As the use of the ass in the economic arts began to decline, the mule or hybrid progeny of this creature and the horse has progressively increased. Although the value of this mongrel has been known, particularly in southern Europe, from very early days, its most extensive employment has been found in the old slave-holding States of the Federal union. The custom of using mules has been almost unknown in England, and has never been generally adopted in the northern part of the United States. It appears to have been introduced into southern regions by the Spaniards and the French, and there to have spread, because of the peculiar fitness of the creature to the climate and the employment it had to endure in that part of America. The mule has the peculiar advantage that it is on the average as large as the horse, is nearly as quick-footed when walking, and has at the same time a considerable share of the patient endurance to hard labor and scant fare which characterizes the donkeys. It matures somewhat more speedily than its nobler kinsman, being ready to meet severe strains perhaps a year earlier. Unless unconscionably abused, its period of fitness for hard work endures about one-third longer, often lasting for thirty years. It is singularly exempt from disease, its sturdy frame withstanding rude usage until the old age time. [Illustration: In the Circus] The mule is especially interesting to the naturalist for the reason that it affords the only certain case in which a hybrid has proved decidedly serviceable to man. It is not unlikely that a similar mixture of the blood of two species occurs in our ordinary cats, and it may exist in the case of the dog and in some of the domestic birds; but so far as we know, there has been no other useful result from the hybridizing, if it has occurred. Moreover, the mule is unique for the fact that the animal is distinctly stronger for its weight, and more enduring than either species which his blood combines. In fact, there is no product of man's industry in relation to domesticated animals which is more interesting than this singular creature. At present, its use appears to be going out of vogue; the evidence goes to show that the hybrid has no place in the affections of mankind, and that it is only likely to be kept in its use in tropical countries, and particularly in regions where the beasts have to be under the care of slaves or other negligent folk. It is a singular fact in connection with this hybrid, that it is nearly absolutely sterile, there being only two or three cases on record in which they have proved fecund. It seems, however, possible that if these rare instances of continued breeding were to be duly used, an intermediate species might be permanently established. This is, indeed, one of the most important lines for experiment which could be undertaken by an institution devoted to the study of problems relating to domestication.