76 PHYSICAL BASIS OF CIVILIZATION 



rationally conceived, except as a tendency to repro- 

 duce conduct by the activity of structures, which 

 on former occasions were modified by conduct 

 producing currents of energy; and to account for 

 character, the mind is forced to assume similar modi- 

 fications in structures whose function it is to co- 

 ordinate and subordinate conduct. See Appendix 

 Note II. 



Exposition of the details of the process by which 

 these changes in nerve structures are produced, and 

 of the parts these changed structures subsequently 

 play in the phenomena of life and mind, belongs to 

 psychology and is too voluminous for these essays ; 

 but an outline of that part only which relates spe- 

 cially to the growth of the human intelligence must 

 be inserted, because it has bearing on subsequent 

 arguments. 



Whenever any kind of complex external conditions 

 for the first time interacts with an organism of the 

 more highly specialized kinds and thereby generates 

 currents of energy in its nervous system, then these 

 currents begin by flowing inwardly along afferent 

 nerves, and thereafter they pass through various 

 ganglia and nerve centers. Next they cause changes 

 in consciousness, such as sensations, feelings, ex- 

 periences, thoughts, comparisons, judgments, pre- 

 ferences, will, etc., etc. By some of these, and mainly 

 by judgment and will, currents of nerve energy are 

 then sent over efferent nerves, etc., into muscular 

 tissues. The muscular tissues are contracted in 



