184 THE PROBLEMS OF EVOLUTION 



primitive form? It would be difficult to pick a 

 more uncertain field of inquiry or one more likely 

 to be complicated by differences of opinion, but 

 even here we have a gratifying morsel of logical 

 probability, if not absolute truth, on which to base 

 tentative conclusions. 



The one fundamental quality of protoplasm on 

 which the development of mind may rest is irrita- 

 bility. The living substance is always sensitive to 

 some conditions of its environment, and is acti- 

 vated by the stimuli which it is thus able to receive. 

 This property is more highly developed in animals 

 than in plants and culminates in the former in the 

 complex nervous development of man, but it is 

 nowhere wholly lacking in the organic world. The 

 development of various organisms results in many 

 degrees of complexity and in corresponding degrees 

 of exactness in the responses of the organism, but 

 difference of kind has not been demonstrated. 

 According to Herrick: "The functions of the hu- 

 man brain, so far as exact physiological knowledge 

 of these processes has gone, can all be reduced to 

 phenomena of excitation and conduction not radi- 

 cally different in fundamental character from those 

 of a protozoan or a polyp." ^ 



The primitive responses of simple organisms to 

 environmental stimuli, commonly called tropisms 

 or taxes, result in favorable orientation of the 



2 Herrick, C. J., Neurological Foundations of Animal Behavior, p. 297, 1924. 



