422 MIGRATION 



breed in the semi-arctic climate which huna: around its 

 skirts, all had to winter as best they could in the already 

 overcrowded Indian and Ethiopian regions, and a few 

 species seem to have made, not simple migrations for a 

 season, but absolute emiorations for orood and all into 

 distant lands, and thus their descendants have become 

 almost cosmopolitan. The migration or irruption of 

 sand-grouse in 1863 was probably an emigration of this 

 nature. 



It must have been a curious state of thino-s in south 

 Europe at this time, when reindeer were destroyed by 

 tigers within sight of a glacier such as now exists at the 

 South Pole. 



After the glacial period had passed its meridian, and 

 the edge of the ice gradually retreated northwards, carry- 

 ing its climate, its swamps, and its mosquitoes with it, 

 the great body of the palaearctic birds followed it, returning 

 every summer farther and farther north to breed. Here 

 and there a colony was left behind, and formed the tropical 

 allies of so many of our species — birds which no longer 

 migrate, but which have the powers of flight, the pointed 

 wings of their ancestors, though they no longer require 

 them. 



The extraordinary emigration of sand-grouse alluded 

 to is doubtless only one of many such great movements 

 which have from time to time taken place. The dis- 

 turbance of bird-life produced by the temporary exter- 

 mination of it in the northern half of the palsearctic region 

 during the glacial epoch must have been very great. The 

 countries to the south of the orreat grlacier must have been 

 overcrowded, and the natural cure for such a state of 

 things must have been emigration on a large scale. It is 

 not difficult to trace some of these movements even after 

 such a lapse of time. Their history is written indelibly 



