NATURE OF THE CHECKS TO INCREASE. 59 



of food, for it allows them rapidly to increase in numbers. 

 But the real importance of a large number of eggs or seed 

 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 num 

 ber 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 destruction 

 inevitably falls either on the young or old during each gen- 

 eration or at recurrent intervals. Lighten any check, miti- 

 gate 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, 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 incomparably better known than any other animal. 

 This subject of the checks to increase has been ably treated 

 by several authors, and I hope in a future work to discuss 

 it at considerable 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 



