This is the reason why all birds with talons walk so
badly, and why they never settle upon rocks. For the character of
their claws is ill-suited for either action.
All this is the necessary consequence of the process of development.
For the earthy matter in the body issuing from it is converted into
parts that are useful as weapons. That which flows upwards gives
hardness or size to the beak; and, should any flow downwards, it
either forms spurs upon the legs or gives size and strength to the
claws upon the feet. But it does not at one and the same time
produce both these results, one in the legs, the other in the claws;
for such a dispersion of this residual matter would destroy all its
efficiency. In other birds this earthy residue furnishes the legs with
the material for their elongation; or sometimes, in place of this,
fills up the interspaces between the toes. 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.
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