Chap. IV.] NATURAL SELECTION. 149 



been supposed to represent a thousand generations, but 

 each may represent 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 Geology, have to refer 

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

 the diagram throws light on the afRnities of extinct be- 

 ings, 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 diagram, we suppose the amount of change, repre- 

 sented by each successive group of diverging dotted lines 

 to be great, the forms marked a^* to p^^, those marked 

 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 

 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 



