204 ANIMAL FORMS 



In very many animals, fishes as well as birds, the tints 

 on the under side of the body are usually relatively light 

 colored, shading gradually into a darker tint above. This 

 is in all probability a protective device, as was recently 

 shown by Mr. A. H. Thayer, an American artist. His ex- 

 periments show that the light from above renders the back 

 less dark, and that the shadow beneath is neutralized by 

 the light color. The bird thus appears uniformly lighted, 

 and this effect, together with streaks and blotches, renders 

 them invisible at surprisingly short distances. 



191. Skeleton. Turning now to the internal organization 

 of birds, we find many points in common with other verte- 

 brates, especially the reptiles, but many interesting modifi- 

 cations are also present that adapt them for flying and for 

 collecting their food. According to the nature of the food, 

 the beak may have a great variety of forms. The skull may 

 be thick and heavy, or thin and fragile, but these are mat- 

 ters of proportion of the various parts possessed by all 

 birds. The neck also is of differing length ; but it is in the 

 trunk region that the greatest changes have arisen, as we 

 may see in any of our ordinary birds. For example, the 

 vertebrae of this part of the body are more or less fused 

 together into rigid framework, to which are attached the 

 ribs that in turn unite with the breast-bone. In the fliers 

 the latter bears a vertical plate or keel, to which the great 

 muscles that move the wings are attached. The tail con- 

 sists, like that of the old-fashioned birds, of several verte- 

 brae, but these are of small size and fused together into a 

 little knob that supports the tail-feathers. The fore limbs 

 are used for flight, but there are the same bones that exist 

 in the fore limbs of other vertebrates one for the upper 

 arm, two for the lower, a thumb carrying a few feathers, 

 and known as the bastard wing, and indications of several 

 bones that form the hand. In the hind limb the resem- 

 blance is equally apparent, though its different parts are 

 of relatively large size to support the body. It is interest- 



