62 THE DURATION OF LIFE. 



by newly formed cells, that is by the process of reproduction in 

 cells 1 . 



"We cannot a priori dispute the possibility of the existence of 

 tissues in which the cells are not worn out by the performance of 

 function, but such an occurrence appears to be improbable when 

 we recollect that the cells of all tissues owe their constitution 

 to a very far-reaching- process of division of labour, which leaves 

 them comparatively one-sided, and involves the loss of many pro- 

 perties of the unicellular, self-sufficient organism. At any rate we 

 only know of potential immortality in the cells which constitute 

 independent unicellular organisms, and the nature of these is such 

 that they are continually undergoing a complete process of re- 

 formation. 



If we did not find any replacement of cells in the higher 

 organism, we should be induced to look upon death itself as the 

 direct result of the division of labour among the cells, and to con- 

 clude that the specific cells of tissues have lost, as a consequence 

 of the one-sided development of their activities, the power of un- 

 ending life, which belongs to all independent primitive cells. We 

 should argue that they could only perform their functions for a 

 certain time, and would then die, and with them the organism whose 

 life is dependent upon their activity. The longer they are occupied 

 with the performance of special functions, the less completely do 

 they carry out the phenomena of life, and hence they lead to the 

 appearance of retrogressive changes. But the replacement of cells 

 is certain in many tissues (in glands, blood, etc), so that we can 

 never seek a satisfactory explanation in the train of reasoning in- 

 dicated above, but we must assume the existence of limits to the 

 replacement of cells. In my opinion, we can find an explanation 

 of this in the general relations of the single individual to its 

 species, and to the whole of the external conditions of life ; and this 

 is the explanation which I have suggested and have attempted to 

 work out in the text. 



1 Boux, in his work ' Der Kampf der Theile im Organismus,' Jena 1881, has 

 attempted to explain the manner in which division of labour has arisen among the 

 cells of the higher organisms, and to render intelligible the mechanical processes 

 by which the purposeful adaptations of the organism have arisen. 



