20 HUMAN PHYSIOLOGY 



Animals have an income and an outgo of matter and the 

 two are strictly balanced unless the body is growing or 

 wasting. They have, similarly, an income and an outgo 

 of energy and the proof that here, too, nothing is lost or 

 gained stands forth as one of the foremost achievements 

 of our science in the last century. It would probably 

 not have surprised a vitalist of the old school if it had 

 been shown that an animal could develop energy inde- 

 pendently of any external supply. This would have 

 made it a creator or generator while we have come to regard 

 it as a transformer. 



Adaptation. Biologists have made many efforts to de- 

 fine life. Their attempts have often been ingenious, 

 though never entirely satisfying. Ignoring the possi- 

 bility of consciousness in the organism observed (though 

 necessarily recognizing its existence for the observer) a 

 great thinker has said that life is "the continual adjust- 

 ment of internal to external relations." It is certainly 

 true that we judge by this standard when we attempt to 

 decide whether a mass of matter is living or dead. If it 

 shows no tendency to protect itself from the changing 

 conditions which are brought to bear upon it we conclude 

 that it is lifeless. This is what we should infer of a dog 

 which lay in the road indifferent to the approach of an 

 automobile. We look to see organisms adapting them- 

 selves to their circumstances so as to preserve their 

 integrity in spite of many hostile forces. The study of 

 adaptation is, accordingly, a large part of physiology. 



The hard thing for the beginner is to regard this famil- 

 iar adaptation as a mechanical process.' He has always 

 thought that the quest of food, the avoidance of enemies, 

 the protection of the body against heat and cold were 

 dictated by intelligence. Introspection encourages such 

 a view by emphasizing the strength of the desire to pro- 

 long one's life. At the same time reflection convinces 

 one that it is impossible to give the attention to all the 

 serviceable reactions which are constantly taking place. 

 Many of these usually pass unnoticed while in others 



