I.] THE DURATION OF LIFE. 29 



even this power, the causes of the loss must be sought outside 

 the organism, that is to sa}'^, in the external conditions of life ; 

 and we have already seen that death can be very well explained 

 as a secondarily acquired adaptation. The reproductive cells 

 cannot lose the capacity for unlimited reproduction, or the 

 species to which they belong would suffer extinction. But 

 the somatic cells have lost this power to a gradually increasing 

 extent, so that at length they became restricted to a fixed, 

 though perhaps very large number of cell-generations. This 

 restriction, which implies the continual influx of new individuals, 

 has been explained above as a result of the impossibility of 

 entirely protecting the individual from accidents, and from the 

 deterioration which follows them. Normal death could not 

 take place among unicellular organisms, because the individual 

 and the reproductive cell are one and the same : on the other 

 hand, normal death is possible, and as we see, has made its ap- 

 pearance, among multicellular organisms in which the somatic 

 and reproductive cells are distinct. 



I have endeavoured to explain death as the result of restric- 

 tion in the powers of reproduction possessed by the somatic 

 cells, and I have suggested that such restriction may conceivably 

 follow from a limitation in the number of cell-generations pos- 

 sible for the cells of each organ and tissue. I am unable to 

 indicate the molecular and chemical properties of the cell upon 

 which the duration of its power of reproduction depends: to 

 ask this is to demand an explanation of the nature of heredity — 

 a problem the solution of which may still occupy many genera- 

 tions of scientists. At present we can hardly venture to propose 

 any explanation of the real nature of heredity. 



But the question must be answered as to whether the kind 

 and degree of reproductive power resides in the nature of 

 the cell itself, or in any way depends upon the quality of its 

 nutriment. 



Virchow, in his ' Cellular Pathology,' has remarked that the 

 cells are not only nourished, but that they actively supply 

 themselves with food. If therefore the internal condition of 

 the cell decides whether it shall accept or reject the nutriment 

 which is offered, it becomes conceivable that all cells may 

 possess the power of refusing to absorb nutriment, and there- 

 fore of ceasing to undergo further division. 



