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  Retrograde 
  Varieties 
  

  

  after 
  pure 
  fertilization. 
  Mendel's 
  law 
  teaches 
  

   us 
  to 
  predict 
  the 
  difficulties, 
  but 
  hardly 
  shows 
  

   any 
  way 
  to 
  avoid 
  them. 
  It 
  lays 
  great 
  stress 
  on 
  

   the 
  old 
  prescript 
  of 
  isolation 
  and 
  pure 
  fertiliza- 
  

   tion, 
  but 
  it 
  will 
  have 
  to 
  be 
  worked 
  out 
  and 
  ap- 
  

   plied 
  to 
  a 
  large 
  number 
  of 
  practical 
  cases 
  before 
  

   it 
  will 
  gain 
  a 
  preeminent 
  influence 
  in 
  horticul- 
  

   tural 
  practice. 
  

  

  Or, 
  as 
  Bailey 
  states 
  it, 
  we 
  are 
  only 
  beginning 
  

   to 
  find 
  a 
  pathway 
  through 
  the 
  bewildering 
  maze 
  

   of 
  hybridization. 
  

  

  This 
  pathway 
  is 
  to 
  be 
  laid 
  out 
  with 
  regard 
  to 
  

   the 
  following 
  considerations. 
  We 
  are 
  not 
  to 
  

   cross 
  species 
  or 
  varieties, 
  or 
  even 
  accidental 
  

   plants. 
  We 
  must 
  cross 
  unit-characters, 
  and 
  

   consider 
  the 
  plants 
  only 
  as 
  the 
  bearers 
  of 
  these 
  

   units. 
  We 
  may 
  assume 
  that 
  these 
  units 
  are 
  

   represented 
  in 
  the 
  hereditary 
  substance 
  of 
  the 
  

   cell-nucleus 
  bv 
  definite 
  bodies 
  of 
  too 
  small 
  a 
  size 
  

  

  / 
  

  

  to 
  be 
  seen, 
  but 
  constituting 
  together 
  the 
  chromo- 
  

   somes. 
  We 
  may 
  call 
  these 
  innermost 
  repre- 
  

   sentatives 
  of 
  the 
  unit-characters 
  pangenes, 
  in 
  

   accordance 
  with 
  Darwin's 
  hypothesis 
  of 
  pan- 
  

   genesis, 
  or 
  give 
  them 
  any 
  other 
  name, 
  or 
  we 
  may 
  

   even 
  wholly 
  abstain 
  from 
  such 
  theoretical 
  dis- 
  

   cussion, 
  and 
  limit 
  ourselves 
  to 
  the 
  conception 
  of 
  

   the 
  visible 
  character-units. 
  These 
  units 
  then 
  

   may 
  be 
  present, 
  or 
  lacking 
  and 
  in 
  the 
  first 
  case 
  

   active, 
  or 
  latent. 
  

  

  