GENERAL PHYSIOLOGY 181 



Whether changes in the nature of protoplasmic response or 

 changes in direction of movement after repeated shocks should be 

 interpreted on the basis of "memory" and "learning" or in some 

 other way is largely a matter of personal idiosyncrasy on the part 

 of the observer. Numerous writers have described processes of 

 food "selection" by Ameba (e.g., Gibbs and Dellinger, 1908; 

 Schaeffer, 1917 and elsewhere; Metalnikoff et al, 1910). Mast and 

 Pusch (1924) interpret an observed change in the protrusion of 

 pseudopodia of Amoeba proteus in respect to a beam of light as 

 evidence of something analogous to "learning" in higher animals, 

 etc. "Learning" involves "memory," and such terms connote 

 processes of an entirely different nature which we associate with the 

 highest types of animals. It is conceivable that fatigue, to use the 

 term in its broad sense implying total or partial exhaustion of pro- 

 toplasmic constituents necessary for a reaction, and therefore a 

 purely physical matter, is adequate for explanation without calling 

 upon any obscure pan-psychic interpretation. Similarly with Kep- 

 ner and Taliaferro's (1913) evidence of "purpose" in methods of 

 food-getting by Amoeba proteus. 



Many of the reactions of Protozoa are bound up with the coor- 

 dinating mechanism of the cell through which the organism acts as 

 a unit. The specific response of an organism to a stimulus is the 

 result of its particular protoplasmic architecture expressed through 

 its coordinating mechanism and motile organs. This has been 

 elaborately worked out by Jennings (1904 to 1909) in connection 

 with the "motor response" of many different kinds of Protozoa. 



The discussions and controversies over the matter of directive 

 stimuli or tropisms in Protozoa have evidently been due in large 

 part to a lack of common understanding of the definition. If by 

 "tropism" is meant the orientation of an organism in respect to the 

 path of a stimulus, then tropisms, as Jennings was the first to point 

 out, play little part in the activities of the Protozoa. If, however, 

 by "tropism" is meant "the direct motor response of an animal to 

 an external stimulus" (Washburn, 1908), then tropisms play a most 

 important part in such activities. The two definitions are not 

 compatible; the former conveys the idea of a directive stimula- 

 tion upon local motor organs or controlling elements; the latter 

 implies the complex reaction of a definite mechanism character- 

 istic of any specific protoplasm, and the same reaction follows upon 

 stimulation by any type of stimulus (Putter, 1903, Jennings, 1909). 

 It follows further that the reaction is called forth regardless of the 

 particular elements first to receive the stimulus. 



We owe Jennings the credit for first clearly distinguishing between 

 these two conceptions, as well as for careful analyses of the move- 

 ments of lower organisms (1904 et seq.), and for demonstrating the 

 particular motor response distinctive of specific types of Protozoa. 



