CHAP. II. RESEMBLE VARIETIES. 59 



difference, for two forms, if differing very little, are 

 generally ranked as varieties, notwithstanding that inter- 

 mediate linking forms have not been discovered; but 

 the amount of difference considered necessary to give 

 to two forms the rank of species is quite indefinite. 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 certain species. 

 Species very closely allied to other species apparently 

 have restricted ranges. In all these several respects the 

 species of large genera present a strong analogy with 

 varieties. And we can clearly understand these analo- 

 gies, if species have once existed as varieties, and have 

 thus originated : whereas, these analogies are utterly 

 inexplicable if each species has been independently 

 created. 



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

 and dominant species of the larger genera which on an 

 average vary most ; and varieties, as we shall hereafter 

 see, tend to become converted into new and distinct 

 species. The larger genera thus tend to become larger ; 

 and throughout nature the forms of life which are now 

 dominant tend to become still more dominant by leav- 

 ing 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. 



