622 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



psychical-social organism, together with his reactions to them, are the ex- 

 pression of the individuality in the sense in which this term is actually 

 applied to higher organisms. Hence individuality is associated with the appear- 

 ance of freedom in his reactions and with the increasing difficulty in estab- 

 lishing causal relations between the environment of the individual and his 

 actions. This is the condition we have in mind when we speak of freedom 

 of the will. The greater the complexity of the constellations into which the 

 individual and his environment enter, the greater become the individual 

 variations in actions. Thus, individuality in the psychical-social sense, the 

 difference between the reactions of different members of the same species, 

 the complexity of the factors determining the behavior and the non- 

 predictability of individual responses resulting from these complexities, 

 increase with increasing complications in structure of the organism as a 

 whole, and especially of the nervous system. 



We may summarize the essential features in which individuality in the 

 more primitive vertebrates differs from that in the higher vertebrates as 

 follows : ( 1 ) The stimuli which call forth a reaction are more simple and 

 stereotyped, (2) the reactions which take place are more limited in number 

 and are likewise more stereotyped, and (3) the degree of modifiability of 

 the reaction as a result of previous experiences is of a lower order in these 

 more primitive forms, and it increases with increasing complexity of struc- 

 tures. With ascending evolution and the more ready formation of condi- 

 tioned reflexes, learning takes place more easily. Structurally, this increase 

 in complexity and modifiability of those conditions of the behavior by which 

 we judge individuality in the psychical-social sense, is paralleled by the de- 

 velopment and increasing differentiation of the cortex in mammals and by 

 the transfer of the control of the most complex reactions from the corpus 

 striatum to the cortex. As Whitman has already pointed out, the develop- 

 ment of instincts, which are so significant in the psychical-social life, 

 especially of the more primitive vertebrates, corresponds to the structural 

 development of various organ systems; both gain in complexity and in this 

 respect take a parallel course during evolution, and it is possible to use for 

 taxonomic purposes instincts as well as organ structures. Instincts and 

 behavior generally are contingent on certain organ functions and they are 

 the direct expression of organ differentials and not of organismal differen- 

 tials, on which they depend only indirectly. Simple changes in texture, color 

 of the skin or of appendages of the skin, and movements may determine 

 species and individual reactions, and special movements and composite series 

 of movements (ceremonies) may function as stimuli in sexual life. Identity 

 of stimuli in several related species may cause identity of reactions of these 

 species towards one another, at least temporarily ; if later other stimuli begin 

 to function, which differ in certain of the species, they may then call forth a 

 differentiation in the reaction of these species towards members of their own 

 and towards members of the strange though related species. 



The complexity of the reactions and the significance of learning, of 

 modifiability of behavior through previous experiences, seem to be in general 



