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 
“ Encyclopedia.” 
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- 
tus, 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 
