50 THE MIGRATION OF BIRDS. 



persal applied to the Charadriid.e/ is not supported 

 by geological evidence. First, no less than three 

 glaciations of the North Polar region are required 

 to account for the present dispersal and differenti- 

 ation of this family of birds into genera and species. 

 But the evidence of more than one North Polar 

 glaciation during Tertiary time is absolutely want- 

 ing ; all that can be gleaned points to one Glaciation 

 only, which, as we have already seen, was Post 

 Pliocene. Secondly, the dominant line of Migra- 

 tion of these birds is unquestionably Inter-Polar, 

 which forces us to the inevitable conclusion that 

 the ancestral forms of this group of birds during 

 remote ages peopled a South Polar area. Another 

 fact strongly confirming Inter-Polar migration is 

 the comparatively cool temperature which appears 

 to be imperatively necessary for the majority of 

 these birds during the breeding season. It seems 

 hard to believe that such Polar birds could ever 

 become so thoroughly acclimatized to tropical zones 

 as to remain isolated in them for thousands of years, 

 as this Glacial theory of dispersal demands, when 

 an Antarctic Polar haunt was open to them, espe- 

 cially as we know that as soon as the North Polar 

 regions became once more habitable a great exodus 

 to them commenced, probably as South Polar 

 haunts became glaciated. Again, it appears to me 

 that too much reliance is placed on the various 

 reputed routes, along which species are said to have 



1 Obviously suggested by Evolution without Natural 

 Selection, chap. I. (published in 1885). 



