Bird Migration from America to Europe 73 



the whips which drove onward to a more temperate region 

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

 tailment 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 uncon- 

 sciously striving for the continuance of the former level 

 temperature 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, 

 when the tops of the Rocky Mountains and the Mexican 

 and Central American Cordilleras were capped with snow 

 and ice, glaciers streaming far down their sides towards 

 the sea, several species found their retreat cut off either 

 by sea or ice. It is my belief that at this time and for 

 this cause our American horses and camels and elephants 

 perished. Their remains are found in vast beds in the 

 warmest parts of the United States, bordering on the Gulf 

 of Mexico. They had reached an impassable barrier, and 

 not being fitted (specialised) they inevitably perished. A 

 different geographical formation in Asia enabled the same 

 creatures to make good their escape to warmer regions. 



As there was a minimum and there will be a maximum 

 to the ice age, so there has been a minimum and there 

 will be a maximum to migrations. The ice fronts of the 

 polar regions must be the limits of the feathered tribes. 

 Mammalian life will find its migratory limit far short of 

 those points. 



In common with other species there is little doubt that 

 earliest man himself was of a migratory habit. The 



