NATURAL SELECTION 113 



will be able to bring some such case before bis 

 mind. 



In the diagram, each horizontal line has hitherto 

 been supposed to represent a thousand generations, but 

 each may represent a million or hundred million 

 generations, and likewise 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, or 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 

 very ancient 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 our diagram, we suppose the amount of change 

 represented by each successive group of diverging 

 dotted lines to be very great, the forms marked a 14 to 

 p M , those marked b 14 and f 1 *, and those marked o u to 

 m 14 , will form three very distinct genera. We shall 

 also have two very distinct genera descended from (I) ; 

 and as these latter two genera, both from continued 

 divergence of character and from inheritance from a 

 different parent, will differ widely from the three 

 genera descended from (A), the two little groups of 

 genera will form two distinct families, or even orders, 

 according to the amount of divergent modification 

 supposed to be represented in the diagram. And the 

 two new families, or orders, will have descended from 

 two species of the original genus ; and these two species 

 are supposed to have descended from one species of a 

 still more ancient and unknown genus. 



We have seen that in each country it is the species 

 of the larger genera which oftenest present varieties or 

 incipient species. This, indeed, might have been 

 expected ; for as natural selection acts through one 



I 



