PHYSIOLOGICAL RHYTHMS. 295 



History shows that races rise, flourish, and disappear, 

 as do the special civilisations that may spring up among 

 them, and it is probable that all the phenomena of 

 development are as surely to be detected in the 

 species as in the individual and his constituent cells. 

 Of late, it has been recognised that the living sub- 

 stance composing the higher organisms may be divided 

 into two portions, the body, or soma, and the germ-plasm, 

 or reproductive cells. In the developing animal certain 

 cells are early set aside to form the reproductive elements, 

 and about these the remaining cells build up a protect- 

 ing body. The great mass of elements which form the 

 soma is, therefore, contrasted with the much smaller mass 

 that forms the germ-plasm, and these subdivisions of 

 the individual have quite different histories. The germ- 

 plasm giving rise through successive generations to new 

 individuals wears the appearance of immortality, while 

 the soma, which shrinks and breaks down after its 

 traditional cycle of " threescore years and ten," has no 

 such claim to distinction. Dependent on slow changes 

 wrought in the germ-plasm must be the long growth 

 period of races and nations to which allusion has just 

 been made, but the rhythms here to be discussed are those 

 occurring within the time limits of somatic life. An 

 analysis of the growth-period shows that in the process 

 of increase in size, phases of rest alternate with those of 

 activity. This rhythm is most rapid at the beginning 

 of development, and probably the vigorous growth at 

 puberty is but the last well-marked acceleration, after 

 the completion of which increase becomes slow, then 

 ceases, and finally gives way to the changes characteristic 

 of old age. Within the life-cycle comes that of repro- 

 ductive capability, with its beginning at adolescence, its 

 maximum at the completion of somatic growth, followed 

 by a slow decline, leading to cessation in old age. 



