22 GENERAL HISTORY OF BIRDS. 



and the diversity of structure that exists throughout the vertebrate series, 

 which of course includes birds, an evident unity of plan and function prevails 

 everywhere. Again we may confine ourselves to the morphology of the ex- 

 tinct forms of birds, — a field of research covered by avine palaeontology. As 

 applied to the group under consideration, we may, too, embrace only the 

 study of the development of birds within and without the egg, a line of 

 study open to the one who undertakes the embryology of birds and their de- 

 velopmental anatomy. In all of these various methods and ways, certain 

 systems alone may be investigated, as the osseous system, constituting the 

 osteology of birds; or the muscular system, or their myology; the nervous 

 system, or neurology, and so on for the vascular, the urogenital, digestive, 

 and several other systems. Some students of the class resort to drawing a 

 hard and fast line between the external or topographical anatomy of a bird 

 and what they please to term the anatomy, or the structure of all of the rest 

 of the organism. Such a division is established but for the sake of conveni- 

 ence, and has no real existence in nature, as, for example, the integumentary 1 

 system and its appendages have quite as much to do with a bird's anatomy as 

 the study of the structure of any of its internal systems or the organs of I 

 special sense, as those of the ear, eye, and others. In the taxonomy or classi- 

 fication of birds, their anatomy constitutes our main reliance, in fact it is] 

 through the structure of birds alone that we can come to a correct knowledge! 

 of their afiinities; place in the system; and their relations to each other, as j 

 well as the limitations of existing and extinct groups, from genus to order. " 

 For one to successfully pursue the anatomy of birds as complete a mastery 

 as possible must be made of a, by no means inextensive glossary now a part of the 

 science; it requires a more or less complete appreciation and understanding of 

 the laws of organic evolution and development; that the latter may mean the 

 developmental history of the class {phylogeny), or that of the individual 

 {ontogeny). These last are extremely important. Structures must likewise 

 be studied in the light of their being analogous or Iwmologous; homology 

 being the true resemblance between parts that are morphologically and often 

 functionally alike, while analogy means that these resemblances are only ap- 

 parent. The wing of a bird and the fore leg of a mammal are homologically 

 the same, while the wing of a bird and that of a bee are but analogically in 

 agreement. 



We say that the homologies are general when the comparisons are re- 

 stricted to a single individual, and they are serially homologous structures or 

 organs when the same, though not essentially exact, representations of each 

 other. The ribs and the vertebrae are thus classed. Homologies again, or 

 homodynamous organs, may be complete or incomplete; for example, 'the 

 femur of a bird and the femur of a squirrel are homologically complete, but 



