xvi LIFE-HISTORIES 333 



vcnescence, as is shown bv its rate of metabolism, that 







being measured, as before, by the degree of suscepti- 

 bility and resistance to cyanides and the like, or by 

 Tashiro's " biometer," an extraordinarily delicate register 

 of the CO 2 output. The new individuals which arise by 

 division or budding are to some extent physiologically 

 younger than the parent. 



In the higher animals the task of rejuvenescence is 

 hopeless as far as the individual is concerned. The evil 

 day may be staved off, but arrears accumulate, and pro- 

 cesses of senescence gain on processes of rejuvenescence. 

 It is interesting to notice, however, that Child finds some 

 evidence that the early development stages of a number 

 of animal types, before bodily specialisation sets in, are 

 conspicuously young in the physiological sense. There 

 may be a rejuvenescence at the beginning of the 

 individual life which keeps the race young while the 

 individuals pass away. 



Death.- -There are three chief kinds of death (a] vio- 

 lent or extrinsic, (b) parasitic or microbic, and (c) natural. 

 (a) A very large number of animals seem in the ordinary 

 course of nature to die a violent or extrinsic death. Thev 



/ 



are devoured bv their enemies or killed bv vicissitudes in 



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their environment, often while still in their prime. 



(b) When an animal enters a new habitat, or when it 

 comes into association with a novel source of infection, 

 it is often fatally attacked by some microbe or by some 

 larger parasite, to which it is unaccustomed, and to 

 which it can offer no internal resistance in the way of 

 counteractive chemical substances (antibodies) or other- 

 wise. Thus we know how man and his stock are often 

 fatally attacked by new parasites in tropical countries. 

 This is well illustrated in the case of the trypano- 

 somes transferred by tse-tse flies from wild animals, 

 which have become accustomed to them, to man and 

 cattle where successful resistance can only be attained 

 artificially. 



ml 



(c) Natural death is more or less successfully evaded by 

 the Protozoa, which have no " body ' to keep up, and 

 can continually recuperate their wear and tear, which 



