Importance  of  the  subject — It  furnishes  a  means  of  analyzing
or  isolating  tlie  influence  of  each  sexual  element — Hybrids
very  variable — Hybrids  from  domesticated  races  more
variable  than  those  from  wild  races — The  descendants  of
hybrids  more  variable  tlian  the  hybrids  themselves— The
offspring  of  a  male  hybrid  and  the  female  of  a  pure  species
are  mucli  more  variable  than  those  of  a  female  hybrid  and
the  male  of  a  pure  species — These  facts  inexplicable  on
any  view,  except  the  one  here  presented — Reciprocal
crosses — They  differ  in  fertility  and  in  structure — The
difference  is  exactly  what  our  theory  requires — Difficulty
in  explaining  transmission  of  characters  without  fusion —
Reversion  caused  by  crossing — Two  kinds  of  reversion —
Summary 118

CHAPTER  VII.
THE  EVIDENCE  FROM  VARIATION.
