This presumptuous, two-legged fighting animal, from whom we are descended, and many of whose instincts we still retain, began to enlarge his warfare, and thereby to improve his organization. For the sake of his own safety, he learned to combine with his fellows, finding strength in numbers, like the wolves in the pack; or, like ants and bees, finding in the combined efforts of many a means of gaining for each individual more food and better shelter than he could win for himself alone. For example, it was possible that a neighboring tribe, instead of waiting to be attacked, was planning an attack upon its own account. It would not do to be surprised at night. Sentries must be established to keep watch while others slept, and to waken their comrades in case of need. Our very word "watch" is derived from the old Anglo-Saxon word "waeccan," meaning "wake." And yet people who tried to watch for long at a stretch would be apt to doze. They must be relieved at regular times; it was a matter of necessity, but how could one measure time at night? Where man has been confronted with a pressing problem he has generally found its solution. Probably in this case the stars gave him a clue. If the sky were clear, their positions would help to divide the night into "watches" of convenient length. Thus did primitive man begin to study the skies. No longer a mere animal, he was beginning, quite unconsciously, to give indications of becoming a student. CHAPTER TWO _The Land Between the Rivers_ Now we must jump over ages so vast in duration that all of our recorded history is by comparison, the merest fragment of time. During the prehistoric period, known to us only by certain bones, drawings, and traces of tombs and dwellings, and by a few rude implements, weapons, and ornaments, we must think of the human family as developing very, very slowly—groping in the dawn of civilization while it ate and slept, hunted, and fought, and, gradually spread over various regions of the earth. It was in this interval, also, that man learned the use of fire and the fashioning of various tools. His club gave place to the spear, the knife, and the arrow-head weapons that were made at first by chipping flakes of flint to a sharp edge. Then, as his knowledge and skill slowly increased, he learned to work the softer metals and made his weapons and his tools of bronze. Meanwhile, he was taught, by observing in nature, to tame and to breed animals for his food and use, and to plant near home what crops he wished to reap, instead of seeking them where they grew in a wild state. Thus, he became a herdsman and farmer.