538 DR, p. CHALMERS MITCHELL ON LONGEVITY AND 



attain so great an age. Nor does the coirelation exist between 

 size and viability, for if the same sets of birds be compared, it 

 will be seen that the average expectation of life is much 

 better in the case of the smaller birds. If, instead of absolute 

 correlation, relative correlation be considered, the case is more 

 striking. An Ostrich must be several hvmdred times larger and 

 heavier than many birds which could ovxtlive it, and which on 

 the average do outlive it. If the comparison between birds and 

 mammals betaken, it is equally clear that, apart from such a case 

 as that of the whale for which a very great age is assumed, and 

 that of man, there are many birds with potential longevities 

 equal to those of the longest-lived mammals. Moreover, if the 

 two Classes be compared from the point of view of longevity in 

 proportion to size, the result is still more striking. On the 

 whole, group by group, mammals are much larger than birds, her- 

 bivorous mammals than herbivorous birds, frugivorous mammals 

 than frugivorous birds, omnivorous or carnivorous mammals than 

 omnivorous or carnivorous birds. And yet, group by group, if the 

 figures be compared, it becomes more and more obvious that birds 

 approach and often surpass mammals in longevity and viabilitj^ 

 Metchnikoff has already called attention to this difference and 

 has associated it with the anatomical fact that the most striking 

 difference between the alimentary tracts of mammals and birds 

 is in the greater relative length, complexity and capacity of the 

 hind-gut in the former group. The facts that mammals with 

 relatively reduced and uncapacious hind-guts and caeca (such as 

 the Oarnivora) tend to have longer lives in proportion to their 

 size, and that birds like the Struthious birds, which have relatively 

 long and capacious hind-guts and cfeca, have short lives in 

 proportion to their size, still further confirm and support 

 Metchnikoii''s general proposition. 



Within certain groups of birds the coiTelation between size and 

 potential longevity appears to exist. It can be traced with more 

 or less clearness in the case of Eagles, Owls, Passeres, and Picarian 

 birds. But even in this limited fashion it is far from being 

 universal. Probably Swans have a higher longevity than Ducks 

 and Geese, but I cannot find any similar difierences if Geese and 

 Dvicks be compared . Kagus appear to live as long as Cranes, Ibises 

 better than Storks or Herons : there is no trace of the correlation 

 in the case of Rails, or Pigeons or Gulls, and amongst Struthious 

 birds the conditions are apparently reversed, and the Kiwi is 

 hardier than the Ostrich. 



I should hesitate to say that the differences in the constitutional 

 longevities of birds could be associated universally with the 

 structure of the alimentary canal, although there are some indi- 

 cations pointing in that direction. The Struthious birds and the 

 Screamers have large and capacious hind-guts and c^ca, and in 

 proportion to their size have very short lives. Sand-grouse 

 differ from Pigeons in having capacious cseca and have relatively 



