A good deal of what has been said thus far in the book applies to nearly all kinds of animals about as well as it does to man. We have now to take up a feature found only in two great groups of the animal kingdom--birds, and the four-legged animals, to which are given the name of mammals. This peculiarity is commonly called warm-bloodedness. What we really mean when we say that an animal is warm-blooded is that the temperature of its body runs about the same summer and winter, day in and day out. An animal that we call cold-blooded, on the other hand, cools down when in a cold place, but is warm when in a warm place. It therefore has a very variable temperature as compared with the almost constant temperature of warm-blooded animals. The maintaining of the warm-blooded condition, which means really the maintaining of a constant temperature, involves only one thing, namely that the amount of heat lost from the body shall exactly balance the amount of heat produced in it. If more heat is produced than is lost, the temperature must rise; if more heat is lost than is produced, the temperature must fall. These are simple facts of physics which apply as well to the body of an animal as to any other source of heat. All animals produce heat, because in all of them metabolism is going on, which means that in all of them energy is being liberated, and it is one of the fundamental laws of physics that whenever any energy is liberated all of it which does not take some other definite form is certain to appear as heat. The other possible forms that the energy liberation of the body may take are chemical energy in the manufacture of various materials and the energy of motion. Whenever either of these processes takes place a large amount of heat is produced in connection; this means, from the standpoint of mechanics, as we have already seen, that the body is an inefficient machine; if it were perfectly efficient, the energy that it liberates might all go into the form either of motion or of the manufacture of materials, with none left over to appear as heat; but since it is only 20 per cent efficient, four-fifths of all the energy that is displayed actually takes the form of heat. While this is a marked inefficiency from the mechanical standpoint, from the standpoint of bodily well-being it is by no means a bad thing, since it is this constant production of heat which makes possible in birds and mammals the maintenance of a constant and rather high temperature. That it is a great advantage to the body to be warm all of the time is clear when we compare the possibilities of our lives with those of cold-blooded animals, like insects or frogs. Whenever the weather cools down they necessarily become inactive since, as we saw in the third chapter, when protoplasm is cooled, its metabolism necessarily slows down, and when the cooling reaches a certain point the protoplasm becomes completely inactive. The temperature of 98½ degrees F., which