340 Cell as Basis of Organic Activity 



An extreme example of isolation is that illustrated by the island 

 continent of Australia with its varied marsupial fauna. The mar- 

 supials at one time in past geological ages were distributed throughout 

 most portions of the world. When Australia and South America were 

 isolated, the marsupials evolved into many different forms. In Aus- 

 tralia, particularly, they radiated into all possible habitats; in South 

 America, they occupied all niches except those taken by the placental 

 herbivores. When South America was again opened for migration, the 

 placental carnivores flooded in and eliminated the marsupial carnivores. 

 Such a migration was impossible in Australia, and the marsupials con- 

 tinued their unique development. 



The present-day fauna of many oceanic islands gives further illus- 

 tration of the effects of isolation. Only forms capable of traversing the 

 great distances of the ocean can arrive and develop. St. Helena, 1,100 

 miles off the coast of Africa and 1,800 miles from South America, has 

 no native vertebrate fauna, but possesses 129 species of beetles of which 

 128 are unique. Islands nearer continents often have species closely 

 related to the neighboring continent but of a unique type. Darwin's 

 study of the birds of the Galapagos Islands illustrated this. At times 

 rather bizarre animals develop on islands due to the lack of competitors. 

 Thus the dodo bird developed on Mauritius in the Indian Ocean, 

 only to be exterminated by thoughtless sailors. It was a huge flightless 

 bird of the pigeon family. 



Wallace's Zoogeographic Regions. — In 1859, the year Darwin's 

 great work appeared, P, L. Sclater attempted to divide the world into 

 zoological regions as based on his studies of perching birds. But it was 

 Wallace in 1876 who, in his book on the geographical distribution of 

 animals, divided the world into the zoogeographical regions essentially 

 as they are known today. He based the delineation of these regions on 

 the present-day distribution of animals. 



These regions were as follows : the Nearctic embracing North 

 America ; the Palaearctic including Europe, North Africa, and most 

 of Asia ; the Ethiopian composed of the portion of Africa south of the 

 Sahara ; the Oriental consisting of India, southeast Asia, and the west- 

 ern portions of the East Indies ; the Australian embracing Australia, 

 New Guinea, and the neighboring islands ; and finally the Neotropical 

 consisting of Central and South America. 



The present-day classification of these regions has been but little 

 altered. Present workers unite the Palaearctic and Nearctic into a single 



