(360 Mast, Orientation in Euglena with some Remarks on Tropisms. 



how orientation occurs. For is not the statement by Loeb that 

 orientation must be a function of the constant intensity in order 

 to be a tropism an attempt to explain the way in which the reac- 

 tion takes place? As a matter of fact Loeb's writings on reactions 

 are full of such attempts 2 ). And does not Bancroft's idea that 

 only "compulsory" orientations are tropisms imply a theoretical 

 explanation? Every limiting adjective of this sort if it means 

 anything at all necessarily implies explanation of some sort for if 

 it did not how would it be possible to distinguish between com- 

 pulsory orientation (tropisms), and orientations which are not com- 

 pulsory. It is evident that both Bancroft and Loeb hold there 

 are such, for if they did not the adjective "zwangsmafiig" and 

 "compulsory" would mean nothing. Orientation and tropism would 

 become synonymous and the phrase "heliotropic orientation" used 

 so much by Bancroft (pp. 413, 414, etc.) would become "helio- 

 orienting orientation", - a senseless combination. 



In order to ascertain then whether or not a given orientation 

 is a tropism it is necessary, in accord with these definitions, to 

 ascertain among other things whether or not it is compulsory. 

 How is this to be accomplished without theory and explanation? 

 Unfortunately Bancroft does not tell us. Loeb, however, if 1 

 correctly, understand him holds that conscious orientation is not 

 compulsory i. e. that psychic phenomena are involved as causal 

 agents in such processes; and if this be true he must believe in 

 free will and material indeterminism. Thus we find that these 

 definitions, supposedly free from theory, involve a question literally 

 steeped in theory, a question which has been discussed for ages 

 and is still being discussed, a question which all experimental 

 methods have failed to solve for any case even under the most 

 favorable circumstances; and yet Bancroft would persuade us that 

 'compulsory orientations" are certain kinds of reactions, that, 

 without theory or explanation, can be separated from orientations 

 which are not compulsory. 



No more convincing illustration of the futility of this definition 

 can be desired than is found in the fact, as I shall show presently, 

 thatEwald, who accepts this definition, does not agree with Ban- 

 croft in classifying the orienting reactions of Euykna, in spite 

 of the fact that he was his colleague and worked in the same 

 laboratory. 



There are, in my opinion, only two definitions of tropism that 

 avoid theory. In one this term is used synonymously with orien- 



2) A number of examples may be found in my book (1911, pp. 28 35). See 

 also Loeb (1912, pp. 3846). 



