APPENDIX. 61 



primitive organisms prove to us, but it has appeared as an adapta- 

 tion 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 metazoan 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, however, 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-de- 

 tritus, 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 community which such cells undertake to perform, involve the 

 death of the cells themselves. But the fact that such functions 

 have appeared, involving as they do the sacrifice of a great num- 

 ber of cells, entirely depends upon the replacement of the old 



