The great multitude of different breeds of the dog and the vast differences in their size, points, and general appearance are facts which make it difficult to believe that they could have had a common ancestry.  One thinks of the difference between the Mastiff and the Japanese Spaniel, the Deerhound and the fashionable Pomeranian, the St. Bernard and the Miniature Black and Tan Terrier, and is perplexed in contemplating the possibility of their having descended from a common progenitor.  Yet the disparity is no greater than that between the Shire horse and the Shetland pony, the Shorthorn and the Kerry cattle, or the Patagonian and the Pygmy; and all dog breeders know how easy it is to produce a variety in type and size by studied selection.

In order properly to understand this question it is necessary first to consider the identity of structure in the wolf and the dog.  This identity of structure may best be studied in a comparison of the osseous system, or skeletons, of the two animals, which so closely resemble each other that their transposition would not easily be detected.

The spine of the dog consists of seven vertebrae in the neck, thirteen in the back, seven in the loins, three sacral vertebrae, and twenty to twenty-two in the tail.  In both the dog and the wolf there are thirteen pairs of ribs, nine true and four false.  Each has forty-two teeth.  They both have five front and four hind toes, while outwardly the common wolf has so much the appearance of a large, bare-boned dog, that a popular description of the one would serve for the other.

Nor are their habits different.  The wolf’s natural voice is a loud howl, but when confined with dogs he will learn to bark.  Although he is carnivorous, he will also eat vegetables, and when sickly he will nibble grass.  In the chase, a pack of wolves will divide into parties, one following the trail of the quarry, the other endeavouring to intercept its retreat, exercising a considerable amount of strategy, a trait which is exhibited by many of our sporting dogs and terriers when hunting in teams.

A further important point of resemblance between the Canis lupus and the Canis familiaris lies in the fact that the period of gestation in both species is sixty-three days.  There are from three to nine cubs in a wolf’s litter, and these are blind for twenty-one days.  They are suckled for two months, but at the end of that time they are able to eat half-digested flesh disgorged for them by their dam—­or even their sire.

We have seen that there is no authenticated instance of a hybrid between the dog and the fox.  This is not the case with the dog and the wolf, or the dog and the jackal, all of which can interbreed.  Moreover, their offspring are fertile.  Pliny is the authority for the statement that the Gauls tied their female dogs in the wood that they might cross with wolves.  The Eskimo dogs are not infrequently crossed with the grey Arctic wolf, which they so much resemble, and the Indians of America were accustomed to cross their half-wild dogs with the coyote to impart greater boldness to the breed.  Tame dogs living in countries inhabited by the jackal often betray the jackal strain in their litters, and there are instances of men dwelling in lonely outposts of civilisation being molested by wolves or jackals following upon the trail of a bitch in season.