I.] THE DURATION OF LIFE. II 



precisely this latter point which is determined by external 

 conditions. 



There is no species of animal which is not exposed to de- 

 struction through various accidental agencies — by hunger or 

 cold, by drought or flood, by epidemics, or by enemies, whether 

 beasts of prey or parasites. We also know that these causes 

 of death are only apparently accidental, or at least that they 

 can only be called accidental as far as a single individual is 

 concerned. As a matter of fact a far greater number of indivi- 

 duals perish 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 count- 

 less minute Crustacea inhabiting our lakes. 



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

 exposed to accidental death when the natural term of its life 

 becomes longer ; and therefore the longer the time required 

 by an individual 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 fol- 

 lows, first, that the number of descendants produced by an^^ 

 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 onl}' 

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



