74 MAN AN ADAPTIVE MECHANISM 



The Tickle Reflex 



Of similar significance is the not infrequent appear- 

 ance of a mechanism of little or no present value to the 

 organism ; for just as the organism is slow in evolving 

 adaptations to newly developed factors in the environ- 

 ment, it is slow in discarding adaptations to an older 

 environment, even such as may be a hindrance to life 

 under present conditions. Such a relic of prehistoric 

 perils is the tickle reflex. It is more strange than ap- 

 pears at first glance that the tickle reflex can be excited 

 only in certain parts of the body, by but two types of 

 tactile impression, and that it is invariably accom- 

 panied by a self -protective reaction. One type of the 

 tickle reflex is elicited by a light running motion on the 

 surface of the skin, which produces a sensation like that 

 produced by a crawling insect, with an irresistible desire 

 to scratch or rub the affected part. A sharp impact 

 causes pain, but if the adequate stimulus of contact 

 which simulates the crawling of an insect be applied again 

 and again in the same spot it will cause each time the 

 same tickling sensation. This reflex was undoubtedly 

 developed at a time when insects were a great menace 

 to life, and when only those individuals which evolved 

 an effective defense were able to prevail. It may even 

 supply an explanation of man's loss of hair in the up- 

 ward march, since the presence of hair would provide 

 ambush for the insect enemy, and its loss, together with 

 the evolution of the tickle sensation, would greatly 

 facilitate defense. 



Although the need of this gross adaptation against 

 the attacks of insects is steadily waning as more subtle 



