194 THE STORY OF THE BIRDS. 
Ceca, a part of the digestive tract, seemg once to 
have been necessary in all birds to the digestion of 
such food as they then used. Now they are very 
variable, and some birds are entirely destitute of 
them, or such vestiges of them as remain are useless. 
Likewise gizzards have been developed and lost, or 
become loose, thin sacs rather; and other internal 
organs have been changed. The very loops and ar- 
rangements of viscera hint much of the route of 
development. 
A very peculiar loss among some birds is that of 
one carotid artery. Usually it is the right that is 
gone, or the two may be merged into one. This 
change may have come about in keeping with the de- 
mand or needs of the brain’s blood supply. Perhaps 
when the bird became hot-blooded with a very active 
heart, the brain received too much blood in the in- 
tense exercise of flying. In mammals this blood sup- 
ply is sometimes regulated by crooking the artery. 
This suppression of the carotids is very variable in 
different groups or different members of the same 
group. It seems to be something easily and recently 
influenced. 
In losing the skin pores, birds, as we have seen, 
found some compensation in the development of the 
oil gland, but some of them have even lost that. 
We have already seen how parts of feathers have 
been lost for beauty. 
In some way not well understood, except that it 
may imply progress away from the reptiles, the 
higher and more songful birds have lost many scales 
