PHASES IN THE LIFE OF THE INDIVIDUAL 721 



animals. Smith 1 remarks that few horses live long enough to show 

 much sign of arterial degeneration. The work they perform is the 

 chief cause of their rapid decay, for their legs wear out before their 

 bodies. But, apart from this, degenerative changes in the teeth, and 

 more particularly the wearing away of the molars, prevent many 

 horses from reaching a real old age. Elaine 2 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, 3 and 

 Metchnikoff in his book of optimistic studies, 4 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 mesenibryan- 

 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. 5 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, the queen ant being known to live for several years (in 

 one case for fifteen years), whereas the male ant survives for only a 

 few weeks. 



1 Smith, loc. cit. 



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



3 Weismann, "The Duration of Life," English Translation, in Essays upon 

 Heredity, etc., 2nd Edition, Oxford, 1891. 



4 Metchnikoff, The Prolongation of Life, English Translation, London, 1907. 



5 Ashworth and Annandale, "On Some Aged Specimens of Sagartia," Proc. 

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



