ANIMAL BEHAVIOUR 191 



words, what is commonly called in higlier animals intelli- 

 gence '^ (Jennings, 190G, p. 241). In many cases we have 

 to do with ^ chain reflexes ', one phase leading on to another, 

 but the fact is that " in most cases the succeeding phase is 

 not invariably and irrevocably called up by the preceding 

 one'' (Jennings, p. 251), the present action depending upon 

 the entire physiological state of the organism, which, again, 

 is determined by various factors. 



One of the clearest results of the modern study of the 

 behaviour of the lower animals is that the kind of action de- 

 pends greatly on the physiological state as a whole, which, 

 again, depends in part on history and experiences That ex- 

 perience can be enregistered in organisms with the simplest 

 of nervous systems or with none is certain. Two individual 

 Planarias (small ciliated fresh-water worms) often react in 

 opposite ways to the same stimulus. What they do varies 

 with their appetite, their freshness or fatigue, their recent 

 stimulation and degree of excitement, and their history. 

 After long study of Planaria, Professor Pearl concludes that 

 " it is almost an absolute necessity that a person should be- 

 come familiar, or perhaps better, intimate, with an organ- 

 ism, so that he knows it in something the same way that 

 he knows a person, before he can hope to get even an approxi- 

 mation of the truth regarding its behaviour'' (quoted by 

 Jennings, 1906, p. 254). 



If a reflex be an invariable reaction to a given stimulus, 

 then there is much more than that in the behaviour of lower 

 animals. For different answers to the same stimulus may 

 be given by the same kind of creature or by the same creature 

 at different times. The answer depends on the organismal 

 condition as a whole. Moreover, the fact that stand? out most 

 clearly in the behaviour of the lower organisms is this: — 



