SEC. m EXTERIOR PARTS OF BIRDS 123 



migratorius Linnaeus ; Planesticus migratorius (Linn.) Bonaparte. The 

 practice still prevails. 



It would take me too far to go fully into the rules of nomen- 

 clature : some few points may be noted. A proper sense of justice 

 to the deseribers of new genera, species, and varieties prompts us to 

 preserve inviolate the names they see fit to bestow, with certain 

 salutary provisions. Hence arises the "law of priority." The first 

 name given since 1758 is to be retained and used, if it can be iden- 

 tified with reasonable certitude ; that is, if we think we know what 

 the giver meant by it. But it is to be discarded, and the next name 

 in priority of time substituted, if it is " glaringly false or of express 

 absurdity," — as calling an English bird " africanus," or a black one 

 " albus." No generic name can be duplicated in zoology, and one 

 once void for any reason cannot be revived and used in any connec- 

 tion. The same specific name cannot be used twice in the same 

 genus. 



The Actual Classifleation of Birds has undergone radical 

 modification of late years, though the same machinery is employed 

 for its expression. This is as would be expected, seeing how pro- 

 foundly the theory of Evolution has affected our principles of classi- 

 fication, how completely the morphological has replaced other 

 systems, and how steadily our knowledge of the structure of birds, 

 and their chronological relations, has progressed. Nevertheless, the 

 ornithological system is still in a transition state. 



With this glance at some taxonomic principles and practices, I 

 pass to an outline of the structure of birds, some knowledge of 

 which is indispensable to any appreciation of ornithological defini- 

 tions and descriptions. It is necessary to be brief, and I shall 

 confine myself mainly to the consideration of those points, and 

 the explanation of those technical terms, which the student needs 

 to understand in order to use any systematic treatise easily and 

 successfully. 



§ 3.— DEFINITIONS AND DESCRIPTIONS OF THE 

 EXTEEIOE PARTS OF BIRDS 



a. Of the Feathers, or Plumage 



Feathers are possessed only by birds, and all birds possess 

 them. Feathers are modified scales ; like scales, hair, horns, plates, 

 sheaths, etc., they are outgrowths of the integument, or skin cover- 

 ing the body, and therefore belong to the class of epidermic (Gr. «rt, 

 epi, upon ; 8ep/j,a, derma, skin), or exoskeletal (Gr. k^, ex, out ; a-KeX- 

 cToV, skeleton, dried ; in the sense of " outer skeleton ") structures. 



