﻿Systematic 
  Atavism 
  637 
  

  

  to 
  occur 
  as 
  an 
  anomaly, 
  incompletely 
  developed, 
  

   often 
  very 
  rare 
  and 
  with 
  all 
  the 
  appearance 
  of 
  

   an 
  accidental 
  variation, 
  but 
  sometimes 
  so 
  com- 
  

   mon 
  as 
  to 
  seem 
  nearly 
  normal. 
  

  

  Coming 
  now 
  to 
  particular 
  instances, 
  we 
  may 
  

   turn 
  our 
  attention 
  in 
  the 
  first 
  place 
  to 
  the 
  genus 
  

   Sisymbrium. 
  This 
  is 
  a 
  group 
  of 
  about 
  50 
  species, 
  

   of 
  wide 
  geographic 
  distribution, 
  among 
  which 
  

   the 
  hedge 
  mustard 
  (S. 
  officinalis) 
  is 
  perhaps 
  the 
  

   most 
  common 
  of 
  weeds. 
  Two 
  species 
  are 
  re- 
  

   puted 
  to 
  have 
  bracts, 
  Sisymbrium 
  hirsutum 
  and 
  

   8. 
  supinum. 
  Each 
  flower-stalk 
  of 
  their 
  long 
  

   racemes 
  is 
  situated 
  in 
  the 
  axil 
  of 
  such 
  a 
  bract, 
  

   and 
  the 
  peculiarity 
  is 
  quite 
  a 
  natural 
  one, 
  corre- 
  

   sponding 
  exactly 
  to 
  what 
  is 
  seen 
  in 
  the 
  inflor- 
  

   escence 
  of 
  other 
  families. 
  Besides 
  the 
  Sisym- 
  

   brium 
  some 
  six 
  other 
  genera 
  afford 
  similar 
  

   structures. 
  

  

  Erucastrum 
  pollichii 
  has 
  been 
  already 
  allud- 
  

   ed 
  to 
  in 
  a 
  former 
  lecture 
  when 
  dealing 
  with 
  the 
  

   same 
  problem 
  from 
  another 
  point 
  of 
  view. 
  As 
  

   previously 
  stated, 
  it 
  is 
  one 
  of 
  the 
  most 
  manifest 
  

   and 
  most 
  easily 
  accessible 
  examples 
  of 
  a 
  latent 
  

   character 
  becoming 
  active 
  through 
  systematic 
  

   atavism. 
  In 
  fact, 
  its 
  bracts 
  are 
  found 
  so 
  often 
  

   as 
  to 
  be 
  considered 
  by 
  some 
  authors 
  as 
  of 
  quite 
  

   normal 
  occurrence. 
  Contrasted 
  with 
  those 
  of 
  

   the 
  above 
  mentioned 
  species 
  of 
  Sisymbrium, 
  

   they 
  are 
  not 
  seen 
  at 
  the 
  base 
  of 
  all 
  the 
  flower- 
  

  

  