HYBRIDITY, the crossing of two individ uals of distinct species. The result of the mtercrossing of species is a hybrid, for exam pie, the mule, which is the result of breeding the horse with the ass. As the mule is invari ably sterile, the infertility has always been supposed to be a test of species. But this is not an invariable rule, as not a few so-called "good" species have been crossed with one another. It may be set down as a general proposition that the difficulty of crossing in creases the more distant the systematic relation ship of the species experimented with. Also these difficulties are, says Hertwig, by no means directly proportional to the systematic diver gence of the species.
Nature tends to keep species separate, in the higher animals, as well as among insects, etc.; mating is usually prevented by the structure of the parts concerned with sexual union; also the principle of preferential mating comes into play among mammals as well as insects, as often between males and females, even of closely allied species or varieties. When there are no structural differences there may exist an aversion which prevents any union of the sexes.
Artificial Hybridization.— Many experi ments have recently been made on the lower marine animals in which the eggs are fertilized in the sea without sexual union, by placing the eggs of starfish and sea-urchins, etc., in a watch-glass and adding the sperm of the males, thus securing artificial fertilization. In this way hybrids have been obtained from species belonging to quite different genera, while it has been found that in some cases closely related species will not cross. For example, among the sea-urchins the spermatozoa of. Strongylocen . trotus lividus readily fertilize the eggs of a species of Echinus, but only rarely those of the more closely allied Sphcerechinus granularis. Hybrids have been obtained from different genera of fishes, as those between the salmon and brown trout. It appears that salmon eggs have been fertilized by trout sperm, but not trout eggs by salmon sperm. According to
Hertwig eggs have been fertilized by sperm belonging to species of different families, orders and possibly classes. For example, the eggs of a flounder (Pleuronectes platcssa) and of Labrus rupcstris have been fertilized by the sperm of the cod; frog's eggs, (Rana arvalis) by sperm of a triton, and even, it is said, the eggs of a starfish by milt from a sea-urchin; in such cases, however, the hybrids die during or at the close of segmentation of the yolk.
Fertility of Hybrids.— While the mule and many other hybrids are sterile, there are some known exceptions. Hybrids of hares and rab bits have continued fruitful for generations, and also hybrids obtained from the wild buck and she-goat, from the Chinese goose (Anser cygnoides) and the common goose (A. domes tints); from Salino salvclinus and S. fontinalis; Cyprinus carpio and Crassius vulgaris, as well as between the two silkworm moths, Philosamia cvnthia and P. ricini, the Arrhindy worm. In this country Caton has hybridized the common Virginian deer with the Ceylon deer and the Acapulco deer, and states that the hybrids seemed perfectly healthy and prolific. Ewart states that the Indian buffalo and the American bison produce fertile hybrids with the European wild ox.
In the human species it is a well-established fact that marriages between remote varieties or races tend to sterility, while crossing between allied races are fertile, and such unions are most beneficial. Thus the most mixed white races are the most fertile and vigorous. Ewart thinks that as there are no definite limits be tween 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 He also states that sterility has in some cases been slowly acquired, in others abruptly, but how it has been acquired is not known.