The Human Machine 
evolution. The response to a sharp 
blow by pain and retreat from the 
offending point; the response to an 
insect-like tickle by the desire to 
scratch; the response to a soft, caressing 
contact by pleasure and approach, are 
all specific to the species and the self- 
protective necessities as a result of 
which they were evolved in the or- 
ganism. Similarly, the more obscure 
and delicate responses of thought and 
sentiment, of ‘study,’ ‘invention,’ ‘am- 
bition,’ ‘industry,’ ‘joy,’ ‘sadness,’ 
aemorse, are, call dependent. upon: 
specific stimuli in the environment and 
are specific to one or another of the 
biologic purposes of self-preservation, 
nutrition or procreation.”’ 
THE USE OF TICKLISHNESS 
Some of these reactions to stimuli 
may have been of use once but are of 
little value under the conditions of 
modern civilization; “for just as the 
organism is slow in evolving adaptations 
to newly developed factors in the en- 
vironment, 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 peril is the tickle reflex. 
It is more strange than appears 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 
accompanied by a self-protective reac- 
tion. 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 sharpimpact 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 re- 
flex was undoubtedly developed at a 
time when insects were a great menace 
to life, and when only those individuals 
who evolved an effective defense were 
able to prevail. It may even supply 
an explanation of man’s loss of hair in 
the upward march, since the presence 
487 
of hair would provide ambush for the 
insect enemy, and its loss, together with 
the evolution of the tickle sensation, 
would greatly facilitate defense.”’ 
“A second type of tickle reflex is 
elicited by heavy penetrating pressure 
in the region of the ribs, the loins, the 
base of the neck and the soles of the 
feet—the pressure simulating the pene- 
trating contact of a tooth-shaped body. 
The reaction in this case is a violent dis- 
charge of energy in the form of laughter 
with cries for mercy and frantic muscu- 
lar efforts to be free if the stimulus be 
continued. If one were tied hand and 
foot and were vigorously tickled for an 
hour, he would probably be as com- 
pletely exhausted as if he had run a 
marathon race or sustained a crushing 
injury; indeed, victims of torture in the 
Middle Ages were often killed by pro- 
longed tickling. 
“The fact that these ticklish areas 
are found in those parts of the body 
which are still and must always have 
been the points most frequently at- 
tacked by savage beasts leaves little 
doubt that this reaction developed at 
a time when man’s progenitors, like 
the carnivora of today, fought their 
enemies face to face with tooth and 
claw, and that this mechanism was 
acquired as a means of protection 
against valiant foes.” 
THREE KINDS OF REACTIONS 
Tears, sneezing, coughing and vomit- 
ing are among the other protective 
adaptations which Dr. Crile describes as 
being based on contact stimuli. There 
are other adaptations which are based 
on chemical stimuli. And finally, there 
is a third class of reactions, much greater 
in extent, which is based on the stimula- 
tion of what he calls “distance ceptors.” 
He explains: 
“Adaptation to environment in some 
species of animals, such as the oyster, 
is secured mainly by reactions to stimu- 
lation of the contact and chemical 
ceptors only; but in most animals there 
has been evolved a third method of 
adaptation to environment by which 
they are directed toward beneficial 
objects in their distant environment and 
away from those that are harmful, thus 
