281 AFFINITIES OF EXTINCT SPECIES. [CUAP. XL 



which have been placed by some naturalists in an order by them- 

 selves, are considered by Professor Huxley to be undoubtedly 

 cetaceans, "and to constitute connecting links with the aquatic 

 carnivora." 



Even the wide interval between birds and reptiles has been 

 shown by the naturalist just quoted to be partially bridged over 

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

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

 nathus, 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 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 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 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 com- 

 pare the older Reptiles 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 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 



