THE GLACIAL RANGE COXTRACTION, ETC. 135 



our area from the south, later emigrants, more delicate 

 of constitution, and requiring a higher winter tempera- 

 ture than the descendants of the earlier emigrants of 

 these self-same species that reached us, it may be, 

 thousands of years before, when the climate was much 

 more rigorous, and which, as we have seen, extended 

 their breeding range into the colder regions of the North 

 and East, after having become thoroughly acclimatized 

 to our winter temperature by long residence therein. 



I may also remark that no birds wintering exclusively 

 south of our area follow this route, a fact which seems 

 to prove that our islands were a base of ancient Eastern 

 Emigration from which resident individuals alone 

 extended their area east. By the time most of our 

 Summer Migrants had reached us, tiie North Sea had 

 probably established a fatal barrier to all Eastern 

 Emigration of southern forms in this direction. Not a 

 single Warbler, in fact not a single Passerine or Picarian 

 species that now visits us only in summer to breed, is 

 by any strange chance observed normally to follow this 

 East to West migration in autumn, or West to East 

 return passage in spring. 



Want of space and time, and the sad deficiency of 

 more exhaustive information, I regret to say, prevent me 

 from entering into greater detail concerning this particu- 

 lar line of Flight, which I may add possesses a singular 

 fascination for me. The subject is too vast and too 

 complicated to be exhaustively treated in the present 

 volume, and in the existing state of our knowledge, but 

 it is a portion of the science of Avian Season Flight 

 that will have to be faced and thoroughly exhausted 

 in the near future. 



