62 ON THE ORIGIN OF SPECIES 



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 

 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 around us 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 in number are most obscure. Look 

 at the most vigorous species ; by as much as it swarms 

 in numbers, by so much will its tendency to increase 

 be still further increased. We know not exactly what 

 the checks are in even one single instance. Nor will 

 this surprise any one who reflects how ignorant we are 

 on this head, even in regard to mankind, so incompar- 

 ably better known than any other animal. This subject 

 has been ably treated by several authors, and I shall, 

 in my future work, discuss some of the checks at con- 

 siderable length, more especially in regard to the feral 

 animals of South America. Here I will make only a 

 few remarks, just to recall to the reader's mind some 

 of the chief points. 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 observations which I have made, I 

 believe that it is the seedlings which suffer most from 

 germinating in ground already thickly stocked with 

 other plants. Seedlings, also, are destroyed in vagi 



