322 PRINCIPLES OF ANIMAL BIOLOGY 



more able to cope with the environment. This supposed superiority of 

 the later mammals has been demonstrated in modern times by the intro- 

 duction of northern true mammals into the southern areas, where they 

 began to replace the primitive forms already there. This has happened 

 very noticeably in Australia, where the dingo and rabbit were intro- 

 duced. Something like a general principle must be involved here, for 

 in other groups of animals northern forms have displaced southern ones 

 when they have been brought together. This has happened in the 

 case of birds (sparrow, starling, blackbird, and others) introduced into 

 Australia, the goldfish in Madagascar, European ants and earthworms 

 in all the southern continents. 



The early mammals were thus driven out of the northern continents 

 which they first occupied. With Australia then joined to Asia, and 

 South America not yet separated from North America, they were free 

 to fill all the southern land masses. Then the sinking of the land cut 

 off Australia, so that the true mammals were not able to follow, and 

 that continent was and is the principal home of the marsupials and 

 monotremes. The severance of the Americas from each other checked 

 the southward migration of the higher mammals, so that primitive 

 types are relatively more common in South America. Restoration of 

 the land connection at Central America has, however, permitted many 

 of the true mammals to reach the southern continent. The traffic was 

 not all in one direction at the isthmus, since the opossums and armadillos 

 reached North America from the south over this restored land connection. 



Primitiveness of Southern Faunas. — The scheme just outlined should 

 have caused the faunas of the southern continents to be on the average 

 more primitive than those of Eurasia and North America. For the 

 mammals of Australia and South America this has already been shown 

 to be true. To a less marked extent it is true also of Africa south of 

 the Sahara; for there is the primitive little deerlike chevrotain, and there 

 are the lemurs, the aardwolf, and the golden mole. In Madagascar is a 

 host of lemurs; and if other groups of animals are to be considered, that 

 island has the most primitive bird of the crane and rail group. Also 

 outside the mammalia, Australia has the most primitive termites, the 

 simplest insects of the butterfly-and-moth order, and some of the most 

 primitive bees. The most primitive land snails are in the southern 

 continents; indeed, the whole mollusk fauna of South America may be 

 characterized as primitive. The three surviving genera of lungfishes are 

 in the three southern continents, one genus in each. The lungfishes are 

 well represented by fossils in North America and Eurasia, and the three 

 living genera are plainly relicts. 



Land Connections. — The connection and separation of land masses 

 postulated in the foregoing account mostly are supported by geological 



