174 Instinct and Intelligence 



which they become adapted to the ever- 

 increasing complexity of their environment, 

 new relations being thus established under 

 new conditions to the advantage of the 

 organism. 



As an example of a simple reflex action we 

 may refer to the increased flow of saliva which 

 accompanies the stimulus of food taken into an 

 animal's mouth; there is no difficulty in col- 

 lecting the saliva which ordinarily finds its way 

 into the mouth, so that the quantity of fluid 

 secreted by the salivary glands can be measured 

 under varying conditions. While a dog is 

 eating his food there is, by reflex action, an 

 increased flow of his saliva. While this is 

 taking place we may proceed to pinch or other- 

 wise stimulate the skin of a definite part of the 

 dog's body. If, when the dog has finished 

 eating and the increased flow of saliva ceases, 

 we then irritate the previously stimulated skin, 

 a fresh flow of saliva takes place, produced 

 this time by a tactile impression made on 

 sensory organs of the skin, in place of one 

 caused by the contact of food with the mucous 

 membrane of the mouth. Again if, while a dog 



