[The  following  letter  refers  to  a  subject  on  which  my
father  felt  the  strongest  interest : — the  experimental  investi-
gation of  the  causes  of  variability.  The  experiments  alluded
to  were  to  some  extent  planned  out,  and  some  preliminary
work  was  begun  in  the  direction  indicated  below,  but  the  re-
search was  ultimately  abandoned.]

C,  Darwin  to  /.  H.  Gilbert*

Down,  February  i6,  1876.
My  dear  Sir, — ^When  I  met  you  at  the  Linnean  Society,
you  were  so  kind  as  to  say  that  you  would  aid  me  with  ad-
vice, and  this  will  be  of  the  utmost  value  to  me  and  my  son.
I  will  first  state  my  object,  and  hope  that  you  will  excuse  a
long  letter.  It  is  admitted  by  all  naturalists  that  no  problem
is  so  perplexing  as  what  causes  almost  every  cultivated  plant
to  vary,  and  no  experiments  as  yet  tried  have  thrown  any
light  on  the  subject.  Now  for  the  last  ten  years  I  have  been
experimenting  in  crossing  and  self-fertilising  plants ;  and  one
indirect  result  has  surprised  me  much ;  namely,  that  by  tak-
ing pains  to  cultivate  plants  in  pots  under  glass  during  several
successive  generations,  under  nearly  similar  conditions,  and
by  self-fertilising  them  in  each  generation,  the  color  of  the
flowers  often  changes,  and,  what  is  very  remarkable,  they  be-
came in  some  of  the  most  variable  species,  such  as  Mimulus,
Carnation,  &c.,  quite  constant,  like  those  of  a  wild  species.
*  Dr.  Gilbert,  F.R.S.,  joint  author  with  Sir  John  Bennett  Lawes  of  a
long  series  of  valuable  researches  in  Scientific  Agriculture.
