﻿HEEITABLE 
  VARIATIONS 
  MULLER 
  361 
  

  

  genes 
  in 
  mature 
  spermatozoa. 
  I 
  find 
  that 
  the 
  genes 
  in 
  the 
  sperma- 
  

   tozoa 
  of 
  the 
  adult 
  male 
  are 
  also 
  more 
  sensitive 
  than 
  those 
  in 
  the 
  germ 
  

   cells 
  of 
  the 
  female, 
  or 
  than 
  those 
  in 
  the 
  germ 
  cells 
  of 
  the 
  larval 
  male. 
  

   There 
  seems 
  to 
  be 
  more 
  difference 
  in 
  their 
  sensitivity 
  to 
  the 
  gene- 
  

   rearranging 
  effect 
  of 
  the 
  rays 
  than 
  in 
  their 
  sensitivity 
  to 
  the 
  transmut- 
  

   ing 
  effect 
  on 
  individual 
  genes. 
  The 
  activity 
  of 
  metabolism 
  however, 
  

   varied 
  by 
  starving, 
  and 
  by 
  feeding 
  and 
  mating 
  the 
  female, 
  had 
  no 
  

   perceptible 
  influence 
  in 
  my 
  experiments, 
  and, 
  as 
  both 
  Stadler 
  and 
  I 
  

   have 
  found 
  independently 
  on 
  barley 
  and 
  flies, 
  respectively, 
  extremes 
  of 
  

   heat 
  or 
  cold 
  applied 
  at 
  the 
  time 
  of 
  treatment 
  have 
  little 
  or 
  no 
  effect. 
  

  

  Thus 
  the 
  work 
  on 
  the 
  physiology 
  of 
  mutation-production 
  is 
  opening 
  

   up, 
  though 
  as 
  yet 
  in 
  a 
  very 
  empirical 
  stage. 
  And 
  meanwhile, 
  X 
  rays 
  

   and 
  their 
  relatives 
  remain 
  the 
  only 
  prime 
  cause 
  of 
  mutations 
  yet 
  

   known. 
  Whether 
  radiation 
  furnishes 
  the 
  exclusive 
  motive 
  power 
  of 
  

   evolution 
  can 
  eventually 
  be 
  ascertained 
  definitely, 
  through 
  pains- 
  

   taking 
  quantitative 
  determinations 
  of 
  the 
  mutation 
  frequencies 
  

   existing 
  in 
  the 
  presence 
  of 
  measured 
  minute 
  amounts 
  of 
  radiation. 
  

   As 
  stated 
  in 
  some 
  previous 
  publications 
  from 
  our 
  laboratory, 
  we 
  have 
  

   experiments 
  projected 
  which 
  we 
  hope 
  will 
  test 
  this 
  question 
  in 
  fhes. 
  

  

  Since, 
  however, 
  mutations 
  in 
  general 
  bear 
  all 
  the 
  earmarks 
  of 
  the 
  

   X-ray 
  mutations, 
  then, 
  even 
  if 
  not 
  all 
  of 
  them 
  have 
  actually 
  been 
  

   produced 
  by 
  radiation, 
  it 
  seems 
  legitimate 
  to 
  use 
  the 
  readily 
  obtain- 
  

   able 
  X-ray 
  (radium, 
  etc.) 
  mutations 
  as 
  the 
  handle 
  by 
  wliich 
  to 
  study 
  

   them. 
  These 
  X-ray 
  mutations 
  are 
  certainly 
  accidental, 
  being 
  produced 
  

   by 
  ultramicroscopic 
  events 
  not 
  individually 
  controllable, 
  that 
  take 
  

   place 
  mthout 
  reference 
  to 
  the 
  outcome 
  or 
  the 
  advantage 
  for 
  the 
  organ- 
  

   ism. 
  The 
  natural 
  mutations 
  — 
  some 
  of 
  which 
  we 
  know 
  must 
  be 
  due 
  to 
  

   radiation 
  — 
  are 
  on 
  the 
  average 
  equally 
  as 
  detrimental, 
  and 
  are 
  of 
  the 
  same 
  

   general 
  nature, 
  so 
  far 
  as 
  their 
  efl'ects 
  are 
  concerned, 
  as 
  the 
  X-ray 
  muta- 
  

   tions. 
  Can 
  we 
  then 
  escape 
  the 
  conclusion 
  that 
  they 
  are 
  accidental 
  in 
  

   the 
  same 
  sense, 
  and 
  that 
  specific 
  mutations 
  are 
  therefore 
  not 
  dictated 
  

   by 
  any 
  "adaptive 
  reactions" 
  or 
  other 
  specific 
  responses 
  of 
  the 
  organ- 
  

   ism 
  to 
  climate, 
  or 
  to 
  an}^ 
  other 
  features 
  of 
  its 
  mode 
  of 
  life? 
  

  

  Due, 
  then, 
  to 
  the 
  tremendously 
  magnified 
  effect 
  which 
  each 
  tiny 
  

   gene 
  can 
  produce, 
  throiigh 
  the 
  processes 
  of 
  growth 
  and 
  development, 
  

   we 
  have 
  a 
  molar 
  indeterminism, 
  in 
  the 
  origination 
  of 
  genetic 
  variations, 
  

   resulting 
  from 
  an 
  ultramicroscopic 
  determinism. 
  (We 
  will 
  not 
  quarrel 
  

   here 
  about 
  whether 
  or 
  not 
  a 
  Heisenbergian 
  "principle 
  of 
  uncertainty" 
  

   lies 
  beneath 
  the 
  latter 
  in 
  turn.) 
  But 
  now 
  "natural 
  selection" 
  sets 
  

   to 
  work, 
  weeding 
  out 
  the 
  many 
  disadvantageous 
  m.utants 
  here, 
  allow- 
  

   ing 
  the 
  multiplication 
  of 
  the 
  few 
  advantageous 
  mutants 
  there, 
  until 
  

   again 
  from 
  all 
  the 
  maze 
  of 
  variants 
  we 
  have 
  organization 
  returning, 
  

   advancing, 
  and 
  so, 
  as 
  a 
  statistical 
  consequence, 
  there 
  results 
  a 
  kind 
  of 
  

   higher 
  molar 
  determinism, 
  finally 
  governing 
  many 
  features 
  of 
  the 
  

   actual 
  evolution 
  of 
  the 
  species. 
  Thus 
  we 
  are 
  sometimes 
  furnished 
  

  

  