FERTILIZATION IN PLANTS AND UNICELLXJLAR ORGANISMS 335 



ovum, but in the modified form of the ' rejuvenating ' power of 

 amphimixis. 



Quite recently an attempt lias been made to modify the idea of 

 the ' rejuvenating ' effect of amphimixis so that it should mean onlj^ 

 an advantage, not an actual condition of persistence. Hartog, in 

 particular, admits so much, that the occurrence of purely asexual and 

 purely parthenogenetic reproduction excludes the possibility of our 

 regarding the process of amphimixis as a condition of the maintenance 

 of life. But then we must also cease to regard the ' ageing ' and 

 dying off of Infusorians which have been prevented from conjugating 

 as an outcome of the primary constitution of the living substance, and 

 should entirely abandon the misleading expression ' rejuvenescence.' 



If we fix our attention on the numberless kinds of cells in higher 

 organisms and on multicellular organisms as intact unities, we see 

 that they all die off, that they are subject to a natural death, that is, 

 a cessation of vital movement from internal causes, yet no one is likely 

 to refer their transitoriness to the fact that they do not enter into 

 amphimixis. We find it quite ' intelligible ' that the cells of our body 

 should be used up sooner or later as a result of their own function, 

 though we are very far from being able to demonstrate the necessity 

 for this, and so really to ' understand ' it. 



It is only from the standpoint of utility that we can understand 

 the occurrence of natural death ; we see that the germ-cells must be 

 potentially immortal like the unicellular organisms, but that the cells 

 which make up the tissues of the body may be transient, and indeed 

 mud be so in the interests of their differentiation — often great and in 

 one direction — which determines the services they render to the body. 

 They required to become so differentiated that they could not con- 

 tinue to live on without limit, and they did become so differentiated 

 because only thus could an ever-increasing functional capacity of the 

 whole organism be rendered possible ; but they die not because ' re- 

 juvenescence through amphimixis is denied them, but because their 

 physical constitution is what it is.' And we must explain the death 

 of the whole many-celled individual in a similar way. When we 

 were trying in a previous study to establish the unlimited continuance, 

 the potential immortality, of unicellular organisms, we noted that an 

 eternal continuance of the life of the body of multicellular organisms 

 could certainly not be a necessity, since the continuance of these 

 forms of life is secured by their germ-cells. A continuance of the 

 body cannot even be regarded as useful from any point of view. 

 And what is not useful for a form of life does not arise as a lading^ 

 adaptation, which is of course not to say that an immortality of 



