﻿< 
  -renetic 
  Segregation. 
  

  

  361 
  

  

  fractional 
  factors. 
  Have 
  they, 
  then, 
  been 
  distributed 
  among 
  different 
  

   chromosomes 
  ? 
  If 
  in 
  Antirrhinum, 
  the 
  colour 
  of 
  the 
  face 
  and 
  of 
  the 
  throat 
  

   were 
  lately 
  parts 
  of 
  a 
  single 
  factor 
  for 
  the 
  total 
  flower-colour, 
  would 
  not 
  

   linkage 
  between 
  them 
  be 
  expected 
  ? 
  ^Nevertheless, 
  in 
  cases 
  of 
  this 
  sort, 
  

   so 
  far 
  as 
  I 
  know, 
  linkages 
  have 
  not 
  been 
  found 
  with 
  any 
  special 
  frequency. 
  

  

  The 
  segregation 
  of 
  a 
  group 
  of 
  differences 
  — 
  and 
  presumably 
  factors 
  — 
  

   in 
  combination 
  has 
  lately 
  been 
  shown 
  by 
  Renner*" 
  to 
  occur 
  with 
  extra- 
  

   ordinary 
  frequency 
  in 
  the 
  Oenotheras, 
  and 
  this 
  peculiarity 
  is 
  without 
  doubt 
  

   at 
  the 
  bottom 
  of 
  the 
  difficulties 
  which 
  have 
  beset 
  the 
  genetic 
  analysis 
  of 
  

   these 
  plants. 
  The 
  complexes 
  are 
  in 
  several 
  forms 
  or 
  species 
  not 
  borne 
  

   equally 
  by 
  the 
  two 
  sexes 
  of 
  the 
  same 
  plant, 
  and 
  most 
  of 
  them 
  are 
  unable 
  

   to 
  exist 
  in 
  the 
  homozygous 
  state. 
  These 
  discoveries 
  greatly 
  elucidate 
  the 
  

   (Enothera 
  problem. 
  For 
  instance, 
  not 
  only 
  (En. 
  lamarckiana, 
  but 
  biennis, 
  

   muricata, 
  and 
  others 
  also, 
  are 
  not 
  homozygous 
  types, 
  but 
  heterozygotes 
  of 
  a 
  

   special 
  kind. 
  Consequently, 
  the 
  production 
  by 
  them 
  of 
  "mutants" 
  is 
  not 
  

   capable 
  of 
  the 
  simple 
  interpretation 
  originally 
  applied 
  to 
  them 
  by 
  de 
  Vries. 
  

   Eenner 
  suggests 
  that 
  the 
  mutants 
  arise 
  owing 
  to 
  some 
  interchange 
  between 
  

   the 
  complexes 
  which 
  at 
  present 
  we 
  cannot 
  investigate, 
  but 
  whatever 
  be 
  

   the 
  exact 
  manner 
  of 
  their 
  origin 
  we 
  cannot 
  regard 
  them 
  as 
  genuine 
  examples 
  

   of 
  the 
  production 
  of 
  novel 
  forms 
  by 
  a 
  homozygous 
  type. 
  

  

  Before 
  leaving 
  this 
  part 
  of 
  the 
  subject, 
  we 
  may 
  notice 
  that 
  the 
  sup- 
  

   position 
  that 
  segregation 
  is 
  concerned 
  solely 
  with 
  characters 
  of 
  a 
  superficial 
  

   or 
  trivial 
  nature 
  has 
  been 
  long 
  ago 
  disproved. 
  Baur's 
  Antirrhinums, 
  the 
  

   study 
  of 
  which 
  was 
  continued 
  by 
  Lotsy, 
  were 
  an 
  excellent 
  demonstration 
  

   to 
  the 
  contrary, 
  for 
  they 
  provided 
  many 
  illustrations 
  of 
  segregation 
  in 
  

   features, 
  the 
  " 
  specific 
  value 
  " 
  of 
  which 
  no 
  systematist 
  would 
  question. 
  If 
  

   further 
  evidence 
  were 
  needed, 
  it 
  may 
  be 
  found 
  in 
  the 
  fine 
  series 
  of 
  experi- 
  

   ments 
  lately 
  published 
  by 
  Heribert-Xilssont 
  in 
  Salix, 
  which, 
  contrary 
  to 
  

   the 
  belief 
  long 
  ago 
  entertained 
  by 
  Wichura, 
  show 
  that, 
  when 
  F2 
  families 
  

   are 
  raised 
  in 
  adequate 
  numbers, 
  very 
  striking 
  segregation 
  occurs 
  in 
  the 
  

   species-crosses. 
  Many 
  geneticists 
  are 
  inclined 
  to 
  the 
  view 
  that 
  segregable 
  

   characters 
  should 
  be 
  pictured 
  as 
  implanted 
  on 
  an 
  irreducible 
  base 
  which 
  

   is 
  outside 
  the 
  scope 
  of 
  segregation, 
  but 
  no 
  means 
  have 
  yet 
  been 
  devised 
  

   for 
  testing 
  the 
  reality 
  of 
  this 
  conception. 
  

  

  2. 
  The 
  Moment 
  of 
  Segregation. 
  

  

  The 
  next 
  question 
  is 
  to 
  determine 
  when 
  in 
  the 
  various 
  life-cycles 
  segre- 
  

   gation 
  can 
  occur. 
  Obviously 
  it 
  is 
  a 
  phenomenon 
  of 
  cell-division. 
  If 
  we 
  

  

  * 
  ' 
  Zts. 
  f. 
  ind. 
  Abstammungs- 
  und 
  Vererbungslehre,' 
  xviii, 
  1917, 
  p. 
  121. 
  

  

  t 
  Lund's 
  ' 
  TJniversitets 
  Arsskrift,' 
  N.F., 
  Avd. 
  2, 
  vol. 
  14, 
  November 
  28, 
  1918. 
  

  

  