VARIATION UNDER NATURE 47 



reputed species range on an average over 6.9 of the provinces into 

 which Mr. Watson has divided Great Britain. Now, in this same 

 catalogue, fifty-three acknowledged varieties are recorded, and 

 these range over 7.7 provinces; whereas, the species to which these 

 varieties belong range over 14.3 provinces. So that the acknowl- 

 edged varieties have nearly the same restricted average range, as 

 have the closely allied forms, marked for me by Mr. Watson as 

 doubtful species, but which are almost universally ranked by Brit- 

 ish botanists as good and true species. 



SUMMARY 



Finally, varieties, cannoLke distinguished, from species, — except, 

 first, by the discovery of intermediate linking forms'f and, sec- 

 ondly, by a certain indefinite amount of difference between them ; 

 for two forms, if differing very little, are generally ranked as vari- 

 eties, notwithstanding that they cannot be closely connected; but 

 the 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. 



