72 MAN AN ADAPTIVE MECHANISM 



they are implanted. The adequate stimulus for each 

 one of these mechanisms is some agent in the environ- 

 ment which originally occasioned the development of 

 that mechanism in the organism. The response to the 

 stimulus is invariably some act by which the organism is 

 protected or the welfare of the species promoted. 



The response to a stimulation of the ceptors of the 

 skin by heat or mechanical injury is a type of pain which 

 is specific to the form of injury. Stimulation of the 

 contact ceptors of the membranous lining of the nose 

 produces a sneeze, leading to the expulsion of the harm- 

 ful obstruction. Stimulation of the membrane of the 

 throat, pharynx or larynx produces a cough, whose pur- 

 pose is the expulsion of the source of irritation. Stim- 

 ulation of the esophagus produces a swallowing move- 

 ment ; of the lining of the stomach and of the muscular 

 wall of the intestine, peristalsis; of the interior walls 

 of the uterus and tubes, of the gall bladder and 

 ducts, of the urinary bladder, of the ureter, of the 

 kidney or of the rectum, contractions which are 

 specific to those regions, resulting in the expulsion of 

 their contents. In no case could any of these mechan- 

 isms, each adequate to the protection of its particular 

 field, be interchanged with another to initiate an act 

 of protection. Stimulation of the cornea of the eye 

 cannot produce a sneeze, irritation of the nose cannot 

 produce a cough ; contact with the skin cannot produce 

 peristalsis. 



In no case is any mechanism superfluous. Although 

 some mechanisms, such as the scratch reflex, may show 

 evidence of a vast antiquity and a dwindling usefulness, 

 the organism as a whole is far from being sufficiently 



