LIGHT AND METABOLISM 



The origin of such rhythms is speculative, but it is interesting to recall the 

 environment of living creatures when first they experienced the drama of a day- 

 night cycle on the earth. For millions of years living organisms never experienced 

 conditions more varied than those of the warm but placid sea, but as the sea- 

 weeds of the swamps spread onto the land, plants became exposed alternately 

 to the stimulating conditions of a humid hot-house during the day and the 

 depression of the comjDarative chill of night. Similarly, as Amphibians emerged 

 to creep upon the land in the heat of the Palfeozoic, and as thej' and the Reptiles 

 matured in the torrid Jurassic and Cretaceous ages, it is difficult to realize the 

 violence of the contrast between the extreme metabolic and nervous activity 

 which must have occurred in the blaze of noon, and the sluggishness of sleep and 

 the reduction of nervous energy which must have prevailed in the cold of night ; 

 for chemical activity and the speed of nervous impulses are both dependent on 

 temperature. It is probable, indeed, that the development of thermostasis and 

 its ultimate evolution into homeostasis were the determining events which made 

 possible the evolution and ultimate supremacy of Birds and Mammals on a 

 cooling globe, and that the lack of the control of temperature was the main cause 

 of the extinction of the Dinosaurs and the retreat of the Amphibians to a few 

 degenerate types. But it is to be remembered that the period during which the 

 primitive creatures which first inhabited the still-warm earth experienced this 

 alternating climax of delirious activity each noon and fatigued torpidity each 

 night, occupied some hundred million years ; and even although their descendants 

 have long acquired the peace of thermostas's, it is not surprising that traces of 

 the early turmoil still remain. 



PHOTOPEEIODISM IN PLANTS 



Over 200 years ago, carl linn.^us (1707-1778) (Fig. 4), who laid 

 the groundwork of scientific botany at Uppsala, noted that many 



Figs. 5 and 6. — Sleep ^Movements in Flowers and Leaves. 



Fig. 5. — Oxalis rosea awake. 



Fig. 6. — Oxalis rosea asleep. 



