The  possibility  that  structures  of  the  greatest  con-
stancy and  importance  may  not  really  be  hereditary  is
well  illustrated  by  Hunter's  well-known  experiments  on
the  sea-gull.  In  pigeons,  and  in  most  birds  which  feed
upon  grain,  the  muscular  wall  of  a  portion  of  the  stomach
is  greatly  developed,  to  form  the  crushing  and  grinding
gizzard^  which  is  lined  with  a  covering  of  tough  mem-
brane, while  the  stomach  of  the  sull  and  of  most  flesh-
feedinsj  birds  is  soft,  and  the  muscular  laver  little  devel-
oped.  Hunter  fed  a  sea-gull  for  a  year  on  grain,  and
he  thus  succeeded  in  hardening  the  inner  coat  of  the
bird's  stomach,  thus  forming  a  true  gizzard;  and  Dar-


A  New  Theory  of  Heredity.  93

win  quotes  from  Dr.  Edmonston  the  statement  that  a
simihir  change  occurs  twice  a  year  in  the  stomach  of  an-
other sea-2:ull  in  tlie  Sliethmd  Ishxnds,  where  this  h'ird
frequents  the  corn-fiekls  and  feeds  on  seeds  in  the  spring,
hut  catches  fish  during  the  rest  of  the  year.  This
ohserver  has  noticed  a  great  change  in  the  stomach  of
a  wren  which  had  long  heen  fed  on  vegetahle  food;  and
Menetries  states  that  wlien  an  owl  was  similarly  treated
the  form  of  the  stomach  was  changed,  and  the  inner
coat  became  leathery,  while  the  liver  increased  in  size.
Semper  states  that  Dr.  Ilolmgrin  has  been  able  to  trans-
form the  gizzard  of  a  pigeon  into  a  carnivorous  stomach
by  feeding  the  bird  on  meat  for  a  long  time.
