﻿HEEITABLE 
  VARIATIONS 
  MULLER 
  357 
  

  

  evidence 
  of 
  being 
  as 
  stable 
  as 
  the 
  original 
  type 
  was 
  before 
  treatment. 
  

   We 
  now 
  have 
  in 
  the 
  laboratory 
  various 
  mutant 
  races 
  of 
  flies, 
  derived 
  

   from 
  our 
  earlier 
  X-ray 
  experiments, 
  which 
  have 
  passed 
  through 
  

   something 
  like 
  75 
  or 
  more 
  generations 
  since 
  the 
  time 
  the 
  mutation 
  

   took 
  place, 
  and 
  there 
  has 
  been 
  no 
  sign 
  in 
  them 
  of 
  any 
  tendency 
  to 
  

   revert 
  back 
  to 
  the 
  originally 
  normal 
  condition. 
  They 
  have 
  their 
  own, 
  

   new 
  norm; 
  they 
  are 
  real, 
  new 
  varieties. 
  The 
  new 
  forms 
  are 
  perma- 
  

   nent, 
  in 
  so 
  far 
  as 
  the 
  word 
  permanent 
  may 
  be 
  applied 
  legitimately 
  to 
  

   living 
  things. 
  When 
  crossed 
  to 
  other 
  forms, 
  the 
  new 
  differences 
  obey 
  

   the 
  same 
  laws 
  of 
  Mendelian 
  and 
  chromosomal 
  inheritance 
  as 
  do 
  the 
  

   gene 
  differences 
  existing 
  between 
  natural 
  varieties. 
  

  

  THE 
  NATURE 
  AND 
  SIGNIFICANCE 
  OF 
  THE 
  GENETIC 
  EFFECT 
  OF 
  

  

  RADIATION 
  

  

  It 
  might 
  perhaps 
  be 
  contended 
  in 
  some 
  quarters 
  that 
  while 
  the 
  

   artificial 
  mutations 
  may 
  be 
  similar 
  in 
  some 
  respects 
  to 
  natural 
  ones, 
  

   and 
  even 
  identical 
  with 
  some 
  natural 
  ones, 
  yet 
  they 
  may 
  not 
  be 
  

   similar 
  to 
  those 
  particular 
  natural 
  mutations 
  which 
  may 
  be 
  termed 
  

   "progressive"; 
  the 
  mutant 
  genes 
  resulting 
  from 
  which 
  survive, 
  

   multiply, 
  and 
  thus 
  become 
  a 
  part 
  of 
  the 
  heritage 
  of 
  an 
  evolving 
  species. 
  

   Such 
  claiments 
  would 
  hold 
  that 
  the 
  X-ray 
  action 
  is 
  necessarily 
  

   destructive, 
  causing 
  only 
  loss 
  and 
  injury, 
  and 
  that 
  thus 
  it 
  can 
  work 
  

   only 
  harm, 
  or 
  at 
  least 
  can 
  cause 
  no 
  indefinite 
  amount 
  of 
  progress 
  in 
  

   organization. 
  Such 
  a 
  contention 
  would 
  rest 
  upon 
  a 
  misconception 
  of 
  

   the 
  action 
  of 
  the 
  X 
  ray, 
  for 
  it 
  can 
  be 
  shown 
  that 
  the 
  speeding 
  electron 
  

   is 
  capable 
  of 
  imparting- 
  energy 
  to 
  other 
  atoms 
  through 
  which 
  it 
  goes, 
  

   and 
  that 
  the 
  resulting 
  chemical 
  changes 
  may 
  be 
  of 
  a 
  synthetic 
  char- 
  

   acter 
  as 
  well 
  as 
  otherwise. 
  However, 
  since 
  we 
  can 
  not 
  analyze 
  

   chemically 
  the 
  real 
  nature 
  of 
  the 
  changes 
  involved 
  in 
  the 
  production 
  of 
  

   mutations 
  by 
  X 
  rays, 
  empirical 
  evidence 
  on 
  the 
  question 
  at 
  issue 
  is 
  

   called 
  for, 
  and 
  that 
  is 
  what 
  we 
  have 
  been 
  trying 
  to 
  obtain. 
  

  

  It 
  is 
  evident, 
  as 
  my 
  wife 
  has 
  suggested, 
  that 
  if 
  the 
  change 
  induced 
  by 
  

   X 
  ray 
  from, 
  say, 
  a 
  gene 
  designated 
  as 
  large 
  A, 
  to 
  a 
  mutant 
  gene 
  of 
  

   different 
  composition, 
  designated 
  as 
  small 
  a, 
  has 
  really 
  involved 
  a 
  

   destructive 
  process 
  or 
  a 
  loss, 
  then 
  the 
  opposite 
  change, 
  from 
  small 
  a 
  to 
  

   large 
  A, 
  must, 
  conversely, 
  involve 
  a 
  constructive 
  process 
  or 
  a 
  gain. 
  

   With 
  this 
  question 
  in 
  mind, 
  Prof. 
  J. 
  T. 
  Patterson 
  and 
  I 
  have 
  been 
  

   engaged 
  in 
  some 
  extensive 
  irradiation 
  experiments 
  involving 
  par- 
  

   ticular 
  characters. 
  The 
  character 
  which 
  we 
  have 
  used 
  most 
  is 
  the 
  

   recessive 
  mutant 
  character 
  termed 
  "forked 
  bristles" 
  (f) 
  as 
  compared 
  

   with 
  the 
  dominant 
  normal 
  straight 
  bristles 
  (F). 
  The 
  evidence 
  is 
  now 
  

   positive 
  and 
  convincing 
  that 
  the 
  X 
  rays 
  not 
  only 
  induce 
  the 
  muta- 
  

   tion 
  of 
  straight 
  bristles 
  to 
  the 
  recessive 
  forked, 
  but 
  also 
  the 
  precisely 
  

   opposite 
  type 
  of 
  change, 
  namely, 
  forked 
  bristles 
  to 
  the 
  dominant 
  

   straight, 
  and 
  abundant 
  controls 
  have 
  shown 
  that 
  it 
  is 
  really 
  the 
  X 
  

  

  