1 66 THE MIGRATION OF BRITISH BIRDS 



treat " to escape the growing inclemency of our English 

 winters. Now if we take the instance of the Dartford 

 Warbler and apply it to a Glacial Epoch, what are 

 the results ? The northern individuals would perish ; 

 the southern representatives of the species would sur- 

 vive, and by Post-Glacial Emigration would expand 

 their previously contracted area, and re-stock the 

 northern lands wherever conditions were favourable 

 to such extension. Had the species no southern 

 base beyond the limits of glaciation or the effects 

 of the severe climate, // vnist have been extenni- 

 natcd. Did it manage to survive in any favourable 

 locality in the south, that locality must have been within 

 its Pre-Glacial range. With Inter-polar (or perhaps 

 Inter-hemisphere) species, however, the conditions were 

 different. These species were probably always migra- 

 tory ; glaciation might curtail their summer or v/inter 

 range, and in combination with equinoctial precession 

 entirely reverse it, but their migratory habits would 

 preserve them from extinction. The Glacial Epoch, 

 then, exerted its baneful influence upon all living things 

 that dwelt permanently within its limits, and mercilessly 

 if slowly exterminated all species, or the northern por- 

 tions of all species, wherever they came within its power. 

 Such species as had no soiitJiern Pi-e-Ghtcial breeding base 

 beyond the limits of glacial influence to sustain tJievi 

 vanished for ever ; only those zvell rooted in more 

 southerly latitudes preserved themselves, and from these 

 have descended all the emigrants that have re-peopled 

 the once devastated northern areas. I may here remark, 

 that by "contraction of range" I mean the complete 

 extermination of all individuals of a species within the 



