AN ESSA Y ON LONGEVITY. 27 



potential longevity appears to be very nearly practi- 

 cally unlimited ; but we may suppose that it has a 

 remote limit which is difficult to observe on account 

 of its distant character. Such organisms are fish, 

 molluscs, large Crustacea, annelids, many trees and 

 sea-weeds. In other organisms, on the contrary, 

 there is distinctly observable a natural inherent limit 

 to life, which is inevitable, however carefully injurious 

 and destructive influences are kept off, which makes 

 -its approach felt with the advance of years, in that 

 state which is called ' natural decay' or ' senility.' 

 Men, other mammals and birds, some reptiles, insects, 

 some lower invertebrata, and many plants, exhibit 

 this condition of things very obviously. In some, 

 as insects, and some low worms and protozoa, the 

 action of this 'natural decay' is far more powerful 

 than it is in the other cases, and we see these crea- 

 tures dying clearly under its influence ; in others it 

 is less obvious, and hence we may suppose that in 

 the former group, where natural decay appears 

 to play no part, its apparent absence is merely a 

 matter of degree, and that it is simply reduced to 

 a minimum. 



That the time of the on-coming of this period of 

 natural decay, i. e. the limit of absolute potential 

 longevity, varies strictly and largely in different 

 species, and proportionately to the normal potential 

 longevity, is difficult of absolute proof in the absence 

 of experiment ; but it will probably be admitted 



