﻿300 
  Retrograde 
  Varieties 
  

  

  But 
  here 
  we 
  will 
  limit 
  ourselves 
  to 
  normal 
  

   cases. 
  

  

  The 
  first 
  example 
  to 
  be 
  considered 
  is 
  obvious- 
  

   ly 
  the 
  assumption 
  that 
  the 
  parents 
  of 
  a 
  cross 
  

   differ 
  from 
  each 
  other 
  in 
  respect 
  to 
  two 
  charac- 
  

   ters. 
  A 
  good 
  illustrative 
  example 
  is 
  afforded 
  

   by 
  the 
  thorn-apple. 
  I 
  have 
  crossed 
  the 
  blue- 
  

   flowered 
  thorny 
  form, 
  usually 
  known 
  as 
  Datura 
  

   Tatula, 
  with 
  the 
  white 
  thornless 
  type, 
  ' 
  desig- 
  

   nated 
  as 
  D. 
  Stramonium 
  inermis. 
  Thorns 
  and 
  

   blue 
  pigment 
  are 
  obviously 
  active 
  qualities, 
  as 
  

   they 
  are 
  dominant 
  in 
  the 
  hybrids. 
  In 
  the 
  

   second 
  generation 
  both 
  pairs 
  of 
  characters 
  are 
  

   resolved 
  into 
  their 
  constituents 
  and 
  paired 
  anew 
  

   according 
  to 
  Mendel 
  's 
  law. 
  After 
  isolating 
  my 
  

   hybrids 
  during 
  the 
  period 
  of 
  flowering, 
  I 
  counted 
  

   among 
  their 
  progeny 
  : 
  

  

  128 
  individuals 
  with 
  blue 
  flowers 
  and 
  thorns 
  

   47 
  " 
  " 
  " 
  " 
  without 
  " 
  

  

  54 
  " 
  " 
  white 
  " 
  and 
  " 
  

  

  21 
  " 
  " 
  " 
  " 
  without 
  " 
  

  

  250 
  

  

  The 
  significance 
  of 
  these 
  numbers 
  may 
  easily 
  

   be 
  seen, 
  when 
  we 
  calculate 
  what 
  was 
  to 
  be 
  ex- 
  

   pected 
  on 
  the 
  assumption 
  that 
  both 
  characters 
  

   follow 
  Mendel's 
  law, 
  and 
  that 
  both 
  are 
  inde- 
  

   pendent 
  from 
  each 
  other. 
  Then 
  we 
  would 
  have 
  

   three-fourths 
  blue 
  offspring 
  and 
  one-fourth 
  in- 
  

   dividuals 
  with 
  white 
  flowers. 
  Each 
  of 
  these 
  

  

  