THE RHYTHMIC NATURE OF LIFE 289 



of the sun. In the intertidal regions of the shores of our oceans, however, 

 the influence of the moon is more than twice as great as that of the sun. 

 The tides of the ocean are produced predominantly by the gravitational in- 

 fluence of the moon and hence the tidal cycles are of lunar frequency. To 

 a lesser extent the tides are affected by the gravitational attraction of the 

 sun. At 15-day intervals the sun's and moon's influences are additive to 

 produce the extra high, so-called spring tides. It is generally conceded 

 that the greater part of the evolution of all animals and plants occurred 

 in the littoral regions of the oceans. Hence, probably for many hundreds 

 of millions of years, ancestors of all present-day living things were sub- 

 jected to the rhythmic ebbing and flowing of the tides. 



Organismic adaptation is one of the most fundamental of biological prin- 

 ciples. Living things tend to become altered structurally and functionally 

 in any environment in such a manner that they come to demand less en- 

 ergy exchange with their surroundings. It is adaptive that any organisms 

 which find some phases of the environmental cycles more favorable than 

 others develop their own intrinsic cycles of the same frequencies. To do so 

 enables them to use the more favorable periods more advantageously and 

 to prepare to defend themselves against the less favorable phases with less 

 effort. I hope to be able to show you during the remainder of this lecture 

 ( 1 ) that living organisms have actually become adapted to their rhythmic 

 external environment, (2) that they have developed means of measuring 

 off solar days, lunar days, and months, and possibly even the year, and 

 (3) that they utilize these capacities very importantly and often critically 

 in the temporal regulation of their normal activities. 



Biologists have known for a long time that, like man, most animals don't 

 carry on their various activities randomly throughout the 24-hour day but 

 typically have a daily behavior pattern of a characteristic sort. Some are 

 nocturnal like the cats, bats, owls, and earthworms ; others are diurnal 

 like the sparrows and butterflies ; still others feed chiefly at twilight and 

 dawn. Animals that live along the ocean shores in the intertidal regions 

 are known to have behavior patterns that are cyclically repeated with the 

 ebb and flow of the tides, with each cycle averaging about 121/2 hours in 

 length. Fiddler crabs scamper to feed at the w'ater's edge only at ebb tides. 

 Oysters and clams actively feed when covered by water at high tide. Some 

 intertidal animals, particularly those that live so high up on the beaches 

 that they are submerged only by the so-called spring tides, exhibit parallel 

 15-day or semilunar periods of activity. 



Great numbers of lower animals and plants dwelling in the seas have bi- 

 monthly or monthly lunar breeding cycles, in which all the members of 

 the species within any given region synchronously become sexually active. 

 This synchrony is essential to the very maintenance of the species, since 



