86 ORIGIN AND CLASSIFICATION 



of the hitherto proposed associations of the orders into larger groups 

 stand the test of critical investigation. All serial arrangements of 

 the orders are therefore perfectly arbitrary ; and although it would 

 be of very great convenience for reference in books and museums 

 if some general sequence, such as that here proposed, were generally 

 adopted, such a result can scarcely be expected, since equally good 

 reasons might be given for almost any other combination of the 

 various elements of which the series is composed. In fact, we have 

 already seen reason to depart in some respects from that used in the 

 " Encyclopaedia." 



The Edentata, Sirenia, and Cetacea stand apart from all the 

 rest in the fact that their dentition does not conform to the general 

 heterodont, diphyodont type to which that of all other Eutheria 

 can be reduced, and which is such a close bond of union between 

 them. In all three orders, however, some indications may be traced 

 of relationship, however distant, with the general type. 



With regard to the Edentata, reasons will be given for believing 

 that both the Sloths and Anteaters are nearly related, and that the 

 Armadillos, though much modified, belong to the same stock, but 

 that the Pangolins and the Aard-varks represent very isolated 

 forms. 



There is no difficulty about the limits of the order Sirenia, com- 

 prising aquatic, vegetable-eating animals, with complete absence of 

 hind limbs, and low cerebral organisation, represented in our present 

 state of knowledge only by two existing genera, Halicore and Mana- 

 ttis, and a few extinct forms, which, though approaching a more 

 generalised mammalian type, show no special characters allying 

 them to any of the other orders. The few facts as yet collected 

 relating to the former history of the Sirenia leave us as much in 

 the dark as to the origin and affinities of this peculiar group of 

 animals as we were when we only knew the living members. 

 They lend no countenance to their association with the Cetacea ; 

 and, on the other hand, their supposed affinity with the Ungulata 

 receives no very material support from them. 



Another equally well-marked and equally isolated, though far 

 more numerously represented and diversified order, is that of the 

 Cetacea, placed simply for convenience next to the Sirenia ; with 

 which, except in their fish-like adaptation to aquatic life, they have 

 little in common. The old association of these orders in one group 

 can only be maintained either in ignorance of their structure or 

 in an avowedly artificial system. Among the existing members of 

 the order, there are two very distinct types, the toothed Whales or 

 Odontoceti, and the Baleen Whales or Mystacoceti, which present 

 as many marked distinguishing structural characters as are found 

 between many other divisions of the Mammalia usually reckoned 

 as orders. Since the extinct Zeuglodonts, so far as their characters 



