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  Retrograde 
  Varieties 
  

  

  In 
  systematic 
  works 
  the 
  positive 
  ones 
  are 
  as 
  a 
  

   rule 
  called 
  " 
  radiate," 
  and 
  the 
  negative 
  ones 
  

   " 
  discoid. 
  " 
  Discoid 
  forms 
  of 
  the 
  ordinary 
  

   camomile, 
  of 
  the 
  daisy, 
  of 
  some 
  asters 
  (Aster 
  

   Tripolium), 
  and 
  of 
  some 
  centauries 
  have 
  been 
  

   described. 
  Radiate 
  forms 
  have 
  been 
  observed 
  

   in 
  the 
  tansy 
  (Tanacetum 
  vulgar 
  -e), 
  the 
  common 
  

   horse-weed 
  or 
  Canada 
  fleabane 
  (Erigeron 
  cana- 
  

   densis) 
  and 
  the 
  common 
  groundsel 
  (Senecio 
  

   vulgaris). 
  Taken 
  broadly 
  the 
  negative 
  varie- 
  

   ties 
  seem 
  to 
  be 
  somewhat 
  more 
  numerous 
  than 
  

   the 
  positive 
  ones, 
  but 
  it 
  is 
  very 
  difficult 
  to 
  come 
  

   to 
  a 
  definite 
  conclusion 
  on 
  this 
  point. 
  

  

  Quite 
  the 
  contrary 
  is 
  the 
  case 
  with 
  regard 
  to 
  

   the 
  color-varieties 
  of 
  red 
  and 
  blue 
  flowers. 
  

   Here 
  the 
  loss 
  of 
  color 
  is 
  so 
  common 
  that 
  every 
  

   one 
  could 
  give 
  long 
  lists 
  of 
  examples 
  of 
  it, 
  Lin- 
  

   naeus 
  himself 
  supposed 
  that 
  no 
  blue 
  or 
  red-col- 
  

   ored 
  wild 
  species 
  would 
  be 
  without 
  a 
  white 
  va- 
  

   riety. 
  It 
  is 
  well 
  known 
  that 
  he 
  founded 
  his 
  

   often 
  criticized 
  prescript 
  never 
  to 
  trust 
  to 
  color 
  

   in 
  recognizing 
  or 
  describing 
  a 
  species, 
  on 
  this 
  

   belief. 
  

  

  On 
  the 
  other 
  hand 
  there 
  are 
  some 
  red 
  varie- 
  

   ties 
  of 
  white-flowered 
  species. 
  But 
  they 
  are 
  

   very 
  rare, 
  and 
  little 
  is 
  known 
  about 
  their 
  charac- 
  

   ters 
  or 
  constancy. 
  Blue 
  varieties 
  of 
  white 
  spe- 
  

   cies 
  are 
  not 
  found. 
  The 
  yarrow 
  (AcJiillea 
  Mil- 
  

   lefolium) 
  has 
  a 
  red-flowered 
  form, 
  which 
  occurs 
  

  

  