tion 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 
Fig. 19. Four Tadpoles of the European Frog Rana fusca. After Oskar Hertwig. The four animals are all of the same age (three days) and raised from the same batch of eggs, but have been kept at different temperatures.
| A at 11.5° Centigrade | B at 15.0° Centigrade |
| C " 20.0°" | D " 24.0°" |
very considerable range of temperature and thrive, apparently. No ultimate injury is done to them by a change of their bodily temperature. Here we have a picture of four young tadpoles, all of which are exactly three days old. The first of these has been kept at a temperature not much above freezing. The fourth, at a temperature of about 24 degrees centigrade; the other two at temperatures between. They are all descendants from the same batch of eggs, and you can see readily that the first one is still essentially nothing but an egg. The second one, which has had a little higher temperature, already shows some traces of organization, and those familiar with the development of these animals can see in the markings upon the surface the first indications of the differentiation of the nervous system. The third has been kept at a considerably warmer temperature, and is now obviously a young tadpole; here are the eyes, the rudimentary gills, the tail, etc. While the fourth tadpole, which was maintained at the best temperature for the growth of these animals, has advanced enormously in its development. Obviously, should we make experiments upon animals of this class it would be
