408 DWIGHT E. MINNICH 



that such an individual usually loops to the left. As it does so, 

 however, the functional eye experiences a pronounced decrease 

 of stimulation during the first half of each loop. According to 

 the 'change of intensity theory,' such decreases should result in 

 swerves toward the opposite side. Such, however, do not occur, 

 as a rule. The animal, instead, completes the loop. It, there- 

 fore, does not respond very strongly to a decrease of intensity. 

 If it did, the performance of circus movements would be quite 

 impossible. If we assume, however, that the looping is produced 

 by the continuous inequality in the stimulation received by the 

 two eyes, the asymmetry of response becomes intelligible at once. 

 For, in an animal with one eye blackened, the functional eye, 

 even when turned farthest fl-bm the light, is the recipient of some 

 stimulation, whereas the covered eye receives none whatever. 



It may be objected, however, that many positive arthropods 

 with one eye blackened are able to overcome their tendency to 

 circle in directive hght, and that this is a response to the decrease 

 of intensity on the functional eye at the beginning of each loop. 

 Such may well be the case, as Holmes ('05, p. 345) has suggested. 

 This is a matter for further experiment to decide. In any case, 

 however, circus movements must be regarded as an established 

 phenomenon of general occurrence among this group of animals. 

 The significant thing, therefore, to an understanding of orienta- 

 tion is to discover what is effective in producing these reactions 

 rather than what is effective in modifying them. 



The evidence afforded by circus movements in directive light 

 as to the natiu-e of the stimulus concerned in their production is 

 thoroughly corroborated by the results obtained in non-directivie 

 light. Under the conditions of non-directive illumination em- 

 ployed by Holmes and McGraw ('13) and by myself, an animal 

 is absolutely free from any pronounced or consistent changes in 

 the intensity of the stimulus to which it is subjected. More- 

 over, the experiments of Dubois ('86) on the beetle Pyrophorus 

 furnish a case in which there is no possibility whatever of inten- 

 sity changes playing a significant role in orientation, since the 

 source of hght is within the animal itseK. Yet in all these cases 

 the elimination of one eye was usually followed by typical circus 



