THE CYCLE OF LIFE 435 



have seen, there is a definite limit of growth, which we 

 regard as the fittest size for the given organization and 

 the given conditions of hfe. Departures from the norm 

 have been persistently pruned off in the course of Natural 

 Selection. Similarly in many animals there is a normal 

 length of hfe (a potential duration of life) which is rarely 

 exceeded, though it may be seldom attained. Many of the 

 facts in regard to unusual length of life refer to animals 

 in captivity, and it is quite hkely that a creature may 

 survive longer in a sheltered Uf e than when it is subject to 

 the struggle for existence. On the other hand, the dura- 

 tion of life in captivity can hardly lead us to over-estimate 

 the potential duration of hfe in nature, since the artificial 

 conditions are bound to be less wholesome. The facts in 

 regard to captive animals tell us that the creatures can 

 hve to such and such an age ; but this may be far above 

 their average length of hfe. It is very unhkely that many 

 wild parrots approach the century which is their potential 

 longevity. In the case of domestic animals, few fowls are 

 allowed to survive for five years, though they might hve 

 for a score; few cattle are allowed to reach the end of 

 their tether, which is about thirty ; and just the same 

 applies to the average length of hfe in Nature, since most 

 wild animals come to a violent end. 



Dr. Chalmers Mitchell's critical revision of the data 

 available in regard to the duration of hfe in mammals 

 and birds goes to show that most of the previous estimates 

 have been too high. Though a hundred years may be the 

 probable hmit for the elephant, twenty to thirty years is a 

 fair average duration. A polar bear hved to thirty-three 

 years in the Zoo. The potential longevity of Uons is between 

 thirty and forty years ; that of some of the largest Ungu- 



