444 Evolution and Adaptation 
very gratifying, for the organisms began immediately to grow 
and to divide, the rate of division rising from five to nine divi- 
sions in successive ten-day periods.” This beneficial effect 
was not lasting, however, and after ten days the paramoecia 
began to die off faster than before, and the renewed applica- 
tion of the liver extract failed to revive them. A number of 
other extracts were then tried without effect. Finally they 
were transferred to the clear extract of lean beef in tap water. 
The effect of this medium was interesting, for, although it 
restored the weakened vitality, there was no rapid increase 
in the rate of division, as when first treated with the teased 
liver. The infusoria were, however, now large and vigorous, 
and did not die unless transferred from the beef medium to 
the usual hay infusion. “When this was attempted, they 
would become abnormally active and would finally die. The 
division rate gradually increased during the month of August 
until, in the last ten days, they averaged six generations. 
Finally, in September, the attempts to get them back on the 
old diet of hay infusion were successful, and then the divi- 
sion rate went up at once to twelve times in ten days, and a 
month later they were dividing at the rate of fifty times a 
month.” 
“These cultures went on well until December, when the 
paramoecia began to die again. They were saved once more 
with the beef extract, and when returned later to the hay 
infusion continued through another cycle of almost three 
months. Some of these were treated, once a week for 
twenty-four hours, with the beef extract, and while the two 
sets ran a parallel course at first, those kept continuously in 
the hay infusion died after a time, but those that had been 
put once a week into the beef extract (which had been 
stopped, however, in March) continued their high rate of 
division throughout the period of decline of their sister cells, 
and did not show signs of diminished vitality until the first 
period in June.” At this time their rate of division increased 
