CH. V] SOUTHWARD MIGRATION OF ANIMALS. 227 



Theory of the Polar origin of Life. 



The theory of the polar origin of life 1 is, as regards 

 terrestrial and fresh water vertebrates rather seductive. 

 On this theory it is clear that the further away from the 

 north pole the more archaic will be the types of animals 

 met with; and on the contrary the newer groups will 

 have moved away td a less distance. Among the Mammalia 

 this is strikingly exemplified of course by the Marsupials, 

 once existing in great variety in Europe and North 

 America. They are now absolutely extinct in Europe and 

 but feebly represented in North America. The survivors 

 have been pushed in to the furthest corner of the world — 

 the Australian continent, and some of the islands to the 

 north. The bulk of the American Opossums are confined 

 to the Neotropical, i.e. the more southern regions of the 

 American continent, only a single species occurring in 

 North America. This it is presumed upon the theory 

 under discussion is due to the fresh development of types 

 of life in the polar regions which have forced the older 

 and less vigorous forms of life to emigrate; a continued 

 efflux of waves of life spreading out from the place of 

 origin push further away the races which have the start. 

 Everyone is agreed that the Struthious birds are in all 

 probability an ancient race, or perhaps races, for the unity 

 of the group is now-a-days rather called in question. 

 Here again we meet with no northern types at all ; North 

 Africa is the northern limit of the birds, and the vast 



1 See Haacke, Biol. CentralblaU vi. 



15—2 



