Canada Goose Branta canadensis (see pp. 34, 134, 162). 



6. Migration and Origins 



THE ICE OF THE LAST and maximal Wisconsin glaciation, it has been 

 earlier pointed out, lay over Canada and southern Alaska for 

 many thousand years (Flint, 1947) and separated an unglaciated area 

 in northern Alaska and Yukon from the southern parts of North 

 America, which during that time remained in habitable condition 

 for birds. It is doubtful that the climate and ice of northeastern 

 Siberia then permitted migration to Alaska from temperate parts of 

 Asia. The maximum stage of this ice period was probably about 

 10,000 years ago and current developments in glacial geology tend 

 to shorten the estimates of the time during which the migrations we 

 now see could have established their present routes to arctic North 

 America. It is evident that the migration of birds to Alaska is a com- 

 paratively modern development and the distances many species have 

 traveled to occupy new areas point out the extent of this recent re- 

 distribution. There must also have been a great multiplication of 

 numbers as new populations developed to occupy the extensive lands 

 in most of Alaska, all of Canada, and some northern States as they 



263 



469496—60 18 



