INTRODUCTION 21 



undue attention is more apt to result in a bungling than 

 a superior performance. We have been led to the belief 

 that they are the inevitable result of structure and not 

 of present intelligence. 



When we are overheated we perspire and the evapora- 

 tion of water cools the skin and the fraction of the blood 

 which is flowing through it. This is an adaptive change 

 but it is obviously one which we can scarcely influence 

 "by taking thought." It is like the adjustment which 

 is made by the pendulum of a clock when the temperature 

 rises. By an ingenious arrangement of different metals 

 the tendency of the pendulum to lengthen, and so to be 

 slowed, is offset. The preservation of the living organ- 

 ism through the summer day and the protection of the 

 clock against the same disturbing agency are both ex- 

 amples of adjustment due to structural characters. It 

 may be urged that the capacity of the clock to regulate 

 its action under these conditions is owing to the wisdom 

 and foresight of its maker and a reverent, parallel infer- 

 ence has often been drawn for the living organism. 



The adaptive changes which an animal must execute to 

 meet emergencies great and small are evidently varied 

 from moment to moment and have no fixed order or suc- 

 cession. There are other life processes which are more 

 constant and monotonous. These can generally be said 

 to be related to maintenance. The mechanisms of breath- 

 ing and the circulation, of alimentation and excretion, 

 may be held to serve primarily for the maintenance of the 

 organism as a whole, though the property of adaptability 

 is frequently illustrated in connection with them. The 

 adaptive mechanisms par excellence are the muscles and 

 the sense-organs together with the central nervous system 

 which correlates the former with the latter. In recent 

 writings the sense-organs are often called the receptors 

 while the muscles and the glands which are played upon 

 through the central nervous system are named effectors. 



The two names just used should explain themselves. 

 A receptor is a structure which is exposed to external 



