1 88 THE MIGRATION OF BIRDS. 



Broadly speaking, in the Northern Hemisphere, 

 a bird's winter quarters are more or less directly 

 south of its breeding-grounds ; whilst in the 

 Southern Hemisphere they are situated to the 

 north of them. Of course there are exceptions 

 to this, as we shall learn in the two following 

 chapters, when we come to deal with the two 

 great seasonal movements; whilst the distance 

 between the winter and summer limits vary to 

 a great extent, according to the length of migra- 

 tion flight and its general direction. The two 

 great summer quarters of migratory birds are 

 situated in the Palaearctic and Nearctic regions. 

 The former region embraces Europe (including 

 Iceland), Africa north of the Great Desert, Asia 

 Minor, Northern Persia, and the remainder of 

 Asia north of India, and the Yangtse valley to 

 Japan and the Aleutian Islands. The latter region 

 comprises Greenland and the entire continent of 

 North America to about north latitude 20° in 

 Southern Mexico. The latter region is by far 

 the poorest in what we may designate Temperate 

 species, probably in the ratio of about two to 

 one. This is owing to the very obvious reason 

 that when birds were banished from the circum- 

 polar zone, glaciation was not only far more severe 

 in the Western than in the Eastern Hemisphere, but 

 the land surface, and consequent accommodation 

 for bird life, directly south of the eastern half 

 of that circumpolar zone, was more than double 

 the area of that directly south of the western 



