WHAT EVOLUTION IS 47 



4. FROM ZOOGEOGRAPHY 



The past and present distribution of 

 animals on the surface of the globe 

 has important bearings on the evolu- 

 tionary problem. Animals are not 

 scattered in a haphazard fashion over 

 the earth, but show a marked regular- 

 ity in their occurrence. This can be 

 well illustrated by what is known of 

 the mammals. The group of mam- 

 mals is made up chiefly of the common 

 beasts of the field and forest, but it 

 includes also such exceptional forms 

 as the bats, among aerial creatures, 

 and the whales and porpoises of the 

 sea. Mammals have warm blood, 

 they produce milk with which they 

 nourish their young, and they are pro- 

 vided with more or less hair. 



Almost all the mammals bring forth 

 their young in a highly developed, 



