296 ON THE ORIGIN OF SPECIES 



day taught that Palaeozoic animals, though belonging to 

 the same orders, families, or genera with those living at 

 the present day, were not at this early epoch limited in 

 such distinct groups as they now are. 



Some writers have objected to any extinct species 

 or group of species being considered as intermediate 

 between living species or groups. If by this term it is 

 meant that an extinct form is directly intermediate in 

 all its characters between two living forms, the objec- 

 tion is probably valid. But I apprehend that in a 

 perfectly natural classification many fossil species would 

 have to stand between living species, and some extinct 

 genera between living genera, even between genera be- 

 longing 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 from each other by a 

 dozen characters, the ancient members of the same two 

 groups would be distinguished by a somewhat lesser 

 number of characters, so that the two groups, though 

 formerly quite distinct, at that period made some small 

 approach to each other. 



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 Reptiles and 

 Batrachians, the older Fish, the older Cephalopoda, 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 preliminary. 



