Mast, Orientation in Euglena with some Remarks on Tropisms. 661 



tation, in the other synonymously with reaction; and if this be 

 true, it would be a simple matter to extricate them, no matter 

 how much they may be mingled with others in my collection of 

 definitions. Accordingly the "inextricable mingling" to which Ban- 

 croft refers is of no serious consequence. 



If, then, it is impossible do distinguish between compulsory 

 and non-compulsory orientation, Bancroft's definition of tropism 

 is impracticable, and there are many others of the same nature, 

 for example, the most recent one by Torrey (1914, p. 120) in 

 which he proposes to make predictability the criterion of tropisms. 

 Thus he would call all processes of orientation which can be 

 predicted tropisms. The impracticability of such a criterion is 

 evident to anyone who is familiar with the activity of animals. I 

 suppose the rational reactions of man can be as accurately predicted 

 as any, and yet the strongest advocates of tropisms have always 

 attempted to exclude such reactions. Is it not true that all responses 

 in all organisms can be predicted within a certain degree of pro- 

 bability, and is it not equally true that none, except in a very 

 general sense can be predicted with certainty? How then is one 

 to know precisely which, in accord with this criterion, are to be 

 considered elect and which not? And what probability is there 

 that two different investigators would select the same? The follow- 

 ing sentence from Parker is apt in this connection (1914, p. 384): 

 "Woe be to him if he begins to tell what a given animal at a 

 given moment will do!" 



Many seem to think that tropisms 3 ) are a specific sort of reac- 

 tions, elementary in character and quite different from those 

 heretofore marshalled under the familiar terms, reflex action, random 

 movement, trial reactions, orientation, etc., and yet every attempt 

 to bring together under a specific definition a group of reactions 

 having these characteristics and differing from others already known 

 has signally failed. The term is in fact, as previously stated, at 

 present used in so many different senses that everyone finds it 

 necessary to indicate in which sense he proposes to use it. Thus 

 it has become a burden rather than a help and it would seem 

 advisable to drop it altogether. There is no more necessity for 

 it in the study of the reactions of lower organisms than there 

 is in the study of the higher forms, and in the study of these we 

 succeed very well without it. If "tropism" is to be used synonym- 

 ously with "orientation" or "reaction", and these senses are the 

 only ones regarding which there is any considerable amount of 

 agreement - why not use in place of it these terms which have 



3) Warren (1914, p. 96) classifies reactions as follows: "tropisms, reflexes in- 

 stincts, intelligent action, and rational volition." 



