136 Journal of Comparative Neurology and Psychology. 



the complex variable series ; there is change from generation to 

 generation in the number and variety of reactions exhibited in 

 the trial and error series. At times these changes are not in 

 the direction of an increase in the scope of reaction, for in cer- 

 tain cases regress in one direction conditions progress in anoth- 

 er : the developmental process is not simple and continuous. 

 In the individual, on the other hand, repetition of a reaction 

 brings about mechanization ; habits are formed ; the number 

 and variety of reactions which were exhibited early in the life 

 of the organism are diminished later by the avoidance of those 

 which are useless or harmful Plasticity in activity is an aspect 

 of phylogenetic development ; fixity in activity, of ontogenet- 

 ic development. 



Precisely such a point of view with reference to aspects of 

 activity as is here taken necessitates a justification of the separ- 

 ation of phylogeny and ontogeny. Why should we speak ot 

 one aspect of actionas characteristic of phylogenetic development, 

 another of ontogenetic ? The answer to this question is found in 

 the history of activity. For if we choose a type of organism 

 which is capable of associative learning we find that any individual 

 of the species during its life exhibits two kinds of action: first, re- 

 flex, instinctive and voluntary acts which are not shaped by ex- 

 perience ; second, instinctive and voluntary acts which are mod- 

 ified by experience, and which in case of the latter give rise to 

 habits. The first represents the hereditary ability of the indi- 

 vidual ; the second, the acquired ability. In the offspring of 

 the individual there appear hereditary activities similar to those 

 of the parent, but not exactly like them, for no two individuals 

 have precisely the same form. There are more or less marked 

 variations, and for the development of activity these variations 

 are of great importance. The reflexes of a species appear in 

 generation after generation with but slight and usually unnoticed 

 changes ; the instinctive acts show greater differences, the old 

 ones are modified and new ones appear, possibly as the result 

 of the inheritance of certain of the tendencies to action which 

 have been established as habits in the parent ; the voluntary 

 acts are strikingly different, unique. It is here that variation 



