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 annually 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 unknown 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 distribution; 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 from spreading to districts apparently well suited for its home, and until recently the turtle dove and great crested grebe were checked in their northward advance. In the evolution of some routes land-bridges certainty appear to have played their part, but once those bridges have ceased to influence direction the shortening of the time occupied by the lengthening of the single oversea flight is only a question of generations when an advantage to a species is to be gained. This subject will be further dealt with in connection with the actual passages performed by certain birds. The study of migration, based on observations at our lighthouses and lightships, shows that even in the comparatively small area of the British Islands there are certain routes followed with regularity. The birds which pass along our western coasts of England and Wales do not as a rule follow the shores round Cardigan Bay or along the eastward tidal scoop of the Irish Sea towards the coasts of Lancashire; the main body passes from Pembroke to the Lleyn Peninsula, and thence to Anglesey and the Isle of Man, on its way to the southern Scottish shores.