24 THE MIGRATIOX OF BRITISH BIRDS 



the size of the birds and the date of their passage pre- 

 venting any possibiUt)- of a wrong interpretation of the 

 facts. The Wheatears or Chats {Saxico/a) are exclusively 

 an Old World group, confined to the Ethiopian and 

 South Pala^arctic regions, with the single exception of the 

 Wheatear, so that there can be no doubt whatever as to 

 the line of emigration followed by that species, or as to 

 the more continuous land surface of its migration route 

 whilst that route was being established. 



The climate from the close of the third glacial period 

 down to the dawn of historic time appears to have 

 suffered considerable changes — alternations in fact of less 

 and less strongly-marked genial and cold conditions, 

 culminating in the fourth (and less extensiv^c) glacial 

 period, the epoch of the great Baltic glacier — as the 

 eccentricity of the earth's orbit gradually diminished 

 and approached that limit which characterizes the pre- 

 sent era. There can be little doubt that the third 

 glacial period was succeeded b)- a warm and genial 

 climate, much more favourable to the growth of large 

 vegetation than now, especially in the north, as is pro\-cd 

 by the remains of forest trees in the peat-bogs of the 

 now sterile Hebrides, Orkneys, Shetlands, and Faroes, 

 l-'ollowing this wc appear to have a relapse to a colder 

 period (the epoch of the Baltic glacier) than marks 

 present time, followed by the submergence which was 

 eventually to isolate our area from continental land, and 

 also to separate Greenland from Europe by the wide 

 water-ways that now exist. Local glaciers and snow- 

 fields probably also appeared on the mountains of 

 England, Wales, and Ireland, as they certainly did on 

 those of Scotland. The climate during this period 



