([15]) The excrements of animals are capable of evolving, by combustion, enormous amounts of heat.
PART II.
ON THE BREEDING AND BREEDS OF STOCK.
SECTION I.
THE BREEDING OF STOCK.
Cross Breeding.—For many years past feeders have zealously occupied themselves in the improvement of their stock, and the result of their labors is observable in the marked superiority of the breeds of the present day over their ancestors in the last century. The improvement of animals designed as food for man is effected by keeping them on a liberal dietary, by selecting only the best individuals for sires and dams, and by combining the excellencies of two or more varieties of a species in one breed. A species consists of a number of animals which exhibit so many points of resemblance, that they are regarded by the great majority of naturalists to be the descendants of a single pair. If we except the believers in the hypotheses relative to the origin of existing varieties of animals and plants, propounded by Lamarck, Darwin, and other naturalists of the "advanced school," there is a general belief in the immutability of species. The individuals of an existing species, say dogs, can never acquire the peculiar features of another species; nor can their descendants, if we except hybrids, ever become animals in which the characteristics of the dog tribe are irrecognisable. By various influences, such as, for example, differences in food and climate, and domestication, a species may be split into varieties, or breeds, all of which, however, retain the more important characteristics of the primordial type. There appears to be no limit to the varieties of dogs, yet one can perceive by a glance that there is no specific difference between the huge Mont St. Bernard dog and the diminutive poodle, or between the sparse greyhound and the burly mastiff. All the varieties of our domestic fowl have been traced to a common origin—the wild Indian fowl (Gallus bankiva). Even Darwin admits that all the existing kinds of horses are, in all probability, the descendants of an original stock; and it is generally agreed that the scores of varieties of pigeons own a common ancestor in the rock pigeon (Columba livia).
As certain individuals are grouped by naturalists into species, so particular species, which in habits and general appearance resemble each other, are arranged under the head of genus. The horse, the ass, and the zebra are formed on nearly the same anatomical plan; they are therefore classed together, and designated the genus Equus, a term derived from the Latin word equus, a horse—that animal being regarded as the type, or perfect member of the group. Thus the horse, in the nomenclature of the naturalist, is termed Equus caballus; the ass, Equus asinus; and the zebra, Equus zebra. By a further extension of this principle of classification, very closely allied genera are united under the term of family.
The different varieties of the same species breed, as might be anticipated, freely together; but it frequently happens that two individuals of different species pair, and produce an animal which inherits some of the properties of each of its progenitors. These half-breeds are termed hybrids, or mules, and we have familiar examples of them in the common mule and the jennet. As a general rule, animals exhibit a disinclination to breed with other than members of their own species; and although the interference of man may overcome this natural repugnance, he can only effect the fruitful congress of individuals belonging to closely allied species, being members of the same genus. Hybrids in the genus Equus are very common. A cross has been produced between the he-goat and the ewe; the camel and the dromedary have bred together; and Buffon succeeded in producing a hybrid in which three animals were represented—namely, the bison, the zebu, and the ox. On the other hand, attempts to effect a cross between animals belonging to different families have generally failed; nor is it at all probable that a cross will ever be produced between the pig and the sheep, between the horse and the cow, or, most unlikely of all, between the dog and the cat.