EVOLUTION OF INTELLIGENCE. Ill 



It was maintained above (p. 18) that the physico- 

 chemical processes of life form an unbroken series of 

 changes ; to these correspond the chain of mental pro- 

 cesses. And we believe that to every mental process, 

 whether of the "highest" kind in the mind of man or of 

 the "lowest" in that of the most primitive organism, 

 there corresponds some physico-chemical change. How 

 intimately connected are the two sets of processes is 

 matter of common knowledge. The slightest disturbance 

 or interruption in the metabolism of the brain, due to an 

 injury, an anaesthetic, a poison, will have its echo in a 

 disturbance of the mental processes. The gradual elabora- 

 tion of the sense organs and nervous system found in the 

 evolutionary series from the ^lo west forms up to man we 

 judge to be accompanied by a corresponding develop- 

 ment of mental powers. So far as we know, neither 

 the mental nor the metabolic processes can take place 

 without the other. Yet, indissolubly bound together 

 as they are, the one is certainly not the product of the 

 other, nor can it interfere with the continuity of the 

 other (p. 18). The mental and the physical series can- 

 not break into each other's continuity because they are 

 not independent of each other ; what exactly is the nature 

 of the connection between them Philosophy may attempt 

 to define, but Science is not called upon to describe. We 

 may point out, however, that they appear to be two 

 aspects of one and the same series of events ; one seen 

 from within and the other from without. It is the 

 artificial separation of these two abstractions, body and 

 mind, from the reality that has led to endless contro- 

 versies about the possible action of one on the other. So 

 in attempting to give a scientific explanation of evolution 

 we can neither speak of mental processes as produced by 

 or guided by physico-chemical processes, nor of meta- 

 bolism as directed aloi'g its course by mind. Several 

 theories of evolution seem to fall into this error, notably 

 the " Mnemic " theory recently supported by Semon. 



