304 SENESCENCE AND REJUVENESCENCE 



other conditions, and the death point is never attained by the indi- 

 vidual, although even in such forms death of cells, apparently from 

 old age, may be a characteristic feature. 



The appearance of death in the course of evolution as the end 

 of the life history of the individual is to be regarded as a result of 

 the increasing physiological stability of the substratum of the 

 organism and the increasing degree of individuation which the 

 greater stability makes possible. These changes determine a 

 greater degree of continuity of progressive development and senes- 

 cence and so less frequent and less extensive regression, reproduc- 

 tion, and rejuvenescence. 



As the evolution of the individual advances with its increasing 

 differentiation and more intimate correlation of parts, death as the 

 termination of the individual life history becomes more and more 

 inevitable. 



SOME THEORIES OF LENGTH OF LIFE 



Most authors who have discussed senescence have regarded 

 death as merely the final termination of the processes of senescence, 

 whatever their view concerning the nature of these. But certain 

 of the theories advanced which concern themselves particularly 

 with the problem of the length of life require special mention here. 



Some thirty years ago Weismann ('82, '84) first stated his view 

 that the cause of death lies in the limitation of capacity for cell 

 reproduction. In the unicellular organisms, according to Weis- 

 mann, this capacity is not limited, therefore the protozoa do not 

 die. In the multicellular organism, however, only the germ cells 

 retain the capacity for unlimited division; in the somatic cells the 

 number of possible cell divisions has been limited by the action of 

 natural selection, which determines in general that life shall not 

 continue long after the reproductive period is completed. In later 

 writings ('92, '04) Weismann has elaborated this idea further, but 

 without essential change. The theory concerns itself with the 

 evolution of length of life and of death rather than with the problem 

 of the nature of the physiological processes involved. Death must 

 of course have occurred before the length of life could be subjected 

 to the action of selection. Weismann maintains, however, that 

 death is not a fundamental characteristic of life, but an adaptation 



