146 THE MEANING OF EVOLUTION 



In the case of the l)rir.k 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 



