328 STRUCTURE OF THE VERTEBRATES 



idence is that it opened and closed regularly during the Tertiary 

 period. The existence of other connections between continents is 

 more hypothetical, except for the island groups in the Pacific. 

 Australasia is undoubtedly a drowned area, the high lands and 

 peaks being left as the large and small islands. 



With the shifting of the earth's surface barriers and bridges 

 may rise or disappear. The repeated shifting of the coast in 

 Italy and other regions during historical times illustrates the 

 (geologically speaking) rapid changes which occur; and the fos- 

 silized sea shells on the rim of the Grand Canyon bear testimony 

 to the great upheavals which have formerly taken place. 



Any change of the earth's surface may be a gradual shifting 

 of the land's weight, or a sudden revolutionary process. A small 

 area or an entire region may sink, leaving a fault line to mark 

 the clean-cut division between the two. Such fault lines are more 

 prominent in volcanic regions where violent earthquakes are 

 more or less frequent; but the most stable lands of the earth 

 show sign of faults where one part of the land sank far below 

 the other. 



Australasia was cut from the continent of Asia by a series of 

 faults. The general course of the major fault was determined by 

 Wallace and is known as Wallace's Line; but more recent re- 

 search has shown that the division is not so regular as was 

 thought half a century ago. However, Bali and Lombok, two 

 islands less than twenty miles apart, have faunas more different 

 than those of Europe and Canada, and even than those of 

 Eastern Asia and the Mississippi Valley. 



The sinking of the lands of the southern Pacific occurred after 

 the marsupials had become widely distributed and before the 

 placentals had arisen. The result is that in Australasia no 

 placentals exist except those carried into the region by man, and 

 a few bats and rodents (and the latter may have gone with the 

 early Malay settlers). The other mammals are monotremes and 

 marsupials. The latter, safe from competition with the more 

 active placentals, evolved in many directions. Almost all the 

 groups of placentals are parallelled in external form. Some have 

 the gnawing incisors of the rodents; others are wolf-like; others 

 resemble insectivores ; and others resemble nothing known among 



