1'ARAMECIUM 225 



infusion, will continue to multiply by binary division for many 

 generations. Careful observation has shown that a Parame- 

 cium, kept under observation in a drop of hay infusion in a 

 moist chamber, will divide two or three times a day for some 

 forty-eight hours, but the rate of division is reduced on the 

 third day, and on the fourth day, when Bacteria begin to dis- 

 appear from the drop, it is greatly diminished. If fresh hay 

 infusion is not added, division ceases and the now numerous 

 progeny of the original Paramecium show evident signs of 

 starvation. But they will recover if supplied with more food, 

 and continue to divide as before. Under carefully devised 

 conditions a numerous stock of Paramecia, the descendants of 

 a single individual, may be cultivated for a considerable length 

 of time. But not for an indefinite period. A series of careful 

 and long-continued experiments have shown that, if fresh hay 

 infusion is constantly added, the stock of Paramecia will mul- 

 tiply at the normal rate of one or two divisions per diem, with 

 slight variations, for the space of about three months. Then 

 a period of depression sets in, not directly attributable to a 

 change of temperature or any other assignable cause. In such 

 a period the rate of division becomes slower and slower and 

 finally ceases ; many individuals die and others appear mori- 

 bund. At such a crisis the survivors can be artificially stimu- 

 lated to fresh activity and endowed, as it were, with a new 

 lease of life. In one case the jolting of a railway journey was 

 observed to have a marked effect in increasing the rate of 

 asexual multiplication by division, but a far more effective 

 stimulus was afforded by a change of diet. When the depressed 

 and moribund Paramecia were fed with beef extract, they 

 rapidly recovered and resumed their normal rate of division. 

 After a time the revived progeny could be restored to a diet 

 of hay infusion and continued to multiply asexually for another 

 period of three months. Then a second period of depression 

 set in, more profound and deadly than the first. The few 

 survivors were with difficulty kept alive on a diet of beef extract, 

 but after a month they had so far recovered that they were 

 again able to maintain themselves and resume the normal 

 rate of multiplication in hay infusion. The experiments were 

 carried on continuously for twenty-three months, at the end 

 of which time the last surviving descendants of the parent 

 individual of the stock died out in the seven hundred and forty- 



