ORDERS 175 
or Ungulata, represented in the actual fauna by the Horses, 
Rhinoceroses, Tapirs, Swine, and Ruminants. Almost as isolated 
as the Proboscidea among existing mammals are the few small 
species constituting the family Hyracide, and in their case palxon- 
tology affords no help at present, and therefore, pending further dis- 
coveries, it has been thought advisable in most recent systems to 
give them the honour of an order to themselves, under the name of 
Hyracoidea. But the number of extinct forms already known allied 
to the Ungulata, though not coming under the definition of either 
of the two groups (Artiodactyla and Perissodactyla) under which all 
existing species range themselves, is so great that either many new 
orders must be made for their reception or the definition of the old 
order Ungulata so far extended as to receive them all, in which 
case both Proboscidea and Hyracoidea may be included within it. 
Again, the Rodentia or gnawing animals—Rabbits, Rats, Squirrels, 
Porcupines, Beavers, etc.—are, if we look only at the present state 
of the class, most isolated. No one can doubt what is meant by a 
Rodent animal, or have any difficulty about defining it clearly, at 
least by its dental characters ; yet owr definitions break down before 
the extinct South American Typotherium, half Rodent and_ half 
Ungulate, which leads by an easy transition to the still more truly 
Ungulate Towodon, for the reception of which a distinct order 
(Toxodontia) has been proposed. It has also been suggested that 
the Rodents are connected by some of the extinct Tillodontia (or 
Teeniodontia) with the Edentates. The Insectivora and the 
Carnivora again are at present quite distinct orders, but they merge 
into one another through fossil forms, and are especially connected 
by the large group of primitive Carnivora, so abundantly repre- 
sented in the Eocene deposits both of America and Europe, to which 
Cope has given the name of Creodonta. The Carnivora also appear 
to have been closely connected with the primitive Ungulates as repre- 
sented by the extinct group called Condylarthra. In another 
direction the step from the Insectivores to the Lemurs is not great, 
and in past times the transition was probably complete. The Bats 
or Chiroptera are allied to the Insectivora in all characters except the 
extraordinary modification of their anterior extremities into wings ; 
but this, like the want of the hind limbs in the Cetacea and Sirenia, 
makes such a clear distinction between them and all other mammals 
that, in the absence of any knowledge of any completely inter- 
mediate or transitional forms, they can be perfectly separated, and 
constitute as well-defined an order as any in the class. We have, 
however, an inkling of the mode in which the Insectivora were 
modified into Chiroptera shown us by the so-called Flying Lemur 
(Galeopithecus). Finally, we have the important and well-character- 
ised group called Primates, including all the Monkeys and Man ; 
and the question is not yet solved as to how and through what 
