I.] THE DURATION OF LIFE. 27 



The objection might perhaps be raised that, if the parent 

 animal does not exactly die, it nevertheless disappears as an 

 individual. I cannot however let this pass unless it is also 

 maintained that the man of to-day is no longer the same indi- 

 vidual as the boy of twenty years ago. In the growth of man, 

 neither structure nor the components of structure remain 

 precisely the same ; the material is continually changing. If 

 we can imagine an Amoeba endowed with self-consciousness, 

 it might think before dividing ' I will give birth to a daughter,' 

 and I have no doubt that each half would regard the other as 

 the daughter, and would consider itself to be the original parent. 

 We cannot however appeal to this criterion of personality in 

 the Amoeba, but there is nevertheless a criterion which seems 

 . to me to decide the matter : I refer to the continuity of life in 

 the same form. 



Now if numerous organisms, endowed with the potentiality 

 of never-ending life, have real existence, the question arises as 

 to whether the fact can be understood from the point of view of 

 utility. If death has been shown to be a necessary adaptation 

 for the higher organisms, why should it not be so for the lower 

 also ? Are they not decimated by enemies ? are they not often 

 imperfect ? are they not worn out by contact with the external 

 world ? Although they are certainly destroyed by other animals, 

 there is nothing comparable to that deterioration of the body 

 which takes place in the higher organisms. Unicellular animals 

 are too simply constructed for this to be possible. If an in- 

 fusorian is injured by the loss of some part of its body, it may 

 often recover its former integrity, but if the injury is too great 

 it dies. The alternative is always perfect integrity or complete 

 destruction. 



We may now leave this part of the subject, for it is obvious 

 that normal death, that is to say, death which arises from 

 internal causes, is an impossibility among these lower organ- 

 isms. In those species at any rate in which fission is accom- 

 panied by a circulation of the protoplasm of the parent, the two 

 halves must possess the same qualities. Since one of them is 

 endowed with a potentiality for unending life, and must be so 

 endowed if the species is to persist, it is clear that the other 

 exactly similar half must be endowed with equal potentiality. 



Let us now consider how it happened that the multicellular 



