78 PHYSICAL BASIS OF CIVILIZATION 



of conduct by which the individual responded to the 

 external conditions, when they first interacted with 

 the organism, have, by repetitions, become organically 

 connected with the changes in the nervous system 

 which these external causes then produced, and that 

 the various parts or sections of these changes have 

 been similarly connected by the same process. 



If the series is simple, consisting of few terms, 

 then very few repetitions suffice; if it be complex, 

 consisting of many terms, then more numerous 

 repetitions are required before the organization is 

 accomplished. 



It follows, then, that the few simple, often re- 

 peated, and similar adjustments by conduct, which 

 creatures below man are either competent or required 

 to perform, will quickly become organized, and that, 

 therefore, the occasions must be rare when their 

 incipient intelligence, will, or judgment is called 

 into action. 



By contrast, the infinite variety of highly complex, 

 co-ordinate actions and conduct, by which alone 

 human life can be maintained, demands the frequent 

 exercise of all these faculties to their utmost capa- 

 city. And since the extent of adaptation must 

 necessarily be proportioned to the degree of develop- 

 ment attained by these faculties, therefore has 

 their higher development important survival value. 

 Thus did the complexity of the human body favor 

 the natural selection of every individual possessing 

 even the smallest advantage in the matter of intelli- 



