EFFECTS OF NATURAL SELECTION 133 



parents, the new species (f") will not be directly interme- 

 diate between them, but rather between types of the two 

 groups; and every naturalist will be able to crJl su^h cases 

 before his mind. 



In the diagram, each horizontal line has hitherto been sup- 

 posed to represent a thousand generations, but each may rep- 

 resent a 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 

 Geolog>', 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 ex- 

 tinct 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 o" to />", those marked &" and f *, and those 

 marked o" to m", will form three very distinct genera. We 

 shall also have two very distinct genera descended from (I), 

 differing widely from the descendants of (A). Those 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 



