8 GENERAL HISTOEl OF BIRDS. 



only lost that power, but the wings have further degenerated, as in the ostriches 

 and related forms, and in some extinct forms (Dinornithids) they had entirely 

 disappeared. The osseous frame work will be considered in a later chapter. 



The entire body is invested in a coat of feathers whose chief modifications 

 deserve a more detailed notice. 



THE PLUMAGE OF BIRDS.* 



The plumage of birds is clearly their distinctive character; the absence of 

 teeth is a character of turtles also, and other animals have horny beaks; the 

 possession of wings is not confined to birds, and the various skeletal characters 

 of the class are to a large extent shared with reptiles. But feathers are unique, 

 and any animal covered with feathers is clearly a bird, while any animal nor- 

 mally lacking them is just as certainly something else. And yet it must be 

 understood that there is a very great variety of structure in feathers, not only 

 in those from different birds, but in those from different parts of the same 

 bird. Moreover there is nearly as wide a range of difference between the 

 plumage of a Penguin and that of an Apteryx as there is between scales and 

 hair. The differences in texture of plumage are generally associated with 

 habit, the long-legged land birds, especially those with reduced wings, having 

 a loose, more or less hair-like plumage, which reaches its extreme in the 

 Apteryx; while most water birds, especially those with short wings and 

 legs, have a very compact plumage, which reaches its extreme in the Penguins. 

 But these differences in texture of plumage are dependent upon three factors, 

 (1) the development of muscle in the skin which control the movements of the 

 feathers, (2) the kinds of feathers, and (3) the arrangement of the feathers. 

 Of the first factor little need be said further than to state the fact that many 

 feathers are provided with subcutaneous muscles, sometimes several to each 

 feather, by which the feathers can be independently moved. The development 

 of this subcutaneous muscle-layer varies much in different groups of birds, but 

 is generally particularly well-marked in those with a very compact plumage. 



THE FEATHERS OF BIRDS. 



There are five quite distinct groups into which feathers can be divided, 

 but intermediate forms constantly occur so that the distinctions are somewhat 

 arbitrary. First and most important of all are the contour -feathers, so-called 

 because they are the ones which give the form or contour to the body. They 

 are the largest and most fully developed of feathers, and the other kinds may 

 be regarded, perhaps, as modified or degenerate contour-feathers. Whether 



*A11 that relates to the plumage of birds and their feathers (pp. 8-14) has been con- 

 tributed by Dr. Hubert Lyman Clark, Professor of Biolog-y in Olivet College, Michigan. 



