REJUVENESCENCE AND DEATH 305 



which has arisen " because unlimited duration of the life of the 

 individual would be a senseless luxury." In other words, death 

 appeared at some time as a chance variation which was inherited 

 and was of such value to the organic world that through the action 

 of natural selection it has become universal in multicellular organ- 

 isms. Death was possible in these forms because somatic and germ 

 cells were separated, while in the unicellular forms they are one and 

 the same cell. 



The problems of death and length of life find no solution in these 

 speculations. The occurrence of death is simply assumed as the 

 foundation of the theory. But it is not true that all multicellular 

 forms necessarily die. As I have endeavored to show, many forms, 

 both plants and animals, may escape death by reproduction and 

 rejuvenescence in exactly the same way as do the protozoa. On 

 the other hand, there is every reason to believe that if the protozoa 

 live long enough without reproduction they too die of old age and 

 the germ cells of the multicellular forms also apparently undergo 

 senescence and die of old age if rejuvenescence is not initiated by 

 fertilization (see pp. 403-6). The evidence also indicates that 

 death occurs in general soon after the period of sexual reproduction 

 is over, not because of advantage to the species, but because sexual 

 maturity is a physiological feature of relatively advanced age. Pro- 

 gressive development, which ends in death, except where interrupted 

 by regression, is -far advanced when sexual reproduction begins. 

 And, finally, it is rather remarkable that natural selection should 

 have succeeded so completely, as Weismann believes it has, in elimi- 

 nating the species in which death does not naturally occur. 



A theory of length of life of a very different sort, based pri- 

 marily upon calorimetric investigations on various domestic mam- 

 mals and man, has been advanced by Rubner ('08, '09). From the 

 available data Rubner has calculated the total energy requirement 

 in calories for a doubling of body- weight after birth and the require- 

 ment per kilogram of body- weight for the whole period of life after 

 growth is completed in a number of the domestic mammals and 

 man. The total calories required for the doubling of weight are 

 given in Table VI, and the total calories per kilogram of body- 

 weight for the period after completion of growth in Table VII. 



