62. THE DURATION OF LIFE. [I. 



complex, higher organisms. Waste does not depend upon the 

 intrinsic nature of the cells, as the primitive organisms prove 

 to us, but it has appeared as an adaptation of the cells to the 

 new conditions by which they are surrounded when they come 

 into combination, and thus form the cell-republic of the meta- 

 zoan body. The replacement of cells in the tissues must be 

 more advantageous for the functions of the whole organism 

 than the unlimited activity of the same cells, inasmuch as the 

 power of single cells would be much increased by this means. 

 In certain cases, these advantages are obvious, as for example 

 in many glands of which the secretions are made up of cast-off' 

 cells. Such cells must die and be separated from the organism. 

 or the secretion would come to an end. In many cases, how- 

 ever, the facts are obscure, and await physiological investigation. 

 But in the meantime we may draw some conclusions from the 

 effects of growth, which are necessarily bound up with a 

 certain rate of production of new cells. In the process of 

 growth a certain degree of choice between the old cells which 

 have performed their functions up to any particular time, and 

 the new ones which have appeared between them, is as it were 

 left to the organism. 



The organism may thus, figuratively speaking, venture to 

 demand from the various specific cells of tissues a greater 

 amount of work than they are able to bear, during the normal 

 length of their life, and with the normal amount of their strength. 

 The advantages gained by the whole organism might more 

 than compensate for the disadvantages which follow from the 

 disappearance of single cells. The glandular secretions which 

 are composed of cell-detritus, prove that the cells of a complex 

 organism may acquire functions which result in the loosening 

 of their connexion with the living cell-community of the body, 

 and their final separation from it. And the same facts hold 

 with the blood corpuscles, for the exercise of their function 

 results in ultimate dissolution. Hence it is not only conceivable, 

 but in every way probable, that many other functions in the 

 higher organisms involve the death of the cells which perform 

 them, not because the living cell is necessarily worn out and 

 finally killed by the exercise of any ordinary vital process, but 

 because the specific functions in the economy of the cell com- 

 munity which such cells undertake to perform, involve the 



