PHASES IN THE LIFE OF THE INDIVIDUAL 679 



a real old age. Elaine l has drawn the following comparison 

 between the age of a horse and that of a man : " The first 

 five years of a horse may be considered as equivalent to the 

 first twenty years of a man ; thus, a horse of five years may be 

 comparatively considered as old as a man of twenty ; a horse 

 of ten years as a man of forty ; a horse of fifteen as a man of 

 fifty ; a horse of twenty as a man of sixty ; of twenty-five as 

 a man of seventy ; of thirty as a man of eighty ; and of thirty- 

 five as a man of ninety." 



THE DURATION OF LIFE AND THE CAUSE OF DEATH 



Weismann, in a famous essay on the duration of life, 2 and 

 Metchnikoff in his book of optimistic studies, 3 have dealt at some 

 length, but from different standpoints, with the factors which 

 determine longevity in the animal kingdom. That the duration 

 of life in the various races of animals is very variable, and that, 

 whereas some species are remarkably long-lived, others die 

 after a relatively brief existence, are facts that are known to all. 

 Both Weismann and Metchnikoff cite numerous instances of 

 longevity among animals, some of the more extreme of which 

 may be mentioned here. 



A sea-anemone belonging to the species Actinia mesembryan- 

 themum is known to have lived for sixty-six years, and to have 

 produced young, though in smaller numbers than formerly, at 

 the age of fifty-eight. Another sea-anemone of the species 

 Sagartia troglodytes, lived to be fifty years old. 4 Certain marine 

 Mollusca are said to live for as many as a hundred years. 

 Among insects there is an extraordinary variability in the 

 duration of life, some living in a condition of maturity for only 

 a few days or even hours, while others (certain Hemiptera) are 

 believed to survive for as many as seventeen years. Moreover, 

 the duration of life is sometimes very different in the two sexes, 



1 Blaine, Encyclopaedia of Rural Sports, London, 1858. 

 * Weismann, "The Duration of Life," English Translation, in Essays 

 upon Heredity, &c., 2nd Edition, Oxford, 1891. 



3 Metchnikoff, The Prolongation of Lif\ English Translation, London, 

 1907. 



4 Ashworth and Annandale, " On Some Aged Specimens of Sagartia," 

 Proc. Roy. Soc., Edin., vol. xxv., 1904. 



