﻿HERITABLE 
  VARIATIONS 
  MULLER 
  349 
  

  

  gray, 
  and 
  from 
  such 
  a 
  union 
  a 
  visibly 
  yellow 
  offspring 
  would 
  emerge 
  

   for 
  the 
  first 
  time. 
  A 
  mutation, 
  when 
  recessive, 
  may 
  accordingly 
  

   fail 
  to 
  manifest 
  itself 
  for 
  many 
  generations, 
  or 
  may 
  never 
  have 
  a 
  

   chance 
  to 
  show 
  itself 
  at 
  all, 
  before 
  the 
  line 
  of 
  individuals 
  carrying 
  it 
  

   becomes 
  extinguished. 
  (It 
  has 
  been 
  shown 
  by 
  Fisher 
  that 
  most 
  

   mutations 
  must 
  meet 
  this 
  mute 
  inglorious 
  fate.) 
  

  

  The 
  new 
  gene, 
  once 
  it 
  has 
  arisen, 
  is 
  ordinarily 
  as 
  stable 
  as 
  the 
  old. 
  

   The 
  change 
  is 
  definite 
  and 
  fixed, 
  obviously 
  of 
  a 
  chemical 
  nature. 
  

   Once 
  it 
  has 
  occurred 
  we 
  have 
  a 
  new 
  mutant 
  gene 
  which 
  will 
  either 
  

   spread 
  throughout 
  the 
  population 
  or 
  be 
  killed 
  oft', 
  according 
  as 
  the 
  

   individuals 
  which 
  carry 
  it 
  reproduce 
  more 
  offspring 
  or 
  fewer. 
  The 
  

   effects 
  of 
  mutations 
  are 
  of 
  course 
  as 
  varied 
  as 
  the 
  gene 
  differences 
  

   which 
  are 
  found 
  to 
  occur 
  within 
  populations, 
  since 
  these 
  gene 
  dif- 
  

   ferences 
  originated 
  by 
  mutation. 
  Some 
  gene 
  differences, 
  some 
  muta- 
  

   tions, 
  produce 
  large 
  and 
  startling 
  effects, 
  like 
  growing 
  a 
  leg 
  on 
  a 
  

   fly's 
  forehead. 
  Some 
  affect 
  the 
  whole 
  body 
  in 
  practically 
  all 
  its 
  

   parts, 
  others 
  change 
  two 
  or 
  three 
  characters, 
  others 
  apparently 
  but 
  

   one. 
  But 
  the 
  less 
  conspicuous 
  changes, 
  the 
  insignificant 
  effects 
  that 
  

   are 
  easily 
  overlooked, 
  or 
  that 
  even, 
  in 
  many 
  individuals, 
  quite 
  overlap 
  

   the 
  normal 
  type, 
  seem 
  at 
  least 
  as 
  apt 
  to 
  occur 
  as 
  do 
  the 
  pyrotechnical 
  

   varieties. 
  Evidence 
  is 
  not 
  lacking 
  that 
  physiological 
  changes, 
  and 
  

   changes 
  that 
  can 
  only 
  be 
  detected 
  physicochemically, 
  are 
  probably 
  

   as 
  frequent 
  as 
  changes 
  in 
  visible 
  structures, 
  but 
  geneticists 
  have 
  till 
  

   now 
  had 
  to 
  have 
  a 
  predominantly 
  morphological 
  training, 
  and 
  anyhow, 
  

   the 
  morphological 
  is 
  easier 
  to 
  see 
  and 
  deal 
  with. 
  It 
  would 
  be 
  absurd 
  

   and 
  scholastic 
  to 
  try 
  to 
  classify 
  mutations 
  according 
  to 
  the 
  nature 
  

   of 
  their 
  effects. 
  A 
  mutation 
  can 
  do 
  practically 
  anything 
  that 
  life 
  

   can 
  do 
  — 
  or 
  at 
  least 
  a 
  little 
  of 
  it, 
  for 
  life 
  is 
  built 
  out 
  of 
  mutations. 
  

  

  THE 
  RANDOMNESS 
  OF 
  MUTATIONS 
  

  

  The 
  statement 
  just 
  made 
  does 
  not 
  necessarily 
  mean, 
  however, 
  

   that 
  the 
  average 
  mutation 
  does 
  very 
  much 
  in 
  the 
  furthering 
  of 
  life. 
  

   The 
  vast 
  majority 
  of 
  observed 
  mutations 
  are 
  positively 
  detrimental, 
  

   and 
  handicap 
  the 
  individual 
  less 
  or 
  more 
  in 
  the 
  struggle 
  for 
  sur- 
  

   vival 
  and 
  reproduction. 
  In 
  fact, 
  as 
  Altenburg 
  and 
  I 
  showed 
  in 
  some 
  

   studies 
  on 
  the 
  fruit 
  fly, 
  Drosophila, 
  in 
  1919, 
  by 
  far 
  the 
  greater 
  number 
  

   of 
  detectable 
  mutations 
  in 
  it 
  are 
  actually 
  lethal; 
  their 
  effect 
  is 
  to 
  kill 
  

   the 
  animal 
  before 
  it 
  becomes 
  adult 
  (though 
  of 
  course 
  their 
  effect 
  

   may 
  be 
  prevented 
  if 
  they 
  are 
  recessive 
  and 
  if 
  the 
  dominant 
  normal 
  

   gene 
  has 
  been 
  received 
  by 
  the 
  individual 
  from 
  its 
  other 
  parent). 
  

   Evidence 
  is 
  accumulating 
  that 
  the 
  same 
  situation 
  probably 
  holds 
  

   true 
  in 
  other 
  forms 
  of 
  life. 
  Now 
  this 
  is 
  just 
  what 
  we 
  should 
  expect, 
  

   and 
  did 
  expect, 
  on 
  the 
  basis 
  of 
  the 
  theory 
  that 
  a 
  mutation 
  is 
  a 
  chemical 
  

   change 
  in 
  a 
  gene, 
  occurring 
  at 
  random, 
  as 
  it 
  were, 
  that 
  is^ 
  without 
  

   reference 
  to 
  the 
  effect 
  that 
  would 
  be 
  produced, 
  a-teleologically. 
  

  

  