192 ANNIVERSARY ADDRESS 



path followed by any species may be called its "Line 

 of Migration," and the coincidence of two or more of 

 these lines may be expressed by the term " Route." 



We find accordingly that the main stream of migration 

 normally flows in a direction more or less southerly in 

 autumn, and more or less northerly in spring, which is 

 precisely what might have been expected from our data ; 

 but the question is complicated by the fact that large 

 numbers of birds fail to steer a course directly to the 

 south or to the north, as for instance in the case of 

 the Curlew Sandpiper, well known, upon our shores on 

 passage, though its breeding quarters appear to be 

 entirely confined to a comparatively small portion of 

 the Arctic Regions of Asia. 



An explanation may, however, be found in the theory 

 of "Lines of least resistance," or in other words we may 

 assume that the flocks will follow that course in which 

 they find their progress least impeded by any geographical 

 or climatic obstructions. High mountains, for example, 

 may prove insuperable obstacles to birds of weaker or 

 more lowly flight, wide oceans may be found impossible 

 to cross, or strong winds may cause some slight divergence 

 in the angle of direction, a divergence which will increase 

 proportionately the further the flocks travel. 



We thus should naturally expect to find, and to some 

 extent we do find, a preference shown by various species 

 for river valleys — especially when of considerable breadth 

 — for the coast-lines of continents, and for open plains, 

 if not of exceptional sterility. But once more we must 

 take into consideration the above-mentioned increase in 

 the breeding area of any form — as far as expansion to 

 the east or west is concerned — which will cause the 

 surplus population, thrust outwards during successive 

 seasons of reproduction, to take its departure in autumn 

 over somewhat diff'erent lines from its predecessors. Con- 

 sequently we are not astonished to discover that the 

 " Lines qf Flight " and " Routes " of birds are many and 



