292 GARY N. CALKINS 
The C, H, N, P, R, U:, Us, and V series had a more extended 
period of age than the average, and series A, C, D, N, P, R, Ui, 
U., and V had more than the average number of divisions during 
the same period. Series in which the periods of both youth and 
age were longer than the average in number of days were C, P, 
U., and V, while series having more than the average length of 
youth and less than the average length of the old-age period 
were A, F, I, and O, both groups indicating a high potential of 
vitality. Series which were below the average in length of the 
period of youth and above the average in the duration of the 
period of age were H, N, R, and U,, while series falling below 
the average both in youth and age were D, J, L, and a. These 
two groups of series thus had a relatively low potential of vitality. 
These statistics, together with the curves of the division rates, 
are indubitable evidence of differences in vitality between the 
various series. It is difficult, however, to measure these differ- 
ences and to determine which of any two or more series is weaker 
in vitality. The division rates for unit periods of time are useful 
in comparing vitality at different periods of the same life-cycle, 
but are practically valueless in determining relative vitality of 
different series because the two factors, time and number of 
divisions, are variables. Thus, if one series lives for 200 days 
and divides 300 times, its division rate for the entire period would 
be fifteen divisions per ten days, while another series which lives 
only 150 days and divides only 225 times would have exactly 
the same division rate. The vitality, however, would not be 
the same in the two series, since the length of the two spans of 
life would be left out of consideration. ‘The division rate repre- 
sents the intensity of metabolic activity; the number of gener- 
ations combines the element of time and the element of metabolic 
activity, and the number of days of life represents what may be 
termed the endurance of the protoplasm. Jor purposes of com- 
parison between different series, it is necessary to take into con- 
sideration all of these factors, which include: 1) the total number 
of days of life; 2) the total number of generations; 3) the number 
of days comprising the period of youth, and, 4) the number of 
divisions during the period of youth. A table suitable for com- 
