ORGANIC EVOLUTION — MENTAL 141 



developing mentally in response to appropriate stimu- 

 lation, and (2) on the amount of appropriate stimulation 

 supplied — as is clearly the case in man, in whom beyond 

 all other animals there has been an evolution of this 

 power. 



In higher animals therefore the power of individually 

 acquiring reason, of varying, of developing mentally, in 

 response to appropriate stimulation, is strictly analogous 

 to their power of individually acquiring corporal traits, 

 of varying, of developing physically in response to ap- 

 propriate stimulation. But just as in lower animals, 

 such as the sponges and coelenterates, the physical 

 structures develop in the absence of all stimulation 

 other than that of sufficient food, so also do such 

 mental traits as they exhibit, reflex action and even 

 instinct, develop in higher animals in the absence of 

 all other stimulation ; and therefore the development 

 of the reflexes and instincts is strictly analogous to the 

 development of the physical structures of lower animals ; 

 that is, appropriate stimulation does not cause variations 

 and developments in reflexes and instincts, but merely 

 calls pre-existing reflexes and instincts into activity; 

 they develop quite apart from stimulation. 



Reflexes and instincts, like the structures of lower 

 animals, and like what I may call the groundwork of 

 many of the structures of higher animals, then, have 

 been developed by Natural Selection to provide re- 

 actions against actions invariably occurring in the simple 

 environment ; but reason, like that part of the physical 

 develoi^ment of higher animals which is achieved only 

 as a reaction to stimulation, is achievable only as a 

 reaction to appropriate stimulation acting on the inborn 

 power to vary, to develop mentally ; by means of it, 

 as by means of their variable physical structures, higher 

 animals are brought into completer harmony Avith a 

 variable and complex environment, in which occur 



