64 STRUGGLE FOR EXISTENCE. 



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 be- 

 tween 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 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 immaterial 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 enti- 

 tled 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 under- 

 standing how species arise in nature. How have all those 

 exquisite adaptations of one part of the organization to an- 

 other part, and to the conditions of life, and of one organic 

 being to another being, been perfected ? We see these beau- 

 tiful coadaptations most plainly in the woodpecker and the 

 mistletoe ; and onty a little less plainly in the humblest par- 

 asite 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 gen- 

 tlest breeze ; in short, we see beautiful adaptations eveiy 

 where and in every part of the organic world. 



Again, t 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 



