The ordinary Australian pouched mammals belong to far less ancient types than ornithorhynchus and echidna, but they too are very old in structure, though they have undergone an extraordinary separate evolution to fit them for the most diverse positions in life. Almost every main form of higher mammal (except the biggest ones) has, as it were, its analogue or representative among the marsupial fauna of the Australasian region fitted to fill the same niche in nature. For instance, in the blue gum forests of New South Wales a small animal inhabits the trees, in form and aspect exactly like a flying squirrel. Nobody who was not a structural and anatomical naturalist would ever for a moment dream of doubting its close affinity to the flying squirrels of the American woodlands. It has just the same general outline, just the same bushy tail, just the same rough arrangement of colours, and just the same expanded parachute-like membrane stretching between the fore and hind limbs. Why should this be so? Clearly because both animals have independently adapted themselves to the same mode of life under the same general circumstances. Natural selection, acting upon unlike original types, but in like conditions, has produced in the end very similar results in both cases. Still, when we come to examine the more intimate underlying structure of the two animals, a profound fundamental difference at once exhibits itself. The one is distinctly a true squirrel, a rodent of the rodents, externally adapted to an arboreal existence; the other is equally a true phalanger, a marsupial of the marsupials, which has independently undergone on his own account very much the same adaptation, for very much the same reasons. Just so a dolphin looks externally very like a fish, in head and tail and form and movement; its flippers closely resemble fins; and nothing about it seems to differ very markedly from the outer aspect of a shark or a codfish. But in reality it has no gills and no swim-bladder; it lays no eggs; it does not own one truly fish-like organ. It breathes air, it possesses lungs, it has warm blood, it suckles its young; in heart and brain and nerves and organisation it is a thorough-going mammal, with an acquired resemblance to the fishy form, due entirely to mere similarity in place of residence.