i84 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. 1 12-13), 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 



