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.