THE RHYTHMIC NATURE OF LIFE 295 



implement inherent tide tables. When their daily clock was set 5 hours 

 slow, they signaled a low tide 5 hours late. 



The clocks are inherited. This was clearly shown a number of years ago 

 by Biinning (1935), and recently confirmed by Brett (1953) in our 

 laboratory and by Pittendrigh (1954) using the fruit fly, Drosophila, 

 which has been responsible for so much of our modern knowledge of 

 heredity. The eggs of the fruit fly develop in one day into minute wormlike 

 maggots or larvae which feed and grow for about 5 days. They then become 

 inactive, secrete a pupal case, and lie dormant for about 4 days. The adult 

 flies then emerge from the pupal cases in greatest numbers just after day- 

 break. Even when the larvae or pupae are placed in a photographic dark- 

 room the great majority still em.erge just after daybreak. Brett found, 

 however, that, when eggs are laid and complete development occurs in 

 darkness, the adult flies emerge randomly at all hours of the day. But one 

 flash of light, as brief as a minute, given to the larvae or pupae is sufficient 

 to cause the flies, days later, to emerge at the same time of day as the time 

 of the light flash. It would appear, therefore, that the fly larvae had in- 

 herent 24-hour clocks but that the clocks were set for various times of 

 day. The flash of light permitted all of the clocks in the population to be 

 set to the same time, the flies treating the light flash as they would a dawn. 

 The single flash of light obviously could not provide any 24-hour clue. 



All this work on the crabs and flies required around-the-clock observa- 

 tions with several investigators spelling one another. In the tidal-rhythm 

 study, about a quarter of a million separate observations were made and 

 analyzed. To reduce the tedium we sought a method for continuous and 

 automatic recording of rhythms other than those of color change. We had 

 good reason to suspect that the same hormones that were responsible for 

 the color changes in the crabs were important in regulating the rate of 

 metabolism of the crabs. And, since in nature the animals tend to rest 

 quietly at high tide and run about actively at low tide, a tidal rhythm of 

 Oo-consumption might reasonably be expected. 



A very simple, automatic, continuously recording respirometer was 

 invented (Brown, 1954a) . It consisted of a stoppered glass flask containing 

 absorbents for carbon dioxide and other wastes eliminated by the crabs. 

 Through the stopper passed a fine hypodermic needle leading into an 

 oxygen-filled collapsible plastic sack. A crab was placed in each flask. The 

 whole was then suspended into a bath, where it acted as a diver. The de- 

 crease in buoyancy of the diver as the oxygen was used was measured by 

 a delicate spring scale which recorded continuously on a slowly moving 

 strip of paper. Obviously, as fast as the crab used up the oxygen in the 

 flask, fresh oxygen flowed down from the sack to replace it, and the diver 

 became correspondingly heavier. For every cubic centimeter of oxygen 



