288 INVERTEBRATE PHYSIOLOGY 



it when I did arrive. Our civilization would be abruptly set backwards by- 

 centuries ; we would revert to a pastoral mode of existence. Under such 

 circumstances the earth would be inhabited by many more people than it 

 could support at the greatly lowered level of operating efficiency and a 

 new population equilibrium consistent with the new mode of life would 

 soon be reached. 



It seems probable that a comparable picture could be painted for innum- 

 erable other animals and plants, as the consequence of having their means 

 of accurately measuring time suddenly abolished. There would be, un- 

 doubtedly, extinction of numerous species, and possibly even extinction of 

 all of them, including man. I add man, because it is highly probable that he 

 also depends upon the type of living clock I shall discuss. At best, there 

 would be a tremendous alteration in the character of the animal and plant 

 populations of the earth. 



For his own clocks and calendars, man has utilized, insofar as he has 

 been able, the natural periods of his cyclic external physical world. The 

 interval between the times of the sun reaching its highest point in the sky 

 or zenith on two consecutive occasions is the period of the solar day. The 

 clocks of man divide up this period into 24 hours ; each of these hours is 

 in turn divided into 60 minutes, and each minute into 60 seconds. The 

 period required for the earth to make one revolution about the sun is the 

 year, which is divided by the calendar into 12 months of about 30 days each 

 and about 52 weeks of seven days. The month is that simple fraction of the 

 year which corresponds closely to the average lunar period from one full 

 moon to the next. This lunar period is the synodic month of about 29.5 . 

 days. 



Animals and plants appear similarly to have utilized the natural cycles 

 of their physical environment in the development of their own clocks and 

 calendars. Their natural periods, however, are chiefly the shorter ones 

 which are responsible for rhythmic alteration in forces in the environment 

 which are difficult or impossible for the organism to ignore. The solar day 

 of 24 hours is associated with the cycle of change in illumination, tempera- 

 ture, and humidity of the day-night cycle. There are also daily rhythms 

 of barometric pressure, the rain of cosmic radiation, and of other factors. 



Another natural period which appears to have been conspicuously used 

 by animals and plants in the measurement of time is the period of the 

 lunar day of 24.8 hours. The lunar day is the period elapsing between two 

 consecutive times when the moon is at its highest point in the sky. In 

 other words, the moon reaches its zenith about 50 minutes later each day. 

 Compared with the sun, the influence of the moon on illumination, tem- 

 perature, and humidity is minor. And its influences in producing rhythms 

 of barometric pressure and cosmic radiation are feeble compared with those 



