﻿262 
  Retrograde 
  Varieties 
  

  

  tried 
  to 
  trace 
  this 
  rule 
  in 
  the 
  single 
  cases, 
  but 
  

   have 
  met 
  with 
  nearly 
  unsurmountable 
  difficul- 
  

   ties, 
  owing 
  chiefly 
  to 
  our 
  ignorance 
  of 
  the 
  units 
  

   which 
  form 
  the 
  differences 
  between 
  the 
  parents 
  

   in 
  the 
  observed 
  cases. 
  

  

  In 
  the 
  case 
  of 
  Oenothera 
  muricata 
  x 
  bien- 
  

   nis 
  the 
  differentiating 
  units 
  reduce 
  the 
  fertility 
  

   to 
  a 
  low 
  degree, 
  threatening 
  the 
  offspring 
  with 
  

   almost 
  complete 
  infertility 
  and 
  extinction. 
  

   But 
  then 
  we 
  do 
  not 
  know 
  whether 
  these 
  charac- 
  

   ters 
  are 
  really 
  units, 
  or 
  perhaps 
  only 
  seemingly 
  

   so 
  and 
  are 
  in 
  reality 
  composed 
  of 
  smaller 
  en- 
  

   tities 
  which 
  as 
  yet 
  we 
  are 
  not 
  able 
  to 
  segregate. 
  

   And 
  as 
  long 
  as 
  we 
  are 
  devoid 
  of 
  empirical 
  means 
  

   of 
  deciding 
  such 
  questions, 
  it 
  seems 
  useless 
  to 
  go 
  

   farther 
  into 
  the 
  details 
  of 
  the 
  question 
  of 
  the 
  

   sterility. 
  It 
  should 
  be 
  stated 
  here 
  however, 
  

   that 
  pure 
  varietal 
  crosses, 
  when 
  not 
  accompan- 
  

   ied 
  by 
  unbalanced 
  characters, 
  have 
  never 
  showed 
  

   any 
  tendency 
  to 
  diminished 
  fertility.. 
  Hence 
  

   there 
  can 
  be 
  little 
  doubt 
  that 
  the 
  unpaired 
  units 
  

   are 
  the 
  cause 
  of 
  this 
  decrease 
  in 
  reproductive 
  

   power. 
  

  

  The 
  genus 
  Oenothera 
  is 
  to 
  a 
  large 
  degree 
  de- 
  

   void 
  of 
  varietal 
  characteristics, 
  especially 
  in 
  

   the 
  subgenus 
  Onagra, 
  to 
  which 
  biennis, 
  mur- 
  

   icata, 
  lamarckiana 
  and 
  some 
  others 
  belong. 
  

   On 
  the 
  other 
  hand 
  it 
  seems 
  to 
  be 
  rich 
  in 
  

   elementary 
  species, 
  but 
  an 
  adequate 
  study 
  of 
  

  

  