INTRODUCTORY 7 



the external periodicities operate. The fundamental 

 problem of the Biology of the Seasons is to trace the relations 

 between the external periodicities and the vital rhythms. 

 Our central thesis is that life is rhythmic, and that it is 

 punctuated by the seasons and by other external periodic 

 influences. 



Let us take a few illustrations. Many vegetable cells, 

 such as simple Algae, feed during the day and divide at 

 night. The deeply rooted inherent contrast between nutri- 

 tion and reproduction is externally punctuated. Flowers 

 open and shut, wake and sleep, periodically. Biological 

 virtuosos with leisure have made " floral clocks." Some 

 flowers are intermittent even in their fragrance. 



The lines of growth on shells and on some bones indicate 

 periodicity, like the rings of growth on a tree, or the rings 

 on the rattlesnake's rattle, and this self-registering of 

 alternations is widespread as the four illustrations may 

 suggest in organic nature. In most cases it seems that 

 the punctuation is from without, while the necessity of the 

 alternation is from within. We can read summer and 

 winter on the scales and otoliths of fishes, just as we 

 can read day and night on a bird's feather. The increase 

 to the scales in the summer period is different from that 

 in the winter period, and the daily variations in the bird's 

 blood-pressure are sometimes registered, when feathers are 

 a-making, by the beautiful cross-bars. There is periodic 

 waxing and waning of venomousness in snakes, and there 

 are well-known periods in some diseases. Even the times of 

 cock-crowing sometimes correspond to external periodicities 

 of temperature. 



Some of the correlations between external and internal 

 changes admit of very direct interpretation ; thus we 

 understand at once that green plants are intensely active 

 during the day and relatively restful at night, the hidden 



