INTRODUCTION. 
HE most conspicuous external characteristic by which the Birds are distinguished 
from all other inhabitants of earth, is the feathery robe which invests their bodies, 
and which serves the double purpose of clothing and progression. For the first 
of these two objects it is admirably adapted, as the long, slender filaments of the 
feathers are not only in themselves indifferent conductors of heat, but entangle 
among their multitudinous fibres a considerable amount of air, which resists the 
ingress or the egress of external or internal heat, and thus preserves the bird in a 
moderate temperature through the icy blasts of winter or the burning rays of the summer sun. 
A similar function is discharged by the furry coats of many mammalia ; but the feathers serve 
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