82 GEOMETKICAL KATIO OF IXCEEASE. [Chap. III. 



tlie world. One fly deposits hundreds of eggs, and 

 another, like the hippobosca, a single one ; bnt this 

 difference does not determine how many individuals of 

 the two species can be supported in a district. A large 

 number of eggs is of some importance to those species 

 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 suffice 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 ensured 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 ecfors or seeds. 



CO 



In looking at Xature, 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 destruction 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. 



