EX VIRIBUS VIVJMUS. 



on them in early stages of existence. Thus from 

 great destruction of young the average longevity 

 may be brought very low, and not indicate directly 

 at all the potential longevity. It is clear that a very 

 high potential longevity will materially raise the 

 average longevity, whilst a low one will somewhat 

 diminish it ; on the other hand, the chances of life 

 may be the better in each individual of the survivors 

 from the fact that the average longevity has been 

 lessened by the destruction of numbers of the weaker 

 and 'unhealthy among the young. It is clear that 

 the subject of mortality is so distinct from that of 

 longevity that it cannot enter largely into considera- 

 tion on the present occasion. 



Whilst we have fixed terms to give us the means 

 of comparing average longevities, what have we that 

 corresponds in the case of potential longevity.' 



This matter has not been fixed by any authority, 

 even in the case of man, who is indeed the only 

 animal of which there are sufficient facts known to 

 enable one to use in any way such a definite indica- 

 tion of potential longevity for comparison. Statis- 

 ticians frame tables for various groups and classes of 

 men, in which the probable ^ after-lifetime or expec- 

 tation of life is calculated for any given age. The 

 expectation of life at birth obviously indicates the 



• Mr. Neison has proposed rather to compare the lifetime, of which 

 there is an equal chance, at different ages, but this proposal has not 

 been made use of. 



