60 THE EVOLUTION OF-BEATH 



ing in both cases the exciting of cell division as the immediate 

 consequence alike of conjugation and fertilization. In both 

 cases there would arise homologous cycles of cell generations. 

 Thus we should have to deal in both types of organisms with 

 individuals in Huxley's sense. The only difference between 

 the two types, which from our present point of view must be 

 regarded as important, is that the cells in the lower type sepa- 

 rate from one another, while in the higher, on the contrary, 

 they unite to form a plant or animal. Death, as we ordinarily 

 observe it, is the breakdown of a multicellular organism, and 

 natural death is a consequence of old age. This considera- 

 tion leads us directly to the question: Do old age and natural 

 death occur in unicellular organisms? Weissmann, who has 

 written several times concerning death, has not conceived 

 the problem rightly, so that his discussions of death go 

 astray in several essential respects. 



The first serious experiments to determine by direct ob- 

 servation whether old age occurs in unicellular animals were 

 carried out by the French investigator, Maupas. 28 " He 

 reared Protozoa through many generations. Of each genera- 

 tion he took a few individuals, allowed them to propagate 

 themselves and noted the rapidity with which the divisions 

 followed upon one another. He found that the rapidity 

 diminished until a new conjugation occurred, whereupon the 

 animals recovered. Later tests of these results have shown 

 that the experiments of Maupas were open to criticism, in 

 part because at that time the great influence of external con- 

 ditions upon Infusoria was unknown, so that the possibility 

 remains that the retardation of the division he observed was 

 conditioned not by internal but by external causes. Further, 

 in order to bring about conjugation, he introduced into his cul- 

 tures newly captured, wild individuals. His cultures, there- 



