Chap. XI.] AFFINITIES OF EXTINCT SPECIES. 109 



or group of species, being considered as intermediate 

 between any two living species, or groups of species. 

 If by this term it is meant that an extinct form is 

 directly intermediate in all its characters between two 

 living forms or groups, the objection is probably valid. 

 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 cha- 

 racters, the ancient members are separated by a some- 

 what 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 dis- 

 covered having affinities directed towards 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 



