AFFINITIES OF EXTINCT SPECIES 379 



quadrupeds, to which the Sirenia are in other respects allied. 

 I'he cetaceans or whales are widely different from all other 

 mammals, but the tertiary Zeuglodon and Squalodon, which 

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

 selves, are considered by Professor Huxley to be undoubt- 

 edly 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 Compsognathus, one of the Dinosaurians — that group 

 which includes the most gigantic of all terrestrial reptiles. 

 Turning to the Invertebrata, Barrande asserts, a higher au- 

 thority 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 

 Jt 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 be- 

 tween genera belonging to distinct families. The most com- 

 mon 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 char- 

 acters, 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 



