PSYCHIC GRADATIONS. 125 



and the fantastic notions of the transcendental dualist 

 philosophy ; though it is precisely these abnormal 

 associations of " faith " and of " revelation " that have 

 often been deemed the greatest treasures of the human 

 mind (cf. chap. xvi.). 



The antiquated psychology of the Middle Ages 

 (which, however, still numbers many adherents) con- 

 sidered the mental life of man and that of the brute 

 to be two entirely different phenomena ; the one it 

 attributed to " reason," the other to "instinct." In 

 harmony with the traditional story of creation, it was 

 assumed that each animal species had received a 

 definite, unconscious psychic force from the Creator at 

 its formation, and that this instinct of each species 

 was just as unchangeable as its bodily structure. 

 Lamarck proved the untenableness of this error in 

 1809 by establishing the theory of descent, and 

 Darwin completely demolished it in 1859. With the 

 aid of his theory of selection he proved the following 

 important theses : — 



1. The instincts of species show individual dif- 

 ferences, and are just as subject to modification 

 under the law of adaptation as the morphological 

 features of their bodily structure. 



2. These modifications (generally arising from a 

 change of habits) are partly transmitted to offspring 

 by heredity, and thus accumulate and are accentuated 

 in the course of generations. 



3. Selection, both artificial and natural, singles out 

 certain of these inherited modifications of the psychic 

 activity ; it preserves the most useful and rejects the 

 least adaptive. 



4. The divergence of psychic character which thus 

 arises leads, in the course of generations, to the 



