Among  these  is  the  general  fact  that  a  great  change  in  nutrition
rarely  takes  place  in  one  organ  at  a  time,  but  usually  afiects  simul-
taneously two  or  more  parts,  between  whose  nutrition  there  is  a  mani-
fest and  constant  connection,  although  there  is   little  or  no  relation


22  CONDITIONS  NECESSARY  TO

between  their  external  functions.  Such,  to  take  an  instance  from  a
large  class,  is  the  connection  between  the  growth  of  various  appendages
of  the  integuments,  and  the  development  or  maintenance  of  the  genital
organs.  This  appears  to  be  a  general  rule.  The  growth  of  the  beard
at  the  period  of  puberty  in  man,  with  which  we  are  so  familiar,  is  more
instructively  represented  in  many  animals — especially  in  birds.  In
these,  as  you  know,  at  the  approach  of  every  breeding  time,  the  genital
organs  begin  to  develope  themselves  for  the  season,  as  in  man  they  do
for  the  whole  time  of  vigorous  life.  And  commensurately  with  this
development  the  plumage  (especially  in  the  male  bird)  becomes  brighter
and  more  deeply  coloured,  both  by  the  growth  of  new  feathers  and  by
the  addition  of  colour  to  the  old  ones.  The  height  and  perfection  of
the  plumage  are  coincident  with  the  full  development  and  activity  of
the  reproductive  organs  ;  but,  as  in  man,  when  the  development  of  the
genital  organs  is  prevented,  that  of  the  beard  and  all  the  other  external
sexual  characters  is,  as  a  consequence,  hindered  ;  so  in  birds,  when  the
breeding  season  ends,  and  the  sexual  organs  pass  gradually  into  their
periodic  atrophy  at  once  the  plumage  begins  to  assume  the  paler  and
more  sober  colours  which  characterise  the  barrenness  of  winter.
