THE METHOD OF TRIAL AND ERROR. 241 



held theories as to the reactions of lower organisms. Just what the 

 organism did was, indeed, fairly clear, but the plan of it all, the gen- 

 eral relations involved in all the details, was not clear. This was 

 partly due, perhaps, to overemphasis of certain phases of the reaction 

 and to a tendency to consider other features unimportant. The beha-_ 

 vior under stimuli is a unit ; each factor must be considered in connec- 

 tion with all the others ; then the general method running through it 

 all becomes strikingly evident. 



Let us now return to the organisms. Sometimes stimuli are received 

 of such a nature that their distribution is not affected by the currents 

 produced by the cilia ; in other words, they cannot be sampled in the 

 currents of water brought to the anterior end or mouth, as shown in 

 Figs. 79 and 80. This is true, for example, of stimulation by light, 

 and of stimulation by contact with solid objects. Under such stimula- 

 tion the behavior is nevertheless still by the method of trial and error. 

 Let us consider first the reaction to a mechanical stimulus. 



When the organism comes in contact with a mechanical obstacle the 

 reaction is exactly the same as that already described. It swims back- 

 ward, swings toward the side X, and this, with the revolution on the 

 long axis, points the anterior end successively in many different direc- 

 tions. The organism then follows one of these directions. If this leads 

 against the obstacle, the reaction is repeated, till finally a direction is 

 found in which the obstacle is avoided. 



In the reaction to light, as it occurs in Stentor or Euglena, experi- 

 ment shows that changes in the intensity of illumination at the sensi- 

 tive anterior end are the agents causing reaction (see the second of these 

 contributions) . The reaction produced is that already described ; by 

 turning toward the side A"" and revolving on its long axis, the organism 

 tries many directions. 



When a negative organism, such as Stentor, comes in its swimming 

 to an area that is more brightly illuminated, or when a positive organ- 

 ism, such as Euglena, comes to an area that is less brightly illuminated, 

 the change in intensity acts as a stimulus. The organism responds in 

 the way already described ; it backs away, then tries many different 

 directions by swinging its anterior end about in a circle. It then 

 starts forward in one of these directions. If this does not lead into the 

 area causing stimulation, well and good ; if it does, the organism 

 repeats the reaction, trying a new set of directions, till it finds one that 

 does not carry it to the area causing stimulation. 



When light coming from a certain direction falls upon one side of a 

 swimming infusorian, the spiral path followed, of course, causes the 

 anterior end to be pointed successively in different directions. As a 

 result, the illumination of the anterior end is repeatedly changed, since 



