ADAPTATION FOR THE INDIVIDUAL lOI 



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 



