BIRDS 



sis .in. considering what these birds were like in 

 their day, and which of existing kinds they most resembled. We 

 must have a knowledge of bones, of osteology, to grapple with 

 the problems which they present. For these reasons I have dealt in 

 the following pages principally with the organs of flight, and with 

 those internal and external characters which are admitted to be of 

 most use in classificatory questions. I have paid less attention to 

 those organs which are not of importance from these points of view. 



Feathers and Feathering. 



It is only a very few birds that have a complete and continuous 

 covering of feathers. The penguins are in this condition; and 

 some of the ostrich-like birds are so, more than most others. But 

 in other birds the feathers are arranged in tracts, between which 

 are patches of quite, or nearly, bare skin. The technical name for 

 the feathered districts is ' Pterylia ' ; that for the bare patches, ' Ap- 

 teria.' If two birds, belonging to different families, are compared, 

 it will often be discovered that they present considerable unlikeness 

 in the mutual arrangement of the feathered and unf eathered tracts. 

 In fact, it was pointed out not far from the beginning of this century 

 that the dispersal of the feathers over the body was one of the very 

 best characters for classifying birds upon. But when the author of 

 this discovery, Professor Nitzsch, of Halle, first published his book 

 on the matter, it was received with some ridicule, and the pictures 

 of birds denuded of their feathers in order to show up clearly the 

 feather tracts were ironically compared to a portion of a poulterer's 

 shop. This ridicule, however, did not do away with the fact that 

 the character is often of great use in settling the mutual relation- 

 ships of birds. "When a bird is carefully skinned, it will be seen 

 that the feather tracts have their own special slips of muscle inserted 

 into the roots of the feathers. These muscles, when they contract, 

 serve to raise the feathers slightly, and must be of at least sub- 

 sidiary importance in flying. This is, perhaps, why the feather 

 tracts are so well marked in birds that fly, and explains the reason 

 for their unmarked character in birds that do not. We can easily 

 understand that the movement of the feathers, if the covering were 

 continuous, would be much more difficult and less pronounced than 

 when there were separate patches far enough away from each other 

 to allow of free and independent movement. In the Penguin, which 



