CONDITIONS OF FLIGHT 237 



was dry land ; their migration route gradually assumed 

 its present aspects as submergence progressed, no single 

 generation of birds having experienced any very sudden 

 change, the sea-passage becoming imperceptibly wider 

 and wider as the land vanished, until the present state 

 of things was reached. 



That migration routes are traversed by experience, 

 and not by inherited impulse, is further proved by the 

 variability of the habit, some individuals going longer 

 distances than others, some remaining stationar}'- alto- 

 gether. Again, if birds are endowed with that mysterious 

 " sense of direction," which popular opinion so readily 

 ascribes to them, how can that sense explain the endless 

 routes, the tortuous journeys, the migrations this way 

 and that to common range bases or centres of dispersal ? 

 How utterly at fault it must be in those species that 

 breed in the far north-east and that winter in the 

 remote south-west ; how impotent in species that breed 

 in Pomerania and journey south-east all the way to 

 India to winter, when just as suitable localities are 

 available directly south and not a quarter of the dis- 

 tance ! Is it not more rational to presume that these 

 migrant birds are following the route of their ancient 

 range extension, a route with which they must be 

 thoroughly familiar, the result of it may be thousands 

 of years of experience ? The very fact that migratory 

 birds, generally speaking, keep so closely to their normal 

 areas of distribution is a most convincing proof that they 

 do not wander from their routes of passage. For this 

 reason I consider it most absurd to say that a wave of 

 migration has been deflected this way or that normally. 

 If it were so, birds would be drifted into country of 



