5.26 BRITISH BIRDS. 



important internal characters, individual variation in many species is so 

 great that there is no lack of material for natural selection to choose 

 from, and that wherever a change could benefit the species, there does 

 not appear to be any adequate reason why an internal change should not 

 be as rapid as an external one. If like causes produce like effects in 

 external characters, why not in internal characters also ? Why may not 

 the superficial resemblance between a Woodpecker and a Nuthatch, and 

 between a Swift and a Swallow, date as far back as, or further than, the 

 osteological resemblances between the Woodpecker and the Swift or 

 between the Nuthatch and the Swallow ? If the Picarian birds be the 

 least-changed descendants of the common ancestors of the Picarian and 

 Passerine birds, why may not the Woodpeckers be the least-changed 

 descendants of the common ancestors of the Woodpeckers and the Nut- 

 hatches, and the Swifts the least-changed descendants of the common 

 ancestors of the Swifts and the Swallows ? Or is it necessary to assume 

 that all the Passerine birds were developed from one branch only of the 

 semi-Picarian ancestors ? Is it not possible that the geological revolutions 

 which changed the food, climate, or other surroundings of the ancestors 

 of the Passerine birds, and was the ultimate cause of their attaining 

 to their present high state of development, affected many of the then 

 existing genera, and produced a rapid advance of development from several 

 centres in parallel directions, and that after all there may be more truth 

 in the conclusions of the casual observer than the anatomist is yet 

 prepared to admit ? 



But whatever may be the genealogy of the Nuthatch, every egg-collector 

 will agree that, in spite of its Woodpecker-like bill, it belongs to the 

 Parinse, together with the curvirostral Creeper. Even in its habits the 

 Nuthatch differs widely from the Woodpeckers. Like the latter birds, 

 the Nuthatch obtains its food almost exclusively on the bark of trees, but 

 also, like them, seeks it occasionally on the ground. The Nuthatch, 

 however, being furnished with a well-developed Passerine hind toe and 

 claw, is able to run up the trunk of a tree with the greatest ease. So 

 does the Woodpecker, in spite of his feebly developed Picarian hind toe, 

 thanks to his having reversed the position of one of his stronger fore toes, 

 and, at the same time, pressed his tail into the service to prevent his 

 slipping back. But however useful the tail of the Woodpecker may 

 be in ascending, it is of no use in descending; so the Woodpecker 

 generally begins at the bottom of a tree and works his way up to the 

 top, and then drops down to the bottom of the next tree and begins 

 again. The Nuthatch, on the other hand, being independent of support 

 from his tail, can descend with as much ease as he can ascend, and con- 

 sequently flits on from tree to tree like a Tit. Another peculiarity in 

 which the Nuthatch agrees with the Tits and differs from the Woodpeckers 



