What can be
plainer than that the webbed feet of ducks and geese are formed for
swimming? yet there are upland geese with webbed feet which rarely or
never go near the water; and no one except Audubon has seen the
frigate-bird, which has all its four toes webbed, alight on the
surface of the sea. On the other hand, grebes and coots are eminently
aquatic, although their toes are only bordered by membrane. What seems
plainer than that the long toes of grallatores are formed for walking
over swamps and floating plants, yet the water-hen is nearly as
aquatic as the coot; and the landrail 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 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 will say,
that in these cases it has pleased the Creator to cause a being of one
type to take the place of one of another type; but this seems to me
only restating the fact in dignified language. He who believes in the
struggle for existence and in the principle of natural selection, will
acknowledge that every organic being is constantly endeavouring to
increase in numbers; and that if any one being vary ever so little,
either in habits or structure, and thus gain an advantage over some
other inhabitant of the country, it will seize on the place of that
inhabitant, however different it may be from its own place.
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