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. In some of these heavy-bodied
birds the legs are furnished with what are called spurs, which replace
the wings as a means of defence. Spurs and talons never co-exist in
the same bird. For nature never makes anything superfluous; and if a
bird can fly, and has talons, it has no use for spurs; for these are
weapons for fighting on the ground, and on this account are an
appanage of certain heavy-bodied birds. These latter, again, would
find the possession of talons not only useless but actually injurious;
for the claws would stick into the ground and interfere with
progression. 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
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