There was not a dog living, according to writers of the eighteenth century, that was not a cross between two other varieties, or even impossible crosses, such as the mastiff being from a cross with the hyena, while some other breed had a dash of the Bengal leopard. The former assertion was made by such eminent naturalists as Pallas and Burchell, and even Lowe stated in his modern "Domestic Animals of Great Britain" that it was very possible. The wild dogs of India were said to be a cross between the wolf and the tiger, and other equally ridiculous statements were made. That the dog and wolf will cross, and that a cross between the fox and dog has been repeatedly claimed, are well-known facts, but these are mules and will breed only with the parent stock, whereas, no matter how widely different are the varieties of dog crossed, the progeny is fruitful inter se.
At Wilton House, England, there is an epitaph, as follows: "Here lies Lupa, whose grandmother was a wolf, whose father and grandfather were dogs, and whose mother was half wolf and half dog. She died on the l6th of October, 1782, aged twelve years." That is the record of an experiment conducted by Lord Clanbrassil and Lord Pembroke. Others have experimented in the same way, but it is the interbreeding of the progeny that is the impossible and proves them to be mules.
-- from The Dog Book: A popular history of the dog, by James Watson (Doubleday, Page and Company, 1906), p. 4.