A FOSSIL CONTINENT. 691 



early descendant of the common ancestor from whom mammals, 

 birds, and reptiles alike are originally derived. 



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 extraordi- 

 nary 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 colors, 

 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 under- 

 lying structure of the two animals, a profound fundamental differ- 

 ence at once exhibits itself. The one is distinctly a true squirrel, 

 a rodent of the rodents, externally adapted to an arboreal exist- 

 ence ; the other is equally a true phalanger, a marsupial of the 

 marsupials, which has independently undergone on his own ac- 

 count 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 resem- 

 ble 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 organization it is a thorough-going mammal, with an acquired 

 resemblance to the fishy form, due entirely to mere similarity in 

 place of residence. 



Running hastily through the chief marsupial developments, 

 one may say that the wombats are pouched animals who take the 

 place of rabbits or marmots in Europe, and resemble them both 

 in burrowing habits and more or less in shape, which closely ap- 

 proaches the familiar and ungraceful guinea-pig outline. The vul- 

 pine phalanger does duty for a fox ; the fat and sleepy little dor- 

 mouse phalanger takes the place of a European dormouse. Both 



