734 GENETICS AND EVOLUTION 



formed. From this center ot origin each species spreads out, under the 

 pressure of an increasing population, until it is halted by a barrier of 

 some kind: a physical one such as an ocean, mountain or desert, an 

 environmental one such as unfavorable climate, or a biologic barrier 

 such as the absence of food or the presence of other species which prey 

 upon it or compete with it for food or shelter. 



As one might expect, regions which have been separated from the 

 rest of the world for a long time, such as South America and Australia, 

 have a unique assemblage of animals and plants. Australia has a mam- 

 malian population of monotremes and marsupials that is found nowhere 

 else. Australia became separated from Malaya during the Mesozoic, before 

 placental mammals evolved, and its primitive mammals were not elim- 

 inated, as were the monotremes and most of the marsupials in the other 

 parts of the world, by the competition of the better adapted placental 

 mammals. The Australian marsupials evolved into a wide variety of 

 forms, each adapted to some particular combination of environmental 

 factors. 



The kinds of animals and plants found on oceanic islands are in- 

 structive. They resemble, in general, those of the nearest mainland, yet 

 they are made up to some extent of species found nowhere else. Darwin 

 studied the flora and fauna of the Cape Verde Islands, some 400 miles 

 west of Dakar in Africa, and of the Galapagos Islands, a comparable 

 distance west of Ecuador. On each archipelago the plants and the non- 

 flying animals were indigenous, but those of Cape Verde resembled 

 African species and those of the Galapagos resembled South American 

 ones. It is clear that species from the neighboring continent migrated or 

 were carried to the island and that by subsequent evolution they became 

 differentiated from their ancestral forms. The animals and plants found 

 on oceanic islands are only those that could survive the trip there. There 

 are, for example, no frogs or toads on the Galapagos, and no terrestrial 

 mammals, even though conditions would favor their survival. 



There are many facts of the present-day distribution of animals and 

 plants which can be explained only by knowledge of their history. Alli- 

 gators, for example, are found only in the rivers of southeastern United 

 States and in the Yangtse River in China. Sassafras, tulip trees and mag- 

 nolias are found only in the eastern United States, Japan, and eastern 

 China. The explanation for these curious patterns of distribution lies 

 in the fact that early in the Cenozoic era the northern hemisphere was 

 much flatter than at present and the North American continent was 

 connected with eastern Asia by a land bridge at what is now Bering 

 strait. The climate of the whole region was much warmer than at present 

 and fossil evidence shows that alligators, magnolia trees and sassafras 

 were distributed over the entire region. Later in the Cenozoic, as the 

 Rockies increased in height, the western part of North America became 

 much colder and drier. During the Pleistocene the ice sheets moving 

 down from the north met the desert and mountain regions of western 

 North America, and the animals and plants that had lived in that region 

 either became extinct or migrated. In southeastern United States and in 



