140 THE BIRDS OF HELIGOLAND 



of the Atlantic between Newfoundland and Ireland. Hence it was 

 believed that the only way in which it could accomplish this 

 journey was by making use of what are called ' diluvial land 

 bridges.' These at the present day are represented by the mere 

 isolated remnants of what, in primary geological periods, were large 

 land connections between different continents. In the case of 

 birds crossing over from America to Europe, such a connection is 

 assumed to be formed by Greenland, Iceland, the Faroes, Shetland, 

 and Orkney Islands. The employment of this path as a migration 

 route is considered to have developed into a habit, and this habit 

 to have passed by hereditary transmission from one generation to 

 another from primitive times down to the present day, so that the 

 birds now in existence are able to find their way with perfect 

 certainty from one to another of these mutilated remnants of a 

 previously contmuous chain of land in spite of the fact that these 

 detached fragments lie far beyond their range of A'ision. Italy, 

 which at one time connected Europe with Africa, dividing the 

 Mediterranean into two inland lakes, is said to have formed a 

 land bridge of this kind, for birds exchanging their habitats between 

 these two continents. 



Having already shown, when speaking of the velocity of the 

 migratory flight, that a bird is able to cross in nine houi'S from 

 Newfoundland to Ireland, the hypothesis of these auxiliary roads 

 loses much of its force. Its intrinsic weakness is, moreover, suffi- 

 ciently displayed in the fact that it can apply to only one or two 

 cases of migration across the sea, and not universally. Among 

 instances which it cannot touch, we maj' once more refer to the 

 autumn migration of the American Plover, Charadrius virginiciis, 

 which extends from Labrador nearly to Patagonia. Besides the 

 innumerable flocks of these birds which every autumn travel rid 

 the Bermudas and Antilles, their migration column stretches from 

 four to six hundred miles south to the east of Bermuda, whence 

 the southern flight of the latter of these migrants extends from 

 Labrador to the north coast of Brazil. Now at no time can there 

 have existed along the whole length of this line a land bridge of 

 the kind referred to above. Throughout the whole of its course 

 the depth amounts to two thousand fathoms, and between latitudes 

 18° and 28° N. it reaches as much as three thousand fathoms. 

 Assuming the depression of the land to be just short of a metre ^ 

 per century, from four to six hundred thousand years must have 

 elapsed since the sea-bottom along that line was level with the 

 surface, or rose above it in the form of a land bridge. Tliis period, 

 comparatively short though it may be in the operation of geological 



1 i.e. 3-39 feet. 



