﻿362 
  Mr. 
  W. 
  Bateson. 
  

  

  knew 
  nothing 
  of 
  the 
  genetics 
  of 
  plants 
  we 
  might 
  confidently 
  adopt 
  the 
  view 
  

   which 
  Morgan 
  has 
  so 
  successfully 
  developed, 
  that 
  normal 
  segregation 
  and 
  

   redistribution 
  happen 
  exclusively 
  in 
  the 
  process 
  of 
  reduction. 
  Though 
  

   unconvinced, 
  I 
  cannot 
  deny 
  that 
  linkage 
  and 
  crossing-over 
  may 
  well 
  be 
  

   represented 
  provisionally, 
  as 
  effected 
  during 
  synapsis. 
  The 
  scheme 
  pre- 
  

   viously 
  offered 
  by 
  Punnett 
  and 
  myself 
  as 
  a 
  diagrammatic 
  plan 
  capable 
  

   of 
  representing 
  these 
  phenomena 
  is 
  certainly 
  far 
  less 
  attractive. 
  There 
  

   is 
  evidence 
  that 
  in 
  certain 
  plants, 
  e.g., 
  Malthiola, 
  the 
  composition 
  of 
  the 
  

   families 
  derived 
  from 
  single 
  pods 
  shows 
  very 
  great 
  and 
  perhaps 
  irregular 
  

   fluctuations, 
  and 
  the 
  normal 
  ratios 
  for 
  those 
  families 
  is 
  only 
  found 
  by 
  

   taking 
  the 
  average 
  of 
  many, 
  but 
  no 
  sufficient 
  statistical 
  examination 
  of 
  such 
  

   cases 
  has 
  yet 
  been 
  made. 
  In 
  some 
  suitable 
  case 
  estimations 
  of 
  the 
  offspring 
  

   derived 
  from 
  individual 
  anthers 
  might 
  be 
  of 
  value 
  in 
  this 
  connexion. 
  

   Eenuer, 
  by 
  examining 
  the 
  starches 
  of 
  the 
  pollen-grains 
  in 
  CEnotheras, 
  has 
  

   lately 
  made 
  visible 
  that 
  di-morphism, 
  of 
  which 
  we 
  had 
  previously 
  genetic 
  

   proof, 
  and 
  perhaps 
  this 
  novel 
  and 
  striking 
  observation 
  might 
  be 
  used 
  for 
  

   the 
  purpose 
  of 
  mapping 
  the 
  distribution 
  of 
  such 
  a 
  character 
  among 
  the 
  

   pollen-grains. 
  Meanwhile, 
  it 
  must 
  be 
  granted 
  that 
  no 
  indication 
  that 
  

   gametic 
  linkage 
  results 
  from 
  somatic 
  differentiation 
  has 
  yet 
  been 
  obtained. 
  

  

  When, 
  however, 
  we 
  examine 
  the 
  view 
  that 
  linkage 
  of 
  factors 
  is 
  a 
  con- 
  

   secpience 
  of 
  their 
  association 
  in 
  a 
  chromosome, 
  we 
  must 
  observe 
  that 
  there 
  is 
  

   no 
  body 
  of 
  evidence 
  that 
  the 
  number 
  of 
  linkage-systems 
  agrees 
  with 
  that 
  of 
  

   the 
  chromosomes, 
  a 
  primary 
  postulate 
  of 
  Morgan's 
  theory. 
  Drosophila 
  is 
  the 
  

   only 
  example 
  which 
  has 
  been 
  adequately 
  investigated. 
  The 
  cytological 
  

   appearances 
  are 
  not 
  readily 
  consistent 
  with 
  the 
  other 
  postulate 
  of 
  Morgan's 
  

   case, 
  that 
  crossing-over 
  is 
  effected 
  by 
  anastomosis 
  of 
  chromosomes 
  and 
  

   exchange 
  of 
  materials 
  between 
  them. 
  In 
  our 
  present 
  ignorance 
  of 
  the 
  

   physical 
  nature 
  of 
  the 
  factors 
  we 
  are 
  not 
  obliged 
  to 
  assume 
  that 
  an 
  actual 
  

   transference 
  of 
  material 
  is 
  an 
  essential 
  condition 
  for 
  an 
  exchange 
  of 
  

   properties 
  ; 
  but 
  since 
  Morgan's 
  suggestion 
  is 
  made 
  in 
  that 
  form 
  we 
  are 
  

   bound 
  to 
  notice 
  how 
  difficult 
  it 
  is 
  to 
  interpret 
  the 
  visible 
  phenomena 
  of 
  

   cytology 
  in 
  accordance 
  with 
  that 
  hypothesis. 
  Without 
  personal 
  familiarity 
  

   with 
  cytology 
  no 
  one 
  can 
  have 
  a 
  confident 
  opinion. 
  I 
  observe, 
  however, 
  

   that 
  in 
  his 
  most 
  recent 
  publication 
  on 
  these 
  subjects, 
  E. 
  B. 
  Wilson* 
  gives 
  

   a 
  very 
  emphatic 
  " 
  counsel 
  of 
  caution," 
  remarking 
  that 
  writers 
  on 
  genetics 
  

   have 
  taken 
  too 
  much 
  for 
  granted, 
  and 
  that 
  for 
  the 
  present 
  the 
  genetic 
  

   development 
  of 
  the 
  chromosome 
  theory 
  has 
  far 
  outrun 
  the 
  cytological. 
  To 
  

   a 
  layman 
  the 
  visible 
  appearance 
  of 
  chromosomes 
  is 
  scarcely 
  suggestive 
  of 
  

   the 
  prodigious 
  material 
  heterogeneity 
  demanded, 
  and 
  the 
  general 
  course 
  

   * 
  ' 
  Amer. 
  Nat.,' 
  p. 
  210, 
  May-June, 
  1920. 
  

  

  