LIFE AND DEATH. 139 



viations presented by natural variation, that it is continually pro- 

 ducing something- new. 



If we consider the introduction of natural death in connection 

 with the foregoing- statements, we may imagine the process as 

 taking place in such a way that, with the differentiation of Hetero- 

 plastids from Homoplastids, and the appearance of division of 

 labour among the homogeneous cell-colonies, natural selection not 

 only operated upon the physiological peculiarities of feeding, moving, 

 feeling, or reproduction, but also upon the duration of the life of 

 single cells. At this developmental stage there would, at any rate, 

 be no further necessity for maintaining the power of limitless 

 duration. The somatic cells might therefore assume a constitution 

 which excluded the possibility of unending life, provided only that 

 such a constitution was advantageous for them. 



It may be objected that cells of which the ancestors possessed 

 the power of living for ever, could not become potentially mortal 

 (that is subject to death from internal causes) either suddenly or 

 gradually, for such a change would contradict the supposition which 

 attributes immortality to their ancestors and to the products of their 

 division. This argument is valid, but it only applies so long as 

 the descendants retain the original constitution. But as soon as 

 the two products of the fission of a potentially immortal cell ac- 

 quire different constitutions by unequal fission, another possibility 

 arises. Now it is conceivable that one of the products of fission 

 might preserve 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 pos- 

 sessed by the parent cell ; again, it is possible that such a cell 

 might extrude a certain amount of organic matter (a true excre- 

 tion) 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 constitution 

 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 spe- 

 cialized a cell becomes, or in other words, the more it is intrusted 

 with only one distinct function, the more likely is this to be the 



