lyo THE MIGRATION OF BRITISH BIRDS 



picture the remote future when the northern movement 

 may die completely away either by a complete change 

 to a warm and equable climate at the North Pole, or by 

 the coming on of a new era of glacial conditions. We 

 should, therefore, if the above remarks are correct, 

 expect to observe instances of northern emigration 

 taking place at the present time. This is precisely what 

 we do find. In many parts of the world, especially in 

 countries where owing to a high state of civilization the 

 phenomenon can be carefully observed. Emigration is 

 undoubtedly still going on. If this Emigration is in 

 progress in civilized countries we have every reason to 

 believe that it continues to progress in other parts of 

 the world where the movement cannot be so easily 

 detected ; but it may be helped or retarded by the 

 presence or absence of civilization. Thus we have 

 many instances where the spread of cultivation has on 

 the one hand assisted Emigration, whilst on the other 

 hand similar cultivation has had a directly opposite 

 effect, and actually decreased or curtailed the range of 

 many species in certain areas, and, as we know, ulti- 

 mately banished them from those areas altogether. 



In the present chapter I propose to deal exclusively 

 with those species of which we have absolute proof of 

 their emigration being actually in progress, within the 

 limits of the British Archipelago. It will perhaps bs 

 best to deal with each species separately, and to give in 

 detail the peculiarities of each emigratory movement, 

 so far as we can trace it, or it has hitherto been observed. 

 The first species whose emigratory movements we will 

 endeavour to trace is the Missel Thrush {Turdus visci- 

 voriis). This species has very considerably increased its 



