566 RAYMOND PEARL. 



previously ineiitioned (pp. 533, 534) tliuro is a sort of cliemo- 

 kinesis. That is to any, the presence of some chemical 

 substauce in the water causes the animals to stop. The 

 evidence for this factor will be taken up with the discussion 

 of the formation of collections. It probably does not play 

 any part in determining where a single individual shall come 

 to rest outside of a collection. 



It must be emphasised that all of these three factors are 

 secondary in importance as compared with the phj^siological 

 condition of the animal, which may be said to prepare it for 

 the resting state. An active animal, in which the tonus is at 

 or near the maximum, will pass through regions of low 

 illumination, uneven surface, or collections of other indi- 

 viduals without stopping. Only when the animal is in the 

 right general condition do these factors come in to determine 

 the precise point where the stop shnll be made. 



1. Formation of Coll ections.— Since the formation of 

 collections is dependent on the animals coming to rest in a 

 certain area, it may propeily be taken up in this section. 

 The collections are fairly well-defined groups of from six or 

 eight up to twenty or more individuals. The general appear- 

 ance of such a group is shown in Fig. 1. The individuals 

 composing it have no definite orientation, but are scattered 

 about with the anterior ends directed in whatever way they 

 happened to be pointed whoi the individuals stopped. The 

 distance separating the individuals varies much in different 

 cases. In some cases it may be as much as a half-centimetre, 

 or again niay be the width of an individual worm or less. 

 This formation of collections of this sort might be considered 

 the result of a " social instinct " by animal psychologists of 

 the Binet school. Actually, it appears to be due to two 

 simple reactions taken in conjunction with the general 

 physiological condition of the individuals composing it. The 

 first of these reactions is that to light. That is to say, when 

 individuals come to a comparatively restricted area of a 

 certain degree of illumination, if they are in a certain 

 condition of reduced tonus, they stop. Those which are 



