Page:Life in Motion.djvu/44

24 which an evil meaning has been attached, as when we speak of a cold-blooded villain! The term "variable temperature" is more correct, as it distinguishes all such animals from those in which the blood has an almost uniform temperature, whatever may be the temperature of the medium in which the animal lives. For example, the temperature of a healthy man in the buruing plains of India or in the snowy wastes of Siberia never varies much from 98.4° Fahrenheit.

It is one of the characteristics of animals of variable temperature, like the frog, that all its tissues are more stable or permanent than those of animals of uniform temperature. The tissues of a frog are not so liable to change as those of a rabbit or of a bird. Now the active phenomena of life all depend on instability of tissue. Imagine you have built a house of cards. You might build it so that the slightest push, or even a whiff of air, might cause it to fall to pieces; or you might so construct it that considerable force would be needed to knock it down. In the first case, the house of cards would be unstable; in the second, it would be stable. The tissues from the two kinds of