Chap. I.] OF A NERVOUS SYSTEM. 25 



than is any one cause of impressions isolated from others 

 with which it may be associated in the complex web of 

 external occurrences. Each acquirement serves as a 

 stepping-stone to the next, and each new response is 

 made easier by those previously rendered possible. In 

 this way the correspondence between the organism and 

 the outside world gradually becomes, as Herbert Spencer 

 has urged, both more precise and more complex. By 

 slow degrees a more and more harmonious relationship 

 between the two is brought about, the degree of complexity 

 of which we are left to gauge, principally by an esti- 

 mation of the character of the movements executed in 

 relation to the stimuli from which they immediately or 

 remotely proceed. We have at first to do with mere 

 simple * reflex ' actions ; in higher forms of life some ot 

 these actions increase so much in complexity as to become 

 worthy of the name ' instinctive *; whilst in still higher 

 organisms we have what are called ' intelligent ' actions 

 in increasing proportion, though always intermixed with 

 multitudes of others belonging to the * instinctive ' and 

 to the * reflex ' categories. 



