BIRDS IN THE GREENWOODS 243 



tree-fluttering. Now, as it appears to me, though it 

 might be easier for a bird to creep up a tree by 

 going round it, it could more easily flutter up it per- 

 pendicularly,* in the way I have described, and, if 

 so, we can understand a bird that is only in process 

 of becoming a tree-creeper, commencing, as it were, 

 at the most advanced end. For it would first have 

 fluttered up perpendicularly, then have both crept 

 and fluttered so, and finally, when it could creep 

 without fluttering, it would do so at first on the old 

 fluttering lines. Then it might begin to adopt the 

 spiral method, but as the effort required became less 

 and less, and structural modification — as seen, for 

 example, in the shape and stiffness of the tail-feathers 

 of the tree-creeper — came to its assistance, this would 

 cease to be a help, and become a habit merely, and 

 when once a habit has lost its rationale, it is in the 

 way of being broken, even in good society. Thus 

 the perpendicular ascents of the tree-creeper may be 

 the final stage in a long process, and the return in 

 ease to what was before done in toil. 



The tree-creeper is assisted in its climbing by the 

 stiff", pointed feathers of the tail, which act as a prop, 

 and also by its small size, which may possibly have 

 been partly gained by natural selection. The great 

 green woodpecker is possessed of the first of these 

 advantages, but not of the second, and it is, I believe, 

 the case that he much more adheres to the spiral 

 mode of ascent than does the tree-creeper, who, as it 

 seems to me, has almost discarded it. It would be 

 interesting, therefore, to observe if the smaller spotted 



* Or rather no particular difficulty would be experienced, so that the 

 shortest course would be the best one. 



