A FOSSIL CONTINENT 101 



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 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 them- 

 selves to the same mode of life under the same general 

 circumstances. Natural selection, acting upon unlike ori- 

 ginal 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 mar- 

 supials, 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 



