I2S THE MIGRATION OF BIRDS. 



which have stimulated and preserved variations in 

 many different directions. 



The great emigration taken by many Polar 

 species, for instance, and caused by a Glacial 

 epoch, often followed different routes from a 

 centre of dispersal, causing a separation of the 

 species into as many groups of individuals. In 

 numbers of cases these several colonies remained 

 isolated from each other sufficient time for the 

 individuals of each to become differentiated from 

 a parent form ; in some cases so completely that 

 w^hen circumstances again brought each group 

 together in a common habitat, they had not 

 only lost the power to interbreed and produce 

 fertile offspring, but had even acquired or developed 

 various characteristics peculiar to each group. In 

 this way emigration has directly led to the origin 

 of new species. When the Polar ice banished 

 every bird from the Arctic regions, emigration 

 took place on a vast scale towards Africa, Southern 

 Asia, and temperate and tropical America. The 

 members of each species by no means kept 

 together. Some followed a western course, others 

 an eastern course ; some emigrated down the coasts 

 of Europe, others down the coasts of Asia ; some 

 down the Pacific coasts of America, others down 

 the Atlantic coasts of that continent; or parties 

 of emigrants were divided by the great mountain 

 chains that stretch from north to south in the 

 Nearctic and Neotropical regions. The result of 

 this emigration and isolation is so indelibly stamped 



