836 AFFINITIES OF EXTINCT SPECIES. 



— that group which includes the most gigantic of all ter- 

 restrial reptiles. Turning to the Invertebrata, Barrande 

 asserts (a higher authority could not be named) that he is 

 every day taught that, although palaeozoic animals can cer- 

 tainly be classed under existing groups, yet that at this 

 ancient period the groups were not so distinctly separated 

 from each other as they now are. 



Some writers have objected to any extinct species, 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 interme- 

 diate 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 dis- 

 tinct 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 char- 

 acters 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 

 reptiles and Batrachians, the older fish, the older cephalo- 

 pods, and the eocene mammals, with the 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 sup- 

 pose 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, ^nim- 



