ANALYSIS OF BEHAVIOR IN LOWER ORGANISMS 307 



(2) In many cases we find that the relation of the movement to 

 the source of stimulation is brought about indirectly through selec- 

 tion from among varied movements. The organism tries moving in 

 many directions, till it finds one in which there is no stimulus to further 

 change. In this way it may become oriented very precisely if the con- 

 ditions require. This is the prevailing method in the infusoria and in 

 various other organisms, as we have seen. It is becoming evident that 

 this method is more common even among higher organisms than has 

 been hitherto set forth. Movements of the head from side to side, such 

 as we find in the flatworm and many other animals, movements of the 

 eyes or other sense organs, such as are common in higher animals, or 

 movements of the body from side to side, as in the swimming of many 

 creatures, give opportunity for determining which movement tends to 

 retain the stimulus, which to get rid of it. In this way they form a basis 

 for the determination of the direction of locomotion through the method 

 of trial. How much part such movements play needs careful study. 



(3) In still other cases the reaction shows a definite relation to the 

 localization of the stimulus, yet it is not due to local reaction of the part 

 stimulated, nor is it brought about by trial. If an infusorian is stimu- 

 lated at the anterior end it swims backward ; stimulated at the posterior 

 end it swims forward. Both these movements are reactions of the 

 entire organisms, all the motor organs of the body concurring to pro- 

 duce them; they are not produced by local reactions of the organs at 

 one end or the other. The flatworm turns toward or away from the side 

 stimulated, by reactions involving the muscles of both sides, as well as 

 transverse and dorso-ventral muscles, all at a distance from the point 

 stimulated. If stimulated on the upper surface of the head, a compli- 

 cated twisting reaction occurs, involving many sets of muscles in vari- 

 ous regions (p. 273), by which the ventral surface is made to face the 

 stimulating agent (p. 236). Innumerable instances of this class of reac- 

 tions could be given; they include perhaps the greater number of the 

 directed movements of organisms. 



In these reactions a stimulus at one side or end evidently produces a 

 different reaction from a stimulus at the opposite side or end, though 

 the reaction is not primarily at the point stimulated. Doubtless the 

 stimulus starts a physiological process of some sort at the point upon 

 which it impinges, and this determines in some way the direction in 

 which the organism shall move. This effect in the region directly acted 

 upon corresponds to the "local sign" in human physiological psychol- 

 ogy. Behavior thus brought about is of course more effective than that 

 of the two preceding classes, permitting more direct and rapid reaction 

 than the method of trial, and meeting the conditions in an incomparably 



