Chap. IV. DIVERGENCE OF CHARACTER. 125 
the extinct species lived at very ancient epochs when 
the branching lines of descent had diverged less. 
I see no reason to limit the process of modification, as 
now explained, to the formation of genera alone. If, in 
our diagram, we suppose the amount of change repre¬ 
sented by each successive group of diverging dotted lines 
to be very great, the forms marked a to ^ those 
marked h and / and those marked to m will 
form three very distinct genera. We shall also have two 
very distinct genera descended from (I) ; and as these 
latter two genera, both from continued divergence of 
character and from inheritance from a different parent, 
will differ widely from the three genera descended from 
(A), the two little groups of genera will form two distinct 
families, or even orders, according to the amount of 
divergent modification supposed to be represented in the 
diagram. And the two new families, or orders, will have 
descended from two species of the original genus; and 
these two species are supposed to have descended from 
one species of a still more ancient and unknown genus. 
We have seen that in each country it is the species 
of the larger genera which oftenest present varieties or 
incipient species. This, indeed, might have been ex¬ 
pected ; for as natural selection acts through one form 
having some advantage over other forms in the struggle 
for existence, it will chiefly act on those which already 
have some advantage; and the largeness of any group 
shows that its species have inherited from a common 
ancestor some advantage in common. Hence, the 
struggle for the production of new and modified de¬ 
scendants, will mainly lie between the larger groups, 
which are all trying to increase in number. One 
large group will slowly conquer another large group, 
reduce its numbers, and thus lessen its chance of further 
variation and improvement. Within the same large 
