﻿Retrograde 
  Varieties 
  153 
  

  

  morphologic 
  entities, 
  but 
  are 
  to 
  be 
  considered 
  

   as 
  physiologic 
  units, 
  present 
  in 
  all 
  parts 
  of 
  the 
  

   organisms, 
  and 
  manifesting 
  themselves 
  where- 
  

   ever 
  occasion 
  is 
  afforded. 
  They 
  are 
  units 
  in 
  

   the 
  sense 
  that 
  they 
  may 
  appear 
  and 
  disappear 
  

   singly. 
  But 
  very 
  often 
  they 
  are 
  combined 
  to 
  

   yield 
  compound 
  characters, 
  which 
  are 
  capable 
  

   of 
  analysis. 
  Opportunities 
  for 
  such 
  an 
  analysis 
  

   are 
  afforded 
  by 
  these 
  groups 
  of 
  cultivated 
  va- 
  

   rieties, 
  of 
  which 
  some 
  members 
  show 
  a 
  single 
  

   distinguishing 
  quality, 
  or 
  a 
  number, 
  of 
  them. 
  

  

  