72 The Outline of Science 



but if the host is out of condition the parasites may get the upper 

 hand, as in the so-called "grouse disease," and become fatal, (c) 

 But besides violent death and microbic (or parasitic) death, there 

 is natural death. This is in great part to be regarded as the price 

 paid for a body. A body worth having implies complexity or 

 division of labour, and this implies certain internal furnishings of 

 a more or less stable kind in which the effects of wear and tear 

 are apt to accumulate. It is not the living matter itself that 

 grows old so much as the framework in which it works the 

 furnishings of the vital laboratory. There are various processes 

 of rejuvenescence, e.g. rest, repair, change, reorganisation, which 

 work against the inevitable processes of senescence, but sooner or 

 later the victory is with ageing. Another deep reason for natural 

 death is to be found in the physiological expensiveness of repro- 

 duction, for many animals, from worms to eels, illustrate natural 

 death as the nemesis of starting new lives. Now it is a very 

 striking fact that to a large degree the simplest animals or 

 Protozoa are exempt from natural death. They are so relatively 

 simple that they can continually recuperate by rest and repair; 

 they do not accumulate any bad debts. Moreover, their modes of 

 multiplying, by dividing into two or many units, are very inex- 

 pensive physiologically. It seems that in some measure this 

 bodily immortality of the Protozoa is shared by some simple 

 many-celled animals like the freshwater Hydra and Planarian 

 worms. Here is an interesting chapter in evolution, the evolution 

 <!' means of evading or staving off natural death. Thus there is 

 the well-known case of the Paloloworm of the coral-reefs where 

 the body breaks up in liberating the germ-cells, but the head-end 

 remains fixed in a crevice of the coral, and buds out a new body 

 at leisure. 



Along with the evolution of the ways of avoiding death 

 should be considered also the gradual establishment of the length 

 of life best suited to the welfare of the species, and the punctua- 

 tion of the life-history to suit various conditions. 



