BIRDS AND LIGHTHOUSES 269 



these places by lines on a map for each species. 

 The lines thus drawn show the equality or inequality 

 of advance made by the species in different 

 longitudes, and, assuming that this advance is right 

 across the isepiptesial lines, or the tract lying 

 between each pair of them, the route of migration 

 is thus clearly indicated. The conclusion at which 

 he arrives is, that in Central Siberia the general 

 direction taken by migratory birds (he enumerates 

 seven species) is almost due north, in Eastern 

 Siberia from south-east to north-west, and in 

 European Russia from south-west to north-east. 



Another Continental naturalist, Herr Palmen, 

 pursuing a similar line of inquiry, published, in 1874, 

 at Helsingfors, an important work (originally in 

 Swedish, afterwards translated into German) on the 

 migration of birds, in which he also endeavoured 

 to trace the general routes taken, and to explain 

 the cause or causes of migration.^ 



The routes usually pursued he believes to be 

 nine in number ; and, if we mark these out on a map 

 of Europe, it must be confessed they appear some- 

 what confusing, as so many of the lines cross, and 

 the evidence that such routes as those indicated are 

 taken seems scarcely strong enough to establish the 

 facts contended for. In fact, these routes are 

 merely conjectural. Moreover, it is evident that 

 the routes must vary, as proved by the absence or 

 scarcity in some years of particular species along a 

 line of route on which they are usually common. 

 Dr Weissmann considers, and so also does 

 ^ Ueber die Zugstrassen der Vdgel. 8vo. Leipzig, 1876. 



