290 GARY N. CALKINS 
weakness owing to constant and close in-breeding. The first 
of these alternatives may be disregarded until the other two have 
been eliminated as possible causes. 
THE EFFECT OF PARENT’S AGE ON VITALITY OF OFFSPRING 
In analyzing the data which I hope will throw light on this 
problem, it is necessary first to define the terms ‘young’ and ‘old’ 
and to establish some standard whereby the relative vitality of 
different series may be measured. Such definitions must be 
based upon a purely arbitrary division between youth and age 
and are useful only for purposes of comparison. The element 
of time is less important than the metabolic activity as measured 
by the division rate, but both are necessary. Using the ordinate 
which marks the rate of ten divisions in ten days as the arbitrary 
dividing line between youth and age in a series, I would define 
the period of youth as that part of the entire life-cycle of a series 
during which the division rate averages more than ten divisions 
in-ten days for sixty-day periods, and the period of old age as 
that part of the life-cycle during which the division rate is less 
than ten divisions in ten days and which ends in natural death. 
Such phases of youth and age may be measured in terms of time 
or activity; if the former, by the number of days; if the latter, 
by the number of generations; while the vitality of the two 
phases may be indicated by the division rate obtained by dividing 
the number of generations by the number of days. Statistical 
data for all of the series are given in table 1. 
According to the arbitrary division line between youth and 
age as adopted above, the period of youth in Uroleptus varies 
from 188 days and 272 generations for the F series, to 69 days 
and 96 generations for the R series, while the Q series had no 
youth at all, but seems to have been ‘born old.’ The average for 
the sixteen series was 139 days and 214 generations; the A, C, F, 
I, O, P, Us, and V series were above the average and the D, H, 
J, L, N, R, Ui and a were below. 
The period of age varied from 127 days and 82 generations for 
the C series, to 40 days and 14 generations for the O series, while 
the average for the sixteen series was 94 days and 65 generations. 
