PRESENT AND PAST. 175 



retreated northward at the close of the glacial 

 period, many animals would follow the retreating 

 ice and thus once more come to inhabit their old 

 localities. Others finding the conditions of their 

 own home well suited to their needs, did not return 

 north, but remained where they were, or even went 

 farther south, And thus the southern continents 

 came to be the habitat of animals and plants previ- 

 ously ranging into the northern continents. The 

 glacial period, by causing extensive migration and 

 extinction has profoundly modified the distribution 

 of animals and plants. 



The distribution of fossils serves in many cases as 

 a key to the present distribution. There exists a 

 wonderful relation between the living and the dead 

 inhabitants of any country. It is often possible to 

 trace quite exactly, by the study of living animals 

 and fossil, remains, where a given order first arose ; 

 how it dispersed in various directions, reaching 

 widely separated regions ; and how, finally, it became 

 extinct in many places, leaving as its scattered repre- 

 sentatives to-day families separated by wide tracts 

 of land or sea. For example, such studies have 

 revealed the following history of marsupials. The 

 order arose in Europe as the earliest mammals with 

 which we are acquainted. From this point of origin 

 they spread all over the eastern hemisphere, even 

 reaching Australia, at that time connected with 

 Asia. They also migrated westward, and succeeded 

 in reaching America, by what means we do not at 

 present completely understand. But reach America 

 in some way they did, and thus the whole world be- 



