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. This is the reason why these birds when they
fly make use of their legs as a tail, stretching them out behind,
and so rendering them serviceable, whereas in any other position
they would be simply an impediment.
In other birds, where the legs are short, these are held close
against the belly during flight. In some cases this is merely to
keep the feet out of the way, but in birds that have talons the
position has a further purpose, being the one best suited for
rapine.
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