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.
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. 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 varies ever so little, either in habits or
structure, and thus gains an advantage over some other inhabitant of the
same country, it will seize on the place of that inhabitant, however
different that may be from its own place. Hence it will cause him no
surprise that there should be geese and frigate-birds with webbed feet,
living on the dry land and rarely alighting on the water, that there should
be long-toed corncrakes, living in meadows instead of in swamps; that there
should be woodpeckers where hardly a tree grows; that there should be
diving thrushes and diving Hymenoptera, and petrels with the habits of
auks.
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