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 



