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 
vertebre 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- 
bree, 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- 
