CHAPTER IV. 
WHY DID THE BIRDS PUT ON SOFT RAIMENT ? 
Nature is a great preserver of her energies. 
Only in the growing and sowing of many seeds does 
she appear wasteful, but this is the truest economy. 
She expects adversity and accident, and prepares 
to meet both, as if she had learned her lesson grad- 
ually by experience. 
The bird was likely her first hot-blooded animal. 
Certainly it was among the first. For the invasion of 
earth and air, as we have seen, she needed a better 
furnace and fanner, and a better fuel carrier than 
any reptile or fish had. So she added more cham- 
bers to the heart (or to her liquid fuel pump) and 
more cells to the lungs (or flues to her boiler), till 
the blood pulsed hot and throbbing with power in 
the new creature. 
Except perhaps the smaller winged insects, no ani- 
mal works at a higher pressure than the flying birds. 
Their normal temperature (104° F.) is high fever heat 
in man. They burn fuel fast. No other vertebrate 
expends so much per hour or for so long a time at a 
stretch. It is like ‘an ocean greyhound ” in minia- 
ture. 
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