THE BIOLOGICAL REGIONS OF THE WORLD 453 



10. The biological regions, as defined above, have Distribution 

 not always been separated as we find them today. The and plants 

 study of fossils shows that in former geological epochs in past ages 

 the climates of various parts of the world were very 

 different from those now found, while both animals and 

 plants have migrated freely. For example, the mar- 

 supials, now characteristic of Australia, were once 

 common over the greater part of the world. The 

 American opossum, a true marsupial, is a relic of this 

 once wide distribution of the group. The camel family 

 was once abundant in North America. The redwood 

 tree, now native only in California, was formerly wide- 

 spread. Thus the biological regions, as we now under- 

 stand them, are valid only for the present epoch ; when 

 we go back to earlier times they must be redefined and 

 limited in quite other ways. Naturally our information 

 concerning the past is not nearly so complete as that for 

 the present, hence the limitations of former regions can- 

 not be exactly stated. 



References 



WALLACE, A. R. Island Life. For the differences between the Palaearctic 

 and Nearctic regions, see Natural Science, June, 1894. 



