The Probert Encyclopaedia of Nature

DOG

The Dog (Canis vulgaris) is a digitigrade, carnivorous animal, forming the type of the genus Canis, which includes also the wolf, the jackal, and, as a sub-genus, the fox. The origin of the dog is a much-debated question, some considering the breed derived from the wolf, an opinion which is based on resemblances of structure, the susceptibility which the wolf shows of being domesticated, the fact of the two animals breeding together and producing fertile young, and the equality in the period of gestation. But all those points are subject to exceptions and reservations which make the matter doubtful. It is generally agreed that no trace of the dog is to be found in a primitive state, the dhole of India, and dingo of Australia being believed to be wild descendants from domesticated ancestors.

Several attempts to make a systematic classification of the varieties of dogs have been made but without much success, it being difficult in many cases to determine what are to be regarded as types, and what as merely mongrels and cross-breeds. Colonel Hamilton Smith divided dogs into six groups as follows: (1) Wolf-dogs, including the Newfoundland, Esquimaux, St Bernard, shepherd's dog, etc; (2) Watchdogs and Cattle-dogs, including the German boar-hound, the Danish dog, the matin dog, etc; (3) Greyhounds, the lurcher, Irish hound, etc; (4) Hounds, the bloodhound, staghound, foxhound, setter, pointer, spaniel, cocker, poodle, etc; (5) Cur-dogs, including the terrier and its allies; (6) Mastiffs, including the different kinds of mastiffs, bull-dog, pug-dog, etc.

Dogs have in the upper jaw six incisors, two strong curved canines, and six molars on each side, the first three, which are small and have cutting edges, being called false molars; in the lower jaw are six incisors, two canines, and on each side seven molars. The fore-feet have five toes, the hind-feet four or five; the claws are strong, blunt, and formed for digging, and are not retractile. The tail is generally long, and is curled upwards. The female has six to ten mammae; she goes with young nine weeks as a rule. The young are born blind, their eyes opening in ten to twelve days; their growth ceases at two years of age. The dog commonly lives about ten or twelve years, at the most twenty.
Research Dog