108 RESULT OF THE ACTION 



new species (f 14 ) will not be directly intermediate between 

 them, but rather between types of the two groups ; and every 

 naturalist will be able to call such cases before his mind. 



In the diagram each horizontal line has hitherto been sup- 

 posed to represent a thousand generations, but each may 

 represent a million or more generations ; it may also repre- 

 sent 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 generally 

 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 

 explained, to the formation of genera alone. If, in the dia- 

 gram, we suppose the amount of change represented by each 

 successive group of diverging dotted lines to be great, the 

 forms marked a u to p u , those marked b li and f u , 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), 

 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 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 belong- 

 ing 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 com- 

 mon. 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 l^ter an4 



