simple fact that throwing sticks on a fire will keep it going, and so no animals, except man, have ever made any real use of artificial heat. When we think that the progress of human civilization has been accomplished wholly in parts of the earth where clothing, shelter, and fire are necessary to human existence, we realize that these, instead of being mere incidents to our life, lie really at the very basis of advancement. Hairless man is evidently a tropical animal; if he had not devised means of maintaining himself outside the tropics, there is no reason to suppose that he would have behaved any differently than have the savages that inhabit those regions at the present time. Clothing, shelter, and fire all operate to prevent us from losing heat too rapidly and so have their great value when the weather is cool; we also can and do bring about an increased heat production in cold weather partly by exercising more actively and partly by increasing the amount of protein in the diet, and so bringing into play the stimulating effect of that substance on metabolism. This latter is seen perhaps most strikingly in the experience of the Eskimos; when one of them comes in from a hunting trip in the depth of winter very much chilled, he scarcely stops to warm himself at all, but with the greatest haste gulps huge quantities of meat that are barely thawed out, not really cooked in our sense of the word; in fact, frequently the meat that is eaten is so cold that it makes the Eskimo chillier than before; as soon, however, as digestion and absorption have commenced, so that the stimulation of metabolism can begin, the rapid production of heat in the body warms it up to the point where complete comfort is obtained. We have no other means of increasing heat production in our bodies except by muscular exercise or eating protein. It is interesting to note, however, that there is a form of involuntary muscular exercise which comes into play when we need to produce more heat in order to balance too rapid loss; this involuntary muscular action is the familiar shivering which one does when chilly. It is reflex in the strict sense, that is to say, the stimulation of cold on the skin arouses nervous disturbances which pass to the muscles in various parts of the body and set them into the violent movements which we call shivering. The functional metabolism of shivering is much greater than we might at first suppose, and the production of heat is correspondingly rapid. In fact, if one who feels inclined to shiver encourages it instead of attempting to keep from doing it, he will very quickly produce enough heat within his body so that he will no longer have a disposition to shiver.