Rhythmicity and Periodicity 351 



rhythmicity should be reckoned among the most general 

 characteristics of the modes of manifestation of vital 

 energy. Apart from the fact that nearly all, and perhaps 

 all external physical stimuli, from the thermal and 

 luminous to the acoustic, are characterized by vibrations; 

 and the other fact, a consequence of the first, of the 

 physiological action exercised by all the rhythmical or 

 periodical manifestations of the most diverse energies, 

 we see that a more or less manifest and more or less 

 regular periodicity is a fundamental character of all or 

 nearly all biological functions. One thinks at once, for 

 instance, of the synchronous rhythm of all the peristomal 

 cilia of an infusorian, a rhythm which manifests itself 

 in the two parts of an animal which has been divided, 

 provided these parts remain connected by a bridge of 

 protoplasm ; of the rhythmicity present in the protozoa in 

 general, present even within the cells, shown by the 

 pulsation of contractile vacuoles, which empty and refill 

 themselves continually at regular intervals; of the beat 

 of the heart, even independent of its connection with the 

 nervous system; of the similar pulsations of the whole 

 vascular system, the entire breathing apparatus, the 

 uterus, and of many other organs; and finally of the 

 periodicity of a whole series of physiological variations, 

 which animals and plants undergo as a result of corre- 

 sponding variations of the outer world, but which persist 

 unaltered for some time even when the outer world, or 

 the periodicity of its variations, may have changed. 



Now it is not difficult to conceive of this rhythmicity 

 or periodicity which nearly all biological functions pre- 

 sent, as a consequence more or less direct or indirect of 

 the vital phenomenon in all its generality, when this 



