Primitive Man, like the savage of to-day, had at night no other light than that of the moon and that of wood fires. For countless ages our ancestors lived without candles, and when they had learned the trick of burning rushes soaked in the fat of neighboring tribesmen their state was not greatly better. Beyond Primitive Man we may dimly discern _his_ ancestors—unmentionable to ears un-Darwinized—who had no artificial light at all. In the darkness of the night and the forest what could these ancient worthies do? They had little enough to do at any time, but even their rudest pursuit—that of one another—could not be carried on in darkness. They did nothing, naturally assuming the most comfortable posture in which to do it, the earlier sort suspending themselves by their tails, the later, having no tails, lying down as we do to-day, or rather to-night. It is a law of nature that when the body, or any organ of it, is inactive a kind of torpor ensues; the blood circulates in it with a more feeble flow; molecular changes take place with a lessened energy—in short, the creature begins to die, and can be restored to full life only by renewal of bodily activity. In the instance of the brain this torpor means unconsciousness—that is to say, sleep. To put the matter briefly, darkness compels inaction, inaction begets sleep. Another law of nature—a rather comical one—is that acts which we do regularly, from choice or necessity, set up a tendency in us to do them involuntarily when we don’t care to; and when the original impulse has been replaced by this new and more imperative one we give it the name of habit and flatter ourselves that we have explained it. Because our pithecanthropoid and autocthonic forefathers, unable by reason of darkness to indulge during the whole twenty-four hours in the one-sided pleasures of the chase and the mutual joy of braining one another, had to sleep, _we_ have to sleep; although we have (by paying sorely for it) plenty of light for many kinds of malign activity. But little by little we are overcoming the sleep habit without loss of health, if not with positive sanitary advantage. As before pointed out, the people of our lighted cities sleep less than the rural population; and this sleeps less than it did before the improvement in lamps. Nothing is more certain, despite popular opinion to the contrary, than that the men of cities are superior in strength and endurance to those of the country, as is abundantly attested in army life, in camp and field. That this is wholly or even greatly due to their nocturnal activity is not affirmed; only that their addiction to the joys of insomnia has not appreciably counteracted the sanitary advantages of city life—amongst which an honorable prominence should be given to defective drainage and drinking-water that is largely solution of dog and hydrate of husband from the city reservoir.