III.] LIFE AND DEATH, 



141 



the physical constitution necessary for immortality, but not the 

 other; just as it is conceivable that such a cell— adapted for 

 unending life— might bud off a small part, which would live 

 a long time without the full capabilities of life possessed by the 

 parent cell ; again, it is possible that such a cell might extrude 

 a certain amount of organic matter (a true excretion) which is 

 already dead at the moment it leaves the body. Thus it is 

 possible that true unequal cell-division, in which only one half 

 possesses the condition necessary for increasing, may take 

 place ; and in the same way it is conceivable that the con- 

 stitution of a cell determines the fixed duration of its life, 

 examples of which are before us in the great number of cells 

 in the higher Metazoa which are destroyed by their functions. 

 The more specialized a cell becomes, or in other words, the 

 more it is intrusted Vv^ith only one distinct function, the more 

 likely is this to be the case : who then can tell us, whether the 

 limited duration of life was brought about in consequence of 

 the restricted functions of the cell or whether it was determined 

 by other advantages ^ ? In either case we must maintain that 

 the disadvantages arising from a limited duration of the cells 

 are more than compensated for by the advantages which result 

 from their highly effective specialized functions. Although no 

 one of the functions of the body is necessarily attended by the 

 limited duration of the cells which perform it, as is proved b}' 

 the persistence of unicellular forms, yet any or all of them 

 might lead to such a limitation of existence without in any way 

 injuring the species, as is proved by the Metazoa. But the 

 reproductive cells cannot be limited in this way, and they 

 alone are free from it. They could not lose their immortality, 

 if indeed the Metazoa are derived from the immortal Protozoa, 

 for from the very nature of that immortality it cannot be lost. 

 From this point of view the body, or soma, appears in a certain 



^ The problem is very easily solved if we seek assistance from the 

 principle of panmixia developed in the second essay 'On Heredity.' 

 As soon as natural selection ceases to operate upon any character, 

 structural or functional, it begins to disappear. As soon, therefore, 

 as the immortaHty of somatic cells became useless they would begin to 

 lose this attribute. The process would take place more quickly, as the 

 histological differentiation of the somatic cells became more useful and 

 complete, and thus became less compatible with their everlasting 

 duration.— A. W., 1888. 



