VITALITY—UROLEPTUS MOBILIS 303 
the P series, in which the relative vitality was 95 per cent. The 
other representatives of this fifth filal generation, the U; and 
Uz series, fell off to 76.3 per cent and 85.9 per cent, respectively, 
but this lower vitality may be accounted for by the fact that 
the parent L series was 208 generations old and in the period 
of old age when the two U series were started; while the high 
vitality of the P series from the same parent L, when younger, 
is evidence that the lower vitality of the U series was not due 
to in-breeding. The sixth filial generation included the V, X, a, 
and Y series. The relative vitality of the V series was 88.1 per 
cent and of the a series 44 per cent. The decrease shown by the 
V series is not significant and probably was due to chance vari- 
ation (cf. the two branches of the U series), while the marked 
decrease of the a series has already been traced to the extreme 
old age of the parents at the time of conjugation. The X and Y 
series are still living and their relative vitalities cannot be com- 
puted, but that their vitality is high is indicated by the fact that 
the X series has already lived and divided longer than any 
previous series (over 300 days), while the Y series is 212 days 
old and is still maintaining a high division rate. 
The other branch from the A series, beginning with the J 
series, has had a continuous record of old-age conjugations so 
that only four filial generations have been produced in the same 
time during which the other branch produced nine filial gener- 
ations. The J series came from the A series when its parents 
were 250 days and 311 divisions old at the time of conjugation; 
the N series came from J when the latter was 120 days and 188 
divisions old; the R series from J when the latter was 160 days 
and 245 divisions old, and the W series came from N when the 
latter was 130 days and 225 divisions old. The W series, in 
spite of its weak ancestry, has shown a remarkable vitality, 
especially in the factor of endurance, for it has already lived 270 
division days, or longer than any other series with the exception 
of C, P, and X. The metabolic activity, as measured by the 
average division rate of 9.7 divisions in ten days, has been con- 
spicuously low. Conjugations did not occur in this series until 
it was 261 days and 255 divisions old. What vitality its filial 
k series will have remains to be seen. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 31, NO. 2 
