116 



George A. Sacher 



captive species (Comfort, 1956, 1958). Second, when data 

 come from animals kept under very different environmental 

 conditions the lifespan is a more stable longevity parameter 

 than is life expectation. This is clearly seen in the life-tables 



1 0,000 c — r 



o 

 o 

 o 



o" 



q: 

 ui 



Q. 



1000 



100 



10 



Thoroughbre,d MarftS 

 A. Comfort, J. Ge^otitol 

 13:342-350, 1958 



X 



12 



16 

 AGE, 



20 

 years 



24 



28 



32 



Fig, la. Logarithm of age-specific rate of mortality 

 plotted versus age (Gompertz plot). Data on thorough- 

 bred mares from Comfort (1958). A life-table for 

 which the Gompertz plot has a straight-line relation 

 to age conforms to the Gompertz equation qx = Ae'^^. 



of human populations in different countries or in the same 

 country in different historical periods. Instances can be found 

 in which life expectations vary by more than a factor of two, 

 but even in these extreme cases the lifespans do not differ by 



