65<S Mast, Orientation in Euglcna with some Remarks on Tropisms. 



of its reactions bear no observable specific relation to the localization 

 of the stimulus, that is, they are of the "trial" type. Moreover it 

 is undoubtedly true that this type of reaction plays a large role 

 in the process of orientation in general. That is, if orientation 

 occurs at all without an increase in the diameter of the spiral, it 

 is probably certain that such occurrences are, under natural con- 

 ditions, in the minority. 



I am consequently unable to understand how, even if orien- 

 tation in Euglcna were found to be direct in all cases, this would 

 prove that the "trial" reactions are not primitive, and that "differ- 

 ential response to localised stimulation" has not been evolved from 

 such reactions, as Jennings maintains. 



C. The Definition of Tropism. 



The term tropism has been applied by different authors to 

 almost every conceivable sort of response. There is so little 

 unanimity in the interpretation of its meaning as applied to reac- 

 tions in animals that practically every one who uses it finds it 

 necessary first to state what sort of reactions he proposes to group 

 under it. In my book "Light and the Behavior of Organisms" I 

 have collected some seventeen different definitions of this term 

 (1911, pp. 53 57). (A number of new ours have appeared since.) 

 Bancroft (p. 384) maintains that in this collection I have "inex- 

 tricably mingled definitions of the term and theories to account 

 for the reactions". He intimates that the confusion in the use of 

 the term is due largely if not entirely to such mingling, and pro- 

 poses to settle the whole matter by distinguishing definition from 

 theory. "Heliotropism", he says (p. 384), ''will be used here as it is 

 used by Loeb, and most authors who use the word, to indicate 

 a certain kind of reaction, entirely apart from the theory which 

 may be adopted to explain the way in which the reaction takes 

 place". He maintains that "Loeb makes his use of the term clear 

 in many places", and gives in support of his contention the follow- 

 ing quotation (Loeb, 1910, p. 452); "Unter den Tropismen der 

 Tiere verstehen wir die zwangsmafiige Orientierung gegen resp. die 

 zwangsmafiige Progressivbewegung zu oder von einer Energiequelle". 

 Finally he gives his own definition which is supposed to be in 

 accord withLoeb's and which he asserts merely "indicates a certain 

 kind of reaction, entirely apart from the theory which may be 

 adopted to explain the way in which the reaction takes place". 

 He writes (p. 384), "In this paper, then, Heliotropism includes 

 those reactions in which there is a compulsory orientation with 

 respect to the light, no matter how that orientation may have been 

 brought about". Is this definition in accord with Loeb's? Is it 

 in reality free from theory? Does it eliminate confusion regard- 



