OLD AGE AND DEATH 367 



In one of his celebrated essays Weismann has sought 

 to show that it is impossible to find the key to the great 

 differences in the ages of animals by any simple reference 

 to size, or to complexity, or to rate of growth, or to rate 

 of life. " The strength of the spring which drives the 

 wheel of life does not solely depend upon the size of the 

 wheel itself, or upon the material of which it is made," 

 or upon its rate of movement. The duration of life has 

 been, in part at least, punctuated from without and in 

 reference to large issues ; it has been gradually regulated 

 in adaptation to the welfare of the species. 



It is true that large animals usually live long, and that 

 small animals are often short-lived : an elephant may 

 live two hundred years, a horse forty, a blackbird eighteen, 

 a mouse six, and many insects only a few weeks or days. 

 But then a cat or a toad may live as long as a horse 

 (forty), a pike or a carp as long as an elephant (two 

 hundred), a crayfish as long as a pig (twenty), and the 

 sea-anemone " Grannie," which died a natural death in 

 Edinburgh on 4th August 1887, was at least sixty-six years 

 old. 



Flourens hazarded the generalisation that the length 

 of life was always five times the period of growth ; but 

 the horse, for instance, matures in four years, and may 

 live to be forty or more. 



On general grounds, already hinted at, we might sup- 

 pose that very active animals wear themselves out quickly, 

 while sluggish creatures live long. Instances of trees 

 living for a millennium rise at once in the mind ; on the 

 other hand, the male ants live only for a few weeks, while 

 the queen-ant may live for thirteen years, and the workers 

 for several years. Birds are exceedingly active animals, 

 but some of them have a long life ; there are cases of 

 ravens, falcons, eagles, vultures, living half more than 



