NATURAL SELECTION 175 



6'* and /'*, and those marked o'* to m'\ will form three 

 very distinct genera. We shall also have two very dis- 

 tinct genera descended from (I), differing widely from 

 the descendants of (A). These two groups of genera will 

 thus form two distinct families, or orders, according to 

 the amount of divergent modification supposed to be rep- 

 resented in the diagram. And the two new families, or 

 orders, are descended from two species of the original 

 genus, and these are supposed to be descended from 

 some still more ancient and unknown form. 



We have seen that in each country it is the species 

 belonging to the larger genera which oftenest present 

 varieties or incij^ieut species. This, indeed, might have 

 been expected; for, _as jiatural 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 com- 

 mon ancestor some advantage in common. Hence, the : 

 struggle for the production of new and modified descend- f 

 ants will mainly lie between the larger groups which are] 

 all trying to increase in number. One large group will I 

 slowly conquer another large group, reduce its numbers, 

 and thus lessen its chance of further variation and im- 

 provement. 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 supplant and destroy the earlier' 

 and less improved sub-groups. Small and broken groups 

 and sub-groups will finally disappear. Looking to they 

 future, we can predict that the groups of organic beings 

 which are now large and triumphant, and which are least 



