RHYTHM IN NATURE. 1 



By F. W. Flattely. 

 Senior Assistant in Zoology, University, Aberdeen. 



That so many phenomena in nature should be of a recurrent or 

 periodic type is not surprising when we consider that the earth on 

 which we live forms part of a rhythmic universe. From the bio- 

 logical point of view, however — and it is with the biological aspects 

 of rhythm that the, writer is concerned — these phenomena, though 

 possibly all of the same nature fundamentally, must, for practical 

 purposes, be regarded as of two distinct kinds. 



We have, in the first place, the numerous cases of periodic behavior 

 in response to, or imposed by, external factors of a cosmic nature, 

 such as the alternation of night and day, the regular and ever-recur- 

 ring sequence of the seasons, the ebb and flow of the tides, and so on. 

 Here the mainspring of the rhythm is obviously external. In con- 

 trast to this, however, are the cases of the second type in which the 

 rhythm is not dependent on the environment, but is of an intrinsic 

 or vital nature, and corresponds to something inherent in the or- 

 ganism. 



The periodicities of this latter type may all be regarded as identical 

 except as regards the time factor. They are of the nature of age 

 cycles. 



In temperate countries, where the difference between the seasons is 

 very marked, the periodical phenomena of plants and animals take 

 place at about the same time in all species. With us, in consequence, 

 the seasons have come to typify the chief stages in the life cycle of 

 plants and animals. Notwithstanding this, the phenomenon of an 

 age cycle must be regarded as something intrinsic in the organism 

 and not merely as the result of environment, the synchronizing of the 

 stages of the age cycle with the seasons being of a secondary nature. 

 " In Europe," says Bates, 2 " a woodland scene has its spring, its sum- 

 mer, its autumnal aspects. In the equatorial' forests the aspect is the 

 same or nearly so every day in the year ; budding, flowering, fruit- 

 ing, and leaf-shedding are always going on in one species or other. 



1 Reprinted by permission from Science Progress, vol. 14, No. 55, January, 1920. 



2 Bates, H. W., The Naturalist on the Amazons. 



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