ADAPTATION FOR THE INDIVIDUAL Io! 
The birds have need for better clothing. To begin 
with, their blood is much warmer, and hence needs 
better protection from outside cold. In addition such 
of them as fly high must be prepared to stand great 
variations in temperature. For these purposes birds 
need a covering of the finest type. This clothing, in 
addition, must be extremely light because the creature 
must carry it into the air in flight. All of the requi- 
site conditions are thoroughly met by the feather, 
which is the lightest and warmest clothing known to 
man. If at night we wish, regardless of expense, to 
keep ourselves warm with the lightest and warmest of 
covering, we send to the Arctic Sea, and from the 
breast of the eider duck we pluck the down which 
lies between the warm blood of the duck with its tem- 
perature of one hundred and seven degrees and the 
water in which the iceberg floats. 
Young mammals and birds, before their clothing 
has well formed, are naturally susceptible to cold; this 
leads to the first genuine approach to a home among 
animals lower than man. Birds lay their eggs long 
before the creatures inside of them are ready to 
emerge. Accordingly they have learned to build nests 
in which to place these eggs, and to protect them 
from the outside air; meanwhile the bird keeps the 
eggs warm by close contact with its own body. The 
lowest of the birds may lay their eggs simply on the 
