid vigour can be produced by the mating of unlike strains within a species. This is often taken advantage of commercially. Pig-breeders, for instance, by mating pigs of distinct breeds, obtain animals which mature more rapidly than either parent stock.
subject is thoroughly discussed and a full Bibliography.-The subject is thoroughly discussed and a full bibliography is given in The Species Problem by G. C. Robson, Oliver & Boyd (1928) . See also C. Darwin, Origin of Species (1859) ; East and Jones, Inbreeding and Outbreeding (1919). (F. A. E. C.)
