^g Struggle for Existence. Chap. hi. 



CHAPTEE III. 



Struggle for Existence. 



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

 trical ratio of increase — Kapid increase of naturalised animals and 

 plants — Nature of the checks to increase — Competition 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 existence hears 

 on Natural Selection. It has been seen in the last chapter that 

 amongst 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 immaterial for us whether a multitude of doubtful forms he 

 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 existence 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 organisation to another part, and to the conditions 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 misletoe ; 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 short, we 

 see beautiful adaptations everywhere and in every part of the 

 organic world. 



Again, it may be asked, how is it that varieties, which I have 

 called incipient species, become ultimately converted into good and 

 distinct species, which in most cases obviously differ from each 

 other far more than do the varieties of the same species ? How do 

 those groups of species, which constitute what are called distinct 



