238 CHARLES HARLAN ABBOTT 



tageous to this group of animals, because it keeps them in a suit- 

 able habitat. Such reversals of reaction as occur are probably 

 adaptive also. Unless other reactions are so important as to 

 overcome the influence of the phototaxis in ways not yet deter- 

 mined, observations and experiments indicate that this reaction is 

 an important ecological character, and, further, that it may have 

 been one of the factors which made possible the migration of the 

 ancestors of the present land isopods from water to land. 



B. THE PROBLEM OF ORIENTATION 



In addition to the ecological analysis, a few observations can 

 be related to the problem of orientation, which has a large place 

 in behavior studies. The subject of orientation has been so thor- 

 oughly discussed, particularly in such general works as those of 

 Jennings ('06), Mast ('11), Loeb ('16), and Hohnes ('16), that 

 no extended introduction is necessary. A distinction which Dar- 

 win ('80) made, according to Mast, is, however of importance. 

 To quote from Mast ('11, p. 47) : "To explain orientation, Darwin 

 said, we do not need to account for movement; it is only neces- 

 sary to account for changes in the direction of movement." 



The problem is well formulated by Bancroft ('13) in two ques- 

 tions which he applies to Eugiena, but which could be asked 

 equally well with regard to any other organism: 



1. Does Eugiena become oriented to light as directly as its method 

 of orientation admits, or does it orient indirectly by the method of trial 

 and error? In either case the reaction will be heliotropie, but the 

 method of orientation will be different. 



2. Is heliotropism in Eugiena brought about by response to tem- 

 poral changes in light intensity, or is it caused by the continuous action 

 of the Ught independent of changes in intensity? 



This division of the subject by Bancroft into two distinct 

 problems avoids a source of confusion which has occurred in 

 most discussions of the question. 



