AN ESSA V ON LONGEVITY. 



23 



average longevity of the group, but at what period of 

 life does the expectation fairly indicate the poten- 

 tial longevity ? It might be answered at once that, 

 •as a matter of course, the highest age attained by any 

 individual of the group, that is, the greatest individual 

 longevity, is the measure of the potential longevity of 

 the group ; but we must remember, in dealing with 

 a large number of cases, not to mistake abnormal or 

 exceptional cases for normal ones, and not to base 

 conclusions for a group on such cases. In the case of 

 man, as-noted again below, this may be of less import- 

 ance, but with the various organisms of the animal 

 and vegetal kingdoms we cannot justly say that the 

 longevity proper to a species is indicated by the 

 greatest longevity attained by an individual of the 

 species. In searching for some terms to be used as 

 indicating the potential longevity where statistics are 

 available (and where they are not, guesses and estima- 

 tions based on the few existing data must take their 

 place), the probable after-lifetime of an individual, 

 when it has attained the average longevity of the 

 species, might be taken arbitrarily as fixing the 

 potential longevity of the species. But it seems better, 

 though less precise, to use the probable after-life- 

 time of an individual at that age when it has passed 

 some crisis, such as the maturity of the reproductive 

 organs, or other similar crises, as the case may be, 

 for the purpose of giving fixed terms of comparison 

 as to potential longevity. It is perhaps scarcely 



