THE NERVOUS FUNCTIONS 33 



behavior which are similarly fixed or stereotyped in their innate 

 organization; but, in addition to these stable reflexes and in- 

 stincts, the higher members of this group have also a consider- 

 able capacity for individual modifiability in behavior, and they 

 are characterized by greater individual plasticity and docility 

 (Yerkes). It appears that the tubular type of nervous system 

 found in vertebrates permits of the development of certain 

 kinds of correlation mechanisms which are impossible in the more 

 compact form of ganglia of the insects. These two branches of 

 the animal kingdom have, therefore, during all of the more 

 recent evolutionary epochs diverged farther from each other, 

 and now, in their highly differentiated conditions, neither type 

 could be derived from the other. The jointed animals (articu- 

 lates) developed from the lower worms, and this branch of the 

 animal kingdom, which may be called the articulate phylum, 

 culminates in the insects. The vertebrates were probably 

 developed from similar lowly worm-like forms along an inde- 

 pendent line of evolution, and this branch of the animal king- 

 dom, the vertebrate phylum, culminates in the human race. 

 Figure 4 illustrates in a rough diagrammatic way the relative de- 

 velopment of the variable and invariable factors of behavior in 

 the articulate and vertebrate phyla. 



In unicellular organisms without nervous systems the general 

 protoplasm, of course, is the apparatus of both the invariable 

 and the variable factors of behavior, and the simpler forms of 

 nervous system likewise possess both of these capacities. But 

 in the more complex forms of nervous system among vertebrates 

 special correlation centers are set apart for the variable activi- 

 ties, particularly those which are intelligently performed, and 

 the most important of these centers are found in the cerebral 

 cortex. This is the part of the brain which is greatly enlarged in 

 mankind, as contrasted with all other animals, and the last three 

 chapters of this work are devoted to the structure and functions 

 of these cortical mechanisms with whose activity the progress of 

 human culture is so- intimately related. 



It should be borne in mind that the higher correlation centers 

 which serve the individually variable or labile behavior in higher 

 vertebrates can act only through the agency of the lower reflex 

 centers. The point is, that all of the elements of behavior are 



3 



