14         MR  W.  YARRELL  ON  THE  LAWS  WHICH  APPEAR  TO  INFLUENCE 
assumes  a  very  different  character  in  the  feathers  of  different  species,  but  is  even  very 
dissimilar  in  the  feathers  of  different  parts  of  the  body  of  the  same  bird.  The  accessory 
plume  is  situated  at  the  distal  end  of  the  quill,  at  the  aperture  through  which  the  shaft 
and  its  lateral  fibres  have  passed  out,  and  at  the  central  point  from  which  the  two  lines 
of  the  web  begin  to  diverge.  In  the  strong  feathers  peculiar  to  the  wings  and  tail,  it 
remains  a  small  tuft  of  down,  as  at  first  mentioned ;  but  in  the  feathers  of  the  body  in 
the  Hawks,  Grouse,  Ducks,  Gulls,  and  some  others,  it  is  to  be  found  of  all  sizes,  aug- 
mented in  some  species  to  the  full  extent  of  the  feather  from  which  it  emanates.  The 
four  species  of  Struthious  birds  afford  remarkable  instances  of  the  variety  that  occurs 
in  this  accessory  plume,  even  in  subjects  so  closely  allied ;  and  the  rich  Menagerie  of 
the  Society  enables  me  to  speak  of  them  from  personal  examination  upon  living  speci- 
mens. In  the  Ostrich  the  feathers  have  no  accessory  plume.  In  the  Rhea  there  is  a 
tuft  of  down.  In  the  Emu  the  accessory  plume  is  augmented  to  the  full  size  of  the 
principal  shaft  and  web,  and  the  feather  of  this  bird  is  constantly  and  correctly  repre- 
sented as  having  two  plumes  on  one  quill.  In  the  Cassowary,  besides  the  double  shafts 
and  webs  from  a  single  quill,  as  in  the  Emu,  there  is  still  an  accessory  plume,  thus 
forming  three  distinct  parts ;  and  a  feather  so  constructed  is  figured  by  M.  Guerin  in 
his  '  Iconographie  du  R^gne  Animal '.  (Oiseaux,  pi.  48.) 
In  young  birds  the  first  feathers  are  preceded  in  their  passage  through  the  skin  by 
filaments  of  down ;  but  after  the  first  plumage,  at  the  regular  period  of  moulting,  each 
old  feather  is  the  pioneer  (gubernaculum)  of  that  which  is  to  follow\  If  the  shaft  of  a 
principal  feather  becomes  broken  off,  the  bird  ejects  the  stump  with  difficulty ;  a 
certain  portion  of  shaft  appears  to  be  necessary  to  enable  the  bird  to  get  rid  of  the 
feather.  Though  perfectly  able  to  throw  off  the  old  feather  if  entire,  it  seems  unable  to 
cast  off  the  smaller  but  mutilated  portion,  and  no  new  feather  comes  through  the  skin, 
the  orifice  being  occupied.  Inflammation  of  the  vessels  in  the  part  of  the  bird,  and 
increased  adhesion  of  the  stump,  are  the  consequences  ;  and  whether  these  portions  of 
the  feathers  are  allowed  to  remain,  or  too  many  of  them  drawn  out  at  the  same  time, 
disease  and  some  risk  to  life  are  the  result ; — in  the  first  instance  from  continued  irri- 
tation, in  the  second  from  too  great  and  sudden  a  demand  upon  the  vital  powers  of  the 
animal.  The  natural  moult  proceeds  by  degrees,  and  the  large  quill-feathers  of  the 
wings  and  tail  are  generally  shed  by  pairs. 
The  state  of  the  plumage  in  birds,  like  that  of  the  productions  of  the  cuticle  in  other 
animals,  man  not  excepted,  is  in  general  a  good  criterion  by  which  to  judge  of  the 
health  of  the  body. 
The  principal  peculiarities  of  the  feather  thus  briefly  premised,  it  may  be  added  that 
the  time  required  to  obtain  that  state  of  plumage  which  is  considered  characteristic  of 
the  adult  bird,  varies  according  to  the  species  from  one  year  to  five ;  and  that  several 
birds  build  nests,  and  rear  young,  before  they  attain  their  adult  plumage. 
Baron  Cuvier  has  stated,  that  when  the  adult  female  bird  differs  from  the  male  in  the 
