GEOGRAPHIC DISTRIBUTION OF ANIMALS: CONTINENTS 261 



In this connection we may note an interesting relationsiiip between the 

 distinctiveness of animals and the length of time they have occupied a 

 given continent. This is particularly evident in South America, as pointed 

 out by Simpson (1950), who called the relationship faunal stratification. 

 The fossil record shows that armadillos and sloths, for example, occurred 

 in South America as long ago as the earliest Cenozoic. There are no sloths 

 anywhere else in the world, and no armadillos either, except as they later 

 spread northward in the Americas. These forms are representative of the 

 oldest "stratum." 



An intermediate stratum is exemphfied by the New World monkeys, 

 which, as we have seen, are unlike the Old World forms in many re- 

 spects. They have lived in South America since mid-Cenozoic. 



As examples of a later stratum Simpson cited the field mice, which are 

 closely allied to those of North America. They have formed part of the 

 South American fauna since late Cenozoic times only. 



Thus we see as a general trend a relationship between the length of time 

 an animal has inhabited a given continent and the amount of differentia- 

 tion that animal has undergone. Such a relationship is eloquent of evolu- 

 tion. If animals were created as they are and remained unchanging such a 

 relationship would be meaningless, or would have to be ascribed to mere 

 coincidence. We shall return to this relationship between elapsed time and 

 amount of differentiation when we discuss the organisms of oceanic islands 

 (see especially pp. 292-293). 



Australia 



The Tropic of Capricorn crosses not only southern Africa and South 

 America but also the continent of Australia. The animal inhabitants of that 

 isolated continent are most unlike those of either of the other two con- 

 tinents crossed by the Tropic. Everyone knows of the Australian kanga- 

 roos, for the young of which a fur-lined pouch on the abdomen of the 

 mother serves as nest and living perambulator. The kangaroos belong to 

 the subdivision of Class Mammalia characterized by possession of such a 

 pouch, or marsupium, and hence called marsupials (see p. 192). Africa 

 has no marsupials; South America has opossums and some tiny creatures 

 known as caenolestids. In passing we might note that marsupials are also 

 absent from Asia, and only one species, the so-called Virginia opossum, 

 occurs in North America. How does it happen that marsupials are found 

 only in such widely separated regions of the earth as America and Aus- 

 tralia? The question suggests something of the complexity of problems 



