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. Thus it is simply a
matter of necessity, that such birds as swim shall either be
actually web-footed, or shall have a kind of broad blade-like margin
running along the whole length of each distinct toe. The forms,
then, of these feet are simply the necessary results of the causes
that have been mentioned. Yet at the same time they are intended for
the animal's advantage. For they are in harmony with the mode of
life of these birds, who, living on the water, where their wings are
useless, require that their feet shall be such as to serve in
swimming. For these feet are so developed as to resemble the oars of a
boat, or the fins of a fish; and the destruction of the foot-web has
the same effect as the destruction of the fins; that is to say, it
puts an end to all power of swimming.
In some birds the legs are very long, the cause of this being that
they inhabit marshes. I say the cause, because nature makes the organs
for the function, and not the function for the organs. It is, then,
because these birds are not meant for swimming that their feet are
without webs, and it is because they live on ground that gives way
under the foot that their legs and toes are elongated, and that
these latter in most of them have an extra number of joints. Again,
though all birds have the same material composition, they are not
all made for flight; and in these, therefore, the nutriment that
should go to their tail-feathers is spent on the legs and used to
increase their size
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