AN ESSAY ON LONGEVITY. 



from the want caused by weakness in old age, pro- 

 tract its life. Parrots, thus, live even one hundred 

 years, and goldfinches twenty- three, which there is 

 good reason to believe is far beyond their length of 

 life when in a ' state of nature.' So lions have lived 

 in menageries to be forty to sixty years old (Haller), 

 being fed after the loss of their teeth and the blunting 

 of their claws. Insects have been so kept for three 

 or four yaars ; and many plants by attention are 

 made perennial or biennial, whereas in natural con- 

 ditions they would be annual. It will probably be 

 admitted that man has this power in many cases 

 without further illustration. 



Hence we must again qualify or analyse potential 

 longevity as applied to species; for there is one 

 period which is proper to the species in its normal 

 conditions, which it cannot by any struggles of its 

 own extend, hedged in as it is by those very con- 

 ditions in relation to which it has either been created, 

 or by which it has been eyolved. There is a second 

 period which is equally proper to a species (as far 

 a~s experiments tell us), which man can make evident 

 by removing some of the natural conditions and sub- 

 stituting others, which however has its limit, beyond 

 which limit no power that is known can extend the 

 life. The first period may be called Normal Potential 

 Longevity, the second Absolute^ Potential Longevity. 



1 Absolute is used for want of a better term ; it is only ' absolute ' 

 within man's experience. 



