16 STRUCTURE AND CLASSIFICATION OF BIRDS 



On theoretical grounds it might fairly be assumed that a 

 continuous covering, without any distinctions between 

 pterylse and apteria, was a primitive condition. But there 

 is evidence to show that where a continuous covering of 

 feathfers exists it is not invariably a mark of ancient stock. 

 Thus the ostrich, as already remarked, which when adult 

 has practically no separation into pterylae and apteria, has, 

 when young, very distinct pterylas and apteria. In this 

 case, therefore, it is clear that the uninterrupted feathering 

 is a secondary character and not a primitive one. With the 

 penguins, on the other hand, it is conceivable that the 

 absence of apteria is a primitive character. As to the value of 

 the various arrangements of apteria in the pterylosis, G-adow 

 lays stress upon the continuous feathering of the neck, or 

 the presence there of lateral spaces, but admits that the 

 Indian painted snipe {BhynchcBa) is an exception which 

 somewhat destroys the value derivable from the considera- 

 tion of the facts. Fuebringee, on the other hand, uses in 

 his tables of characters the dorsal tract and its modifications. 

 But the variations which occur here in a single and surely 

 well-marked family (e.g. Picidse) tend to shake our faith in 

 the value of the exact way in which the hypothetically con- 

 tinuous feathering has lost its continuity. No doubt G-adow 

 is right in saying that it is of taxonomic importance ' more 

 in the investigation of small than of large groups.' 



NiTZSCH, for instance, lays some stress upon the ' furcate 

 division and degradation of the portion of the spinal tract 

 situated between the shoulder blades ' in the Accipitrinse ; 

 this division includes the owls. 



But on turning over his plates one is struck by the fact 

 that the peculiarity in question is by no means confined to 

 that group, occurring as it does in such widely removed 

 forms as Caprimulgidse, Charadriidae, and Psophia. Nor is 

 an undivided dorsal tract a distinctive mark of affinity, 

 since it is to be found in such a diversified assemblage as 

 that including Pavo, Alcedo, Certhia, Todus, and various 

 passerines. The ventral tracts divide each of them upon 

 the breast into an outer and an inner division in Pernis 



