GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 117 



is, by so much the more it tends to connect by some 

 of its characters groups now widely separated from each 

 other. This remark no doubt must be restricted to those 

 groups which have undergone much change in the course 

 of geological ages; and it would be difficult to prove the 

 truth of the ^proposition, for every now and then even a 

 living animal, as the Lepidosiren, is discovered having 

 affinities directed toward very distinct groups. Yet if we 

 compare the older Eeptiles and 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 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 in italics 

 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 con- 

 sidered as extinct. The three existing genera a", g*'*, p'*, 

 will form a small family; 6'* and /'* a closely allied 

 family or sub-family; and o'*, e", m'*, 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 pro- 

 genitor. On the principle of the continued tendency to 

 divergence of character, which was formerly illustrated 



