OF NATURAL SELECTION. 115 



mediate in character between existing groups; and we can 

 understand this fact, for the extinct species lived at various 

 remote epochs when the branching Hues 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 the 

 diagram, we suppose the amount of change represented 

 by each successive group of diverging dotted lines to be 

 great, the forms marked a" to p^, those marked 5" and 

 /", and those marked o" to m", will form three very dis- 

 tinct genera. We shall also have two very distinct genera 

 descended from (I), differing widely from the descendants of 

 (A). These two groups of genera will thus form two dis- 

 tinct families, or orders, according to the amount of diver- 

 gent modification supposed to be represented in the dia- 

 gram. And the two new families, or orders, are descended 

 from two species of the original genus, and these are sup- 

 posed to be descended from some still more ancient and 

 unknown form. 



"We have seen that in each country it is the species be- 

 longing to the larger genera which oftenest present varie- 

 ties or incipient species. This, indeed, might have been 

 expected; 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 pro- 

 duction of new and modified descendants 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 number, and thus lessen its chance 

 of further variation and improv;ement. Within the same 

 large group, the later and more highly perfected sub- 

 groups, from branching out and seizing on many new 

 places in the polity of nature, will constantly tend to sup- 

 plant and destroy the earlier and less improved sub-groups. 

 Small and broken groups and sub-groups will finally disap- 

 pear. Looking to the future, we can predict that the 

 groups of organic beings which are now large and trium- 

 phant, and which are least broken up, that is, which have 

 as yet suffered least extinction, will, for a long period, con- 



