90 THE ORIGIN OF THE NERVOUS SYSTEM 



within its range into a definite physiological relation. 

 The differences in electric potential which have been 

 found to exist between different levels afford one means 

 of actual correlation, and, if Lillie's theory is correct, the 

 resulting electric currents must be the primary factors in 

 determining and mamtaining the conditions at different 

 levels. Conceived in these terms, organismic integration 

 is primarily electrical, that is to say, the factor actually 

 concerned in the first instance in bringing different proto- 

 plasmic regions into physiological relation with each 

 other is the electric current. But this is only the first 

 step in organismic integration. As soon as differences 

 in protoplasmic constitution appear at different levels of 

 a gradient, chemical or transportative correlation begins 

 to play a part in integration, and still later various 

 mechanisms of correlation and integration develop, e.g., 

 the various types of vascular system, the supporting 

 and other mechanical tissues, the organs of secretion 

 and excretion, and in most animals the nervous system 

 as the organ of integration par excellence. 



THE PHYSIOLOGICAL GRADIENT AS A BASIS FOR 

 DIFFERENTIATION 



Granting that the physiological gradient is the basis 

 of axiate pattern, it is evident that it must constitute 

 the starting-point of the orderly, progressive complica- 

 tion in differentiation and physiological correlation 

 which we call development. In some way qualitative 

 differences must arise in the protoplasm at different 

 levels of the gradient which was essentially quantitative 

 in its origin. The origin of differentiation from differ- 

 ences which are primarily quantitative may appear to 



