AN ESS A Y ON LONGEVITY. 



to define what are the limits of the period to which 

 we refer, yet, in speaking of ' longevity ' of groups of 

 beings, we usually mean the potential longevity — or 

 'lease of life,' as Mr. Grindon terms it — and do not 

 allow the average longevity, affected as it is by 

 disease and accident at all periods of life, to enter 

 into our consideration. 



The term ' mortality' is usually applied to the ques- 

 tion of average longevity, and hence, in accordance 

 with general convention, longevity may be under- 

 stood to refer to potential longevity. Once for all, 

 here it may be pointed out how slightly these quan- 

 tities can affect each other, though they are to a 

 certain extent related. Mortality has been largely 

 studied in the case of man, and much more is known 

 of it than of longevity in his case ; but among animals 

 and plants generally, vastly important as mortality is 

 in regard to the necessities of life and of organisms, 

 there is as little known as in the matter of longevity. 

 That the average longevity of a group of individuals 

 is but slightly related to the potential longevity, 

 appears from these considerations. From enemies 

 preying upon the ' young ones,'^ or from disease, or 

 from a severe struggle for food; or from the accidents 

 of dispersion, vast numbers of the individuals of the 

 group may die at a very early age ; those, on the 

 other hand, which do survive, may live to a period of 

 time quite unaffected by the conditions which acted 

 * ' Young ones.' — Herbert Spencer. 



