Page:The New International Encyclopædia 1st ed. v. 10.djvu/417

* HYBRIDITY. 361 HYBKIDITY. Tlie hybrids are interesting because of the curious blending of characters, derived apparently partly from their actual and partly from their remote ancestors, and because they shed new light on many questions of general interest, such as the origin of stripes, reversion, interbreeding, and prepotency. Reversion. He concludes that "some of the hybrids in make and disposition strongly suggest their zebra sire, others their respective dams; but even the most zebra-like in form are utterly unlike their sire in their markings. It is not a matter of taking after a grandparent, but after an ancestor in all probability thousands of gen- erations removed, an ancestor probably far more like the Somali than any of the Burehell zebras." (This confirms Standfuss's conclusion stated above.) Ewart adds (p. 21): "The reversion may be to recent, remote, or intermediate ances- tors, and the tendency will in most cases be to revert to sports that here and there mark the route along which the development has pro- ceeded." He also, by breeding pigeons, restored the rock pigeon, or primitive form. "In the case of my most typical bird there is. as far as an external examination can show, practically complete re- version." Reversion, he says, "seems to lead to a form of rejuvenescence." He regards polydac- tylism in the horse as in part due to reversion to the llipparion or the three-toed fossil horse. As he well puts it, the cases of reversion that he cites "must be more or less accurate restorations of their comparatively remote ancestors." Re- version also indicates that in some cases "varie- ties and breeds which have, through interbreed- ing, undergone senile degeneration, may be regenerated without the loss of their best and most prized characteristics." Pkepotency. It is generally assumed that an old species or variety is prepotent over a more recent species or variety. It is impossible to say whether zebra hybrids in their markings take after a remote zebra ancestor, or after an ances- tor common to both zebras and horses, or after a hypothetical mid-parent combining the characters of the less remote ancestors of both zebras and horses. There is, however, no difliculty in seeing that while some zebra hybrids, apart from their stripes, closely resemble the zebra parent, others take after their horse parent, thus showing that the wild sire is not necessarily the most prepo- tent. "But even when the hybrids are distinctly horse-like they never repeat recently acquired pe- culiarities, such as a blaze, or short ears, high withers, or a small head and long neck." He believes that sports and certain marked variations are often prepotent ; and also that in- breeding (see Cro.ss-Fektii.ization) is "common among wild animals, and that by inducing prepo- tency it plays an important part in the origin of species." It may be here observed that Ewart's experiments lead him to reject telegony (q.v.). and to explain such occurrences by cases of rever- sion. The zebra hybrids are also interesting as being "in some respects almost intermediate lietween their parents." Each parent, he says, bands on its most fixed individual ehariieters. His zebra hybrids are neither new creations nor yet intermediate*forms. Sterility in Hybrids. — Ewart concludes that, as there is no hard and fast line between species and varieties, there can be "no fundamental difference between a hybrid and a cross, nor yet any a priori reason why any given hybrid should be sterile, or any given cross fertile." "Sterility has doubtless been acquired in some cases slowly, in others ab- ruptly, but how it has been acquired it is im- possible in most cases even to guess." Mendel's Law. Great interest has been ex- cited among biologists by Mendel's law, and his discovery of certain principles of heredity which will prove of much practical value to breeders of plants and animals, and lend a new phase to the theory of heredity and also of evolution, since the results thus far obtained point to dis- continuing evolution and the absence of inter- mediate forms. Mendel's original paper was published at Briinn, Austria, in 1805, but was overlooked until De Vries in 1900 published an e.xact counterpart of Jlendel's theory, while Correns and also Bateson hit upon the same law. As stated briefly by Spillman, it is as fol- lows: In the second and later generations of a hj'brid, every possible combination of the parent character occurs, and each combination appears in a definite proportion of the individuals. A parent character which is fully developed in the h3'brid is said to be 'dominant:' if it is appar- ently absent it is said to be 'recessive.' A ease in illustration is thus stated by Bateson: "In breeding crested canaries, the kind of crest de- sired for exhibition can, according to canary- fanciers, be produced most easily by mating crested birds with non-crested, or 'plain-beads,' as they are called. If it be suppo.sed that the crested character is usually dominant, we have a simple explanation. Wlien crested birds are bred together, a number of birds are produced whose crests are coar.se and stand up, and others without crests. The latter are the reccssives; the former we may suppose to be the pure domi- nants." The fact that the hornless breeds of goats will give off some horned oflfspring is probably due to the fact that they are what Bateson terms 'heterozygotes,' under what is usually called 're- version.' From the analogy of cattle, it may be anticipated that the hornless form is domi- nant. Consult Bateson. Mendel's Principles of Heredity, with a translation of Mendel's <nig- inal papers on hybridization (London. in02). Mendel's more important discoveries are also thus stated by Castle: "(1) His law of domi- nance: when, for example, the offspring of two parents differing in respect of one character all resemble one parent, and possess therefore the dominant character, that of the other parent being latent or recessive. (2) In place of simple dominance, there may be manifest in the im- mediate hybrid offspring an intensification of character, or a condition intermediate between the two parents ; or the offspring may have a peculiar character of their own (heterozygotes). (.■?) A segiegation of characters inited in the hybrid takes place in their offspring, so that a certain per cent, of these offspring possess the dominant character alone, a certain per cent, the recessive character alone, while a cer- tain per cent, are again hybrid in nature." Bibliography (Plants). Darwin, Variation of. . . Plants Under Domestieation (2d ed., London, 1875) : Bailey, Plant Hreediny (New York. 1890) : Focke," Die Pflatnen-Misehlinge (Berlin, 1881): Swingle and Webber, "Hybrids and Their Utilization in Plant Breeding,'" in Yearbook of the Department of Agriculture for