VITALITY—UROLEPTUS MOBILIS 297 
Contrasting these extremely weak series with the markedly 
strong ones, such as series C (97.7 per cent), P (95 per cent), 
I (89 per cent), F (94.1 per cent), and V (88.1 per cent) (the 
parentage of A was unknown), we find that all came from young 
parents in the prime of metabolic activity. The parents of both 
C and P were 70 days old; of F, 50 days old; of V, 90 days old, 
and of I,100 days old. There is strong evidence, therefore, that 
the vigor of these series was due to conjugation of relatively 
young parents. 
The remaining series, U;, Us, O, H, L, and D, with intermediate 
relative vitality, came, as a rule, from parents of middle age. 
The two U series came from the same epidemic of conjugation 
which occurred in the L series when the latter was 140 days and 
208 generations old. The O series, with relative vitality of 79.5 
per cent, came from the M series when the latter was 60 days 
and 108 generations old. It was started, therefore, during the 
period of youth of its parents M and should have had a higher 
relative vitality. Its parents, however, came from a cyst instead 
of from conjugation, and this may have had something to do 
with its low relative vitality. The H and D series are interesting 
in that both came fromthe same ancestor A series. The D series 
was started when the parent protoplasm was 70 days and 100 
generations younger than when the H series was started, yet 
its relative vitality (73.6 per cent) was less than that of the H 
series (78.6 per cent). Its average division rate for the entire 
life, however, was higher (D 12.4 divisions per ten days, H 10.9), 
and higher also for the period of youth (D 16.1, H 15.8) and higher 
also for the period of age (D 7.7, H 4.9), showing that the meta- 
bolic activities of the D series were more virile than were those 
of the H series. Its weakness was due to its low endurance, 
for it lived only 215 days, while the H series lived 245 days. 
This may have been due to chance variation resulting from 
amphimixis. Unfortunately, ex-conjugants were not taken 
either from D or H. 
The L series, finally, with its low relative vitality of 74.5 per 
cent, came from the I series while the latter was in the period 
of youth, or 120 days and 199 generations old. Its protoplasm 
