AN ESSAY ON LONGEVITY. 15 



A. Longevity in Organisms generally. 



3. Longevity defined. 



It is very necessary to have a clear perception of 

 the meaning of the principal terms involved in the 

 consideration of the duration of life. By ' longevity ' 

 must be understood the length of time during which 

 life is exhibited in an individual.^ Unless we intro- 

 duce the term ' individual,' and assign to it a definite 

 meaning, we become involved in numerous difficulties 

 when making a comparison of the length of life in 

 different species of organic beings. However im- 

 portant in a zoological sense the definition of 'in- 

 dividual' may be which regards the various forms and 

 existences appertaining to a species between ovum 

 and ovum, as the individual of that species, for 

 physiological purposes, such a definition cannot be 

 accepted. The whole product of a fertilized germ, 

 whilst it no doubt, in many cases, agrees with all 

 requirements as a definition of the individual of a 

 species, is yet, in many other cases, open to much 



' Though longevity is thus limited on the present occasion, in ac- 

 cordance with the ordinary usage of the term, it is only right to remind 

 the reader that there is a vastly important and most interesting aspect of 

 longevity which has been but little written on or considered as yet, and 

 which it is to be regretted we cannot now enter upon. The longevity of 

 rac6s and species (as such) is the subject to which allusion is made : in 

 connection with the Darwinian theory and struggle for existence, the 

 longevity of species will prove a most fertile field of research. 



