Agriculture, however, involves the raising of cattle, particularly of oxen. These male cattle are castrated, usually when very young. They are thus made tractable, so that they may be hitched to the plough and used for agricultural purposes more easily than is possible in the case of bulls, which are never completely manageable. What, then, were the motives which led to the raising of cattle, an occupation which, in many places at least, is carried on solely in the interests of agriculture? What motives led to the castration of male cattle, a practice which everywhere obviously serves agricultural purposes? 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