MAMMALS 67 



body. The proper movement is brought about in this 

 way. In our example the wave of the hand would be 

 the sense-impression ; this would be conducted by the 

 sensory nerves to the brain, and from here the closing 

 of the eyelid would be brought about by means of 

 the motor nerves. 



But the reflex action does not always consist of a 

 single movement; often many follow upon a single 

 impression, and one of these brings about the other. 

 This is the case, for instance, when we trip over a stone. 

 The shock to the foot is followed by a whole series 

 of movements a stretching of the arms, a throwing 

 back of the upper part of the body, and several quick 

 movements of the legs. 



Here we have reached the transition to instincts; 

 these also are characterised by the fact of a number of 

 movements following upon sense - impression. It is, 

 therefore, only a complicated reflex action when a 

 young, untaught kitten springs instinctively after a 

 mouse as soon as it sees it. 



Thus the basis of both reflex and instinctive actions 

 is corporeal, and is found in the nerve-tracks. And as 

 all the organs of the body can be affected by natural 

 selection, it can also bring about changes in the 

 connections of these tracks and can increase the 

 sensitiveness of the nerves. When we thus discover 

 the material groundwork of natural selection, it becomes 

 clear that instincts are subject to it, and modified by it, 

 and that new ones may be brought into existence by it. 



Anyone who tries to catch a fly with his hand will 

 see that the little creature is generally off before the 



