684 THE PHYSIOLOGY OF REPRODUCTION 



simple stage to the more complete organisation, they lose some- 

 thing of their vitality, of their power of growth, and of their 

 possibilities of perpetuation. Just as senescence depends upon 

 the increase and differentiation of the cytoplasm, so, conversely, 

 rejuvenation depends upon the increase of the nuclear material ; 

 and consequently the alternation of the two phases of the life 

 cycle (the early brief one when the young material is formed, 

 and the later prolonged one when the process of differentiation 

 is going on) is due to an alternation in the proportions of nucleus 

 and protoplasm. In criticism of this theory, it may be urged 

 that it is in reality nothing more than a descriptive account of 

 a general type of cellular change, and that it provides no sort 

 of explanation as to why this type of change occurs, nor how 

 it is that differentiation is apparently correlated with a reduction 

 of vitality leading eventually to death. 



MetchnikofE has laid great stress on the idea that natural 

 death is a rare phenomenon, at least among the higher animals. 

 That death with Man is frequently, if not generally, caused by 

 disease or accident is a fact about which there can be no disagree- 

 ment, and Karl Pearson^ has worked out statistically the chances 

 of death occurring in the different phases of human life. " We 

 have five ages of man," he says, " corresponding to the periods 

 of infancy, childhood, youth, maturity or middle age, and 

 senility or old age. In each of these periods we see a perfectly 

 chance distribution of mortality centring at a given age and 

 tacking ofi on either side according to a perfectly clear mathe- 

 matical law." It was found also that the curve of mortality, 

 as deduced from a study of the deaths per annum of a thousand 

 persons born in the same year, " starts very high in infancy, 

 falls to its least value at thirteen or fourteen years with only 

 236 deaths. It then slowly increases till it reaches a maximum 

 in the seventy-second year of life, and falls more rapidly than it 

 rose, till scarcely two isolated stragglers of the thousand reach 

 ninety-one." It is' clear, therefore, that death from old age is 

 far from being the rule in the human species, but according to 

 Metchnikoff it seldom occurs at all.^ 



^ Pearson, The Chances of Death, &c., vol. i., London, 1897. 

 2 Metchnikoff, foe. cil.; and The Nature of Man, Mitchell's Translation, 

 London, 1903. 



