236 DOCTRINE OF EVOLUTION 



alteration of the brain as the result of diseased growth 

 causes- the mind to grow dim and incapable. Some- 

 times infants are born which are so deficient mentally 

 as to be idiots, and an examination of the brain in such 

 a case reveals certain correlated defects in physical 

 organization. These and similar facts form the basis 

 for the dictum that the development and evolution of 

 the brain mean the growth and evolution of human 

 intellect. 



The further question as to the nature of the connec- 

 tion is interesting, but it relates to matters of far less 

 consequence to the natiu'alist than the central fact of 

 the invariable relation which does exist. Throughout 

 the centuries many philosophers and naturalists of 

 numerous peoples have endeavored to explain the con- 

 nection in question in ways that have been largely 

 determined by the changing states of knowledge of 

 various periods, as well as by differences in individual 

 temperament. Three general conceptions have been 

 developed : first, that the material and mental phe- 

 nomena interact; second, that they are parallel; and 

 third, that they are one. 



According to the first view, the individual thoughts 

 and feelings forming elements in the chain of consecu- 

 tive consciousness are affected by the events in the 

 material physiology of the brain as a physical structure ; 

 the latter in turn react upon the psychical or mental 

 elements. Thus there would be two complete series 

 of phenomena, which are interdependent and inter- 

 acting at all times, although each would be in itself a 

 complete chain of elements. 



The second interpretation is that the two series of 



