504 S. O. MAST 



to such an extent that, while doubtless many more facts of a qual- 

 itative nature will be discovered, this is not likely to result in 

 the formulation of principles of much importance. Advance in 

 behavior undoubtedly depends largely upon intensive quantita- 

 tive research. 



This paper deals with the beginning of a series of quantitative 

 investigations concerning changes in the nature of responses. 

 Reversion in the sense of orientation was selected for this study 

 because it appears to be among the simplest of the changes in 

 responses, and if it occurs at all it is always fairly complete and 

 fairly well defined, so that it serves well for work requiring pre- 

 cise measurements. The selection of an organism that can be 

 cultivated in the laboratory is also of importance, for in organisms 

 that thrive in the laboratory normal behavior can be much more 

 readily ascertained than it can in those which do not. Thus 

 difficulties such as Esterly ('17), for example, encountered in work 

 on the marine copepods are readily avoided. 



At present we are interested primarily only in reversal in 

 orientation in light. The literature on this subject has recently 

 been fairly thoroughly reviewed. (Mast, '11, pp. 265-287; 

 Hohnes, '16, pp. 93-119; Washburn, '17, pp. 200-208). Refer- 

 ence to these reviews and to a few investigations not mentioned 

 in them leads to the following conclusions : 



1. Organisms are usually positive in weak and negative in 

 strong illuminations. Holmes ('01, '05), however, maintains 

 that the opposite holds for Orchestia and Ranatra, and there are 

 many organisms in which reversion cannot be induced by Ught. 



2. Reversal in the sense of orientation in light is probably 

 usually dependent upon the amount of light energy received. 

 This has been fairly clearly demonstrated by Arisz ('15) for plants 

 and Mast ('07, pp. 154-162) for Volvox. Under certain condi- 

 tions, however, reversion may depend upon the time-rate of 

 change in light intensity. Gamble and Keeble ('03, p. 397) 

 maintain that sudden increase in illumination causes positive 

 Convoluta to become temporarily negative. Ostwald ('07) con- 

 tends that the same reversion is produced in Daphnia by either 

 a sudden increase or a sudden decrease in illumination. Ewald 



