HYBRID 103 ovum and spermatic fluid of different animals, may be incapable of fertilization, owing to pe- culiarities of their own internal constitution ; and consequently their physical contact would produce no result. But there are other rea- sons upon which the non-occurrence of hy- brids in nature may partly depend. Among animals there is an instinctive preference for | sexual union with their own species rather than with others, and a similarity of habits, of locality, and general disposition, corroborates this preference, and alone makes it much more likely that sexual union, as a matter of fact, will take place between animals of the same species. A certain degree of similarity in the physical structure of the parents is essential to the fertility of their sexual union. Thus all the most frequent and most useful forms of hybridity occur between different species be- longing to the same genus. The horse, for ex- ample, will breed with the ass, the zebra, and the quagga ; the dog has been certainly known to breed with the wolf, and probably with the fox ; the goat with the sheep, the ram with the roe; and it has been comparatively easy to obtain hybrids from the union of the rabbit and the hare. _But a cross union is not neces- sarily fertile, even between species of the same genus ; between those of different genera it is still more exceptional; and it is doubtful whether hybridity, either natural or artificial, has ever occurred beyond these limits. The second question of interest relating to hybrid- ity is that of the fertility of hybrids among themselves. As a rule it may be said that hy- brids are not fertile. Thus the mule does not reproduce itself, but is only obtained by a repe- tition of the union of the ass and the mare. The female mule will sometimes reproduce by union with either the horse or the ass; but in this case the offspring is no longer a mule, but re- verts to the type of the original stock in pre- cise proportion to the admixture of blood re- sulting from the union. Notwithstanding, therefore, that the mule and its mode of pro- duction have been known from time immemo- rial, and notwithstanding the recognized use- fulness of its qualities in some respects, we have never been able to obtain an indepen- dent and self- reproductive breed of mules; that is, the hybrid has never acquired the physiological characters of a natural species. The terms hybrid and hybridization are of- ten vaguely used as applied to plants, and many are called hybrids which are only crosses be- tween varieties. The name hybrid should be restricted to plants resulting from the seeds of one species fertilized by the pollen of another species ; those forms produced by cross breed- ing between varieties of the same species should never be called hybrids, but crosses. It is to be regretted that horticulturists generally ig- nore this distinction and use the terms hybrid and cross as synonymous. Hybrid plants some- times occur in nature, and are frequently pro- duced artificially. In hybridizing, it is neces- sary to prevent the flower used as the mother, or seed-bearer, from being fertilized by its own pollen both before and after the artificial appli- cation of the strange pollen ; the operator is favored by the fact that pollen retains its vital- ity for some time after it is removed from the flower which produced it. It is probable that with this, as with seeds, the duration of vitality varies in different species ; at all events, it is known that some pollen will keep for weeks and even months. The flower selected as the seed-bearer is taken just as it is about to open and before any insects can have visited it ; the envelopes are carefully opened or removed, and if a perfect flower its still unopened stamens are cut away with a delicate pair of scissors, the foreign pollen applied to the stigma with a small brush, and the flower or flowers enclosed in a bag of gauze to prevent the access of in- sects, which would probably bring pollen of the same kind to interfere with the action of the strange pollen. This is a brief outline of the pro- cess ; there are details which can be learned by practice. It is not possible to know beforehand whether two species will hybridize ; two species of a genus that seem to be the most nearly related will sometimes refuse to be hybridized, while other two that seem most unlike will readily form a union. It makes a difference also which plant is chosen as the seed-bearer and which as the pollen-bearer; for instance, the pistil of A will refuse to be fertilized by the pollen of B, while the pistil of B will readily accept the pollen of A. Seeds from the flowers thus fertilized may produce plants quite intermediate between the two parents, or may more strongly resemble the one or the other. Sometimes a hy- brid will have the leaves of one parent and the flowers and fruit of the other. By this means horticulturists have produced useful varieties of fruit, notably in grapes and strawberries, and some of the finest flowers are the result of hybridizing. Among hardy flowers, the rho- dodendrons and azaleas are striking examples of the improvement that may be effected in this manner; the fine rhododendrons are hy- brids between the hardy R. Catawbieme of the southern Alleghanies and Jf. Ponticum, a greenhouse species from Asia Minor. It is a singular fact that the English hybrids, in which B. Catawbiense is the mother plant, are gen- erally hardy, while the Belgian hybrids are very much less hardy for the reason that the Belgian florists use R. Ponticvm as the seed- bearer. When a desirable form is obtained by hybridizing, it can be continued and multiplied indefinitely by means of layers, cuttings, or grafts. Hybrid plants are sometimes fertile; the progeny from them shows a tendency to revert to the one or the other parent, and in a few generations all trace of the admixture is obliterated ; sometimes the progeny is too weak to bear seeds, and thus becomes extinct. More generally hybrid plants are wholly or partly sterile ; the degeneration shows itself most prominently in the anthers, which fail to pro-
