﻿356 
  ANNUAL 
  EEPOET 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  2 
  9 
  

  

  since 
  the 
  shooting 
  electrons 
  let 
  loose 
  by 
  the 
  X 
  rays 
  are 
  coursing 
  

   helter-skelter 
  through 
  the 
  cell, 
  quite 
  blindly, 
  and 
  are 
  just 
  as 
  apt 
  to 
  

   hit 
  one 
  gene 
  as 
  another, 
  to 
  strike 
  it 
  either 
  on 
  its 
  left 
  or 
  its 
  right 
  side, 
  

   through 
  its 
  heart 
  or 
  its 
  appendix, 
  so 
  to 
  speak, 
  and 
  so 
  will 
  cause 
  one 
  

   change 
  or 
  another 
  indiscriminately. 
  We 
  have 
  in 
  the 
  X-ray 
  muta- 
  

   tions, 
  then, 
  a 
  group 
  of 
  variations 
  which 
  seem 
  necessarily 
  to 
  be 
  random, 
  

   and 
  hence 
  would 
  necessarily 
  be 
  mostly 
  detrimental. 
  In 
  view 
  of 
  

   this, 
  it 
  is 
  interesting 
  to 
  compare 
  with 
  them 
  in 
  this 
  respect 
  the 
  natural 
  

   mutations, 
  and 
  to 
  note 
  that, 
  so 
  far 
  as 
  our 
  evidence 
  goes, 
  the 
  natural 
  

   mutations 
  have, 
  on 
  the 
  average, 
  every 
  bit 
  as 
  much 
  tendency 
  to 
  be 
  

   detrimental 
  as 
  the 
  X-ray 
  mutations 
  have. 
  The 
  obvious 
  conclusion 
  

   is 
  that 
  the 
  natural 
  mutations 
  too 
  are 
  random 
  changes, 
  in 
  the 
  same 
  

   sense 
  that 
  the 
  X-ray 
  mutations 
  are. 
  

  

  As 
  in 
  the 
  studies 
  on 
  natural 
  mutations, 
  so 
  too 
  among 
  our 
  artificial 
  

   ones, 
  the 
  great 
  majority 
  were 
  lethal 
  — 
  they 
  killed 
  the 
  fly 
  before 
  it 
  

   ever 
  hatched 
  except 
  where 
  there 
  was 
  a 
  normal 
  gene 
  from 
  the 
  other 
  

   parent 
  to 
  dominate 
  over 
  the 
  lethal 
  and 
  save 
  the 
  fly's 
  life, 
  so 
  that 
  it 
  

   could 
  be 
  bred 
  and 
  the 
  transmission 
  of 
  the 
  lethal 
  studied. 
  The 
  changes 
  

   in 
  wings, 
  eyes, 
  etc., 
  previously 
  mentioned 
  were 
  only 
  the 
  exceptional 
  

   visible 
  changes, 
  culled 
  from 
  out 
  of 
  a 
  great 
  mass 
  of 
  lethals. 
  Thus, 
  

   although 
  the 
  great 
  majority 
  of 
  the 
  offspring 
  of 
  X-rayed 
  flies 
  that 
  

   lived 
  looked 
  normal, 
  many 
  of 
  them 
  carried, 
  hidden 
  by 
  the 
  domi- 
  

   nant 
  normal 
  gene, 
  a 
  recessive 
  lethal 
  gene. 
  And 
  if 
  we 
  count 
  up 
  all 
  

   these 
  lethals 
  we 
  find 
  that 
  the 
  majority 
  of 
  the 
  oft'spring 
  of 
  heavily 
  

   X-rayed 
  flies 
  are 
  not 
  really 
  normal 
  in 
  their 
  genes 
  after 
  all, 
  for 
  some- 
  

   thing 
  over 
  50 
  per 
  cent 
  of 
  them 
  contained 
  some 
  kind 
  of 
  lethal 
  mutation 
  

   that 
  would 
  not 
  work 
  its 
  destruction 
  until 
  a 
  still 
  later 
  generation. 
  

   This, 
  too, 
  deserves 
  being 
  considered 
  in 
  its 
  bearing 
  on 
  X-ray 
  effects 
  

   in 
  the 
  case 
  of 
  human 
  beings. 
  Now, 
  previous 
  studies 
  of 
  Altenburg 
  

   and 
  myself 
  on 
  natural 
  mutations 
  have 
  shown 
  that 
  among 
  them 
  too, 
  

   although 
  the 
  total 
  frequency 
  of 
  mutations 
  is 
  so 
  much 
  smaller, 
  never- 
  

   theless 
  the 
  number 
  of 
  lethals 
  is 
  just 
  as 
  large, 
  relatively 
  to 
  the 
  number 
  

   of 
  other, 
  visible 
  mutations 
  which 
  occur 
  naturally, 
  as 
  it 
  is 
  among 
  the 
  

   X-ray 
  mutations. 
  As 
  the 
  lethals 
  differ 
  from 
  the 
  others, 
  after 
  all, 
  

   merely 
  in 
  being 
  more 
  detrimental, 
  this 
  result 
  simply 
  means 
  again 
  

   that 
  natural 
  mutations 
  are 
  just 
  as 
  apt 
  to 
  be 
  very 
  detrimental, 
  i. 
  e., 
  

   lethal, 
  as 
  are 
  X-ray 
  mutations, 
  thus 
  confirming 
  what 
  I 
  have 
  caffed 
  

   the 
  "accidental" 
  character 
  of 
  the 
  natural 
  mutations. 
  

  

  The 
  descendants 
  of 
  the 
  X-rayed 
  flies 
  have 
  been 
  bred 
  through 
  many 
  

   subsequent 
  generations. 
  It 
  has 
  been 
  found 
  that, 
  where 
  a 
  gene 
  was 
  not 
  

   caused 
  to 
  mutate 
  in 
  the 
  first 
  place, 
  it 
  wiff 
  not 
  show 
  a 
  subsequent 
  

   tendency 
  to 
  mutate, 
  without 
  further 
  treatment, 
  i. 
  e., 
  there 
  is 
  no 
  

   perceptible 
  after-effect 
  on 
  the 
  genes 
  that 
  escaped 
  an 
  immediate 
  hit. 
  

   On 
  the 
  other 
  hand, 
  those 
  genes 
  that 
  were 
  hit 
  and 
  mutated 
  now 
  breed 
  

   true 
  to 
  their 
  new 
  type, 
  which 
  in 
  the 
  great 
  majority 
  of 
  cases 
  gives 
  

  

  