Chap. XL Affinities of Extinct Species. 303 



But in a natural classification many fossil species certainly stand 

 between living species, and some extinct genera between living 

 genera, even between genera belonging to distinct families. The 

 most common case, especially with respect to very distinct groups, 

 such as fish and reptiles, seems to be, that, supposing them to be 

 distinguished at the present day by a score of characters, the ancient 

 members are separated by a somewhat lesser number of characters ;. 

 so that the two groups formerly made a somewhat nearer approach 

 to each other than they now do. 



It is a common belief that the more ancient a form 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 towards very distinct groups. Yet if we compare the 

 older Keptiles and Batrachians, the older Fish, the older Cepha- 

 lopods, 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 some- 

 what 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 un- 

 important 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 14 , q u , j> 14 , will form a small family; b u and/ 14 a closely allied 

 family or sub-family; and o u , e 14 , m 14 , 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 progenitor. On the principle of the continued 

 tendency to divergence of character, which was formerly illus- 

 trated by this diagram, the more recent any form is, the more 

 it will generally differ from its ancient progenitor. Hence we 

 can understand the rule that the most ancient fossils differ most 

 from existing forms. We must not, however, assume that diver- 

 gence of character is a necessary contingency ; it depends solely 



