RHYTHMIC NATURE OF ANIMALS AND PLANTS—BROWN 391 
Fiddler crabs in nature romp most actively on the beaches seeking 
food about the time of low tide. When these crabs are brought into 
the laboratory and kept in vessels in a photographic darkroom, then, 
like those still left on the beaches, they continue to show their greatest 
running activity at the times when it is ebb tide on their native beaches, 
and remain relatively quiet at the time of flood tide. The time of 
maximum running occurs, therefore, later each day at the expected 
lunar-tidal rate. If in the same darkroom at the same time we place 
CALENDAR DAYS 
Ficure 2.—Daily rhythm of color change in fiddler crabs and some of its experimentally 
altered phase relationships. 
two groups of crabs, taken from two different beaches on which the 
time of low tide was different by several hours, each group of crabs 
may continue to signal the time of low tide on its own beach. So, 
simultaneously in the same crabs deprived of all ordinary cues relative 
to time of day or tide, there persists, on the one hand, a solar-day 
rhythm of color change and, on the other, a lunar-tidal cycle of spon- 
taneous activity. 
Studies comparable to these, using numerous other kinds of animals 
and plants, have demonstrated quite clearly that, like the daily, the 
