Much has depended on the specialization of the horse into different breeds, made possible by the iron shoe. By reconciling the creature to uses--agriculture, which depends on draught animals, and the commerce of importance, which can only be effected by means of wagons--the rapid economic development of our civilization was made possible. By developing a horse capable of bearing an armored man, Europe was brought into a condition in which organized armies took the place of mere forays, and so the development of centralized states was promoted. In the warfare between the Mohammedans and the Christian states of Europe, in the campaigns with the Turks and the Saracens, it is easy to see that the powerful breeds of horses reared in western and northern Europe were a mighty element in determining the issue of the contest. The battles of these momentous campaigns represented, not only a struggle between the Christian Aryans and the Semitic followers of Mahomet, but, in quite as great a degree, the war was waged between the light and agile steeds of the Orient and the massive and powerful animals that bore the mail-clad warriors of the West. On the field of Tours, when the fate of Christian Europe for hours hung in the balance, we may well believe that the strong and enduring horses of the northern cavalry did much to give victory to our race. Along with our general account of the place of the horse in civilization, it is fit to give something to the story of his near, though inferior, kinsmen, the ass and the mule, both of which have played a subordinate, though important, part in the same field of endeavor in which the nobler species has done so much for man. The original progenitors of our donkeys differed from the ancestral form of the horse by variations of good specific value. So far as we can determine from visible features, these forms were more distinctly parted than the dog and the wolf, or either of these animals from the jackal. Nevertheless, these equine forms are clearly closely akin, for they may be bred together. Although the original stock of the ass may possibly have been lost, it seems most likely that the wild forms which exist in Asia have not wandered off from captivity, but are the remnants of the original wilderness form.