our bodies maintain, is a temperature which is very well suited for an active metabolism; by keeping this temperature all the year around they are able to show this metabolism all the time instead of only in summer, as in the case of the cold-blooded animals. As we said a moment ago, the maintaining of a constant temperature is altogether a matter of making the loss of heat balance the production of heat; the production of heat is altogether a matter of the metabolism; practically the whole of the resting metabolism takes the form of heat, since apparently the expenditure of energy in keeping the protoplasm alive is extremely wasteful; nearly all the energy that is actually liberated takes the form of heat, almost none of it being used in actual manufacture of material. This is, as has already been said, a very constant metabolism; the functional metabolism on the other hand is extremely variable depending on how hard we use our muscles; the total amount of heat produced from hour to hour is scarcely ever the same except when we are asleep; this means that the loss of heat, which has to balance the production of heat, must vary from hour to hour exactly parallel with the latter. In birds and in all mammals, except man, the adjustment of heat loss to heat production is almost wholly automatic; the animal or bird does very little to control it. We do see, however, a few examples of effort on the part of animals either to prevent heat from being lost too rapidly or to favor its more rapid loss. Thus, on a cold day, a cat or dog is apt to lie curled up or with its legs bunched under it; on a very warm day, on the other hand, it will lie stretched out as much as possible, and in the case of the dog, with the tongue hanging out full length. Birds sometimes are seen with feathers ruffled up apparently in an effort to keep warm. Both birds and mammals seek sheltered places in which to sleep, where they will be as little exposed as possible to cold winds or rains. In man the automatic adjustment of heat loss to heat production is very imperfect, in fact, it would not enable men to live naked except in the tropics. It is true that outside the arctic regions men might go naked and still make the heat production equal the heat loss during the waking hours by very vigorous exercise, but during the hours of sleep the loss of heat would be certain to be much more rapid than its production and so the body temperature would fall in severe weather enough to cause death. Man maintains himself outside the tropics, then, by using artificial aids for maintaining the balance between heat production and heat loss; these are of three sorts; first the use of clothing, second the use of shelter, and third the use of artificial heat. It is really a very curious fact when one stops to think of it that, although many animals enjoy artificial heat and gladly bask in it when opportunity offers, no animal has ever discovered the