Chap. IV. DIVERGENCE OF CHAEACTEE. 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 14 to p 14 , those 

 marked b ,4 and/ 14 , and those marked o 14 to m 14 , 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 



