It has generally been accepted that the feathers in the bird's wing are the outcome of nature's effort to provide a plane surface and that the resulting wing acts as a simple plane when in use either for soaring or propelling.

As egards soaring the explanations even under this supposition appear adequate, but when the wing is used as a propeller or for lilting the bird, the usual explanations have a1 ways appeared to the author as extremely unconvincing.

On examining the wing of a bird it will be noticed that the feathers are so arranged that the leading feather is the lowest.