368 yournal of Comparative Neurology and Psychology. 



is, strictly speaking, a physiological and not a morphological prob- 

 lem." In his illustration derived from a study of regeneration 

 in Leptoplana after mutilation, he says: "The regionsof the body 

 develop in a characteristic form because they function or attempt to 

 function in a characteristic manner." This is, with certain reser- 

 vations, a very pregnant statement of the basic principle of dynamic 

 realism as applied to biology. But it must be noticed that the 

 methods involved in the study of multilations present us with 

 little more than a diagram, and a more or less arbitrary one at 

 that, which serves merely to illustrate the principle. The appar- 

 ent deduction that the reproduced parts are what they are simply 

 because the attempts at locomotion after mutilation tend to focus 

 or direct the growing activity in a certain direction, is probably 

 misleading. This is, as the author himself admits, only a small 

 part of the process. 



It would not be claimed that the leg of a newt regenerates in a 

 characteristic form because the growing member kicks or attempts 

 to kick in a characteristic manner. The kicking in a characteristic 

 manner is a late coordination of types of coordinated activity of 

 great complexity which when the organism is complete, form a 

 balanced elastic system whose origin can be traced backward 

 through heredity indefinitely. Structure is simply the way the 

 system reveals itself to one of our methods of research while the 

 gross forms of activity (kicking, etc.) are extreme molar perturba- 

 tions of the equilibrium along lines marked out by the nature of 

 more subtle coordinations. The structure of a striped muscle 

 fiber is itself the revelation of certain types of (molecular) activi- 

 ties. The leg of a newt regenerates in the characteristic form 

 because the plastic energic system bears the impress of the leg in 

 all its past relations to that system and is adjusted to its trophic 

 activities. Every activity of the remainder of the body after muti- 

 lation continues to attempt to react as if the leg were still present 

 and this phase of the locus formula corresponding to the leg part 

 of the whole complex is converted into trophic tendencies. While 

 the leg was present a certain factor in physiological equilibrium 

 was devoted to nutrition of a member of such and such a type. 

 This factor does not cease to operate but, by what might be called 

 physiological inertia, continues this nutritive tendency. Let us 

 figure an elastic bag filled with a viscid fluid from which a tube 

 is conducted. The bag is under pressure and fluid constantly 



