REPTILES, BIRDS, AND MAMMALS 



461 



Protected by their isolation from competition with higher types, they 

 underwent an adaptive radiation, which we have already cited as a classical 

 illustration of this phenomenon. 



The ancestral marsupial was apparently a small, arboreal, opossum- 

 like animal. From such a stem form, a great variety of types evolved, 

 adapted to almost every sort of habitat and mode of life. Among living 

 Australian marsupials (Figs. 24.5 and 16; 28.28 and 29) there are "mice," 

 "shrews," "squirrels," "cats," a "wolf," "moles," an "anteater," sloth- 



Fig. 28.28. Representative Australian marsupials. A and B, two kinds of wombats, Phas- 

 colomys. C, anteater, Myrmecobius. D, koala, Phascolarctus. E, kangaroo, Macropus, with 

 young in pouch. F, marsupial "wolf," Thylacinus. G, "tiger cat," Dasyurus. H, phalanger 

 or "opossum," Trichosurus. (A and E, courtesy Zoological Society of Philadelphia, remainder, 

 American Museum of Natural History.) 



like "bears," badgerlike wombats, the wolverinelike Tasmanian devil, and 

 other types like nothing else on earth, including kangaroos, bandicoots 

 (suggesting rabbits with long tails), and the flying phalangers. Many 

 of these show extraordinarily close resemblance to the unrelated placental 

 mammals whose names they were given by the European settlers of 

 Australia. On the main continents the placental squirrels occupy one 

 ecological "niche," or way of life, the placental moles another, and so 

 on; in Australia the corresponding niches are occupied by marsupial 

 "squirrels," marsupial "moles," etc. In the one region the various niches 

 have been filled by adaptive radiation of the placentals, in the other by 

 adaptive radiation of the marsupials, and the superficially similar end 

 products exemplify the phenomenon called adaptive convergence. 



