226 COMPARATIVE ANATOMY 



second generation. The final result of these very interesting 

 and laborious experiments was to show that under certain con- 

 ditions the life cycle of Paramecium exhibits periodic cycles of 

 vigour and depression, as measured by increment or decrement 

 in size and in the rate of multiplication by asexual reproduc- 

 tion. At the end of every three months there was a period 

 of diminished vitality, but not so profound as to require more 

 than a slight stimulus for complete recovery. At the end of 

 every six months, however, the period of depression was pro- 

 found and lasting and the stock was only maintained by the 

 application of a strong stimulus. As time went on the six- 

 monthly periods of depression became more and more pro- 

 nounced. Beef extract failed to revive the moribund individuals, : 

 and recourse was had to extracts of brain and pancreas. 

 Finally these stronger stimuli failed in their effects and the ' 

 race died out altogether. The conclusions to be drawn from 

 these facts are sufficiently obvious. A swarm of Paramecium 

 is incapable of maintaining itself by asexual reproduction for 

 an indefinite period, even when supplied with an abundance 

 of suitable food and maintained under the most favourable 

 conditions of temperature. At regularly recurring periods the 

 vitality of the race diminishes, but can be restored by artificial 

 stimulation. After a time, however, stimulation ceases to be 

 effective, and the race dies out as a result of what, for want of 

 a better term, we may call senile decay. 



In the case of Paramecia living in a state of nature, the 

 available food supply must vary considerably from time to 

 time, and it can only be by the rarest coincidence that a change 

 of diet affording the necessary stimulus is provided at the 

 onset of a period of depression. The animalcules would 

 therefore perish, either for want of food or because their vital 

 energies are exhausted, if the onset of degeneration were not 

 arrested by the natural stimulus afforded by the act of con- 

 jugation. Indeed, the experiments of which mention has 

 been made show that conjugation is the natural means by I 

 which these organisms renew their vitality whenever a depres- ' 

 sion period sets in, for in the control cultures which were kept 

 going alongside of the series under observation, numerous 

 cases of conjugation were observed to synchronise with the 

 depression periods. Prior to conjugation the individuals 

 move hither and thither in a rapid and excited manner as if 



