REJUVENESCENCE AND DEATH 307 



complex reaction, as Rubner suggests, but by changes in the sub- 

 stratum which retard metabolism, the domesticated mammals might 

 certainly be expected to require somewhere near similar amounts 

 of energy to attain the death point. 



Rubner fails entirely to take into account the fact that in all 

 the species under consideration the length of life of different cells 

 is very different. Some die after a life which is short compared 

 with the life of a whole organism, and are replaced by others, so 

 that in some tissues growth and development continue throughout 

 the Ufe of the animal. Other cells apparently persist as long as the 

 animal Hves, and it is probably these, e.g., the cells of the nervous 

 system, which are primarily responsible for natural death, as sug- 

 gested above. Rubner's theory also does not admit the possibiUty 

 of rejuvenescence except in connection with fertilization, nor does 

 it show how the starting-point of the complex reaction is again 

 attained at the beginning of each generation. As regards the 

 exceptional position of man, Rubner beheves that the human living 

 substance is different from that of other mammals and requires a 

 much larger amount of energy for a given amount of growth. 

 These data compare man with domesticated mammals; if it were 

 possible, it would be of considerable interest to determine whether 

 the energy requirements are the same in wild as in domesticated 

 animals. It seems probable that they would be higher in the wild 

 forms. 



In a number of papers Loeb has discussed the nature of the 

 processes which bring about death in the mature egg when it is not 

 fertilized and has described certain methods by which its life can 

 be prolonged. In two papers, however (Loeb, '02, '08), he has 

 dealt with the problems of death and length of life in a more general 

 way. The starfish egg, if not fertihzed, dies, usually within a few 

 hours after maturation, but if it is prevented by lack of oxygen 

 from undergoing maturation its life may be prolonged for days. 

 From these facts Loeb concludes that natural death in these cases 

 is due to specific destructive processes which are set going by 

 maturation. These processes cannot be identical with the pro- 

 cesses underlying development, because they are inhibited or 

 delayed by the fertihzation of the egg. 



