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.

He who believes in separate and innumerable acts of creation may say, that in these cases it has pleased the Creator to cause a being of one type to take the place of one belonging to another type; but this seems to me only restating the fact in dignified language.