96 THE RIDDLE OF MIGRATION 



middle of the Pliocene. The climate was then 

 cooling and was probably not very different from 

 that of today. Here, let us suppose, the birds bred 

 in the spring and remained in the winter. If con- 

 ditions were favorable, their numbers must have 

 shown a steady increase and their breeding area must 

 have slowly extended in all directions, north, south, 

 east, and west, but later only north and east, the 

 mountains in the west and the Gulf of Mexico to the 

 south providing barriers in these directions. That 

 such spreading would actually have occurred we 

 can hardly question, for this is exactly what is 

 happening today in other cases. Introduced birds in 

 all parts of the globe have illustrated the principle 

 many times over. The European starling (Sternus 

 vulgaris) in the eastern States and the Hungarian 

 partridge (Perdix perdix) in western Canada are 

 two cases in point at the moment. The starling has 

 found an effective barrier in the Atlantic on its 

 eastern front; the partridge has met the mountains 

 on the west. Spread has therefore been in the other 

 directions only. The partridge was introduced at 

 Calgary, Alberta in 1908 and 1909. By 1930 it had 

 gone north to Fort McMurray (450 miles) and east 

 to central Saskatchewan (300 miles) and south at 

 least 200 miles. Every suitable locality traversed 

 in its spread now boasts a heavy population of 



