ROUTES 43 



and water since the time when birds began to be 

 migratory is indeed probable, but unless crossing 

 the sea means a distinct advantage it implies the 

 retention of a habit which would not only be useless 

 but might be a positive danger to the species. 



In the Gulf of Mexico and the Caribbean Sea there 

 is evidence of perhaps the most recent land-bridge 

 in the chain of islands from Florida to Venezuela, 

 collectively known as the West Indies. Although 

 vast numbers of North American birds winter in 

 South America only a few of the species which an- 

 nually pass from one continent to the other make 

 use of this comparatively easy passage. One might 

 naturally conclude that the final severance of England 

 from the Continent was in the neighbourhood of the 

 Straits of Dover, yet this short passage is only used 

 by a comparatively small number of our migrants. 



Mr Dixon indeed argued that there is no greater 

 barrier to migration than even a narrow arm of the 

 sea (26). He refers to many Continental species 

 which are common breeders in France but are un- 

 known as nesting species in the British Islands, and 

 others which are found in England but not in 

 Ireland. But this is surely but an incident of dis- 

 tribution ; the narrow strait or even river may for 

 a time mark the limit of expansion of a species, just 

 as at the present time the westward and northward 

 unseen barrier prevents the range of the nightingale 



