148 THE STORY OF THE BIRDS 



the significant fact that to enter such areas a 

 movement in a direction opposed to this sug- 

 gested law of dispersal would have to be made ; 

 but we also find that, however remote the 

 breeding grounds of a migratory species may 

 be from the original centre of dispersal of that 

 species (or one certain portion of it), it returns 

 unerringly to that base, notwithstanding the fact 

 that winter quarters are available by travelling 

 in some cases less than one-third the distance. 

 The distribution of our own Nightingale in 

 England is an admirable example of the first- 

 named fact, the absence of that bird from the 

 south-west peninsula of England being, we 

 believe, entirely due to that Law which forbids 

 emigration or range expansion in the Northern 

 Hemisphere to take a southern course ; whilst 

 such a species as the Arctic Willow Wren (Phyl- 

 loscopus borealis), that actually breeds as far 

 west as Finmark, but is not known to winter 

 any nearer that country than Burma (the range 

 base of the species), may be instanced as an 

 equally admirable example of the second fact. 

 It might be thought that such volant creatures 

 as birds would not be deterred in their colo- 

 nising or range-expanding movements by such 

 obstacles as narrow seas in not a few cases 



