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 
