AN ESSA V ON LONGEVITY. 19 



looked upon as abnormal, and not credited with a 

 false significance. The life of a dried Rotifer is 

 rather potential or latent life than efficient life, and 

 it is with efficient life that our definition of longevity 

 deals. 1 Doubtless efficient life, by approximating 

 more closely, from time to time, or continuously, to 

 this merely potential life, has its term extended in 

 various cases ; '^ but the normal and natural course 

 of events must not be confounded with what is 

 abnormal and accidental. With these qualifications, 

 longevity may be defined as the length of duration 

 of life of an individual. With this meaning of the 

 term we may now endeavour to see how the longevity 

 of various organisms may be compared. 



4. The various kinds of Longevity. 



Great as is our ignorance.with regard to longevity 

 in all that relates to accuracy and detail, yet there are 

 a few patent facts within everyone's experience which 

 it is well to consider at once. Firstly, various indi- 

 viduals enjoy various durations of life. That men, cats, 



' Professor Owen remarks in a recent note (' Monthly Microsc. Journal,' 

 vol. i. p. 294) — 'There are organisms (Vibrio, Rotifer, Macrobiotus, &c.) 

 which we can devitalize and revitalize — devive and revive — many times. 

 As the dried animalcule manifests no phenomenon suggesting any idea 

 contributing to form the complex one of " Ufe " in my mind, I regard it 

 to be as completely lifeless as is the drowned man whose breath and 

 heat have gone, and whose blood has ceased to circulate.' 



° Ex. gr. Parasitic worms which become encysted : hybernation ap- 

 proaches this condition in great measure. 



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