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  Retrograde 
  Varieties 
  

  

  formerly 
  lost. 
  Some 
  may 
  originate 
  in 
  a 
  

   negative, 
  others 
  in 
  a 
  positive 
  manner, 
  but 
  in 
  

   both 
  cases 
  nothing 
  really 
  new 
  is 
  acquired. 
  

  

  This 
  distinction 
  holds 
  good 
  for 
  all 
  cases 
  in 
  

   which 
  the 
  relationship 
  between 
  the 
  forms 
  in 
  

   question 
  is 
  well 
  known. 
  It 
  seems 
  entirely 
  justi- 
  

   fiable 
  therefore 
  to 
  apply 
  it 
  also 
  to 
  cases 
  in 
  

   which 
  the 
  systematic 
  affinity 
  is 
  doubtful, 
  as 
  well 
  

   as 
  to 
  instances 
  in 
  which 
  it 
  is 
  impossible 
  to 
  ar- 
  

   rive 
  at 
  any 
  taxonomic 
  conclusions. 
  The 
  ex- 
  

   treme 
  application 
  of 
  the 
  principle 
  would 
  no 
  

   doubt 
  disturb 
  the 
  limits 
  between 
  many 
  species 
  

   and 
  varieties 
  as 
  now 
  recognized. 
  It 
  is 
  not 
  to 
  be 
  

   forgotten 
  however 
  that 
  all 
  taxonomic 
  distinc- 
  

   tions, 
  which 
  have 
  not 
  been 
  confirmed 
  by 
  physi- 
  

   ologic 
  tests 
  are 
  only 
  provisional, 
  a 
  view 
  ac- 
  

   knowledged 
  by 
  the 
  best 
  systematists. 
  Of 
  course 
  

   the 
  description 
  of 
  newly 
  discovered 
  forms 
  can 
  

   not 
  await 
  the 
  results 
  of 
  physiologic 
  inquiries, 
  

   but 
  it 
  is 
  absolutely 
  impossible 
  to 
  reach 
  definite 
  

   conclusions 
  on 
  purely 
  morphologic 
  evidence. 
  

   This 
  is 
  well 
  illustrated 
  by 
  the 
  numerous 
  dis- 
  

   cords 
  of 
  opinion 
  of 
  different 
  authors 
  on 
  the 
  sys- 
  

   tematic 
  worth 
  of 
  many 
  forms. 
  

  

  Assuming 
  the 
  above 
  mentioned 
  principle 
  as 
  

   established, 
  and 
  disregarding 
  doubtful 
  cases 
  as 
  

   indicated, 
  the 
  term 
  progressive 
  evolution 
  is 
  

   used 
  to 
  designate 
  the 
  method 
  in 
  which 
  element- 
  

   ary 
  species 
  must 
  have 
  originated. 
  It 
  is 
  the 
  

  

  