440 THE WONDER OF LIFE 



any conjugation or primitive pairing ; another reason being 

 that the medium is or becomes in some way abnormal. 

 But Weismann's doctrine postulates natural conditions, 

 which would, of course, include the possibility of conjuga- 

 tion, and an ever fresh medium. A recent worker, Mr. 

 G. T. Baitsell, reports that he has discovered an optimum 

 medium in which one of the Infusorians will thrive and 

 multiply indefinitely without conjugation and without 

 introduced tonics. 



It is a familiar fact that in the history of a hay infusion, 

 one kind of Protozoon succeeds another, which disappears 

 before it. But this disappearance is sometimes due to 

 violent death, and is sometimes not more than passing into 

 a latent state, as the result of deficient food or accumulated 

 waste-products. And again, it may be admitted that 

 when a Protozoon divides into two or many individuals, there 

 is, in a sense, a disappearance of a particular individuality 

 which went through a particular sequence of experiences ; 

 yet we cannot speak of death when one creature directly 

 turns into two or into many, and when there is nothing 

 left to bury. 



It is not improbable that very simple multicellular 

 animals, such as the freshwater Hydra, may go on living 

 indefinitely if the natural conditions are altogether pro- 

 pitious. The structure and the multiplication of Hydra 

 are alike so simple, that there seems no good reason why 

 it should die a natural death. But as the body became 

 more complex, death was instituted as a tax on progress. 

 In discussing senescence we have mentioned some of the 

 facts which more or less certainly involve natural death, 

 but they are mostly reducible to two : (1 ) That the effects of 

 wear and tear in the body are not readily made good with 



