302 AFFINITIES OF EXriNCT SPECIES. [Chap. XI 



down the intervals between existing genera. Cuvier ranked the 

 Euminants and Pachyderms, as two of the most distinct orders of 

 mammals ; but so many fossil links have been disentombed that 

 Owen has had to alter the whole classification, and has placed 

 certain pachyderms in the same sub-order with ruminants ; for 

 example, he dissolves by gradations the apparently wide interval 

 between the pig and the camel. The Ungulata or hoofed quad- 

 rupeds are now divided into the even-toed or odd-toed divisions; 

 but the Macrauchenia of S. America connects to a certain extent 

 these two grand divisions. No one will deny that the Hipparion is 

 intermediate between the existing horse and certain older ungulate 

 forms. What a wonderful connecting link in the chain of mammak 

 is the Typotherium from S. America, as the name given to it by 

 Professor Gervais expresses, and which cannot be placed in any 

 existing order. The Sirenia form a very distinct group of mammals, 

 and one of the most remarkable peculiarities in the existing dugong 

 and lamentin is the entire absence of hind limbs, without even a 

 rudiment being left ; but the extinct Halitherium had, according 

 to Professor Flower, an ossified thigh-bone " articulated to a well- 

 defined acetabulum in the pelvis," and it thus makes some approach 

 to ordinary hoofed quadrupeds, to which the Sirenia are in other 

 respects allied. The 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 undoubtedly ceta- 

 ceans, " and to constitute connecting links with the aquatic car- 

 nivora." 



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 Compso- 

 gnathus, one of the Dinosaurians — that group which includes the 

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

 brata, Barrande asserts, and 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 be- 

 tween two living forms or group 1 ?, the objection is probably valid. 



