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CHAPTER III. 



STRUGGLE FOR EXISTENCE. 



Its bearing on natural selection — The term used in a wide sense — 

 Geometrical ratio of increase — Rapid increase of naturalized 

 animals and plants — Nature of the checks to increase — Com- 

 petition universal — Effects of climate — Protection from the 

 number of individuals^ — Complex relations of all animals and 

 plants throughout nature — Struggle for life most severe between 

 individuals and varieties of the same species: often severe 

 between species of the same genus— The relation of organism 

 to organism the most important of all relations. 



Before entering on the subject of this chapter I must 

 make a few preliminary remarks to show how the struggle 

 for e^xistence bears on natural selection. It has been seen 

 in the last chapter that among organic beings in a state of 

 nature there is some individual variability: indeed I am 

 not aware that this has ever been disputed. It is imma- 

 terial for us whether a multitude of doubtful forms be 

 called species or sub-species or varieties; what rank, for 

 instance, the two or three hundred doubtful forms of 

 British plants are entitled to hold, if the existence of any 

 well-marked varieties be admitted. But the mere exist- 

 ence of individual variability and of some few well-marked 

 varieties, though necessary as the foundation for the work, 

 helps us but little in understanding how species arise in 

 nature. How have all those exquisite adaptations of one 

 part of the organization to another part, and to the condi- 

 tions of life and of one organic being to another being, 

 been perfected? We see these beautiful co-adaptations 

 most plainly in the woodpecker and the mistletoe; and only 

 a little less plainly in the humblest parasite which clings to 

 the hairs of a quadruped or feathers of a bird; in the 

 structure of the beetle which dives through the water; in 

 the plumed seed which is wafted by the gentlest breeze; in 



