6 3 8 



HISTORICAL GEOLOGY 



The effect of isolation upon the animal life depends somewhat upon 

 the size of the region and the diversity of the topography. If the 

 topography, climate, and vegetation are varied, a diversified mam- 

 malian fauna will arise to take advantage of every opportunity of 

 securing food ; and the body, limbs, and feet will become adapted to 

 a great variety of conditions ; some will become adapted for burrow- 

 ing, some for life in the water, some for rapid motion, and some for 

 tree life. The larger the region and the more diverse the conditions, 

 the greater will be the variety of mammals that will result. When 



MIOCENE 



Fig. 563. — The continents, with the exception of South America, were broadly 

 united during the Middle and Upper Miocene, permitting widespread migrations. 

 (After W. D. Matthew.) 



after long periods of isolation animals which, because of the physical 

 conditions under which they lived or because of the fierceness of the 

 competition with other forms, had become especially fitted for life, 

 were able to migrate to other regions where for various reasons 

 evolution had not been so effective in producing such successful types ; 

 the better fitted quickly possessed the new regions, either forcing the 

 former inhabitants into subordinate positions or causing their extinc- 

 tion. 



No better example of the effect of such isolation can be found than 

 in Australia to-day, where only mammals of a low type (marsupials) 



