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  Descent 
  

  

  tary 
  qualities. 
  These 
  are 
  to 
  be 
  deduced 
  from 
  

   the 
  composition 
  of 
  the 
  progeny, 
  and 
  in 
  order 
  

   to 
  obtain 
  complete 
  evidence 
  on 
  this 
  point, 
  two 
  

   successive 
  generations 
  are 
  often 
  required. 
  The 
  

   investigation 
  must 
  ascertain 
  the 
  average 
  condi- 
  

   tion 
  of 
  this 
  offspring 
  and 
  the 
  occurrence 
  of 
  any 
  

   deviating 
  specimens, 
  and 
  for 
  both 
  purposes 
  it 
  

   is 
  necessary 
  to 
  cultivate 
  them 
  in 
  relatively 
  large 
  

   numbers. 
  It 
  is 
  obvious 
  that, 
  properly 
  speak- 
  

   ing, 
  the 
  whole 
  family 
  of 
  a 
  mutated 
  individual, 
  

   including 
  all 
  its 
  nearer 
  and 
  more 
  remote 
  rela- 
  

   tives, 
  should 
  be 
  known 
  and 
  recorded. 
  

  

  Hence 
  pedigree-book-keeping 
  must 
  become 
  

   the 
  general 
  rule. 
  Subordinate 
  to 
  this 
  are 
  two 
  

   further 
  points, 
  which 
  should 
  likewise 
  be 
  stated 
  

   here. 
  One 
  pertains 
  to 
  the 
  pure 
  or 
  hybrid 
  

   nature 
  of 
  the 
  original 
  strain, 
  and 
  the 
  other 
  to 
  

   the 
  life-conditions 
  and 
  all 
  other 
  external 
  in- 
  

   fluences. 
  It 
  is 
  manifest 
  that 
  a 
  complete 
  under- 
  

   standing 
  of 
  a 
  mutation 
  depends 
  upon 
  full 
  infor- 
  

   mation 
  upon 
  these 
  points. 
  

  

  All 
  experiments 
  must 
  have 
  a 
  beginning. 
  The 
  

   starting-point 
  may 
  be 
  a 
  single 
  individual, 
  or 
  a 
  

   small 
  group 
  of 
  plants, 
  or 
  a 
  lot 
  of 
  seeds. 
  In 
  

   many 
  cases 
  the 
  whole 
  previous 
  history 
  is 
  ob- 
  

   scure, 
  but 
  sometimes 
  a 
  little 
  historical 
  evidence 
  

   is 
  at 
  hand. 
  Often 
  it 
  is 
  evident 
  that 
  the 
  initial 
  

   material 
  belongs 
  to 
  a 
  pure 
  species, 
  but 
  with 
  re- 
  

   spect 
  to 
  the 
  question 
  of 
  elementary 
  species 
  it 
  is 
  

  

  