|Standard Cyclopedia of Horticulture|
Hybrids are the products of crossing between species. Of late, the word hybrid has been used by most writers to comprise all crosses, whether between species or varieties. The justification of this usage is the fact that there are no hard and fast lines between varieties and species, and therefore that hybridism in the old sense is incapable of exact delimitation. The opponents to this usage, however, contend that so long as it is customary to speak of species and varieties as different classificatory categories, it is equally allowable and useful to speak of hybrids as between species and of crossbreeds as-between varieties; moreover, historical custom favors this usage. Common-language terms rarely if ever express absolute or ideal truth: they grow up by custom. Whenever new ideas and discoveries render them inexact, it may be quite as well to invent new terms as to give new and technical meanings to old terms which are thoroughly established in literature. The word hybrid has always been a specific term, and it were a pity now to make it a generic one, particularly since there is a well established generic term. The generic word, both substantive and verb, is cross. Specific kinds of crosses are hybrids, between species; crossbreeds, between plants of the same species; half- hybrid, between a species and a variety of another species; bigener, between plants of different genera. There are technical terms to designate the various kinds and degrees of crossing. The .word hybrid has now become so flexible, however, and other standards of measurement are so much in vogue, that these special terms are little used.
It was formerly held that inability to make fertile hybrids is proof that the forms are distinct species; and contrarywise, that plants which make fertile crosses are of one species. Hybridization has also been made a test of genera. These notions arc now given up, for crossing and classification belong to two unlike categories of facts. Species and genera are not entities in themselves, but are mere artificial groups made by men for their convenience when writing and speaking of living things. Crossing is a biological phenomenon.
Hybrids are unusual facts in nature; that is, they are rare compared with the whole number of plants. On the other hand, cross-breeds are usual. Most flowers are so constructed as to favor cross-pollination. Cross-breeding is one of the prune means of inducing slight variations and of invigorating a type. Upon the variations which arise from crossing and other means, natural selection operates in the production of new forms. But it is significant that these new forms usually come about slowly and gradually. It is the desire of the cultivator to produce new forms quickly and of pronounced distinctness. He therefore employs crossing between unlike types, or species, hoping thereby to secure wider departures. In nature, the cross-breed is the beginning of a process of breeding: it starts off the variation. Man is often tempted to look upon the hybrid as the end. If the products of a given cross are not to his liking, he throws them away and tries again. The most expert plant-breeders, however, now hybridize to get a "break," and thenceforth depend chiefly on selection to realize their clear-cut ideals, particularly in seed-propagated plants.
To man hybrids are of no value unless they can be propagated. By seeds they usually vary immensely: it is difficult to "fix" them so that they will come true. By cuttings or layers or division, however, the character of the parent may be propagated with practical certainty: the original plant is divided, and the parts are put on the market. Nearly all commercial hybrids are of plants which are thus propagated by asexual parts: Kieffer pear, hybrid grapes, Wilson blackberry, Wild Goose plum, cannas, roses, begonias, anthuriums, fuchsias, pelargoniums, rhododendrons. Since the hybrid is variable when propagated by seeds, continued selection, or plants-breeding, must be employed to fix and establish a desirable type.
It is thus seen that hybridization rarely gives rise to dominant horticultural seed-races, but rather to an indjvidual plant which may be disseminated by some divisional means of propagation. The seeds of hybrids —as of the modern cannas—may give rise to good varieties, and they may not; but these new varieties are, in their turn, usually propagated by means of asexual parts if they are to be kept true.
Practically there is no certainty in hybridization. Rarely can a man picture to himself an ideal variety, and then by means of hybridization produce it. He hybridizes plants which possess some of the characteristics of the desired or ideal variety, and then takes his chances. True plant-breeding sets an ideal, and then reaches it by working along certain definite lines. It seeks first to secure a variation in the desired direction: this may be secured by means of crossing, change of soil, modification of food-supply, and other changed conditions. It seeks, then, to preserve or augment the form by means of definite selection.
We are not yet able to formulate positive laws of hybridization. Every hybrid is a law unto itself. By the study of many examples of hybridization, one is able to construct an average of probabilities as to what will or what will not occur in a given case: but the given case may contradict all the probabilities without apparent cause. Hybridization is an empirical subject.
One cannot tell what species will or will not hybridize except by trying. Hundreds of species have been tried, and for them the knowledge is more or less exact. Plants hybridize most freely which are the subjects of much care and coddling: the orchids are the best examples. In these groups, hybrids are chiefly fanciers' plants, valuable often only because they are hybrids or are rare and curious. One cannot tell beforehand whether the products of any hybridization will be exact intermediates, or in what way or degree they will carry over or blend the parental characters. As a rule, the more closely akin the species, the more perfect will be the blending or amalgamation of the two. See Breeding of Plants, Vol. I.
The literature of hybridization is extensive but scattered. The possibilities of hybridization as a factor in plant-breeding are presented in many aspects in the Hybrid Conference Report" of the Royal Horticultural Society, London, 1900. There are special books devoted to orchid hybrids (see Orchids). See an excellent paper by Swingle and Webber. "Yearbook of the United States Department of Agriculture," 1897; papers in "American Gardening," 1899, pp. 397, 413, 431; Bailey & Gilbert's "Plant-Breeding;” De Uries' "Plant-Breeding." L. H. B.