■14 U. S. NATIONAL MUSEUM BULLETEST 217 



any event it does not seem to have been a suitable intercontinental 

 route for animals from a warm climate. 



It is not to be suspected that during the glacial periods birds could 

 make annual migrations across the American ice cap. Whether mari- 

 time and oceanic birds could migrate to Bering and Arctic sea coasts 

 from the north Pacific would have depended, for one condition, upon 

 the amount of ice-free coastal and inland waters. It does not seem 

 likely that extensive nesting grounds were open to them on the coast 

 or in the interior. Ice free corridors through the east Siberian 

 glaciers probably existed (Flint, 1947), but for long distances the 

 routes to warm areas for wintering in Asia were in proximity to ice 

 fields. The routes to Alaska and Siberia were either impassible or 

 difficult, and the limited unglaciated arctic area and its conditions 

 would seem to have provided insufficient attraction for the annual 

 migTation of birds to the arctic. 



When the ice caps melted, the American lands suitable for animals 

 were increased by the opening of most of Alaska, Canada, and the 

 northernmost states. Populations have extended to occupy these new 

 lands but new species have not evolved. The source of the present 

 avian populations may have been in survivors in the ice-free refuge 

 and in those which have come in from the south as migratory routes 

 developed. While it seems possible that some species of birds and 

 mammals might have survived the last maximum glaciation in the 

 unglaciated parts of Alaska and Yukon, the annual migrations of 

 birds to nest in the Arctic have certainly developed since the ice 

 caps melted. In each spring the migrants now pass northward over 

 lands which have only gradually become occupied durmg the last 

 10,000 years. Many of the populations that pass annually from 

 a warm wintering place to nest in the cold arctic spring encounter 

 cooling climatic gradients by their own movements, thus reversing 

 the seasonal transitions in temperature encountered by northern 

 sedentary populations. All the expanding populations of Canada 

 and most of Alaska have had to adjust to the new and changing condi- 

 tions of the environments in which they have settled. They have also 

 had to adapt social habits to the requirements of moving and develop- 

 ing populations. The animals which live near the arctic limits of life 

 must have possessed a great range of adaptability toward environ- 

 mental and social conditions to have succeeded during the changes of 

 the last 10,000 years. 



