THE DURATION OF LIFE. 11 



through the operation of these agencies than by natural death. 

 There are thousands of species of which the existence depends upon 

 the destruction of other species ; as, for example, the various kinds 

 of fish which feed on the countless minute Crustacea inhabiting 

 our lakes. 



It is easy to see that an individual is, ceteris paribus, more ex- 

 posed to accidental death when the natural term of its life becomes 

 longer ; and therefore the longer the time required by an in- 

 dividual for the production of a sufficient number of descendants to 

 ensure the existence of the species, the greater will be the number 

 of individuals which perish accidentally before they have fulfilled 

 this important duty. Hence it follows, first, that the number of 

 descendants produced by any individual must be greater as the 

 duration of its reproductive period becomes longer ; and, secondly, 

 the surprising result that nature does not tend to secure the 

 longest possible life to the adult individual, but, on the contrary, 

 tends to shorten the period of reproductive activity as far as 

 possible, and with this the duration of life ; but these conclusions 

 only refer to the animal and not to the vegetable world. 



All this sounds very paradoxical, but the facts show that it is 

 true. At first sight numerous instances of remarkably long life . 

 seem to refute the argument, but the contradictions are only 

 apparent and disappear on closer investigation. 



Birds as a rule live to a surprisingly great age. Even the 

 smallest of our native singing birds lives for ten years, while the 

 nightingale and blackbird live from twelve to eighteen years. 

 A pair of eider ducks were observed to make their nest in the 

 same place for twenty years, and it is believed that these birds 

 sometimes reach the age of nearly one hundred years. A cuckoo, 

 which was recognised by a peculiar note in its call, was heard in 

 the same forest for thirty-two consecutive years. Birds of prey, 

 and birds which live in marshy districts, become much older, for 

 they outlive more than one generation of men. 



Schinz mentions a bearded vulture which was seen sitting on 

 a rock upon a glacier near Grindelwald, and the oldest men in 

 Grindelwald had, when boys, seen the same bird sitting on the 

 same rock. A white-headed vulture in the Schonbrunn Zoo- 

 logical Gardens had been in captivity for 118 years, and many 

 examples are known of eagles and falcons reaching an age 



