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 of cats and seals and whales insistently recall the analogies of
the locomotive and the ship employed at the outset. All these animals,
like the mechanical examples, have come to differ in their derivation from
the same original parents, and their lines of descent have diverged so as
to fit the products of evolutionary modification to diverse circumstances.
Even the vestigial organs of animals have their counterparts in the
machines. The cowcatcher was a large and important structure in the early
days of railroading, but it has become relatively useless with the
decrease of grade crossings and the construction of more complete lines of
fence. The structure still persists, sometimes in a greatly reduced form.