The water-hen and landrail are members of this order, yet the first is nearly as aquatic as the coot, and the second is nearly as terrestrial as the quail or partridge.

In such cases, and many others could be given, habits have changed without a corresponding change of structure.

The webbed feet of the upland goose may be said to have become almost rudimentary in function, though not in structure.

In the frigate-bird, the deeply scooped membrane between the toes shows that structure has begun to change.