the hippopotamus has four which
reach the ground, with a vestige of a fifth, so this animal has apparently
descended from a typical mammal with the full number along a different
line from that taken by the odd-toed forms. A pig has a cloven hoof, made
up of what we may call the third and fourth members of a series of five
digits, but the second and fifth fingers and toes are present, though they
are withdrawn from the ground so as to be no longer functional; this
animal seems to have proceeded further along the same line taken by the
hippopotamus. A deer, with still smaller rudiments at the sides of its
double foot, leads in the comparative series to the camel with a cloven
hoof devoid of any such relics.

We must pass with only brief mention the lower orders of mammalia, like
the insect-eating forms to which armadillos and ant-bears belong. Of
greater interest are the pouched mammals like the kangaroo and opossums,
which live almost exclusively in the Australian realm. The kangaroo is
endowed with a head somewhat like that of a goat, and well-developed hind
legs that enable it to make leaps of astonishing length. Some of its
relatives, such as the bandicoot, are like rats, or like bears, as in the
case of the wombat. The Tasmanian wolf is another true marsupial, even
though divergent adaptation has brought it to resemble the carnivora of
the dog tribe in general appearance and in special structures like the
teeth. Finally at the very bottom of the mammalian scale are two small
forms living in the Australian faunal region. The duckbill or
_Ornithorhynchus_ is the better known animal, with its close fur, webbed
feet, and flattened ducklike beak, while its only other near relative, the
_Echidna_, is somewhat similar to the spiny hedgehog in external
appearance. A unique peculiarity of these two forms is that they produce
eggs much like those of reptiles and birds, and this fact, together with
others of a structural nature, brings the whole group of mammals near to
the lower classes of the Vertebrata.

Looking back on the several orders of mammals, it will be seen that the
last mentioned are much less differentiated or specialized in their
general organization. Above the level of the egg-layers and the pouched
mammals, the higher orders branch out in different directions and reach up
to various levels of the scale of animal organization.

The foregoing structural evidences of organic transformation in the past
histories