THE IMMORTALITY OF THE PROTOZOA. 339 



without blemish, without exhaustion, and without 

 any sign of old age. Plenty of food and. simple 

 drugs have successfully resisted senility and the 

 train of atrophic degenerations which it involves. 



Causes of Senescence. — As for the causes of senes- 

 cence which have been remedied with such success, 

 they are not exactly known. Calkins thinks that 

 senescence results from the progressive losses to the 

 organism of some substance essential to life. Con- 

 jugation or intensive alimentation would act by build- 

 ing up again this necessary compound. G. Loisel 

 believes on the contrary that it is a matter of the 

 progressive accumulation of toxic products due to a 

 kind of alimentary auto-intoxication. 



