HIGH SCHOOL ZOOLOGY. 121 



CHAPTER V. 

 The Birds. 



1. From what we learned in last chapter it is neither the 

 power of flight nor the supporting of the body by the hind 

 limbs which constitutes a bird, for both these characteristics 

 were present in certain fossil reptiles. It is the peculiar epider- 

 mal clothing of feathers, which we must regard as marking off 

 the birds from the flying reptiles to which they are allied. 

 Yet there is not so much difference between the reptilian scales 

 and the avian feathers as might at first sight appear. If we 

 watch the development of the feathers in a bird, we may see 

 that they arise 'at first very much like scales, along regular 

 tracts, and that they are simply thickenings of the epidermis 

 over papilla? of the cutis. But as the feather is developed it is 

 retracted into a follicle in the skin, and the epidermis gives rise 

 to the feather proper, the dried-up cutis-papilla to the pith. 



y 2. In most birds we distinguish two different elements in 

 the plumage, the feathers proper or contour-feathers and quills, 

 and the down-feathei-s ; but in certain birds destitute of the 

 power of flight down-feathers alone are present. Both kinds 

 resemble each other in having a quill by which they are in- 

 serted into the feather-follicle, a shaft, and a vane which is 

 composed of two rows of barbs, each provided with projecting 

 barbules. If the barbules are so arranged that those on con- 

 tiguous barbs interlock with each other, then we have a contour- 

 feather, or if it is of special use in flight, is a quill-feather, but 

 if the barlniles are soft, and do not interlock, we have the 

 down-featlier. It is obvious that a creature only possessed 

 of the latter cannot fly ; the interlocking of the barbules is neces- 

 9 



