Seite - 379 - in The Origin of Species

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