40 THE WOODPECKERS 



that the internal difference, whatever it is, is in 

 favor of the tree that is dying ; while the only 

 external difference appears to be the marks left 

 by the sapsucker. While one tree is sparingly 

 marked by him, the other is tattooed with his 

 punctures, placed in single rings and in belts 

 around trunk and branches beneath every fork. 

 It is a law of reasoning that, when every condi- 

 tion but one is the same and the effects are dif- 

 ferent, the one exceptional condition is the cause 

 of the difference. If these trees are alike in 

 everything except the work of the sapsucker (the 

 only internal difference apparently offsetting his 

 work in part), what inference do we draw as to 

 the effect of his work ? 



We presume that he is killing the tree, with- 

 out as yet knowing how he does it. What is 

 his object? Good observers have stated that 

 he draws a little sap in order to attract flies and 

 wasps ; that the sap is not drawn for its own 

 sake, but as a bait for insects. Is this theory 

 true ? 



The first objection is that it is improbable. 

 The sapsucker is a retiring, woodland bird that 

 would hesitate to come into a town garden a 

 mile away from the nearest woods unless to get 

 something he could not find in the woods. Had 

 he wanted insects, he would have tapped a tree 



