The traditional mode of explanation would lead us to suppose that man foresaw the effects of castration, that he knew beforehand that if the bull were subjected to this operation he would become an animal fitted to draw the plough. The impossibility of this supposition is evident. Such an effect could be learned only from experience, prior to which, therefore, it could not have been known. The problem relating to the cultivation of the soil by means of the plough, therefore, divides into two questions: How may we account for the ox? How for the plough? These questions are closely related, and yet they lead us back to divergent explanations. For in all probability the plough was originally drawn by man. Moreover, the plough was not the first implement to be thus drawn; it was anteceded by the _wagon_. Even on the early Babylonian and Assyrian monuments there were figures of a wagon bearing either an image of a god or else the king or chief priest, both of whom were probably regarded as uniting in _one person_ the function of their offices with that of representative of the deity. Thus, the question as to the origin of the plough carries us back directly to that of the origin of the wagon. Now, the earliest wagon had but two wheels; the four-wheeled wagon came as a later discovery or as an improvement. The two-wheeled wagon, however, presupposes the wheel. But how did the wheel come to be recognized as a useful object of locomotion? The first traces of a wheel or of wheel-like objects are to be found in the latter part of the stone age. A number of such objects have been discovered in Europe; in their centre is a hole, and there are spokes that radiate to the circumference. The fact that these wheels are of small size indicates that they may have been worn about the neck as amulets. But even in early culture the wheel was also put to an entirely different use. Widely prevalent over the earth and probably connected with ancient sun worship, is the custom of kindling a fire to celebrate the festival of the summer solstice. In ancient Mexico, tradition tells us, this fire was started by turning a notched disk of wood about a stake until the heat thus generated gave rise to fire--the same method of producing fire by friction that is still in use among primitive peoples. This fiery wheel was then rolled down a hill as an image of the sun, and later, when the custom had lost its original magical significance, as a symbol of the sun moving in the heavens. According to the report of W. Mannhardt, a remarkably similar custom existed in East Prussia not so very long ago. Perhaps the wheel that was worn about the neck as an amulet or article of adornment likewise had some connection with the idea that the sun was a celestial wheel rolling across the heavens. After the early sun cults had once created the rolling wheel in imitation of the sun and its movements, it was but a short step to the idea of securing regular, continuous movements by means of which some sort of work might be performed. An early application of this idea is to be found in the practice of spinning with distaff and whorl. This invention was credited even by the ancients to prehistoric times. Doubtless its origin belongs to the beginnings of the heroic age. This same early period, however, probably also used the wheel for transporting heavy articles. This was the original purpose of the one-wheeled barrow. It alone enabled the ancient Babylonians and Egyptians to overcome the difficulties of transporting by human agencies the mighty blocks of stone required for their temples and pyramids. From this it was not a far advance to the two-wheeled wagon. The barrow was pulled or pushed by men. The wagon, in contrast to the barrow, was apparently from the beginning an aristocratic mode of transit, never used by the common people. The two-wheeled wagon was in the first instance a vehicle of the gods. Later it served as the vehicle of the ruler, the terrestrial counterpart of the deity. Finally, the nobleman employed it in war, in going forth to battle. A vivid portrayal of battles in which such two-wheeled wagons played a part is presented in the Iliad. True, the wagon is here also, as a rule, only a means for carrying the hero to the scene of combat. The fighting itself is seldom done from it. Upon its arrival at the appointed place, the warrior dismounts, to try his strength, shield against shield, with his opponent. The general populace, however, always goes on foot.