944. FIRST PRINCIPLES OF NATURAL CLASSIFICATION. 
longing to the crow, the whole structure becomes adapted 
for one particular purpose—that of climbing trees, and 
extracting from them the allotted food. The energies 
of nature are concentrated, as it were, to the production 
of that form most adapted for one especial purpose. 
Every part of the structure of a woodpecker, not im- 
mediately essential to its peculiar habits, is found to be 
in a subordinate state of developement,—for the best of 
reasons, because their greater developement is unneces- 
sary. Its flight is comparatively feeble, for it merely 
journeys from tree to tree even in its migrations: ‘the 
feet almost incapacitate the bird from moving upon the 
eround, for there its food does not exist: the position 
of its legs, placed very far back, gives it an awkward 
appearance on an even surface ; but what is this to a 
bird which usually sits in a perpendicular position? Its 
food is almost entirely restricted to particular insects ; 
and to procure these it is gifted with powers which are 
withheld from all other birds. It is the type of the 
climbing race; and, consequently, exhibits the scansorial 
structure, and no other, in the highest degree of per- 
fection. Great muscular strength, for striking blows 
which may be heard half a mile ; a bill as hard as iyory, 
formed on the model of a perfect wedge ; a strong rigid 
tail, acting as a buttress to the body when the bird is at 
labour ; short robust feet, armed with strong claws for 
grasping the bark, and a long spear-shaped tongue 
for inserting within its clefts,—these are the typical 
distinctions of the family before us, which, however 
inferior to the crow in the number of its perfections, 
far exceeds it in one, that one being the characteristic 
of its whole tribe. The union of many properties is, 
therefore, the usuai character of types of typical groups ; 
while the highest developement of some one property 
is the distinguishing mark of types belonging to aber- 
rant groups. In the former, this perfection of structure 
is seen in the mammalia among yertebrated animals ; 
in the winged insects, or the Ptilota of Aristotle, in the 
sub-kingdom Annulosa ; in the testaceous  shell-fish 
