Significance of the Internal Secretory System 155 



by internal secretions, when as a matter of fact they are 

 only partly explained thus. 



The surprising thing about the confusion of this group 

 is that wholly irreconcilable positions are held with impun- 

 ity, such for instance as those according to which thyroid 

 substance is held to be the organ-forming substance of frogs' 

 legs in one part of a discussion, and mesenchyme cells are 

 acknowledged to be of this nature in another part. The 

 contradiction is, to be sure, often of such character as 

 easily to escape the uncritical reader; but as to the au- 

 thors of such contradictions no other explanation seems 

 possible than that of wrong habits of scientific thought be- 

 gotten of untenable a priori conceptions. For example, a 

 hasty reading of the discussion under review might lead one 

 to suppose that thyroid substance is not, after all, regarded 

 by Loeb as anything more than one contributing cause of 

 frogs' legs, mesenchyme cells being recognized as another 

 cause. Close attention to the text does not, however, war- 

 rant this generous interpretation of the author's position. 

 Going back to his espousal of the theory of Sachs and other 

 botanists as to organ- forming substances, we read : "Specific 

 shoot-producing substances are carried to the apex, while 

 specific root-producing substances are carried to the base of 

 a plant. When a piece is cut from a branch of willow the 

 root-forming substances must continue to flow to the basal 

 end of the piece, and since their further progress is blocked 

 there they induce the formation of roots at the basal 

 end." n 



If this means anything it means that the shoots and roots 

 are actually built up by material carried about in the wil- 

 low branch. There is nothing in the language that can be 

 interpreted as meaning that shoot-and-root-forming sub- 

 stances are mere stimulators of some other substances which 

 become the actual shoots and roots. Yet it is with forma- 

 tive substances of this sort that Loeb in some parts of 



