270 THE ORIGIN OF THE NERVOUS SYSTEM 



in its various stages of development, up to the cerebral 

 cortex of man, is in these terms a physiological conse- 

 quence of the establishment of the physiological gradient 

 or gradients in the cell or cell mass from which the indi- 

 vidual develops. 



The distinction between pattern and material, 

 between the physiological gradient and the hereditary 

 constitution of the protoplasm in which it arises, has 

 been pointed out repeatedly and may be emphasized 

 once more. The course of events in relation to a gra- 

 dient, for example, the particular kind of nervous sys- 

 tem which develops in a particular organism is primarily 

 a matter of hereditary constitution, but the gradients 

 determine how the potentialities of the protoplasm as 

 regards nervous development shall be realized. 



This conception of organismic pattern and of the 

 relation of nervous pattern to it is fundamentally a 

 physiological conception and requires no assumptions 

 nor special hypotheses concerning heredity and evo- 

 lution, except in so far as it involves rejection of the 

 more extreme preformistic types of theory. It does 

 not, however, in any sense represent the extreme epi- 

 genetic viewpoint. Moreover, it does not, as some of 

 its critics have asserted, involve acceptance of Lamarck- 

 ian doctrine, though it is possible of course to develop 

 a Lamarckian conception of inheritance of organismic 

 pattern from the conception of the physiological gradient. 

 The gradient is primarily nothing more than an ordering 

 and integrating factor in developmental physiology. 



Objection may be, in fact has been, made to the use 

 of the conception of the physiological gradient as the 

 foundation of a theory of organismic and nervous pattern 



