Physiological Basis of Restitiiiiou of Lost Parts 495 



cited above is in fact a demonstration that functional processes 

 may be less sharply localized than the structures which represent 

 them. The anatomical structure known as the head in Planaria 

 is not the only region where "head-reactions" are possible, but 

 it does represent the region where they occur with greatest energy 

 and frequency in the normal animal. Admitting this, the question 

 arises as to why heads do not form all along the body in Planaria, 

 i. e., as to why structure should be thus more narrowly locaHzed 

 than function. The answer is not far to seek. If two parts, one 

 of which is capable of reacting in a certain manner more rapidly 

 and with greater energy than the other are correlated, the reaction 

 to a given stimulus will occur in the first part earlier and with 

 greater energy than in the second. The fact that a reaction has 

 occurred in the first part must bring about changes in the system 

 m consequence of which the character of reaction in the second part 

 is altered. If structure is, as I believe, the visible expression of 

 functional or dynamic conditions, we cannot expect that the second 

 part, even though it possesses in some degree the same functional 

 capacities as the first should exhibit the same structure, for the 

 very fact of its correlation with the first part which possesses these 

 capacities in greater degree determines that the functional con- 

 ditions in it shall be different from those in the first part. In gen- 

 eral terms we may say that the region where a particular functional 

 complex occurs with greatest energy, frequency or rapidity domi- 

 nates so far as this particular complex is concerned all other parts 

 of the organism which possess the same capacity in less degree, 

 and modifies their activities to a greater or less extent. Conse- 

 quently the structure with which a particular functional complex 

 is associated in the normal animal may be much more narrowly 

 locaHzed than the functional complex. In Planaria, for example, 

 the head-structure is hmited to the anterior end of the animal, 

 while the functional capacities commonly regarded as charac- 

 teristic of the head exist at all levels of the body. These other 

 regions are capable of producing a head-structure, hut only when 

 isolated from the original head. 



It is evident then from this consideration that localization of visi- 

 ble structure is not necessarily an exact criterion of localization of 



