298 ON THE ORIGIN OF SPECIES 



successive formations, were discovered at several points 

 low down in the series, the three existing families on 

 the uppermost line would be rendered less distinct 

 from each other. If, for instance, the genera a 1 , a 5 , 

 al0 > f S ) mZ ) m *> m9 , were disinterred, these three 

 families would be so closely linked together that they 

 probably would have to be united into one great family, 

 in nearly the same manner as has occurred with 

 ruminants and pachyderms. Yet he who objected to 

 call the extinct genera, which thus linked the living 

 genera of three families together, intermediate in 

 character, would be justified, as they are interme- 

 diate, not directly, but only by a long and circuitous 

 course through many widely different forms. If many 

 extinct forms were to be discovered above one of the 

 middle horizontal lines or geological formations — for 

 instance, above No. VI. — but none from beneath this 

 line, then only the two families on the left hand 

 (namely, a 14 , etc., and & 14 , etc.) would have to be 

 united into one family ; and the two other families 

 (namely, a 14 to f u now including five genera, and o u 

 to m li ) would yet remain distinct. These two families, 

 however, would be less distinct from each other than 

 they were before the discovery of the fossils. If, for 

 instance, we suppose the existing genera of the two 

 families to differ from each other by a dozen characters, 

 in this case the genera, at the early period marked VI., 

 would differ by a lesser number of characters ; for at. 

 this early stage of descent they have not diverged in 

 character from the common progenitor of the order, 

 nearly so much as they subsequently diverged. Thus 

 it comes that ancient and extinct genera are often in 

 some slight degree intermediate in character between 

 their modified descendants, or between their collateral 

 relations. 



In nature the case will be far more complicated than 

 is represented in the diagram ; for the groups will 

 have been more numerous, they will have endured for 

 extremely unequal lengths of time, and will have been 

 modified in various degrees. As we possess only the 



