AN ESSA Y ON LONGEVITY. 41 



how death may be brought about by mechanical 

 causes and external agencies in those organisms 

 whose period of natural decay is very remote. There 

 is of course the chance of accident, which is greater 

 in a long life than a short one. But there are two 

 examples of self-adjusting, or rather self-destroying 

 tendency in the organism, to which allusion may be 

 made. Trees increasing in size as they grow older, 

 expose a larger surface to the wind, whilst the roots 

 cannot penetrate beyond the limited soil ; they thus 

 are more liable to get blown over year by year. 

 Again, increasing as they do and being stationary in 

 position, they encroach on each other's area, and 

 exhaust the limits of soil and space by their united 

 action, what is enough for one being not enough 

 for five or six. In the case of animals, the same 

 mechanical limit appears ; where the food is diffused 

 and taken in numerous but small mouthfuls (i. e. as 

 in herbivorous and scavenger animals, not praedaceous 

 animals), five small mouths will be more efficient in 

 supporting five pounds of an animal than one big 

 one. It is thus that the Maori fly is expelled by 

 the smaller European house-fly. It is thus that 

 large fish, large molluscs, large Crustacea of species 

 with diffuse food receive a limit to their life. The 

 greater danger of all kinds involved in increased 

 surface also tends to limit life in such organisms. 



We have yet to ask how the exact or approximate 



