DISTRIBUTION OF MAMMALS 159 



broken up in the Age of Reptiles or not later than very early 

 in the Age of Mammals, a hundred million years ago. No indi- 

 cation of such a continental origin of these islands is to be found 

 in the mammals of this area. Similarly, New Zealand has been 

 supposed by some to have been separated from Australia by 

 the sinking of the great area now covered by the Tasman Sea. 



Oceanic islands, built up from the bottom of the ocean, are 

 of several kinds. Just as on land, the crust of the earth cov- 

 ered by the oceans may be wrinkled and folded. Great under- 

 water ranges of mountains are known, the highest points of 

 which project above the water. In the case of New Caledonia 

 and probably New Zealand, the projecting mountain-tops form 

 large islands and reach high above the ocean level. Other is- 

 lands are volcanoes, built up by great eruption from the depths. 

 Combinations of mountain range and volcano may explain the 

 arc-like chains of islands, such as that formed by the Palaus, 

 Marianas, and Bonin Islands. Another such arc includes the 

 Solomons and New Hebrides, while the Marshalls, Gilberts, and 

 Ellice Islands suggest a third. If Celebes, the Moluccas, and 

 the Philippines are not remains of an ancient continental mass, 

 they may well be ridges pushed up from the ocean floor. The 

 first two seem to have been revolved like a vast eddy between 

 the masses of southeastern Asia and Australia. Great dis- 

 turbances of the earth's crust must have occurred in this region ; 

 certain parts have been heaved up, others have sunk. 



Coral barrier reefs and coral atolls are built on and around 

 volcanic bases throughout the tropical Pacific. In some cases 

 their peculiar shapes are explained by the gradual sinking of 

 the heavy volcanic islands around which they formed. In other 

 cases the change of sea level during the ice age may have been 

 important. 



Oceanic islands, even if situated near the continent, harbor 

 relatively few mammals ; but then, their nearness to the main- 

 land is important if winds and water currents are likely to 

 bring them drifting organisms. Often ancient types, "living 



