62 THE EVOLUTION OF DEATH 



tion that impregnation has the purpose of increasing varia- 

 bility in order to offer room for the play of natural selection 

 originated with Weissmann. (It is said that Treviranus had 

 previously expressed this view, but I have as yet been unable to 

 personally confirm this statement.) Impregnation has also 

 certainly to care both for heredity and for the initiation 

 of further development. We know now that these functions 

 may be separated experimentally. If sexual reproduction be 

 conceived as a modification of conjugation, then we may assume 

 that the function of initiating development was acquired later. 

 Returning to the Infusoria we encounter in them, so far as the 

 available observations go, a so-called depression indeed, but 

 no senescence in the strict sense. (Quite conclusive as to 

 the absence of senescence are the experiments of L. L. Wood- 

 ruff,* who has maintained a pedigreed race of Paramecium 

 for five years without conjugation. If all the possible indi- 

 viduals had survived, they would have made a volume of 

 protoplasm many million times the volume of the earth). 

 Calkins, as said above, originally interpreted depression 

 as a true senescence and declared the diminution of metab- 

 olism to be the essential characteristic of old age. This view 

 has been adopted by C. M. Child 31 and E. G. Conklin. 32 Pro- 

 fessor Child has made experiments with a simple worm, Plana- 

 ria. He treated these animals with alcohol by placing them 

 in water to which i per cent, of alcohol had been added. 

 The results he obtained are interesting and valuable. He 

 seems to me, however, to go too far when he asserts that if 

 the metabolism diminishes the animals become old. It is 

 true that in the higher animals, when they become old, met- 



* L. L. Woodruff: Biologisches Centralblatt, XXXIII, p. 34, 1913. Professor 

 Woodruff has informed me that on April 3, 1913, he had the 36501!! generation of 

 the mentioned Paramecium colony. 



