244 FIRST PRINCIPLES OF NATURAL CLASSIFICATIOK. 



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 

 ground, 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 ivory, 

 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 usual 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 formei , this perfection of structure 

 is seen in the mammalia among vertebrated animals ; 

 in the winged insects, or the Ptilota of Aristotle, in the 

 sub-kingdom Annulosa ; in the testaceous shell-fish 



