ROUTES OF MIGRATION 211 



by an extension of breeding area, which in many cases 

 has now ceased to be a breeding area in the extreme 

 south, owing to a change in the climate, due probably 

 to equinoctial precession ; but in many more cases the 

 breeding range and the winter range do still continue 

 to overlap at the southern extremity of the one and the 

 northern extremity of the other. The birds that breed 

 exclusively in the high north required a Polar temper- 

 ature, and were among the first to cease breeding in 

 temperate latitudes ; but there is perfectly conclusive 

 evidence to show that when more southerly areas were 

 Arctic in climate, the breeding range extended over 

 them, as witness so many Polar birds wintering, if in 

 small numbers, in comparatively high latitudes, as, for 

 instance, the Knot in the British Isles. 



It is only too popularly believed that the Glacial 

 Epoch was a sudden phenomenon ; that birds left the 

 Arctic regions at once, as if overtaken by a change of 

 climate as quick as that which now marks the change 

 from summer to winter in the Arctic regions ; and that 

 parties of a species, or the entire species, immediately 

 emigrated this way and that, along that coast-line or 

 down that valley, far south to a refuge from its rigours. 

 No more erroneous view of the facts could be con- 

 ceived. Ages most probably elapsed before any per- 

 ceptible difference could be observable in the northern 

 range of species, and no one generation of birds would 

 ■experience any perceptible change in the climatic con- 

 ditions. As the glacial climate slowly came on, the 

 breeding range in response slowly became more and 

 more southerly as the cold spread southwards. Species 

 simply retreated by extermination from those glacial 



