Finally,  as  soon  as  we  have  recognised,  on  theoretical  grounds,
the  existence  of  ids  at  all,  the  fact  that  a  number  of  them  exist
in  the  cell  is  supported  by  direct  observation.  For  whether
they  correspond  to  the  ^  chromosomes '  of  other  writers,  which
I  speak  of  as  idants,  or  to  the  'microsomes,'  of  which  the
chromosomes  are  composed,  as  I  assume  to  be  the  case,  a  large
number  of  ids  may  always  be  observed  to  be  present  in  the  cell.
^w  ■  m    ^^^■'^ir^


THE   PHENOMENA   OF  REVERSION  299


CHAPTER  X

THE  PHENOMENA  OF  REVERSION  IN  THEIR  RELATION

TO  AMPHIMIXIS

1.  Reversion  to  Racial  Characters  in  Plant-Hybrids

By  the  term  reversion,  is  meant  the  appearance  of  characteris-
tics which  existed  in  the  more  remote  ancestors,  but  were  absent
in  the  immediate  ancestors  —  i.e.^  the  parents.
The  facts  relating  to  these  phenomena  are  familiar  enough,
and  I  shall  therefore  only  refer  to  as  many  of  them  as  are  nec-
essary for  the  further  development  of  my  theory.
The  simplest  case  of  reversion  occurs  in  hybrids.  It  occa-
sionally happens  that  hybrids  which  have  been  fertilised  by
their  own  pollen,  produce  offspring  some  of  which  more  or  less
resemble  only  one  of  the  two  ancestral  species.  In  such  cases,
therefore,  a  simple  reversion  to  a  grandparent  takes  place.  In-
stances of  this  kind  certainly  occur,  though  not  in  all  hybrid-
piants;  nor  are  they  often  met  with  even  in  those  species  in
which  they  do  occur.  On  this  point  Darwin  quotes  two  contra-
dictory statements  made  by  Wichura  and  Na\idin  respectively,
the  former  of  whom  never  observed  instances  of  reversion  in  his
specimens  of  willow-hybrids,  while  the  latter  insisted  strongly
on  the  frequent  occurrence  of  reversion  in  the  CucurUtacea.
