218 METAMORPHOSIS AND 



As we have tried to show, this dogma is no longer 

 credible in face of the discoveries concerning 

 hormones. The hormone theory supposes that the 

 somatic modifications due to external stimuli — in the 

 case of the Flat-fish the disappearance of pigment 

 from the lower side, the torsion of the orbital region 

 of the skull, and the extension of the dorsal fin — 

 modify the hormones given off by these parts, 

 increasing some and decreasing others, and that these 

 changes in the hormones affect the determinants, 

 whatever they are, in the gametocjrbes within the 

 body. 



Here arises an interesting question — namely, how 

 does the hormone theory explain the phenomenon of 

 metamorphosis any better than the mutation theory ? 

 It might be agreed that if the determinants are 

 stimulated or deprived of stimulation, the effect of 

 the change should logically show itself from the 

 beginning of development, and that therefore the 

 process of metamorphosis or indirect development 

 does not support the hormone theory any more than 

 the theory of gametogenic mutations. This objec- 

 tion may be answered in the following way. The 

 reason why the determinants give rise to the original 

 structure first and then change it into the new 

 structure is probably the same as that which causes 

 secondary sexual characters to develop only at the 

 stage of puberty. By the hypothesis the new habits 

 and new stimuli begin to act at some stage after the 

 complete development of the original structure of the 

 body. The differences in the original hormones of 

 the modified parts are therefore acting simultaneously 

 with the hormones, that is, the chemical substances 

 derived from all other parts of the body in its fully 



