300 ELEMENTS OF BIOLOGY 



the theory; the fact that senescence is metaboUc and general has 

 again been lost sight of, or ignored. 



Duration of Life. Duration of life, barring accident or disease, 

 varies greatly among animals. Accurate data of the maximum age 

 reached by common animals are not easily obtained, but we are 

 familiar with the fact that some insects may live but a few days, 

 some of the smaller mammals for a few years only, and that very 

 exceptionally does a human live for more than a hundred years. 

 We are also familiar with the fact that duration of life is greater 

 among some human families than others. Thus duration of life is a 

 diagnostic feature, a part of the attributes of the several phyla, 

 and is an inheritable character within the species. So, the length of 

 a human life is determined at its very beginning, within certain 

 limits, by inheritance. 



Death in an animal body may result from one of two conditions: 

 An accident of the environment, including destruction by disease, 

 and the ceasing of the fundamental metabolism by reason of changes 

 brought on by time. Mammals are thus in double jeopardy; acci- 

 dent, disorders, or infection may destroy at any time in life; if these 

 are avoided death due to gradual arrest of metabolism is inevitable. 

 But in a complex animal body all cells do not cease to function 

 simultaneously. Experimentally one may, by proper handling, keep 

 some tissues and organs alive for a considerable time after they 

 have been removed from the body. The ultimate death of a meta- 

 zoon body is reached when all the cells have died; life, then, may 

 persist for some time after the body as a whole has ceased to 

 exhibit the characters that are commonly recognized as evidence 

 of life. The maintenance of life is therefore conditioned upon the 

 interdependence of the cells, an expression of organization. Death 

 must represent some sort of disorganization. 



Death of Protoplasm. In last analysis death means the death 

 of cells and cell death is brought about by protoplasmic disorgani- 

 zation. As we do not know the exact organization of living proto- 



