526 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-lke bill, it belongs to the 
Parinze, 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 im 
which the Nuthatch agrees with the Tits and differs from the Woodpeckers 
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