66 THE BOOK OF MIGRATORY BIRDS 



zonal distribution of climates. Birds and beasts travel 

 thousands or hundreds of miles at the oncoming of spring 

 or fall. 



During all the latter part of the long tertiary period the 

 process of mountain building or subsidence was changing 

 the face of Nature. The fauna which had previously 

 enjoyed so wide a range of equable temperature and a 

 general food supply discovered that those ranges had been 

 restricted in certain directions, while perhaps they had 

 become more extended in others. The land bridges which 

 had previously given them passage had sunk beneath the 

 encroaching waters. Mountain ranges had reared their 

 heads into the clouds and were snow-covered, forming 

 impassable barriers in the paths of certain species. The 

 true migration began to take form, keeping pace with the 

 constantly changing climatic conditions, and in general 

 terms they may be said to have begun with the ice age. 



The direct burning rays of the sun near the Equator were 

 the whips which drove onward to a more temperate region 

 both birds and beasts. The increase of cold and the curtail- 

 ment of their food supply drove them back again. The 

 sheep and goat families accomplished their migrations in 

 altitude and became specialised, remaining there to-day. 

 Certain of the largest birds followed their example. Others 

 of the feathered tribe, needing no land bridges, scorning 

 even the mountain barriers, "took to themselves the wings 

 of the morning and fled to the uttermost parts of the 

 earth.*' 



Food supply was not the only cause for these flights, 

 but comfort and well-being, and after the passing of ages 

 the newly acquired habit of life. They were unconsciously 

 striving for the continuance of the former level tempera- 

 ture condition which had been, during millions on millions 

 of generations, the whole life history of their species. 

 Many varieties, however, failed to pass this extreme test 

 of their powers. 



When the ice age reached its maximum of cold and the 

 glacier front had crept far down the Mississippi valley, 



