288 THE MIGRATION OF BRITISH BIRDS 



the Glacial Epoch, but a result of range extension from 

 more southerly areas during favourable intervals of 

 climate in either hemisphere. Such species were, and 

 probably always had been, Inter-hemisphere or Inter- 

 polar, with a more or less ancient Equatorial range base. 

 An Inter-hemisphere species, for instance, like the Swal- 

 low, or an Inter-polar species like the Hudsonian Godwit, 

 could never be exterminated by a Glacial Epoch, As 

 the summers became colder, the breeding range would 

 gradually become lower — the species probably suffering 

 considerably meantime, owing to the increasing adverse 

 conditions under which the young of the most northerly 

 breeding birds would be reared, in some seasons perhaps 

 none at all surviving — and sink back upon itself, until 

 it was driven as far north or south as the winter quarters 

 commenced, which represent the centre of dispersal or 

 range base of the species. If the winter range or range 

 base of a species did not extend south of the Equator 

 the species would never emigrate south, but continue to 

 occupy that area until a return of more favourable 

 conditions for northern extension or range expansion 

 again took place. If the winter range or range base of 

 a species did not extend north of the Equator, the species 

 would never emigrate north, but remain stationary until 

 favourable conditions returned, and permitted a southern 

 extension or range expansion. If the range base 

 extended both north and south of the Equator, that 

 Polar or Temperate area would be occupied by those 

 Inter-hemisphere and Inter-polar species which pre- 

 sented the most favourable conditions for reproduction. 

 At the present time these conditions are almost exclu- 

 sively in the Northern Hemisphere, but during the last 



