THE WOODPECKER'S TOOLS: HIS TAIL 93 



Which shape brings the most feathers into use 

 in all positions? Which positions bring most 

 feathers into use? We see at once that the 

 rounded end has a decided advantage, that the 

 middle pair of feathers is used in all possible 

 positions, that the pair next outside is the next 

 important, and that the spread tail curving 

 downward at the edges and the closed tail in dif- 

 ferent planes are the two shapes which give the 

 best support. There is therefore a reason for the 

 rounded end which we said was the rule among 

 the woodpeckers. 



Our little experiment is what we call a deduc- 

 tio7i. It shows us what we ought to expect 

 under certain imaginary conditions. But it does 

 not show us what actually exists, so there often 

 comes a time when our deductions are faulty be- 

 cause Nature has done some unexpected thing, as 

 when we found the single exception of the log- 

 cock's foot upsetting a fine theory of ours. A 

 deduction must always be compared with facts, 

 and is worth little or nothing if a single fact of 

 the series we are studying is not explained by 

 it. This time all the facts do agree ; for I had, 

 before we made our experiment, examined the 

 tails of every species of woodpecker ever found 

 in North America, and there was no exception to 

 the rounded end. I had already drawn my con- 



