28 THE DUKATION OF LIFE. 



As the complexity of the Metazoan body increased, the two 

 groups of cells became more sharply separated from each other. 

 Very soon the somatic cells surpassed the reproductive in number, 

 and during this increase they became more and more broken up 

 by the principle of the division of labour into sharply separated 

 systems of tissues. As these changes took place, the power of 

 reproducing large parts of the organism was lost, while the power 

 of reproducing the whole individual became concentrated in the 

 reproductive cells alone. 



But it does not therefore follow that the somatic cells were 

 compelled to lose the power of unlimited cell-production, although 

 in accordance with the law of heredity, they could only give 

 rise to cells which resembled themselves, and belonged to the same 

 differentiated histological system. But as the fact of normal 

 death seems to teach us that they have lost even this power, the 

 causes of the loss must be sought outside the organism, that is 

 to say, in the external conditions of life ; and we have already 

 seen that death can be very well explained as a secondarily ac- 

 quired 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 indi- 

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

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

 appearance, among multicellular organisms in which the somatic 

 and reproductive cells are distinct. 



I have endeavoured to explain death as the result of restriction 

 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 possible 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 



