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 to 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. Centrulblutt vi. 



152 



