DISHARMONIES AND OTHER SHADOWS 591 



after early stages in development. We do not get any ad- 

 ditions to our nerve-cells after birth. But why might not 

 nerve-cells have retained the power of regeneration that they 

 have in some of the lower animals ? 



A reason for old age and natural death has been found 

 in the slow accumulation of poisonous waste-products, of the 

 results of incomplete combustion, of the results of bacterial 

 activity, and so on. The fire of life may be smothered in its 

 own ashes. But it must be recognised that there is no neces- 

 sity for this, that we can conceive of more perfect arrange- 

 ments for purification. Isolated pieces of tissue can be kept for 

 a long time living if waste-products are carefully eliminated. 



Similarly it has been pointed out that ageing is associated 

 with the diminishing activity of glands of internal secretion, 

 with a cumulative disproportion between cytoplasm (cell- 

 substance) and nucleoplasm, with the occurrence of organi- 

 cally expensive modes of reproduction, and so on. But these 

 suggestions seem to disclose what are merely symptoms of 

 some more fundamental imperfection. 



What that is may be discovered by asking whether it is 

 really the case that all living creatures grow old and die. 

 We know that an insect may live for days, another for weeks, 

 another for months; that a fish may live for years, man 

 for scores of years, and a Big Tree for centuries; but are 

 there any creatures that need not die ? It seems that natural 

 death is more or less successfully evaded by most of the 

 Protozoa, which, being unicellular or non-cellular, have no 

 1 body ' to keep up, which have very inexpensive modes 

 of multiplication, which can continually recuperate their 

 wear and tear. There is good reason to suspect that the 

 same is true of multicellular animals like Hydra and Plana- 

 rian worms. 



