194 THE MIGRATION OF BRITISH BIRDS 



to their requirements from North France to Denmark, 

 those areas being entirely non-mountainous, flat, and 

 probably of a dense forest or swampy character. That 

 the dominant line of Post-Glacial Emigration north- 

 wards of this party of Grouse (whether completely 

 differentiated then or not being a matter of no con- 

 sequence) was only across Britain, and did not extend 

 in that direction to Scandinavia, is proved by the 

 absence of this bird from the Shetlands. That it 

 emigrated across our area slowly seems also suggested 

 by the fact that no trace of an emigration in the 

 direction of Greenland by way of the Faroes is to be 

 found. Probably by the time this Grouse had reached 

 the Orkneys, this north-westerly route was considerably 

 broken by areas of water,^ or conditions favourable 

 to range extension in this direction were wanting. 

 The Willow Grouse that now dwell in Scandinavia 

 have reached that area by a north-westerly line of 

 emigration, and there is no trace that this extension 

 was ever west of E. long. lo". The almost sedentary 

 habits of these Grouse have been the means of keep- 

 ing the two colonies distinct, and by isolating the 

 individuals to allow of the differences — due to changed 

 conditions of life — to become constant characters of 

 full specific value. The brown dress of the Red 



1 Even had the Red Grouse reached Scandinavia by an emigra- 

 tion across the British Area (of which there is no evidence) or by 

 way of Holland and Denmark (which is by no means improbable), 

 the species must have been exterminated again by the fourth 

 Glacial Period — the epoch of the Great Baltic Glacier — which, it 

 will be remembered, only slightly affected the British Area, not 

 sufficiently to cause the extinction of such a hardy tundra species 

 as the Red Grouse. 



