112 Herbert Eugene fValter 



ture animal behavior, particularly among the lower forms, is 

 ordinarily referred to in its specific or even generic aspect. The 

 distinctive actions of individuals, as such, it seems, have usually 

 been outside of the purpose of the observer.'^ 



Individuals, however, even among such comparatively simple 

 forms as planarians, do not always act with machine-like uniform- 

 ity. Until it is possible to predict with exactness what behavior will 

 result under any given set of conditions, an accurate knowledge of 

 the behavior of any kind of organism must be based upon repeated 

 observations of individuals as such rather than as representatives 

 of species and genera. 



Individual variations in behavior constantly appeared through- 

 out the course of the present investigation. It will be sufficient, 

 however, for the purpose of making clear their importance to cite 

 only a few instances of such variations. 



It should be noticed that whenever "exceptions to the rule" of 

 behavior occur, as in the case of negative planarians coming to 

 rest in the light or becoming positively phototactic for a time (see 

 the two cases cited on p. 78), they are ordinarily simply abnor- 

 mal cases of individual behavior standing out against the back- 

 ground of average specific or generic behavior. Exceptional cases 

 ot this kind, however, are not so typical of what really constitutes 

 individual behavior as the less aberrant actions making up the 

 majority of the movements which the animal performs. 



The main point to be recognized, then, is that the individual 

 presents unknown factors, which, even in the simplest forms of 

 life, where the range of variation is least, have never yet been 

 reduced entirely to chemico-physical terms, a fact which impairs 

 somewhat the conclusions of those writers who would draw a 

 complete parallel between an organism and a machine. 



Rate on Successive Days. When the rate of locomotion of cer- 

 tain isolated individuals is averaged from four trials, for example, 

 and the same experiment is repeated on the following day with the 

 same individuals, thereby eliminating the effects of fatigue, under 

 as nearly identical conditions as possible, the two sets of figures 



^Frandsen ('oi), who was impressed by individual differences in the phototaxis of Limax, and 

 Smith ('02), who worked with the earthworm, are exceptions to this generalization. 



