250 University of California Puhlicaiions in Zoology [Vol. 11 



B. MODIFIABILITY IN THE BEHAVIOR OF THE LEECH 

 DIN A MICROSTOMA MOORE 



I. INTRODUCTION 



As a rule, the factors which determine the behavior of an 

 animal at a given moment are many and complex. It has been 

 shown conclusively by numerous workers that these factors are 

 internal as well as external. Consequently, behavior is the re- 

 sultant of the action of these two classes of stimuli : sometimes 

 the internal stimuli preponderate ; again, it is the external stimuli 

 which exercise the greatest determining influence. The complex 

 interrelations of these two classes of stimuli make the problem 

 of analysis of behavior a difficult, though not impossible, one. 

 The various factors can be intensified or diminished and the 

 effects of these changed relations observed. Observations secured 

 under these controlled conditions will provide data which are 

 of such a nature as to contribute much to the solution of the 

 problem of modifiability. 



With such a number of factors involved, it is evident that 

 a wide range of variation in both degree and type of response 

 will be the result. However, there is a limit to this modifiability, 

 which usually is in direct proportion to the needs placed upon 

 the organism by the varying conditions of its environment. To 

 discover the physiological bases for these reactions, and the limits 

 of power of adjustment of these "physiological bases," consti- 

 tutes the problem of modifiability. 



As to the importance of this phase of animal behavior, Jen- 

 nings (1905) says: "A thorough study of the modifiability of 

 reactions to external stimuli in lower organisms seems at present 

 one of the great desiderata in the study of animal behavior. Re- 

 cent work has been devoted largely to the study of sharply defined 

 forms of reaction and to the discovery of conditions under which 

 these forms appear in a typical way. As a result, there is a 

 widespread impression that the behavior of lower organisms is 



