ANALYSIS OF BEHAVIOR IN LOWER ORGANISMS 305 



animal. In regard to these injurious substances Paramecium thus 

 makes what we would call in ourselves a "mistake." The whole 

 scheme of reaction by the selection of the results of varied movements 

 is not a set, perfected, final one, but is a tentative plan, based on the con- 

 fusing world taken as it comes ; it is liable to mistakes, and is capable of 

 development. Progress in this method of behavior takes place largely 

 through increase in the accuracy of discrimination of different stimuli. 

 This may occur through the law of the increased readiness of resolution 

 of physiological states after repetition, in the way that we shall attempt 

 to set forth later (Chapter XIX). 



E. Adaptivcness of Movements 



The second chief factor on which depends the effectiveness of 

 behavior by selection of overproduced movements lies in the relative 

 fitness of the movements to relieve the organism from the unfavorable 

 conditions. This, of course, depends on many things. If a powerful 

 chemical is diffusing from a certain direction, the rapid movements of 

 Paramecium are more likely to save than is the slow motion of Amoeba. 

 There are two factors on which the effectiveness of the movements 

 depends, that are worthy of special consideration. 



In what we may call the pure method of trial, a most important re- 

 quirement for effectiveness is that the movements shall be so varied as 

 to give much opportunity for finding other conditions. There are great 

 differences in the behavior of different organisms from this standpoint. 

 This may be illustrated by a comparison of the reactions of Paramecium 

 and Bursaria to heat, as previously described. When a portion of the 

 area containing the organisms is heated, these two infusoria react in 

 accordance with essentially the same plan, yet practically none of the 

 Paramecia are injured, while a large proportion of the Bursariae are 

 killed. The difference is due chiefly to the fact that Paramecium 

 rapidly repeats its reactions and revolves on its long axis as it turns, so 

 that in a short time it has tried in a really systematic way many different 

 directions, and is practically certain to find one leading away from the 

 heated region, if such exists. Bursaria, on the other hand, changes its 

 direction of movement only at longer intervals, and usually soon ceases 

 to revolve on its long axis as it turns toward the aboral side. This fail- 

 ure to turn on the long axis deprives it of the great advantage of being 

 directed successively in many different directions in the different planes 

 of space. The result is that it is likely to be destroyed by the heat 

 before it has found a direction leading to a cooler region. 



