Chap. X. AFFINITIES OF EXTINCT SPECIES. 331 



Batrachians, the older Fish, the older Cephalopods, and 

 the eocene Mammals, with the more recent members 

 of the same classes, we must admit that there is some 

 truth in the remark. 



Let us see how far these several facts and inferences 

 accord with the theory of descent with modification. As 

 the subject is somewhat complex, I must request the 

 reader to turn to the diagram in the fourth chapter. 

 We may suppose that the numbered letters represent 

 genera, and the dotted lines diverging from them 

 the species in each genus. The diagram is much too 

 simple, too few genera and too few species being 

 given, but this is unimportant for us. The horizontal 

 lines may represent successive geological formations, 

 and all the forms beneath the uppermost line may 

 be considered as extinct. The three existing genera, 

 a u , q u , p u , will form a small family ; b u and / 14 a 

 closely allied family or sub-family ; and o u e u , m H , a 

 third family. These three families, together with the 

 many extinct genera on the several lines of descent 

 diverging from the parent-form A, will form an order ; 

 for all will have inherited something in common 

 from their ancient and common progenitor. On the 

 principle of the continued tendency to divergence of 

 character, which was formerly illustrated by this dia- 

 gram, the more recent any form is, the more it will 

 generally differ from its ancient progenitor. Hence we 

 con understand the rule that the most ancient fossils 

 differ most from existing forms. We must not, how- 

 ever, assume that divergence of character is a necessary 

 contingency ; it depends solely on the descendants from 

 a species being thus enabled to seize on many and dif- 

 ferent places in the economy of nature. Therefore it is 

 quite possible, as we have seen in the case of some 

 Silurian forms, that a species might go on being slightly 



