140 LIFE AND DEATH. 



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 l ? 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 ne- 

 cessarily attended by the limited duration of the cells which per- 

 form it, as is proved by 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 sense as a secondary appendage of the real bearer of 

 life, the reproductive cells. 



Just as it was possible for the specific somatic cells to be differen- 

 tiated from among the chemico-physical variations which presented 

 themselves in the protoplasm, by means of natural selection, until 

 finally each function of the body was performed by its own special 

 kind of cell ; so it might be possible for only those variations to 

 persist the constitution of which involved a cessation of activity 

 after a certain fixed time. If this became true of the whole mass 

 of somatic cells, we should then meet with natural death for the 

 first time. Whether we ought to regard this limitation of the 

 life of the specific somatic cells as a mere consequence of their 

 differentiation, or at the same time as a consequence of the powers 

 of natural selection especially directed to such an end, appears 

 doubtful. But I am myself rather inclined to take the latter view, 

 for if it was .advantageous to the somatic cells to preserve the un- 

 ending life of their ancestors the unicellular organisms, this end 



1 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 selec- 

 tion ceases to operate upon any character, structural or functional, it begins to dis- 

 appear. As soon, therefore, as the immortality 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 com- 

 plete, and thus became less compatible with their everlasting duration. A.W. 1888. 



