as the result of careful experiments made by very competent experimentalists. On the whole, the results of some of these experiments, although so few in number, must be regarded as making out a strong case in favour of the possibility of graft-hybridism. For it must always be remembered that, in experiments of this kind, negative evidence, however great in amount, may be logically dissipated by a single positive result.
Theory of Hybridism.—Charles Darwin was interested in hybridism as an experimental side of biology, but still more from the bearing of the facts on the theory of the origin of species. It is obvious that although hybridism is occasionally possible as an exception to the general infertility of species inter se, the exception is still more minimized when it is remembered that the hybrid progeny usually display some degree of sterility. The main facts of hybridism appear to lend support to the old doctrine that there are placed between all species the barriers of mutual sterility. The argument for the fixity of species appears still stronger when the general infertility of species crossing is contrasted with the general fertility of the crossing of natural and artificial varieties. Darwin himself, and afterwards G. J. Romanes, showed, however, that the theory of natural selection did not require the possibility of the commingling of specific types, and that there was no reason to suppose that the mutation of species should depend upon their mutual crossing. There existed more than enough evidence, and this has been added to since, to show that infertility with other species is no criterion of a species, and that there is no exact parallel between the degree of affinity between forms and their readiness to cross. The problem of hybridism is no more than the explanation of the generally reduced fertility of remoter crosses as compared with the generally increased fertility of crosses between organisms slightly different. Darwin considered and rejected the view that the inter-sterility of species could have been the result of natural selection.
"At one time it appeared to me probable," he wrote (Origin of Species, 6th ed. p. 247), "as it has to others, that the sterility of first crosses and of hybrids might have been slowly acquired through the natural selection of slightly lessened degrees of fertility, which, like any other variation, spontaneously appeared in certain individuals of one variety when crossed with those of another variety. For it would clearly be advantageous to two varieties or incipient species if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate. In the first place, it may be remarked that species inhabiting distinct regions are often sterile when crossed; now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile and, consequently, this could not have been effected through natural selection; but it may perhaps be argued that, if a species were rendered sterile with some one compatriot, sterility with other species would follow as a necessary contingency. In the second place, it is almost as much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male element of one form should have been rendered utterly impotent on a second form, whilst at the same time the male element of this second form is enabled freely to fertilize the first form; for this peculiar state of the reproductive system could hardly have been advantageous to either species."
Darwin came to the conclusion that the sterility of crossed species must be due to some principle quite independent of natural selection. In his search for such a principle he brought together much evidence as to the instability of the reproductive system, pointing out in particular how frequently wild animals in captivity fail to breed, whereas some domesticated races have been so modified by confinement as to be fertile together although they are descended from species probably mutually infertile. He was disposed to regard the phenomena of differential sterility as, so to speak, by-products of the process of evolution. G. J. Romanes afterwards developed his theory of physiological selection, in which he supposed that the appearance of differential fertility within a species was the starting-point of new species; certain individuals by becoming fertile only inter se proceeded along lines of modification diverging from the lines followed by other members of the species. Physiological selection in fact would operate in the same fashion as geographical isolation; if a portion of a species separated on an island tends to become a new species, so also a portion separated by infertility with the others would tend to form a new species. According to Romanes, therefore, mutual infertility was the starting-point, not the result, of specific modification. Romanes, however, did not associate his interesting theory with a sufficient number of facts, and it has left little mark on the history of the subject. A. R. Wallace, on the other hand, has argued that sterility between incipient species may have been increased by natural selection in the same fashion as other favourable variations are supposed to have been accumulated. He thought that "some slight degree of infertility was a not infrequent accompaniment of the external differences which always arise in a state of nature between varieties and incipient species."
Weismann concluded, from an examination of a series of plant hybrids, that from the same cross hybrids of different character may be obtained, but that the characters are determined at the moment of fertilization; for he found that all the flowers on the same hybrid plant resembled one another in the minutest details of colour and pattern. Darwin already had pointed to the act of fertilization as the determining point, and it is in this direction that the theory of hybridism has made the greatest advance.
The starting-point of the modern views comes from the experiments and conclusions on plant hybrids made by Gregor Mendel and published in 1865. It is uncertain if Darwin had paid attention to this work; Romanes, writing in the 9th edition of this Encyclopaedia, cited it without comment. First H. de Vries, then W. Bateson and a series of observers returned to the work of Mendel (see Mendelism), and made it the foundation of much experimental work and still more theory. It is still too soon to decide if the confident predictions of the Mendelians are justified, but it seems clear that a combination of Mendel’s numerical results with Weismann’s (see Heredity) conception of the particulate character of the germ-plasm, or hereditary material, is at the root of the phenomena of hybridism, and that Darwin was justified in supposing it to lie outside the sphere of natural selection and to be a fundamental fact of living matter.
Authorities.—Apellö, "Über einige Resultate der Kreuzbefruchtung bei Knochenfischen," Bergens mus. aarbog (1894); Bateson, "Hybridization and Cross-breeding," Journal of the Royal Horticultural Society (1900); J. L. Bonhote, "Hybrid Ducks," Proc. Zool. Soc. of London (1905), p. 147; Boveri, article "Befruchtung," in Ergebnisse der Anatomie und Entwickelungsgeschichte von Merkel und Bonnet, i. 385-485; Cornevin et Lesbre, "Étude sur un hybride issu d’une mule féconde et d’un cheval," Rev. Sci. li. 144; Charles Darwin, Origin of Species (1859), The Effects of Cross and Self-Fertilization in the Vegetable Kingdom (1878); Delage, La Structure du protoplasma et les théories sur l’hérédité (1895, with a literature); de Vries, "The Law of Disjunction of Hybrids," Comptes rendus (1900), p. 845; Elliot, Hybridism; Escherick, "Die biologische Bedeutung der Genitalabhänge der Insecten," Verh. z. B. Wien, xlii. 225; Ewart, The Penycuik Experiments (1899); Focke, Die Pflanzen-Mischlinge (1881); Foster-Melliar, The Book of the Rose (1894); C. F. Gaertner, various papers in Flora, 1828, 1831, 1832, 1833, 1836, 1847, on "Bastard-Pflanzen"; Gebhardt, "Über die Bastardirung von Rana esculenta mit R. arvalis," Inaug. Dissert. (Breslau, 1894); G. Mendel, "Versuche über Pflanzen-Hybriden," Verh. Natur. Vereins in Brünn (1865), pp. 1-52; Morgan, "Experimental Studies," Anat. Anz. (1893), p. 141; id. p. 803; G. J. Romanes, "Physiological Selection," Jour. Linn. Soc. xix. 337; H. Scherren, "Notes on Hybrid Bears," Proc. Zool. Soc. of London (1907), p. 431; Saunders, Proc. Roy. Soc. (1897), lxii. 11; Standfuss, "Études de zoologie expérimentale," Arch. Sci. Nat. vi. 495; Suchetet, "Les Oiseaux hybrides rencontrés à l’état sauvage," Mém. Soc. Zool. v. 253-525, and vi. 26-45; Vernon, "The Relation between the Hybrid and Parent Forms of Echinoid Larvae," Proc. Roy. Soc. lxv. 350; Wallace, Darwinism (1889); Weismann, The Germ-Plasm (1893).
(P. C. M.)
HYDANTOIN (glycolyl urea), C3H4N2O2 or
the ureïde of glycollic acid, may be obtained by heating allantoin or alloxan with hydriodic acid, or by heating bromacetyl urea with alcoholic ammonia. It crystallizes in needles, melting at 216° C.
When hydrolysed with baryta water yields hydantoic