Preliminary Definitions and Illustrations. 7 



there certain nervous centres are called into activity, as 

 the result of which an orderly (co-ordinated) set of out- 

 going nerve-currents are distributed to the muscles which 

 are concerned in moving the leg. If a frog be killed by 

 the rapid extirpation of the brain, and then its flank be 

 sharply pinched, the hind leg of the same side will be 

 raised, and the foot will scratch at the irritated spot. This 

 again is a reflex action ; and the fact that the brain has 

 been extirpated shows that spinal centres alone are 

 sufficient for its due and accurate performance. Now, it 

 is by no means easy, if indeed it be possible, to draw 

 any sharp and decisive line of demarcation between 

 instinctive activities and reflex acts. Instinct has been 

 well described by Mr. Herbert Spencer as compound reflex 

 action ; hence the distinction between them turns in large 

 degree on their relative complexity. It would seem, 

 therefore, that, whereas a reflex act — such, for example, 

 as the winking of the eye when an object is seen to 

 approach it rapidly — is a restricted and localized response, 

 involving a particular organ or a definite group of 

 muscles, and is initiated by a more or less specialized 

 external stimulus ; an instinctive activity is a response 

 of the organism as a whole, and involves the co-operation^ 

 of several organs and many groups of muscles. Initiated 

 by an external stimulus or group of stimuli, it is, at any 

 rate in many cases, determined also in greater degree than 

 reflex action by an internal factor which causes uneasiness 

 or distress, more or less marked, if it do not flnd its normal ' 

 instinctive satisfaction. Take, for example, the before- 

 mentioned instinct of the great water-beetle to leave the 

 pond and burrow in its bank when the time for pupation 

 is at hand. There is something more here than a local 

 response to an external stimulus ; something more, it would 

 seem, than mere reflex action. There are activities 



