8o4 LIFE : OUTLINES OF GENERAL BIOLOGY 



or his environment to determine whether the inherited tendency is 

 realised or not. Just as stature is a heritable qualit}^ so is potential 

 longevity; but the degree of expression is in part determined by 

 "nurture" in the widest sense. 



We may usefuUy recognise four categories of phenomena in con- 

 nection with age. (i) The first is that of the immortal unicellular 

 animals which never grow old, which seem exempt from natural 

 death. (2) The second is that of many wild animals, which reach 

 the length of their life's tether without any hint of ageing, and pass 

 off the scene — or are shoved off — ^victims of violent death. In many 

 fishes and reptiles, for instance, which are old in years, there is not 

 in their organs or tissues the least hint of age-degeneration. (3) The 

 third is that of the majority of civilised human beings, some 

 domesticated animals, and some wild animals, in which the decline 

 of life is marked hy normal senescence. (4) The fourth is that of many 

 human beings, not a few domesticated animals, e.g. horse, dog, cat, 

 and some semi-domesticated animals, notably bees, in which the 

 close of life is marked by distinctly pathological senility. It seems 

 certain that wild animals rarely exhibit more than a slight senescence, 

 while man often exhibits a bathos of senility. What is the reason of 

 this? 



There is, as we have hinted, reason to believe that natural death 

 is not to be simply regarded as an intrinsic necessity — the fate of all 

 life : we can carry the analysis further, and say that it is incident on 

 the complexity of the bodily machinery, which makes complete 

 recuperation wellnigh impossible, and almost forces the organism 

 to accumulate arrears, to go into debt to itself; that it is incident on 

 the limits which are set to the multiplication and renewal of cells 

 within the body : thus the number of brain cells in higher animals 

 cannot be added to after birth ; and it is incident on the occurrence 

 of organically expensive modes of reproduction, for reproduction 

 is often the beginning of death. At the same time, it seems difficult 

 to rest satisfied with these and other physiological reasons ; and to 

 a considerable extent we have to fall back on the selectionist view 

 that the duration of life has been, in part at least, punctuated from 

 without and in reference to large issues; it has been gradually 

 regulated in adaptation to the welfare of the species. 



The majorit}^ of wild animals seem to die a violent death before 

 there is time for senescence, much less senility. The character of old 

 age depends upon the nature of the physiological bad debts, some of 

 which are more unnatural than others, much more unnatural in 

 tamed than in wild animals, much more unnatural in man than in 

 animals. Furthermore, civilised man, sheltered from the extreme 

 physical forms of the struggle for existence, can live for a long time 

 with a very defective hereditary constitution, which may end in a 

 period of very undesirable senility. Man is also very deficient in the 



