176 THE AGE OF MAMMALS 



tions by subsidence has undoubtedly been a potent cause both of exter- 

 mination in certain localities and of the survival of very primitive forms. 

 It may be said at once that most of the causes both of survival and of 

 extinction which prevail upon continents are intensified on islands.' Wal- 

 lace attributed the widespread extinction which occurred in Australia in 

 early Pleistocene times partly to the increased competition and struggle 

 for existence caused by the progressively contracted land area due to sub- 

 sidence.- Wallace also rightly attributed the survival of certain primitive 

 mammals among the monotremes and marsupials to the practically in- 

 sular condition of the Australian region. On the other hand, there is 

 reason to believe that the introduction of new forms of life on islands 

 causes more rapid and profound modifications in the fauna than similar 

 introductions on continents. 



' Osborn, H. F., The Causes of Extinction of Mammalia, Amer. Natural., Vol. XL, 

 no. 479, Nov., 1906, pp. 769-795; no. 480, Dec, 1906, pp. 829-859. (See especially pp. 

 773-774.) 



2 Wallace, A. R., The Geographical Distribution of Animals (1876). Vol. I, pp. 158-159. 



