146 THE MEANING OF EVOLUTION 
In the case of the bank swallow his nest may be 
a very simple contrivance, consisting only of a tunnel 
running back into a bank, and widening at the back. 
Some material that will soften the bed upon which 
eggs are to be laid must be placed in this cavity. The 
whole home is a very simple and crude affair. But 
little better is the arrangement which the woodpecker 
calls a home. This has been cut into the dry wood of 
a defective tree. No woodpecker can make his home 
in absolutely solid sapwood. Hence the first labor of 
the woodpecker must consist in finding a place in 
which it can dig. If there is an old stump of a limb 
sticking up, the problem is readily solved. Such 
wood has no sap in it, and is brittle enough to be eas- 
ily dug out. But, if there be no such stub, the wood- 
pecker will find a suitable place in most trees. At 
some time or other almost every tree loses a big limb. 
When such accident occurs there will always be in the 
old trunk a region through which sap once went to 
this limb. This region, deprived of its function, goes 
completely dry, like the heartwood of the tree, and it 
is into such material as this that the woodpecker suc- 
ceeds in drilling his well-protected home. 
As birds rise higher in the scale the nest-building 
becomes a more complicated affair, and after a while 
we find a well-woven substantial nest, through which 
even the air will not chill the eggs enough to prevent 
