Diurnal Periodicity 217 



tion of day and night, such as the opening and closing of flowers and 

 the folding of leaves. 



Although often not conspicuous, a very large number of the lower 

 animals are controlled in their activities by the change in light from 

 day to night. This is often a matter of simple photokinesis. The 

 animals are more active in the light, less active in darkness or vice 

 versa, but behavior may also be modified by concomitant diurnal 

 changes in other factors such as temperature and humidity. Among 

 the higher animals the reactions are often more elaborate, and some- 

 times the daily rhythm of activity is only indirectly related to light. 

 Many of the relatively defenseless forms, such as mice, come out to 

 forage chiefly at night when they are less likely to be detected by their 

 enemies. As a further complication many of the predaceous animals 

 are nocturnal as an adaptation to the night activity of their prey or 

 to the fact that they can stalk their prey more successfully under 

 cover of darkness. A patch of woods may be inhabited by two sets 

 of animals which practically never meet because one set is active 

 only during the day and the other active only during the night (Park, 

 1940). 



The fact that a diurnal rhythm in the activity of an organism is 

 sometimes deeply ingrained can often be demonstrated by bringing 

 the organism into the laboratory and observing it under constant or 

 under changed conditions of illumination. In one such experiment 

 the deer mouse, Feromijscus, which is normally active at night and 

 quiescent during the day, continued to display a diurnal rhythm in 

 its behavior after seven months in continuous darkness. This and 

 other experiments summarized by Welsh (1938) and by Park (1941) 

 indicate that certain internal physiological processes have become 

 attuned to a 24-hour cycle. In many organisms this rhythmicity per- 

 sists for long periods and tides over periods when the usual controlling 

 environmental stimulus fails to occur. Under normal conditions the 

 timing of the periodicity is re-enforced by changes in light or other 

 factors each day. 



The diurnal shift in the activity of animals frequently results in 

 significant changes in their position in the community. Perhaps no 

 better illustration of this phenomenon could be found than the vertical 

 migration of zooplankton in the sea and in lakes. In general, vast 

 numbers of copepods and other planktonic animals tend to swim to- 

 ward the surface at night and to move downward to deeper levels 

 during the day. The diurnal changes in vertical distribution of fe- 

 males of the copepod, Calanits, in the Clyde Sea area are indicated 

 in Fig. 6.17. The time of ascent in the evening and of descent in the 



