is as large and heavy as a whale, because the bone and muscle of which they are made would be incapable of supporting so great a weight. In the water, however, it is different; the huge mass is evenly supported by the water in which it almost floats, thus relieving the anatomy of the whole from nearly all stresses due to gravity.
The same cause has operated to make all inhabitants of the air small. No very large bird, say as large as a horse, is known, and not even the extraordinary creations of past geologic ages show us any examples of very large flying creatures. The necessary relations between velocity of wing movement, weight, and size might be found for a flying elephant, but the intrinsic strength of living tissues would prove too weak to sustain so large a mass in the air by muscular exertion. So it is apparent that it is the intrinsic strength of living tissue, and not weight alone, which limits the size of aërial creatures.
The second general relation between substance and structure may be stated thus: "The nature of matter puts a limit to the intensity of action."
This proposition is nearly self-evident, and needs only one or two illustrations. All living structures consist largely of water. In the actively growing portions of vegetables upward of sixty per cent of the weight is water, while in animals more than seventy-five per cent of the weight of the whole body is represented by the same liquid. Physically and chemically speaking, life is chiefly an aqueous phenomenon. Now, as water forms steam of a quite sensible pressure at temperatures a little over 100º F., while it becomes a solid at 32º, we see that this property of water would alone be sufficient to account for the fact that living creatures can not grow and propagate outside of these temperature limits, while if they are somewhat exceeded, even the smallest and most resisting forms of life, the so-called germs, are permanently killed.
Again, the rate of nerve transmission in warm-blooded animals is about one hundred and fifty feet per second. A peripheral sensation takes a sensible time to reach the brain, another interval for the brain to act, and a third for the order to be executed. Herein lies the explanation why we are burned by unintentional contact with fire. All the time during which the message to and from the nerve center is being transmitted, the finger is passively lying in the flame and chemical destruction of tissue is going on, so that by the time the finger gets the order to move it has become badly injured.
If the nerves could take up and transmit a stress with the intensity and velocity of a copper wire carrying electricity; if the brain could act with the promptitude of a Leyden jar, and the