22 BEHAVIOR OF THE LOWER ORGANISMS 



actions are, as in Amoeba, important factors in determining reactions 

 to present stimuli. In Amoeba we see in the simplest way the effects of 

 past stimuli and past reactions in determining present behavior. 



As a result of this interplay of external and internal factors in deter- 

 mining movement, the avoidance of a stimulating agent usually occurs 

 in Amoeba by a process which we should call in higher animals one of 

 trial. If the movement were directly and unequivocally determined by 

 the localization of the stimulus, there would be nothing involved that 

 could be compared to a trial. The direct withdrawal of the part stimu- 

 lated is a factor due immediately to the localization of the external agent. 

 But the sending forth of a pseudopodium in a new direction is not forced 

 by the external agent, but is an outflow of the internal energy of the 

 organism, and the position of this new pseudopodium is, as we have 

 seen, determined by internal conditions. The latter factors are those 

 which correspond to the activities that we call trial in higher animals. 

 If the new direction of movement leads to further stimulation, a new 

 trial is made. Such trials are repeated till either there is no further 

 stimulation, or if it is not possible to escape completely, until the stimu- 

 lation falls on the posterior end, and the animal is retreating directly 

 from the source of stimulation. 



The entire reaction method may be summed up as follows: The 

 stimulus induces movement in various directions (as defined by internal 

 causes). One of thece directions is then selected through the fact that 

 by subjecting the animal to new conditions, it relieves it from stimula- 

 tion. This is our first example of "selection from among the conditions 

 produced by varied movements," a phenomenon playing a large part, 

 as we shall see, in the behavior of organisms. 



The method of reaction above described gives, with different stimuli, 

 two somewhat differing classes of results. In the reactions to mechani- 

 cal, chemical, and thermal stimuli, different directions are "tried" until 

 the organism is moving in such a direction that it is no longer subjected 

 to the stimulating agent ; in this direction it continues to move. But in 

 the reactions to light and to electricity new directions are tried merely 

 until the stimulation falls upon the posterior end, and the organism is 

 retreating directly from the source of stimulation. There is no possibil- 

 ity of escaping the stimulating agent completely. In the reactions to the 

 two stimuli last mentioned the long axis of the animal must after a time 

 take up a definite orientation with respect to the direction from which 

 the stimulus comes, while in the reactions to other stimuli there is usually 

 no such orientation. This difference is due, not to any essentially dif- 

 ferent method of reacting in the two cases, but merely to the peculiar 

 distribution of the stimulating agents ; light and electricity act continu- 



