﻿HEEITABLE 
  VARIATIONS 
  MULLER 
  355 
  

  

  some 
  which 
  has 
  been 
  subjected 
  to 
  the 
  most 
  intensive 
  study 
  (the 
  X- 
  

   chromosome) 
  the 
  majority 
  of 
  all 
  the 
  well-known 
  mutations 
  that 
  had 
  

   previously 
  been 
  found 
  by 
  the 
  dozen 
  or 
  so 
  active 
  investigators 
  in 
  the 
  

   course 
  of 
  15 
  years, 
  now 
  were 
  found 
  to 
  have 
  arisen 
  over 
  again 
  in 
  the 
  

   cultures 
  of 
  X-rayed 
  flies 
  here. 
  Besides 
  these 
  reappearances 
  there 
  were 
  

   of 
  course 
  many 
  new 
  types 
  also, 
  more 
  new 
  types 
  than 
  old, 
  but 
  it 
  

   should 
  be 
  remembered 
  in 
  this 
  connection 
  that 
  new 
  types 
  are 
  con- 
  

   tinually 
  being 
  found, 
  though 
  with 
  far 
  lesser 
  frequency, 
  in 
  the 
  untreated 
  

   material 
  also. 
  

  

  The 
  new 
  types 
  of 
  mutations, 
  like 
  the 
  old, 
  conformed 
  in 
  their 
  general 
  

   expression 
  and 
  mode 
  of 
  inheritance 
  to 
  certain 
  general 
  principles, 
  

   which 
  I 
  had 
  previously 
  observed 
  to 
  hold 
  in 
  the 
  case 
  of 
  the 
  mutations 
  

   occurring 
  in 
  untreated 
  material. 
  One 
  of 
  these 
  principles 
  was 
  that 
  the 
  

   great 
  majority 
  of 
  the 
  mutations 
  of 
  X 
  ray 
  as 
  well 
  as 
  of 
  natural 
  origin 
  

   are 
  recessive 
  to 
  the 
  normal 
  type, 
  despite 
  the 
  presence 
  of 
  a 
  rather 
  small 
  

   minority 
  of 
  dominants. 
  Thus 
  the 
  techniquej'^of 
  in 
  breeding 
  through 
  

   a 
  number 
  of 
  generations 
  in 
  order 
  to 
  find 
  the 
  mutations, 
  was 
  found 
  to 
  

   be 
  justified. 
  It 
  may 
  be 
  remarked 
  here 
  that 
  if 
  human 
  beings 
  are 
  

   affected 
  by 
  X 
  rays 
  in 
  the 
  same 
  way 
  as 
  flies, 
  we 
  can 
  not 
  expect 
  to 
  find 
  

   much 
  evidence 
  of 
  a 
  mutational 
  effect 
  of 
  X 
  rays 
  on 
  them 
  from 
  data 
  

   derived 
  only 
  from 
  the 
  first, 
  or 
  even 
  the 
  first, 
  second, 
  and 
  third 
  human 
  

   generations, 
  and 
  such 
  a 
  negative 
  result 
  will 
  therefore 
  by 
  no 
  means 
  

   indicate 
  a 
  lack 
  of 
  significant 
  genetic 
  effect. 
  

  

  The 
  second 
  principle 
  observed 
  was 
  that 
  the 
  X-ray 
  mutations, 
  like 
  

   the 
  natural 
  ones, 
  included 
  both 
  inconspicuous 
  as 
  well 
  as 
  conspicuous 
  

   changes, 
  changes 
  of 
  slight 
  or 
  almost 
  impreceptible 
  degree 
  as 
  well 
  as 
  

   striking 
  changes 
  of 
  structure 
  or 
  quality, 
  and 
  changes 
  that 
  registered 
  

   their 
  effect, 
  so 
  far 
  as 
  could 
  be 
  determined, 
  only 
  in 
  slight 
  lowerings 
  of 
  

   the 
  general 
  vitality, 
  as 
  well 
  as 
  those 
  that 
  were 
  more 
  graphically 
  de- 
  

   scribable. 
  If 
  anything, 
  the 
  more 
  easily 
  overlooked 
  effects 
  were 
  the 
  

   more 
  frequent. 
  

  

  A 
  third 
  principle 
  noted 
  was 
  that 
  most 
  of 
  the 
  X-ray 
  mutations 
  were 
  

   in 
  some 
  way 
  detrimental 
  to 
  the 
  animal 
  in 
  living 
  its 
  life 
  — 
  they 
  were 
  

   steps 
  in 
  the 
  wrong 
  direction 
  in 
  the 
  struggle 
  for 
  existence. 
  This 
  find- 
  

   ing 
  has 
  already 
  been 
  discussed 
  in 
  the 
  case 
  of 
  the 
  natural 
  mutations, 
  

   and 
  it 
  has 
  been 
  explained 
  that 
  this 
  is 
  just 
  what 
  is 
  to 
  be 
  expected, 
  on 
  the 
  

   whole, 
  of 
  changes 
  that 
  occur 
  at 
  random, 
  accidentally, 
  by 
  "chance" 
  — 
  

   I 
  care 
  not 
  what 
  term 
  you 
  wish 
  to 
  use 
  to 
  describe 
  the 
  idea 
  that 
  they 
  

   occur 
  without 
  reference 
  to 
  theu* 
  consequences, 
  unadaptively, 
  and 
  

   hence 
  are 
  more 
  likely 
  to 
  be 
  "wrong" 
  than 
  "right" 
  changes, 
  just 
  be- 
  

   cause 
  there 
  are 
  more 
  wrong 
  roads 
  than 
  right 
  roads 
  to 
  follow, 
  and 
  

   because, 
  as 
  is 
  well 
  known, 
  the 
  right 
  road 
  is 
  apt 
  to 
  be 
  the 
  narrower. 
  

   In 
  the 
  case 
  of 
  the 
  X-ray 
  mutations 
  it 
  is 
  easily 
  seen 
  that, 
  if 
  the 
  change 
  

   occurs 
  as 
  I 
  have 
  pictured 
  it, 
  it 
  must 
  occur 
  accidentally, 
  without 
  refer- 
  

   ence 
  to 
  the 
  possible 
  advantage 
  or 
  disadvantage 
  it 
  would 
  confer, 
  

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