HYBRIDITY. 359 HYBRIDITY. getlier, many breeders conclude that by syste- matic hybridization most species of plants will be found capable of almost indefinite improve- ment. In connection with the artificial production of liybrids, the phenomenon of 'xenia' has re- cently attracted much attention. This term is applied to cases in which characters of the pol- linating parent appear directly in the fruit or seed. It was once thouglit that only the embryo could show hybrid characters, which would become evident, therefore, only upon the germination of the seed. In cases of xenia, how- ever, the regions of the seed outside of the em- brj'o show hybrid characters, so that it is evident that they too have been influenced by the pol- linating parent. The crossing of the races of maize has proved to be most favorable for study- ing this phenomenon. If a race of maize nor- mally producing white grains be pollinated by one producing red grains, the resulting grains will show the red coloration in various ways. The same result is also obtained in crossing races of sweet and dent com. This phenomenon was inexplicable until the recent discovery of 'double fertilization' among flowering plants. This phrase means that both of the male cells were brought by the pollen-tube to the ovule function — one fertilizing the egg, which then produces the embryo, the other uniting with the 'endosperm nucleus.' which then produces the endosperm or food substance of the seed about the embryo. It is evident, therefore, that where double fertiliza- tion occurs in connection with hybridizing, the endosperm as well as the embryo is a hybrid, and that the resulting seed may show characters of the pollinating parent. See Angiospeems ; Feb- TrLlz.Tiox ; Plaxt-Breedixg. ilETHODS OF Hybridizing. — The methods of hy- bridizing plants are simple. If the flower is hermaphroditic its stamens must be cut away before they are ripe — usually before the corolla opens — and the flower must be tied up in a paper bag. When the stigma is ripe the pollen is placed en it. and the bag tied over the flower again and not removed until the seed begins to form. If the flower bears no stamens, it requires, of course, no operation except the tying up while in the bud to prevent the access of foreign pollen. Among animals, much less experimentation in producing hybrids for commercial purposes has been done. HTBKIDITY L ANIMAIS. The number of animal hybrids known to have been produced or to actually exist is not large. The only special work on hybrids is that of Suchetet, the first volume (1001 pages) being devoted to birds alone. Of hybrids produced by nature we have on rec- ord comparatively few examples, hut the number is probably quite considerable. It is knonTi that fish, especially the Salmonidae, readily hybridize. There are on record over a hundred cases of hybrids among insects, and the number probably runs into the thousands. Of amphibians, species of diverse frogs have been seen mating, but it is not definitely known that such unions have given rise to hybrids. Xatural hybrids amonsr birds have been described — e.2. between the flicker of the Kastem United Stntes and the Mexican flick- er, between two species of warblers of the genus Helminthophila, between species of geese, various European grouse, and certain .Asiatic pheasants. Hybrids Pboduced Artiticiaixt. By artificial fertilization of the ova of difterent spiecies, and even genera and families, of sea-urchins, the early larval stages have been reared, but never carried to maturity; the resulting pluteus stages are more or less monstrous. In one ease early lar%-al hybrids between species of two different classes of echinoderms, starfish and sea-urchins, were produced. The cases of fertile hybrids of birds and mammals mentioned below were pro- duced in captivity; besides these there have been instances of hj'brids between the brown and polar bear, the leopard and jaguar, the lion and tiger, the sea-lion and fur-seal, between the zebra and horse, the horse and ass, and the common ass and varieties of the kiang. Hybrids are also known to occur between the dog and fox (Darwin), but this case is doubted by Ewart. Vabiation.s of Hybrids. Danvin has shown that hybrids are more variable and have a greater tendency to vary than the products of pure race of both parents. He also has shovn that the products of h3'brids are much more variable than the hybrids themselves. Results of Recent Experiments. As a result of experiments by Standfuss on thousands of specimens of the higher Lepidoptera, the follow- ing results have been obtained: ( 1 ) The intercrossing of two species may re- sult in anything, from complete sterUity to the production of the normal number of fertile eggs. "These extremes may even occur as the result of crossing different individuals of the same two species. Hence a cross must not be pronounced infertile on the evidence of a single failure. (2) Individual differences in the structure of the genital apparatus may prevent effective cross- ing between some members of two given species, though other members of the same two species may pair freely. (3) Some crossings have resulted entirely in male, others entirely in female, offspring. A third class has given both sexes in various propor- tions. As regards the fertility of hybrids, in no case observed by Standfuss or known to him has the female of a true hybrid been shown to be fertile. But the occurrence of undoubted cases of fertility in male hybrids has been proved by crossing the male hybrids with the females of both parent species, and in one case with the female of a third species. The silk-moth hybrids of Philo.samia cynthia and Philosamia ricina. the arrhindy moth, two closely allied species, are also said to have been fertile for eight generations. Fertility of the Hybrids, It is well known that the mule is infertile, and though there are reported to be three cases where a mule has pro- duced young, they are doubtful. Ewart. in re- ferring to the instance of the Paris .Jardin de rAcclimatation mule, dovibts whether these ani- mals are mules, quoting Ayrault's opinion that the Paris mule was an ordinary mare. Darwin states that he knows of no thoroushly well authenticated oases of perfectly fertile hybrid animals, though he adds: "I h.ive reason to be- lieve that the hybrids from Cervulus vaginalis and Reevesii and from Phasianus colehicus with Phasianus torquatus are perfectly fertile." The hare and rabbit are supposed to have fertile off- spring; the hybrids of the common and Chinese
