Ixxvi 



THE PROCESS OF DISPERSAL. 



of a species. Or the lines of least resistance may be influenced by 

 temperature either of a temporary or a more permanent character ; 

 or in the case of birds which come to our country from great dis- 

 tances — from Scandinavia and the continental areas of their distri- 

 bution — a line of greatest resistance, and, per contra^ a line of least 

 resistance, may be caused by lower or higher currents of wind, or 

 other meteorological phenomena taking place either at the time of 

 their departure on the migratory movement, or occurring during the 

 actual passage across sea. Yet another influence may be found 

 simply in the fact that, after being driven out from the area of first 

 occupation by the aforesaid congestion, the various species may have 

 utterly failed to find suitable haunts along the new line of depar- 

 ture. This of course — if my views are correct — would be most 

 noticeable among such species as ornithologists have been accus- 

 tomed to consider resident in our country, because, as I have tried 

 to explain, they would have less experience, or have acquired less 

 knowledge of these new lands than truly migratory species would 

 have done, or be less affected thereby. 



It may at once be asked : " Why, then, is it that such species as 

 the Fieldfare and the Eedwing and the Brambling, and many others, 

 which have been known to visit and revisit our country for a very 

 long period of time, have never taken up their summer home 

 here ? " Perhaps that may be answered : The areas occupied, 

 say in Scandinavia and the Continent, are not yet congested, and 

 it may even take many more years before they are. 



Somewhere Seebohm has written the remark — I write from 

 memory — that the climate of Scotland is really Arctic in 

 character in midsummer — i.e. that the isothermal line of 60° at 

 mid- J uly, or slightly under that temperature, passes through Scot- 

 land; and that this accounts satisfactorily for the far-southerly 

 range of such species in the breeding season as the Snow-Bunting, 

 Ptarmigan, Eed-necked Phalarope, Whimbrel, Greenshank, Great 

 Skua, Black-throated Diver, and Fulmar Petrel, and others, as well 

 as several species of Ducks which are really Arctic breeding species, 

 and for their extension further south in Britain than on the Con- 

 tinent. However that may be, I think that the land-temperatures 

 over even so small an insular area as Scotland are assuredly influenced 

 by the presence of the Gulf Stream, as also are the breeding quarters 



