ORIENTATION IN COLONIAL ORGANISM 13 



tinued orientation; it is consequently not necessary to assume 

 that the orienting stimulus continues to act after orientation 

 as well as during the process of orientation, as is demanded by 

 the continuous-action theory. Organisms are partially isolated 

 dynamic systems and much that they do is dependent upon 

 changes within, quite independent of immediate environmental 

 factors. 



5. DISCUSSION 



We have in this and in previous publications presented a con- 

 siderable amount of evidence in favor of the change-of-intensity 

 theory of orientation. Let us now briefly consider the evidence 

 that favors the continuous-action theory. 



In the reactions of the unicellular and the colonial forms ver}- 

 little has been discovered that supports this theory. In fact 

 practically all of the favorable evidence is found in Bancroft's 

 work on Euglena ('13). Bancroft maintains that he has demon- 

 strated that in this form orientation occurs in accord with Loeb's 

 continuous-action theory; at any rate that the change-of-inten- 

 sity theory does not hold, I need not here enter upon a discus- 

 sion of Bancroft's interesting observations, for I have elsewhere 

 shown ('14) that his results must be confirmed under conditions 

 more thoroughly controlled before much dependence can be 

 placed upon them and that if his contentions are valid they ac- 

 tually oppose the theory that he substitutes for the one he claims 

 to have overthrown; that is, they oppose the continuous-action 

 theory. 



Thus we see that the evidence in support of the continuous- 

 action theory found in the reactions of the forms mentioned is 

 exceedingly weak. In the reactions of some of the more com- 

 plex organisms there is, however, some evidence indicating that 

 this theory holds at least in part. Blaauw ('08), Froschel ('08 

 and '10), Arisz ('11), and Clark ('13) have demonstrated that 

 photic orientations, in a number of different seedlings, is within 

 ♦ certain limits, dependent upon the amount of light energy received ; 

 that is, that long exposure in weak light produces the same effect 

 as short exposure in strong light. Mast ('11, p. 163) reached 



