mming, and of flying when
{185} unwillingly it takes flight, would be mistaken by any one for an auk
or grebe; nevertheless, it is essentially a petrel, but with many parts of
its organisation profoundly modified. On the other hand, the acutest
observer by examining the dead body of the water-ouzel would never have
suspected its sub-aquatic habits; yet this anomalous member of the strictly
terrestrial thrush family wholly subsists by diving,--grasping the stones
with its feet and using its wings under water.

He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not at all in agreement. 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 {186}
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. Hence it will cause him no surprise that
there should be geese and frigate-birds with webbed feet, l