424 MIGRATION 



Asia, whence they were gradually driven south as the 

 polar ice extended its area. The European ground- 

 thrushes took refuge in Africa, and overspread that 

 continent. A small part of them remained ground- 

 thrushes, and their descendants now form the African 

 species of the genus Geocichla. But by far the larger 

 portion developed into true thrushes, some of whom 

 permanently settled in Africa, whilst others crossed the 

 then warm South Pole and spread over South America, 

 some even emigrating as far as Central America and 

 South Mexico. We thus find that the true thrushes of 

 the Ethiopian and the Nearctic regions are very closely 

 allied, and have by some writers been separated from 

 the genus Turdus, and associated together under the 

 name of Planesticus. During the warm period at the 

 North Pole which followed the glacial epoch, the true 

 thrushes of North Africa appear to have followed the 

 retreating ice, and to have spread over Europe, penetrat- 

 ing eastwards into Turkestan and Kashmir, and north- 

 wards across the pole into North America as far south as 

 Mexico. 



In Asia a similar emigration must have taken place. 

 The original ground-thrushes of Siberia were driven 

 across the Himlayas into the Indo-Malay region, where 

 a few of them still retain their original generic character. 

 It would appear that one or two species found a retreat 

 across Bering Strait into America, one being found in 

 Alaska and one in Mexico. The Alaska species probably 

 crossed over after the glacial period, as it is very nearly 

 allied to the East Siberian species. The Mexican species 

 is nearly allied to that found on Bonin Island, and 

 probably crossed over before the glacial period, and was 

 driven southwards by the ice, never to return. The 

 greater number, however, of Asiatic ground-thrushes 



