THE EVOLUTION OF DEATH 6 1 



fore, were not kept strictly pure. In America a long series of 

 researches on the rapidity of division in Protozoa has been 

 made, largely upon the instigation of G. N. Calkins,^^ who 

 discovered that Infusoria may suffer "depression " a result 

 which has been confirmed by further investigations of his own, 

 of his pupUs, and of other American investigators. The de- 

 pression arises gradually, the animals become inert, nourish 

 themselves poorly, and divide slowly or even not at all. If 

 the depression lasts too long the animals may die off Calkins 

 considered the depression to be senescence, or a growing 

 old. (He has since himself questioned the justness of this 

 interpretation.) 



Our conception of senescence is based on the observation 

 of the higher animals and plants, and comprises not merely 

 the increasing weakness, but also alterations in structure 

 which go far and are very striking. The Infusoria during 

 their depression show no corresponding alterations of their 

 organization; hence in my belief we cannot homologize this 

 phenomenon in the Protozoa with the senescence of higher 

 animals. In this belief we are confirmed by the fact that the 

 newer investigations of conjugation make it improbable that 

 it serves to renew and hasten the growth and division of 

 unicellular organisms. Indeed, it is possible that conjuga- 

 tion does not have this function at all. Significant here are 

 the studies of the very talented investigator, H. S. Jennings,^" 

 which demonstrate that conjugation serves to increase varia- 

 bility. Jennings observed that Paramaecium exhibit consid- 

 erable variability. During ordinary division the individuals 

 remain more ahke, but after conjugation their variation in- 

 creases. His careful statistics leave no doubt as to his results. 

 It is probable that sexual reproduction also has the purpose 

 of maintaining the variability of the forms. The interperta- 



