Page:Popular Science Monthly Volume 71.djvu/200

194 of the animal or in the changes age produces. The animals which belong to the vertebrate sub-kingdom, of which we ourselves are members, can be grouped in two large divisions according to the natural temperature of their bodies. The lower vertebrates, the fishes, frogs and their kin, are animals' which depend for their body temperature more or less on the medium in which they live. The other division of vertebrate animals, which includes all the higher forms, are so organized that they have within certain limits the power of regulating their own body temperature. Now it is easily to be observed—and any one who has made observations upon the growth of animals can confirm this—that animals otherwise alike will grow at different speeds at different temperatures. There are animals, like the frogs and salamanders, which will live at a