STRUGGLE FOR EXISTENCE 105 



which depend on a fluctuating amount of food, for it 

 allows them rapidly to increase in number. But the real 

 importance of a large number of eggs or seeds is to 

 make up for much destruction at some period of life; 

 and this period in the great majority of cases is an early 

 one. If an animal can in any way protect its own eggs 

 or young, a small number may be produced, and yet the 

 average stock be fully kept up; but if many eggs or 

 young are destroyed, many must be produced, or the 

 species will become extinct. It would sufl&ce to keep 

 up the full number of a tree, which lived on an average 

 for a thousand years, if a single seed were produced once 

 in a thousand years, supposing that this seed were never 

 destroyed, and could be insured to germinate in a fitting 

 place. So that, in all cases, the average number of any 

 animal or plant depends only indirectly on the number 

 of its eggs or seeds. 



In looking at Nature, it is most necessary to keep the 

 foregoing considerations always in mind — never to forget 

 that every single organic being may be said to be striving 

 to the utmost to increase in numbers; that each lives by 

 a struggle at some period of its life; that heavy destruc- 

 tion inevitably falls either on the young or old, during 

 each generation or at recurrent intervals. Lighten any 

 check, mitigate the destruction ever so little, and the 

 number of the species will almost instantaneously increase 

 to any amount. 



Nature of the Checks to Increase 



The causes which check the natural tendency of each 

 species to increase are most obscure. Look at the most 

 vigorous species; by as much as it swarms in numbers, 



