This section is from the book "The Dogs Of Great Britain, America, And Other Countries. Their Breeding, Training, and Management in Health and Disease", by John Henry Walsh (Stonehenge). Also available from Amazon: The Dogs Of Great Britain, America And Other Countries.
The varieties of the dog are extremely numerous, and, indeed, as they are apparently produced by crossing, which is still had recourse to, there is scarcely any limit to the numbers which may be described. It is a curious fact that large bitches frequently take a fancy to dogs so small as to be incapable of breeding with them; and in any case, if left to themselves, the chances are very great against their selecting mates of the same breed as themselves. The result is, that innumerable nondescripts are yearly born, but as a certain number of breeds are described by writers on the dog, or defined by "dog-fanciers," these "mongrels," as they are called from not belonging to them, are generally despised, and, however useful they may be, the breed is not continued. This, however, is not literally true, exceptions being made in favor of certain sorts which have been improved by admixture with others, such as the cros6 of the bulldog with the greyhound; the foxhound with the Spanish pointer; the bulldog with the terrier, etc., etc., all of which are now recognized and admitted into the list of valuable breeds, and not only are not considered mongrels, but, on the contrary, are prized above the original strains from which they are descended.
An attempt has been made by M. F. Cuvier to arrange these varieties under three primary divisions, which are founded upon the shape of the head and the length of the jaws, these being supposed by him to vary in accordance with the degree of cunning and scenting powers, which the animal possessing them displays. The following is his classification, which in the main is correct, and I shall adhere to it, with trifling alterations, in the pages of this book.