STRUGGLE FOR EXISTENCE 1471 



So that, in all cases, the average number of any ani- 

 mal 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 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. 



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, by so much will it tend to increase still 

 further. We know not exactly what the checks are 

 even in a single instance. Nor will this surprise any 

 one who reflects how ignorant we are on this head, 

 even in regard to mankind, although so incom- 

 parably better known than any other animal. Eggs 

 or very young animals seem generally to suffer most, 

 but this is not invariably the case. With plants there 

 is a vast destruction of seeds, but, from some observa- 

 tions which I have made, it appears that the seedlings 

 suffer most from germinating in ground already 

 thickly stocked with other plants. Seedlings, also, 

 are destroyed in vast numbers by various enemies; 

 for instance, on a piece of ground three feet long 

 j two wide, dug and cleared, and where there 



