264 AGE, GROWTH, AND DEATH 



question, with his assertion that unicellular organisms are im- 

 mortal. The assertion remains, but the proof of the assertion 

 is lacking. In order to justify the assertion, it must be demon- 

 strated that there does not occur in Protozoa a true senescence, 

 showing itself gradually through successive generations of cells. 

 Has Weismann furnished this demonstration? Certainly not. 

 He has, strictly speaking, not discussed the subject. It is clear 

 that we must first determine whether natural death from senes- 

 cence occurs in Protozoa or not, before we can pass to a scientific 

 discussion of the asserted immortality of unicellular beings. 

 The problem cannot be otherwise apprehended. Weismann has 

 not thus conceived it, therefore the judgment stands against 

 him : he misses the real problem" 



E. Maupas 1 has maintained that among unicellular animals 

 loss of vitality (senescence) and actual rejuvenation could be 

 demonstrated. He was the first to follow a colony of Protozoa 

 through a long series of generations with a view to determining 

 the changes in the life cycle. His conclusion is that there is an 

 actual exhaustion of the cells going on with the progress of the 

 generations, and that conjugation must occur to effect " rejeu- 

 nissement " (rejuvenescence) otherwise the cells of the cycle die 

 off. Similar experiments have been made in this country by 

 G. N. Calkins, 2 who likewise concludes that the development of 

 Protozoa is cyclical, the end of the cycle coming through senile 

 degeneration of the cells, and new cycles beginning by a re- 

 juvenation effected by conjugation. If these conclusions are 

 correct we must expect to find proof of cyclical development in 

 other Protozoa. 



Maupas and Calkins leave a fundamental question undecided. 



1 E. Maupas, Archives de ZooL Expdr., i., 299 ; *., 427 (1883); vi., 165 

 (1888) ; vii., 149(1880). 



2 G. N. Calkins, " Studies in the Life History of Protozoa," Arch. fur. En- 

 twickelungs mechanik, xv., 139186, also Biol. Bulletin, iii., 192205, and 

 Jotirn. Exp. Zool., i. , 423461 (1904). A comprehensive and later presentation 

 of Calkins's views on the protozoan life cycle is given by him in chapter xvii. 

 of the first volume of Osier's Modern Medicine (1907); see especially pp. 

 361-367. 



