VITALITY—-UROLEPTUS MOBILIS 299 
period of youth was only half as long as the average for all series 
(ef. table 1). Its first division occurred on August 13, 1919, and 
its last on January 8, 1920. Other series living during these five 
months and subject to the same environmental factors were 
series Ui, Us, V, W, Y, Z, and all of the X series (regeneration 
experiments). The Y series started on August Ist and is still 
living; the W series started on May 31, 1919, and is still living; 
the Z series and seven experimental X series with controls, 
started in September and October and all are living; the Ui, 
U., and V series divided for the last times on November 15th, 
December 3rd, and January 2nd, and all died a natural death 
after 285, 301, and 317 generations, respectively. Environ- 
mental factors, had they been responsible for the weakness of 
the a series, should have shown some effects on these other 
contemporaneous series, but as no ill effects were shown by them 
we must conclude that the low vitality of the a series was not 
due to environmental causes. 
Analyzing the R series and the J series in the same way, we 
find that environmental factors were not responsible for their 
low vitality. The R series lived from January 23 to July 27, 
1919, a period during which the L, O, P, Ui, Us, V, W, and some 
of the X series were under cultivation and showed no ill effects 
of adverse environmental conditions. The J series divided for 
the first time on August 20, 1918, and for the last time on March 
5, 1919; during this period the F, H, I, L, M, N, O, and P series 
were also under cultivation without showing any effects of ad- 
verse external conditions. 
Since the low vitality of the weak series cannot beattributed 
to adverse external conditions; since all weak series agree in 
respect to one condition, viz., extreme old age at the time of the 
parents’ conjugation; and since all series with high relative 
vitality agree in coming from parents whose protoplasm .was 
comparatively young, we must conclude that weakness of the 
parental protoplasm, whether due to age or to congenital causes, 
is the primary cause of the low vitality of the offspring. The 
effect on offspring of congenital weakness of parents is not clearly 
shown by any of the above series, although indications of it 
