212 THE STORY OF BIRD-LIFE. 



in pairs, as is done by most birds to-day. The 

 usefulness of this claw would at once begin to 

 decline, and its size would begin to decrease. 

 Then it would tend to disappear as soon as the 

 nestling period was over. Then even the nestling 

 would manage to dispense with it — as do most 

 modern birds — and it would appear for a time as 

 a tiny vestige ; at last it would crop up only in 

 the embryo, and here in time it would vanish too. 

 We can trace all these gradations in the develop- 

 ment of modern birds. It is a long time since 

 the nestling game-birds climbed about in trees, so 

 that the claw has nearly ceased to exist ; it appears 

 for a short time in the embryo. The peculiar 

 order of the succession of the quill-feathers, how- 

 ever, is still retained as we have already shown 

 on p. 206. Thus when we suspect that an animal 

 has lost such and such an organ, still possessed 

 by its fellows, we look for it in the nestling or 

 the embryo, and generally with success. 



The next most primitive type that we have is 

 that to which the ostrich, rhea, cassowary, apteryx 

 and emu belong. They are generally grouped 

 together under one head as "ratites," because 

 they have a raft- like breast-bone, but this ar- 

 rangement is rather one for convenience' sake. 



Concerning this breast-bone you will remember 

 that we have already pointed out its chief dis- 

 tinctions (p. 72). We need only say here that in 

 the ratite and the stringops-parrot there is no 

 ^'keel" running down the middle of the breast- 

 bone as there is in all other birds. The breast- 

 bone of the fowl or pigeon looks rather like the 

 -wide and shallow hull of a ship with a huge keel ; 



