4  See  Castle,  Heredity  oj  Sex.
5  See  §  288.  6  See  §  257.
I92  THE  FUNCTION  OF  SEX

this  problem,  we  must  bear  in  mind  that  sexual  reproduction  is
almost  universal ;  whereas,  to  judge  by  facts  that  have  been
actually  observed,  not  merely  imagined,  Mendelian  reproduction
is  one  of  the  rarest  things  in  nature.  Speaking  practically,  it
occurs  only  when  mutations  are  reproduced ;  and  all  the  evidence
indicates  that  mutations  occur  relatively  seldom,  and,  even  then,
are  very  seldom  preserved,  except  when  the  selection  is  artificial.
Mendelian  reproduction,  therefore,  is,  in  effect,  unknown  in
normal — that  is,  zVz/ra-varietal — breeding.  Natural  varieties  very
rarely  cross,  and  only  rarely  display  Mendelian  traits  when  man
crosses  them.  Such  traits  are  common  only  when  artificial
varieties,  in  which  mutations  have  been  accumulated  by  man,  are
crossed  by  man.  They  seem  common  to  experimental  workers
only  because  the  materials  for  thought  to  which  they  have  limited
themselves  have  been  obtained  almost  exclusively  in  this  latter
way.  I  think,  therefore,  that  no  one  who  has  been  accustomed
to  found  his  judgments,  not  on  the  exception  but  the  rule,  will
hesitate  to  conclude  that  Mendelian  reproduction  is  a  variety  of
sexual  reproduction,  and  that  the  converse  is  as  absolutely  untrue
as  if  we  had  declared  that  mammals  are  a  variety  of  dogs.
