112 EVOLUTION OF LIVING ORGANISMS. 



Now, if we only knew the whole series of physico- 

 chemical changes which correspond to the mental pro- 

 cesses, the whole story of evolution could be told from 

 beginning to end from the point of view of the scientific 

 abstraction as an unbroken chain of metabolic changes. 

 But unfortunately we are still very far from having 

 obtained this complete knowledge, except perhaps in 

 the case of plants ; and so, in describing the behaviour 

 of animals, we constantly pass over from the physico- 

 chemical to the psychological side, and describe be- 

 haviour in terms of mental processes we know only in 

 ourselves, but infer from external evidence to take place 

 in animals also. The gaps in the physico-chemical 

 description are thus filled in from the other series. The 

 physiologist is, however, daily extending our knowledge 

 of the metabolic processes, and as fast as he advances 

 he discards the psychical "explanation." But in doing 

 so he often ignores the passage from one abstraction to 

 the other, and erroneously concludes that mental pro- 

 cesses are insignificant, superfluous, or non-existent. 

 He imagines he has reduced them to mere physics and 

 chemistry ; an error into which we must be careful not 

 to fall. If the whole chain of physico-chemical meta- 

 bolic processes going on in even the human body and 

 brain be some day discovered, the mental processes will 

 still remain untouched and incapable of being described 

 in the same terms. 



The current method of dealing with mental processes, 

 even morals and emotions, in a scientific description of 

 evolution is then a makeshift, a provisional but con- 

 venient method ; so the words feeling, memory, choice, 

 and will, constantly creep into our descriptions of animal 

 behaviour no harm is done so long as their foreign 

 origin is recognised. Since the two sets of processes 

 correspond, we need not be astonished at finding that 

 the " laws " of variation, inheritance and natural selection 

 hold good in mental as in material evolution. 



