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  Retrograde 
  Varieties 
  

  

  It 
  emerges 
  from 
  concealment 
  only 
  very 
  rarely 
  

   and 
  only 
  on 
  its 
  own 
  initiative. 
  Such 
  instances 
  

   of 
  atavism 
  have 
  been 
  described 
  in 
  previous 
  lec- 
  

   tures, 
  and 
  their 
  existence 
  has 
  been 
  proved 
  be- 
  

   yond 
  doubt. 
  

  

  Systematic 
  latency 
  explains 
  the 
  innumerable 
  

   instances 
  in 
  which 
  species 
  are 
  seen 
  to 
  lack 
  def- 
  

   inite 
  characteristics 
  which 
  ordinarily 
  do 
  not 
  

   fail, 
  either 
  in 
  plants 
  at 
  large, 
  or 
  in 
  the 
  group 
  

   or 
  family 
  to 
  which 
  the 
  plant 
  belongs. 
  If 
  we 
  

   take 
  for 
  instance 
  the 
  broom-rape 
  or 
  Orobanclne, 
  

   or 
  some 
  other 
  pale 
  parasite, 
  we 
  explain 
  their 
  

   occurrence 
  in 
  families 
  of 
  plants 
  with 
  green 
  

   leaves, 
  by 
  the 
  loss 
  of 
  the 
  leaves 
  and 
  of 
  the 
  green 
  

   color. 
  But 
  evidently 
  this 
  loss 
  is 
  not 
  a 
  true 
  one, 
  

   but 
  only 
  the 
  latency 
  of 
  those 
  characters. 
  

   And 
  even 
  this 
  latency 
  is 
  not 
  a 
  complete 
  one, 
  as 
  

   little 
  scales 
  remind 
  us 
  of 
  the 
  leaves, 
  and 
  traces 
  

   of 
  chlorophyll 
  still 
  exist 
  in 
  the 
  tissues. 
  Nu- 
  

   merous 
  other 
  cases 
  will 
  present 
  themselves 
  to 
  

   every 
  practical 
  botanist. 
  

  

  Taking 
  for 
  granted 
  that 
  characters, 
  having 
  

   once 
  been 
  acquired, 
  may 
  become 
  latent, 
  and 
  that 
  

   this 
  process 
  is 
  of 
  universal 
  occurrence 
  through- 
  

   out 
  the 
  whole 
  vegetable 
  and 
  animal 
  kingdom, 
  we 
  

   may 
  now 
  come 
  to 
  a 
  more 
  precise 
  and 
  clear 
  con- 
  

   ception 
  of 
  the 
  existing 
  differences 
  between 
  spe- 
  

   cies 
  and 
  varieties. 
  

  

  For 
  this 
  purpose 
  we 
  must 
  take 
  a 
  somewhat 
  

  

  