THE RHYTHMIC NATURE OF LIFE 293 



again warmed to room temperature, a daily rhythm was still evident, but 

 now it was retarded in each phase by about a quarter cycle. In other words, 

 the clock appeared to have been stopped or greatly slowed at the very low 

 temperature, but when the temperature rose again, it resumed its normal 

 24-hour cycles but was now about 6 hours slow. The rhythmic change then 

 continued indefinitely in its reset time with no tendency to return to the 

 original time. By such low-temperature treatment, it was possible to get 

 the cycle of the animals any desired degree out of phase with the normal 

 one, and thereafter it was as stable in the new state as it had been in the 

 original. 



The clock can also be reset by illumination changes at sensitive times in 

 the daily cycle (Brown and Webb, 1949) . For example, fiddler crabs which 

 have been kept in a very bright continuous illumination for about ten days 

 stop changing color and remain continuously black. It can be shown that 

 these animals possess a daily rhythm of sensitivity to stimuli which can 

 reset the clock. If these rhythm-inhibited animals are placed in a darkroom 

 either at 12 :00 noon or 6 :00 p.m., a normal rhythm of color change com- 

 mences at once. If, however, they are placed in the darkroom at 6 :00 a.m., 

 the rhythm which now reappears in darkness is set forzvards by about a 

 quarter cycle, or about 6 hours. 



If animals having a normal rhythm in darkness are illuminated on three 

 consecutive days from midnight to 6 :(X) a.m., the phases of the cycles are 

 set backwards by about a quarter cycle (Webb, 1950). If these same ani- 

 mals are exposed to illumination from 6 :00 p.m. to midnight for three con- 

 secutive days, their cycles are set backwards still another quarter cycle, 

 and the cycles are then inverted. Before the first shift in the cycle, illumi- 

 nation from 6 :00 p.m. to midnight would have produced by itself no per- 

 sistent shift. 



One can also invert the phases of the rhythm by a few periods of illumi- 

 nation by night and darkness by day. The number of cycles of light change 

 necessary to produce the inversion becomes fewer, the brighter the illumi- 

 nation. 



It is impossible to impress upon the crabs any cycle other than the nor- 

 mal 24-hour cycle. Exposure to many 32-hour "days" of alternating light 

 and dark finally produced a 96-hour cycle of change, the smallest com- 

 mon denominator of the imposed 32-hour and the normal 24-hour cycles. 

 But just as soon as the crabs were returned to constant darkness they 

 reverted to normal 24-hour cycles. 



During this study of the persistent daily rhythm of color change in the 

 fiddler crab, it became evident that the degree of darkening during the 

 daytime varied from day to day (Brown, Fingerman, Sandeen, and Webb, 

 1953). Sometimes the time of greatest darkening occurred in the morning, 



