12 THE DURATION OF LIFE. 



of over 100 years. Finally, we must not forget Humboldt's 1 

 Atur parrot from the Orinoco, concerning which the Indians said 

 that it could not be understood because it spoke the language of 

 an extinct tribe. 



It is therefore necessary to ask how far we can show that such 

 long lives are really the shortest which are possible under the 

 circumstances. 



Two factors must here be taken into consideration ; first, that 

 the young of birds are greatly exposed to destructive agencies ; 

 and, secondly, that the structure of a bird is adapted for flight and 

 therefore excludes the possibility of any great degree of fertility. 



Many birds, like the stormy petrel, the diver, guillemot, and 

 other sea-birds, lay only a single egg, and breed (as is usually the 

 case with birds) only once a year. Others, such as birds of prey, 

 pigeons, and humming-birds, lay two eggs, and it is only those 

 which fly badly, such as jungle fowls and pheasants, which produce 

 a number of eggs (about twenty), and the young of these very 

 species are especially exposed to those dangers which more or less 

 affect the offspring of all birds. Even the eggs of our most 

 powerful native bird of prey, the golden eagle, which all animals 

 fear, and of which the eyrie, perched on a rocky height, is beyond 

 the reach of any enemies, are veiy frequently destroyed by late 

 frosts or snow in spring, and, at the end of the year in winter, the 

 young birds encounter the fiercest of foes, viz. hunger. In the 

 majority of birds, the egg, as soon as it is laid, becomes exposed to 

 the attacks of enemies ; martens and weasels, cats and owls, buzzards 

 and crows are all on the look out for it. At a later period the 

 same enemies destroy numbers of the helpless young, and in winter 

 many succumb in the struggle against cold and hunger, or to the 

 numerous dangers which attend migration over land and sea, 

 dangers which decimate the young birds. 



It is impossible directly to ascertain the exact number which 

 are thus destroyed ; but we can arrive at an estimate -by nn 

 indirect method. If we agree with Darwin and Wallace in 

 believing that in most species a certain degree of constancy 

 is maintained in the number of individuals of successive gene- 

 rations, and that therefore the number of individuals within 

 the same area remains tolerably uniform for a certain period of 

 1 Huinboldt's 'Ansichten der Natur.' 



