THE GLACIAL RANGE COXTRACTION, ETC. 143 



following the river valleys, would therefore have been 

 under the necessity of breaking the Law of their dispersal 

 by increasing their area south-west had they kept to the 

 river ; but following the normal conditions of their 

 dispersal, they left the valley entirely at the apex of this 

 southern trend and began to colonize the tundras north- 

 wards. To this day their descendants miss the river 

 valley probably entirely, after reaching its most northerly 

 trend ! These birds were all emigrants from South-west 

 Asia or South-east Europe. On the other hand, the 

 great body of migrants to the tundras did pass through 

 Ust Zylma, but they either did not follow river valleys 

 so closely (as for instance in the case of the Siberian 

 Chiffchaff), or most certainly came from more south- 

 westerly areas {co)if. footnote, p. 127). 



We will now endeavour to trace more minutely the 

 emigration of birds within the British Archipelago. We 

 will deal first with the species that are resident in the 

 British Islands. For the most part these consist of 

 hardy birds, numbering amongst them some of the 

 earliest to reach our area after the third cold period of 

 the Glacial Epoch. From these early arrivals, as we 

 should naturally expect to be the case, the dominant 

 avifauna of Ireland has descended. They represent 

 species whose emigrations to the British Area took 

 place whilst that area was much more compact than 

 it is now ; when St. Georges Channel and the English 

 Channel did not exist, and extension northwards and 

 westwards was not retarded or absolutely prevented 

 by wide areas of water. Our resident birds are 

 tabulated below. 



