Thus primitive man must, from a very early period, have observed that the sun gives light and heat. It required but a slight extension of this observation to note that the changing phases of the seasons are associated with the seeming approach and recession of the sun. This observation, however, could not have been made until man had migrated from the tropical regions, and had reached a stage of mechanical development enabling him to live in subtropical or temperate zones. Even then it is conceivable that a long period must have elapsed before a direct causal relation was felt to exist between the shifting of the sun and the shifting of the seasons; because, as every one knows, the period of greatest heat in summer and greatest cold in winter usually comes some weeks after the time of the solstice. Yet the fact that these extremes of temperature are associated in some way with the change in the sun's place in the heavens must in time have impressed itself upon even a rudimentary intelligence.
That the sun, moon, and stars move across the heavens must obviously have been among the earliest scientific observations. It required a relatively high development of the observing faculties, yet a development which man must have attained ages before the historical period, to note that the moon has a secondary motion which leads it to shift its relative position as regards the stars; that the stars themselves, on the other hand, keep a fixed relation as regards one another, with the notable exception of two or three of the most brilliant members of the galaxy, the latter being the bodies which came to be known finally as planets, or wandering stars. The wandering propensities of such brilliant bodies as Jupiter and Venus cannot well have escaped detection. We may safely assume, however, that these anomalous motions of the moon and planets found no explanation that could be called scientific until a relatively late period.
Turning from the heavens to the earth, and ignoring such primitive observations as that of the distinction between land and water, we may note that there was one great scientific law which must have forced itself upon the attention of primitive man. This is the law of universal terrestrial gravitation. The word gravitation suggests the name of Newton, and it may excite surprise to hear a knowledge of gravitation ascribed to men who preceded that philosopher by, say, twenty-five or fifty thousand years. Yet the slightest consideration of the facts will make it clear that the great central law that all heavy bodies fall directly toward the earth cannot have escaped the attention of the most primitive intelligence.
It further helps to connect us in sympathy with our primeval ancestor if we recall that in the attempt to explain this fact of terrestrial gravitation Newton made no advance, and we of to-day are scarcely more enlightened than the man of the Stone Age. Like the man of the Stone Age, we know that an arrow shot into the sky falls back to the earth. We can calculate, as he could not do, the arc it will describe and the exact speed of its fall; but as to why it returns to earth at all, the greatest philosopher of to-day is almost as much in the dark as was the first primitive bowman that ever made the experiment.
Other physical facts, going to make up an elementary science of mechanics, that were demonstratively known to prehistoric man, are such as these: The rigidity of solids and the mobility of liquids; the fact that changes of temperature transform solids to liquids and vice versa; that heat, for example, melts copper and even iron, and that cold congeals water; and the fact that friction, as illustrated in the rubbing together of two sticks, may produce heat enough to cause a fire. The rationale of this last experiment did not receive an explanation until about the beginning of the nineteenth century of our own era. But the experimental fact was so well known to prehistoric man that he employed this method, as various savage tribes employ it to this day, for the altogether practical purpose of making a fire; just as he employed his practical knowledge of the mutability of solids and liquids in smelting ores, in alloying copper with tin to make bronze, and in casting this alloy in moulds to make various implements and weapons. Here, then, were the germs of an elementary science of physics.
In the field of what we now speak of as biological knowledge primitive man had obviously the widest opportunity for
