VITALITY—UROLEPTUS MOBILIS 293 
puting the relative vitality of series which have completed their 
life-cycles as well as for future series when the data are available, 
can be made only on the basis of common factors of age in days 
and in number of generations. <A table of constants from 0 
to 350 is adopted here as the basis for computing the relative 
vitality of any series which has completed its life-cycle. On 
such a table, under the appropriate headings, the data for a 
given series are entered opposite the nearest numbers. The 
sum of the four numbers thus indicated gives the vitality index 
for the series, and this may be expressed on a percentage basis 
by dividing by an assumed constant, which, on the basis of 
results obtained thus far, represents a perfect vitality. Thus, 
an ideal life-cycle, from first to last division, extends through 
300 days and through 350 generations by division. An ideal 
period of youth is 175 days long and includes 275 generations. 
The sum of these four (1100) gives the constant index or numerical 
expression of vitality, of an ideal life-cycle. 
Table 2 includes the data for all series which have completed 
their life-cycles. 
On the basis of the vitality index obtained from the above 
table and expressed in percentages of the ideal total, we have the 
following order in relative vitality of the series to date: C 97.7 
per cent, P 95 per cent, F 94.1 per cent, A 90.9 per cent, I 89 per 
cent, V 88.1 per cent, U2 85.9 per cent, O 79.5 per cent, H 78.6 
per cent, U; 76.3 per cent, L 74.5 per cent, D 73.6 per cent, N 
71.3 per cent, J 69 per cent, R 48.1 per cent, a 44 per cent, and 
Q 5.4 per cent. 
With such a key. to the relative vitality of different series, 
it is not difficult to ascertain the effects of parents’ age on vitality 
of the offspring. The statistical data are given in table 3. 
Series with relative vitality lower than 70 per cent may safely 
be regarded as weak. Such are the J, R, a, and Q series. They 
all agree in coming from parents in the old-age period. Thus 
the J series came from A when the latter was 250 days and 311 
generations old. The weakness of the parent protoplasm is 
indicated by the fact that it lived only seventeen days and 
divided only twice after the J series was started by conjugation. 
