﻿HERITABLE 
  VARIATIONS 
  MULLER 
  351 
  

  

  lacking, 
  on 
  the 
  part 
  of 
  numerous 
  investigators, 
  to 
  find 
  the 
  cause, 
  

   or 
  a 
  cause, 
  of 
  visible 
  mutations, 
  by 
  trying 
  all 
  sorts 
  of 
  maltreatments 
  

   in 
  the 
  attempt 
  to 
  produce 
  such 
  changes. 
  In 
  the 
  course 
  of 
  this 
  work, 
  

   animals 
  and 
  plants 
  have 
  been 
  drugged, 
  poisoned, 
  intoxicated, 
  ether- 
  

   ized, 
  illuminated, 
  kept 
  in 
  darkness, 
  half 
  smothered, 
  painted 
  inside 
  and 
  

   out, 
  whirled 
  'round 
  and 
  'round, 
  shaken 
  violently, 
  vaccinated, 
  muti- 
  

   lated, 
  educated, 
  and 
  treated 
  with 
  everything 
  except 
  affection, 
  from 
  

   generation 
  to 
  generation. 
  But 
  their 
  genes 
  seemed 
  to 
  remain 
  oblivious, 
  

   and 
  they 
  could 
  not 
  be 
  distracted 
  into 
  making 
  any 
  obvious 
  mistake 
  in 
  

   the 
  reproduction 
  of 
  daughter 
  genes 
  just 
  like 
  themselves. 
  The 
  new 
  

   genes 
  were 
  exact 
  duplicates 
  of 
  the 
  old 
  ones, 
  showing 
  no 
  demonstrable 
  

   mutations, 
  or 
  at 
  most 
  such 
  a 
  scattering 
  few 
  as 
  might 
  have 
  occurred 
  

   anyhow. 
  

  

  Either 
  the 
  technique 
  used 
  for 
  finding 
  the 
  mutations 
  was 
  inadequate, 
  

   or 
  the 
  treatments 
  had 
  little 
  or 
  no 
  effect 
  upon 
  the 
  composition 
  of 
  the 
  

   genes, 
  or 
  both, 
  and 
  I 
  am 
  inclined 
  to 
  think 
  the 
  latter 
  is 
  correct. 
  And 
  

   yet 
  mutations 
  certainly 
  do 
  happen, 
  even 
  though 
  rarely. 
  In 
  the 
  

   examination 
  of 
  over 
  20,000,000 
  fruit 
  flies, 
  not 
  specially 
  maltreated, 
  

   over 
  400 
  mutations 
  have 
  been 
  found. 
  These 
  mutations 
  must 
  have 
  

   causes. 
  What 
  then 
  can 
  the 
  causes 
  be? 
  What 
  subtle 
  conditions 
  are 
  

   they, 
  apparently 
  so 
  independent 
  even 
  of 
  violent 
  injury 
  and 
  of 
  other 
  

   drastic 
  and 
  obvious 
  changes 
  in 
  the 
  physiological 
  or 
  pathological 
  state 
  

   of 
  the 
  organism? 
  In 
  going 
  over 
  the 
  data 
  on 
  mutational 
  occurrences 
  

   in 
  Drosophila 
  the 
  present 
  writer 
  reported 
  in 
  1920 
  the 
  finding 
  of 
  evi- 
  

   dence 
  that 
  in 
  this 
  fly, 
  when 
  a 
  mutation 
  occurred 
  in 
  a 
  given 
  gene 
  of 
  a 
  

   cell, 
  not 
  only 
  did 
  the 
  hundreds 
  or 
  thousands 
  of 
  genes 
  of 
  other 
  kinds 
  in 
  

   that 
  cell 
  remain 
  unchanged, 
  but 
  even 
  the 
  twin 
  gene 
  of 
  the 
  other 
  set 
  

   in 
  the 
  same 
  cell 
  — 
  i. 
  e., 
  the 
  originally 
  identical 
  gene 
  that 
  the 
  individual 
  

   had 
  received 
  from 
  its 
  other 
  parent 
  — 
  remained 
  unchanged 
  also. 
  

   Here, 
  then, 
  are 
  two 
  genes 
  of 
  identical 
  chemical 
  composition, 
  lying 
  

   very 
  close 
  to 
  one 
  another 
  in 
  the 
  same 
  ceU 
  — 
  on 
  the 
  average 
  less 
  than 
  

   a 
  thousandth 
  of 
  a 
  millimeter 
  apart 
  — 
  and 
  one 
  of 
  them, 
  but 
  not 
  its 
  

   duplicate, 
  is 
  caused 
  to 
  mutate. 
  Neither 
  do 
  the 
  identical 
  genes 
  in 
  

   neighboring 
  cells 
  mutate. 
  Evidence 
  for 
  this 
  same 
  kind 
  of 
  occurrence 
  

   has 
  been 
  adduced 
  in 
  other 
  organisms. 
  Why 
  do 
  not 
  the 
  same 
  condi- 
  

   tions, 
  acting 
  on 
  the 
  same 
  materials, 
  produce 
  everywhere 
  the 
  

   same 
  results? 
  If 
  events 
  in 
  this 
  sphere 
  are 
  apparently 
  so 
  indeter- 
  

   ministic, 
  is 
  it 
  any 
  wonder 
  that 
  we 
  could 
  not 
  in 
  our 
  previous 
  trials, 
  

   by 
  the 
  application 
  of 
  definite 
  conditions, 
  produce 
  definite 
  mutational 
  

   results? 
  

  

  In 
  view 
  of 
  these 
  accumulating 
  findings, 
  the 
  conclusion 
  seemed 
  to 
  me 
  

   to 
  become 
  increasingly 
  probable, 
  not 
  that 
  mutations 
  were 
  causeless 
  

   or 
  expressions 
  of 
  "the 
  natural 
  cussedness 
  of 
  things," 
  or 
  of 
  the 
  devil, 
  

   but 
  that, 
  as 
  Troland 
  had 
  suggested 
  prior 
  to 
  the 
  finding 
  of 
  this 
  evidence, 
  

   they 
  were 
  not 
  ordinarily 
  due 
  directly 
  to 
  gross 
  or 
  molar 
  causes, 
  but 
  

  

  