Chap. II. 
EESEMBLE VABIETIES. 
59 
difference, for two forms, if differing yeiy 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 
or 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. 
