44 STRUGGLE FOR EXISTENCE. [CHAP. III. 



amount of difference considered necessary to give to any two forms 

 the rank of species cannot be defined. In genera having more than 

 the average number of species in any country, the species of these 

 genera have more than the average number of varieties. In large 

 genera the species are apt to be closely, but unequally, allied 

 together,, forming little clusters round other species. Species very 

 closely allied to other species apparently have restricted ranges. 

 In all these respects the species of large genera present a strong 

 analogy with varieties. And we can clearly understand these 

 analogies, if species once existed as varieties, and thus originated : 

 whereas, these analogies are utterly inexplicable if species are 

 independent creations. 



We have, also, seen that it is the most flourishing or dominant 

 species of the larger genera within each class which on an average 

 yield the greatest number of varieties ; and varieties, as we shall 

 hereafter see, tend to become converted into new and distinct 

 species. Thus the larger genera tend to become larger ; and 

 throughout nature the forms of life which are now dominant tend 

 to become still more dominant by leaving many modified and 

 dominant descendants. But by steps hereafter to be explained, 

 the larger genera also tend to break up into smaller genera. And 

 thus, the forms of life throughout the universe become divided into 

 groups subordinate to groups. 



CHAPTER III. 

 STRUGGLE FOR EXISTENCE. 



Its bearing on natural selection The terra used in a wide sense Geometrical 

 ratio of increase Rapid 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 

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

 that amongst organic beings in a state of nature there is some indir 

 vidual 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 entitled to hold, if the existence of any well-marked varieties 



