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  Retrograde 
  Varieties 
  

  

  group 
  the 
  cross 
  is 
  not 
  at 
  all 
  distinguished 
  from 
  

   a 
  normal 
  fertilization, 
  and 
  ordinarily 
  these 
  

   characters 
  are 
  simply 
  left 
  out 
  of 
  consideration. 
  

   But 
  it 
  should 
  never 
  be 
  forgotten 
  that 
  they 
  con- 
  

   stitute 
  the 
  enormous 
  majority, 
  amounting 
  to 
  

   hundreds 
  and 
  thousands, 
  whereas 
  the 
  differen- 
  

   tiating 
  marks 
  in 
  each 
  case 
  are 
  only 
  one 
  or 
  two 
  

   or 
  a 
  few 
  at 
  most. 
  The 
  whole 
  discussion 
  is 
  to 
  

   be 
  limited 
  to 
  these 
  last-named 
  exceptions. 
  We 
  

   must 
  consider 
  first 
  what 
  would 
  be 
  the 
  nature 
  of 
  

   a 
  cross 
  when 
  species 
  are 
  symmetrically 
  com- 
  

   bined, 
  and 
  what 
  must 
  be 
  the 
  case 
  when 
  varieties 
  

   are 
  subjected 
  to 
  the 
  same 
  treatment. 
  In 
  so 
  do- 
  

   ing, 
  I 
  intend 
  to 
  limit 
  the 
  discussion 
  to 
  the 
  most 
  

   typical 
  cases. 
  We 
  may 
  take 
  the 
  crosses 
  between 
  

   elementary 
  species 
  of 
  the 
  same 
  or 
  of 
  very 
  nar- 
  

   rowly 
  allied 
  systematic 
  species 
  on 
  the 
  one 
  side, 
  

   and 
  on 
  the 
  other, 
  limit 
  treatment 
  to 
  the 
  crossing 
  

   of 
  varieties 
  with 
  the 
  species, 
  from 
  which 
  they 
  

   are 
  supposed 
  to 
  have 
  sprung 
  by 
  a 
  retrograde 
  

   modification. 
  Crosses 
  of 
  different 
  varieties 
  of 
  

   the 
  same 
  species 
  with 
  one 
  another 
  obviously 
  

   constitute 
  a 
  derivative 
  case, 
  and 
  should 
  only 
  be 
  

   discussed 
  secondarily. 
  And 
  crosses 
  of 
  varie- 
  

   ties 
  with 
  positive 
  or 
  degressive 
  characters 
  have 
  

   as 
  yet 
  so 
  rarely 
  been 
  made 
  that 
  we 
  may 
  well 
  

   disregard 
  them. 
  

  

  Elementary 
  species 
  differ 
  from 
  their 
  nearest 
  

   allies 
  by 
  progressive 
  changes, 
  that 
  is 
  by 
  the 
  ac- 
  

  

  