CHAPTER XX 



THE CROWD OP ANIMALS AND THE STRUGGLE FOR 

 EXISTENCE 



247. The crowd of animals. All animals feed upon living 

 organisms, or on their dead bodies. Hence each animal 

 throughout its life is busy with the destruction of other 

 organisms, or with their removal after death. If those 

 creatures upon which others feed are to hold their own, there 

 must be enough born or developed to make good the drain 

 upon their numbers. If the plants did not fill up their 

 ranks and make good their losses, the animals that feed 

 on them would perish. If the plant-eating animals were 

 destroyed, the flesh-eating animals would in turn disappear. 

 But, fortunately, there is a vast excess in the process of 

 reproduction. More plants sprout than can find room to 

 grow. More animals are born than can possibly survive. 

 The process of increase among animals is correctly spoken 

 of as multiplication. Each species tends to increase in 

 geometric ratio, but as it multiplies its members it finds 

 the world already crowded with other species doing the 

 same thing. A single pair of any species whatsoever, if not 

 restrained by adverse conditions, would soon increase to 

 such an extent as to fill the whole world with its progeny. 

 An annual plant producing two seeds only would have 

 1,048,576 descendants at the end of twenty-one years, if 

 each seed sprouted and matured. The ratio of increase is 

 therefore a matter of minor importance. It is the ratio of 

 net increase above loss which determines the fate of a spe- 

 cies. Those species increase in numbers whose gain exceeds 

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