RHYTHM IN NATURE 



By F. W. FLATTELY, 



Senior Assistant in Zoology, University, Aberdeen. 



I 



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. 

 ^rom the biological 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 funda- 

 mentally, must, for practical purposes, be regarded as of two 

 distinct kinds. 



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

 behaviour in response to, or imposed by, external factors of a 

 cosmic nature, such as the alternation of night and day, the 

 regular and ever-recurring sequence of the seasons, the ebb 

 and flow of the tides, and so on. Here the mainspring of the 

 rhythm is obviously external. In contrast 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 organism. 



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. Not- 

 withstanding 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 synchronising of the stages 

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

 " In Europe," says Bates, 1 " a woodland scene has its spring, 

 its summer, its autumnal aspects. In the equatorial forests 

 the aspect is the same or nearly so every day in the year : 

 budding, flowering, fruiting, and leaf-shedding are always 

 going on in one species or other. The activity of birds and 



1 Bates, H. W., The Naturalist on the A masons. 



418 



