DISTRIBUTION OF BIRDS IN TIME AND SPACE. 189 



Next comes the ostricli. As in rhea, the 

 primary and secondary quill-feathers still per- 

 sist and retain their full strength in point of 

 numbers. They are however too weak and 

 feeble to serve the purposes of flight. The 

 New Zealand apteryx furnishes the next stage, 

 having greatly reduced the number of quill- 

 feathers, and the size of the wing. So dimin- 

 utive is this last that it cannot be found 

 without careful search. 



Lastly follow the emu and the cassowary. In 

 both, primary feathers are greatly reduced in 

 number. In the former they have lost the stout 

 ''quill" portion by degeneration, and retained 

 the feathery vane. In the latter the "quills" 

 have become extremely specialised, and project 

 beyond the wing as huge spines, whilst the 

 "vane" has been thrown off". This is an in- 

 terestiog example of the specialisation of a 

 degenerate structure. 



Not only then have the feathers in these birds 

 degenerated, but the skeleton too shows every 

 gradation of decrease and decay by gradual 

 absorption. We may gather from this the 

 almost absolute certainty that we are, in these 

 wings, studying so many stages in the decline of 

 a once functional organ — an organ of flight or, 

 as we call it, a wing. For additional and even 

 more convincing evidence we might turn to the 

 anatomy of the soft parts, e.g. the muscles. 

 These, we should find, were in every way 

 arranged as in the birds that fly, proving, 

 by this disposition, that they have undergone 

 a similar adaptation to the mechanical require- 



