Of all arrangements, then, the only possible, and so
the necessary, one is that birds shall have two feet; for this with
the wings will give them four points for motion. The breast in all
birds is sharp-edged, and fleshy. The sharp edge is to minister to
flight, for broad surfaces move with considerable difficulty, owing to
the large quantity of air which they have to displace; while the
fleshy character acts as a protection, for the breast, owing to its
form, would be weak, were it not amply covered.
Below the breast lies the belly, extending, as in quadrupeds and
in man, to the vent and to the place where the legs are jointed to the
trunk.
Such, then, are the parts which lie between the wings and the
legs. Birds like all other animals, whether produced viviparously or
from eggs, have an umbilicus during their development, but, when the
bird has attained to fuller growth, no signs of this remain visible.
The cause of this is plainly to be seen during the process of
development; for in birds the umbilical cord unites with the
intestine, and is not a portion of the vascular system, as is the case
in viviparous animals.
Some birds, again, are well adapted for flight, their wings being
large and strong. Such, for instance, are those that have talons and
live on flesh.
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