EX VIRIBUS VIVIMUS. 



An Essay on the Comparative Longevity of 

 different species of lower animals, and 

 the longevity of man in different states 

 OF Civilization. 



I. Introductory. 



The periodic phenomena observable in organisms 

 have always a special interest for students of Nature 

 on account of the extreme obscurity of their rela- 

 tions, as well as from the practical importance which 

 they possess for mankind. There is probably but 

 small room for doubt that ultimately the various 

 recurring periods of death, of reproduction, of sleep, 

 of hybernation, of gestation, of puberty, are all re- 

 lated to or derive their origin from those great astro- 

 nomical cycles of change in the relative positions 

 of sun, moon, aiid earth — ^which we know as year, 

 month, and day. Whilst in some of the periodic 

 phenomena of organisms this relation is clear and 

 direct, in others it is obscured by the introduction 



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