148 THE ORIGIN OF THE NERVOUS SYSTEM 



and hereditary potentialities of protoplasm are altered 

 in the course of evolution, however we suppose such 

 alteration to occur, the form, slope, or curve which a 

 physiological gradient will take and the rate or intensity 

 of physiological activity at its high end may also undergo 

 change. In other words, the reaction of the protoplasm 

 to the conditions giving rise to a gradient, or in the 

 presence of a persistent or inherited gradient, must 

 undergo modification in the course of evolution, like 

 other protoplasmic reactions. In whatever terms we 

 may prefer to interpret the problem of cephalization 

 with respect to inheritance and evolution, I believe the 

 physiological basis of the differences in different forms 

 will be found to consist in the differences in the charac- 

 ter of the physiological gradients in the different pro- 

 toplasms. 



The localization of the "central" nervous system in 

 more or less distinct regions of the body in the pelecy- 

 pods and gastropod mollusks indicates that the physio- 

 logical conditions concerned in the localization of 

 nervous structure in these forms are somewhat different 

 from those in most other invertebrates. As regards 

 the gradients in development some data have been 

 obtained for certain gastropods, and these indicate that 

 the primary polar gradient of early embryonic stages is 

 more or less completely obliterated in larval stages and 

 that in place of it several regions of high physiological 

 activity arise, the foot, the visceral mass, the mantle 

 folds, and the branchial region. It may be suggested 

 that the peculiar ganglionic form of the nervous system 

 in these groups is associated with these physiological 

 facts, the differentiation of nervous structure being 



