SUMMARY 75 



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 varie- 

 ties. 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 appar- 

 ently have restricted ranges. In all these respects the spe- 

 cies 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 dom- 

 inant species of the larger genera within each class which on 

 an average yield the greatest number of varieties ; and varie- 

 ties, 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. 



