species from far eastern regions* which are met with in Heligo- 

 land, must stand to the quantity of these erratics passing through 

 the whole of Central Europe every autumn, in the same relation 

 as the size of this island does to that of the continent." There- 

 fore, if in Heligoland alone from eighty to one hundred examples 

 of the Yellow-browed Warbler, have been shot or seen during the 

 last fifty years, it foUows that the same species, but in proportion- 

 ately greater numbers, must also have appeared in Germany or 

 other parts of Europe during the same period. 



The above extracts make it sufficiently clear that the author 

 wishes his definition of a broad migration column to be taken 

 in its most literal sense. 



In comparing the area of Heligoland with that of Central 

 Europe, one naturally only compares the area of the former, 

 with a like area occupying the same longitude in the latter, as 

 the flight is supposed to travel east to west. Otherwise the 

 same individuals would be liable to be enumerated over and 

 over again at various different points. But if we are to apply 

 the before-mentioned theory to the rarer species, we are surely 

 entitled to do the same in the case of the commoner ones. 



To fully realise the significance of such a theory, we have 

 only to take Herr Gatke's estimates of the numbers, comprising 

 the flocks of Hooded Crows, Starlings, Skylarks and others, 

 which pass the island, and multiply them by the vastly greater 

 extent of Central Europe in comparison with Heligoland, to 

 find that the results are something incredible and altogether 

 impossible. It must not be forgotten, too, that migration for 

 the most part, in the opinion of the author, takes place at 

 altitudes beyond the powers of ordinary vision, and that such 

 of it that comes within our cognisance is due to disturbances 

 of the normal flight. It is only fair to mention, however, that 

 the above-named species are amongst the few which usually 

 perform their journeys at a very moderate elevation. There 

 still remain the great majority, however, whose normal flight 

 is performed at heights beyoijd our vision. 



As an illustration of what in Herr Gatke's idea is the breadth 

 of the migration front, his own words may be quoted. He writes 

 as follows : — " In the case of the most different species, and in 



