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 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.  Even more obvious is the change of structure in the case of masts of vessels, which originally bore the sails for propelling the ship.  When steam engines were employed to give motive power, masts did not disappear.  They now provide the derrick supports of trading steamers; in battleships their function is changed to that of fighting tops and signal yards.  Even the poles carried by canal boats to bear windmills must be regarded as the reduced vestiges of masts originally constructed to carry sails; and their adaptive evolution, like that of countless structures in animals, has been accomplished by degeneration.