20 EX VIRIBUS VIVIMUS. 



mice, bees, and buttercups live for different periods 

 of time, is matter of experience, and not only this, 

 but all men do not live equally long, nor all cats, nor 

 all bees and flowers. Hence every individual has its 

 own longevity, if we understand that term to mean 

 duration of life. On looking a little further, we 

 readily discover that there is a closer agreement as 

 to duration of life (though we cannot deal with accu- 

 rate numbers) between the individuals of the same 

 species than there is between the individuals of dif- 

 ferent species ; and though the individuals of the 

 same species exhibit great variation in their length 

 of life, yet there is a probable duration which cha- 

 racterizes the species, and is the same therefore for 

 all the individuals. We thus, then, have individual 

 and specific longevity. But when we try to form 

 some more definite notion of this ' specific longevity,' 

 great difficulties have to be encountered. 



By 'specific longevity' we may mean the average 

 longevity of the individuals of a species, that is, the 

 average duration of life of all the individuals born ; 

 and had we data for various organisms as we have 

 for some groups of mankind, we should speak of this 

 period as the expectation of life at birth, and could 

 assign to it a fixed quantity, as is done for men. On 

 the other hand, a very different term is that which 

 we usually speak of as the ' longevity ' pf this or that 

 race, family, or species. Howsoever ignorant we are 

 of numbers in this matter, though it is even difficult 



