CHAPTER VI 
FORM AND FUNCTION 
Digestive Apparatus 
WHAT is the cause of the wonderful vitality of 
birds? How is it that the Golden-crested Wren, 
apparently so weak and helpless, can fly all across 
the North Sea from Norway? What are the pro- 
cesses of life that go on within the bird and make 
it so different from its lethargic reptilian ancestors? 
To these questions I hope to give some answer in 
the present chapter. 
To begin with, a bird has a very large appetite, and 
a reptile a very small one. I have found twenty-two 
acorns in the crop of an unusually small wood-pigeon, 
and this was probably quite an ordinary meal to him. 
They had not made him torpid, like a boa-constrictor 
after his weekly rabbit. He was flying with all his 
usual vigour when the shot brought him down. To 
speak of an animal as an engine, the supplies of fuel 
1As many as sixty-three have been found. See Badminton 
Library, Shoodzng, p. 229. 
