AUTUMN MIGRATION IN BRITISH AREA 267 



another movement going on amongst the same species 

 to a great extent. These are the individuals that breed 

 in our islands and pass to more southern areas to winter. 

 The cause of this double movement has already been 

 dwelt upon. 



It has been asserted that some species reverse their 

 routes almost entirely in autumn, and travel south by 

 quite a different fly-line from that which they followed 

 north in spring. Unfortunately, with too great a respect 

 for the opinions of other naturalists, I myself have alluded 

 to this movement in the Migration of Birds as though it 

 were a fact. There cannot be the slightest doubt that 

 this change of route is purely imaginary, for if we look 

 closely into the facts they will be found to admit of a 

 very different construction. When I wrote that volume 

 I regret that the Law of Dispersal which I have 

 attempted to explain and illustrate in the present work 

 was then entirely unknown to me. I had accepted the 

 general belief that a glacial epoch could cause southern 

 emigration, and I was also labouring under the very 

 general, if quite erroneous, idea that species were driven 

 this way and that across the world without any govern- 

 ing impulse. Let us deal with the few instances known 

 to me, and which I gave as examples of route reversal. 

 The first species was the Nightingale. As a proof that 

 this bird travels by a different route in autumn from 

 that which it traverses in spring, it is shown that it 

 passes Heligoland in April and May, but has never been 

 caught there in autumn. Now Heligoland is situated at 

 or near the very northern limits of this bird's distribution. 

 There is nothing then very remarkable about a few birds 

 overshooting the mark in spring and visiting the island, 

 just as we know many southern birds visit our islands 



