ON THE ORIGIN OF DEATH. 12/ 



the partridge's two thousand ; therefore the partridge 

 produces ten times as many young as the eagle, and 

 it is safe to say that the partridge has ten times as 

 many enemies. If the life of either were shortened, 

 the race would die out unless the power of reproduc- 

 tion were increased or the struggle for existence became 

 less severe. 



Many sea-birds, such as the petrel, auk, and gannet, 

 lay but one or at the most two eggs a year. Any one 

 who visits a locality where these birds breed must be 

 struck with the enormous number of eggs or young 

 which are destroyed. The eggs are often laid on the 

 bare rock on projecting ledges of a cliff, so that the 

 slightest movement will precipitate them to the beach 

 below. Every disturbance among the breeding birds 

 is marked by a small avalanche of eggs or young, so 

 that the beach below is strewn with broken eggs and 

 mangled remains. If these birds were not long-lived 

 they would soon become extinct. Now all of these 

 birds live much longer than mammals of a much larger 

 size. The lion lives thirty-five years, the sheep fifteen, 

 the fox fourteen, the squirrel or mouse about six. Most 

 of these animals are much more fertile than the birds, 

 and the young are much less exposed to dangers. The 

 bird's egg is exposed from the time it is laid, while 

 the young mammal is protected during its development. 



Only the very largest of the mammals, such as the 

 whale, the elephant, and possibly the rhinoceros, live as 

 long as these birds. The elephant may live for a 

 hundred or perhaps a hundred and fifty years, and 

 reaches maturity when about thirty. A pair produce 

 but a single calf about every ten years ; hence, during 



