68 BIOLOGY 



that of the higher animals where the cells remain attached. But 

 the process of division is the same and may continue for a long 

 time. Eventually, however, as we have already seen, this 

 power of division by the simple process of fission becomes ex- 

 hausted, and the multiplication tends to die out. We can per- 

 haps compare this with the old age of a larger animal, for in old 

 age we find division becoming less and less vigorous, until it 

 finally ceases altogether and the whole generation of cells dies. 

 Among the larger animals, to prevent the extermination of the 

 race, a single cell, an egg, is set aside to start the process over 

 again, thus beginning the new cycle. In the case of the Parame- 

 cium, after the ordinary reproduction has gone on for a long time 

 it becomes impaired in vigor and seems to be started over again 

 by this process of conjugation. The process of conjugation, 

 therefore, corresponds to reproduction by an egg in one of the 

 larger animals or plants. Hence one life cycle of the Parame- 

 cium lasts from one period of conjugation, through all the nu- 

 merous successive divisions by ordinary fission, until again the 

 conjugation occurs to start a new cycle. One generation, then, 

 consists of all the members that arise between one conjugation 

 and the next; and inasmuch as these animals may multiply 

 almost indefinitely "by ordinary division, it is evident that one 

 generation of Parameda may consist of thousands of organisms 

 scattered over a wide territory. It is evident, therefore, that 

 the term individual in the case of the Paramecium cannot have 

 the same significance that it has with the higher animals, since 

 the individual of one of the higher animals would correspond to 

 a combination of all of the different Parameda that arise from 

 the division of any single cell that comes from a process of con- 

 jugation, until again it enters into a process of conjugation with 

 another cell. Conjugation thus starts a new generation or a 

 new individual. 



We do not know how long a time may elapse between two 

 successive conjugations in the case of a Paramecium, nor do we 

 know the conditions which bring about the process. We are 



