THE GLACIAL RANGE CONTRACTION, ETC. 155 



owing to an amelioration of climate ; and that as the 

 range becomes more and more northerly, and less and 

 less southerly, the intermediate areas are only passed on 

 migration or occupied by comparatively small numbers 

 of individuals in summer and winter — a few individuals 

 breeding in the area, a few wintering ; but even then 

 there is every probability that the former draw south in 

 winter, and are replaced by individuals breeding further 

 north. We may thus assume that the species that pass 

 our islands on migration were the earliest to enter our 

 area after the Glacial Epoch, and have followed the 

 ameliorating climate northwards, none of them breeding 

 in our islands now. We may also assume that the 

 species that return to our islands to winter therein are 

 the descendants of birds that were somewhat later in 

 emigrating northwards after glacial conditions had 

 lapsed, and as is usual in such cases a few individuals 

 remain to breed in our area — direct descendants, we 

 must regard them, of the very last individuals that 

 moved north to that area, and which we can have 

 no doubt in many if not in all cases move south of 

 our islands to winter. 



That both the resident species in the British Area, as 

 well as those that winter therein, or pass over it on 

 migration, were early emigrants is still further confirmed 

 by the fact of their being so dominantly distributed in 

 Ireland. They reached that area when it was joined to 

 England in the south, and before much of the land was 

 submerged that is now covered by St. Georges Channel, 

 the Bristol Channel, and the English Channel. They 

 continue to winter in Ireland in such proportionate 

 abundance probably because the land continues to be 



