EMIGRATION AND EVOLUTION. 129 



upon the birds of the Pahrarctic and Nearctic 

 regions of to-day, that we can trace with absolute 

 certainty not only the route many of those ancient 

 bands of emigrants followed, but the ancestral forms 

 from which they sprung. Probably few if any 

 of the species living in the circumpolar region 

 during Prae-glacial times survive at the present 

 day. Some of them were doubtless exterminated ; 

 others became segregated into two or more species. 

 Take, for instance, the great number of Palaearctic 

 birds, that are represented in the Nearctic region 

 by closely allied forms, or that are divided into 

 eastern and western races due to emigration and 

 isolation in past ages ; or yet again the eastern 

 and western species of Nearctic birds whose areas of 

 distribution are separated by the Rocky Mountains, 

 the result of diverging routes of emigration during 

 the Glacial Epoch. In fact throughout the 

 Northern Hemisphere the ornithological student 

 is continually discovering fresh evidence of the 

 vast influence of emigration on the origin of 

 avian species in these regions. Many instances 

 might be given in support of these statements, 

 did space permit ; many I have already recorded 

 in Evolution ivitlwut Natural Selection, a work 

 to which I would refer any reader sufficiently 

 interested to follow the subject further. Precisely 

 the same vast emigration has taken place from 

 South Polar areas due to glaciation. 



Again, many island species of birds owe their 

 origin almost entirely to emigration. From a 



