98 THE ORIGIN OF SPECIES 



million or more generations ; it may also represent a section of the 

 successive strata of the earth's crust including extinct remains. 

 We shall, when we come to our chapter on geology, have to refer 

 again to this subject, and I think we shall then see that the diagram 

 throws light on the affinities of extinct beings, which, though gen- 

 erally belonging to the same orders, families, or genera, with those 

 now living, yet are often, in some degree, intermediate in character 

 between existing groups; and we can understand this fact, for the 

 extinct species lived at various remote epochs when the branching 

 lines of descent had diverged less. 



I see no reason to limit the process of modification, as now ex- 

 plained, 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 14 

 to £ 14 , those marked b 14 and / 14 , and those marked 14 to w 14 , 

 will form three very distinct genera. We shall also have two very 

 distinct genera descended from (I), differing widely from the de- 

 scendants 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 represented 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 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 production 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 improvement. 

 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 the future, we 

 can predict that the groups of organic beings which are now large 

 and triumphant, and which are least broken up, that is, which 

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



