GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 309 



in the most unexpected manner, on the one hand, by the ostrich 

 and extinct Archeopteryx, and on the other hand by the Comp- 

 sognathus, one of the Dinosaurians — that group which includes 

 the most gigantic of all terrestrial reptiles. Turning to the Inverte- 

 brata, Barrande asserts (a higher authority could not be named) 

 that he is every day taught that, although palaeozoic animals can 

 certainly be classed under existing groups, yet that at this an- 

 cient 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 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 be- 

 tween 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 an- 

 cient 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 toward very distinct groups. Yet if we compare the older 

 reptiles and Batrachians, the older fish, the older cephalopods, 

 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 suppose that the num- 

 bered 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 



