SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 379 



rather than homologous modifications. The classic example of this com- 

 mon phenomenon is the convergence between the marsupial mammals 

 and the placental mammals. In Australia, protected by isolation, the 

 more primitive marsupial stock underwent adaptive radiation. A great 

 variety of types evolved, fitted for nearly every mode of life available to 

 mammals. Meanwhile in the other continents a similar adaptive radiation 

 occurred among the placentals, which produced an even greater variety 

 of adaptive types. Many of the Australian marsupials came to resemble 

 closely their placental counterparts (or vice versa), though the similarities 

 are analogies. Adaptive radiation within the marsupials and placentals 



Fig. 24.16. Adaptive convergence. Left, the flying squirrel Glaucomys volans, a gliding 

 placental mammal; right, the flying phalanger Petaurus sciurus, a gliding marsupial mam- 

 mal. (Courtesy American Museum of Natural History.) 



was accompanied by convergence between those species that adopted 

 similar modes of life. 



BIOGEOGRAPHY AND EVOLUTION 



Everyone knows that species are not universally distributed. Lions do 

 not occur in North America except in zoos, nor snakes in Ireland. In part 

 the reasons are obvious; we should not expect to find alligators in deserts 

 or sagebrush in wet regions. The ecological requirements of organisms 

 account for much of their distribution. On the other hand, hardly any 

 species is found in all parts of the world where it could exist. The chief 

 exceptions are man and the creatures that man has carried about with 

 him, on purpose or by accident. There were no rabbits in Australia until 

 they were brought there by white men. This was not because they could 

 not live in Australia ; as soon as they were introduced they multiplied and 

 became a plague. Evidently something besides ecological factors deter- 

 mines where species occur. 



