1 84 INDIVIDUALITY IN ORGANISMS 



in the equally barren neo-vitalistic assumptions of some 

 non-mechanistic controlling or determining principle, 

 entelechy, or whatever we please to call it. 



The head of Planaria will serve to illustrate the 

 point. I have shown that a series of different forms 

 of head occur in reconstitution, ranging from the normal 

 to the headless condition (pp. 106-8). These differ- 

 ent forms represent various degrees of inhibition and 

 they result, not only from the inhibitory influence of 

 other parts (pp. 108-14), but can be produced experi- 

 mentally by a great variety of conditions. In a lot of 

 similar pieces from animals in similar physiological 

 condition a decrease in head-frequency or a shift toward 

 the headless condition can be induced by low tempera- 

 ture, narcotics, carbon dioxide, etc., although in certain 

 cases, as we have seen (pp. 112-13), the results are com- 

 plicated by the metabolic relations between the head- 

 forming region and other parts of the piece. On the 

 other hand, conditions which accelerate metabolism, 

 such as high temperature or increased motor activity, 

 increase the head-frequency or shift it toward the normal 

 end of the series. We cannot believe that differences 

 in temperature or motor activity alter the fundamental 

 " organization " in the head-forming region, but it is 

 a fact that such conditions according to their degree 

 may determine any or all of the various kinds of head 

 between the normal and headless extremes. 



Again, how does either an "organization" or an 

 entelechy aid us in interpreting the structures formed 

 on rings in Harenactis (pp. 146-49) ? Here results 

 range from various bilateral arrangements of parts to 

 the characteristic radial symmetry, and from single 



