How much knowledge has increased does not appear to be
generally realized. I say this because lectures have been recently delivered in which a number of the exploded theories of ten years ago have been repeated. I have been comparing books by noted authorities published in 1908 and 1926; and it is astonishing how far out of date the older books are— under three heads : the maps of the routes, the average height at which migrant birds fly, and the speed of flight. How immensely Glitke (whose account of migrants descending on Heligoland is classic) exaggerated the height of voyaging birds ! The old accounts of fantastic speeds have no authority whatever. We now know that 2,000 yards is much above the average height and sixty miles an hour above the average speed even of the speediest fliers. As to route, evidence is accumulating that in autumn our summer visitors fly more directly south and less east than we used to think : but on this head more evidence is wanted, especially from West Africa.
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