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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 neces- 

 sary 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 aerial 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 sev- 

 enty-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 100 F., 

 while it becomes a solid at 33, we see that this property of water 

 would alone be sufficient to account for the fact that living crea- 

 tures 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 perma- 

 nently killed. 



Again, the rate of nerve transmission in warm-blooded ani- 

 mals is about one hundred and fifty feet per second. A peripheral 

 sensation takes a sensible time to reach the brain, another inter- 

 val 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 



