﻿Theories 
  of 
  Evolution 
  25 
  

  

  not 
  rarely 
  open 
  to 
  doubt. 
  Large 
  numbers 
  of 
  

   hybrid 
  plants 
  and 
  hybrid 
  races 
  are 
  in 
  existence, 
  

   concerning 
  the 
  origin 
  of 
  which 
  it 
  is 
  impossible 
  

   to 
  decide. 
  It 
  is 
  impossible 
  in 
  many 
  in- 
  

   stances 
  to 
  ascertain 
  whether 
  they 
  are 
  of 
  

   hybrid 
  or 
  of 
  pure 
  origin. 
  Often 
  there 
  is 
  only 
  

   one 
  way 
  of 
  determining 
  the 
  matter; 
  it 
  is 
  to 
  

   guess 
  at 
  the 
  probable 
  parents 
  in 
  case 
  of 
  a 
  cross 
  

   and 
  to 
  repeat 
  the 
  cross. 
  This 
  is 
  a 
  point 
  which 
  

   always 
  requires 
  great 
  care 
  in 
  the 
  interpretation 
  

   of 
  unusual 
  facts. 
  

  

  Three 
  cases 
  are 
  to 
  be 
  distinguished 
  as 
  to 
  

   heredity. 
  Many 
  plants 
  are 
  so 
  constituted 
  as 
  to 
  

   be 
  fertilized 
  with 
  their 
  own 
  pollen. 
  In 
  this 
  

   case 
  the 
  visits 
  of 
  insects 
  have 
  simply 
  to 
  be 
  ex- 
  

   cluded, 
  which 
  may 
  be 
  done 
  by 
  covering 
  plants 
  

   with 
  iron 
  gauze 
  or 
  with 
  bags 
  of 
  prepared 
  paper. 
  

   Sometimes 
  they 
  fertilize 
  themselves 
  without 
  

   any 
  aid, 
  as 
  for 
  instance, 
  the 
  common 
  evening- 
  

   primrose; 
  in 
  other 
  cases 
  the 
  pollen 
  has 
  to 
  be 
  

   placed 
  on 
  the 
  stigma 
  artificially, 
  as 
  with 
  La- 
  

   marck's 
  evening-primrose 
  and 
  its 
  derivatives. 
  

   Other 
  plants 
  need 
  cross-fertilization 
  in 
  order 
  to 
  

   produce 
  a 
  normal 
  yield 
  of 
  seeds. 
  Here 
  two 
  

   individuals 
  have 
  always 
  to 
  be 
  combined, 
  and 
  

   the 
  pedigree 
  becomes 
  a 
  more 
  complicated 
  one. 
  

   Such 
  is 
  the 
  case 
  with 
  the 
  toad-flax, 
  which 
  is 
  

   nearly 
  sterile 
  with 
  its 
  own 
  pollen. 
  But 
  even 
  in 
  

   these 
  cases 
  the 
  visits 
  of 
  insects 
  bringing 
  pollen 
  

  

  