﻿HERITABLE 
  VARIATIONS, 
  THEIR 
  PRODUCTION 
  BY 
  

   X 
  RAYS 
  AND 
  THEIR 
  RELATION 
  TO 
  EVOLUTION 
  ' 
  

  

  By 
  H. 
  J. 
  Mtjller 
  

   University 
  of 
  Texas 
  

  

  Biological 
  evolution 
  is 
  composed 
  of 
  a 
  succession 
  of 
  individual 
  vari- 
  

   ations, 
  which, 
  being 
  heritable, 
  become 
  accumulated 
  to 
  form 
  an 
  in- 
  

   creasingly 
  complex 
  living 
  fabric. 
  The 
  long-disputed 
  questions 
  con- 
  

   cerning 
  the 
  method 
  of 
  evolution 
  can 
  therefore 
  be 
  decided 
  only 
  through 
  

   a 
  study 
  of 
  the 
  mechanism 
  whereby 
  these 
  individual 
  variations 
  are 
  

   produced. 
  What 
  is 
  needed 
  here 
  is 
  more 
  precise 
  and 
  analytical 
  data 
  

   regarding 
  the 
  nature 
  of 
  those 
  differences 
  which 
  distinguish 
  one 
  gener- 
  

   ation 
  of 
  individuals 
  from 
  its 
  predecessors, 
  and 
  which 
  they 
  in 
  turn 
  tend 
  

   to 
  transmit 
  as 
  a 
  heritage 
  to 
  their 
  descendants. 
  We 
  must 
  not 
  remain 
  

   content 
  to 
  view 
  evolution 
  from 
  afar, 
  but 
  must 
  view 
  close 
  up, 
  as 
  through 
  

   a 
  microscope, 
  the 
  transitions 
  now 
  occuring 
  out 
  of 
  which 
  the 
  evolution- 
  

   ary 
  story 
  is 
  pieced 
  together. 
  The 
  science 
  which 
  essays 
  this 
  study 
  is 
  

   "genetics." 
  

  

  THE 
  FUNCTION 
  OF 
  THE 
  GENE 
  

  

  During 
  the 
  present 
  century 
  genetics, 
  building 
  upon 
  the 
  earlier 
  dis- 
  

   coveries 
  of 
  Mendel, 
  has 
  practically 
  solved 
  the 
  problem 
  of 
  the 
  method 
  of 
  

   inheritance 
  of 
  the 
  differences 
  referred 
  to, 
  once 
  they 
  have 
  arisen. 
  All 
  

   modern 
  genetic 
  work 
  converges 
  to 
  show 
  that 
  the 
  heritable 
  differences 
  

   between 
  parent 
  and 
  offspring, 
  between 
  sister 
  and 
  sister, 
  in 
  fact 
  between 
  

   any 
  organisms 
  which 
  can 
  be 
  crossed, 
  have 
  their 
  basis 
  in 
  differences 
  in 
  

   minute 
  self-reproducing 
  bodies 
  called 
  the 
  genes, 
  located 
  in 
  the 
  nucleus 
  

   of 
  every 
  cell. 
  The 
  genes 
  themselves 
  are 
  too 
  small 
  to 
  be 
  separately 
  

   visible, 
  but 
  hundreds 
  or 
  thousands 
  of 
  them 
  are 
  linked 
  together 
  into 
  

   strings, 
  and 
  these 
  strings 
  of 
  genes, 
  together 
  probably 
  with 
  some 
  access- 
  

   ory 
  material, 
  are 
  large 
  enough 
  to 
  be 
  seen 
  through 
  the 
  microscope 
  by 
  

   the 
  cytologist; 
  they 
  constitute 
  the 
  sausage-shaped 
  bodies 
  called 
  chro- 
  

   mosomes. 
  We 
  know 
  that, 
  ordinarily, 
  each 
  individual 
  gene 
  in 
  a 
  string 
  

   is 
  different 
  from 
  every 
  other 
  gene 
  in 
  the 
  same 
  string, 
  and 
  has 
  its 
  own 
  

   distinctive 
  role 
  to 
  play 
  in 
  the 
  incomparably 
  complicated 
  economy 
  of 
  

  

  > 
  Based 
  on 
  an 
  article 
  in 
  the 
  Scientific 
  Monthly, 
  December, 
  1929; 
  here 
  printed 
  by 
  permission 
  of 
  that 
  

   Qurnal. 
  

  

  345 
  

  

  