PHYSIOLOGICAL DOMINANCE 123 



head in relation to other parts is determined primarily 

 by its ability to grow at their expense. In a shorter 

 piece there is less material available for such growth 

 than in a longer piece, consequently a smaller head 

 develops. Essentially the same relation exists as 

 regards other parts. Where an excess of nutritive 

 material is available the relation is not necessarily very 

 different, for each part uses nutrition instead of the 

 substance of other parts according to its metaboHc 

 activity, i.e., according to its position in the axial 

 gradients, so that in this case also the chief factors in 

 determining the proportions of parts characteristic of 

 each form are the metabolic relations between them. 

 In the early stages of development in nature the simple 

 quantitative gradation in size from the apical toward 

 the basal region appears, but as specialization occurs 

 and the differences in metabolic rate at different levels 

 bring about changes in metabolic character the size rela- 

 tions must of course become more complex. 



The return or approach to the characteristic form of 

 the species which very commonly takes place in the 

 reconstitution of pieces has been regarded by Morgan 

 and others as largely a matter of the physical rearrange- 

 ment of the substance of the piece. That changes in 

 shape may be brought about in soft-bodied forms like 

 the flatworms by mechanical conditions connected 

 with motor and other functional activities of the ani- 

 mals, I have shown. ^ Wherever such factors play a 



^ Child, "Studies on Regulation. IV," Jour, of Exp. ZooL, I, 1904; 

 VII, ibid., II, 1905; "Studies on Regulation. IX, X," Arch, fiir 

 Entwickelungsmechanik , XX, 1905; "The Regulatory Change of Shape 

 in Planaria dorotocephala,^' Biol. Bull., XVI, 1909. 



